Laminated metal composites (LMCs) can be produced nicely by the accumulative roll bonding process ARB, where different metallic sheets are stacked and rolled together. Other than the different processes used earlier to produce LMCs the ARB-process allows to obtain so-called nanolaminates with layer thicknesses in the range of a few micrometers down to some hundred nanometers. With such an approach materials can be tailored to achieve optimum properties. For example stacks of different Al alloys have been produced where a corrosion resistant alloy is on the outside and a high strength alloy in the inside of the sheet. Also nanolaminates of Al/Ti, Cu/Al and Al/steel have been produced. By stacking sheets of different purity of the same metal it is also possible to create an inhomogeneous laminate, where the grain size inside the different lamellae varies significantly due to faster recrystallization of the purer layer. By such a process, a microstructure is obtained which is very similar to bimodal microstructures obtained after annealing nanostructured materials. Graded sheets can be processed by using particle spraying between the different rolling cycles or by local heat treatments, where the sheet is heated from the outer surface, leading to a graded microstructure.
The mechanical properties of such nanolaminates have been investigated by compression and nanoindentation testing. Both test methods allow also measuring the strain rate sensitivity, which is very important for the ductility of these materials. Also the fatigue behavior of such nanolaminates produced by ARB has been investigated for the first time, where crack propagation mechanisms at the interfaces is important and will be discussed in the presentation.

We employ a microstructure design concept to simultaneously improve strength and ductility by optimizing the internal stress distribution in a deforming sheet. It can be realized for different applications in a wide variety of microstructure configurations, although we here demonstrate its validity by designing a graded ultrafine-grained ferrite martensite dual-phase steel, in which a smooth microstructure gradient is introduced by selective shear during thermo-mechanical treatment. Compared to the reference case with the same composition and mean grain size but no microstructure gradient, this novel microstructure demonstrates simultaneous increases in strength and ductility, and hence, confirms the proposed concept.

The recent progress in multi-material additive manufacturing technologies brings about a new opportunity for materials science and engineering emerging on the mesoscopic scale, a scale between the microstructure (microscale) and that of an engineering component (marcoscale). For example, by delivering a mixture of multiple materials as the feedstock and by varying the volume ratio of each material during the manufacturing process, components with controlled composition and property gradients can be made, e.g. with metals and alloys using direct laser deposition, or with polymers using inkjet-based 3D printing. Here we explore the problem of “mesostructure optimization”, that is, to determine the best materials distribution for a given mechanical application through computation design and optimization. The concept of mesostructure optimization is illustrated with an example problem, in which a functionally graded cylinder is designed for maximum resistance to contact loading. By optimizing the mesostructure of the cylinder, the Hertzian stress fields are redistributed, and the safety factor against mechanical overload is significantly improved as compared to any possible monolithic counterpart. In addition, mesostructure optimization is shown to be an effective approach for multi-objective optimization of materials performance, including e.g., improved mechanical performance at lower density. The optimization methods used in this work present a generalizable approach to design and optimization of functionally graded of materials; these methods are not limited to mechanical applications, but can also be applied to multi-physics applications.

The microstructure evolution, tensile properties and corrosion resistance of large dimensional nanocrystalline 316 stainless steel after rolling with different thickness reduction were characterized in details. The results showed that nanocrystalline phase size decreased from 100~800nm to 20~100nm and dispersed more uniform in the microcrystalline phase in the 316L steel with the thickness reduction, the strength, ductility and corrosion resistance increased with the thickness reduction, whereas elongation increased a little. When the thickness reduction was 70%, the tensile strength, yield strength and elongation was about 985MPa, 800MPa and 20%, respectively, the combination of strength and elongation was the best in the reported value of stainless steels. This new nanostructure designing route can be used to all of alloys to get high tensile strength and ductility.

The development of materials with gradients in structure and composition from the surface to the interior is an innovative route to design materials to satisfy the requirement of enhanced resistance to wear, fatigue and corrosion, i.e. a demand for multifunctionality. The gradient in structure e.g. grain size can be achieved through surface plastic deformation processes such as shot peening and deep rolling. Recently it been shown that addition of a small amount of alloying elements in metals can enhance the structure grain refinement and surface structures with a grain size as small as 5 nm can be produced by surface plastic deformation [1]. The gradient in composition can be achieved through chemical/physical coating or chemical-thermal treatment such as nitriding and carburizing. In order to investigate the possibility to design the mechanical, chemical and physical properties of the surface layer od metals, in this work we explore new approaches to produce gradient nanosurfaces by combining coating or chemical-thermal treatment with plastic deformation. Two examples are shown to demonstrate a generation of a large depth of a nanosurface layer with gradient in structure and composition by selected combined processes. The hardness profiles and enhancement in wear resistance and surface toughness are examined for the produced gradient nanosurfaces. The relationship between the gradients in structure and chemistry and the enhancement in mechanical properties are discussed based on quantitative microstructural characterization and hardness measurement in order to analyze key strengthening mechanisms.
[1] D. A. Hughes and N. Hansen, PRL 112, 135504 (2014)

In this study, the effects of Ultrasonic Nano-crystal Surface Modification (UNSM) on residual stresses, microstructural changes and mechanical properties of austenitic stainless steel 304 were studied. Due to high level of plastic strain induced by the multiple strikes during UNSM, surface nanocrystallization and transformation to martensite has been achieved and high magnitude of compressive residual stresses are generated on material surface. Highly dense deformation twins have been generated at material subsurface to a depth of 100 microns. The sandwich microstructure with two strong surface layers and a compliant interior embedded with highly dense nanoscale deformation twins leads to both high strength and high ductility. The work-hardened surface layers and high magnitude of compressive residual stresses lead to significant improvement in fatigue performance. The results indicate that UNSM is a powerful surface processing technique that can improve component properties and performance.

Thin walled metallic tubes are used in various medical device applications. These devices require specific mechanical properties which could vary from the inside of the tube to the outer surface. Manipulation of the properties can be done by producing a gradient microstructure across the tube wall.
The process of High Pressure Tube Shearing (HPTS) developed by the authors is a severe plastic deformation technique which enhances the strength of metallic tube walls in the bulk, i.e. throughout the entire wall thickness or only in near-surface regions. Through the choice of process parameters, it is possible to control the thickness of the HPTS-modified layer or to produce a specific gradient in microstructure. A severe shear strain is imposed within the thickness of the tube wall due to a difference in magnitude of the material flow velocities at the inner and outer surfaces of the tube, which results from the difference in rotational speed of the mandrel and the confining die. Concurrently, a high hydrostatic pressure is produced by means of a special geometry of the mandrel designed to reduce the tube wall thickness. The combined severe shear strain and high hydrostatic pressure are applied within a localized zone, which moves progressively along the tube. This type of deformation processing results in a significant grain refinement of the tube material leading to a pronounced increase in strength. By using a zero rotation speed either of the mandrel or the die, a gradient in microstructure can be achieved assisted by the difference in kinematic and static friction.
The feasibility of this technique was demonstrated on three types of steel tubes with different level of carbon, which were processed with two different values of the ratio of rotation-to-translation speeds. All samples were found to exhibit substantial grain refinement. The minimum grain size close to the mandrel was as small as ~100 nm (within a limited depth). A pronounced grain size gradient was established, the thickness of lamellar grains varying from the order of 100 nm to ~1 mu;m towards the outer edge. Higher ratios of rotation-to-translation speeds caused a greater extent of grain refinement and variation in the depth of the ultrafine grained layer for all steel grades tested. As a result of grain refinement, the ultimate tensile strength was substantially increased over that of the as received steel grades.

