Thermal and Mechanical Stability of Austenite in Metastable Austenitic Stainless Steel
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METASTABLE AUSTENITIC stainless steel (MASS) has a face-centered cubic (FCC) crystal structure and low stacking fault energy (SFE). It finds
A.A. TIAMIYU, A.G. ODESHI, and J.A. SZPUNAR are with the Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, SK, Canada. Contact e-mail: [email protected] SHITENG ZHAO is with the University of California, Berkeley, Berkeley, CA. ZEZHOU LI is with the University of California, San Diego, La Jolla, CA 92093. Manuscript submitted January 3, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
application in energy (chemical and nuclear) and transportation sectors. MASS could be exposed to various degree of conditions that could result in the instability of its austenite phase in service, i.e., martensitic phase transformation could occur. This could affect the properties of MASS, e.g., loss of non-magnetic property of the MASS when a¢-martensite developed.[1] Some of these stability-deteriorating conditions include a cryogenic environment and exposure to an external load, either at low or high strain rates. In a cryogenic environment, the austenitic phase may become unstable, leading to the evolution of a¢-martensite. Generally, the martensitic transformation is displacive with definite crystallography,[2] and they are categorized into
athermal and isothermal transformations depending on the kinetics of transformation.[3–5] While the amount of martensites formed during athermal transformation depends solely on temperature, the amount of those that form during isothermal transformation is a function of both temperature and time.[6] For athermal transformation to occur and proceed, the thermal activation is not necessary.[7] In other words, only the thermodynamic driving force obtained by lowering temperature is adequate for athermal martensitic transformation. This thermodynamic driving force must overcome the elastic energy that opposes initiation at specific sites at and below the martensite start (Ms) temperature. Athermal martensitic transformation involves two steps. The first step is ‘‘barrier-less’’, and it entails the evolution of martensite units, i.e., martensite unit starts to form without the need to overcome any form of barrier. The second step is the growth of the martensite units which involves the migration of a glissile interface without thermal activation.[2] Meanwhile, the isothermal transformation has no definite Ms temperature, but it occurs with time (incubation) during isothermal holding. The amount of the product phase (martensite) in athermal transformation does not depend on time, but on temperature due to its intrinsic nature.[3] This is because no diffusion is involved in athermal transformation, and the composition of the product is the same as that of the parent (austenite) phase. While thermal activation implies statistical probability (i.e., the same site will not always be the first to initiate the transformation process), the same site during a non-thermal activation process (athermal martensites) tends to repeatedly ini
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