Elsevier

Acta Materialia

Volume 115, 15 August 2016, Pages 35-44
Acta Materialia

Full length article
Synchrotron high energy X-ray diffraction study of microstructure evolution of severely cold drawn NiTi wire during annealing

https://doi.org/10.1016/j.actamat.2016.05.039Get rights and content

Abstract

Microstructure evolution of a cold-drawn NiTi shape memory alloy wire was investigated by means of in-situ synchrotron high-energy X-ray diffraction during continuous heating. The cold-drawn wire contained amorphous regions and nano-crystalline domains in its microstructure. Pair distribution function analysis revealed that the amorphous regions underwent structural relaxation via atomic rearrangement when heated above 100 °C. The nano-crystalline domains were found to exhibit a strong cold work induced lattice strain anisotropy along 〈111〉, which coincides with the crystallographic fiber orientation of the domains along the wire axial direction. The lattice strain anisotropy systematically decreased upon heating above 200 °C, implying a structural recovery. Crystallization of the amorphous phase led to a broadening of the angular distribution of 〈111〉 preferential orientations of grains along the axial direction as relative to the original 〈111〉 axial fiber texture of the nanocrystalline domains produced by the severe cold wire drawing deformation.

Introduction

NiTi shape memory alloys (SMAs) are known for their functional properties such as the shape memory effect and pseudoelasticity. Both properties originate from martensitic transformations occurring in these alloys. Being a mechanical lattice distortion process as well as a phase transformation, the martensitic transformation is sensitive to the metallurgical and mechanical conditions of the alloy matrix, thus understanding these materials requires knowing the thermal and mechanical treatment history. NiTi alloys are often subjected to severe cold working during material production to control, or tune, their properties. It is known that NiTi has a high tendency to be amorphized upon cold working [1], [2], [3]. Therefore, a knowledge of the structural evolution of amorphized NiTi upon heating is of fundamental importance for NiTi materials design and processing. In addition, nano-crystalline NiTi SMAs have also attracted much attention in recent years because their transformation behavior and mechanical property characteristics are quite different than the conventional coarse-grained NiTi SMAs [4], [5], [6], [7], [8], [9], [10]. A feasible processing technique for large quantity production of nano-crystalline NiTi SMAs is to anneal amorphous NiTi formed after severe plastic deformation (SPD). Common techniques used for severe plastic deformation include cold rolling [3], high pressure torsion [11] and wire drawing [12].

Annealing upon heating of severely deformed NiTi alloys involves structural relaxation, crystallization and grain growth [13]. Some studies have been conducted to understand the kinetics of crystallization and grain growth, which has a significant influence on microstructure control, transformation behavior and the mechanical properties of NiTi alloys [13], [14], [15]. Peterlechner et al. studied the crystallization kinetics of cold-rolled amorphous NiTi [13]. They found that the nucleation and grain growth kinetics obey the Johnson-Mehl-Avrami (JMA) model, which is typical for a diffusion controlled process. Delville et al. studied the process of grain growth of cold drawn amorphous NiTi wires during pulsed electric current annealing by means of in-situ transmission electron microscopy (TEM) studies. They observed very fast grain growth within a few milliseconds (e.g., up to 1 μm growth within 14–18 ms) and found superelastic properties in partially crystallized microstructures having grain sizes in the range of 25–50 nm [15]. Peterlechner et al. carried out in-situ TEM studies of the grain size evolution in amorphous NiTi produced by high pressure torsion [16]. They found that the NiTi alloy deformed by high pressure torsion contained medium-range-ordered (MRO) domains on the order of 1–3 nm dispersed in a continuous amorphous matrix, and that heterogeneous nano-crystallization occurred during heating via nucleation from these pre-existing MRO domains.

