Polytypic transformations in Laves phases

The Cr2X (X=Ti, Zr, Hf, Nb or Ta) Laves phases have the hexagonal (2H or 4H) structure at high temperatures and the cubic (3C) structure at room temperature. Their compositional ranges are dictated by their e/a ratios. Experimental observations of two-phase alloys in several of these systems demonstrate that the hexagonal structure is often retained at room temperature. Subsequent annealing at elevated temperature transforms the metastable hexagonal structure to the stable cubic structure. In single-phase Cr2Ti and Cr2Ta, the transformation is extremely sluggish whereas in Cr2Nb, it is rapid. In two-phase alloys, the transformation is almost always sluggish (except in the Cr–Nb system) and is accompanied by compositional changes. Transmission electron microscopic observations are consistent with the transformations being achieved by synchroshear behind glissile synchroshockley dislocations. The synchroshear mechanism is described and the transformation kinetics are discussed in terms of long-range diffusion in two-phase alloys and in terms of the mobility of synchroshockley dislocations in single-phase alloys.