Role of Chemical Driving Force in Martensitic Transformations of High-Purity Fe-Cr-Ni Alloys
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AUSTENITIC stainless steels are an important family of steels, which are widely used in engineering applications due to their excellent corrosion resistance, good weldability, excellent formability, and high strength.[1] Composition of austenitic stainless steels mainly consists of Fe, Cr, and Ni elements with a small amount of other alloying elements such as Mo, Ti, Nb, Al, Si, and Mn. Their microstructure is composed of the metastable austenite (c) phase. Austenitic stainless steels are prone to transformation from the initial face-centered-cubic (fcc) c phase to the body-centered-cubic a¢ martensite phase through spontaneous transformation or plastic deformation. Spontaneous transformation occurs when the alloy is rapidly cooled below the martensite start temperature (Ms). Further, plastic deformation of the c phase slightly above the Ms can give birth to the formation of stressinduced martensite. The temperature above which strain-induced martensite is not formed by plastic deformation is referred to as Md. It was proposed that the most probable way of the martensitic transformations in Fe-Cr-Ni alloys is c fi e fi a¢.[2] The e phase has hexagonal-close-packed (hcp) crystal structure. In-situ studies carried out by Brooks et al.[3,4] show that the e-martensite occurs in regions where, appropriately, but usually irregularly, spaced stacking faults are formed. Most earlier investigations reveal that the nucleation of a¢-martensite embryos is always confined to microscopic shear band intersections.[5] However, Sato et al.[6] found that in Fe-Mn-Al alloys, high
stacking fault energy (SFE) (>20 mJ/m2) promotes direct transformation c fi a¢; low SFE (
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