Effect of Solutes on Phase Stability in A13 Nb

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EFFECT OF SOLUTES ON PHASE STABILITY IN A13 Nb P..R. SUBRAMANIAN*, J. P. SIMMONS*", M. G. MENDIRATTA*, AND D. M. DIMIDUK*** *Universal Energy Systems, Inc., Dayton, OH 45432-1894 "**Carnegie-Mellon University, Pittsburgh, PA 12513 ***AFWAL/MLLM, Wright-Patterson AFB, OH 45433-6533

ABSTRACT Alloying transition metal trialuminides such as A13 Nb with appropriate ternary elements may stabilize the ordered cubic L12 structure relative to the DO22 structure. This may enhance the symmetry related contributions to plastic flow, thereby aiding in overcoming the low ductility in the trialuminide systems. The present investigation was directed towards exploring the potential for obtaining new ternary Al 3 Nb-based intermetallics with the L12 structure. The effects of various ternary elements on the phase stability of A13 Nb were studied by SEM, XRD, and EPMA. Results of the A13 Nb-X phase relationship studies are presented. Preliminary microhardness results on the ternary DO 22 AI 3 Nb-X phases are also reported. The predictive capabilities of Pettifor's structural stability scheme are evaluated in the light of the present investigation.

INTRODUCTION Transition metal trialuminide based intermetallic compounds may offer a combination of properties which are of interest for elevated temperature structural applications. A major factor limiting the development of these binary intermetallics is their apparently poor low-temperature ductility [1,21, which could be partially attributed to the low symmetry DO 22 crystal structure. Work on a range of alloy systems, in general, has demonstrated that the deficiency in low temperature ductility may be overcome by macroalloying to change to a high symmetry crystal structure. For example, alloying of the DO 2 2 trialuminide A1 3 Ti with appropriate ternary elements is known to result in ternary A13 Tibased L12 phases, some of which show an improvement in mechanical properties [3]. Semi-empirical schemes such as those proposed by Villars [4] and Pettifor [5] offer the possibility of stabilizing high symmetry crystal structures by suitable alloying additions. Ternary additions can also be utilized to produce fine, two-phase microstructures, which may provide an attractive combination of mechanical properties at elevated temperatures. The objectives of the current efforts were: (1) to explore the possibility of synthesizing ternary Al 3 Nb-based compounds with the ordered cubic L12 structure, (2) to examine the possibility of. forming two-phase alloys with a fine eutectic microstructure, (3) to ascertain the effect of ternary additions on the extent of the DO22 AI 3 Nb phase field, and (4) to investigate solution softening of DO2 2 A13 Nb. This paper represents a progress report of our on-going investigations on selected Al-Nb-X systems. Mat. Res. Soc. Symp. Proc. Vol. 133.

-1989

Materials Research Society

52

EXPERIMENTAL Table I lists the Al-Nb-X compositions selected for the phase stability investigations. In each system, two alloy compositions were chosen close to the trialuminide