8:40 AM - *CT08.01.03
On the Role of Stress in Microstructure Evolution During Thermo-Chemical Surface Engineering
Marcel Somers1,Thomas Christiansen1
Technical University of Denmark1
Thermochemical surface engineering of metals is characterized by a deliberate and targeted modification of the (sub)surface region of metals, with the aim to improve materials performance with respect to fatigue, wear, corrosion and combinations thereof. Generally, thermochemical surface engineering is understood in terms of thermodynamics and diffusion kinetics to describe the evolution of the microstructure under the influence of the chemical modification at elevated temperature. Associated with the change in composition in the surface-adjacent region strains and stresses are introduced. These strains/stresses affect the thermodynamics and the kinetics of the ingress of chemical species and, consequently, the microstructural evolution. This contribution illustrates several of the effects of composition-induced stresses/strains during thermochemical surface engineering with interstitials in metals through examples from activities in the authors’ research group. The presentation includes experimental as well as modelling aspects. The following examples are covered.
1. The dissolution of N and/or C into stainless steels and high entropy alloys (HEAs) at temperatures below 725 K for N and below 825 K for C is associated with the development of a supersaturated solid solution of interstitials in the fcc phase. This supersaturation is “kinetically stabilized” by sluggish decomposition, because of slow diffusion of substitutionally dissolved components. The lattice expansion caused by dissolution of N or C is accommodated elastically for low interstitial contents, leading to strengthening. As solid solution strengthening scales with the cube root of composition squared and biaxial compressive stresses caused by lattice expansion scale linearly with composition, plastic accommodation will occur above a threshold composition. Also, with an increase in N content long range ordering (LRO) of N atoms is encountered. The combination of elasto-plastic accommodation of lattice expansion and LRO leads to anisotropic growth of the developing case.
2. Hcp titanium can dissolve high contents of oxygen and is applied for surface hardening of Ti and its alloys. The thermo-chemical treatment of Ti in CO above the hcp-bcc transition temperature leads in principle to the uptake of both O and C. Since C has a very low solubility as compared to O, hence, primarily O is dissolved, thereby stabilizing a case of hcp-Ti atop bcc-Ti. The conversion of bcc to hcp is associated with the introduction of tensile stresses in the hcp case, leading to crack development in the hard and brittle part of the case close to the surface. Upon cracking, CO ingress leads to the development of a mixed interstitial compound Ti(C,O) along the crack surface, thereby converting this part into an extremely wear resistant Ti-surface, with a hardness of up to 3,000 HV.
3. Oxidizing of ZrCuAl-based bulk metallic glass (BMG) below the glass transition temperature is a potential thermo-chemical surface engineering treatment of BMGs. Within the BMG an internal oxidation zone (IOZ), consisting of nano-crystalline ZrO2, develops. The volume expansion caused by zirconia formation leads to compressive residual stresses in the IOZ. The theoretical stresses are 30 times higher than the experimentally determined stress levels, indicating that stress relaxation has occurred. This is accomplished by shear band formation in the BMG adjacent to the IOZ, which later affects the ZrO2 development of the advancing oxidation front. In addition, the compressive stresses induce outward diffusion of “noble” elements, as for example Ag (if present) and Cu. Surprisingly, segregation of these noble elements at free surfaces, as the outer surface, but also crack surfaces, leads to crystalline metallic regions, which can be considered as self-healing in the case of crack formation. Depending on the oxygen partial pressure, Cu can oxidize upon arrival at the surface.