Effects of Solution and Aging Treatments on Corrosion Resistance of As-cast 60NiTi Alloy
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JMEPEG DOI: 10.1007/s11665-016-2402-z
Effects of Solution and Aging Treatments on Corrosion Resistance of As-cast 60NiTi Alloy Qiuhui Qin, Yuhua Wen, Gaixia Wang, and Lanhui Zhang (Submitted June 20, 2016; in revised form October 6, 2016) 60NiTi alloy has become a competitive candidate for bearing applications due to its shape memory effect, superelasticity, high strength, hardness, excellent abrasion resistance and corrosion resistance, etc. However, the relationship between its corrosion resistance and heat treatment is not clearly understood. Therefore, we used OM, XRD, SEM and EDS to study the evolution of microstructure in as-cast, solutiontreated and aged 60NiTi alloy. Besides, the potentiodynamic polarization and salt spray test were used to compare corrosion resistance of 60NiTi alloy and 316 stainless steel and to study the effect of microstructures on corrosion resistance of 60NiTi alloy. The results show that the corrosion resistance of ascast 60NiTi alloy is comparable to that of 316 stainless steel, but the corrosion resistance of solution-treated and aged 60NiTi alloys is much superior. The significantly reduced Ni3Ti phase after the solution and aging treatments is responsible for the remarkable improvement in the corrosion resistance of as-cast 60NiTi alloy. Keywords
60NiTi alloy, corrosion resistance, microstructure, Ni3Ti phase
1. Introduction NiTi shape memory alloys were developed by Buehler et al. at Naval Ordnance Laboratory (NOL) in the early 1960s, also known as Nitinol, and have found more and more applications in areas of biomedicine, aerospace, navigation and so on. The most commonly used and studied Nitinol is 55NiTi (wt.%), whose atomic ratio is 1:1 (Ref 1). In fact, 60NiTi alloy, whose Ni content is 60 wt.%, was discovered along with 55NiTi, but the studies of 60NiTi were hindered due to its brittleness and difficulty in formation (Ref 2). In the late 1980s, Nitinol Technologies Inc. (NiTech) developed methods to hot roll 60NiTi into plate. Fabrication techniques such as laser cutting and abrasive water jet machining were also developed. In addition, the heat treatment procedure was also developed to significantly reduce its notch sensitivity (Ref 3-5). Recent studies showed that 60NiTi alloy exhibits not only good superelasticity and shape memory effect, but also high tensile strength, excellent abrasion and corrosion resistance (Ref 6-8). Besides, the hardness of 60NiTi alloy ranges from approximately 33 HRC for furnace cooled specimens to 63 HRC for water-quenched specimens, indicating the possibility for readily attaining any desired intermediate hardness through appropriate heat treatments (Ref 9). These excellent properties combination makes it become a competitive candidate for the tribological applications in which some corrosive media exist (Ref 7, 10). In1968, Buehler et al. tested corrosion resistance of 60NiTi alloy in marine environment (Ref 2), but they did not indicate the Qiuhui Qin, Yuhua Wen, Gaixia Wang, and Lanhui Zhang, College of Manufacturing Science and Engineer
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