Kinematic and thermal characteristics of Lüders and Portevin-Le Châtelier bands in a medium Mn transformation-induced plasticity steel

Abstract

This paper focuses on experimental investigations on the kinematic and thermal aspects of the propagative plastic instabilities, namely Lüders band and Portevin–Le Châtelier (PLC) band, observed in a medium Mn transformation-induced plasticity (TRIP) steel. The full-field strain and heat source measurements are used concurrently to investigate in-situ the propagative bands. The kinematic analysis provides the real-time strain field evolution allowing the observation and quantification of the band nucleation and propagation. The local Lüders strain is demonstrated both space and time independent, and its magnitude is equivalent to the macroscopic Lüders strain. The spatio-temporal independent characteristic is also verified on the local strain in the PLC bands. Concerning the thermal field analysis, the heat sources are estimated and used to obtain the geometrical and dynamic parameters of the propagating bands, including the band width, orientation, propagation velocity, and appearance moment. Then the estimated heat sources are applied to establish an experimental energy balance during the plastic deformation. The quantitative energy analysis reveals that the strain-induced martensitic transformation takes place in the Lüders band but barely in the PLC bands. The ex-situ X-ray diffraction measurement on the phase content and surface hardness testing of the material confirm well this determination.