Structural characterisation of melt-spun Ti50Ni25Cu25 ribbons

Abstract

The structure of melt-spun Ti50Ni25Cu25 ribbons after various heat treatments has been studied by X-ray and TEM analyses. As-received amorphous Ti50Ni25Cu25 ribbons, when annealed at 500°C for a few minutes, crystallise into the B2 structure, which fully transforms into the orthorhombic martensite upon cooling. The X-ray scans of Ti50Ni25Cu25 ribbons annealed for 5 min. at 500°C show the well developed orthorhombic pattern, which was used for refinement of the orthorhombic martensitic structure of TiNi-Cu alloys. The refined structure of the orthorhombic martensite in TiNi-Cu alloys differs from the standard B19 type suggested previously. X-ray investigation reveals a shift of atoms in the (010)ORT layers from the centro-symmetric positions towards the [001]ORT direction. The longer annealing of Ti50Ni25Cu25 ribbons at 500°C causes TiCu precipitation and results in appearance of untransformed B2 grains at room temperature. Annealing at 700°C causes Ti3Cu4 precipitation with significant Cu depletion of the matrix resulting in the appearance of the mixed orthorhombic and monoclinic martensite.