Effect of electroplastic rolling on deformability and oxidation of NiTiNb shape memory alloy

doi:10.1016/j.jmatprotec.2012.08.001

Journal of Materials Processing Technology

Volume 213, Issue 1, January 2013, Pages 30-35

https://doi.org/10.1016/j.jmatprotec.2012.08.001 Get rights and content

Abstract

Ni₄₇Ti₄₄Nb₉ strips were successfully processed by electroplastic rolling at relatively low temperature compared to the traditional hot rolling. The result shows the deformability of Ni₄₇Ti₄₄Nb₉ is improved by electropulse, with the maximum thickness reduction of 24% in a single pass. X-ray spectrum and TEM analysis indicates the main structure of Ni₄₇Ti₄₄Nb₉ after electroplastic rolling is polycrystalline and Vickers hardness test shows electropulse can reduce the work hardening. In addition, electroplastic rolling can improve the surface quality of Ni₄₇Ti₄₄Nb₉ owing to the short heat treatment time and low heat treatment temperature.

Introduction

In all the shape memory alloys, Ni₄₇Ti₄₄Nb₉ (numbers indicate at.%) alloy is famous for its excellent property of the wide transformation temperature hysteresis, which makes it widely used in coupling the pipes. Yan et al. (2010) presented that hot working was usually employed for deformation of NiTi alloys since they are difficult to be processed at room temperature. Ni₄₇Ti₄₄Nb₉ alloy is not the exception and often rolled at about 850 °C. Tao et al. (2005) also found that much oxygen is absorbed into the Ni₄₇Ti₄₄Nb₉ alloy in the processing, which leads to the oxidation of surface and produces oxide impurities due to the high temperature during rolling and the activation of Ti element. As a result, even though it is careful to control the processing condition in production, the successful rate of qualified products is still low. Moreover, a lot of time, energy and efforts are spent in heat treatment in the traditional rolling of NiTi alloy process.

Recently, Guan et al. (2010) proved that electropulse with the duration of less than 100 μs and high power density of 10³–10⁵ A/cm² can be used to reduce the deformation resistance. The effect of electropulse on the flow stress during processing is called electroplasticity (EP). EP is observed in many alloys, such as Fe, Zn and W alloys. Conrad (2000) explained that the interaction of high velocity electron drift and dislocation could activate more dislocations for movement, thereby resulting in the improvement of plasticity. By employing EP, metals can have good plasticity even though the temperature of metals is low.

The rolling with the high energy density electropulse conducted to the deformation zone in the process is called electroplastic rolling (EPR). It is a novel method to manufacture thin strips. EPR has been successfully used to roll AZ31 strip at temperature of 200 °C with the thickness reduction of 23% done by Xu et al. (2007). The dynamic recrystallization phenomenon occurred in Mg alloy during EPR leads to the sharp drop of the rolling separation force compared to the rolling without electropulse. In this research, EPR is used in processing of Ni₄₇Ti₄₄Nb₉ to improve its plasticity and reduce the oxidation in processing.

Section snippets

Experiments

The Ni₄₇Ti₄₄Nb₉ (hereafter annotated as NiTiNb) alloy, which has a composition of 47 at.% Ni, 44 at.% Ti and 9 at.% Nb, was produced by an induction vacuums melting furnace, and then was hot-forged and hot-rolled to the plates with the thickness of 1.89 mm at 850 °C. The strips with the width of 5 mm, the thickness of 1.89 mm and the length of 1500 mm for EPR processing, were made by spark cutting. The surface of samples was polished by the abrasive papers in order to get good electrical contact

Results

The samples before and after EPR processing are presented in Fig. 2(a) and (b), respectively. Fig. 2 (c) shows the cracked strips after cold rolling with the total thickness reduction of 47% for three passes rolling, which is the same as the first three passes of EPR. TiNi alloy is an intermediate alloy, which is very brittle and hard to deform at low temperature.

In the cold rolling, the dislocation sliding is difficult for NiTiNb alloy, therefore, the inner stress is produced and cracks

Effect of electropulse on the oxidation reduction of NiTiNb alloy in EPR process

Electropulse has a famous application in powders sintering, which uses electropulse to heat powder during pressing. Due to the short heat time, it is beneficial to gaining fine crystalline material, improving efficiency and saving energy, which was reported by Biswas et al. (2007). In EPR processing of NiTiNb strips, electropulses directly contact the rolled metals, so the metals only needs 9.6 s to be heated to the destined temperature. Moreover, the temperature between two electrodes is

Conclusions

NiTiNb strip was successfully rolled from 1.89 to 0.5 mm by EPR at a relatively low temperature compared to the traditional hot rolling. The cold rolling with the total thickness reduction of 47% for three passes makes NiTiNb strip cracked, while in EPR the deformability rate can reach 74%. Meanwhile, it only takes 9.6 s to heat the sample to the required temperature and only a small segment of strips have the temperature be more than 500 °C for 2–3 s. Consequently, good surface can be obtained by

Acknowledgement

The work is supported by National Natural Science Foundation of China (No. 50571048).

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Effect of electroplastic rolling on deformability and oxidation of NiTiNb shape memory alloy

Abstract

Introduction

Section snippets

Experiments

Results

Effect of electropulse on the oxidation reduction of NiTiNb alloy in EPR process

Conclusions

Acknowledgement

Materials Science and Engineering A

Acta Materialia

Materials Science and Engineering

Acta Metallurgica

Scripta Materialia

Journal of Materials Processing Technology