Element segregation and α2 formation in primary α of a near-α Ti-alloy

Abstract

Alloy TIMETAL®834 is a near-α Ti-alloy typically processed to have a complex bimodal microstructure that provides a good combination of mechanical properties at temperatures in excess of 450 °C. Due to the high Al content, typical ageing procedures result in the formation of intragranular and coherent nano-scale Ti3Al precipitation (α2), which increase strength but also promotes slip planarity. The present study focuses on chemical partitioning as a result of sub-β-transus heat treatment and the consequences for the two different constituents in the bimodal microstructure. The detailed chemical and structural analysis were carried out by combining Electron Probe Micro Analysis (EPMA), Wavelength Dispersive Spectroscopy (WDS), Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) for investigating local compositional variations and their effect on the formation of α2 precipitates. Detailed microchemical analysis shows a core-shell composition arrangement of α-stabilisers with the shell composition similar to the one of secondary α. Selected area electron diffraction in the TEM and APT analysis demonstrates that those local variations in α stabilisers affect the level of α2 precipitation. In addition, EPMA maps show that while Zr and Sn are often considered to be neutral alloying elements in Ti-alloys, they do segregate to the β-phase during sub-β-transus heat treatment.