Dislocation structures in fatigued copper-aluminum alloys

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Abstract
Low‐temperature (1.5–30 K) thermal conductivity measurments are used for a nondestructive determination of dislocation structures generated by cyclic deformation in two copper‐aluminum alloys (1 and 15 at.% Al in Cu). A torsional strain mode is adopted with an amplitude of a few percent. Fatigue life is estimated to be approximately 1000 cycles. In both alloys the change in lattice thermal resistivity due to deformation follows a T−2 dependence above 10 K, and a T−1 dependence below 2 K. At intermediate temperatures it is found to be composed of a T−2 and a T−3 term. This temperature variation is interpreted as due to phonon scattering by dislocations and by dislocation structures. Electron microscope observations by others indicate that these structures form cell walls in the lower‐concentration alloy, and dislocation bands in the higher‐concentration alloy. A dipole wall model of these substructures is proposed, and the corresponding lattice thermal resistivity calculated using a simple extension of Klemens’s theory of phonon scattering by the various lattice defects. These calculations give reasonable agreement with the experimental results. Dislocation densities are estimated to be of the order of 1012 cm−2 in the substructures, and of the order of 1010 cm−2 in the crystal matrix. It is found that dislocation substructures are formed during the first quarter ‐cycle of straining. Subsequent strain reversal tends to ’’condense’’ dislocations into cell walls and dislocation bands. As a result, there is a vary rapid saturation of dislocations in the substructures, which is almost complete after two cycles of deformation.