Chemical compositions of ultrafine lamellae in the water-quenched Ti-48Al alloy

Abstract

As Ti rich Ti-Al alloy shows ductile nature with the α2 + γ lamellar structure [1], the mechanism of microstructural evolution in Ti-Al alloys has attracted much interest. Since the lamellar structure is formed by the continuous cooling in industrial processes, cooling rate dependence of α → γ and α → α2 + γ transformations has been extensively studied by many researchers [2-5]. Wang et al. [2,3] reported that α transformed to γ massively, when Ti-48Al alloy was water-quenched from the single phase α. As a nature of the massive transformation, they reported that there was no detectable difference in composition between massively transformed γ (hereafter, denoted as γm) and α2 phase, where α2 is a result of ordering of untransformed α phase. Jones and Kaufman [4] constructed continuous cooling transformation (CCT) diagrams for Ti-Al with various concentrations and suggested that lamellar microstructure occurred when cooling rate was slow and massive γm is formed when cooling rate was fast. Since massive transformation is nucleated at grain boundaries, α can be fully transformed to γm massively when grain size is small. However, Kumagai et al. [6] reported that massive reaction does not complete when Ti-48Al was solution treated for an extended period of time. This is attributed to larger size of α grains which has grown during the solution heat treatment. In these untransformed regions, they found extremely fine lamellar of α2 + γ with an orientation relationship {111}γ//{0001}α2 and γ / / α2 . This lamellar microstructure is very unusual in view of the following two points: (1) the lamellar distance is extremely fine with an order of 5 to 10 nm, and (2) the α2 thin lamellae are completely reverted to γ lamellae upon annealing at 1273 K. Based on these observations, Kumagai et al. concluded that the fine lamellar structure observed in the untransformed region in the water-quenched Ti-48Al alloy is a precursor phase to the γm. Based on the ultrafine scale of the lamellar structure, Abe et al. [7] suggested that the ultrafine lamellae of α2 + γ are formed by a displacive (martensitic) transformation without change in chemical compositions. However, in this study, they did not determine the chemical compositions in the lamellae, and no evidence to prove the displacive transformation mechanism was provided. Hence, this study aims to measure the local chemical concentration changes across the fine lamellae directly and to discuss the mechanism of microstructural evolution of the ultrafine lamellae in water-quenched Ti-48at.%Al alloy. For local concentration determination of the ultrafine lamellae, a unique microanalytical technique with an atomic spatial resolution, atom probe field ion microscopy (APFIM), was employed.