David Armstrong, University of Oxford
David Bahr, Purdue University
Megan Cordill, Erich Schmid Institute of Materials Science
Corinne Packard, Colorado School of Mines
Symposium Support Hysitron, Inc.
T3: Time-Dependent Behavior and Testing I
Monday PM, November 30, 2015
Hynes, Level 1, Room 102
2:30 AM - T3.01
Ultra-Small-Scale High Cycle Fatigue Testing Using Micro-Cantilevers
Jicheng Gong 1 Angus J. Wilkinson 1
1Univ of Oxford Oxford United KingdomShow Abstract
A new method has been developed for testing high cycle and very high cycle fatigue properties of materials at the micro-scale based on micro-cantilevers. Focused ion beam was employed to cut micro-cantilevers with triangular cross-section into the surface of a selected grain in a bulk polycrystalline commercial pure Titanium. The bulk specimen was pre-examined by EBSD, so the crystal orientation of all micro-cantilevers were known. The bulk sample containing the micro-cantilevers was then attached to a high power ultrasonic generator, which can generate mechanical vibration at the frequency 20KHz. The bulk of the specimen moves with the ultrasonic generator, but the micro-cantilever lags somewhat behind. The resulting deflections generate cyclic stress in the micro-cantilevers. The high vibration frequency means it can easily test into the high cycle and very high cycle regime. The design challenge is to generate enough stress to cause fatigue in these ultra-small specimens because the stress amplitude achieved in vibration is inverse to the cantilever size. Previous finite element modelling and experiments had shown that the classic micro-cantilever with uniform cross-section can only generate stress of a few MPa, even with the acceleration as high as 107m/s2. Instead, we designed a new ‘hammer-shaped' micro-cantilever with a widened region at the free end to significantly increase the inertia. This design now generates sufficient stress and enables fatigue testing even in Titanium, which is a challenging material due to the high strength to weight ratio (both high stress and low density require higher acceleration).
SN curves in the testing range from 105 to 108 cycles have been obtained for 2 mu;m wide micro-cantilever single crystal Ti samples using a step test protocol. The stress to failure decreases systematically as the number of cycles to failure increases. However, there is strong dependence on the crystal orientation with the fatigue strength at 107 cycles for test pieces cut along the direction being approximately twice that of those cut in the direction. The fatigue strength of micro-fatigue test is significantly lower than the static strength measured on micro-cantilevers of identical size using a nanoindenter. Due to the small specimen size, it is suggested that the results reflect the behavior of fatigue nucleation rather than propagation.
2:45 AM - *T3.02
In Situ TEM Stress Relaxation and Cracking Investigation of Nanocrystalline Ultrathin Films
Ehsan Hosseinian 1 Saurabh Gupta 1 Marc Legros 2 Olivier N. Pierron 1
1Georgia Inst of Technology Atlanta United States2CEMES-CNRS Toulouse FranceShow Abstract
This talk presents an experimental investigation of the stress relaxation and cracking mechanisms in ultrathin nanocrystalline metals (Au and Al). Tensile tests that include stress relaxation segments were performed on ultrathin (<100 nm) specimens with micron-sized dogbone shapes, using a MEMS-based nanomechanical testing setup. The setup allows quantification of stress and strain (and their evolution with time) as well as in situ TEM observations of the governing mechanisms. The results (based on in situ TEM observations and activation volume calculation as well as evolution of strain rate with applied stress) show that the transient relaxation deformation in 100-nm-thick Au (average grain size: 75 nm) is highly localized and dominated by intergranular and intragranular dislocation activities. These dislocation activities slow down within a timescale of 1 hour, leading gradually to a diffusion-dominated deformation (steady-state creep). In contrast, the 30-nm-thick Au films (with a much smaller average grain size) do not show these sustained dislocation activities; instead their deformation behavior appears to be dominated by grain boundary activities. In addition to the stress relaxation properties, this talk will discuss the cracking behavior in ultrathin nanocrystalline gold and aluminum films.
3:15 AM - T3.03
Fatigue-Induced Abnormal Grain Growth in Nanocrystalline Metals: An In Situ Synchrotron XRD Approach
Timothy Allen Furnish 1 Brad Boyce 1 Apurva Mehta 2
1Sandia National Laboratories Albuquerque United States2Stanford Synchrotron Radiation Lightsource Menlo Park United StatesShow Abstract
The mechanical behavior of nanocrystalline metals has been the subject of extensive research, especially in terms of their high strengths and wear-resistance. However, the fatigue performance of these metals has received much less attention, despite initial reports of exceptional fatigue strengths. Particularly lacking is a comprehensive understanding of the cyclically-induced deformation mechanisms, structural evolution, and fracture behaviors in nanocrystalline metals. Recent studies using various post-fracture microscopy techniques have identified extensive regions of abnormally large grains near and along macro-scale cracks - but, the exact origins of these features and their overall effects on the fatigue performance and fracture are unclear. This is due, in part, to the general difficulties in performing non-destructive in situ characterization of through-thickness microstructures during fatigue testing. In the current study, a newly developed method of detecting abnormal grain growth (AGG) using synchrotron x-ray diffraction was employed to “watch” the microstructural evolution of various nanocrystalline Ni alloys during high-cycle fatigue. This in situ fatigue approach allowed us to verify with statistical confidence that the abnormally large grains do, in fact, grow during the fatigue process (i.e. they are not pre-existing) and that they develop well before final fracture of the material. The effects of the fatigue stress conditions on the microstructural evolution and AGG were also evaluated using a range of fatigue testing conditions. Additionally, traditional cross-sectional microscopy was performed after the onset of AGG, but before final fracture, to further explore the deformation and micro-scale crack mechanisms that lead to the macro-scale fracture during fatigue.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
3:30 AM - T3.04
Uniaxial Compression of Cellular Materials at a 10-1 Strain Rate Simultaneously with Synchrotron X-Ray Computed Tomographic Imaging
Brian M Patterson 1 Nikolaus L Cordes 1 Kevin Henderson 1 Mathew Robinson 2 Xianghui Xiao 3 Angel R Ovejero 4 Tyler Stannard 4 Jason Williams 4 Nikhilesh Chawla 4
1Los Alamos National Laboratory Los Alamos United States2Atomic Weapons Establishment Aldermaston United Kingdom3Argonne National Laboratory Argonne United States4Arizona State University Tempe United StatesShow Abstract
Understanding the effects of a material&’s morphology upon the overall material performance requires a detailed understanding of its initial morphology and how it changes under external stimuli. Laboratory based X-ray computed tomography (CT) systems can image polymer foams as they are compressed, in an interrupted in situ modality. Due to the lower flux of a laboratory X-ray source, stress relaxation must be allowed to occur in the material before a tomogram can be collected. Otherwise, the residual sample motion leads to significant image blurring. This relaxation process requires ~20 minutes before the tomogram can be collected over 1+ hours. Important information regarding compressive performance and changes in morphology is lost during this stress relaxation period.
