Order-hardening in CuAu

Single crystals of CuAu were used to examine the changes in hardness, flow stress, and microstructure resulting from heat treatment at 150 to 340° C, which produces the tetragonal superlattice. The crystals, quenched from a high temperature, were in the disordered cubic form to begin with; some were also cold-rolled, to permit comparison of the ageing characteristics of as-quenched and of cold-worked specimens. The asquenched crystal hardened much more than the cold-rolled ones; a peak hardness was attained and followed by a fall in hardness. At 150° C, no microstructural changes were seen; at 240° C, spontaneous recrystallisation took place at grain boundaries; while at 340° C, there was twinning and spontaneous grain-boundary fracture. The observations are rationalised in terms of the various mechanisms for relief of the large microstrains that accompany ordering in this alloy