Order-softening of Cu-10at.%Au

The critical resolved shear stress τc of Cu-10at.%Au-single crystals has been measured as a function of their state of order. This state has been varied by heat-treatments between 473 and 723 K. X-ray diffraction experiments and the observation of dislocation pairs in the transmission electron microscope led to the interpretation that all specimens were L12-long-range-ordered; the antiphase domains were, however, small: around 5 nm. τc has been found to decrease as the degree S of long-range order increases; that is, there is order-softening. This surprising result is interpreted on the basis of the elastic interaction of the large Au-atoms with edge dislocations. If S is high, the resulting interaction forces tend to average out.