On the role of stress in microstructure evolution during thermo-chemical surface engineering

Abstract

<h2>Abstract</h2> Thermochemical surface engineering of metals is the deliberate and targeted modification of the (sub-)surface region of metals, with the aim to improve materials performance. Generally, thermochemical surface engineering is understood in terms of thermodynamics and diffusion kinetics to describe the evolution of the microstructure during chemical modification at elevated temperature. Associated with the change in composition, strains and stresses are introduced. These strains/stresses affect the thermodynamics and the kinetics of microstructure evolution. This contribution illustrates several examples from activities in the authors' research group. The following examples are covered:<ul><li>•The dissolution of nitrogen (N) or carbon (C) into austenitic stainless steels and high entropy alloys at temperatures below 725 K for N and below 825 K for C. Strong supersaturation with interstitials of the austenite leads to lattice expansion which is accommodated elasto-plastically and residual stresses of several GPas.</li><li>•Oxidizing of ZrCuAl-based bulk metallic glass (BMG) below the glass transition temperature leads to an internal oxidation zone (IOZ), consisting of nano-crystalline ZrO₂. Compressive residual stresses induce outward diffusion of "noble" elements, as for example Ag (if present) and Cu, which segregate at free surfaces. This segregation leads to crystalline metallic regions, which can provide a mechanism for self-healing of cracks.</li></ul>