Surface mechanical attrition treatment induced phase transformation behavior in NiTi shape memory alloy
Introduction
The near equiatomic NiTi shape memory alloys have attracted more attention over the past two decades due to their superior properties, e.g. shape memory effect, superelasticity, high damping capacity, excellent biocompatibility and fatigue resistance etc. These properties enable the successful applications of NiTi SMAs in various engineering practices [1], [2], [3], [4]. Almost all applications of NiTi SMAs involve transformation-related deformation via either martensite reorientation or stress-induced martensite transformation and shape recovery of deformed martensite [5], [6]. Therefore, the transformation behaviors of the materials under various deformation conditions are of great concern. To predict the transformation behavior of NiTi SMAs as a function of various deformation factors, namely temperature [7], loading mode [8], [9], strain [10] and grain size [11], [12], much work has been conducted in the past few years. Lin and co-workers [8], [13] investigated the effects of cold-rolling on the transformation behavior of equiatomic NiTi SMAs and found the parent B2 phase and stabilization effects on thermoelastic martensite can be induced by cold-rolling. Liu [5], [14], [15] studied the deformation of NiTi SMAs via martensite reorientation (MR) and considered that the internal elastic energy stored in the reoriented martensite may impede the reverse M → A transformation giving rise to stabilization effects on martensite. More recently, the transformation behavior of severely deformed NiTi SMAs has been studied [11]. It is observed that heavily deformed B2 and B19′ accompanied with martensite stabilization co-exist due to the formation of the twin-related nanograins by severe plastic deformation. However, the experimental work conducted so far seldom analyzed the transformation behavior of NiTi SMAs deformed under high strain rates. In some cases of application, e.g. nanocrystallization [16], NiTi SMAs are subjected to severe plastic deformation at high strain rate. If we are able to predict the transformation points and phases constituent, we will have a good chance to predict the subsequent shape recovery process accurately. Thus, to understand the transformation behavior of NiTi SMAs deformed at high strain rates is a pre-requisite.
The surface mechanical attrition treatment (SMAT) is an effective technique to generate severe plastic deformation in surface region of the treated sample at high strain rate [17]. This technique has been used to enhance the mechanical properties and generate nanocrystalline structures in metals [18], [19], [20]. In SMAT, a large number of metallic balls with 1–10 mm in diameter are resonated ultrasonically. The vibrating balls impact the sample surface from random directions over a short time. Each impact may result in plastic deformation with high strain rates estimated to be as high as 102–103 s−1 in the top surface layer and decreasing to zero in the bulk. Consequently, the repeated multidirectional impacts give rise to severe plastic deformation with gradient variation in the stress and strain from the surface to the substrate. In the present work, we report for the first time the deformation of NiTi SMAs by SMAT. The phase constituents and transformation behavior of the treated NiTi SMAs are investigated.
Section snippets
Experimental
A Ti–50.7 at.% Ni plate with a thickness of 3 mm was used in the present work. Samples were hot rolled and annealed at 973 K for 30 min, followed by water quenching to room temperature. The transformation temperatures of the as-received specimens were determined by DSC to be As = 339 K, Af = 360 K, Ms = 329 K and Mf = 309 K, respectively. Prior to SMAT, the surface was polished with silicon carbide papers. The detailed SMAT setup and procedures can be found in Ref. [17]. During SMAT, the stainless steel balls
Results and discussion
Fig. 1 shows the surface XRD spectra of the NiTi specimens after undergoing SMAT for different periods of time. The typical XRD spectrum of the as-received NiTi specimen shows the main phase of B19′ in addition to a small amount of precipitates identified as Ti2Ni, Ni3Ti and Ni4Ti3 phases. After SMAT for 5 min, the intensity of the B19′ peak diminishes, whereas the B2 (1 1 0) diffraction peak corresponding to austenite emerges obviously. With further deformation by SMAT, the B2 (1 1 0) and B2 (2 1 1)
Conclusions
SMAT results in the formation of the parent B2 phase from martensite B19′ in the surface layer of NiTi alloy. The amount of B2 phase decreases with depth and a graded phase structure layer is observed. SMAT also induces the stabilization effect on martensite in the sub-surface layer up to 300 μm deep in the NiTi alloy but no martensite stabilization effect can be observed in the bulk.
Acknowledgement
Financial support by Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No. CityU 112307 is acknowledged.
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