显微组织不均匀性对富Ti含量Ti-Ni合金负热膨胀行为的影响

Influence of anisotropic microstructure on negative thermal expansion behavior of Ti-rich Ti-Ni alloys

采用真空电弧熔炼技术结合水冷铜模快速凝固工艺制备出在一定温度范围内具有负热膨胀行为的系列成分富Ti含量Ti_(100-x)Ni_x(x=38~50,摩尔分数,%)熔铸态与吸铸态合金。结果表明:合金的负热膨胀行为主要源于升温过程中B19′→B2逆马氏体相变所引起的体积变化。富Ti含量Ti-Ni合金在室温下的主要物相为B19′-NiTi相和Ti_2Ni相,通过热膨胀实验数据和混合定则(ROM)计算所得的B19′相、Ti_2Ni相和B2相的热膨胀系数值与文献报道实验结果非常接近。所制备合金的显微组织具有明显择优取向生长特征,导致合金的负热膨胀行为呈现显著的各向异性。当合金中Ni含量增加时,样品的负热膨胀系数沿最大冷却速率方向增大,而在垂直于最大冷却速率方向上先增大后减小。

The conventional cast and suction-cast Ti_（100-x）Ni_x（x=38.0~50.0（mole fraction, %）） alloys with negative thermal expansion（NTE） behavior were fabricated by vacuum arc melting process combining with rapid solidification technique. The results show that the NTE behavior of the alloys originates from the volume change accompanying the B19′→B2 reverse martensitic phase transformation during heating process. B19′-Ni Ti and Ti_2Ni are the two major phases in Ti-rich Ti-Ni alloys at room temperature, and values of the calculated coefficient of thermal expansion（C_（TE）） of B19′, Ti_2Ni and B2 phases by using the rule of mixtures（ROM） agree with the experimental data published in literatures. The significant anisotropic NTE response is attributed to the microstructure of the alloys which exhibits typical preferred grain growth characteristics. With increasing the Ni content, the absolute values of C_（TE） along the maximum cooling direction of the alloys increase, while the C_（TE） perpendicular to the maximum cooling direction increases initially and then decreases.