Design of Supercompatible Shape Memory Alloys

The decisive influence of the geometry of microstructure on the reversibility of martensite was appreciated since shape memory alloys emerged in the 1950s. Recently, new restrictions on lattice parameters have been derived (λ₂ =1; cofactor conditions). These restrictions can be considered as the analog for microstructure of nongeneric conditions on the geometry of a truss at which it suddenly becomes flexible. Tuning lattice parameters to satisfy conditions of supercompatibility by compositional changes leads to diverse alloys with thermal hysteresis of 1-2°C. The strongest such conditions delivers alloys with multi-million cycle repeatability of full transformation under high-stress superelastic cycling. In pillar compression experiments supercompatible alloys overcome the usual competition between bulk and interfacial energy, and exhibit complex microstructures even at micron scale. We review alloy design by supercompatibility, and also discuss the emerging story of how it interacts with other alloy optimization procedures based on the manipulation of precipitates.