Crystallographic relationships for the Lee-Leighly mechanism in UAI 4
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IN a metallographic study of aluminum-uranium alloys, Bramfitt and Leighly ~observed crystals of UAI 4 that were derived from the peritectic reaction of UA13 + Liq --o UA1a. These crystals frequently contained angularly shaped cores, which were thought originally to be UA13, because such cores often occur in alloys in which the peritectic reaction is incomplete. However, it was determined by microprobe analyses that the cores had the composition of an aluminum rich terminal solid solution. Kaufman and Gordon 2 demonstrated that UAI 3 is cubic and isotypic with C u 3 A u and has four atoms per unit cell. Boric 3 determined that UA14 has an orthorhombic unit cell containing 20 atoms. Boric 3 suggested that UA13 could be transformed to UA14 if one separated the UA13 crystals into slabs on nominal (011)u,a ~ planes. After this separation, the slabs can be moved m a (211)UAh direction. To achieve the correct stoichiometry, aluminum atoms must diffuse into a crystal along the (011)u~a3planes to form planes of aluminum atoms that approximate the { 110) face centered cubic plane as found in pure aluminum. This plane of atoms produces the eight-fold aluminum atoms referred to by Borie, 3 and the planes of aluminum atoms give the displaced slabs of UA13 the continuity to form UA14 crystals. Lee and Leighly4 studied the morphology of UA14 and found that the typical UA14 crystal tends to be needle-like with a'oo^aparallel to the long axis. The cores described above usually appear when sections are cut through the crystals normal or nearly normal to the long axes. Depending on the angle between the sectioning plane and the long axis of a crystal, a core will have a shape of a square, a rectangle, a rhombus, or a parallelogram. A section cut parallel to the long axis will reveal a form that has a rather sharp point at one end and a reentrant angle at the other. Lee and Leighly,4 who proposed a mechanism to explain this observed morphology, showed that the mechanism involves a shear step, which is followed by the diffusion of aluminum into the crystal. In this manner, the proper stoichiometry and crystallographic relationships ae attained. This mechanism substantiates the model proposed by Borie. 3 H. P. LEIGHLY, JR. is Professor of Metallurgical Engineering and D. R. EDWARDS is Professor of Nuclear Engineering at University of Missouri-Rolla, Rolla, MO 65401. Manuscript submitted April 3, 1979. METALLURGICAL TRANSACTIONS A
THE M E C H A N I S M Figure 1 is a section through a UA13 lattice normal to the (100)u~3 direction. The axes Xi, X2, and X3 form the orthogonal axis system from the UA13 lattice, whereas X~, X~, and X~ represent the axis system for the UA14 lattice. The intercepts of the (111)u~a, and (1 ]-l)uAt3 planes with the (100)UA~3 plane are labeled BB. The [01]-]Uhl3 and the [011]uA13 directions in the (100)~A~3 plane of the aluminum atom are parallel to b'o,~ and Co'uA~4 respectively. In the mechanism, slabs of the UAI 3 crystals separate into blocks of atoms along planes parallel to nominal (011)Uh~~planes. The tra
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