Effects of TiFe Intermetallic Compounds on the Tensile Behavior of Ti-4Al-4Fe-0.25Si Alloy
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Al-Fe (a + b) titanium alloys are of interest for both the aerospace and biomedical industries. These alloys have been developed to replace Ti-6Al-4V, which is currently the most widely used titanium alloy in these sectors. The addition of Fe instead of V results in a better cost efficiency. Developments in Ti-Al-Fe (a + b) alloys, such as TIMETAL 62S (Ti-6Al-2Fe-0.1Si),[1] Tikrutan LT35 (Ti-5Al-2.5Fe),[1] and Super-TIX 51AF (Ti-5Al-1Fe)[2] have resulted in comparable or better mechanical properties relative to Ti-6Al-4V. The tensile behavior of Ti-Al-Fe (a + b) alloys is strongly dependent on processing and heat treatment methods of solution treatment (ST) and aging (STA). Metastable b (BCC structure) leads to b fi a¢
SANG WON LEE and CHAN HEE PARK, Senior Researchers, and JEA KEUN HONG and JONG-TAEK YEOM, Principal Researchers, are with the Titanium Alloys Department, Metal Materials Division, Korea Institute of Materials Science (KIMS), Changwon 642-831, Republic of Korea. Contact e-mail: geegee@ kims.re.kr KYONG MIN KIM, Ph.D. Student, is with the Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki, 305 8573, Japan. DONALD S. SHIH, Technical Fellow, is with Boeing Research & Technology, The Boeing Company, St. Louis, MO, 63166-0516. Manuscript submitted July 20, 2016. Article published online November 16, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
(hexagonal structure), b fi a¢¢ (orthorhombic structure) and b fi x (metastable hexagonal structure) transformations during water quenching.[2] In addition, a TiFe intermetallic compound (B2, ordered BCC structure) can nucleate during aging at temperatures lower than 873 K (600 C). The presence of TiFe in an alloy increases the strength but substantially decreases the ductility[3]; the nucleation of TiFe is thus avoided for Ti-Al-Fe (a + b) alloys for a more desirable balance of properties.[4] b Ti-Fe alloys exhibiting a eutectic transformation (b fi b+TiFe) have previously demonstrated extremely high strengths.[5–7] A high concentration of Fe (>20 pct) is required for the b fi b + TiFe transformation to occur because of the high solubility of Fe in this b phase. Contieri et al.[5] reported the compressive behavior of a Ti-32.5Fe alloy with a lamellar structure via b fi b + TiFe. As the interphase spacing between b and TiFe decreased from 1.5 to 0.7 lm, the strength increased substantially from 1844 to 3000 MPa. Zeitz et al.[8] investigated the effect of morphology and distribution of TiFe on the tensile properties of Ti-(0 to 4 pct)Al-(8 to 15 pct)Fe alloys. These alloys exhibited a eutectoid transformation (b fi a + TiFe) upon aging at temperatures lower than 863 K (590 C). The alloy featuring TiFe platelets was extremely brittle at room temperature, but in contrast the presence of spheroidized and homogeneously distributed TiFe resulted in improved ductility. Very few studies are available on the effect of TiFe nucleation on the tensile properties of Ti-Al-Fe (a + b) alloys. The low Fe (Tb), 1173 K (900 C, 300 lm) when the TAFS was annealed at 122
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