Flow Behaviors and Processing Maps of NiTi Shape Memory Alloy with Microstructural Observations on Austenite Phase durin
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JMEPEG https://doi.org/10.1007/s11665-020-05182-1
Flow Behaviors and Processing Maps of NiTi Shape Memory Alloy with Microstructural Observations on Austenite Phase during Hot Compression Chaojun Li, Jiao Luo, and Miaoquan Li (Submitted May 26, 2020; in revised form August 10, 2020) The flow stress–strain curves, strain rate sensitivity (m) and processing maps of Ni50.8Ti49.2 shape memory alloy were analyzed in the present study. The microstructure of deformed samples at some processing parameters was observed via optical micrograph (OM) and electron backscatter diffraction (EBSD) techniques. The results showed that an early yield reduction phenomenon were observed at the deformation temperatures 800 and 900 °C and strain rates of 1 and 0.1 s21, and it extended at 600 °C as the strain rate was 0.01 s21 due to the noticeable softening effect after escaping the pinning site. The maximum m-value of 0.29 occurred at a deformation temperature of 800 °C due to the occurrence of complete dynamic recrystallization. The instability region of NiTi shape memory alloy increased with increasing strain, in which the austenite grains were obviously elongated and showed an oriented characteristics. Keywords
flow behavior, mechanism of DRX, NiTi shape memory alloy, processing maps, strain rate sensitivity
1. Introduction Near-equiatomic NiTi alloys are well known due to their shape memory effect and superelasticity or pseudoelasticity (Ref 1). Moreover, NiTi alloys also show good biocompatibility (Ref 2) and high energy densities, which makes that the alloys are widely exploited in many engineering (e.g., pipe joint and micro-actuators) and medical applications (e.g., stents) (Ref 3–5). Due to their unique properties, NiTi alloys are very promising for practical applications (Ref 6-8). When the content of nickel in the alloy is 49.3–51 at.%, the alloy exhibits excellent shape memory effect, but the high amount of Ni will promote that the alloy becomes very hard and brittle (Ref 9). Thus, its workability is poor at room temperature or lower temperature. Hot working could improve the plasticity of the alloys, and provide a effective way to gain the desired microstructure-mechanical property characteristics. Therefore, it is necessary to study the deformation behavior of the alloy and its dynamic response characteristics to processing parameters during hot working. Nowadays, a number of researchers have studied NiTi shape memory alloys in various aspects, including the analysis of the flow stress–strain curves and processing maps et al, which was widely used to characterize the deformation behavior of materials. For instance, Zhang et al. (Ref 10, 11) investigated the microstructure evolution of Ni50Ti50 alloy, and established the constitutive model using Arrhenius-type equation and processing map. Chaojun Li, Jiao Luo, and Miaoquan Li, State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xian 710072, Peoples Republic of China. Contact e-mail: [email protected].
Journal of Materials Engineering
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