JOURNAL OF MATERIALS SCIENCE LETTERS 18 (1 9 9 9 ) 927 – 929 Diffusion ledge mechanism of massive γ transformation in quenched TiAl alloys J. G. LIN, C. E. WEN, Y. G. ZHANG, C. Q. CHEN Department of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, People’s Republic of China TiAl intermetallic alloys, with low density, superior strength, high creep and oxidation resistance at elevated temperature, has attracted a great deal of attention from the aerospace community and the automobile industry. In recent years, a large number of research has been car- ried out to explore the effect of microstructure changes on mechanical properties. Among the transformations occurring in TiAl alloy, one is the massive transforma- tion from α to γ . Many authors have reported the mas- sive transformation in TiAl alloys and discussed the mechanism by which it develops [1–3]. However, there still are a number of controversial issues, such as the mechanism of nucleation and growth; The orien- tation relationship and the nature of interface between the product massive phase and the parent phase being consumed and the development of the structure defects. In this paper, the mechanism of the massive transfor- mation of Ti-48at %Al alloys was discussed based on the experimental observation and results analyses. The results indicate that the massive transformation is a dif- fusion ledge mechanism. The samples of nominal compositions Ti-48at %Al were prepared by none-consumable arc melting of sponge Ti and high purity Al. The buttons were ho- mogenized at 1473 K for 48 h followed by furnace cooling. Samples for heat treatment were encapsulated in quartz capsules to a vacuum of about 1 × 10 −3 Pa and back-filled with argon to 1 × 10 5 Pa. They were solution treated at 1638 K for 1 h and then quenched into the brine. Foils for electron transmission microscopy (TEM) observation were prepared by cutting slices from quenched samples and then ground down to a thick- ness of about 200 m, and final electropolishing was performed in a twin-jet polisher with a voltage of 30 V at −35 ◦ C. Scanning electron microscope (SEM) and electron microprobe analyses were carried out on S-530, and TEM observations were conducted on a H-800 transmission electron microscope. Fig. 1 is the optical micrograph of the sample after heat treated in the single phase field at 1683 K for 1 h followed by brine quenched. Two different regions are shown in the optical micrograph. One is dark and deeply etched, containing different morphologies of patchy phase, and the other is the bright. The TEM observation confirms that the dark patchy microstructures are mas- sive γ phase (γ m ), while the bright matrices are α. It was also found that a large number of defects are in γ m , including microtwins, anti-phase boundaries (APBs), stacking faults (SF), and dislocations. The lattice parameters of γ m are determined by TEM to be a = 4.06 A, c = 4.138 A and c/a = 1.009. In com- parison with that of the stoichiometrical TiAl, which are a = 4.0, c = 4.06 and c/a = 1.015 [4]. It indicated that during quenching process, equilibrium γ phases were suppressed due to the rapid cooling rate, and a new massive γ with slightly smaller c/a ratio was formed. The composition of regions across the α/γ interface over a distance of about 600 μm was detected by micro- probe analysis. The result reveals that the compositions of the two phases are practically identical, as shown in Fig. 2. It indicates that, during the α → γ m transforma- tion, the crystal structure of parent phase changes to Figure 1 Optical micrograph of the sample heat treated at 1683 K for 1 h and brine quenched. Figure 2 Composition of the regions cross the interface of γ m /α. 0261–8028 C  1999 Kluwer Academic Publishers 927