Epitaxial formation of AlNi on Al 3 Ni during electropolishing
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T H E final thinning of metallic specimens for t r a n s mission electron microscopy is generally accomplished by electropolishing. This technique does not present any difficulties for single component m a t e r i a l s , but for multicomponent alloys t h e r e is a possibility that the less noble component will be preferentially dissolved, leading to the formation at the specimen surfaces of m a t e r i a l which may differ in composition and s t r u c ture from the original material, 1-4 This possibility must therefore be considered in the interpretation of electron mierographs of multicomponent alloys. In the present paper w e describe such an effect, observed in the preparation of thin foils of an A1-A13Ni eutectic alloy. EXPERIMENTAL
PROCEDURE
1
A1-7 wt pct Ni r o d s , ~- in. in diam and 9 in. long, were melted in a graphite boat and solidified vertically upwards at r a t e s of 1 cm per hr to 14 cm per hr by lowering the boat, in vacuum, through the work coil of an induction furnace. By this procedure, crystals with a fiber composite structure, AlaNi f i b e r s in an alum i n u m m a t r i x , were prepared. The microstructures and the relative orientations of the fibers and m a t r i x (A13Ni [010] II A1 [110] LIgrowth direction and AlaNi [001] at about 14 deg from A1 [ h 0 ] ) were as observed by Lemkey et al s Transverse and longitudinal s l i c e s 300 to 400p thick were spark-cut from various positions in the crystals u s i n g an Agie type AB-15K EDM machine. By e l e c trolytic jet machining, disks were cut from these s l i c e s and dished to a thickness of 50p. F i n a l thinning was accomplished by electropolishing in a solution containing 56 ml perchloric a c i d , 70 ml ethylene glycol monobutyl e t h e r and 490 ml of methanol at temperatures of -40° to 20°C and at an applied potential of 15 to 25 v between a stainless s t e e l cathode and the disc. A number of other solutions gave s i m i l a r results. This wide r a n g e of polishing conditions was explored in o r d e r to find the best polishing conditions for the preparation of thin foils of A1-AlsNi eutectic specim e n s , for experiments to be reported elsewhere. After thinning, the specimens were examined in a Philips EM 300 electron microscope.
J. T. JUBB and E. E. LAUFER are Research Scientists, Department of Energy,Mines, and Resources, Ottawa, Ontario, Canada. Manuscript submitted November 18, 1970. METALLURGICAL TRANSACTIONS
OBSERVATIONS
AND DISCUSSION
A typical electron diffraction pattern from a t r a n s v e r s e section is shown in Fig. 1. The pattern is made up of the AlsNi (010) reciprocal lattice plane and one plane from each of two foreign reciprocal lattices, identified in the f i g u r e as A1Ni, " A " and " B " variants. Occasionally diffraction patterns were observed which showed r i n g s in addition to the AINi s p o t s ; t h e s e r i n g s were identified as NiAleO4. The foreign lattice planes exhibit three-fold symmetry and occupy crystallographically equivalent positions w i t h respect to the AlsNi latt
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