Synchrotron light sources, such as the Advanced Photon Source, provide a significant increase in photon flux, compared to lab-based X-ray tubes. Coupling this high flux with a high speed camera allows for X-ray radiographs to be acquired every millisecond. This generates tomographic data in ~1 second. By coupling a custom loading stage to the X-ray microscope, it is possible to study the dynamic in situ deformation of polymeric foams at a 10-1 s-1 strain rate.
In this study, synchrotron-based X-ray CT, at beam line 2-BM, was used to capture the morphology changes in polymeric foam materials during dynamic uniaxial compression. Twenty tomograms of each sample were acquired while simultaneously compressing the samples up to 60%, within a timeframe of 100 seconds. The materials studied included hydrogen blown silicone foam (LK3626), prilled urea silicone foam, syntactic Sylgard-184, and additively manufactured foams.
With this technique, we can measure and correlate the differences in mechanical performance of polymeric foams to their physical/morphological changes. For example, LK3626 is a soft-structured hydrogen blown silicone foam. It has a weak ligament structure which quickly buckles with almost no bending resistive force, even at low compressive strains. Also, it compresses with no Poisson effect. Syntactic Sylgard 184 is a much stiffer material, due to the reinforcement from the 50 mu;m glass microballoon pore former; therefore, it exhibits a stress/strain curve with definitive bending/buckling/compressing regions. Importantly, the true stress can be calculated from CT data by measuring the full 3D change in area for each tomographic step. Additionally, the tomographic data can be converted into a hypermesh for importing into Abaqus for explicit analysis. Direct comparison of the reconstructed slices to the modeled result adds a high level validation to the modeled uniaxial compressive performance of the material that cannot be acquired with other techniques.
3:45 AM - T3.05
Dynamic Fracture Tests of Brittle Microspheres and Development of a Pulverization Parameter
David F. Bahr 1 Wayne Chen 1 Mohamad B. Zbib 1 Niranjan D. Parab 1
1Purdue Univ West Lafayette United StatesShow Abstract
Impact loading of spherical particulates occurs during materials handling, processing, and service conditions. This can lead to fracture of the particles and commutation into smaller particles. The dynamic fracture behavior of micro spherical particles of 5 brittle spherical solids, ranging from soda lime glass, to polycrystalline silicon and yttrium-stabilized zirconia (YSZ) was characterized with high speed, in situ X-ray phase-contrast imaging to examine the failure mechanisms in situ for spheroidal particles with diameters (d) from 0.600 to 2 mm. A new pulverization parameter was developed and is shown to predict the failure mechanism of the materials under fixed grips loading (i.e. overdriven in load) relating the hardness, toughness, modulus, and the size of the given particulate. The high speed behavior will be compared to quasi-static loading of particulates to demonstrate the effects of strain rate on particle fracture.
T4: Structural Materialsmdash;Mechanical Behavior of Materials for Extreme Environments
Monday PM, November 30, 2015
Hynes, Level 1, Room 102
4:30 AM - T4.01
Onset of Plasticity in Zirconium due to Hydrides Formation
Wojciech Szewc 1 Sandrine Brochard 1 Emmanuel Clouet 2 Laurent Pizzagalli 1
1Institut Pprime Chasseneuil France2SRMP - CEA Saclay FranceShow Abstract
Zirconium is a material of great importance in nuclear applications, as it is commonly used for fuel cladding. One of the known factors potentially degrading the mechanical properties is hydrogen precipitation, leading to the formation of brittle zirconium hydride needles. Different hydride phases form, depending on the amount of hydrogen in these needles. The associated volume expansion of hydrides leads to an increasing applied strain on the zirconium matrix. Available observations suggest that such strains may induce the nucleation of dislocations in zirconium from the zirconium/hydrides interface, thus further altering the mechanical properties of the material. But these observations are rather limited, and little is known about the elastic limits, and the nucleation mechanisms.
To address this issue, we have performed molecular dynamics simulations of a strained Zr surface, using two different interatomic potentials (EAM and COMB). Different loading modes were tested, in order to mimic the influence of the hydride precipitate. We found that the onset of plasticity is controlled by heterogeneous nucleation of partial dislocations in the first-order pyramidal planes, immediately followed by partial dislocations in the basal plane. This surprising result can be understood from the analysis of generalized stacking fault energy surfaces . The apparent blocking of the prismatic-plane slip, which unquestionably dominates in bulk zirconium, follows from the loading conditions imposed by the precipitate. The elastic limit depends on the geometric features of the surface making contact to the precipitate. In particular, it is lowered by the presence of surface steps. In specific circumstances, it is possible to favor the basal slip over the pyramidal one.
 C. Varvenne et al., Acta Mater. 78 (2014) 65 - 77
4:45 AM - T4.02
New Experimental Methods for Determining the Fracture Toughness in Brittle Metals: Tungsten Case
Teresa Palacios 1 Jens Reiser 2 Jose Ygnacio Pastor 1
1Universidad Politeacute;cnica de Madrid Madrid Spain2Karlsruhe Institute of Technology Karlsruhe GermanyShow Abstract
The objective of this work is to show some new and original methods to determine the fracture toughness in brittle metals. In this case the analysis was performed for tungsten due to its thermo-physical properties, tungsten is a suitable material for plasma facing applications in the future fusion reactors. However, its structural use is compromised due to its inherent brittleness. Therefore, to improve this critical feature is relevant to accurately measure its fracture toughness as a real property independent of geometrical parameters.
