High-strength superelastic as-cast Ni50.9Ti49.1-TiB2 in-situ composites

Highlights: • The as-cast Ni_50.9Ti_49.1−TiB₂ in-situ composites were prepared in the VIM process. • The B content has a great influence on the morphology of TiB₂ in the NiTi composites. • Adding a small amount of boron can significantly strengthen NiTi. • Adding 1 at.% B has little effect on the superelasticity and shape memory effect. In this study, as-cast Ni_50.9Ti_49.1−TiB₂ in-situ composites were prepared by adding boron in the vacuum induction melting (VIM) process. With boron content of 1 at.%, TiB₂ phases appear in the form of flakes at grain boundaries of the NiTi alloy matrix. These TiB₂ flakes are two dimensional in geometry, featured with an ultrafine thickness less than 200 nm and two other dimensions up to ~20–60 μm. Increasing boron content to 2.4 at.% alters the morphology of TiB₂ from discrete flakes to agglomerative clusters. This morphological dissimilarity has noticeable impact on resulting mechanical properties, superelasticity and shape memory effect. Overall, the NiTi-1at%B composite outperforms both the monolithic Ni_50.9Ti_49.1 alloy and the NiTi-2.4 at%B composite. More specifically, it exhibits complete superelasticity for pre-strains below 4% and up to 4.3% recoverable strain for 6% pre-strain, which is comparable to the pure as-cast Ni_50.9Ti_49.1 alloy. Meanwhile, the corresponding strength under 6% pre-strain reaches 1800 MPa, 20% higher than that of Ni_50.9Ti_49.1. In compression, the yield strength is 65% higher than that of pure Ni_50.9Ti_49.1. Moreover, a shape memory recovery ratio of 45.5%, slightly below 48.6% of the monolithic NiTi, was also achieved in NiTi-1at%B. This indicates that the addition of 1 at.% B can effectively improve the strength of NiTi while maintaining its superelasticity and shape memory effect.