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A. F. CRAWLEYand B. LAGOWSKIare Research Scientists, NonFerrous Metals Section, PhysicalMetallurgy Division, Mines Branch, Department of Energy, Minesand Resources,Ottawa, Canada. Manuscript submitted July 10, 1973. METALLURGICAL TRANSACTIONS

trolyte was - 55~ During polishing, the discs were held in platinum-tipped tweezers. The polished foils were examined in a Philtps EM300 electron microscope equipped with a goniometer stage and double-tilt holder. Selected a r e a diffraction patterns of the matrix and individual precipitates were photographed and measured. A computerized analysis of electron diffraction patterns developed by Milliken 5 was used to determine the precipitate-matrix orientation relationship. The output data from this program whose use is described elsewhere, 8 also includes in-

Fig. 1--Electron m i c r o g r a p h of AZ91 after aging for 96 h at 100~ followed by 192 h at 140~ Photograph shows two r e gions of precipitation--continuous on the left and discontinuous on the right.

Fig. 2 - - E l e c t r o n m i c r o g r a p h of AZ91 after aging for 96 h at 100~ followed by 192 h at 140~ showing continuous p r e cipitation. VOLUME 5, APRIL 1974-949

formation from which the lattice s y m m e t r y and ceil dimensions can be determined to within 0.5 pct. The results of the micrographic examination show that in AZ91, as in A9, 3 two types of precipitates appear. These are: 1) general or continuous precipitates which grow by heterogeneous nucleation; this precipitate is responsible for age hardening; and 2) discontinuous precipitates which nucleate at high angle grain boundaries and grow by a diffusion mechanism; this type of precipitate, massive in form, is detrimental to age hardening of the alloy. The precipitate in both cases is the bcc Mg~TAI~2. No evidence of a precipitate of another composition was found. Fig. 1 shows an electron micrograph of a double-

aged specimen where regions of two types of precipitation are evident--the platelets of continuous precipitate and the lamellar discontinuous precipitate. The matrix of the discontinuous precipitate appears lighter because of preferential thinning during polishing. Figs. 2 and 3(a) present electron mtcrographs of general precipitation in a double-aged alloy (Fig. 2) and the alloy aged at 140~ (Fig. 3(a)). The distinct difference in precipitate size and distribution are immediately obvious. Subsequent examination of the p r e cipitates showed that two step aging did not result in a change in the type of orientation relationship between precipitate and matrix. By selected area diffraction, the precipitate-matrix orientation relationships were established. The general precipitate forms as platelets with a basal growth habit. Using the computer program, the precise orientation relationship between the bcc precipitate and the close-packed hexagonal magnesium matrix was determined as [lll]pp t II [2ii0]matrix (0il)pp t N(0001)matrix Fig. 3(b) shows the diffraction patterns of a [2ii0]matri x and a [lll]ppt zone axes. It should be noted that by t