Kinetics of Ordering and Disordering of Alloys

In the first part, the ordering kinetics of reversibly ordered phases such as Ni₄Mo and Ni₃Fe are examined; these can be initially disordered either by quenching from above T_c (which < T_m for a ‘reversibly’ ordered alloy) or by cold-working the ordered alloy. The relationship of ordering kinetics to diffusivities is outlined. The interrelation of ordering kinetics and the kinetics of recovery and recrystallization of reversibly ordered alloys is briefly discussed. — In the second part, the reordering of Ni₃Al, an alloy of the ‘permanently’ ordered variety, is analysed. This alloy is ordered, in equilibrium, up to the melting temperature; it can be substantially disordered either by severe cold work or by ultrarapid quenching. Some recent work on disordering of Ni₃Al by ball-milling is described; the process is associated with unexpected changes in lattice parameter. The effect of annealing and gradually reordering such a mechanically disordered Ni₃Al upon its hardness is briefly treated.