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镍钛形状记忆合金样品分别在573、723和873 K时效2 h。在573和723 K时效的镍钛样品中,可以观察到2个R相变体,而且这2个R相变体与B2基体之间的取向关系可以确定。在573和723 K时效的镍钛样品中,细小均匀的Ni4Ti3沉淀相与B2奥氏体基体之间保持共格关系。在873 K时效的镍钛样品中,可以观察到Ni4Ti3沉淀相在晶粒内部和晶界处析出,不均匀的Ni4Ti3沉淀相与B2基体之间保持共格、半共格和非共格关系。对于873 K时效的镍钛样品,加热时发生B19’马氏体向B2奥氏体转变的一阶相变,但在冷却时发生B2-R-B19’的二阶相变。723 K时效的镍钛样品在冷却时也表现出B2-R-B19’的二阶相变,但在加热时发出B19’-R-B2的二阶相变。对于573 K时效的镍钛样品,由于Ni4Ti3沉淀相周围的局部应力不均匀和局部成分不均匀,则其发生三阶相变。
NiTi shape memory alloy samples aged at 573, 723 and 873 K for 2 h, respectively. In the NiTi samples aged at 573 and 723 K, two R phase variants were observed, and the orientation relationship between the two R phase variants and the B2 matrix could be determined. In the NiTi samples aged at 573 and 723 K, there is a coherent relationship between the fine and uniform Ni4Ti3 precipitates and the B2 austenite matrix. In 873 K Ni-Ti samples, Ni4Ti3 precipitates were observed to be precipitated in the grain boundaries and grain boundaries. The inhomogeneous Ni4Ti3 precipitates and B2 matrix remained coherent, semi-coherent and non-coherent. For the 873 K aged NiTi samples, the first order transformation of B19 ’martensite to B2 austenite transformation occurred on heating, but the second order transformation of B2-R-B19’ occurred on cooling. The 723 K aged Ni-Ti samples also exhibited a second-order phase transition of B2-R-B19 ’when cooled, but a second-order phase transition of B19’-R-B2 occurred upon heating. For 573 K aged NiTi samples, the third-order phase transformation occurs because of the local stress nonuniformity and local composition around Ni4Ti3 precipitates.