The most common methods to produce cracks such as fatigue, indentation microfracture or electro-discharge machining cannot be used here because they are difficult to apply, do not produce reproducible cracks or induce extended thermal damage in the material. For these reasons, in a first part of this work four experimental methods, that gradually approach a crack-like notch, were compared to determine the effect of the notch root radius on the measured fracture toughness of a nanostructured bulk tungsten material. Three-point bending tests were performed on these four types of specimens with notches introduced with: a classical diamond disc, a diamond wire, a razor blade and an ultra-short pulsed laser. The results showed that the best alternative is to introduce a notch with a femtosecond pulsed laser. This method, which produces crack-like notches without thermal damage, possesses good reproducibility, high accuracy and reliable fracture toughness. It was previously used on ceramics but there is no evidence of its use on metals.
Additionally, the ultra-short pulsed laser was used to introduce notches in tungsten foils of 0.1 mm thickness to perform in situ tensile tests inside a scanning electron microscope and determine the fracture toughness. Two types of geometries were also produced in the foils: single-edge and double-edge notched specimens. The introduction of the notches is a challenging task because of the reduced size of the samples and their slenderness. Single-edge notched specimens, easier to produce, were not suitable to provide accurate fracture toughness since the slenderness and the non-symmetric geometry resulted in complex stresses and momentum apart from the uniaxial tensile being applied to the sample. Double-edge notched specimens, however, produced accurate fracture toughness results, although to introduce both notches aligned at both sides of the specimens involves higher complications during the notching process.
5:00 AM - T4.03
In Situ Micro-Cantilever Testing of the Deformation and Fracture of Neutron Irradiated Graphite
Dong Liu 1 Peter Flewitt 1
1University of Bristol Bristol United KingdomShow Abstract
Irradiation in a fast neutron environment is known to degrade the mechanical properties of materials, therefore, to ensure safe operation it is necessary to undertake measurements of these properties. To minimise doses to personnel during post irradiation testing it is appropriate to minimise the sample mass and select small test specimens. In addition, in materials with a complex microstructure, it is necessary to undertake these measurements at the required length-scale to provide input parameters for computer modelling. In this paper, we undertake measurements using micro-metre length scale cantilever beam specimens prepared by ion beam milling in a Helios Dualbeam workstation. These cantilever beams were loaded using a customised system developed at the University of Bristol. This allows load-displacement to be measured and the associated mechanical properties, such as the elastic modulus and flexural strength to be determined. With these techniques, it is possible to observe specimens throughout the duration of a test.
Pile Grade A (PGA) graphite was used as moderator and structural material within the core of UK gas-cooled Magnox reactors. It is an anisotropic, polygranular graphite comprising aligned filler particles in a matrix of graphitised binder with around 20 vol.% porosity caused by the manufacturing process. Irradiated and radiolytically oxidised PGA graphite samples were extracted from the fuel channels as part of an overall monitoring programme to support safe operation. Three cylindrical samples (7 mm height by 12 mm dia.) trepanned from bricks with increasing distance from the centre of the reactor were selected to provide a decreasing neutron dose. The microstructure, in particular the fine-scale porosity has been characterised using serial sectioning, high spatial resolution tomography. To extrapolate small-scale testing data to size of reactor core bricks, a microstructure-based multi-scale modelling approach has to be adopted. This requires the input parameters at a suitable length-scale to eliminate size effects introduced by the heterogeneity of the multi-scale microstructure. Measurement of load-displacement, elastic modulus and fracture strength have been made using 2 µm x 2 µm x 10 µm micro-cantilever test specimens in the dualbeam workstation to evaluate the changes in properties with neutron dose. This has revealed the effect of irradiation damage, independent of the macro-sized pores present in the PGA graphite. The data are discussed with respect to the possible mechanisms leading to changes in properties and their benefit as input parameters to multi-scale computer models.
5:15 AM - T4.05
Evolution of Microcrack Density under Tensile Loading in beta;-Eucryptite
Ryan C Cooper 1 Giovanni Bruno 2 Amit Shyam 1
1Oak Ridge National Laboratory Oak Ridge United States2BAM, Federal Institute of Materials Research and Testing Berlin GermanyShow Abstract
The microcracking phenomenon in ceramics initiates above a critical grain size due to anisotropy in the coefficient of lattice thermal expansion. This microcrack network can result in an increase in light scattering, electrical and thermal resistance, and a decrease in the bulk coefficient of thermal expansion-even to the point of negative thermal expansion. The tunable material properties of microcracked ceramics provide a unique design opportunity but in order to utilize these materials, there is a need to understand the effect of mechanical loading on microcrack populations. In the current study, we probe the effects of microcracks in ceramics by varying the microstructure in beta-eucryptite samples. We mounted specimens with varying grain sizes into a microtensile apparatus and loaded until failure. The resulting stress-strain curves exhibit nonlinear behavior and irrecoverable strain accumulation. Digital image correlation (DIC) was used to measure the two-dimensional strain fields under applied stress for noncontact microstrain measurements. We develop a constitutive model that predicts the evolution of microcrack density within a linear elastic continuum and then applied the model to fit the experimental tensile curves. The constitutive model accounts for the accumulation of irrecoverable strain due to the increasing crack density and the evolving effect of linear-elastic modulus due to changes in crack density.
