Effect of SHS conditions on microstructure of NiTi shape memory alloy

Highlights

• Conditions of SHS for preparation of low-porosity NiTi were determined.

• Rapid heating and use of coarse particles can be recommended.

• Product contains NiTi and Ti₂Ni phases.

Abstract

This work aims to optimize the parameters of the SHS process for the preparation of NiTi shape memory alloy to obtain a high-purity, low-porosity material. For this purpose, high heating rate (over 300 °C min⁻¹), temperature of 1100 °C and process duration of 20 min were determined as the optimal parameters. To prevent nickel depletion of the product and to minimize the porosity, the use of coarse titanium particles (200–600 μm) with fresh surface can be recommended. SHS products were composed of NiTi with the admixture of Ti₂Ni phase. The formation of this phase cannot be fully avoided in SHS preparation route.

Introduction

Approximately equimolar Ni–Ti alloy is the most widely known shape memory alloy. Shape memory effect in this alloy is connected with the transformation between austenite (high-temperature modification having B2 cubic structure) and low-temperature monoclinic martensite [1]. For the practical application of these alloys, superelasticity is very important. This phenomenon occurs when the NiTi alloy is deformed slightly above the martensite → austenite transformation temperature. Deformation induces the formation of the martensite phase, which is continuously transformed to austenite during unloading. Due to this phenomenon, this alloy behaves like enormously elastic material [1].

The transformation temperatures in NiTi alloy depend greatly on the content of nickel as well as on the amount of impurities. The impurities in NiTi alloy can be also present in the form of inclusions, such as TiN of TiC, that arise during the metallurgical processing [2], [3].

The most commonly applied techniques in industrial production of nitinol alloy are melting metallurgy processes – vacuum induction melting (VIM) [3], [4] and vacuum arc re-melting (VAR) [5]. In VIM of Ti-containing alloys, there is a serious danger of a strong contamination of the melt due to high reactivity of molten titanium. Therefore, special zirconia (ZrO₂) or yttria (Y₂O₃) bulk or coated crucibles have to be applied [6]. However, even these materials contaminate the molten NiTi alloy partially, causing the presence of oxide inclusions. The VAR technique enables to prepare alloys with higher purity, but the problem is homogeneity. To obtain sufficiently homogenous product, the VAR process has to be repeated even more than 4 times [4], [5].

A promising alternative to these melting metallurgy production routes is powder metallurgy (PM). However, the application of conventional PM processes using NiTi alloy powders is complicated due to their poor compressibility and sinterability [4]. As the compaction methods, advanced consolidation techniques such as hot isostatic pressing or spark plasma sintering are required [4]. Alloyed powders prepared by the methods involving melting (e.g. melt atomization) can be also contaminated by the same elements as the melting metallurgy product.

A simple alternative production technology is reactive sintering. In general, reactive sintering is a densification process, where initial components in powder form are transformed to a compact product via thermally-activated chemical reactions [7]. Since the intermetallics-forming reactions are strongly exothermic, the heat evolved by the reaction sustains and propagates the reaction through the reaction mixture. Therefore the process is called SHS (Self-propagating High-temperature Synthesis) [7], [8], [9]. The route from powders to the compact usually concerns powder blending, cold pressing and sintering [7], [10]. In some processes, cold pressing and reactive sintering are replaced by reactive hot isostatic pressing [4]. The energy for the activation of chemical reactions during reactive sintering is supplied by heating in vacuum or protection-atmosphere furnace [7], [8], [9] or by electric discharge [11]. When pure metallic powders are applied and the process is carried out in vacuum or protective atmosphere to prevent oxidation, this technique enables to yield high-purity materials. In addition to high purity, controlled porosity can be also achieved, when NiTi is produced by this process [12], [13]. The porosity in SHS process was found to be formed by several phenomena: thermal migration, volume change [14] due to the crystal lattice re-ordering [14], [15], Kirkendall effect [15], gas porosity [7] or residual porosity after pressing [16]. On the contrary to the papers dealing with porous NiTi, this work aims to develop the SHS production route for low-porosity high-purity NiTi alloy.