Processing, Microstructure, Texture, and Tensile Properties of the Ti-6Al-4V-1.55B Eutectic Alloy
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STRUCTURALLY efficient materials provide a direct path for reducing mass via substitutions with thinner and lighter components, thus improving performance and affordability. The requirement for higher structural efficiency (a combination of specific stiffness and specific strength) provides motivation for the development of titanium (Ti) materials and processes for aerospace applications. Boron (B) is the most efficient stiffener of Ti alloys, and addition of 0.5 B gives the same modulus improvement as 6.5Al (all compositions are in weight percent).[1] The modulus increase is due to the stiff, strong TiB phase that precipitates in situ upon B addition. The TiB is thermodynamically and thermally stable in Ti-rich alloys, so that mechanical property improvements derived from TiB are not expected to degrade with extended exposure at elevated temperatures. Other advantages of the TiB phase in Ti alloy are that its coefficient of thermal OREST M. IVASISHIN, Associate Director, ROMAN V. TELIOVYCH, Head, Laboratory of Heat Treatment of Steels and Titanium Alloys, VOLODYMYR G. IVANCHENKO, Head, Department of Phase Equilibria, are with Institute for Metal Physics, Kiev, Ukraine. SESHACHARYULU TAMIRISAKANDALA, Senior Scientist, is with FMW Composite Systems, Inc., Bridgeport, WV 26330. Contact e-mail: seshacharyulu.tamirisakandala.ctr@ wpafb.af.mil DANIEL B. MIRACLE, Senior Scientist, is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, OH 45433. Manuscript submitted March 21, 2007. Article published online December 28, 2007 402—VOLUME 39A, FEBRUARY 2008
expansion matches that of Ti so that residual stresses are not generated due to thermal cycling, and the density of TiB is essentially equal to that of Ti, which eliminates segregation during casting and does not cause weight increase penalty. Boron is completely soluble in the liquid phase of Ti but has negligible solid solubility in the hightemperature bcc b and low-temperature hcp a-Ti phases.[2] The intermetallic TiB phase forms via the eutectic reaction L fi b + TiB as shown in the Ti-rich end of the binary Ti-B phase diagram[3] (Figure 1). Boron-modified Ti alloys, denoted generally as Ti-B alloys, can thus be classified as hypoeutectic, eutectic, or hypereutectic.[4] Increasing the B content increases strength and stiffness due to a higher volume fraction of TiB. The ductility and fracture toughness of the base alloy is retained for hypo-eutectic Ti-B alloys,[5] but the ductility decreases in hypereutectic Ti-B alloys as a result of fracture that initiates at large, primary TiB particles that grow unconstrained in the L + TiB phase field to sizes as large as 100 to 500 lm.[6] Therefore, hypoeutectic alloys with B concentration as close to the eutectic composition as possible are of specific interest for fracture-critical applications, since they allow for maximal structural efficiency without sacrificing fracture related properties. An additional advantage of eutectic Ti-B alloys is the ability to produce aligned or unidirectionally grow
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