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

Abstract

The tensile behavior of Ti-Al-Nb alloys with Al concentrations between 12 and 26 at. pct and Nb concentrations between 22 and 38 at. pct has been investigated for temperatures between 25 ЊC and 650 ЊC. Several microstructural features were evaluated in an attempt to identify microstructureproperty 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 singlephase microstructures, and the results provided insight into the microstructure-property relationships of the two-phase orthorhombic (O) ϩ body-centered-cubic (bcc) microstructures. The disordered fully-bcc (␤) Ti-12Al-38Nb microstructure, produced through heat treatment above the ␤-transus, exhibited a room-temperature (RT) elongation of more than 27 pct 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-Ti 2 AlNb alloys, exhibited fracture strengths up to 672 MPa and low elongations-to-failure ( f Յ 0.6 pct). Thus, increasing the Al content, which favors ordering of the bcc structure, significantly reduces the ductility of the bcc phase. Similar to the ordered B2 microstructure, the ordered fully-O Ti 2 AlNb microstructures exhibited intermediate RT strength (Յ 704 MPa) and f (Յ1 pct). The O ϩ 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 twophase microstructures compared to their single-phase counterparts. A RT of 1125 MPa was measured for the finest-grained two-phase microstructure. The O ϩ bcc microstructures containing greater bccphase 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 f for the Ti-12Al-38Nb alloy. However, large RT elongations ( f Ͼ 12 pct) were maintained for aged Ti-12Al-38Nb microstructures, which contained 28 vol pct 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 ϩ bcc microstructures achieve attractive strengths and elongations. The YS vs temperature behavior is discussed in light of other Ti-alloy systems.

Effect of Hot Isostatic Pressing and Heat Treatment on Microstructure and Mechanical Properties of Pm Ti‐22Al‐24Nb‐0.5Mo (At.%) Alloy

“…In this study, we choose HIP scheme as 1030°C/140MPa/3h. act as the barrier [12][13][14][15] to hinder the growth of B2 phase. The phase constituents are 2 , O and B2 phase when the solution temperature is below 930 o C, O phase formed preferentially as a rim around the 2 phase and is lamellar inside the B2 phase.…”

“…Higher volume fraction of B2 phase is preferred because the strength of B2 is stronger than that of 2 and O phase. For the equiaxed microstructure, room temperature strength is controlled by two factors, (1) the ordering of B2 phase, (2) B2 phase volume fraction [12][13]15] . Room temperature strength increases when solution temperature increases to the range of 900~1020 o C, because the ordering degree and volume fraction of B2 phase increase.…”

“…B2 phase in solution state Ti 2 AlNb alloys is metastable; to improve microstructure stability, aging treatment is usually suggested for this kind of intermetallic alloy. Aging treatment can also increase the comprehensive mechanical properties of Ti 2 AlNb alloys such as creep and fatigue properties [12][13]15] . In this study, as-HIP'ed compacts are solution treated at 980 o C, held for 2 hours, air cooled followed by aging treatment.…”

“…Detailed aging conditions are listed in Table 1. There are two different kinds of reversible phase transition ( 2 B2 and O B2) [12][13]15] during aging heat treatment. The 2 phase has high strength but low ductility.…”

The Effect of Molybdenum on the Microstructure and Creep Behavior of Ti-24Al-17Nb-xMo (at %) and its Composites

“…Orthorhombic-based intermetallic alloys, such as Ti-25Al-17Nb (at%), have displayed excellent intermediate-temperature strength [1] and creep resistance [2]. However, further improvement in creep behavior may be obtained by altering the microstructure.…”