Change in Resistivity of a Cu-4 wt%Ti Alloy during Tensile Deformation at 77°K

Abstract

The change in resistivity during extension at 77°K was measured on a Cu-4%Ti alloy aged at 150°, 200°, 250°, 350° and 450°C. It was found that the resistivity of each aged specimen increased with straining at 77°K. The increment of resistivity for a strain of 5% became maximum for the specimen aged for a certain period of time at each temperature. The ageing time giving the maximum increase in resistivity of the specimen, t_max, tended to be shorter as the ageing temperature was raised. It was shown that the resistivity increase observed was due largely to the destruction of composition modulation or the modulated structure present in the aged specimen. It was suggested that the time t_max indicated a characteristic time t_s at which the spinodal decomposition, i.e. the growth of amplitude of the composition modulation was over. The time t_max coincided well with t_s which was determined from small-angle scattering experiments on this alloy. Based on Cahn’s spinodal theory, the dependence of t_s on the ageing temperature T was found to be given by the relation:
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where T_s was the coherent spinodal temperature and E the activation energy for the diffustion of Ti atoms in the alloy. Hence, with the observed values t_s\simeqt_max, T and T_s\simeq350°C, we obtained E\simeq0.8 eV. The value of E thus obtained was reasonable for the diffusion of the solute atoms in the alloy during the early stages of ageing at relatively low temperatures.