Stressed microstructures in thermally induced M9R-M18R martensites

We revisit the phase transformation that produces ‘long-period stacking’ M9R–M18R martensites in Cu-based shape-memory alloys and analyze some associated microstructures, in particular, the typical wedge-shaped configuration. Our basic premise is that the cubic-to-monoclinic martensitic phase change in these alloys is, geometrically, but a slight modification of the well-known bcc-to-9R transformation occurring in various elemental crystals, whose lattice strain is, at the microlevel, the same Bain strain as for the bcc-to-fcc transformation. For the memory alloys we thus determine the ‘near-Bain’ microstrain, thereby analyzing the faulted, long-period stacking martensite as a mesoscale structure derived from compatibility with the austenite. We compute the transformation-twin systems, habit planes, average deformation and stacking-fault density of the 9R, 18R, M9R or M18R martensites, as they arise from the compatibility conditions between the parent and product lattices. We confirm earlier conclusions that a stress-free wedge is not kinematically compatible in these materials. However, we show that this microstructure is ‘close enough’ to compatibility, finding that its stress levels are low and should cause only minimal plastification and damage in the crystal. The wedge is therefore rationalized as a viable path for the transformation also in these substances. We verify this to hold for all the lattice parameters reported for Cu-based alloys. In general, we conclude that martensitic microstructures can be stressed to a degree also in good memory materials. Furthermore, we find that the lattice-parameter relations, guaranteeing the zero-stress compatibility of special configurations favoring the transformation and its reversibility, do not need to be strictly enforced in these crystals, because the residual stresses in microstructures are low regardless of lattice-parameter values.