Observations on the β→ζ m massive transformation in two-phase (β+ζ) alloys of the Cu−Ga−Ge system
Optical metallography, transmission electron microscopy, and thermal arrest measurements, have been used to study the transformation, on quenching, of an initial two-phase (β+ζ) microstructure, in the Cu−Ga−Ge system. The two phases bcc (β) and hcp (ζ) coexist in a narrow temperature range near 635°C in a ternary Cu-21.0Ga-1.5Ge alloy. The microstructure at high temperature typically consists of β regions outlining ζ grains. Upon rapid quenching of such a structure, the hexagonal ζ grains remain stable, but each β region transforms via a massive transformation to an hcp ζ m structure without change of composition. Growth of such ζ m grains takes place only from certain original β/ζ boundaries, but not from others. A ξ m grain adopts the same crystallographic orientation as the adjacent ζ grain at which it initiated. However, the coherent ζ/ζ m “boundary” which develops exhibits a lattice parameter discontinuity across it, due to the difference in composition. The process amounts to massive growth without conventional nucleation. The significance of these findings, particularly in terms of the desirability of the presence or absence of certain initial crystallographic relationships between the β and ζ grains is discussed.
KeywordsMisfit Dislocation Massive Transformation Impact Boundary
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