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The aim of this study is to examine the effect of solution treatment temperature (STT) on the microstructure, the micromechanical properties, and the kinetic parameters of the β → α phase transformation during continuous cooling of the dual phase titanium alloy Ti-6Al-4V. Increasing the STT from 1050 to 1200 °C delays the formation of the α phase during cooling and increases the value of its activation energy. The microstructural analysis reveals the emergence of αW platelets from protuberances on the αGB/αW interface. The investigation of the morphology of the αW platelets reveals the presence of ledges on their longest side showing a sharp extremity. The micromechanical properties determined by nanoindentation and microhardness tests are almost insensitive to the cooling rate but are strongly affected by the STT; the higher the STT, the lower the overall microhardness of the Ti-6Al-4V alloy. In addition, the STT affects the microhardness and the Young's modulus of both α and β phases differently; when the STT increases, the microhardness and the Young's modulus of the α phase decrease, whereas those of the β phase increases. [ABSTRACT FROM AUTHOR]

A mixed-control model was developed to study the transformation character of ferrite formation by a ledge mechanism. A numerical two-dimensional diffusion-field model was combined to describe the evolution of the diffusion field ahead of the migrating austenite/ ferrite interface. The calculation results show that the bulk diffusion-controlled model leads to a deviation from experimental results under large solute supersaturation. In the mixed-control model, solute supersaturation and a parameter Z together determine the transformation character, which is quantified by the normalized concentration of carbon in austenite at the austenite/ferrite interface. By comparing with experimental data, the pre-exponential factor of interface mobility, M, is estimated within the range from 0.10 to 0.60 mol·m·J·s for the alloys with 0.11wt%-0.49wt% C at 700-740°C. For a certain Fe-C alloy, the trend of the transformation character relies on the magnitude of M as the transformation temperature decreases. [ABSTRACT FROM AUTHOR]

Abstract: Ledge expansion and the concomitant growth of a stacking fault tetrahedron (SFT) are investigated in Al and Cu by molecular dynamics (MD) simulations by addition of vacancy rods with selected lengths. Ledge expansion is largely governed by the site preference of vacancies on the SFT edges resulting in distinct stable ledged SFTs. Both edge- and corner-facing ledge configurations may be adopted. The growth of SFTs, especially large ones, is controlled by thermal agitation. The mobile part of the ledges consists of a dipole of Shockley partials generally oriented in the 60° mixed orientation that move in a thermally activated manner, reflecting a certain core reorganization of the Shockley dipole. [ABSTRACT FROM AUTHOR]

Diffusional phase transformations often progress by the lateral motion of growth ledges. We develop an approximate treatment for the effect on ledge migration of a solute which is attracted to the ledge, and which is also rejected into the parent phase by volume diffusion. [Copyright &y& Elsevier]