Influence of Fine-Scale Alpha Precipitation on the Mechanical Properties of the Beta Titanium Alloy Beta-21S
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METAL 21S (b-21S), which has the nominal weight percent composition 15Mo, 3Al,3Nb, and 0.2Si, is a metastable beta titanium alloy that offers a unique combination of high strength, good elevated temperature properties, and extraordinary environmental degradation resistance.[1–6] It was developed by TIMET in 1988 as a foil matrix material for titanium metal matrix composites for the NASP, though the primary aerospace application has been on the Boeing 777 exhaust nozzle.[3,6] Furthermore, b-21S is known to withstand operating temperatures to 923 K (650 C)
S.A. MANTRI, Graduate Student, D. CHOUDHURI, PostDoctoral Research Assistant, and R. BANERJEE, Professor, are with the Department of Materials Science and Engineering, University of North Texas, Denton, TX 76207, and also with the Center for Advanced Research and Technology, University of North Texas, Denton, TX 76207, and also with the Center for Advanced NonFerrous Structural Alloys, University of North Texas, Denton, TX 76207. Contact e-mail: [email protected] A. BEHERA, Graduate Student, and N. KUMAR, Post-Doctoral Research Assistant, are with the Department of Materials Science and Engineering, University of North Texas. J.D. COTTON, Technical Fellow, is with the Center for Advanced Non-Ferrous Structural Alloys, University of North Texas, and also with the Metallic Materials, Boeing Research & Technology, Seattle, WA 98124. Manuscript submitted December 1, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
(for short durations), which is employed by Boeing and P&W.[6] In addition to high-temperature performance, for structural applications at primarily ambient conditions, one desires an optimum balance of strength and ductility at room temperature. To achieve such a balance, mechanical behavior of b-21S alloy has been examined via thermomechanical processing and isothermal annealing with the objective of producing finer b grain size.[3,5] Another approach for obtaining strength– ductility balance is to form fine-scale alpha (a) precipitates within the parent b matrix. According to Lutjering and Williams, a homogeneous distribution of fine-scale a can be achieved by annealing within the b + omega phase (x) stability regime [~673 K (400 C)], followed by high-temperature [e.g., ~873 K (600 C)] annealing.[7] Such fine-scale precipitation further resulted in improved mechanical properties.[1–3,6,7] The literature reports indicate that fine-scale a increases the tensile strength (~1150 to 1350 MPa) while maintaining reasonable tensile elongations (~6 to 8 pct).[1–3,6] Notably, the spread in the reported data[1–3,6] suggests the possibility of an optimum fine-scale alpha microstructure that exhibits a reasonable strength–ductility combination. Toward that end, we have compared and contrasted the mechanical response of microstructures containing fine-scale a with those containing coarser a microstructures. Earlier work on the effect of a size scales on b-21S mechanical response was primarily limited to reporting only the mechanical properties.[2,3] Therefore, this w
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