A lattice parameter study of the precipitates in Al-rich Al-Ag-Zn alloys
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Precipitation of the metastable and equilibrium phases has been studied by X - r a y diffraction in t e r n a r y aluminum rich A1-Ag-Zn alloys. The precipitation sequence was found to be G P zones -- r -- E Lattice mismatch between the G P zones, s and E phases was found to depend upon the solute content of the alloy. The lattice mismatch between the metastable precipitates and the matrix could be altered by alloying or aging at 150~ OUR present understanding of precipitation in t e r n a r y aluminum based alloys has evolved from studies p e r formed on binary alloys as well as studies done on some t e r n a r y systems. The information obtained from the binary precipitation studies has proven to be helpful for t e r n a r y alloy design; however, precipitation in t e r n a r y alloys is usually so complex that the extension of our knowledge from binary alloys to t e r n a r y alloys is by no means straightforward. This investigation was initiated in order to study several aspects of precipitation in A1-Ag-Zn alloys. The principal purpose of our study was to develop a basis for understanding precipitation in a model t e r n a r y alloy system. Following similar reasoning, Bates and Gould have previously investigated z several A1-Ag-Zn alloys and were able to show that: 1) The precipitation sequence is: GP zones--~ e ' ~ E. 2) Only one type of GP zone was present in the alloys which were studied. 3) There is probably a distortion of the matrix in the vicinity of the GP zones. 4) Both e' and ~ are hcp. Another study by K6ster e t al ~ indicated the following: 1) The equilibrium h c p e phase exists over a wide range of temperatures and compositions. 2) The phase has lattice p a r a m e t e r s which monotonically dec r e a s e with increasing alloy content of the precipitate. In light of the above two studies it appeared that by measuring the lattice p a r a m e t e r s of the precipitates in A1-Ag-Zn alloys it would be possible to develop a further insight into precipitation in a relatively simple t e r n a r y alloy system.
I. ALLOY DESIGN CONSIDERATIONS Since it is known that the F e r m i energy is a factor which can influence phase stability, s five alloys were prepared having a constant electron concentration. There was an indication that electron concentration had some influence on phase stability in binary A1-Ag and A1-Zn alloys ;4 Fig. 1 shows the apparent c o r r e l a tion for the binary phase diagrams when based on electron concentration. The five alloys used in the investigation were p r e pared at 2.9 e / a by using valences of the various cornD. E. PASSOJA is Research Scientist, Union Carbide Corporation,
Tarrytown Technical Center, Tarrytown, N.Y. 10591. S. POPOVI(~is Scientist, Institute Ruder Boskovic,Zagreb, Yugoslavia.P. BARRAND, formerly Lecturer at the Universityof Manchester, Manchester, England is deceased. Manuscript submitted April 17, 1973. METALLURGICAL TRANSACTIONS
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