Mechanical and Civil Engineering Seminar

Martensitic materials, although traditionally known as shape memory alloys, are exploited for many new applications in microelectronics and energy conversion devices when the change of lattice parameters are linked to the sensitivity of multiferroic properties. However, the functionalities of martensitic materials usually degrade significantly after only few transformation cycles. The origin of such degradation comes from the formation of microstructure consisting of stressed-transition layers due to lattice mismatch at heterophase boundaries. It has been proven that when the lattice parameters satisfy a set of geometric conditions of compatibility, both hysteresis and reversibility can be optimized, thus long life-time is achieved for these applications. New alloys discovered by satisfying these conditions exhibit nearly zero functional migration up to millions of cycles [Song et. al, Nature (2013) and Chluba et al. Science (2015)]. In sharp contrast to ordinary martensitic materials, a variety of hierarchical microstructures arise in these highly reversible materials: riverine curved interfaces, sharp zig-zag interfaces as well as large single variant bands. In addition, the material loses the usual reproducibility and acoustic emission traces from cycle to cycle. Here, I would like to present the formulation of the geometric conditions of compatibility and their roles in discovering highly reversible materials. By in situ micro-Laue scan across the austenite/ martensite interface, we can quantify both morphology and coherency quantitatively. Combined the quantitative measurement from the micro-Laue scan with the theoretical calculation, the mysteries of unusual hierarchies in the highly reversible material can be understood.