Reaction Scheme and Liquidus Surface of the Ternary System Aluminum-Chromium-Titanium
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TRODUCTION
THE importance of Al-Cr-Ti alloys for many applications led to a steady interest in the constitution of the Al-Cr-Ti system and needs no introduction. Thus, isothermal sections over the entire composition range are available in the literature for 600 C,[1] 800 C,[2] 1000 C,[2–5] 1050 C,[6] and 1150 C.[4,5] The partial isothermal sections reported encompass even the temperature range between 497 C (Al corner)[7] and 1400 C (Ti corner).[8] The only ternary phase, cubic L12-type s, has at 1150 C a composition range 56 to 66 at. pct Al and 6 to 14 at. pct Cr.[5] The binary high-temperature C14type Laves phase cCr2Ti is stabilized by small additions of Al (which essentially substitutes for Cr) down to all temperatures investigated. The maximum solubility of aluminum in cCr2Ti(C14) is near 40 at. pct.[4,5] The continuous series of bcc b-phase solid solutions b(Ti,Cr,Al) existing at high temperatures splits at lower temperatures into three branches: b(Ti,Al) and b(Cr) with W-type crystal structure as well as a ternary CsCl-type ordered phase labeled b(B2). The agreement between these numerous investigations generally is very high. On the other hand, for >50 at. pct Al, qualitatively disagreeing liquidus surface projections are reported by HAILIN CHEN, Graduate Student, and YONG DU, University Professor, are with the State Key Lab for Powder Metallurgy, Central South University, 410083 Changsha, Hunan, People’s Republic of China. FRANZ WEITZER, Assistant Professor, and JULIUS C. SCHUSTER, University Professor, are with the Innovative Materials Group, Universita¨t Wien, 1090 Wien, Austria. Contact e-mail: [email protected] NATALIYA KRENDELSBERGER, Senior Research Fellow, is with the CEST–Center of Electrochemical Surface Technology, 2700 Wr. Neustadt, Austria. Manuscript submitted February 6, 2009. Article published online October 9, 2009 2980—VOLUME 40A, DECEMBER 2009
Shao and Tsakiropoulos[9] and Raghavan,[1] respectively. In the article of Shao and Tsakiropoulos, the ternary phase s is shown to form peritectically from L + TiAl + Ti2Al5, while Raghavan proposed a reaction L + b(Ti,Cr,Al) + Ti5Al11. In the binary system Ti-Al accepted for the present article,[10] both Ti2Al5 and Ti5Al11 are considered to be at high temperature part of the TiAl homogeneity region. Thus, the formation of s has to be either from L + TiAl + TiAl3 (according to Shao and Tsakiropoulos[9]) or from L + TiAl + b(Ti,Cr,Al) (according to Raghavan[1]). The invariant temperature for this reaction is given by Raghavan as 1370 C. This apparently was read from the vertical section at 25 at. pct Ti by Mabuchi et al.[11] The only other temperatures for invariant reactions reported were 1342 C for L + Ti7Al3 = TiAl3 + s (with the presently accepted Ti-Al binary, this should read L + TiAl = TiAl3 + s) and 1183 C for L + s = TiAl3 + Cr9Al17.[12] The last phase is labeled c1(Cr,Al) in the following. The present article started with the aim to determine the correct reaction scheme and accurate temperatures for the invariant reac
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