Abstract
Microhardness of stoichiometric CuAu single-crystal was measured for two different types of isochronal cooling from the disordered state above T_O/D = 410 °C. It is shown that there is a difference between the microhardness of the ordered sample measured in the regime of conventional isochronal cooling and the microhardness obtained after an isochronal annealing/quench step, following immediately after a standard disorder annealing of the sample. This difference shows that the state of the sample is influenced by the heating process from room temperature to the annealing temperature in the conventional isochronal regime and explains the absence of thermal hysteresis effects near order-disorder temperature in earlier isochronal resistometric and microhardness experiments on CuAu.

Key words
microhardness, copper-gold intermetallic compound, long-range order, isochronal cooling, order-disorder transformation

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