In recent decades, intense research efforts have been made for strengthening of existing commercial structural materials to fulfill the increasing demand of materials with improved load bearing capacity and superior in-service performance. In particular, significant efforts were made to strengthen metallic materials, i.e. metals and alloys, via grain refinement. The resulting homogeneous fine-grained microstructures, consisting of sub-micron or nanometer sized grains, exhibited significant improvement in the strengths. However, grain refinement also yielded extremely poor ductility at room temperature. Therefore, the inability of the homogeneous fine/coarse grained microstructures to fulfill the requirement of materials with high strength and reasonable ductility led to the development and evaluation of heterogeneous microstructures in recent years. Especially, bulk metallic materials consisting of bimodal grain size distribution have demonstrated the ability to provide a combination of high strength together with sufficient ductility. However, preparing bulk materials with controlled and reproducible bimodal structure remains a challenge. As a result, the reliability of properties and performance of such materials has been an important issue. In order to deal with the aforementioned issues, Ameyama and co-workers have proposed a unique heterogeneous microstructural design based on bimodal grain size distribution, called Harmonic Structure, for the strengthening of metallic materials. The harmonic structure design is essentially a bimodal microstructure with a specific topological distribution of fine and coarse grained regions, wherein coarse-grained areas are interlinked through a three dimensional interconnected network of fine-grained regions. Moreover, an attractive and efficient powder metallurgy (PM) processing approach was also developed to prepare bulk material with harmonic structure. The harmonic structure design proved extremely efficient in achieving a combination of high strength and high ductility in a variety of metals and alloys. It also resulted in excellent reproducibility of microstructure and properties. The present work deals with the synthesis and evaluation of harmonic structured Ti-6Al-4V (Ti-64) and Co-Cr-Mo (CCM) alloys which are the most important commercial bio-structural materials. It has been demonstrated that the creation of harmonic structure led to significant improvements in the mechanical properties of these important commercial materials also. An effort has also been made to correlate the effect of such a typical microstructure on the deformation behavior and properties of Ti-64 and CCM alloys. Particularly, it was observed that the network structure suppresses the localization of stress and strain, and thus, the work hardening became enhanced and the uniform elongation was increased.

We summarize our recent works on the advanced metallic nanomaterials with exceptional dual mechanical properties using multiscale metallurgical structure-driven design combined with advanced mechanical simulation. The effect of nanostructured materials on the mechanical behavior and on the failure mechanism of metallic material shows the possibility to develop a new strength gradient composite. The computational models and experimental results successfully provide valuable information about the nanomaterials properties as a function nanostructure configuration (nanograins and nanotwins). The processing of nanomaterials using mechanical processing and heat treatment has been studied at nanoscale and atomic scale. The material studies using nanomechanics based experimental investigations (nanoindentation and nano-pillar tests) can reveal the effects of the atomic structure and nanostructure gradient on the mechanical behaviors. The failure mechanisms studies at nano-, micro- and macroscopic scale can provide efficient ways to enhance the ductility of materials using the general approach of strain non localization. We shall specially introduce the progress on the development of the high strength and high ductility industrial alloys with high density nano twinned stainless steels and hierarchical nanotwinned TWIP steels. The potential applications in light weight land transportation systems will be presented. The integration of nanomaterials using advanced design tools with associated processing development will be introduced.

The outmost surface of engineering materials typically faces the most severe risk of damage. When such boundary conditions are given, employment of graded structures is desired to enhance the safety of the structure in an economic way. Mechanically, adopted gradient structures typically involves change in Young&’s modulus or strength, and are beneficial to crack shielding, reducing stress concentration, retarding shear localization, etc. In this of discussion, we present experimental observation on how gradient twins could enhance the strength of a twin-induced plasticity steel while retaining its tensile ductility. The mechanisms accounting for the exceptional mechanical properties were investigated at the microscopic level. The switch of twinning systems at the grain level during different deformation stages and the subsequent formation of hierarchical nanotwinned structure are responsible for the strengthening and toughening of the material.

Usually, strength and ductility properties are opposite to each other for structural materials, and hence methodology of producing such materials with both high strength and ductility has been a long-standing challenge in materials science. Here, a novel structure consisting of Ni with grain size ranging over three magnitudes from 28 nm to 4 µm is prepared by electrodeposition. Moreover, the grain size distributions (degree of gradient) were adjusted. It is found that the gradient Ni shows a good combination of high strength and plasticity. At optimum gradient, the yield strength is 700 MPa, which is twice higher than that of coarse grained Ni, while uniform elongation is 7%, that is almost unchanged. It is found that the extra deformation ability is attributed to the deformation induced twinning and grain growth in the gradient materials, which could effectively absorb the energy and accommodate the large plastic strains. The novel structure not only provides ways to fabricate the materials with optimum structure/property, but also offers an ideal model to investigate the deformation mechanism in gradient materials.

The trade-off between strength and ductility has been a long-standing problem in material research. Nano-grained metals have high strength but limited ductility, which limits their applications. Extensive effort has been devoted to increasing the strength of materials while retaining their ductility. Recent experiments have shown that a gradient nano-grained layer on a coarse-grained substrate will lead to an extraordinary combination of high strength and high ductility. In addition, nanotwinned materials are also found to achieve simultaneous high strength and high ductility. Here, we report preliminary modeling and simulation studies aimed to investigate the relationship between the tensile ductility and strength of a gradient-grained material with various grain size and twin thickness distribution profiles.

Abstract: Mechanically-induced grain coarsening dominated plastic deformation has been observed in many nano-grained (NG) materials. However, the intrinsic mechanism for mechanically-induced grain coarsening is not well explored yet. Here in this study, we focused on the gradient nano-grained (GNG) surface layer on coarse-grained (CG) Cu substrate and provided systematic studies of the effects of temperature and strain rate on the tensile properties as well as microstructure stability of the GNG Cu layer during tensile deformation, respectively. It is demonstrated that continuous and homogenous grain coarsening with increasing strain was detected with a variety of temperature and strain rate. However, at a given strain, the extent of grain coarsening was less significant at lower deformation temperature and higher strain rate, although higher strengths were achieved in GNG Cu layer at these two conditions. The results demonstrated clearly that the homogenous grain coarsening accompanying the plastic deformation of GNG Cu layer was thermally activated and not exclusively controlled by the stress.

A gradient nanostructured (GNS) surface layer has been produced on AISI 316L stainless steel by means of surface mechanical rolling treatment (SMRT). The mean grain size is ~30 nm at the topmost surface layer and increases gradually with depth. Tension-compression fatigue tests showed that the fatigue strengths in both high-cycle and low-cycle fatigue regimes are significantly enhanced by the GNS surface layer. In comparison with the initial sample, the fatigue limit increases from 180 MPa to 420 MPa of the SMRT rod sample with a diameter of 3 mm, and an increment of ~50% is achieved in the fatigue strength at the fatigue lifetime ~10³ cycles. Meanwhile, the fatigue ratio (fatigue limit vs tensile strength) significantly increases with tensile strength in the SMRT sample.
The correlations between microstructure, mechanical and fatigue properties of the GNS sample have been clarified by investigating aspects of crack initiation and propagation, cyclic deformation behaviors, residual stresses, etc. The results emphasized the GNS surface layer enhances the fatigue property by suppressing the initiation of cracks and accommodating a remarkable cyclic plastic strain amplitude. Deformation-induced martensite formed in the sample interior also contributing to the enhanced fatigue strength. Nevertheless, the effect of the pronounced compressive residual stresses is insignificant in the enhancement.

Surface mechanical attrition treatment (SMAT) has demonstrated its ability to refine the grain size of polycrystalline AZ31 Mg alloy with gradient distribution from top surface to inside, hence effectively improved the alloy&’s mechanical performances. However, there is a need to better understand the microstructure-property relationship for their different fracture mechanisms due to grain refinement. Here, by using tensile testing with in situ scanning electron microscopy (SEM) method, we examined two types of SMATed AZ31 samples with different grain sizes at ~10 um (“coarse grain sample”) and ~5um (“fine grain samples”), respectively, and compared with the raw AZ31 Mg alloy (with grain size ~15-20 um). Through quantitative tensile testing with one-to-one correspondence to the real-time SEM imaging, we showed that fracture of “fine grain samples” was mainly initiated by inter-granular crack and dominated by “tortuous crack propagation”, which effectively enhanced their toughness. While for the coarse grain samples, it showed clearly that trans granular crack lead to the final failure. Both inter-granular and intra-granular cracks were activated in the raw material and their nucleation resulted in the growth of main crack, led to the final failure. It is expected that this mechanisms could be applicable for some other grain refined metals and alloys, and we plan to use this powerful technique to study their corresponding strengthening and failure mechanisms.

In this talk we will present and discuss the perspective, prospects, and problems of gradient structures, especially in the nano-scale. Gradient structures are formed and evolved naturally in many biological systems such as bones and plant stems for superior mechanical and functional properties. They have been engineered into mechanical parts via thermochemical routes such as nitriding and surface heat treatments. Recently gradient structures with grain size gradient from nano-scale to micro-scale were introduced into metals, producing excellent strength and ductility. Materials with gradient structures represent an emerging academic field with many fundamental and engineering problems to be solved by researchers from the communities of materials science and materials mechanics. Some of the issues can only be solved by team effort from both experimentalists and theorists.

