The knoop-hardness yield locus of an orthorhombic titanium aluminide alloy
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p-Hardness Yield Locus of an Orthorhombic Titanium Aluminide Alloy P.D. NICOLAOU and S.L. SEMIATIN Extensive research has been conducted recently to develop an understanding of the properties and processing of an emerging class of titanium aluminide alloys based on the orthorhombic Ti2AlNb phase. These materials possess a good combination of room- and high-temperature mechanical properties.[1,2] Microstructure evolution during various thermomechanical processes in the single- or two-phase field has been established.[3,4] The effect of the mechanical and crystallographic texture developed during primary processing on ambient and elevated temperature properties has also been investigated.[4,5] The objective of the work reported here was to establish the plastic anisotropy of a Ti-21Al-22Nb (atomic percent) sheet material in the as-fabricated condition as well as after subsequent hot-tension deformation. For this purpose, plane-stress yield loci were determined using a Knoop microhardness method and interpreted in terms of measured
P.D. NICOLAOU, Research Scientist, is with the Materials and Processes Division, UES, Inc., Dayton, OH 45432. S.L. SEMIATIN, Senior Scientist, Materials Processing/Processing Science, is with Materials and Manufacturing Directorate, Air Force Research Laboratory, AFRL/ML, Wright-Patterson Air Force Base, OH 45433. Manuscript submitted October 31, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 1—Pole figures for the ordered cubic (B2) matrix phase of the orthorhombic titanium aluminide sheet alloy.
tensile properties and aggregate theory calculations based on the measured textures in the sheet. The processing and resulting microstructure of the sheet used in this study have been described in detail elsewhere.[6] In brief, the sheet was fabricated by hot isostatic pressing of a layup of cold-rolled-and-annealed foils. Its microstructure comprised three phases—an ordered cubic beta (B2) matrix phase with approximately 20 vol pct of orthorhombic (O) phase platelets and approximately 30 vol pct of alpha-two particles elongated along the rolling direction. The beta transus of the material was determined to be 1080 7C via a series of heat treatments. Because prior work[7] has demonstrated that the plastic anisotropy of orthorhombic titanium aluminides is controlled primarily by the crystallographic texture of the ordered beta phase, pole figures for the B2 phase in the present material were measured and are summarized in Figure 1. (Pole figures for the O and alpha-two phases were also measured[5] but were not used to interpret the present results). The pole figures shown in Figure 1 were determined via an X-ray back reflection technique (using copper Ka radiation) with data analysis performed using the computer program POPLA.[8] The textures exhibit a moderate (4.4 3 random) cube-type component with the (002) planes lying in the plane of the sheet with a ‘‘smearing’’ of approximately 530 deg. There is also a relatively weak (211) texture component. The Knoop microhardness method[9] was use
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