Effects of Defect Development During Displacive Austenite Reversion on Strain Hardening and Formability

Martensite that is mechanically induced from metastable austenite can be reversed to austenite upon annealing. The reversion transformation can be either diffusive or displacive, and the defect substructure development, in either case, has mechanical consequences. Here, to better understand the effects of microstructure development during displacive phase transformations, we focus on the influence of the initial plastic deformation on the austenite reversion (α′ → γ) in a transformation-induced plasticity-maraging steel. The phase transformation kinetics and the developing defect structure within the reversed γ phase are characterized by carrying out differential scanning calorimetry measurements, electron-backscattered diffraction, and electron channeling contrast imaging analyses. The resulting mechanical behavior is investigated by uniaxial and biaxial tension experiments. These investigations demonstrate that the defect development during sequential deformation-annealing treatments can help increase the overall strain hardening capacity of the alloy, which in turn increases the accumulative uniform elongation, and the formability. While the necking can be progressively delayed to higher strain levels following such treatments, the local fracture strain apparently cannot be, due to damage accumulation.