Effect of Boron on the Elevated-Temperature Tensile and Creep Behavior of Cast Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Weight Percent)
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TRODUCTION
TITANIUM alloys are among the most heavily used metallic alloys for critical components in jet engines. Titanium is an attractive choice because of its low density, high specific strength and modulus, good creep and fatigue behavior, and good fracture toughness, along with its ability to be processed economically using conventional methods. Ti-6Al-2Sn-4Zr-2Mo-0.1Si is used for parts and cases of jet engine compressors and also for aircraft skin components.[1] As design demands continue to increase for jet engines, there is a need to improve the physical and mechanical properties of Ti alloys. The presence of trace levels of impurities can have a profound effect on the elevated-temperature creep behavior of Ti alloys. In particular, the following impurities have a significant effect on the creep resistance of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (wt pct): Si, Fe, O, and Ni.[2–6] Small amounts of boron have also significantly improved the creep resistance of as-cast Ti-6Al-4V.[7,8] In fact, conventional Ti alloys containing small additions of B have received considerable interest due to their attractive mechanical properties, including high roomtemperature specific stiffness and strength along with reasonable elongation to failure (ef).[9–17] Recent work has shown that the addition of small amounts (0.1 W. CHEN, Graduate Student, and C.J. BOEHLERT, Associate Professor, are with the Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824-1226. Contact e-mail: [email protected] Manuscript submitted August 27, 2008. Article published online May 12, 2009 1568—VOLUME 40A, JULY 2009
wt pct) of B to Ti-6Al-2Sn-4Zr-2Mo-0.1Si (wt pct)* and *Henceforth, all alloy compositions are given in weight percent.
Ti-6Al-4V decreases the as-cast grain size by approximately an order of magnitude.[12] Boron is an innoculant that provides heterogeneous nucleation sites leading to smaller grain sizes.[12] In fact, the average grain size decreased from approximately 1700 lm for the Ti-6Al4V to approximately 200 lm for Ti-6Al-4V-1B (wt pct). This drastic reduction in the as-cast grain size leads to significant benefits, including increasing yield strength. The significant increase in strength and stiffness in B-modified Ti alloys arises from the strong and stiff TiB whiskers that precipitate in-situ during solidification.[11,15,18] The TiB-phase precipitates form with B additions greater than 0.02 wt pct.[12] Due to the similarity of the thermal expansion coefficients (8.6 9 10 6/C for Ti[19] and 7. 5 9 10 6/C for TiB[19]), only small residual stresses result at the TiB/Ti interface, which does not exhibit significant reactivity,[20] after the cooldown from processing. Small B additions do not result in significant density changes; thus, the higher strength and stiffness of B-modified Ti alloys provides important increases in specific strength and stiffness. Although B-modified Ti alloys are currently used for various ambient-temperature commercial applications (e.g., intake and exhaust valves of automotive e
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