On the influence of heterogeneous precipitation on martensitic transformations in a Ni-rich NiTi shape memory alloy

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Abstract

Recently, a new explanation was given for the evolution of DSC chart features after ageing of Ni-rich NiTi alloys. It was shown that the 2–3–2 evolution of DSC chart features can be fully rationalised on the basis of a microstructural scenario, where metastable Ni-rich Ni4Ti3 particles precipitate on and near grain boundaries in a material, which was solution annealed and subsequently aged. A detailed description of the multiple step martensitic transformation has been given in the literature. However, this was only experimentally validated for one ageing temperature (500 °C). In the present study we show that heterogeneous precipitation is also a dominant microstructural feature when ageing is performed at 400 and 450 °C and that the microstructural scenario described previously also accounts for the phase transformation characteristics after lower temperature ageing. Long-term ageing results in large and widely spaced precipitates which no longer directly affect martensitic transformations.

Introduction

It was recently shown how heterogeneous precipitation of Ni4Ti3 particles near grain boundaries affects martensitic transformations in Ni-rich NiTi shape memory alloys [1], [2]. As the precipitation microstructure evolves with ageing time, the number of peaks observed in a DSC chart recording the martensitic transformations of the alloy can change from 2 to 3 and back to 2 (“2–3–2 transformation behaviour”). The precipitation of Ni-rich particles affects the Ni-concentration of the matrix of the material and it is well known that Ms temperatures strongly increase with decreasing Ni-content. Therefore, an earlier explanation invoked an evolving Ni-concentration profile between growing precipitates [3] to rationalise the changes in DSC chart features with ageing time; this could not be confirmed by subsequent microstructural work [1], [2], where clear evidence was presented for heterogeneous precipitation being responsible for multiple step transformations. However, so far only one ageing condition at 500 °C was considered [1], [2]. In the present study we analyse whether ageing at lower temperatures (further away from the B2 phase field) also results in heterogeneous precipitation and whether an evolution of DSC chart features can also be observed after lower temperature ageing. In the present study we extend our investigations to lower ageing temperatures (400 and 450 °C). The objective of the present work is to find out whether our microstructural explanation for multiple step martensitic transformations after 500 °C ageing [1], [2] also applies after the material was subjected to ageing treatments at lower temperatures.

Section snippets

Experimental

In the present study a binary Ni-rich NiTi alloy with a nominal composition of 50.7 at.% Ni alloy was investigated. It was purchased from Memory Metals, Weil am Rhein in the form of cylindrical rods of 1 m length and 13 mm diameter. The material was solution annealed at 850 °C for 15 min and subsequently water quenched. Then ageing of the solution annealed material states was performed at 400 and 450 °C for 1, 10 and 100 h; the microstructures after ageing were investigated in the transmission

Results and discussion

Fig. 1, Fig. 2 show a DSC chart of the solution annealed and water quenched material and a TEM micrograph of this material, respectively. It can be clearly seen that there are one-step transformations on heating and cooling, Fig. 1. The microstructure consists of single phase matrix material with oxide and carbide inclusions, Fig. 2. The results shown in Fig. 1, Fig. 2 have already been reported previously and we highlight them here to introduce the precipitate free material as a reference

Summary and conclusions

The present study shows that stress free ageing of Ni-rich NiTi alloys between 400 and 500 °C results in heterogeneous precipitation of Ni-rich particles near grain boundaries. There is an evolution of DSC chart features with ageing time. The present results show that the behaviour is even more complex than previously assumed and may well have a 1–2–3–2–1 characteristic (and not just a 2–3–2 evolution as previously suggested [3]). Severe ageing treatments are characterised by large and widely

Acknowledgements

The authors acknowledge funding by the DFG (Deutsche Forschungsgemeinschaft) in the framework of projects A6 and A8 of Sonderforschungsbereich 459 (Research Center on Shape Memory Technology at the Ruhr-University Bochum).

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