Part III. The tensile behavior of Ti-Al-Nb O+Bcc orthorhombic alloys
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s have involved the microstructure–tensile property relations for orthorhombic (O; Cmcm symmetry based on Ti2AlNb) alloys.[1–26] Work on alloys containing both the bcc and ␣2 (hcp Ti3Al, D019 structure) phases, in addition to the O phase, has noted phase volumefraction effects on room-temperature (RT) tensile properties.[9–13,21] In these studies and others,[5,22] it was discovered that adjacent ␣2 grain boundaries provide crack initiation sites, which lead to failure and are detrimental to strength and elongation. In one study,[21] a greater than 2.5 times increase in elongation to failure (f) was achieved, without a loss in strength, by reducing the volume fraction of ␣2 phase and consequently reducing the concentration of ␣2/␣2 grain boundaries through post-processing heat treatment. C.J. BOEHLERT, formerly a postdoctoral research associate with Los Alamos National Laboratory, Los Alamos, NM 87545, is Assistant Professor, New York State College of Ceramics, School of Ceramic Engineering and Materials Science, 2 Pine Street, Alfred, NY 14802. Manuscript submitted August 7, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
Similar to the ␣2 phase, cracks typically nucleate at equiaxed-O/equiaxed-O grain boundaries for O-dominated microstructures, and intergranular fracture results. This leads to poor RT fracture toughness, of the order of 6 MPa 冪m,[2] and f values of approximately 1 pct.[20,23,24] On the other hand, due to its crack-blunting capability and ductile dimpling characteristics along with its wavy slip character, the bcc phase plays a critical role in imparting ductility to O alloys. It is in this respect that the bcc phase provides a unique combination of both strengthening and toughening for O alloys. In addition, transmission of slip from the O to the bcc phase has been found between adjacent O and bcc grains. Slip traces have been shown to transmit from O to bcc to O without deflection, as well as to be jogged at bcc laths before transmission back to O.[9,10] Slip transmission is thought to reduce stress-concentration effects, which are usually heightened at grain boundaries, and this is partially responsible for the significantly reduced grainboundary cracking at adjacent O/bcc grains compared with O/O grains. Along with differing slip characteristics, the slip systems of the of the ␣2 and O phases differ significantly from that for bcc. While the bcc slip character is wavy, planar slip is VOLUME 32A, AUGUST 2001—1977
Table I. Orthorhombic Alloy RT Tensile Properties Alloy Composition (Reference) [21]
Ti-23Al-16Nb Ti-22Al-23Nb[13] Ti-23Al-23Nb[16] Ti-22Al-20Nb-5V[4] Ti-22Al-24Nb[4] Ti-22Al-25Nb[1] Ti-22Al-25Nb[1] Ti-22Al-27Nb[1] Ti-25Al-21Nb[1] Ti-15Al-45Nb[32]
Heat Treatment
0.2 Pct YS (MPa)
UTS (MPa)
f (Pct)
1050 ⬚C/1 h/WQ ⫹ 850 ⬚C/2 h/FC 1050 ⬚C/2 h ⫹ 815 ⬚C/8 h/FC 760 ⬚C/100 h 815 ⬚C/24 h ⫹ 760 ⬚C/100 h 815 ⬚C/4 h 1000 ⬚C/1 h/Ar ⫹ 815 ⬚C/2 h/Ar 1125 ⬚C/1 h/BC ⫹ 815 ⬚C/2 h/Ar 815 ⬚C/1 h/Ar 1050 ⬚C/1 h/Ar ⫹ 815 ⬚C/2 h/Ar 1050 ⬚C/4 h ⫹ 800 ⬚C/24 h
691 836 472 900 1257 1245 1134 1294 847
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