Grain orientation dependence of the forward and reverse fcc ↔ hcp transformation in FeMnSi-based shape memory alloys studied by in situ neutron diffraction

Abstract

The grain orientation dependence of the deformation-induced forward fcc→hcp and reverse hcp→fcc martensite transformation of a FeMnSi-based shape memory alloy was studied by in situ neutron diffraction during cyclic loading. A deformation-induced fcc→hcp transformation is observed during tensile straining to +2%. The hcp martensite phase that forms under tension partially reverts to fcc austenite upon subsequent compression from +2% → −2% for the {220}, {331} and {111} grain families aligned with respect to the loading direction but not for the {200} grain family. The martensite formation and the reversion of the individual grains can be explained by considering grain orientation dependent Schmid factors of the {111}<112> slip system underlying the fcc to hcp transformation. While for post-yield elastically compliant grains the Schmid factor of the leading partial dislocation is larger than that of the trailing partial dislocation, the opposite is true for post-yield elastically stiff grains. The former grains show a phase reversion, i.e. hcp→fcc upon compression, the latter grains do not transform back to fcc. EBSD characterization confirms the phase reversion for a <541> orientated grain by the disappearance of hcp bands. Martensite bands, which have not reverted to austenite during compression, showed a thickening. The thickening of existing bands during compression is associated with the activation of a second slip system.