Effects of ageing on microstructure, martensitic transformation and shape memory performance of a Ni-rich Ti49.5Ni34.5Cu11.5Pd4.5 alloy
Introduction
For decades, shape memory alloys (SMAs) have been used as a class of actuators with larger power density, in comparison with other active materials [1,2]. Because of the large output displacement and force, Ti–Ni alloys become to the most significant SMAs for the purpose of practice [[3], [4], [5], [6], [7]]. However, the transformation hysteresis is large in the martensitic transformation of Ti–Ni alloys with a value of 20–40 °C, which blocks the functionality as actuators. In addition, through 100 thermal cycles, there will be a significant reduction of the martensitic transformation temperature (about 30 °C) [8,9]. Recent works have shown that the transformation hysteresis is associated with the degree of compatibility between the martensite and austenite phases [[10], [11], [12], [13]]. A geometric non-linear theory of martensite (GNLTM) has been built to find the way to improve compatibility in many alloy systems [14,15]. Based on the GNLTM, Ti–Ni–Cu–Pd alloys with near-zero hysteresis have been designed recently [[16], [17], [18]]. According to the preliminary works of our group, the transformation temperature reduction of Ti–Ni–Cu–Pd alloys with near-zero thermal hysteresis is merely less-than-1 oC through 5000 thermal cyclings [19]. Moreover, through underload thermal cycles, it also manifests excellent functional stabilities [20].
Despite the remarkable performances, the shape memory effect (SME) of the solution-treated Ti–Ni–Cu–Pd alloy with near-zero thermal hysteresis is small, which shows a fully recoverable strain of less than 3% [20]. Studies have manifested that the precipitates induced by ageing can strengthen the shape memory performance. In addition, the ageing effects on Au–Cd, Cu–Al–Ni and Ti–Ni SMAs have been investigated [[21], [22], [23], [24], [25], [26], [27], [28]]. Whereas, the relevant studies about the ageing effects on the Ni-rich Ti–Ni–Cu–Pd alloys with near-zero thermal hysteresis are few. In the present paper, we systematically and comprehensively study ageing effects on the microstructure, martensitic transformation and shape memory performance of a Ni-rich Ti49.5Ni34.5Cu11.5Pd4.5 alloy with near-zero thermal hysteresis as a function of temperature to develop high quality SMAs with both excellent SME and near-zero thermal hysteresis.
Section snippets
Experimental
The fabrication of the Ni-rich Ti49.5Ni34.5Cu11.5Pd4.5 alloys was to melt ingot using an Ar-arc furnace with 99.97% Ni, 99.9% Ti, 99.95% Pd, 99.99% Cu. Subsequently the ingot was homogenized at 950 °C for 7.2 ks. Finally, specimens were treated with ageing heat-treatment at a series of temperatures between 400 and 650 °C for 3.6 ks. Philips-CM12-type TEM was used to observe the microstructures, with a high tension of 120 kV. The voltage and current used in the electrolytic double injection were
Results and discussions
The microstructural evolution of the specimens aged at 400, 500, 600 and 650 °C for 1 h are presented in Fig. 1. When ageing at 400 °C for 1 h, small precipitated phase particles about 2 nm in length appeared in the matrix as shown in Fig. 1a. Ageing at 500 °C for 1 h gives rise to the formation of massive fine precipitated phases with lamellar shape about 30–50 nm in length, showing significant increase in the precipitate number and dimension (see Fig. 1b). When the electron beam comes in from
Conclusions
In conclusion, TEM, DSC and tensile tests were used to investigate the ageing effects on the microstructure, martensitic transformation and shape memory performance of a Ni-rich Ti49.5Ni34.5Cu11.5Pd4.5 alloy with near-zero thermal hysteresis as a function of temperature. The ageing treatment causes the formation of (Ni,Cu)2Ti-type precipitates at the aged Ni-rich Ti49.5Ni34.5Cu11.5Pd4.5 alloys. By increasing the ageing temperature, the recoverable strains show an increase first followed by a
CRediT authorship contribution statement
Hang Li: Conceptualization, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Xianglong Meng: Conceptualization, Resources, Writing - review & editing. Wei Cai: Resources, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The work was supported by 973 projects of China (2012CB619400 and 2011CB012904), Natural Science Foundation of China (No. 51322102 and No. 51171052) and the Fundamental Research Funds for the Central Universities (HIT. BRET 201201).
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