Part I. The microstructural evolution in Ti-AI-Nb O + Bcc orthorhombic alloys

C. J. Boehlert, B. S. Majumdar, V. Seetharaman, D. B. Miracle

Research output: Research - peer-reviewArticle

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Abstract

Phase transformations and the resulting microstructural evolution of near-Ti2AlNb and Ti-12A2Al-38Nb O + bec orthorhombic alloys were investigated. For the near-Ti2AlNb alloys, the processing temperatures were below the bcc transus, while, for Ti-12Al-38Nb, the processing temperature was supertransus. Phase evolution studies showed that these alloys contain several constituent phases, namely, bcc, O, and α2; when present, the latter was in small quantities compared to the other phases. The transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray investigations of samples that were solutionized and water quenched were used to estimate the phase fields, and a pseudobinary diagram based on Ti = 50 at. pet was modified. The agingtransformation behavior was studied in detail. For solutionizing temperatures between 875°C and the bcc transus, the phase composition and volume fraction of the near-Ti2AlNb alloys adjusted through relative size changes of the equiaxed B2,0, and α2 grains. The aging behavior followed three distinct transformation modes, dependent on the solutionizing and aging temperatures. Widmanstätten formation was observed when a new phase evolved from a parent phase. Thus, Widmanstätten O phase precipitated within the B2 phase for supertransus fully B2 microstructures, as well as for subtransus α2 + B2 microstructures. Similarly, Widmanstätten B2 phase can form from a fully O microstructure, a transformation that has not been observed before. In the case of equiaxed O + B2 solutionized and water-quenched microstructures, Widmanstatten O-phase formation occurred only below 875°C. For the subtransus-solutionized and water-quenched microstructures, a second aging transformation mode, cellular precipitation, was dominant below 750°C. This involved formation of coarse and lenticular O phase that grew into the prior B2 grains from the grain boundaries. A third transformation mode involved composition-invariant transformation, where the fully B2 supertransus-solutionized and water-quenched microstructure transformed to a fully O microstructure at 650°C. This microstructure reprecipitated B2 phase out of the O phase with continued aging time. For Ti-12Al-38Nb, Widmanstätten O precipitation remained the only transformation mode. It is shown that subtransus processing offers flexibility in controlling microstructures through postprocessing heat treatments.

LanguageEnglish (US)
Pages2305-2323
Number of pages19
JournalMetallurgical and Materials Transactions A: Physical Metallurgy and Materials Science
Volume30
Issue number9
StatePublished - 1999
Externally publishedYes

Profile

Microstructural evolution
Microstructure
microstructure
Aging of materials
Water
Temperature
water
temperature
Processing
Phase composition
Volume fraction
Grain boundaries
Phase transitions
Heat treatment
Transmission electron microscopy
X rays
Scanning electron microscopy
Chemical analysis
phase transformations
flexibility

ASJC Scopus subject areas

  • Materials Science(all)
  • Metals and Alloys

Cite this

Part I. The microstructural evolution in Ti-AI-Nb O + Bcc orthorhombic alloys. / Boehlert, C. J.; Majumdar, B. S.; Seetharaman, V.; Miracle, D. B.

In: Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, Vol. 30, No. 9, 1999, p. 2305-2323.

Research output: Research - peer-reviewArticle

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AU - Miracle,D. B.

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N2 - Phase transformations and the resulting microstructural evolution of near-Ti2AlNb and Ti-12A2Al-38Nb O + bec orthorhombic alloys were investigated. For the near-Ti2AlNb alloys, the processing temperatures were below the bcc transus, while, for Ti-12Al-38Nb, the processing temperature was supertransus. Phase evolution studies showed that these alloys contain several constituent phases, namely, bcc, O, and α2; when present, the latter was in small quantities compared to the other phases. The transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray investigations of samples that were solutionized and water quenched were used to estimate the phase fields, and a pseudobinary diagram based on Ti = 50 at. pet was modified. The agingtransformation behavior was studied in detail. For solutionizing temperatures between 875°C and the bcc transus, the phase composition and volume fraction of the near-Ti2AlNb alloys adjusted through relative size changes of the equiaxed B2,0, and α2 grains. The aging behavior followed three distinct transformation modes, dependent on the solutionizing and aging temperatures. Widmanstätten formation was observed when a new phase evolved from a parent phase. Thus, Widmanstätten O phase precipitated within the B2 phase for supertransus fully B2 microstructures, as well as for subtransus α2 + B2 microstructures. Similarly, Widmanstätten B2 phase can form from a fully O microstructure, a transformation that has not been observed before. In the case of equiaxed O + B2 solutionized and water-quenched microstructures, Widmanstatten O-phase formation occurred only below 875°C. For the subtransus-solutionized and water-quenched microstructures, a second aging transformation mode, cellular precipitation, was dominant below 750°C. This involved formation of coarse and lenticular O phase that grew into the prior B2 grains from the grain boundaries. A third transformation mode involved composition-invariant transformation, where the fully B2 supertransus-solutionized and water-quenched microstructure transformed to a fully O microstructure at 650°C. This microstructure reprecipitated B2 phase out of the O phase with continued aging time. For Ti-12Al-38Nb, Widmanstätten O precipitation remained the only transformation mode. It is shown that subtransus processing offers flexibility in controlling microstructures through postprocessing heat treatments.

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