采用液态金属冷却定向凝固方法制备了Ni-44Ti-5Al-2Nb-1Mo(原子分数, %)合金,分析了加热温度为1 550 ℃,抽拉速率为0.3、 1.2、 3.0、 6.0、 18.0 mm/min 时的定向凝固组织特征。结果表明,定向凝固没有改变合金的相组成,但改变了组成相的形态。定向凝固组织均由初生的NiTi相与晶间析出的Ti2Ni相、富Nb的Ti2Ni相组成,与铸态组织相同。定向凝固后,NiTi相发生了强烈的[100]择优取向。随着抽拉速率的增大,NiTi相沿[100]择优取向趋势增强,固液界面形态由发达的树枝晶变为胞状树枝晶,NiTi枝晶及析出相Ti2Ni、富Nb的Ti2Ni明显细化。定向排列的NiTi枝晶组织和分布在晶间的细小析出相将有益于合金高温强度和室温塑性的提高。
Alloy Ni-44Ti-5Al-2Nb-1Mo (atomic fraction,%) is directionally solidified (DS) at a constant temperature (1 550 ℃) in a range of withdrawal rates (0.3,1.2,3.0,6.0,18.0 mm/min) by liquid metal cooling technology. An investigation of the microstructural characteristics indicates that the DS processes do not affect the phase composition, but they do change the phase morphology of the alloy. The phase composition of DS samples remains the same as that of the as-cast specimen, which is composed of NiTi (B2), Ti2Ni and Nb-rich Ti2Ni phases. After DS, [100] becomes the preferred orientation in the NiTi matrix. With the withdrawal rate increasing, orientation [100] is significantly enhanced, and the solid/liquid interface morphology changes from coarse dendrite to cellular dendrite. At the same time, both the primary dendrite arm of NiTi and the precipitates are greatly refined. The directional dendrites and tiny precipitation distribution will be beneficial to the high temperature strength and room temperature ductility of the alloy.
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