Laminate microstructures contribute to strength of materials in a wide range of systems. We review some recent results on pearlitic steels, Cu-Nb and nano-twinned metals. The Cu-Nb laminates are made by two methods, namely accumulated roll bonding (ARB) and physical vapor deposition (PVD) with lamellae thicknesses in the range 10 µm to 10 nm. Nano-twinned copper and silver are electrodeposited, for which a considerable range of texture type and strength is accessible. The pearlitic steels are self-evidently a naturally occurring nano-laminate composite. The strength of the composites, as measured by nano-indentation (and other methods) increases with decreasing lamella thickness as has long been known.
The analysis of the pearlitic steels used the viscoplastic FFT model and focused on a) the influence of the different orientation relationships that have been postulated for this system and b) the influence of the geometry of the two phases e.g. lamellar versus globular cementite. The main findings are that a) the deformation is unaffected by the choice of the orientation relationship whereas b) the properties are strongly sensitive to the phase morphology, which suggests that load transfer across the hard (cementite) phase is an important feature.
Similar strengths in the Cu-Nb composites are seen in ARB and PVD material across the length scale, with ARB strength being slightly lower. The PVD composites are tested as-deposited whereas the ARB materials are the result of substantial plastic deformation, so it is surprising that the latter are less hard. Strength varies with average layer thickness following a Hall-Petch relationship. The nano-twinned copper & silver follow a similar trend, albeit at substantially lower strengths. The more physically based confined layer slip (CLS) model describes the variations in strength across the range of thicknesses. However there are other models that also can explain the variations.

The laminates Ti6Al4V/AA2519 were fabricated by explosive welding in two versions: with and without an aluminium alloy 1050 interlayer plate. In the present communication, the focus is on interface phenomena such as diffusion and phase transformations during bonding and subsequent annealing as well as mechanical behaviour of the laminate structures. The results demonstrated that both Ti6Al4V/AA2519 and Ti6Al4V/AA1050/AA2519 composite plates exhibit good quality bonding. Scanning electron microscopy observations and energy-dispersive X-ray spectrometry (EDS) results revealed that bimetallic compounds of Ti/Al form at the bonding interface in AA 1050 interlayer. Ti/Al interlayer had a thickness of about 5 µm and consisted of nano-sized grains formed along entire bonding interface. In the samples without aluminium interlayer the thickness of bimetallic Ti/Al interlayer was significantly smaller. However, near the joint one can see the grid in the grain boundaries rich in copper. These intermetallic inclusions were often accompanied by nano-cracks. The value of microhardness decreased when the distance from the interface increased. Tensile tests revealed that in both types of joints the interfacial zone wasn&’t the weakest part of this composites.

In many interface-dominated nanostructured materials the role of interfaces during deformation is not yet completely clarified. Very fine spacing of interfaces leads to a competition between dislocation controlled and grain boundary sliding based plasticity. To improve our understanding of this competition we have to investigate the atomistic origin of deformation in the interface region.
We present the results of molecular dynamics simulations of sliding at γ/γ interfaces in lamellar TiAl alloys, which can explain existing, seemingly contradictory, experimental results on the role of interfacial sliding during creep. We suggest that the origin of the controversy lies in the pronounced in-plane anisotropy of the shear strength of the individual interfaces, which is observed in the simulations. Experimentally, the orientation of in-plane directions with respect to the loading axis has not been monitored so far.
A multi-scale concept is introduced to capture effects of both the electronic and the atomistic level. On the one hand we carried out quasi-static calculations of multi-planar generalized stacking fault energy (MGSFE) surfaces of the interface plane as well as the adjacent layers. On the other hand molecular dynamics simulations guided by ab initio GSFE-surface calculations were carried out for different bicrystal cells under different shear loading conditions. The critical stresses for shearing some of these interfaces, which were derived from these bicrystal shear simulations, are of the same order of magnitude or even lower as those for dislocation motion in a γ-single crystal, showing that these mechanisms are competitive.
In total four shear mechanisms, twin nucleation and migration/absorption, interfacial partial dislocation nucleation, rigid grain boundary sliding and grain boundary migration were observed. The comparison with the MGSFE surfaces allows to create a link between (quasistatic) physical properties such as stacking fault energies, and the (dynamic) deformation mechanisms, and hence between the results of ab-initio calculations and molecular dynamics simulations.

Magnesium and its alloys, the lightest structural materials, have a huge potential as structural materials in the aerospace and automotive industries and have been actively investigated for the past few decades. However, Mg with a hexagonal close packed structure lacks the ductility and formability of cubic materials due to scarcity of easy slip systems and the localized shear characteristic of twinning. Therefore, for their applications as structural materials, there is a strong desire to improve the deformability while maintaining the high flow strength of Mg and Mg alloys. Multilayered metallic materials containing a high density of interfaces show promising enhancements in both the yield strength and ductility of layered composites. The mechanical strength of the Mg/Nb multilayers measured according to hardness can achieve a high value of ~1.0 GPa when the layers are a few nanometers thick. Besides such high strength, we also demonstrated such layered structure can enhance fracture toughness associated with highly activated basal slip and interface shearing between Mg and Nb interfaces. We performed in situ three-point bending experiments in SEM and found that Mg/Nb interfaces could block crack propagation accompanying with blunting crack tip. Stress concentration at crack tip is significantly released by shearing interfaces parallel to Mg basal planes.

Magnesium and its alloys, the lightest structural materials, have attracted a lot of attention of the automotive industry for reducing the vehicle&’s weight to improve its fuel efficiency. However, Mg has a hexagonal close packed (HCP) structure, which lacks the ductility and formability of cubic materials due to scarcity of easy slip systems to accommodate an arbitrary plastic deformation mode. Layered composites are promising because presence of high density of interfaces in such materials that can enhance both yield strength and ductility. In this work, we present first-principles study of the energetics and atomic solubility of coherent interface in Mg/Nb multilayers. We find that solutes Zn, Cd, Sc, Ti and Zr show better solubility at Mg/Nb interface compared to the bulk Mg and the bulk Nb and would segregate at the interface. We also search for solutes that can stabilize Mg/Nb interfaces and suppress deformation twinning in Mg to improve the mechanical properties of Mg/Nb multilayer systems.

Nanolayered composites such as Cu-Ag, Cu-Nb and Zr-Nb with layer thicknesses less than 100 nm exhibit enhanced material properties such as ultra-high strength, high ductility, and radiation damage resistance. At such fine length scales, the atomic structure of the interface plays a dominant role in its response to defects and dislocations. In this work, a combined approach of experiment and atomistic and mesoscale modeling elucidates the effects of the atomic structure of bimetal interfaces on the propensity for deformation twinning. We utilize Accumulative Roll-Bonding (ARB) to process bulk Cu-Nb and Zr-Nb nanolamellar composites from 1 mm thick high-purity polycrystalline sheet into bulk nanocomposites with individual layer thicknesses below 100 nm. The ARB process imposes thousands of percent strain to refine the microstructure of ordinary coarse-grained composite metals down to submicron and nanoscales. TEM analysis showed that the stable ARB interface structure at layer thicknesses less than 50 nm maintains the conventional Kurdjumov-Sachs (KS) orientation, but joins the Cu and Nb crystals at interface planes different from those observed in Physical Vapor Deposited materials investigated previously: {112}Cu || {112}Nb and <110>Cu || <111>Nb. In addition, twin formation was experimentally observed in copper on a plane that lies 19.5 degrees from the {112} interface plane, which had not been previously observed in rolled physical vapor deposited (PVD) nanolayered Cu-Nb composites. Moreover, the ability of interfaces in the Cu-Ag system to aid in twinning processes at fine length scales is investigated and compared to those found in Cu-Nb. Cu-Ag nanoscale multilayers, in bulk cast eutectic form, have a cube-on-cube or twinned orientation relationship with interfaces consisting of common {111} planes. These differing interface types will be discussed in terms of the influence of interfacial geometry, dislocation kinetics, interfacial energetics, and atomic structure on the propensity for different deformation mechanisms.

Interfaces in materials can play an important role in determining mechanical performance of the materials through the complicated interaction between defects and interfaces. In metallic nanolayered composites, interface barrier strength (IBS), which refers to the resistance for dislocations to cross an interface, is a key factor to determine the ultrahigh strength of nanolayered composites. Although several theoretical models have been proposed to describe strengthening mechanisms in multilayers and primary contributions to the IBS, the variation of interface properties caused by element interdiffusion at atomic scales and its influence on the IBS of nanolayered composites are still not well understood. In this talk, we will present experimental and theoretical investigations of the IBS of Cu/Au multilayers, emphasizing effects of interface properties and structures. The Cu/Au multilayers with individual layer thickness ranging from 25 to 250 nm were annealed at different temperatures to modulate Cu/Au interface properties. Experimental results from nanoindentation testing show that the Hall-Petch slope in the relation between the strength and the individual layer thickness of the multilayers gradually decreases with increasing annealing temperature, indicating a decrease in the IBS. TEM and HRTEM characterization reveals that element interdiffusion between the Cu layer and the adjacent Au layer leads to a compositional gradient at the interface. A detailed analysis for effects of the compositional gradient on the resistance to dislocation crossing the interface was conducted. It is demonstrated that the interface structure of the Cu/Au multilayers has become the most important factor in governing the IBS.

