Precipitation of $alpha$-phase in metastable $beta$-phase Ti alloys
The isothermal decomposition of metastable $beta$-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 $alpha$-phase are observed as decomposition products. Type 1 $alpha$ obeys the well-known Burgers orientation with respect to the $beta$-phase matrix. Type 2 $alpha$, 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 $alpha$. The Type 1 $alpha$ forms initially at aging temperatures in the range of 400 to 650$sup 0$C, and is replaced by Type 2 $alpha$ after longer holding times at the aging temperature. The Type 1 $alpha$ 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 $alpha$ from Type 1 $alpha$ are considered. The first of these is mechanical twinning of Type 1 $alpha$ 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. (auth)
- Research Organization:
- Rockwell International, Thousand Oaks, CA
- NSA Number:
- NSA-33-015285
- OSTI ID:
- 4072902
- Journal Information:
- Metall. Trans., A, v. 6A, no. 11, pp. 2103-2114, Journal Name: Metall. Trans., A, v. 6A, no. 11, pp. 2103-2114; ISSN MTTAB
- Country of Publication:
- United States
- Language:
- English
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BINARY ALLOY SYSTEMS
BURGERS VECTOR
CRYSTALLOGRAPHY
ELECTRON MICROSCOPY
N50230* --Metals
Ceramics
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Structure & Phase Studies
PRECIPITATION HARDENING
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TWINNING