Microstructures and phase relationships in the Ti3Al + Nb system

Majumdar

Seetharaman

et al. 1999

Metall Mater Trans A

271

Phase transformations and the resulting microstructural evolution of near-Ti 2 AlNb and Ti-12Al-38Nb O ϩ bcc orthorhombic alloys were investigated. For the near-Ti 2 AlNb 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 ␣ 2 ; when present, the latter was in small quantities compared to the other phases. The transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Xray investigations of samples that were solutionized and water quenched were used to estimate the phase fields, and a pseudobinary diagram based on Ti ϭ 50 at. pct was modified. The agingtransformation behavior was studied in detail. For solutionizing temperatures between 875 ЊC and the bcc transus, the phase composition and volume fraction of the near-Ti 2 AlNb alloys adjusted through relative size changes of the equiaxed B2, O, and ␣ 2 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 ␣ 2 ϩ 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 ϩ B2 solutionized and water-quenched microstructures, Widmanstatten O-phase formation occurred only below 875 ЊC. For the subtransus-solutionized and water-quenched microstructures, a second aging transformation mode, cellular precipitation, was dominant below 750 ЊC. 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 supertransussolutionized and water-quenched microstructure transformed to a fully O microstructure at 650 ЊC. This microstructure reprecipitated B2 phase out of the O phase with continued aging time. For Ti12Al-38Nb, Widmanstätten O precipitation remained the only transformation mode. It is shown that subtransus processing offers flexibility in controlling microstructures through postprocessing heat treatments.

Section: Introductionmentioning

confidence: 97%

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

Majumdar

Seetharaman

et al. 1999

Metall Mater Trans A

271

“…A fourth possibility is that the nature of the bcc phase, which depends on alloy composition and heat treatment, is important. In particular, the bcc phase chemistry determines ordering, where at lower Al and higher Nb contents the ordered B2 structure undergoes a composition-induced disordering, which preferentially occurs during long aging times at low temperatures (Ref [10][11][12]. This is of practical importance in view of the substantially higher diffusivity expected for the disordered bcc (β) phase compared with B2, as this may adversely affect the creep resistances of such alloys.…”

mentioning

confidence: 99%

The phase evolution and microstructural stability of an orthorhombic Ti-23Al-27Nb alloy

1999

JPE

The phase evolution and microstructural stability were studied for an orthorhombic Ti-23Al-27Nb alloy. Monolithic sheet materials were produced through conventional thermomechanical processing techniques comprising nonisothermal forging and pack rolling. Phase evolution studies showed that, depending on the heat treatment schedule, this alloy may contain several constituent phases including: α 2 (ordered close-packed hexagonal D0 19 structure), B2 (ordered body-centered cubic, bcc), β (disordered bcc), and O (ordered orthorhombic based on Ti 2 AlNb). Differential thermal analysis studies indicated that the B2 transus temperature was 1070 °C. Heat treatment and transmission electron microscopy studies showed that the α 2 + B2 phase field extended between 1010 and 1070 °C. From 875 to 975 °C, a two-phase O + B2 field existed. Sandwiched between these two-phase regimes was a narrow three-phase α 2 + B2 + O field. Below 875 °C, an O + β field existed. All heat treatments at or above 875 °C, followed by quenching, resulted in equiaxed microstructures. However, below 875 °C, the B2 phase transformed into a mixture of O and bcc phases with lenticular morphologies. Cellular precipitation of O + β platelets at O/B2 and α 2 /B2 grain boundaries occurred depending on solutionizing and aging temperatures, which is explained by the compositional gradient between the bcc phases.

“…[1][2][3][4][5] Such alloys have shown potential to exceed the elevatedtemperature capabilities of conventional a + b Ti alloys. In particular, a Ti-15Al-33Nb (at.…”

Section: Introductionmentioning

confidence: 99%

Microstructure, Tensile, and Creep Behavior of Boron-Modified Ti-15Al-33Nb (at.%)

Cowen

2008

Metall Mater Trans A

The effect of boron on the microstructure, tensile, and creep behavior of a Ti-15Al-33Nb (at. pct) alloy was investigated. In addition to the normal constituent phases present in the unmodified alloy, the boron-modified alloys contained borides enriched in titanium and niobium. These borides made up to 9 pct of the volume and were present in the form of needles/ laths. Boron additions of 5 at. pct resulted in significant strengthening and stiffening and reduced elongation-to-failure. Smaller boron additions of 0.5 at. pct did not as significantly impact the RT tensile properties, but reduced the 650°C yield strength by 45 pct. Constant load, tensile-creep experiments were performed in the stress range of 150 to 400 MPa and the temperature range of 650°C to 710°C, in both air and vacuum environments. The addition of 5 at. pct boron significantly improved the creep resistance, whereas the addition of 0.5 at. pct boron degraded the creep resistance. In-situ tensile-creep experiments indicated that localized grain boundary cracking was prevalent at the prior-b grain boundaries.