The thermal stability of Mg/Ti multilayer nanofilms was investigated by examining their microstructure and nanoindentation hardness after annealing at various temperatures and time periods. The multilayers with individual layer thickness hge;5 nm exhibit excellent capability of maintaining the lamellar microstructure and high strength up to 200 °C for annealing time up to 2.0 hours. The annealed multilayer films with h=2.5 nm are still highly textured but characterized with discontinuous layer interfaces, in which the transition of atomic arrangement from hexagonal close-packed (HCP) to body-centered cubic (BCC) structure was observed at columnar boundaries. The degradation of uniform lamellar microstructure is related to the decrease of hardness with annealing temperature at this size scale. A diffusion based instability mechanism was proposed for this typical HCP-based nanoscale multilayer system.

Tri-component multilayers, such as Cu/Ni/Nb. that contain both coherent and incoherent interfaces have a greater capacity for strain hardening than a tri-component bilayer (Cu-Ni/Nb) system, in which only incoherent interfaces are present. However, the thermal stability of these systems is not as well studied as other bi-metallic multilayer systems. The current study examines both the mechanical response and the microstructure and interfacial structure of sputtered Cu/Ni/Nb tri-layers and Cu-Ni/Nb in the as-deposited and annealed conditions. In general it may be expected that coherent interfaces would provide relatively easy diffusion and homogenization and hence softening of the multilayer was expected following annealing at 573 and 773 K for several hours. However, post-annealing nanoindentation of both the bi and tri-layer films showed an increase in hardness of the tri-layer film after elevated temperature testing had been conducted, indicating the likelihood of a change in the microstructure. X-ray diffraction experiments suggest that microstructural changes are taking place in the coatings following even a modest anneal; peaks from Cu and Ni in the tri-layer system are observed to begin to merge and there is some evidence for new peaks forming. Transmission electron microscopy revealed that while the grain size is relatively stable, a Ni-Nb intermetallic forms at both the Ni/Nb, Cu-Ni/Nb and Cu/Nb interfaces. It was also observed that considerable interdiffusion of Cu and Ni took place above 700K. This formation of precipitates appears to add a strengthening component above and beyond that predicted from the confined layer slip model.

Multi-layered aluminum clad sheets are used for a variety of applications in airplane structural parts and automobile heat exchangers. Different alloys with different properties can be joined together to one sheet and it show improved properties or additional properties rater than each alloy. In this study, 6022/5023/6022 aluminum clad sheets were fabricated by roll bonding process and the mechanical properties of the annealed sheets were examined. The bonding strength of the sheets increases with increasing a thickness reduction rate of roll bonding, 6022/5023/6022 clad sheets can be fabricated successfully by roll bonding at room temperature with the thickness reduction rate per pass of 50%. After annealing at 500 ^oC for 1 hour, the sheets have a well bonded interface without a void and 2^nd phase and the microstructure of the sheets consisted of fully recrystallized grains. Stress-strain curves show same strain hardening characteristics with core Al5023 alloy sheets. Meanwhile, the elongation of the sheets was higher than that of commercial Al6022 or Al5023 alloy sheet. The best thickness combination for the elongation was 1:2:1 thickness ratio of 6022:5023:6022. In addition, the sheets had very low tensile anisotropy due to different deformation characteristics of each layer. The low anisotropy of the sheets brought with excellent drawability and the minimum earing at cupping test. It comes from a combination of different texture characteristics of each layered sheets. Numerical simulation results on the formability were corresponded well with the experimental result.

A severe plastic deformation (SPD) technique was applied to an Al-7075 alloy reinforced with 10 vol.% Al₂O₃. This processing method of high-pressure torsion (HPT) was performed at room temperature under a pressure of 6.0 GPa through a total number of up to 20 turns. The metal matrix composite (MMC) showed a significant grain refinement from an initial average grain size of ~8 mm to ~300 nm after processing by HPT through 20 turns which led to an increase in the average values of Vickers microhardness and strength at room temperature and an increase in the elongations to failure at elevated temperatures.

Graded nanocrystalline materials with spatial grain size gradients are promising to achieve new maxima in the strength-toughness space. Microstructure evolution of these emerging heterogeneous materials directly affects their thermomechanical stability, but is not understood mechanistically and has not been correlated with quantitative measures of nanoscale structural gradient. This talk will present mesoscale modeling of nanoscale gradient evolution in graded nanocrystalline materials with various types of spatial grain size gradients. We in particular focus on elucidating the correlation and interplay between the spatial grain gradient evolution and the evolution in the Grain Boundary Character Distribution (GBCD). Preliminary results reveal gradient-dependent grain kinematics and homogenization, as well as a transient state exhibiting spatial gradient in GBCD. Our work could offer quantitative insights into nanoscale grain heterogeneity design.

Fully annealed ultrafine grained metallic materials with mean grain sizes smaller than 1-2 miro-meter(s) universally exhibit discontinuous yielding characterized by yield-drop phenomenon. This is probably because dislocation multiplication within small volumes (fine grains) becomes difficult in ultrafine grained materials. At the same time, trans-grain propagation of plastic deformation is necessary for macroscopic yielding in polycrystalline materials. In the present study, 99% purity aluminum (2N-Al) and 99.99% purity aluminum (4N-Al) sheets were highly deformed by the accumulative roll bonding (ARB) process under different lubrication conditions. The ARB processed sheets were annealed and ultrafine grained microstructures having mean grain sizes around 1 micro-meter but different grain-size distributions could be obtained. It was found that the materials having broader grain-size distributions showed less obvious yield-drop even though the mean grain sizes were nearly the same. The results indicate that grain-size distribution or gradation would significantly affect yielding behaviors of polycrystalline materials, especially when the grain size is very fine.

Gradient microstructure materials have demonstrated unique properties based on the interactions of microstructures from the nano- to microscale from the surface to the core of a material. For gradient microstructures formed via deformation processing, the microstructural evolution is intrinsically tied to the level of deformation seen as a function of depth. In this study, we investigate the shear strain as a function of depth using deformed carbides in a low carbon steel as strain markers. The microstructural evolution, textural formation and hardening will then be correlated with the amount of shear strain induced as a function of depth. The results will forecast more efficient processing for desired properties based on specific knowledge of the amount of shear strain needed for optimal microstructures.

In this paper, Cu and Cu alloys were respectively processed by surface mechanical attrition treatment (SMAT) at liquid nitrogen temperature condition (LNT) and at room temperature (RT) for 5min, 15min and 30min, respectively. Due to the gradient strain and strain rate impacted into the sample, gradient microstructures were obtained from the treated surface to the central area of Cu and Cu alloy.
The strength and ductility Cu and Cu alloy are simultaneously increased.The yield strength of Cu-30%Zn and Cu-4.5%Al processed by the LNT-SMAT were ~325 MPa and 478 ~MPa, respectively. Moreover, The accepted uniform elongation of about 37% and 22% were also obtained.

Strain hardening is critical for structural materials to secure desired ductility, especially for high-strength metals that often suffer from poor ductility. Here we report the gradient nano-grained (GNG) surface layers sandwiching a coarse-grained core exhibit render an extra strain hardening. The grain size gradient in the nano-micro-scale induced a notable strain gradient under tension that converts the applied uniaxial stress to multi-axial stresses. Thereby the accumulation and interaction of dislocations are promoted in the GNG layers, resulting in an extra hardening and an obvious strain hardening rate up-turn. Such a unique extra strain hardening inherent to the GNG structures, which does not exist in homogeneously-structured materials, provides a novel strategy to develop strong-and-ductile materials by architecturing heterogeneous nanostructures. The work uncovers the intrinsic large uniform tensile elongation of the nanostructures and paves the way toward a combination of high strength and good ductility for their structural application.

