Theory of flow localisation due to dynamic strain ageing

Local strain hardening and nonuniformity of plastic deformation

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 dσ/d∅, 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.

Strain hardening and transformation mechanism of deformation-induced martensite transformation in metastable austenitic stainless steels

Solid Solution Strengthening

The paper reviews the main dislocation mechanisms relevant for plastic instabilities in metals and the patterns involved. The observability of plastic instabilities by various methods and the classification of micromechanisms (M-, h-, S-, T-type instabilities) as well as propagating modes of shear are summarized. Short excursions deal with instabilities by diffusionless structural transformations and with unstable localized deformation in amorphous alloys. Finally, the possibilities for describing the deformation kinetics on micro- and macroscale by the approaches of viscous glide and of generation rate of dislocations are discussed.

J. Schlipf

Papers

Stable and unstable modes in tensile deformation