Martensitic transformation and plastic deformation in iron alloy single crystals

Abstract

The characteristics of plastic deformation of austenite single crystals have been studied in Fe-16Cr-25Ni, Fe-16Cr-12Ni and Fe-30Ni alloys and related to the crystallographic mechanism of the martensite transformation. The 16 25 alloy is completely stable down to 4°K and its plastic characteristics appear to be identical with other concentrated, f.c.c. solid solutions of low stacking fault energy. From 350°C to 200°C the plastic behavior of the 16 12 alloy crystals is similar to the stable 16 25 alloy. Below 200°C transformation begins under stress and the critical resolved shear stress first increases and then decreases with temperature and appears to go to zero at −32°C due to the dilation associated with the spontaneous transformation at this temperature. The transformation in the 16 12 alloy when under an applied stress proceeds through the sequence of structures, f.c.c. → h.c.p. → b.c.c., originating on the {111} A plane of maximum resolved shear stress. Other significant characteristics of the transformation were observed and related to the structure, namely: crystallographic reversibility of the transformation, stabilization by prior deformation, and isothermal transformation to h.c.p. and b.c.c. products at room temperature following deformation at a lower temperature, and the effects of transformation products on the plastic properties of crystals. Significant differences between the behavior of Fe-Cr-12Ni and Fe-30Ni crystals were observed and attributed to the difference in stacking fault energy and the associated differences in the transformation.