The correlation of austenite stability and sequence of strain accommodation during room temperature deformation of a duplex lightweight steel

The effect of austenite stability on the sequence of room temperature strain accommodation has been investigated in Fe-0.07C-11.15Mn-5.6Al-0.12Si lightweight steel through employing interrupted tensile tests coupled with electron backscattered diffraction analysis. It has been found that in the microstructure holding finest grain size, the strain is mainly accommodated through austenite via transformation induced plasticity (TRIP) effect during the first stage of deformation (up to the true strain of 0.03), which hinders Kernel Average Misorientation (KAM) increment in constituent phases. Although, high stability of austenite limits the first deformation stage to low strains and the contribution of ferrite along with austenite is traced in the following stages as indicated by KAM increment, leading to higher TRIPing saturation strains, due to this simultaneous strain accommodation. However, by lowering the austenite stability, strain accommodation behaves differently in which case, low stability of the austenite leads to intensified TRIP effect which accommodates the imposed strains to larger strains (up to the true strain of 0.08) during the first stage. Respectively, due to intense TRIPing during the mentioned stage, KAM of both constituent phases remains low, but this severe solo accommodation decreases the fraction of austenite dramatically and saturates the capability for TRIP effect. Subsequently, load transfers from austenite to the ferrite during the second deformation stage leading to a sharp reduction in work hardening and increment in KAM value. Despite the microstructure holding higher austenite stability which represents simultaneous accommodation, sequential strain accommodation deteriorates the hardening capability and the strength/ductility balance of the material.

Mots clés

Strain accommodation sequences; Austenite stability; Work hardening rate; Transformation induced plasticity, (TRIP); Hardening stage.