Conventional metals are routinely hardened by reducing their grain size, or by cold working. However, this strengthening comes at the expense of ductility. Recent nanostructuring strategies have attempted to evade this strength versus ductility trade-off, but the paradox persists. It has never been possible to combine the strength reachable in nanocrystalline metals with the large uniform tensile elongation characteristic of coarse-grained metals. Here we attempt a new route of defect engineering on the nanoscale to approach this ultimate combination. For an elemental metal Ni, nanoscale domains (average 7 nm in diameter) were produced during electro-deposition, occupying only ~2.4% of the total volume. Yet the resulting Ni achieves a yield strength approaching 1.3 GPa, on par with the strength previously reported for nanocrystalline Ni with uniform grains. Simultaneously the material exhibits a uniform elongation as large as ~30% in uniaxial tension, close to the hallmark ductility of ductile face-centered-cubic metals. Electron microscopy observations and molecular dynamics simulations demonstrate that the spread-out nanoscale domains effectively block dislocations, akin to the role of precipitates for Orowan hardening in alloys. In the meantime, the abundant (mostly low-angle) domain boundaries provide effective dislocation sources and trapping sites of running dislocations for efficient dislocation multiplication and storage in the grain interior to sustain a pronounced strain hardening rate to large uniform elongations. The extraordinary product of strength and ductility sets the nanodomained Ni apart from all previous (from coarse-grained all the way to nanocrystalline) Ni with a homogeneous grain structure. The heterogeneously architected nanostructure thus enables access to a hitherto unoccupied regime in the property space for single-phase metals.

Many materials found in Nature have shown the ability to maximize their structural performance by implementing a gradual change in composition from the core to the surface. The advances in several manufacturing techniques such as surface mechanical attrition treatment (SMAT) are a driving force for development of new materials which are mimicking such naturally architectured materials. However, the prediction of the mechanical behavior of such materials is challenging and new computational models are required to better understand the mechanisms responsible for the improvement of mechanical properties and accelerate the design of new materials with architectured microstructure.
One potent model, which can be adopted for modeling such metallic gradient materials, is based on microstructure-related constitutive description in which the dislocation cell walls and the dislocation densities cell interior, entering the model as scalar internal variables, [1]. The resulting &’phase mixture&’ model can be extended to include the strain gradient theory that allows to account for a strengthening effect associated with microstructural gradient present in the material. Two types of strain gradient model are proposed arising from two distinct physical mechanisms. One is associated with the occurrence of geometrically necessary dislocations and inclusion of the gradient of first order in the Taylor type flow stress formulation [2]. The second one is associated with the reaction stresses due to plastic strain incompatibilities between neighboring grains and incorporate strengthening effect due to second derivative (Laplacian) of the equivalent strain, [2]. Both models have the advantage that the intrinsic length scale parameter depends on microstructural features. The numerical results will be validated by comparison with experimental data from high pressure torsion [2]. Other case studies related to materials processed by surface mechanical attrition treatment (SMAT) and three roll planetary milling [3] will also be presented.
References
[1] L.S. T#972;th, A. Molinari and Y. Estrin, Journal of Engineering Materials and Technology, Vol. 124, p. 71, 2002
[2] Y. Estrin, A. Molotnikov, C. H. J Davies, and R. Lapovok, Strain gradient plasticity modelling of high-pressure torsion, Journal of the Mechanics and Physics of Solids, Vol. 56, p. 1186, 2008
[3] Ya Li Wang, Andrey Molotnikov, Mathilde Diez, Rimma Lapovok, Hyoun-Ee Kim, Jing Tao Wang, Yuri Estrin, Materials Science & Engineering A 639, 165-172, 2015.

Recent experiments have demonstrated the intriguing effects of gradient grain sizes on the mechanical properties and deformation mechanisms in FCC and BCC polycrystalline metals. We have developed both crystal plasticity and atomistic models to investigate the gradient nanograins in FCC Cu. We adapt the classical crystal plasticity model to explicitly account for the grain size dependent yield strength and strain hardening. The associated finite element simulations reveal the non-linear effects of gradient nanograins on the global and local stress-strain behaviors. Moreover, our molecular dynamics simulations reveal the competing deformation mechanisms of grain growth and dislocation plasticity. Our simulation results are further coupled with experiments of gradient nanograined Cu to gain insights into the design of optimal gradient nanograins.

Copper-based high strength and high electrical conductivity nanocomposite wires reinforced by Nb nanofilaments are prepared by severe plastic deformation, applied with an Accumulative Drawing and Bundling process (ADB), for the windings of high pulsed magnets. The ADB process leads to a multi-scale Cu matrix containing up to N=85⁴ (52.2 10⁶) continuous parallel Nb filaments with diameter down to few tens nanometers. After heavy strain, The Nb nanotubes exhibit a homogeneous microstructure with grain size below 100 nm. The Cu matrix presents a multi-scale microstructure with multi-modal grain size distribution from the micrometer to the nanometer range.
The use of complementary characterization techniques at the microscopic and macroscopic level (in-situ tensile tests in the TEM, nanoindentation, in-situ tensile tests under high energy synchrotron beam) shed light on the role of the multi-scale nature of the microstructure in the recorded extreme properties.
We will present here how these experimental parameters can be exploited to derive relevant simulations at different length scales (from atomistic to crystal plasticity) to assess the respective roles of microstructure refinement and architecture in the high strength of these nanocomposite metals.
Acta Materialia, 57 (2009), 3157
Acta Materialia, 58 (2010), 1418
Acta Materialia, 59 (2011), 7744
Advanced Engineering Materials, 14-11 (2012), 998

A twinning-induced plasticity (TWIP) steel strengthened by elegant arrays of laminated nanotwins was manufactured by a simple thermomechanical treatment consisting of cold rolling and recovery annealing. Different to other lab-scale methods making nano-structured materials, the present simple thermomechnical treatment is suitable for large-scale production in the steel industry using existing facilities, which makes the present steel being an attractive structure material. The steel achieved a high yield strength (1450 MPa), high ultimate tensile strength (1600 MPa) and considerable uniform tensile elongation (20%). During the recovery annealing, the nanotwins are thermally stable so that they remain in the sample after recovery annealing. On the contrary, the dislocation density was reduced greatly after recovery annealing. The deformation mechanism of the present nanotwinned steel is investigated by synchrotron X-ray diffraction, transmission electron microscopy, nanoindentation and electrical resistivity, illustrating that the dislocation density increases dramatically with strain while the volume fraction of nanotwins remains constant. Therefore, the work hardening behaviour of the nanotwinned steel is mainly provided by the accumulation of dislocations.

This talk reviews microstructure-property correlations of multifunctional nanocrystalline materials. First part of this talk focuses our work on the introduction of metallic elements into ceramics to control mechanical, electrical, thermal and magnetic properties in a systematic way. We have chosen a model Ni-doped MgO system, where Ni can be located into substitutional sites or reduced and clustered into metallic precipitates. By using novel processing method, these relative fractions can be precisely varied and new mechanical and magnetic properties obtained. Metallic precipitates in ceramic bodies can be selectively heated and melted by high-power pulsed laser irradiation to create Frank-Reed sources for dislocations and provide a continuous source for ductility and fracture toughness of ceramics. In-situ TEM, studies on crack propagation clearly illustrate the role of metallic precipitates in arresting the cracks and enhancing dislocation generation, thus enhancing fracture toughness and ductility simultaneously. In the second part, we address processing of nanocrystalline materials of uniform size with controlled doping of interfaces. Our careful experiments in Cu, Zn and WC:NiAl systems clearly show that there is hardening effect with decreasing grain size in accordance with Hall-Petch relationship and intragrain deformation mechanism. However, below a critical size, intergrain deformation sets in, which leads to softening. These two regimes have been modeled and there is a good agreement with our experimental results of nanocrystalline materials of uniform size. Using these techniques, it is possible to create functionally gradient materials by changing the grain size and hardness with depth.

Metallic/ceramic composite nanostructures are a new type of materials that it is shown to exhibit high strength and relativity high ductility compared to its bulk counterparts. Because of the presence of the ceramic layer, they can serve as advanced coatings in high service temperature structures as these found in the cladding of nuclear reactors. In such applications, the existance of the interface can be of outmost importance, since they act as sinks to irradiation defects. However, not much work has been done so far on how these nanocomposites behave under irradiation. The present work uses molecular dynamics simulations to study the radiation damage resistance of a representative Nb/NbC metallic/ceramic systems and compared to that of the single ceramic. The damage is induced by a primary kinetic atom (PKA) with energies ranging between 1 and 40 keV at various temperatures. Both the Nb and C atoms were used as PKA atoms to study the effect of the different atoms. The results showed that the radiation damage resistance of the metalic/ceramic nanocomposite has been improved compared to the single ceramic. Furthermore, the damage of the interface has been studied and the results has been related to the energy of the PKA atom to identify the maximum energy that the interface can withstand without destroying the structure.

