Ni60Ti40形状记忆合金的热变形行为

为获得Ni₆₀Ti₄₀形状记忆合金热变形的最佳工艺参数,利用等温恒速率热压缩试验研究了在温度为800~1 000 ℃、应变速率为0.005~5.000 s-1条件下Ni₆₀Ti₄₀合金的热变形行为,通过探究不同变形温度和应变速率对Ni₆₀Ti₄₀合金流变行为的影响创建本构关系,并以动态材料模型为基础构建热加工图。结果表明,Ni₆₀Ti₄₀合金的流变应力随变形温度的升高而减小、随应变速率的升高而增大。温度为900~1 000 ℃、应变速率为0.005~0.500 s-1时,流变应力较快达到稳态,且所需的变形量较少。采用Arrhenius双曲正弦模型构建的Ni₆₀Ti₄₀合金热变形的流变应力本构关系模型可基本准确地预测实际流变应力随工艺参数的变化趋势,计算得到Ni₆₀Ti₄₀合金的平均热变形激活能为213 kJ/mol。Ni₆₀Ti₄₀合金的热变形有3个稳定变形区和1个失稳区,适宜变形的区域为800~870 ℃/0.005~0.080 s-1、870~950 ℃/0.080~0.500 s-1和950~1 000 ℃/0.050~5.000 s-1;不适合进行热加工的区域为800~850 ℃/0.220~5.000 s-1。

Thermal deformation behavior of Ni60Ti40 shape memory alloy

To obtain optimum conditions of Ni₆₀Ti₄₀ shape memory alloy during hot deformation, the thermal deformation behavior at 800-1 000 ℃ and strain rate 0.005-5.000 s-1was studied by isothermal compression experiment with constant strain rate. The constitutive relationship of Ni₆₀Ti₄₀ alloy was established by exploring the effect of different conditions on the rheological behavior, and the processing maps were constructed based on the dynamic material model. As the results showing, the flow stress of Ni₆₀Ti₄₀ alloy decreases with the increase of deformation temperature and increases with the increase of strain rate. When the compression test involving the wide temperatures of 900-1 000 ℃ and strain rates of 0.005-0.500 s-1, the flow stress reaches the steady state quickly, and requires less deformation. The flow stress constitutive model of Ni₆₀Ti₄₀ alloy hot deformation constructed using Arrhenius hyperbolic sinusoidal model can basically predict actual flow stress variations with process parameters accurately, and the average activation energy of Ni₆₀Ti₄₀ hot deformation is 213 kJ/mol. During hot deformation of the Ni₆₀Ti₄₀ alloy, three stable deformation area and one instability area are confirmed in the processing maps. The suitable deformation domains are 800-870 ℃/0.005-0.080 s-1, 870-950 ℃/0.080-0.500 s-1 and 950-1 000 ℃/0.050-5.000 s-1. The range of instability area that should be avoided during hot working is 800-850 ℃/0.220-5.000 s-1.