Unexpected deformation-induced martensitic phase transformations in Ni–Cr and Ni–Cr–Fe alloys

Deformation of Ni–Cr and Ni–Cr–Fe alloys has historically been understood to occur through dislocation slip, while deformation-induced martensitic transformations are believed to be suppressed by the high stacking fault energy (SFE). The present study challenges these longstanding beliefs with experimental and theoretical evidence of deformation-induced martensitic transformations in Alloy 625 and Ni–20Cr, respectively. Systematic investigations are conducted along the three principal grain orientations; experiments employ nanoindentation coupled with post mortem transmission electron microscopy (TEM) analysis, while molecular dynamics (MD) simulations are conducted in uniaxial tension. Experimental results reveal γ→ε and γ→ε→α^′ martensitic transformations in Alloy 625 consistent with the Bogers-Burgers-Olson-Cohen (BBOC) intersecting shear mode. The orientation relationship is characterized as a distorted Shoji-Nishiyama OR between FCC/HCP and Kurdjumov-Sachs OR between FCC/BCC. By contrast, Alloy 690 does not exhibit martensitic transformations despite having a lower SFE than Alloy 625, due to Mo solute strengthening in Alloy 625. MD simulations reveal consistent transformation mechanisms across grain orientations, but orientation-dependent differences in Schmid factors and critical resolved shear stress affect the strain evolution and extent of the transformation. This study concludes that SFE alone may not comprehensively dictate whether deformation-induced martensitic transformations will occur; other factors such as free energy and strain rate also influence transformability.