T1: Thin-Film Mechanical Behavior
Monday AM, November 30, 2015
Hynes, Level 1, Room 102
9:15 AM - T1.01
Surface Effects on Anelastic Relaxation in Sub-Micron Gold Thin Films Using the Bulge Technique
Jeffrey Smyth 1 Patrick Holmes 1 Richard P. Vinci 1 Walter Brown 1 Nicholas Strandwitz 1 Roderick Marstell 1 Ling Ju 1
1Lehigh University Bethlehem United StatesShow Abstract
Stiction failures in radio frequency (RF) micro-electro-mechanical systems (MEMS) based devices can sometimes be attributed to stress relaxation in the metallic thin films that make up the architecture. Efforts to mitigate the extent of the relaxation through novel manufacturing and engineering techniques are critical to production of highly reliable MEMS devices. We have shown previously via thin film bulge mechanical testing that both viscoplastic and viscoelastic deformation behavior are present during the stress relaxation of a typical FCC metal film. Our prior results indicate that the mechanism responsible for the viscoelastic component is dislocation-based, but literature reports based on substrate curvature measurements imply that surface diffusion could be responsible for the viscoplastic component. In the present work, the mechanism responsible for relaxation in both regimes was explored using thin film bulge testing of metal films with and without surface layers. Surface modifications to gold films such as TiO2 passivation layers, Al2O3 atomic layer deposition (ALD) coatings, and SiNx support layers were each evaluated in an iso-strain relaxation test on the bulge tester. It was found that surface diffusion was not the dominant mechanism for relaxation under the small-strain isothermal conditions typically experienced by a MEMS device in service, but rather it is hypothesized that a dislocation double-kink nucleation mechanism controls both components of deformation
9:30 AM - T1.02
Investigation into Delamination Problems in Multilayered Systems by Means of Puncture Testing
Sara Reynaud 1 Evan Crocker 1
1Arkema King of Prussia United StatesShow Abstract
Adhesion and bonding of multilayer systems continue to be the topic of interest in the coatings and membrane industry. Generally, simple techniques such as peel testing have been used to evaluate film adhesion properties. The ability to perform controlled, repeatable, and rapid assessment of the adhesion properties in film systems, especially systems with more than two layers, is of utmost importance for formulation and application development.
In this work we present a new analytical approach to evaluate layer to layer adhesion strength for multilayered systems. A high-throughput mechanical testing (HTMECH) technique is used to measure the impact performance of polymeric films under a wide range of strain conditions. Delamination in multilayered systems is induced by puncturing the film at increasing impact speeds. Ongoing study on the mechanism of failure in the systems with varying levels of adhesion will be discussed.
9:45 AM - T1.03
Correlative Nanomechanical Measurements for Complex Engineered Systems
Douglas D. Stauffer 1 Eric Hintsala 2 S.A. Syed Asif 1
1Hysitron, Inc. Eden Prairie United States2University of Minnesota Minneapolis United StatesShow Abstract
Nanomechanical measurements, particularly nanoindentation, have transitioned from a purely academic research instrument into a tool for examination of complex industrial processes. In an academic setting proper design of experiments allows for the simple elimination of variables. This is not possible in an industrial setting where a complex engineering material such as the multilayers in a hard disc film stack are the accumulation of 1000&’s of engineering hours. For these complex cases both statistical information and data on the failure mechanisms are required. Here, both ex and in situ measurements are performed and then correlated, using a co-deposited hard drive film stack. Indentation depths range from 0.5 to 10 nm, with the analysis utilizing both the approach and retract curves to describe the crack growth/separation between the diamond indenter and the carbon film. Additionally, the elastic-plastic transition is thoroughly explored in an effort to understand the effects of a protective coating on a substrate and the loss of magnetic information during plastic deformation.
10:00 AM - *T1.04
Fracture Patterns in Thin Brittle and Ductile Functional Coatings - The Interplay of Extrinsic and Intrinsic Defects
Ralph Spolenak 1
1ETH Zurich Zurich SwitzerlandShow Abstract
Functional coatings protect substrates from wear, add special electronic and optical properties to the substrates and change their gas permeability. Their functionality, however, is often limited by fracture. This contribution focuses on the flavor of fracture patterns as they arise from an interplay between the cleanliness of the substrate, the adhesion between the two substrates, the amount of ductility that the coating exhibits and the coating microstructure. The examples given range from brittle materials such as diamond like carbon, silica, titania, silicon, ink-jet printable materials to ductile coatings such as Cu, Au, Al and some of their alloys. Special analysis methods are based on in-situ tensile testing and include the measurement of electrical resistivity, AFM, SEM, optical microscopy, Raman microscopy as well as reflectance difference spectroscopy.
10:45 AM - T1.06
Sputter Deposited Nickel-Molybdenum-Tungsten Thin Films for Use in Metal MEMS Applications
Gi-Dong Sim 1 K.Madhav Reddy 1 Jessica Krogstad 2 Timothy P. Weihs 3 Kevin J. Hemker 1
1Johns Hopkins University Baltimore United States2University of Illinois at Urbana-Champaign Urbana United States3Johns Hopkins University Baltimore United StatesShow Abstract
Currently the majority of commercial MEMS devices are fabricated out of silicon, but the development of metal MEMS alloys that possess enhanced mechanical and functional properties, requisite dimensional stability, and the ability to be shaped on the microscale would greatly expand the design space for MEMS in applications ranging from microturbine engines, to power generation, high frequency switching, microheaters and high temperature sensors. Advanced metallic alloys are especially attractive in MEMS applications that require high density, electrical and thermal conductivity, strength, ductility and toughness. Here we report the mechanical behavior of sputter deposited Nickel (Ni)-Molybdenum (Mo)-Tungsten (W) thin film alloys. The as-deposited films go down as single-phase nanocrystalline solid solutions and possess ultra high tensile strengths of approximately 3 GPa, but negligible ductility. Subsequent heat treatments resulted in grain growth and the nucleation of Mo-W precipitates. Films annealed at 600oC or 800oC for 1hour still showed brittle behavior similar to the as-deposited film. Interestingly, films annealed at 1,000oC for 1hour were found to exhibit perfect elastic-plastic deformation behavior with strength greater than 1.2 GPa and approximately 10% tensile ductility. The ultra high strength observed in the as-deposited films and significant ductility measured in the annealed films suggest that sputtering and subsequent heat treatment may offer an attractive option for depositing metallic MEMS materials with tailorable mechanical properties.
T2: Structural Materialsmdash;Hydrogen in Metals
Monday AM, November 30, 2015
Hynes, Level 1, Room 102
11:30 AM - *T2.01
Nanomechanical Aspects of Hydrogen Embrittlement
Afrooz Barnoush 1 Nousha Kheradmand 1 Tarlan Hajilou 1 Yun Deng 1
1NTNU Trondheim NorwayShow Abstract
Hydrogen embrittlement (HE) affects the cracking behavior of metals by influencing the crack nucleation and growth process, which leads to disastrous consequences. Considering that dislocations are the carrier of plastic deformation, which avoids catastrophic failure by brittle fracture, the crucial role of dislocations in HE is enlightened. Hence, in order to understand the fundamental process governing HE, it is essential to clarify the effect of HE on dislocation activity in the fracture process zone of the crack. In an elastic-plastic solid, the dislocation activity in the plastic zone of the crack is limited, either by the dislocation nucleation rate, or by the dislocation mobility.
