The Effect of Static Recrystallisation of Parent Austenite on the Lath Martensite Intervariant Boundary Network

Abstract

The current study investigated the effect of static recrystallisation (SRX) of parent austenite microstructure on the lath martensite intervariant boundary network. In general, the misorientation angle distribution of lath martensite exhibited a bimodal distribution in the range of 10–22 deg and 47–60 deg for all thermomechanical conditions, closely matching the misorientations expected from the ideal Kurdjumov-Sachs orientation relationship. Nevertheless, the population of 60 deg misorientation angle initially reduced with the extent of softening up to 50 pct beyond which it progressively increased to a maximum in the fully recrystallised condition. This was closely consistent with the trend noticed for the 60 deg/[110] intervariant boundaries having twist character. This was explained through the distinct variant selection mechanism occurring due to the change in the parent austenite grain characteristics (i.e., size and deformed state) upon static recrystallisation. The recrystallisation initially led to the formation of fine dislocation-free parent austenite grains up to 50 pct softening, promoting a 4-variant (V1V2V3V4) clustering arrangement to decrease the strain related to the martensite transformation. In turn, this promoted the formation of symmetric tilt 60 deg/[111] intervariant boundaries, which ultimately reduced the 60 deg/[110] boundaries. Further softening, accompanied with the growth and/or coalescence of parent austenite grains, altered the variant clustering to a 3-variant arrangement (V1V3V5) and the increase in 60 deg/[110] twist intervariant boundaries. Indeed, the martensite boundary network was largely controlled by the SRX parent austenite characteristics (i.e., size), which can be controlled to enhance the materials performance for a given application.