Influence of annealing on NiTi shape memory alloy subjected to severe plastic deformation

doi:10.1016/j.intermet.2012.07.025

Intermetallics

Volume 32, January 2013, Pages 344-351

https://doi.org/10.1016/j.intermet.2012.07.025 Get rights and content

Abstract

The amorphous phase with the retained nanocrystalline phase, the deformation bands and the amorphous bands coexist due to inhomogeneous plastic deformation in nickel–titanium shape memory alloy (NiTi SMA) subjected to severe plastic deformation (SPD) based on local canning compression. Transmission electron microscopy, X-ray diffraction and differential scanning calorimetry are used to investigate microstructural evolution and phase transformation of NiTi sample subjected to SPD in the case of annealing for 2 h at 300 °C, 450 °C and 600 °C, respectively. Annealing at 300 °C and 450 °C leads to nanocrystallization of amorphous NiTi sample, while annealing at 600 °C results in the coarse-grained NiTi sample, where the (001) martensite compound twins are found. The precipitation phases such as Ni₄Ti₃ and Ni₃Ti are suppressed in NiTi sample formed as a result of crystallization of the amorphous structure and thus occur more easily in the deformation bands in the case of annealing. Martensitic phase transformation is influenced by the grain size and is suppressed with the decrease in the nanocrystalline grain size.

Highlights

► Local canning compression leads to severe plastic deformation of NiTi sample. ► Deformation bands and amorphous bands are observed in NiTi sample subjected to SPD. ► Martensitic transformation is suppressed with decreasing nanocrystalline grain size. ► The precipitates are suppressed due to crystallization of amorphous NiTi phase. ► The precipitates occur more easily in the deformation bands during annealing.

Introduction

Nickel–titanium shape memory alloy (NiTi SMA) is widely used in engineering fields due to its shape-memory effect as well as superelasticity [1]. Cold working plays an important role in engineering application of NiTi SMA and has a significant influence on shape-memory effect as well as superelasticity of NiTi SMA. In general, cold plastic deformation leads to a high density of dislocations in NiTi SMA, which makes a contribution to superelasticity of NiTi SMA [2], [3], [4]. However, severe plastic deformation (SPD) based on cold working is able to lead to nanocrystallization or amorphization of NiTi SMA. So far, plenty of work with respect to SPD of NiTi SMA in the case of cold working has been done over the last decade by means of high-pressure torsion (HPT) [5], [6], cold rolling [7], [8], cold drawing [9], [10] and surface mechanical attrition treatment (SMAT) [11], [12]. SPD of NiTi SMA followed by subsequently appropriate heat treatment contributes to improving the mechanical properties and the functional properties of NiTi SMA, such as high ultimate strength and high elongation at the elevated temperatures [13] and perfect superelasticity [14], [15], which is attributed to nanocrystallization of amorphous NiTi SMA prepared by SPD in the case of annealing. Many researchers investigated crystallization mechanism of amorphous NiTi SMA subjected to SPD and phase transformation behaviour of NiTi SMA after crystallization. Waitz et al. [16], [17] induced nanocrystallization of an amorphous NiTi SMA based on HPT in the case of annealing and found that martensitic phase transformation is suppressed with decreasing the nanocrystalline grain size and phase transformation behaviour is closely related to the nanocrystalline grain size. Peterlechner et al. [18] studied relaxation and crystallization kinetics of amorphous NiTi SMA made by repeated cold rolling and revealed that nucleation and growth of the crystalline phase is based on mixed nucleation or nucleation with a decreasing rate as well as three-dimensional growth. Srivastava et al. [19] investigated the influence of the different annealing temperatures on crystallization of amorphous bands of cold-rolled NiTi SMA and demonstrated that annealing for 30 min at or above 350 °C leads to crystallization of the amorphous regions and the grain size increases with the increase in the annealing temperature.

In the present study, local canning compression was used in order to lead to SPD of NiTi SMA, and subsequently the influence of annealing on microstructural evolution, precipitation and phase transformation of NiTi samples subjected to SPD was investigated systematically.

Section snippets

Materials and methods

As-received NiTi alloy with a nominal composition of Ni_50.9Ti_49.1 (at.%) was prepared by means of vacuum induction melting method, and then was rolled at 800 °C and finally was drawn to NiTi bar with the diameter of 12 mm. The transformation temperatures of NiTi bar are as follows: M_s = −27.2 °C, M_f = −41.7 °C, A_s = −17.3 °C, A_f = −4.1 °C. Therefore, microstructure of as-received NiTi bar possesses a typical B2 austenite structure at room temperature. NiTi samples with the diameter of 4 mm and the height of 6

NiTi sample subjected to SPD under local canning compression

Fig. 1 demonstrates the schematic diagram of deformation mode of NiTi sample under local canning compression and the corresponding TEM photographs taken from the different deformation zones of NiTi sample subjected to SPD. Fig. 1(a) shows the schematic diagram of deformation mode of NiTi sample subjected to SPD based on local canning compression. It can be seen from Fig. 1(a) that NiTi sample is caused to be in a three-dimensional compressive stress state during plastic deformation. In order to

Discussion

Inhomogeneous plastic deformation is unavoidable during SPD of NiTi alloy under local canning compression at room temperature. An almost complete amorphous matrix with a small amount of nanocrystalline phase occurs in the principal deformation zone of NiTi sample. In the plastic deformation zones other than the principal deformation zone, there exist a lot of deformation bands in which there are plenty of crystal defects, such as dislocations, stacking faults, and twins. At the low annealing

Conclusions

(1)
SPD of NiTi sample based on local canning compression leads to the complicated microstructural morphologies, including deformation bands, amorphous bands and amorphous phase in which the retained nanocrystalline phase is embedded in the amorphous matrix.
(2)
Annealing at 300 °C and 450 °C leads to nanocrystallization of amorphous NiTi sample, but is unable to result in complete elimination of deformation bands and amorphous bands. Annealing at 600 °C results in complete crystallization of NiTi sample

Acknowledgements

The work was financially supported by National Natural Science Foundation of China (No. 51071056) and the Fundamental Research Funds for the Central Universities of China (No. HEUCFR1132 and No. HEUCF121712).

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Influence of annealing on NiTi shape memory alloy subjected to severe plastic deformation

Abstract

Highlights

Introduction

Section snippets

Materials and methods

NiTi sample subjected to SPD under local canning compression

Discussion

Conclusions

Acknowledgements

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