Recent nanomechanical testing methods, based on nanoindentation, provide a versatile tool for studying both dislocation nucleation and motion. Especially with integration of an electrochemical setup into the nanoindenter, we investigated the hydrogen effect on the activation energy and activation volume for dislocation nucleation as well as interaction force of dislocation with each other that controls their mobility. In this paper, we will present the application of this method, the so called electrochemical nanoindentation, for studying the nanomechanical aspects of HE.
12:00 PM - T2.02
On the Implications of Hydrogen on Dislocation Pattern in f.c.c. Metals
Xavier Feaugas 1 Abdelali Oudriss 1 David Delafosse 2
1University of La Rochelle La Rochelle France2Ecole des Mines de Saint Etienne Saint Etienne FranceShow Abstract
Dislocation organizations and slip band patterns developed under strengthening nickel crystals at room temperature was studied with different content of hydrogen. Complementary observations by AFM and TEM carried out at the same plastic strain yield correlations between different structural parameters of dislocation organization in strengthening nickel oriented for single slip, <135> and multiple slip, <001>. The impact of these correlations on the slip localization and the length scale (geometric necessary boundary, GNB spacing) which affects the hardening rate is demonstrated and discussed in relation with the wavelength of long range internal stresses and the mean free path of mobile dislocations. Additionally, it is shown that hydrogen content reduces the length scale because of a decrease of the stacking fault energy and cross-slip probability on the one hand and a shielding effect on internal stresses on the other. A similitude law between hardening rate and inverse of GNB spacing is evidenced and offers the opportunity to discuss the effect of solutes on hardening rate. In opposite, the impact of hydrogen on the equiaxe cell size and IDB (incidental dislocation boundary) size seems to be moderated for <001> multiple slip orientation. If our work gives a first quantitative insight on the impact of hydrogen on the characteristic dimensions of dislocation cell structures, the dynamics of these effects remain unclear and require more investigations.
 A. Oudriss, X. Feaugas, Length scales and scaling laws for dislocation cells developed after monotonic deformation of (001) nickel single crystal, Inter. J. of Plasticity, (2015), in revision.
 G. Girardin, C. Huvier, D. Delafosse, X. Feaugas, Correlation between dislocation organization and slip bands: TEM and AFM investigations in hydrogen-containing nickel and nickel-chromium, Acta Materialia, 91 (2015) 141-151.
 A. Oudriss, J. Creus, J. Bouhattate, F. Martin, X. Feaugas, Impact of dislocation distribution on hydrogen diffusion and trapping in tensile strengthening nickel (100) single crystal, Acta Materialia, (2015) in progress.
12:15 PM - T2.03
In situ TEM Investigation of Blister Formation on Aluminum Surface in Hydrogen Environment
Degang Xie 1 Zhangjie Wang 1 Jun Sun 1 Ju Li 1 2 Evan Ma 1 3 Zhiwei Shan 1
1Xi'an Jiaotong University Xi'an China2Massachusetts Institute of Technology Cambridge United States3Johns Hopkins University Baltimore United StatesShow Abstract
The corrosion resistance of metals relies on a passivating surface layer of dense and adherent oxide. However, the integrity of such a protective oxide is compromised in the presence of excess hydrogen which can cause blistering and eventual spallation of the oxide film. So far it remains unclear as to how a nanoscale gas bubble manages to reach its critical size at the metal/oxide interface before the oxide layer can deform. Using in situ environmental transmission electron microscopy, we have discovered that once the aluminum metal/oxide interface is weakened by the segregating hydrogen, rampant surface-diffusion of Al atoms sets in to form numerous gas-accumulating cavities on the metal side driven by Wulff reconstruction. The morphology and growth of these metal-side cavities are found to be highly orientation sensitive. The surface oxide layer remains unyielding until the metal-side cavities grow to a critical size above which the accumulated gas pressure become strong enough to blister the oxide layer. Our findings have broad implications for coating performance in nuclear, petroleum, and transportation industries, and can help optimize the material design strategies to alleviate a broad range of hydrogen induced interface failures. ( Xie et al, Nature Materials, 2015)
12:30 PM - T2.04
The Interaction of Dislocations and Hydrogen-Vacancy Complexes and Its Importance for Deformation-Induced Proto Nano-Voids Formation in alpha;-Fe
Suzhi Li 1 Yonggang Li 2 Yuchieh Lo 2 Thirumalai Neeraj 3 Rajagopalan Srinivasan 3 Peter Gumbsch 1 Ju Li 2
1Karlsruhe Institute of Technology Karlsruhe Germany2MIT Cambridge United States3ExxonMobil Research and Engineering Annandale United StatesShow Abstract
By using multi-scale simulation techniques, we probed the role of hydrogen-vacancy complexes on nucleation and growth of proto nano-voids upon dislocation plasticity in α-Fe. Our atomistic simulations reveal that, unlike a lattice vacancy, a hydrogen-vacancy complex is not absorbed by dislocations sweeping through the lattice. Additionally, this complex has lower lattice diffusivity; therefore, it has a lower probability of encountering and being absorbed by various lattice sinks. Hence, it can exist metastably for a rather long time. Our large-scale atomistic simulations show that when metals undergo plastic deformation in the presence of hydrogen at low homologous temperatures, the mechanically driven out-of-equilibrium dislocation processes can produce extremely high concentrations of hydrogen-vacancy complex (10-5~10-3). Under such high concentrations, these complexes prefer to grow by absorbing additional vacancies and act as the embryos for the formation of proto nano-voids. The current work provides the possible route for the experimentally observed nano-void formation in the context of hydrogen-induced failure and also bridge the link from the atomic-scale events to the macroscopic failure.
