Zr <sub>50</sub>CU <sub>25</sub>NI <sub>7.5</sub>CO <sub>17.5</sub> High-Temperature Shape Memory Alloy with Excellent Thermal Stability and Large Recovery Strain, and the Associated Microstructural Deformation Mechanism

Abstract

High-temperature shape memory alloys are receiving increasing interest. However, their use is currently limited due to the high cost of raw materials, and poor thermal stability and shape memory effect of low-cost alternatives. The thermal stability and shape memory effect must both be improved while maintaining low material cost to achieve the commercialization of such materials. In this study, we produced a novel Zr50Cu25Ni7.5Co17.5 high-temperature shape memory alloy with the aim of achieving high thermal stability and a good shape memory effect. Differential scanning calorimetry and compression cycle were utilized to determine the phase transformation temperature and shape memory effect of the alloy, and transmission electron microscopy (TEM) was employed to investigate the microstructure and clarify corresponding deformation mechanism. We found that the Zr50Cu25Ni7.5Co17.5 alloy exhibited excellent thermal stability and achieved the best maximum recovery strain of 6.87% (8% pre-strain) observed to date. Thus, it has great potential. The TEM results indicated that, of the numerous contraction twins that formed, a small number of detwinning (001) compound twins provided the recovery strain during the primary stage. As the stress increased, increasing numbers of nanoscale (021) and (111) type-I deformation twins formed, and the shift displacement of the type-B planar defect has been expanded from 3d~(001) to 8d~(001). Furthermore, the newly found (111) type-I deformation twin and the extension of the type-B planar defect that contributes to the large recovery strain in Zr50Cu25Ni7.5Co17.5 alloy was observed.