Design of NiTi-based shape memory microcomposites with enhanced elastocaloric performance by a fully thermomechanical coupled phase-field model

The non-transforming intermetallic Ni3Ti phase generated in NiTi matrix by additive manufacturing was previously reported to create elastocaloric composites with a great coefficient of performance (COP) between 11 and 22 [Hou et al., Science 366 (6469) (2019) 1116–1121]. In this work, we use a fully thermomechanical coupled phase-field model to design microarchitectures considering the effects of all the possible non-transforming intermetallics (Ni4Ti3, Ni3Ti, and Ti2Ni) in NiTi. Our simulations show possibilities of increasing the COP by guiding the type, shape and volume fraction of intermetallics, which are controllable by processing parameters. With 50% intermetallic fraction arranged in strips of 500 nm width perpendicular to the loading direction, the Ti2Ni intermetallic induces higher COP (67.13) than Ni3Ti (16.18) and Ni4Ti3 (14.29), all surpassing that of the bulk NiTi without intermetallics (12.92). Additionally, the COP increases to 79.94 for 65% volume fraction of Ti2Ni and decreases to 56.31 for 35% Ti2Ni content. Even nontrivial designs with 50% of circular or square transforming NiTi domains display high COP of 40.06 and 29.22, respectively. A high COP is achievable by introducing intermetallics having high modulus (for low input energy), thermal conductivity (for temperature change) and heat capacity (for the output energy).