12:45 PM - T2.05
Characterization of Hydrogen Embrittlement Related to Martensitic Transformation in a Type 304 Austenitic Stainless Steel
Yoji Mine 1 Ryo Matsuoka 1 Kaoru Koga 2 Kazuki Takashima 1 Oliver Kraft 3
1Kumamoto Univ Kumamoto Japan2Kumamoto University (Currently: RYOBI LIMITED) Kumamoto Japan3Karlsruhe Institute of Technology Eggenstein-Leopoldshafen GermanyShow Abstract
Metastable austenitic stainless steels such as type 304 suffer from severe hydrogen embrittlement (HE), whereas the type 310S stable austenitic steel exhibits little degradation in ductility because of deformation localization in the presence of hydrogen. This difference between these steels is related to the intrinsic tendency towards deformation-induced martensitic transformation. Moreover, fractographic observation has often shown quasi-cleavage and planar fracture features in hydrogenated austenitic stainless steels. The cleavage is presumably formed near martensite laths and/or annealing twin boundaries. However, the role of martensite and twin boundaries in the HE of metastable austenitic steels is controversial. Therefore, this study using microtension testing has been employed to analyze the HE in single crystals and twinned bicrystals, with a particular focus on the effect of the deformation-induced martensite transformation.
The material used in this study was a solution-treated 304 stainless steel with an average grain size of 60 mu;m. Microtension specimens with gauge section dimensions of 20 mu;m × 20 mu;m × 50 mu;m were fabricated using a focused ion beam. Single-crystalline specimens were prepared so that the loading direction (LD) is close to the  and  directions, denoted as specimen A and B, respectively. For the bi-crystalline specimen, the twin boundary was arranged perpendicular to the LD. A set of specimens was electrochemically charged with hydrogen prior to microtension testing. A microtension test was performed at room temperature in air and at a loading rate of 0.1 mu;m sminus;1. After failure, the deformed microstructure was studied by electron back scatter diffraction (EBSD).
In the uncharged single-crystalline specimen A, yielding occurred at a stress of 170 MPa, and an ultimate strength of 490 MPa and 130% strain-to-failure were attained through a three-step strain hardening process. Specimen B exhibited a high yield stress but a low strain-to-failure owing to significant strain hardening. For both orientations, the yield stress increased but the ductility decreased drastically when pre-charged with hydrogen. For the hydrogen-charged single- and bi-crystalline specimens, the fracture morphology exhibited quasi-cleavage and a planar facet fracture, respectively. The EBSD observation of the deformation microstructures suggests that hydrogen-induced fractures occurred along the martensite habit plane and the austenite twin boundary, confirming that these microstructural features play indeed a crucial role for the hydrogen embrittlement.
David Armstrong, University of Oxford
David Bahr, Purdue University
Megan Cordill, Erich Schmid Institute of Materials Science
Corinne Packard, Colorado School of Mines
Symposium Support Hysitron, Inc.
T7: Time-Dependent Behavior and Testing II
Tuesday PM, December 01, 2015
Hynes, Level 1, Room 102
2:30 AM - *T7.01
Characterizing the Strain Hardening Behavior of Nanoscale Mulitlayers during Wear Testing
Marian Siobhan Kennedy 1 David Ross Economy 1 Nathan Mara 2 Raymond Robert Unocic 3
1Clemson Univ Clemson United States2Los Alamos National Laboratory Los Alamos United States3Oak Ridge National Laboratory Oak Ridge United StatesShow Abstract
In complex loading conditions, mechanical properties, such as strain hardening and initial hardness, will dictate the long-term performance of materials systems. In this study, the strain hardening behaviors of metallic and metallic/ceramic nanoscale metallic multilayer systems were examined by performing nanoindentation tests within nanoscratch wear boxes. Both the architecture and substrate influence were examined by utilizing three different individual layer thicknesses (2, 20, and 100 nm) and two total film thicknesses (1 and 10 µm). After nano-wear deformation, metallic multilayer systems with thinner layers showed less volume loss. Strain hardening exponents for multilayers with thinner layers were less than was determined for the thicker layers. These results suggest that single-dislocation based deformation mechanisms observed for the thinner systems limit the extent of achievable strain hardening.
3:00 AM - T7.02
Nano Scale In Situ X-Ray Microscopic Imaging of Mechanical Deformation
Brian M Patterson 1 Kevin Henderson 1 Nikolaus L Cordes 1 Amy J Clarke 1 Bryce Tappan 1 Virginia Manner 1
1Los Alamos National Laboratory Los Alamos United StatesShow Abstract
Understanding mechanical failure, crack propagation, and compressive behavior at the sub-micrometer scale is essential for tailoring material properties for structural performance. Typically, tension or compression loading is needed to understand these processes. Here, we demonstrate the coupling of a custom compression/tension load stage with a laboratory-based nano-scale X-ray transmission microscope. With this technique, 3D images of a material are collected, non-destructively, providing a probe into its internal structure. This provides a better understanding of both its manufactured and after-experiment morphologies. For some materials, it is possible to operate in an ‘interrupted in situ&’ modality to probe morphological changes during experimentation. The use of uniaxial loading (tension or compression) is needed to understand these processes. Here, we demonstrate the coupling of a custom compression/tension load stage with a nano-scale X-ray transmission microscope.
The solidification conditions used in the manufacturing of metal alloys determines the material&’s resultant properties. In order to relate the relationship between the processing (e.g., cooling rates) and morphological results (e.g., dendritic spacing, crystal structure) and how they affect the mechanical properties requires a thorough suite of characterization techniques. These techniques must capture the multi-scale phenomenon that occurs from the micro- to mesoscopic defects, solidification structures and properties. Using aluminum-copper alloys as a test, X-ray microscopy can not only directly image the resultant structure but also probe failure pathways with ~150 nm spatial resolution. Uniaxial tension experiments using micro-dogbones can image the 1-2 micrometer lamella while applying the loads. The results show that fracture failure occurs along the aluminum-copper interface. Not only does the fracture occur there, but it changes directions to follow this interface. Additionally, understanding dislocation formation within high explosive crystals is important in understanding hotspot formation which can affect performance. We will demonstrate the in situ imaging of 100 micrometer (or less) diameter single crystals of high explosive. Using phase contrast X-ray microscopy, we can directly image these dislocations and fracture progression with compression loading.
