Past exam of the materials science course of the University of Cambridge / 2019 / Paper 1 / 3 / d

At a common test temperature and otherwise comparable microstructure, chemical order generally raises the yield strength. An ordinary FCC dislocation displaced by $a⟨110⟩/2$ can interchange Au and Cu sublattices, creating an antiphase boundary behind it. This additional energy resists slip; paired dislocations can restore the order after the second member passes. The disordered phase does not pay the same ordering penalty. Comparing specimens actually tested at their different phase-stability temperatures also includes temperature-dependent mobility and recovery, so the ordering argument alone does not predict a unique measured strength ratio.