采用Gleeble-3500热模拟试验机对锻态Ni55Ti45合金进行等温恒应变速率压缩实验,研究了该合金在变形温度为650 ～850℃、应变速率为0.01～1 s-1范围内的高温变形特性,并分析其变形机制.结果表明,应力-应变曲线呈“应变软化”型,热变形激活能为317.304 kJ/mol,根据双曲正弦函数建立了峰值流变应力本构方程.微观组织观察表明,在650℃、1 s-1和700℃、1 s-1时发生了局部塑性流动的失稳变形,主要由变形热效应导致.高温、低应变速率利于动态再结晶的发生,但动态再结晶晶粒尺寸更大.动态再结晶形核机制以晶界弓出机制为主,同时伴随有少量的PSN机制.
The hot deformation characteristic and mechanism of as-forged Ni55 Ti45 shape memory alloy were investigated at deformation temperature range of 650 ℃ to 850 ℃ and strain rate range of 0.01 s-1 to 1 s-1 by a Gleeble-3500 thermal simulation machine.The results show that the flow stress of the alloy is greatly affected by strain rate and deformation temperature,and the flow stress decreases with the increasing of deformation temperature and the decreasing of strain rate.The deformation activation energy of the alloy is 317.304 kJ/mol.A peak flow stress constitutive equation is established based on the hyperbolic sine equation.Microstructure observation shows that the local plastic flow instability deformation occurs at 650 ℃ and 700 ℃C when strain rate is 1 s-1,which is mainly caused by deformation thermal effect.Higher temperature and lower strain rate are favorable for dynamic recrystallization,but the grain size of dynamic recrystallization is larger.The nucleation mechanism of dynamic recrystallization is dominated by the mechanism of grain boundary bulging,accompanied by a small amount of particle stimulated nucleation (PSN) mechanism.
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