Recent investigations have shown that layered structures with spatial variations of the Young&’s modulus can greatly improve the fracture resistance and strength of materials if the composite architecture fulfills certain design rules [1, 2]. The reason for this effect is that the crack driving force is reduced when the crack grows into the compliant interlayer, which leads to crack arrest. This mechanism is responsible for the high fracture resistance of certain biological materials, such as deep-sea glass sponges, although they consist to 95% of brittle bio-glass [1].
In the current presentation, the idea of enhancing the fracture resistance by introducing compliant interlayers is transferred to technical composite materials where both, the matrix and the interlayer material, behave elastic-plastic. In a first step, it is shown that the crack driving force is also reduced, if the crack enters a region where the yield stress strongly increases [3]. From the results of numerical studies with the configurational forces concept, optimum interlayer configurations are deduced for different types of composites so that the interlayers work as effective crack arresters. It is shown that yield stress and thickness of the interlayer have to fulfill certain conditions. This knowledge is then used to derive criteria for the optimal design of fracture resistant composites based on the yield stress inhomogeneity effect.
References
[1] O. Kolednik, J. Predan, F.D. Fischer, P. Fratzl, Bioinspired design criteria for damage-resistant materials with periodically varying microstructure. Adv. Funct. Mater. 21, 2011, 3634-3641.
[2] O. Kolednik, J. Predan, F.D. Fischer, P. Fratzl, Improvements of strength and fracture resistance by spatial material property variations, Acta Mater. 68, 2014, 279-294.
[3] M. Sistaninia, O. Kolednik, Effect of a single soft interlayer on the crack driving force. Eng. Fract. Mech. 130, 2014, 21-41.

This research presents the formulation of a phase field dislocation dynamics model designed to treat a system comprised of two materials differing in moduli and lattice parameters, meeting at a common interface. We use it to investigate the critical stress required to transmit a perfect dislocation across a bimaterial interface with a cube-on-cube orientation relationship. The calculation of the critical stress accounts for the effects of: 1) the stresses induced at the interface due to the lattice mismatch (misfit or coherency stresses), 2) the elastic moduli mismatch (Koehler forces or image stresses) and 3) the formation of the residual dislocation. Our results show that the critical stress associated with the transmission of a dislocation from material 1 to 2 is not the same as from material 2 to 1. We find that the transmission from the material with the lower shear modulus is easier than the reverse and the degree of asymmetry in critical stress is directly proportional with the lattice mismatch. An analytical model for the critical stress is also presented. It is based on the formation energy of the residual dislocation and shows good comparison with the simulated results in the limit of large mismatch for coherent interfaces. The analytical model predicts a scaling factor for the critical stress that is based on the shear moduli and lattice parameters of both materials that will also be discussed.

The extraordinary sensitivity and extremely small thermal mass of nanocalorimetry sensors allow the study of solid-state reactions in thin films over a broad range of heating rates, from isothermal to 10⁵ K/s. Ab initio simulations provide insight in the phase transformation and diffusion behavior of a material at the atomistic scale.
We have investigated the reaction kinetics of Zr/B multilayers using a combination of nanocalorimetry and ab-initio calculations. On the experimental side, we examine the effects of heating rate (from 3,000 to 10,000 K/s) and bilayer period on the multilayer reactions. The microstructural evolution of the multilayers during the reaction is revealed using transmission electron microscopy. The simulations elucidate the effects of concentration (ZrB_x, 0Both experimental and simulations results show that addition of B to the Zr lattice amorphizes the structure, which enhances B transport in the alloy. The diffusion energy barriers determined using nanocalorimetry (0.52±0.17eV) are in good agreement with the value determined from the ab-initio calculations (0.75±0.07).

Metallic nanolaminates composed of periodically alternating nanocrystalline and amorphous layers represent hierarchically structured materials that simultaneously exhibit exceptional strength and ductility, which are often mutually exclusive behaviors in monolithic nanocrystalline and amorphous metals. While a number of pioneering studies have shown that the amorphous layers act as both a source and sink for defects operating within the crystalline regions, how these processes can be precisely tuned for enhancing mechanical stability and toughness remains unresolved. In this presentation, results from atomistic simulations on alloy nanolaminates containing columnar nanocrystalline structures accessible in experimental films will be presented. Focus was placed on elucidating the mechanisms in which dislocations are emitted into the crystalline layers from the intersection of grain boundary planes with the amorphous-crystalline interface (ACI), and how to manipulate these mechanics for optimizing mechanical behavior. Enhanced shear transformation zone (STZ) activity in the amorphous layer directly adjacent to the grain boundaries acted as nucleation sites for lattice dislocations in the crystalline layers. Following emission at one ACI, dislocations were observed to traverse the grain and be subsequently absorbed at an adjacent ACI, in turn triggering new STZ activity and providing a new nucleation site for a lattice dislocation. The distribution of plastic strain among the disparate mechanisms operating in the nanolaminates was quantified using continuum deformation metrics. From this analysis, it was determined that the coupling between dislocation and STZ plasticity significantly suppressed the amount of plastic strain accommodated at grain boundaries relative to equiaxed nanocrystalline metals. By manipulating the columnar grain size collectively with the structural length scales of the nanolaminate, the deformation physics were precisely tuned to optimize mechanistic coupling at ACIs. In these optimized structures, a unique crossover in the deformation behavior was uncovered where delocalized strain accommodation mechanisms promoted a more homogenous plastic response.

Tantalum thin films are used in a wide variety of applications, including microelectronic devices and biocompatible implants. Tantalum films can be deposited in one of two phases: the alpha phase is the stable bulk equilibrium phase and has a body centered cubic structure, while the beta phase is metastable, has a complex tetragonal structure, and is found only in thin films and other nanoscale structures. Because beta Ta is metastable, a beta film will transform to alpha on heating. Alpha films formed using this technique have a very unusual microstructure: Where typical polycrystalline metals have discrete grains, each with a single orientation, phase-transformed Ta films show long-range orientation gradients—that is, the orientation of the film rotates smoothly with respect to film position. Thus far, attempts to model the details of this microstructure and the mechanism by which it forms have been difficult because experimental orientation data has proved too noisy for precise analysis. We have therefore generated a model which replicates several noteworthy features of the orientation gradient structure, and which can be analyzed to give insight to the unique microstructure seen in our phase-transformed Ta thin films.

Multilayered structures offer unique opportunities for enhancing the strength and ductility of structural materials at extended interval of temperatures partly due to the interactions of dislocations and layer boundaries. In present study we have selected a Ni-Fe system on the basis of two engineering arguments. Firstly, it was already shown that by controlling parameters of electrodeposition (ED) we are able to process alternating layers with a modulated microstructure, where sub-layer thicknesses can be controlled from 5 µm down to 100 nm. Secondly, Ni-Fe alloys are of interest owing to their excellent high temperature stability. To investigate the mechanical response and understand the underlying deformation mechanisms uniaxial tensile tests were performed at various temperatures (25-400°C) and strain rates (10^-2 - 10^-5). Before mechanical tests the thermal stability of specimens were studied using differential scanning calorimetry. Structural studies prior to and following tensile deformation were conducted using electron microscopy. Reported properties are discussed in context of a bimodal microstructure of the multilayered samples as well as the contribution of each grain size regime to strength and ductility. We report that at elevated temperatures, the ductility of the ED specimens with small layer thicknesses increased significantly and became uniform. This investigation is supported by separate ONR grants to UCD, Texas University and Integran Technologies.

We performed in situ indentation in a transmission electron microscope on Al-TiN multi-laminates with individual layer thicknesses of 50 nm, 5 nm and 2.7 nm to explore the effect of length scales on the plastic co-deformability of a metal and a ceramic. At 50 nm, plasticity was confined to the Al layers with easy initiation of cracks in the TiN layers. At 5 nm and below, cracking in TiN was suppressed and post mortem measurements indicated a reduction in layer thickness in both layers. In addition, micro indentation and 3-point bending tests have been performed to demonstrate the enhanced fracture toughness of Al/TiN multi-laminates with individual layer thickness less than 5 nm. The results demonstrate the profound size effect in enhancing plastic co-deformability in nanoscale metal-ceramic multilayers.

