The effect of annealing temperature on the martensitic transformation of a Ti49.2Ni50.8 alloy processed by equal channel angular pressing (ECAP) was investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The as-ECAP processed and subsequently annealed Ti49.2Ni50.8 alloys consist of B2 parent phase, Ti4Ni2O phase and B19′ martensite at room temperature. Upon cooling, all samples show B2→R→B19′ two-stage transformation. Upon heating, when the annealing temperature is less than 400 °C, the samples show B19′→R→B2 two-stage transformation; when the annealing temperature is higher than 500 °C, the samples show B19′→B2 single-stage transformation. The B2–R transformation is characterized by wide interval due to the dislocations introduced during ECAP.

•Addition of Nb leads to the presence of β-Nb phase.•Transformation hysteresis of (Ti50Ni40Cu10)100−xNbx alloys increases as the Nb content increases.•Addition of Nb is effective in improving mechanical properties and shape recovery.In present work, microstructure, martensitic transformation behavior and mechanical properties of (Ti50Ni40Cu10)100−xNbx (x = 0, 5, 10, 15 at.%) alloys were investigated as a function of Nb content. The addition of Nb to TiNiCu alloy leads to the presence of β-Nb phase. During cooling and heating, the alloys show one-step B2 ↔ B19 transformation. As the Nb content increases, the transformation temperatures almost linearly decrease and the transformation hysteresis monotonously increases due to the decrease of middle eigenvalue of the phase transformation matrix. The addition of Nb is effective in improving the elongation because of the introduction of β-Nb phase. With the increase of Nb content, both the yield strength and the critical stress to induce martensitic transformation increase, resulting in the improved superelastic strain.

•Microstructure of Ti44Ni47−xNb9Bx alloys depend on B content.•With increasing B content, transformation temperature increase.•The optimized mechanical properties can be obtained by the proper addition of B.In present work, microstructure, martensitic transformation and mechanical properties of Ti44Ni47−xNb9Bx (x = 0, 0.5, 1, 5 at.%) alloys were investigated as a function of B content. The results show that the addition of B significantly influences the microstructure of the alloys. The microstructure of Ti44Ni47Nb9 alloy consists of B2 parent phase matrix and β-Nb phase. When the B content is 0.5 at.%, Nb3B2 phase presents. With further increasing B content to above 1 at.%, TiB and NbB phases present instead of Nb3B2 phase. With increasing B content, the transformation temperatures increase due to the reduced Ni/Ti ratio and Nb content in the matrix. The mechanical properties can be optimized by the addition of 1 at.% B.

•Microstructure of the as-ECAP processed TiNiNb alloy is inhomogeneous.•Transformation temperatures are lower than those of the initial material.•Transformation stability and mechanical properties can be considerably improved.Microstructure, martensitic transformation and mechanical properties of an ultrafine-grained Ti44Ni47Nb9 shape memory alloy processed by equal channel angular pressing were investigated. The as-ECAP processed sample is characterized by an inhomogeneous and refined microstructure. In β-Nb phase-rich region, the grains of matrix are elongated with high density dislocations. In β-Nb phase-free region, the microstructure is partial recovery and characterized by near-equiaxed grains. The heterogeneous microstructure is attributed to presence of β-Nb phase. Martensitic transformation behavior of the as-ECAP processed sample is characterized by a single-stage transformation. The thermal cycling stability of transformation and the mechanical properties are considerably improved due to a strengthening effect resulting from refined grain size and high dislocation density.

•The ECAPed sample shows a wider transformation hysteresis than the initial alloy.•Shape memory effect and its cycling stability were obviously improved by ECAP.•ECAP is effective in enhancing the reliability of pipe coupling made of TiNiNb alloy.In present work, transformation hysteresis and shape memory effect of an ultrafine-grained Ti44Ni47Nb9 alloy processed by ECAP were studied. After deformation, the ECAPed sample showed a much wider transformation hysteresis than the initial sample due to the enlarged strength mismatch between matrix and β-Nb phase. The shape memory effect and its cycling stability of the ECAPed sample were obviously improved.

Superelasticity and cycling stability of an ECAP-processed Ti49.2Ni50.8 alloy were investigated as a function of annealing temperature. After annealing at 400 °C for 30 min, Ti3Ni4 precipitates formed to strengthen the matrix and yet the grain size kept constant. Such an annealing favors to optimize the superelasticity.

In this study, effect of aging treatment on microstructure, deformation behavior, and superelasticity of Ti48.8Ni50.8V0.4 alloy was investigated. After aging at 400 °C for 30 min, Ti3Ni4 precipitates formed. With increasing aging temperature from 300 to 450 °C, the yield strength of reoriented martensite increased due to the strengthening effect of Ti3Ni4 phase, thus improved the shape recovery ratio and reduced the stress hysteresis. Further increasing the aging temperature, the size of Ti3Ni4 precipitates increased and the coherency between precipitate and matrix gradually lost, leading to the decreasing yield strength of reoriented martensite and shape recovery ratio. Simultaneously, the stress hysteresis increased resulting from the hinder of plastic deformation to the interfacial movement during phase transformation. The critical stress to induce martensitic transformation continuously decreased with increasing aging temperature.

In the present work, effect of precipitation on two-way shape memory effect and deformation induced martensite stabilization of Ti50Ni25Cu25 ribbon was investigated. The two-way shape memory effect developed as a result of martensite deformation is affected by formation of precipitates. After straining martensite to 5.5%, a two-way shape memory strain of 1.25% was obtained in a sample annealed at 800 °C for 300 s that does not introduce B11 precipitates. Results also show that the deformed Ti50Ni25Cu25 ribbon containing precipitates has multiple-stage transformation characteristics during the first heating.

In present work, the microstructure and martensitic transformation of Ti49Ni51 − xHfx (x = 3–15) alloys were studied. The microstructure of Ti49Ni51 − xHfx alloys consists of B19′ martensite and (Ti,Hf)2Ni phase at room temperature. The martensitic transformation behavior is characterized by a single-stage transformation. With increasing Hf content, the transformation temperature increases from 75 to 279 °C resulting from the reduced valence electron concentration, indicating that the replacement of Hf for Ni is effective in increasing the transformation temperatures. The results suggest that the Ti49Ni51 − xHfx shape memory alloy is one of potential candidates for high temperature applications.

•Transformation sequence of as-ECAPed Ti49.2Ni50.8 alloy depends on initial aging treatment.•As-ECAPed alloy initially aged at 450 °C for 60 min shows three-stage transformation.•Transformation sequence was discussed based on microstructure evolution.Multiple-stage transformation of Ti49.2Ni50.8 alloy processed by equal channel angular pressing (ECAP) was investigated as a function of pass number and aging treatment before ECAP. When the pass number is no more than four passes, three stage transformation, namely A→R, R1→M1 and R2→M2, occurs in the as-ECAP processed alloy initially aged at 450 °C for 60 min. Only the A→R→M forward transformation occurs provided that the aging duration was decreased/increased to 10/600min. The transformation sequence was discussed based on the microstructure evolution of as-ECAP processed alloy with different initial microstructure and pass number.