Constituent phases, microstructures, and martensitic transformation of β-type Zr-Nb-Sn alloys

Abstract

Constituent phases, shape memory and superelastic properties and microstructure in β-type Zr–(6–10.5)Nb–(2–5)Sn (at.%) alloys were investigated. Zr–(8–10.5)Nb–(2–4)Sn alloys exhibited shape memory effect and partial superelastic recovery. Zr–(8–9)Nb–5Sn alloys exhibited distinct superelasticity. The alloys with low Nb content (x < 8 at.%) exhibited an α” phase and a β phase. For the alloys with high Nb content (x ≥ 8 at.%), in addition to the matrix of a β phase, a β′ phase with a body-centered tetragonal structure was observed. Lattice deformation strains were calculated based on the lattice parameters of the β phase and the α” phase. The distribution of the β′ phase showed no significant Nb content dependence, whereas the addition of Sn caused a sparse distribution of the β′ phase. In situ X-ray diffraction measurements under heating and cooling revealed that transformation between the β phase and the β′ phase is reversible. The β′ phase was absent in the deformation-induced α” phase, suggesting that β’→α” phase transformation occurred during the deformation together with β → α” phase transformation. TEM observation revealed that dislocations remained in the deformation-induced α” phase closed to the β′ phase. The presence of the β′ phase was considered to be the cause for a larger stress hysteresis and a smaller superelastic recovery strain in the Zr-based superelastic alloys than those in Ti-based superelastic alloys. The effect of the β’ phase on the superelastic properties of Zr-based alloys was discussed in comparison with Ti-based alloys.