STRAIN-HARDENING AND TRANSFORMATION MECHANISM OF DEFORMATION-INDUCED MARTENSITE-TRANSFORMATION IN METASTABLE AUSTENITIC STAINLESS-STEELS

Austenitic stainless steels with lower chromium or nickel contents can transform to martensite when they are plastically deformed at sufficiently low temperatures. The correspondingly obtained stress-strain curves show an unusual strain hardening behaviour, e.g. minima and maxima of the strain hardening coefficient d-sigma/d-phi, in contrast with normal materials without transformation. A unified model is proposed on the basis of dislocation theory, thermodynamic and experimental observations, which can explain the strain hardening abnormality and give a quantitative relation between the flow stress and martensite transformation. It is shown that deformation-induced martensite formation closely correlates with the homogeneity and stability of plastic deformation. The martensite transformation is initiated by inhomogeneous deformation. The transformation proceeds by orientated rapid dislocation movement and multiplication. Its onset takes place spontaneously when a sufficiently high stress is reached, rather than cumulatively with increasing strain. The increase in the martensite volume fraction is linearly proportional to the square of the flow stress.