Whereas some knowledge has been accumulated of the crystallization and grain growth of amorphous NiTi upon heating, much less is known about the process of structural relaxation prior to crystallization. This is largely due to the technical difficulty in characterizing the structural features of an amorphous matrix. It has been reported that heating at low temperatures (e.g., 400 °C) increases the Young’s modulus and hardness of NiTi amorphous thin films, and both effects are attributed to the reduction of free volume in the matrix associated with relaxation caused by heating [17]. It has also been reported that the crystallization nucleation rate is increased after a pre-relaxation treatment, leading to a more homogeneous grain size distribution. This observation is attributed to the formation of short-range ordered clusters during structural relaxation, which favor nucleation. However, it is not clear how the short-range ordered clusters are formed by atomic rearrangement [17]. Peterlechner et al. [13] studied the relaxation of cold rolled NiTi containing amorphous and highly deformed crystalline domains by means of modulated DSC measurements. They demonstrated that a commonly observed exothermic event upon continuous heating is associated with the relaxation of the amorphous phase. However DSC measurements cannot probe the physical structure and evolution of the relaxed amorphous state. In general, very little is known of the intrinsic behavior of relaxation or the structure of the relaxed alloy.

In this work, we studied the microstructure evolution, including structural relaxation, recovery, grain growth and texture evolution, of severely cold drawn NiTi wires during annealing by means of in-situ high energy X-ray diffraction and scattering. The experimental data were analyzed by means of the Williamson-Hall technique to elucidate the process of grain size evolution and internal stress relaxation of nanocrystalline domains. Pair distribution function (PDF) measurements were used to characterize the structural relaxation and atomic shuffling of the amorphous phase during heating.

Section snippets

Experimental procedure and methods

A 28 kg NiTi ingot with a nominal composition of Ni50.2Ti49.8 (at. %) was prepared by vacuum induction melting. The ingot was hot forged at 850 °C into a 16 mm diameter rod and further hot drawn at 750 °C to a 2 mm diameter wire. The hot-drawn wire was then cold drawn to 0.5 mm in diameter at room temperature with intermediate anneals at 700 °C. The 0.5 mm wire was subjected to a final cold drawing to 0.23 mm in diameter (corresponding to an area reduction of 79%).

Synchrotron high-energy X-ray

Microstructural and calorimetric features

Fig. 2 shows TEM images of a NiTi wire before and after crystallization. Fig. 2a is a high resolution TEM image of the wire before annealing. The sample contained crystalline nano-domains (indicated by the yellow circles) embedded in a continuous amorphous phase. The volume fraction of the nanocrystalline domains surviving the cold working may be roughly estimated to be <10% based on the TEM observations and synchrotron XRD measurements. The inset shows a selected area electron diffraction

Discussion

It is evident in Fig. 2d that there is a continuous exothermic event starting at Tr = 125 °C upon heating prior to crystallization at Tx = 335 °C. A similar exothermic effect has also been observed for severely cold rolled NiTi, and the effect was attributed to a structural relaxation of the amorphous phase in the matrix [13]. In this study, we discovered that, within this temperature range, active atomic rearrangement occurred. These atomic rearrangement activities caused a volume expansion of

Conclusions

This study analyzed the microstructural evolution process of a cold drawn NiTi wire upon heating by means of in-situ high-energy X-ray diffraction and high resolution transmission electron microscopy. The main findings are summarized as follows:

  • 1.

    The severely cold drawn NiTi wire contains largely an amorphous matrix with a small quantity of nanocrystalline domains surviving the cold work. The nanocrystalline domains have an apparent fiber texture within a narrow range between 〈223〉 and 〈445〉

Acknowledgments

The authors wish to thank Prof. R. Wenk, Prof. L. Lutterotti and Dr. W Kanitpanyacharoen for their help with texture analysis. This work was supported by the National Natural Science Foundation of China (51231008, 11474362 and 51401240), the National 973 program of China (2012CB619403), the Australian Research Council (Grant No. DP140103805), and the Institute for NanoScience, Engineering, and Technology (INSET) of Northern Illinois University. Use of the Advanced Photon Source at Argonne

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