外应力对NiTi合金中共格Ni_4Ti_3沉淀相长大行为影响的相场法模拟

PHASE FIELD SIMULATION OF THE EFFECT OF APPLIED EXTERNAL STRESS ON GROWTH KINETICS OF COHERENT Ni_4Ti_3 PRECIPITATE IN NiTi ALLOY

运用相场模型模拟了外加应力对NiTi形状记忆合金中Ni_4Ti_3沉淀相组织形貌及长大行为的影响.无外加应力作用的时效过程中,4组Ni_4Ti_3变体均沿着各自对应的(111)_(B2)惯习面从NiTi基体相中析出;而在〈111〉_(B2)方向施加压应力时,只有一组Ni_4Ti_3变体从NiTi基体相中析出,其惯习面法线方向平行于外加应力方向;单轴压应力作用下时效时,Ni_4Ti_3形核数量明显增加.外加单轴压应力可促进Ni_4Ti_3变体的形核和长大,但并不改变变体的面积分数、长度、宽度以及长度-宽度比等参数随时效时间变化的趋势;压应力增大使变体长度及宽度均略微增加,但均小于无应力作用下的情形;变体的面积分数随外加应力的增大而增加.相场模拟结果与实验观察结果吻合较好.

The presence and distribution of Ni4Ti3 particles in NiTi alloys have a significant influence on martensitic phase transformation path by favoring the formation of R-phase rather than B19＇ phase since the latter produces larger lattice deformation.To deeply understand the above,some experimental studies have been done by using differential scanning calorimetry（DSC） and in situ transmission electron microscopy（TEM）.Meanwhile,some preliminary simulations have also been performed focusing on the morphology evolution of single and multiple Ni4Ti3 variants in single NiTi alloy system as well as considering the effect of external loads on selective precipitate growth.Whereas, in engineering application,the NiTi alloys often undergo the external load which may affect the growth kinetics of Ni4Ti3 precipitates.Therefore,it is necessary to investigate the effect of applied load on growth kinetics of Ni4Ti3 precipitates.In this paper,the phase field method has been extended to study the microstructure evolution and growth kinetics of Ni4Ti3 precipitates in NiTi alloys during zero-stress and stress-assisted aging.The simulation results show that during stress-free aging,four groups of the variants precipitate along the corresponding（111）_（b2） habit plane;when the NiTi matrix is under〈111〉_（b2） compressive stress-assisted aging,there is only one group of the variants with the normal lines parallel to〈111〉_（b2） to be precipitated.Although the uniaxial compressive stress apparently promotes the nucleation and slightly accelerates the growth of Ni4Ti3 variants in each group,the trends of aging time dependences of the area fraction,variant length,variant width and length-width ratio seem unchanged.The larger stresses can cause length and width of the variant slightly larger,but the area fraction of the Ni4Ti3 particles increases with increasing stress level.The simulation results are in good coincidence with the experimental results available.