Scripta METALLURGICA Vol. 23, pp. 327-331, 1989 Pergamon Press plc Printed in the U.S.A. HIGH TEMPERATURE PHASE EQUILIBRIA OF THE Lip COMPOSITION IN THE AI-Ti-Ni, AI-Ti-Fe, AND AI-Ti-C~ SYSTEMS S. Mazdiyasni, D. B. Miracle, and D. M. Dimiduk,* M. G. Mendiratta and P. R. Subramanian** *AFWAL/MLLM, Wright-Patterson AFB, Ohio 45433-6533 **Universal Energy Systems, Inc., Dayton, Ohio 45432-1894 (Received October 19, 1988) (Revlsed December 15, 1988) Introduction The addition of a few atomic percent of selected transition metal elements to the inter- metallic compound Al3Ti has been shown to destabilize this compound's D022 crystal structure in favor of the Ll2 structure (I-8). In the AI-Ti-Ni, AI-Ti-Fe and AI-Ti-Cu systems (referred to as the Ni, Fe and Cu systems, respectively, throughout this paper), no experimental data have been published on the stability or extent of the Ll 2 phase field above 800°C. The primary interest in this investigation was to verify the extent of the Ll2-type phases in the Ni, Fe, and Cu systems, respectively, at 800°C, and to establish the extent of these phase fields at 1200°C. The phase fields surrounding the Ll 2 compositions at 1200°C have also been investi- gated. Experimental Procedure Small (50-10O g) ingots of Ni, Fe and Cu alloys of the compositions given in Table l were made by the nonconsumable vacuum arc melting of elemental constituents on a water-cooled copper hearth. In the Ni and Fe systems, the 1200°C isotherms were determined by heat treating the arc cast ingots at 1200°C for 500 h in argon (Ar) and quenching. Identical compositions were obtained on alloys exposed at 1200°C for only lO0 hours. Therefore, in the Cu system, the alloys were heat treated at 1200°C for lOO h and quenched. For the 800°C isotherm, the Ni, Fe, and Cu alloys heat treated at 1200°C were given an additional heat treatment in Ar at 800°C for 300 h and quenched. Cross-sections of the specimens were prepared for metallographic examina- tion and the compositions of the various phases were determined by electron probe micro- analysis. Intensity corrections were made with a standard ZAF iterative correction program. Eight to ten individual analyses were done for each phase. The standard deviation of each sampling was found to be < l at.% as calculated for the operating conditions used. Impurity analyses showed only mino-r trace elements. Oxygen and nitrogen levels were each maintained below 500 ppm by weight. X-ray diffraction was conducted on powder samples of the single phase compositions. The powders were annealed at lO00°C prior to x-ray analysis to remove any strain induced during preparation of the powders. Results and Discussion Figures l(a) - l(c) show the Al-rich corners of the Ni, Fe, and Cu equilibrium diagrams determined from this investigation. The oval-shaped regions outlined by a solid line represent the extent of the Ll2 phase fields in the Ni, Fe, and Cu systems, respectively, at 1200°C. The inner dashed circle in the Fe system represents the extent of the Ll 2 phase field estimated at 800°C. The dotted lines separating the Ll2 phase boundary from the liquid phase boundary in the Fe system are two-phase tie lines. In the Ni system, precipitation within the Ll 2 phase at 800°C occurred on such a fine scale [Figures 2(a) and 2(b)] that exact determination of the phase boundary was not possible. This observation suggests that the width of the phase field decreases with a decrease in temperature. A similar decrease in the Ll 2 phase field width was observed in the Fe system. However, only alloy Fe-3 exhibited a structure too fine for accurate quantitative analysis [Figures 3(a) and 3(b)]. The Ll2 phase field not only decreases in size, but also shifts toward higher content Al as the temperature is lowered in the Fe system. Data at 800°C in the Cu system are not yet complete. 327 0036-9748/89 $3.00 + .00