It is well known that ultrafine-grained (UFG) materials exhibit superior fatigue properties compared to coarse grained materials. The accumulative roll bonding (ARB) process enables the production of fine laminated structures and sheets. Previous investigations already showed that ARB laminated sheets show superior fatigue properties compared to their UFG monomaterial sheets due to crack deflection at the interfaces.
In this work the influence of the interfaces on the fatigue properties has been studied by varying the layer architecture of ARB sheets (i.e. Al-Al or Al-steel). The laminates were characterised by a microstructural analysis and nanoindentation experiments. Fatigue properties were determined by 3-point bending tests performed on a vibrophore testing machine and the crack path was analysed afterwards. The crack path is strongly influenced by the interfaces. The crack propagates straight through the interface if the hardness on both sides is similar. In contrast if the hardness difference between both materials is high the fatigue crack stops close to the interface from the softer to the harder layer and propagates parallel to the interface within the softer layer. This cracking behaviour is more pronounced at higher stress amplitudes, which is explained by the larger plastic deformation zone in front of the crack tip. As the fatigue life at higher stress amplitudes is predominantly governed by crack propagation the fatigue life is significantly higher for laminates with a high hardness difference at the interfaces. This is due to the observed pronounced crack branching in these laminates. At lower stress amplitudes, where crack initiation is dominating, the fatigue life mainly depends on the properties of the outer layer. Laminates with the same outer layer, but different interlayers, show very similar fatigue lives. Thus, by layer architecturing laminates with significant higher fatigue properties compared to the monomaterial reference sheets could be achieved.

Laminated structures have long been viewed as desirable candidates to deflect cracks at interfaces and thereby enhance damage resistance, as well as to provide enhanced creep resistance in high temperature structural materials. Laminated metallic structures can occur naturally as a eutectic product or artificially created by vapor deposition processes or diffusion bonding. In this presentation, the crack growth in such lamellar structures will be examined; specifically, the behavior of polycrystalline lamellar TiAl/Ti₃Al two-phase alloy will be used as an example to first demonstrate the role of interfaces in affecting crack growth behavior and how they can provide enhancement of damage resistance. As a second example, the crack growth behavior in Zn single and twist-oriented bicrystals will be considered to demonstrate the role of anisotropic fracture energy in determining fracture path and influencing fracture resistance. Within this context, the presence of macro and nano twins in materials and their ability to form at crack tips (deformation twins) or pre-exist(annealing/growth twins) and affect crack growth behavior will be discussed as well.

Seashells are biological laminate materials with superior mechanical strength and eminent toughness. What is the recipe that Mother Nature uses to fabricate seashells? What roles do the nanoscale structures play in the strengthening and toughening of seashells? Can we learn from this to produce seashell-inspired laminate materials? The recent discoveries of nanoparticles in seashell&’s individual laminates are summarized, and the roles these nanoparticles play in seashell&’s strength and toughness are elucidated. It was found that seashells are constructed with multi-scale hierarchical architectures. The basic building blocks are aragonite nanoparticles. Rotation and deformation of aragonite nanoparticles are the two prominent mechanisms contributing to energy dissipation in seashells. The biopolymer spacing between nanoparticles facilitates the particle rotation process. Individual aragonite nanoparticles are deformable. Dislocation formation and deformation twinning were found to play important roles in the plastic deformation of individual nanoparticles, contributing remarkably to the strength and toughness of seashells upon dynamic loading.

Both, plastic behaviour and microstructural stability of nanolamellar metallic composites are of paramount importance for practical applications since most of the beneficial mechanical properties are related to the diminished length scales in the material. The model system Cu/Nb produced by accumulative roll bonding with layer thickness of 63 nm is chosen for investigations of the mechanical properties by tensile tests at low temperatures - including 4 K tests. Based on stress relaxation experiments thermal activation analysis was used to determine the athermal influence of grain size (or rather layer thickness) on the total stress measured in the experiment. Moreover, thermal activation analysis results in precise information about the elementary deformation mechanisms dominating the plastic behaviour of the material. At very low temperatures at and below 77 K the 63 nm Cu/Nb nanolamellar composite shows a fcc like response at low stresses and a sharp transition towards an bcc like behaviour at high stresses. This is obviously due to the different slip systems operating, namely <110>{111] in fcc Cu and <111>{110] in bcc Nb, respectively. At low stresses the behaviour of the composite is determined by conventional dislocation storage and thermally activated “forest cutting” by the easily mobile edge and screw dislocations in the Cu layers and the edge dislocations in the Nb layers. The bcc like behaviour at higher stresses is probably due to enhanced “kink pair formation” needed to mobilize also the <111> screw dislocations in the bcc Nb layers. The results clearly show that the individual Cu and Nb layers in the 63 nm Cu/Nb laminate at low temperatures behave more or less like their bulk counterparts, i.e. like bulk copper and bulk niobium, respectively. It seems that at higher temperatures above 77 K a “mixing” of fcc and bcc behaviour dominates the plastic deformation from the beginning.

Laminated metallic composites (LMCs) produced by accumulative roll bonding (ARB) offer a new way for light-weight materials design by tailoring of grading materials properties. Other than different processes known to be able to produce LMCs the ARB-process allows to adjust the layer thickness rather easily and to realize manifold layer designs (sheet architectures). In addition, the ARB-process also leads to an ultrafine-grained microstructure resulting in extraordinary high strength of the materials. In this context, being able to develop tailored laminates it is essential to understand in detail how and to which extent the different layers and the interfaces affect the mechanical properties and the related deformation behaviour.
In this study the influences of the layer architecture on the mechanical properties were investigated. Pure copper and the aluminum alloy AA6014 were roll bonded at room-temperature and constant layer thicknesses without fragmentation have been achieved. To investigate the architectural influences of the layer structure the outer layers of the sheets were alternated. Thus two kinds of laminated materials were produced, one with the aluminum alloy and one with the copper as the outer layer. The different mechanical properties of the two primary materials show a great influence on the mechanical properties of the produced sheets. Ex- and in-situ tensile tests in a Large-Chamber SEM reveal different behavior for the sheet designs. For sheets with aluminum as the outer layer a superior tensile strength was observed. Also the elongation to failure is significantly higher for these sheets. The strain rate sensitivity (SRS) was also strongly influenced by the sheet architecture. It was observed that the SRS for the sheets with copper as the outer layer is higher than for the sheets with aluminium as the outer layer. Obviously, the global SRS of the sheets is significantly influenced by the type of the outer layer. In order to shed some light on this behavior, digital image correlation was used in in-situ tensile tests in the Large-Chamber SEM to visualize the differences in the deformation behavior between the two designs.

Strain-induced grain refinement in metals generally saturates in the submicron regime due to the enhanced rate of structural restoration. The present investigation is aimed to study the structural evolution beyond the saturation and to explore the dominant processing parameters that might govern structural evolution toward the nanoscale. Gradient nanostructures with a spatial variation in both size and boundary characters are fabricated in nickel by means of surface mechanical grinding treatment. In the subsurface layer of 10-80 mm deep, 2-dimensional laminated structures with low angle boundaries and strong deformation textures were formed of which the average thickness is ~20 nm, one order of magnitude smaller than that of the ultrafine structures in Ni induced by conventional severe plastic deformation. The extraordinary grain refinement was ascribed to the high strain rates and strain gradients that enhance accumulation of geometric necessary dislocations with a suppressed recovery dynamics. Deformation of the nano-laminated structures is governed by dislocation slip and supplemented by deformation twining at the nanoscale, eventually leading to fragmentation into nano-sized equiaxed grains.

Bulk graphene-reinforced Al matrix composites of various reinforcement concentrations were fabricated via a modified powder metallurgy approach. These composites possess a nanolaminated, brick-and-mortar architecture, where layers of ~200nm-thick pure Al platelets are stacked in a staggered arrangement, and are separated by graphene sheets, each containing 4-5 graphene monolayers. The composite containing 1.5 vol. % graphene were shown to have an uniaxial tensile strength of 302±3MPa, about 50% higher than that of unreinforced Al matrix prepared using the same fabrication route (201±6MPa). Moreover, the composite possess a uniform elongation of 3.4±0.2%, only slightly lower than that of the Al matrix (4.3±0.4%), and have a significantly lower strain hardening capability. Combined with post-mortem and in situ transmission electron microscopic (TEM) analysis, our findings were interpreted in terms of the uniform distribution of graphene in the Al matrix, the effective load transfer between the graphene sheets and Al platelets, and the interaction between mobile dislocations and the graphene-Al interfaces.

Strain-gradient plasticity under a nanoindent is used to quantify the resolved shear stress for dislocation motion in single crystalline TiN thin films. During in situ indentation in a transmission electron microscope (TEM), dislocations are observed to nucleate and glide on {110}<110> slip systems in TiN. Under the gradient stress field generated by nanoindentation, the gliding dislocations stop at a point where presumably the local stress drops below the critical resolves shear stress (CRSS). The strain gradients are measured from the acquired high-resolution TEM images and used to compute the CRSS corresponding to partial or full dislocation motion in high Peierls stress materials under nanoindentation.

