Bulk NiTiCuCo shape memory alloys with ultra-high thermal and superelastic cyclic stability

Shape memory alloys sustaining stable functional properties for millions of cycles have been reported in NiTiCuCo thin films. The mechanism behind such ultralow functional fatigue was proposed to originate from the epitaxy strains surrounding coherent Ti2Cu precipitates. Despite having a well-understood microstructural origin, such fatigue-resistant property has never been achieved in bulk NiTiCuCo counterparts. In this study, we first show that the semi-coherent Ti2Cu precipitates help satisfy closely the compatibility criteria under thermal phase transformation. Assisted by aging at 600 degrees C for 1 h, an unprecedented thermal stability is reported in bulk coarse-grained Ti54Ni3.7Cu123Co2, where the transformation temperatures migrate by similar to 0.1 degrees C for 200 cycles. To achieve stable superelastic response, a thermomechanical processing route is proposed that results in a uniform nanocrystalline microstructure with embedded Ti2Cu precipitates. Such a microstructure exhibits much improved superelastic cyclic stability as evidenced by similar to 22 MPa shakedown of plateau stress for 200 superelastic cycles. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.