| The use of high strength Al alloys for numerous aerospace applications has been severely restricted by poor elevated temperature performance. Suitable dispersed phases in high temperature Al alloys should be thermodynamically stable and exhibit partial coherency with the matrix at temperatures of interest. Minimum lattice disregistry across the precipitate/matrix boundary yields a low interfacial energy and thus, a low driving force for Ostwald ripening.; The extents of solid solubility in the equilibrium, tetragonal Al(,3)X-type dispersed phases (X represents binary and ternary combinations of Hf, Ti, V and Zr) were examined using powder x-ray diffraction methods. Minimum lattice disregistry ((delta)) with the Al(ss) matrix was achieved by maximizing the amount of Ti or V added to the Al(,3)Hf and/or Al(,3)Zr phases without forming a co-intermetallic compound exhibiting a DO(,22) crystall structure. In comparison to Al(,3)Zr, with a ((delta)) equal to 2.88%, the addition of V to Al(,3)Zr in the ratio 7:1, i.e., Al(,3)(v(,0.875)Zr(,0.125)), results in a decrease in ((delta)) by approximately 17%. Electron diffraction revealed that the addition of V to the metastable cubic (L1(,2)) Al(,3)Zr phase also resulted in a reduction in the lattice disregistry across the precipitate/matrix boundary. In comparison with the cubic Al(,3)Zr phase, with a ((delta)) of approximately 1.0%, the Al(,3)(V(,0.875)Zr(,0.125)) phase exhibits a mismatch of approximately -0.14%.; The cubic Al(,3)(V(,0.875)Zr(,0.125)) phase was observed by TEM to be substantially more stable, i.e., resist transformation to the equilibrium tetragonal phase, when compared to the cubic Al(,3)Zr phase. It is proposed that a reduction in lattice disregistry results in a decrease in the strain energy component of the system's total free energy.; A systematic decrease in the coarsening rate with a reduction in the lattice disregistry for the cubic Al(,3)Zr, Al(,3)(V(,0.725)Zr(,0.275)) and Al(,3)(V(,0.875)Zr(,0.125)) and tetragonal Al(,3)Zr and Al(,3)(V(,0.875)Zr(,0.125)) phases is proposed to be representative of a decrease in the interfacial energy across the precipitate/matrix boundary. In comparison, the coarsening rates measured for the tetragonal and cubic Al(,3)(V,Zr) phases were measured to be two and three orders of magnitude slower than the coarsening rates measured for the dispersed phases present in Alcoa's Al-Fe-Ce and Pratt & Whitney's Al-Fe-(Mo) alloys at 425(DEGREES)C. |
