Phase formation kinetics of titanium aluminides produced from elemental powder mixtures

An elemental powder composite Ti-48 at.% Al was prepared by cold extrusion. The kinetics of annealing-induced phase formation at 800 C was studied using quantitative X-ray diffraction, and the results were analyzed in terms of a one-dimensional multiphase diffusion model. During heating to 800 C, a reaction takes place which results in a multiphase mixture containing Ti_3Al, TiAl, Al_2Ti and Al_3Ti. Subsequent annealing at 800 C leads to the growth of the phases TiAl and Al_2Ti at the expense of Ti_3Al and Al_3Ti. In the competitive growth stage, the diffusion through Al_2Ti is 1.4 times quicker than through TiAl. After consumption of Al_3Ti, the growth of TiAl at the expense of Al_2Ti and Ti_3Al can be described by a classical #sq root#t-law where t is the annealing time. Due to the statistical nature of the original microstructure, the final stages of TiAl growth are slowed down according to Johnson-Mehl-Avrami growth kinetics with exponent n=0.61. (orig.)Reprint from: Z. Metallkd. v. 88(2), 1997, p. 125-130Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Phase equilibria involving the #tau#-L1_2 and TiAl_2 phases in the Ti-Al-Cr system

An investigation into the phase equilibria of the Al-rich portion of the Ti-Al-Cr ternary system was carried out using both bulk alloy and diffusion couple samples. Combination of the data from the samples enabled the construction of partial isothermal sections at 1000 and 800 C. Further, the Ti(Cr, Al)_2+#tau##->##beta#(Cr)+TiAl_2 invariant reaction was found to occur between 1000 and 800 C. In addition to the above reaction a precursor invariant reaction, TiAl+#tau##->#Ti(Cr, Al)_2+TiAl_2, also occurs between those temperatures. Due to the recession of the #beta#(Cr) phase with decreasing temperature and the formation of the Cr_2Al phase, the #beta#(Cr)+#tau##->#Cr_2Al+Al_8Cr_5 invariant reaction was observed at about 890 C. (orig.)Special print from: Intermetallics 4(1996), p. 543-556. Published in 1996 by Elsevier (Netherlands)Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman