ABSTRACT Isothermal compression tests of NiTi shape-memory alloy (NiTi SMA) were carried out on a Gleeble-3800 thermal simulating tester in the temperature range 750–1050°C and strain rates of 0.005–10 s−1 with a deformation of 0.7. The mechanical behaviour characteristics and microstructure evolution during the hot compression process are the main items of study. The hot-processing maps under five different instability criteria (Prasad, Gegel, Malas, Murty, and Semiatin instability criteria) were constructed based on hot-compression experimental data and dynamic material model (DMM) theory. Suitable forming processing regions and flow instability regions of the NiTi SMA are predicted by the processing maps in the process of hot deformation. The results indicate that: the prediction for the alloy intersect and supplement each other because of the different theoretical basis of the above instability criteria. The prediction range of the above five instability criteria is verified by combining variation of strain-rate sensitivity exponent, microstructure characterisation and actual hot-deformation results in the process of hot deformation. The result predicted by the Prasad instability criteria is the most accurate for the alloy. Plastic flow bands and adiabatic shear cracks mainly occur in the flow-instability regions, while dynamic recovery (DRV) and dynamic recrystallisation (DRX) mainly occur in the processing regions.

中文翻译：

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NiTi形状记忆合金在不同失稳判据下的加工图研究

摘要 NiTi 形状记忆合金 (NiTi SMA) 的等温压缩试验在 Gleeble-3800 热模拟试验机上进行，温度范围为 750-1050°C，应变速率为 0.005-10 s-1，变形为 0.7。热压缩过程中的力学行为特征和微观结构演变是主要研究项目。基于热压缩实验数据和动态材料模型 (DMM) 理论构建了五种不同不稳定标准（Prasad、Gegel、Malas、Murty 和 Semiatin 不稳定标准）下的热加工图。通过热变形过程中的加工图预测NiTi SMA的合适成形加工区域和流动不稳定性区域。结果表明：由于上述失稳判据的理论基础不同，对合金的预测相互交叉、相辅相成。结合热变形过程中应变率敏感指数的变化、显微组织表征和实际热变形结果，验证了上述五个失稳判据的预测范围。Prasad 不稳定性准则预测的结果对于合金来说是最准确的。塑性流动带和绝热剪切裂纹主要发生在流动不稳定区，而动态恢复（DRV）和动态再结晶（DRX）主要发生在加工区。微观结构表征和实际热变形导致热变形过程。Prasad 不稳定性准则预测的结果对于合金来说是最准确的。塑性流动带和绝热剪切裂纹主要发生在流动不稳定区，而动态恢复（DRV）和动态再结晶（DRX）主要发生在加工区。微观结构表征和实际热变形导致热变形过程。Prasad 不稳定性准则预测的结果对于合金来说是最准确的。塑性流动带和绝热剪切裂纹主要发生在流动不稳定区，而动态恢复（DRV）和动态再结晶（DRX）主要发生在加工区。