Structural characterisation of melt-spun Ti₅₀Ni₂₅Cu₂₅ ribbons

Abstract

Abstract. The structure of melt-spun Ti5 0 Ni 25 Cu 2 5 ribbons after various heat treatments has been studied by X-ray and TEM analyses. As-received amorphous Ti 50 Ni 2 5Cu 2 5 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 Ti5"Ni 25 Cu25 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 Ti 50 Ni 25 Cu 25 ribbons at 500"C causes TiCu precipitation and results in appearance of untransformed B2 grains at room temperature. Annealing at 700"C causes Ti,Cu 4 precipitation with significant Cu depletion of the matrix resulting in the appearance of the mixed orthorhombic and monoclinic martensite.

Physical metallurgy of Ti–Ni-based shape memory alloys

“…They found that most of the ribbons are amorphous with a small amount of spherical crystalline particles of /3 lm to /22 lm, which consist of either (0 1 1) o and (1 1 1) o twins of B19 martensite. Thereafter the research on Ti 50 Ni 25 Cu 25 melt-spun ribbons became quite active [401][402][403][404][405][406][407][408][409][410][411]. Rösner et al [401,402] found plate-like precipitates by heat-treating the ribbons at 410°C for 22-48 h, which is 45°C lower than the crystallization temperature of 455°C, as shown in Fig.…”

Origin of stabilization of macrotwin boundaries in martensites

“…6, enlargement of the circled region on Fig. 4(c) is shown together with a HRTEM image of a macrotwin boundary in NiAl [17] for a convincing comparison between the nonlinear modeling and the experimental observation. We indeed observe that the tapering and bending of alternating variants is well reproduced by the nonlinear model.…”

“…We want here to investigate the dynamics and morphology of macrotwins, such as those observed in Ni 65 Al 35 [15,16,17], a shape-memory alloy which undergoes a cubic to tetragonal martensitic transition upon cooling (see Fig. 1).…”

“…the angle θ is given by tan θ = |α−β| α+β . In the case of Ni 65 Al 35 [16,17], the entries of the transformation strain have been estimated to α 0.93 and β 1.15, which leads to θ ∼ 6 • . Another possibility for the same variants I and II is to accommodate along twin walls originating from former (110) planes of the austenite, as in ones involved in the first plate.…”