Effect of coherency strain fields on the flow stress of a copper-3·12% cobalt alloy

The reversible flow stress ratio in a polycrystalline Cu-3·12% Co alloy was investigated by temperature changes below 303°K as a function of the strain and of the precipitation structure determined by transmission electron microscopy. Shear modulus values determined with a torsion pendulum were used to correct the data. Coherent precipitates of about 15 Å diameter produced by ageing at 923°K resulted in an anomalous increased temperature dependence of the flow stress, whereas those over about 25 Å diameter gave an anomalous reversed temperature dependence, the alloy becoming softer on cooling. The maximum anomalous softening occurred in the peak-hardened condition, and at zero strain and 0°K was estimated to be 16% of the flow stress at 303°K. Straining progressively removed the effect. The proportional decrease in flow stress (for zero strain) on cooling from 303° to 77°K for the peak aged alloy is in excellent agreement with the proportional decrease in mis-match determined by x-ray parameter measurements on a Cu-20% Co alloy homogenized at 923°K. The anomalous softening effect disappeared on gross overageing which resulted in loss of coherency. The results support the concept of coherency hardening.