Microstructure and mechanical properties of an austenite NiTi shape memory alloy treated with laser induced shock
Introduction
Shape memory alloys (SMAs) are one of the most popular active materials that derive their unique properties such as shape memory and superelastic effects from a thermoelastic martensitic transformation [1]. They have been increasingly used as candidate materials for medical devices and microelectromechanical systems (MEMS). The wear resistance and fatigue performance are two of major concerns in those applications [2]. For conventional engineering alloys such as stainless steels and titanium alloys, several surface treatment techniques such as shot peening have been widely used to improve those two properties in metallic components of bulk materials [3], [4], [5]. For SMAs, besides the improvement of wear and fatigue performance, the shape memory characteristics of the materials often require being preserved in their applications. Moreover, for small-scale materials and complex structural geometry used in medical devices and microsystems, processing techniques with precise control of localized treatment are needed. Recently developed laser shock peening (LSP) technique seems to have advantages over those issues [6], [7]. In LSP process, a shock wave is generated and propagates into the target through the interaction of a pulsed high-intensity laser beam and absorption layer on the metallic target surface. If the amplitude of the shock wave exceeds the Hugoniot elastic limit (HEL) of the target material, plastic deformation occurs and residual compressive stresses are induced near the material surface, resulting in the enhancement of fatigue life. Since the beam size could be easily adjusted, localized micro-scale peening technique has been developed and applied on the metal films in semiconductor industry [8], [9]. In addition, LSP is suitable in processing materials with complex structural geometry by manipulating laser beam and utilizing liquid confining media like water. Therefore, LSP has great potential to treat SMAs and modify their surface properties for practical applications. Most recently, Liao et al. [10] used laser induced shock to generate residual deformation induced martensite (DIM) in a NiTi alloy. However, the thermo-mechanical properties of NiTi alloys after the LSP process and their relationships with the microstructure evolution have not been reported yet. Moreover, in a LSP process, the peak pressure of laser induced shock is at the level of several GPa, and the shock duration is in the nanosecond scale. The strain rate achieved on the target surface can be as high as 105–107/s. The existing literature on high strain rate behavior of SMAs is not extensive and has controversy. Using the Hopkinson pressure bar technique, Nemat-Nasset et al. [11] reported the existence of a critical strain rate (around 104/s) above which the austenite phase deforms by direct dislocation-induced plastic slip instead of transforming to martensite. The ultra-high strain rate response of SMAs needs more experimental investigation to clarify this issue.
In this study, an austenite NiTi alloy was processed by LSP with various laser power densities. The effects of laser power density on residual stress, microhardness, and superelastic stress–strain behavior were investigated. The microstructure is characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM).
Section snippets
Sample preparation
Commercially available NiTi polycrystalline sheets were purchased from GEE Shape Memory Alloy Inc. (Beijing, China). The nominal alloy composition was Ni-50.9% and Ti-49.1% (at%). The size of the grain is about 30 μm as observed by optical microscopy (see Fig. 1a). As shown in Fig. 1b, the microstructure examination with transmission electron microscopy (TEM) shows the NiTi specimen consists of austenite NiTi phase with Ni4Ti3 precipitates. The transformation temperatures determined by
Results and discussion
In Fig. 4, Vickers indentation tests on the cross-section of peened sample shows that the measured hardness increases in the surface layer, and then decreases to the value of untreated specimens at a depth about 250–300 μm, which indicates that the thickness of the LSP affected layer is about 250–300 μm for the laser parameters in our study. The surface hardness of untreated specimen is 277±5 kg mm−2. The surface hardness of peened specimen are 304±9, 304±9 and 303±7 kg mm−2 for laser power density
Conclusion
In this study, microstructure and mechanical properties of austenite NiTi alloy treated with laser induced shock was investigated. It was found that the thickness of laser affected layer is about 250–300 μm. The hardness of the specimen is increased by approximately 10% after LSP. Laser induced shock introduces slightly residual compressive stress in the peened specimen. The ultrahigh-strain-rate plastic deformation by LSP results in dislocation substructure and amorphization underneath the
Acknowledgments
The authors would like to thank the National Natural Science Foundation of China (Grant nos. 11002150 and 10972228) and the Basic Research Equipment Project of Chinese Academy of Sciences (YZ200930) for financial support.
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