Multistep evolution of βo phase during isothermal annealing of Ti-42Al-5Mn alloy: Formation of Laves phase

Abstract

Intermetallic β-solidifying γ-TiAl alloys exhibit improved hot workability owing to the presence of soft β phase at elevated temperature. However, through the incorporation of essential β-stabilizers the metastable ordered β phase (βo) is retained and hence an understanding of its phase transformation behavior at target temperature is indispensable. Here we report new insights into the multistep evolution of the βo phase during isothermal annealing of Ti-42Al-5Mn (at.%) alloy at 800 °C. Our observations reveal that both irregularly shaped α2 and γ platelets are firstly precipitated from the interior of βo phase, accompanied with Mn rejection to the surrounding. And this contributes to the local enrichment of Mn, which thereupon leads to intragranular precipitation of secondary phase particles (SPPs). More specifically, an ellipsoidal submicrometre-scale Mn-rich Laves phase with a C14 hexagonal structure formed at the expense of α2 precipitates via a shuffle of atoms is detected. The Laves phase has an orientation relationship (OR) with the parent α2 phase described as [101¯0]α2//[112¯0]Laves, (0001)α2//(11¯00)Laves. It is due to the instability of partial pre-formed γ precipitates that assists the solution of Al in Laves phase forming the formula Ti(Al,Mn)2, thereby facilitating the growth of Laves phase.