Nanostructured metallic multilayers provide unique opportunity to investigate the influence of layer interfaces on mechanical properties of metallic nanocomposites. High strength is often achieved at small (several nm) individual layer thickness (h). Recently we discovered high-density stacking faults in FCC Co in highly (100) textured Cu/Co multilayers. In contrast in (111) textured Cu/Co nanolayers, Co remained its stable HCP structure at large h. The two Cu/Co systems have very different size dependent strengthening behavior. HCP Cu/Co has much greater peak strength than FCC Cu/Co. The large discrepancy in their strengthening mechanisms is discussed and compared to those of highly textured Cu/Ni multilayer systems. In another highly textured nanolayers system, Ag/Al, epitaxial interfaces were observed across various h (1-200 nm). High-density nanotwins and stacking faults appear in both Ag and Al layers, and stacking fault density in Al increases sharply with decreasing h. At smaller h, hardness of Ag/Al nanolayers increases monotonically and no softening was observed. These studies allow us to investigate the influence of layer interfaces, stacking faults and nanotwins on strengthening mechanisms of metallic nanolayers. This research is funded by DOE-OBES.

Deformation and fracture set important limits for the use of nanostructured metals in applications, providing strong motivation to understand the roles of interfaces and length scale on dislocation motion and crack propagation. Here, we use a variety of micromechanical and in-situ electron microscopy methods to investigate shear failure (Mode II) and crack propagation (Mode I) in nanoscale multilayers. Our studies have focused on films of polycrystalline titanium and amorphous zirconium oxide layers where we can tune the layer thicknesses between 10 and 100 nm. We find that the strength of the polycrystalline Ti increases with decreasing layer thickness until a critical value is reached at which point deformation diverts into the interfaces. We estimate the toughness from the crack tip opening displacement and argue that Ti layer strength, cropping of the plastic zone, and constraint of the crack path all play a role in explaining the layer thickness dependence of the toughness. We also present several recent observations of a clear effect of hydrogen gas on the deformation behavior. In the end, our goal is to identify the controlling mechanisms of deformation and fracture in nanostructured materials and to suggest various tactics for the design of tough and recyclable nanoscale composites.

Nanoscale metallic multilayers exhibit strong size effects in their mechanical behavior such as increasing hardness with decreasing layer thickness as well as good fatigue properties and wear resistance, high thermal stability, and excellent irradiation tolerance. Previous investigations have revealed that the strength and plastic deformation are length scale dependent but also strongly controlled by the interface structure, which determines the interface barrier strength and slip transmission. However, a more detailed description of the interface effect and length-scale dependent deformation behaviors are still needed for our understanding of these superior properties.
In this study, two types of nanoscale multilayers, i.e. Cu/Au and Cu/Cr, were investigated. Cyclic sliding experiments with up to 1000 cycles were conducted using a nanoindenter. The microstructures underneath the sliding tracks were investigated by scanning electron and transmission electron microscopy of cross sections prepared by focused ion beam milling.
For the Cu/Au multilayers, grain growth and a thickness reduction of the individual layers occur at the early stages of deformation. Dislocation glide, dislocation pile-up at the interface and mechanically-driven grain coarsening are suggested as the controlling mechanisms. With continuing deformation, increasing waviness of the interface together with strain rate gradients underneath the sliding track result in a Kelvin-Helmholtz instability and a vortex structure develops. Finally, vorticity-driven mechanical mixing of Cu and Au as well as the formation of nanocrystalline grains is observed. In case of the Cu/Cr multilayers, only the Cu layers deform plastically at the early stages of deformation also accompanied by grain coarsening. At higher cycle numbers, fracture of the Cr layers was observed. While a nanostructured deformed zone developed, no vortices were observed. The deformation behaviors of the two different material systems and the deformation mechanisms will be discussed.

The creation of ductile ferrite films has been the focus of electronic material research in recent years due to their application in flexible/bendable electronics. Conventional methods used in the production of ferrite films creates high quality (e.g. high-permittivity) films at the expense of scalability, thickness, fabrication speed, or cost. The incorporation of these films onto flexible substrates is also very limited, often resulting in failure at the interface from a mismatch between mechanical properties of the stiff ferrite and flexible polymer substrate. To solve this problem, we have developed a low-cost, scalable method of creating ductile thin ferrite films (100 µm -1 mm). Individual films can be created with tunable mechanical properties which, when laminated together into a graded composite, create a substrate with the desired electrical and mechanical properties for incorporation into flexible electronics.
We have developed a processing method that creates ductile thin films of ferrite/polymer composite. The individual films are created by solvent casting a homogenous colloidal solution of ferrite in a liquid polymer phase. This colloidal mixture is easy to handle and can be cast into a variety of forms including molds, screen-printing, or other methods. The mechanical properties of the films are controlled by changing the volume fraction of the ferrite, polymer molecular weight, colloid viscosity, and/or post-processing techniques.
Ductile and tensile tests were performed on film and bar samples of the ferrite composites to determine the mechanical properties of different formulations. When the percentage of polymer in the composite is increased, both the ultimate strength and the yield strain increase. As the ductility of the samples increases and the ferrite content decreases, the electrical properties suffer. To overcome this, we have created a
laminated composite comprised of films with varying ferrite loading giving improvements in both permittivity and ductility.
We are able to fabricate a graded structure by laminating several layers of the ferrite films with differing mechanical and electrical properties creating a final composite with high-permittivity and ductility, properties inaccessible by conventional methods. The lamination process involves the interpenetration of the polymeric components of the films into each other, reducing the occurrence of interfacial defects, resulting in a stronger final composite. The methods of production for the ferrite/polymer films and their graded laminate composites are easily scalable making them ideal for industrial applications.

Gradient and laminate materials are an emerging area with the potential to impact a wide range of technologies including structural materials, energy storage, and defense applications. However, fine control of gradients is a challenge for many material types. This work aims to demonstrate the capability to tune the properties of ceramic-metal composites (cermets) by using electrophoretic deposition (EPD) to create compositional/microstructural gradients within the cermet part. Controlling the outcome of microstructural features and phase composition enables the tailoring of the final product for desired applications. B₄C green body compacts comprised of tailored microstructures through EPD are then infused with Al during pressureless sintering.

Metal-matrix ceramic-reinforced composites are used extensively in industry in applications requiring high strength and wear resistance. While these properties are often desirable, enhancements of strength typically lead to detrimental reductions in ductility and toughness of the material. Manufacturing of layered and functionally graded composite structures provides an opportunity to produce materials with strengths approaching those of uniformly dispersed composites while mitigating losses in ductility. We have developed a novel composite manufacturing technique using electrochemical deposition to create nickel-matrix alumina-reinforced layered, graded, and uniformly dispersed composite structures. This system is demonstrated to produce crack-free films with alumina nanoparticle volume fractions of up to 25%. Layer thicknesses and gradation scales can be controlled with sub-micron scale resolution with total film thicknesses of mm scales attainable. Microhardness and microtensile testing of these hierarchically structured composite materials demonstrate enhanced mechanical strength while maintaining good ductility when compared to uniformly dispersed composites and pure nickel samples. Some layered microstructures are even shown to have toughnesses exceeding those of pure nickel films while displaying significant strength enhancements. Applications of this technique to other matrix and reinforcement materials is also discussed.

Abstract—The macroscopic electromechanical behavior of ceramic/ceramic lead-free ferroelectric 2-2 composites was characterized at room temperature and compared to 0-3 composites of the same composition. Composites consisted of a nonergodic relaxor, (Bi_1/2Na_1/2)TiO₃-0.07BaTiO₃, with an irreversible electric-field-induced relaxor-to-ferroelectric transition, and an ergodic relaxor, Bi_1/2(Na_1/4K_3/4)TiO₃-6BiAlO₃, which undergoes a reversible electric-field-induced relaxor-to-ferroelectric transition. Microstructural investigations of the multilayer composites, including compositional analysis, porosity and grain size measurements, were performed and differences in these properties as a function of composition were observed. Results indicate that the 2-2 composite structure provides an accurate approximation of the ferroelectric response of mixed composites. By taking advantage of the different macroscopic strain- and polarization-electric-field responses of the constituents, internal mechanical and electrical fields can be developed that enhance the unipolar strain over that expected by a rule of mixtures approximation, thereby improving the properties needed for application of such materials to actuator systems. It is possible through further tailoring of sample processing such as sintering parameters and properties of the constituents to optimize the electromechanical properties of multilayer lead-free ferroelectrics.