3:15 AM - *T7.03
Nanomechanical Testing of Feedstock Powders, Cold Spray Coatings and Their Third Bodies
Richard R Chromik 1 J. Michael Shockley 1 YinYin Zhang 1 Sima Alidokht 1 Praveena Manimunda 1
1McGill University Montreal CanadaShow Abstract
Cold spray is a process that produces thick, fully dense metallic coatings. More recently, it has been used to deposit metal matrix composites containing ceramic or solid lubricants phases for improved tribological properties. Optimization of the cold spray process to obtain robust tribological coatings involves both tailoring of the feedstock powder properties and process conditions such as gas temperature and pressure. Coatings typically have a non-homogenous microstructure and thus local mechanical properties vary somewhat. When the coatings are tested for their wear resistance, the material is modified mechanically and sometimes chemically, leading to ‘third bodies&’ with properties different from the as-prepared coating. In all of these settings, from powder to coatings to third bodies, nanomechanical testing is useful to determine mechanical properties, which are important for cold spray process optimization and understanding the material modifications while the coating is in service.
In this work, we report on nanoindentation measurements of feedstock powders, cold spray coatings and third bodies produced from the coatings after wear testing. For powder, special care was taken to produce reliable test conditions and analysis strategies for micron sized powders embedded in a soft matrix. For cold spray coatings, mapping of mechanical properties are presented and tied to various microstructural features observed by electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). And for third bodies, mechanical property changes are correlated to the microstructural modifications induced by wear. Materials systems reported on include Al-Al2O3, Cu-MoS2 and Ni-WC coatings.
3:45 AM - T7.04
On-Chip Irradiation Creep Testing of Copper Films
Pierre Lapouge 1 Renaud Vayrette 2 3 Fabien Onimus 1 Thomas Pardoen 3 Jean-Pierre Raskin 2 Yves Brechet 4
1CEA/Saclay Gif-Sur-Yvette France2UCL Louvain-la-Neuve Belgium3UCL Louvain-La-Neuve Belgium4INPG St Martin Drsquo;Hegrave;res FranceShow Abstract
Metallic alloys used as structural materials in the nuclear core of pressurized water reactors suffer from irradiation creep deformation. A proper understanding of the mechanisms which control the deformation is essential in order to predict the dimensional changes under irradiation. At the macroscopic scale, many experimental data are available. However, the microscopic mechanisms are still not yet fully understood.
In this study, a novel approach based on the testing of on-chip thin freestanding structures is evaluated. This on-chip test method, developed at Université catholique de Louvain, is for the first time used in the context of irradiation studies. An elementary test structure is composed of three main elements: (i) a thin specimen of the material of interest, (ii) an actuator layer of silicon nitride with strong internal tensile stresses to deform the attached specimen layer and (iii) a sacrificial layer of silicon dioxide to release the test structure from the underlying substrate. The small thickness of the material, below a few hundreds nanometers, allow full and homogenous irradiation by heavy ions with a kinetic energy of a few hundreds keV.
After adapting the method to in situ irradiation conditions, sets of test structures were successfully used to assess the room temperature creep behaviour of PVD copper films. The contribution of the irradiation creep to the deformation has been quantified by performing several irradiation steps up to a dose of 1 dpa and measuring after each step the deformation of the specimens.
Preliminary results show that while the creep behaviours before and after irradiation are very similar, the strain rate under irradiation is several times higher than out of flux. The activation volumes and the strain rate sensitivity were extracted from these experiments. Theses quantities were found to be nearly identical for the unirradiated samples and the irradiated ones. Hence the same mechanisms are thought to take place in both cases. The study of the mechanisms responsible for irradiation creep is currently in progress.
In parallel to these nanomechanical tests, the microstructure of the copper film before and after irradiation is characterized by Transmission Electron Microscopy. In particular, the influence of the grain size on the irradiation defects is investigated. No significant effect on the irradiation defects for grain size between 100 nm and 10 µm is found in this study.
T8: Mechanics of Micromachined Materials and Structures
Tuesday PM, December 01, 2015
Hynes, Level 1, Room 102
4:30 AM - *T8.01
Lessons Learned from Nano Scale Specimens Tested by MEMS Based Apparatus
M Taher A Saif 1
1University of Illinois at Urbana-Champaign Urbana United StatesShow Abstract
Materials at small scale behave differently from their bulk counterparts. This deviation originates from the abundance of interfaces at small scale. Quantifying the properties and revealing the underlying mechanisms requires experiments with small samples in situ in analytical chambers. However, small size poses the challenge of sample handling, but offers the opportunity of in situ inspection of mechanism during testing in analytical chambers. In order to overcome the challenge and take advantage of the opportunity, we developed a MEMS based micro scale testing stage where the sample and the stage are co-fabricated. The stage suppresses any misalignment error in loading by five orders of magnitude. The stage allows in situ inspection of samples during testing in SEM and TEM. We employed the stage in two scenarios. (1) Exploring the effect of microstructural heterogeneity, such as grain size and orientation, on the deformation mechanisms in nano grained polycrystalline metals. Here the test specimens are free standing thin films subjected to uniaxial tension. We found that heterogeneity introduces two apparently dissimilar, but fundamentally linked, anomalous behaviors. The samples undergo plastic deformation during unloading, i.e., exhibit Bauschinger type phenomenon. Upon unloading, they recover a significant part of plastic deformation with time. The underlying mechanism, verified by in situ TEM inspection, is as follows: during loading, the relatively larger grains undergo plastic deformation and relax by employing dislocations, while the smaller grains remain elastically deformed. During unloading, the smaller grains apply reverse stress on the larger grains causing reverse plasticity resulting in a deviation from linear stress-strain response. Upon complete unloading, the residual stress of the elastically strained small grains continue to apply reverse stress on the larger grains resulting in biased jumps of dislocation in the larger grains and strain recovery. (2) Exploring the effect of size on brittle to ductile transition (BDT) temperature (540C) in single crystal silicon. Here the sample is a micro scale single crystal silicon beam subjected to bending which limits the high stress region to a small volume in the sample, and minimizes the probability of premature failure from random flaws. We found that silicon indeed deforms plastically at small scale at temperatures much lower than 540C. Smaller the size, lower is the temperature, e.g., 290C for 800nm sample. Ductility is achieved through a competition between fracture stress and the stress needed to nucleate dislocations from surface. Small size offers high flaw tolerance - lower the size of the sample, higher is the fracture stress. Nucleation stress is temperature dependent - higher is the temperature, lower is the nucleation stress. With decreasing sample size, fracture stress exceeds the nucleation stress even at lower temperatures resulting in a size dependent BDT temperature.
