Precipitation of α-phase in metastable β-phase Ti alloys

The isothermal decomposition of metastable β-phase in Ti-- 14 wt percent Mo-6 wt percent Al and Ti--11.6 wt percent Mo alloys has been studied by electron microscopy. Two distinct types of α-phase are observed as decomposition products. Type 1 α obeys the well-known Burgers orientation with respect to the β-phase matrix. Type 2 α, which is characterized by arced reflections in the electron diffraction patterns, does not obey the Burgers orientation relation, but rather can be described as being in a [10 anti 1 2] (10 anti 1 1) twin orientation with respect to Burgers orientation α. The Type 1 α forms initially at aging temperatures in the range of 400 to 6500C, and is replaced by Type 2 α after longer holding times at the aging temperature. The Type 1 α precipitates form as monolithic plates in the ternary alloy and as monolithic needles in the binary. The Type 2 precipitates in the ternary alloy occur as very fine, contiguous particles in a colony-like arrangement wherein the colony has a plate morphology. The Type 2 precipitates in the binary alloy form as a peripheral shell around the Type 1 needle. Two possible mechanisms describing the formation of Type 2 α from Type 1 α are considered. The first of these is mechanical twinning of Type 1 α plates, and the second is a nucleation and growth mechanism which is driven by a composition difference between Types 1 and 2. The former is concluded to be less likely than the latter