STRUCTURAL TRANSFORMATIONS DURING DECOMPOSITION IN CU-BE ALLOYS

In this paper we consider the decomposition that occurs in Cu-rich CuBe alloys. The decomposition process is regarded as an isostructural decomposition of the f.c.c. phase into Be-rich and Be-depleted regions followed by the transformation of the Be enriched f.c.c. regions into Be enriched b.c.c. regions via the Bain strain. Concomitant with and after this process, the b.c.c. regions atomically order to form the CsCl (B2) structure. Consideration of the coherency strain results in the conclusion that the habit plane of the emerging B2 phase changes during decomposition to minimize the elastic strain energy. The habit change is accompanied by a change in crystal structure induced by the coherency. The changes are rationalized in terms of material constants. The predicted habit planes and lattice parameters agree with the X-ray and electron microscopy observations. The morphology and structure of the intermediate γ″ and γ′ phases are interpreted as constrained states of the same precipitate B2 phase (γ) with different habit planes. © 1990.