Orientation dependence of the γ-α′ martensitic transformation in single crystals of austenitic stainless steels with low stacking-fault energy

The development of the gamma-alpha' martensitic transformation (MT) upon tensile deformation of single crystals of austenitic stainless steels of compositions (wt %) Fe-17% Cr-12% Ni-2% Mn-0.75% Si (I) and Fe-18% Cr-12% Ni-2% Mo-0.015% C (II) has been studied as a function of the crystal-axis orientation and test temperature by X-ray diffraction and electron microscopy. It has been established that the orientation dependence of the slip deformation preceding the gamma-alpha' MT is determined by two factors, namely, the orientation dependence of slip deformation preceding the gamma-epsilon MT and the orientation dependence of the work U necessary for the formation of the alpha'-martensite crystals. The orientation dependence of slip deformation preceding the gamma-epsilon MT leads to the gamma-alpha' MT in the [(1) over bar 11], [(1) over bar 23], [011], and [012] crystals with different defect densities and, correspondingly, at different stress levels. In the [001] crystals, no gamma-alpha' MT is observed macroscopically since the gamma-epsilon MT in these crystals is suppressed. It has been established that the gamma-alpha' MT in the [(1) over bar 11], [011], [(1) over bar 23], and [012] crystals can be developed at T = 300 K after preliminary deformation at T = 77 K. The development of the gamma-alpha' MT at T = 300 K is physically related to the growth of the alpha'-martensite nuclei formed upon plastic deformation at T = 77 K.