Novel mixed-mode phase transition involving a composition-dependent displacive component.
ABSTRACT Solid-solid displacive, structural phase transformations typically undergo a discrete structural change from a parent to a product phase. Coupling electron microscopy, three-dimensional atom probe, and first-principles computations, we present the first direct evidence of a novel mechanism for a coupled diffusional-displacive transformation in titanium-molybdenum alloys wherein the displacive component in the product phase changes continuously with changing composition. These results have implications for other transformations and cannot be explained by conventional theories.
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ABSTRACT: We performed plane wave-based first principles calculations using the projector augmented wave (PAW) potential under the generalized gradient approximation (GGA) within the density functional theory to study the formation of ordered omega (B82-structured) Zr2Al phase in �-Zr3Al alloy. The transformation involves both replacive and displacive processes. We investigated two possible paths for the transformation where steps involving replacive (diffusive) and displacive processes occur in succession with their sequence of occurrence being different in the two paths. From this study, it was possible to show that the initial chemical ordering facilitates the displacive process leading to the transformation. It was also possible to correlate instability with respect to omega-type displacements in Zr2Al alloy with the number of Zr–Al bonds present in the unit cell. Electronic structure analysis indicated that stronger Zr–Al bonding plays an important role in the formation of chemically ordered omega phase.Philosophical Magazine 11/2012; 92(33):4040. · 1.60 Impact Factor