In situ observations on the mechanical stability of austenite in TRIP-steel

Abstract

In-situ tensile deformation tests have been performed on a high Al TRIP steel (composition 0.26 wt. % Si, 1.5 wt. % Mn, and 1.8 wt. % Al) displaying the transformation-induced plasticity (TRIP) effect, while monitoring the phase transformation by means of X-ray microdiffraction in transmission geometry. Due to the small beam size (25 μm x 25 μm) every retained austenite grain appears as a discrete spot on the diffraction patterns. The diffraction patterns are treated like a powder pattern for different η-angles, with η representing the angle between the tensile direction and the normal direction of the diffracting {200}planes. The disappearance of austenite {200} reflections is analyzed during as a function of the imposed stress and orientation. Grains with η = 0 or 90° tend to transform to martensite more easily. A unique feature of this microdiffraction experiment is the possibility of detecting the average carbon concentration of the retained austenite as a function of stress. Direct proof has been obtained that austenite with a lower carbon content x c transforms at lower stress levels. The average X c increases from 1.0 to to 1.05 wt.%. This increase indicates a relatively narrow distribution of the carbon content.