Part III. The tensile behavior of Ti-Al-Nb O+bcc orthorhombic alloys

The tensile behavior of Ti-Al-Nb alloys with Al concentrations between 12 and 26 at. pet and Nb concentrations between 22 and 38 at. pet has been investigated for temperatures between 25 degreesC and 650 degreesC. Several microstructural features were evaluated in an attempt to identify microstructure-property relationships. In particular, the effects of the phase volume fraction, composition, morphology, and grain size were examined. In addition, the constitutive properties were evaluated using single-phase microstructures, and the results provided insight into the microstructure-property relationships of the two-phase orthorhombic (O) + body-centered-cubic (bee) microstructures. The disordered fully-bcc (beta) Ti-12Al-38Nb microstructure, produced through heat treatment above the beta -transus, exhibited a room-temperature (RT) elongation of more than 27 pet and the lowest yield strength (YS-553 MPa) of all the alloys studied. The ordered fully-bcc (B2) microstructures, produced through supertransus heat treatment of near-Ti2AlNb alloys, exhibited fracture strengths up to 672 Mpa and low elongations-to-failure (epsilonf less than or equal to 0.6 pct). Thus, increasing the Al content, wl-&h favors ordering of the bee structure, significantly reduces the ductility of the bee phase. Similar to the ordered 132 microstructure, the ordered fully-O Ti2AlNb microstructures exhibited intermediate RT strength (:: 704 MPa) and epsilon (f) (less than or equal to1 pct). The O + bcc microstructures tended to exhibit strengths greater than both the fully-O and fully-bcc microstructures, and this was attributed to the finer grain sizes in the two-phase microstructures compared to their single-phase counterparts. A RT of 1125 MPa was measured for the finest-grained two-phase microstructure. The O + bcc microstructures containing greater bcc-phase volume fractions tended to exhibit greater elongations yet poorer elevated-temperature strengths. A higher Al content typically resulted in larger elevated-temperature strengths. For the Ti-12Al-38Nb bcc-dominated microstructures, fine O platelets, which precipitated during aging, provided significant strengthening and a reduction in ef for the Ti-12Al-38Nb alloy. However, large RT elongations (epsilon (f) > 12 pet) were maintained for aged Ti-12Al-38Nb microstructures, which contained 28 Vol pet O phase. Morphology did not appear to play a dominant role, as fully-lath and fully-equiaxed two-phase microstructures containing the same phase volume fractions exhibited similar RT tensile properties. The slip and cracking observations provided evidence for the ductile and brittle characteristics of the single-phase microstructures, and the slip compatibility exhibited between the two phases is an important part of why O + bee microstructures achieve attractive strengths and elongations. The YS vs temperature behavior is discussed in light of other Ti-alloy systems.