5:00 AM - T8.02
Combinatorial Approach for Testing the Fracture Toughness of Graded NiAl Single Crystals by Microcantilevers
Mathias Goeken 1 Ralf Webler 1 Steffen Neumeier 1 Karsten Durst 1
1Friedrich-Alexander-University Erlangen-Nuuml;rnberg, FAU Erlangen GermanyShow Abstract
Testing of the local mechanical properties as hardness, Young&’s modulus, fracture toughness etc. by nanomechanical testing methods is now well established. Therefore these methods are often used in combinatorial studies for testing the properties of diffusion couples, where many different compositions are studied. Developing of new advanced materials with complex compositions requires a very good knowledge on the influence of the chemical compositions on the properties of the constituent phases.
A very simple method of generating a graded single crystalline NiAl sample consists of annealing a prior homogeneous crystal at high temperatures since Al evaporates out and an Al depleted zone is formed at the sample surface. By such an approach the graded sample remains single crystalline which is especially important for determining the fracture toughness, which is highly anisotropic. In addition microstructural influences are avoided which often complicates the analysis of polycrystalline diffusion couples. The fracture toughness is then determined by FIB-milled microcantilevers in dependence of the chemical composition. In previous work it has been shown that the fracture toughness of single crystals can be reliably tested with the microcantilever method.
This new approach has been used to study the properties of Ni-rich NiAl single crystals where it is found that the fracture toughness decreases with increasing the Ni content from the stoichiometric composition whereas the hardness increases with the number of constitutional defects. The results of the microcantilever tests are compared with nanoindentation measurements and will be discussed in terms of plastic deformation in the microcantilever experiments.
5:15 AM - T8.03
Effect of Focused Ion Beam Species on Microcompression Deformation of Ultrafine-Grained Aluminum
Yuan Xiao 1 3 Verena Maier 2 Sandra Korte-Kerzel 3 Ralph Spolenak 1 Jeff Wheeler 1
1ETH Zurich Zurich Switzerland2Erich Schmid Institute of Materials Science Leoben Austria3RWTH Aachen Aachen GermanyShow Abstract
Since its introduction by Uchic et al. , micro-compression testing of pillar samples has risen to be one of the primary techniques for interrogating the deformation behavior of small volumes. The technique offers site specific interrogation of microstructural features and a simple uniaxial stress state. This uniaxial stress state provides several advantages over other techniques such as nanoindentation, which imposes complex triaxial stresses. However, fabrication of micropillars is usually performed using focused ion beam (FIB) machining techniques. These have been shown to impose significant ion damage into small pillar and cause changes in deformation behavior in metallic samples compared to pillars produced without FIB techniques . Even the differences in ion beam dosage caused by different milling techniques has been shown to introduce significant variations in deformation behavior . Polycrystalline aluminum micropillars are expected to be especially susceptible to gallium ion damage  due to the well-known embrittlement of aluminum by gallium caused by the segregation and concentration of gallium at the grain boundaries . Therefore, polycrystalline aluminum samples are an ideal case for comparing the relative damage introduced by focused ion beam machining using xenon ions as an alternative to gallium ions for machining of metallic materials. In this project, micropillars will be manufactured by FIB using both ion species to produce pillars of a range of sizes. The grain boundary content of the pillars will be varied using by varying both the extrinsic pillar diameter and the intrinsic grain size of the polycrystalline aluminum. The strengths and rate sensitivity of the resulting deformation behavior will finally be investigated by strain rate jump microcompression testing of the pillars.
 Uchic MD, Dimiduk DM, Florando JN, Nix WD. Sample dimensions influence strength and crystal plasticity. Science. 2004;305:986-9.
 Bei H, Shim S, George EP, Miller MK, Herbert E, Pharr GM. Compressive strengths of molybdenum alloy micro-pillars prepared using a new technique. Scripta Materialia. 2007;57:397-400.
 Hütsch J, Lilleodden ET. The influence of focused-ion beam preparation technique on microcompression investigations: lathe vs. annular milling. Scripta Materialia. 2014;77:49-51.
 Kiener D, Motz C, Dehm G, Pippan R. Overview on established and novel FIB based miniaturized mechanical testing using in-situ SEM. International Journal of Materials Research. 2009;100:1074-87.
 Schmidt S, Sigle W, Gust W, Rühle M. Gallium segregation at grain boundaries in aluminium. Z Metallk. 2002;93:428-31.
5:30 AM - T8.04
A Weakest Size for Precipitated Alloys?
Rui Gu 1 Alfonso H.W. Ngan 1
1Univ of Hong Kong Hong Kong ChinaShow Abstract
Duralumin, a representative precipitated aluminium alloy, is a strong aircraft alloy that exhibits high tensile strength of well over 400MPa, and would not creep significantly at room or moderately elevated temperatures in the bulk condition. Here, we report a “larger-being-weaker” size effect of strength in micron-sized duralumin. Compression tests on duralumin micro-pillars in the ~1 to 6.5 microns size range at room temperature revealed that their proof strength drops from ~300MPa for the 1 micron pillars to ~120 MPa for the 6.5 micron pillars. Creep experiments conducted on duralumin micro-pillars of this size range also showed that larger samples exhibited more significant creep at room temperature than smaller counterparts. Such a “larger-being-weaker” size effect should not continue for larger specimens since their strength should revert towards that of the bulk as the specimen size increases in the tens-of-microns regime or beyond. Therefore, a critical size of duralumin, possibly in the tens-of-microns regime, at which the strength of duralumin reaches a minimum, is evident from these preliminary findings. Such a weakest-size behaviour of precipitated alloys is a significant discovery as precipitate strengthening is a deep-rooted concept in physical metallurgy, yet there has been no previous report suggesting that the specimen size in the tens-of-microns range can be an important factor hampering the effect of precipitate strengthening. A complete understanding of the weakest-size effect is therefore important when precipitated alloys are used as miniaturized structural components.
T9: Poster Session
Tuesday PM, December 01, 2015