Effect of aging temperature on ordered structure of Ni-Cr-Al-Fe precision resistance alloy

The electrical resistivity of Ni-Cr-Al-Fe precision resistance alloy is quite sensitive to aging temperature, due to the resulting evolution of ordered structures. For the first time, the dependence of the electrical resistivity and microstructure on aging temperature has been investigated via an isochronal aging procedure at different temperature. Electrical resistivities of the as-quenched and -aged samples were measured in order to identify the typical samples that were used to reveal the effect of aging temperature on the microstructure. Transmission electron microscopy (TEM) and atom probe tomography (APT) were used to study the microstructure. The L12 ordered structure formed during aging process was identified by selective area electron diffraction (SAED). Based on the results of APT, composition distributions in the samples were analyzed and compared with each other. It was found that the L12 long range order (LRO) was formed during aging at 460 °C, and grew up with the increase of aging temperature. In the superlattice, Ni atoms occupy primarily the face center sites, while Al atoms take the face edge sites more than Cr and Fe atoms do. No compositional difference was found between the LRO clusters and the matrix in the sample aged at 460 °C. With the increase of aging temperature, Cr and Fe atoms tend to be excluded from the ordered clusters and reside in the disordered matrix. Therefore, the composition of the ordered clusters is different than the matrix in the samples aged at 595 °C and 670 °C.