Electron Diffraction Study on the Crystal Structure of a Ternary Intermetallic Compound Co 3 AlC x
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O
Fig.1 The ideal E21 ordered crystal structure for
T
T3 MX compound.
oX In the case of Co-Al-C system, Co and Al atoms construct the LI 2 structure and C atoms occupy the octahedral interstice which is surrounded by six Co atoms. Based on this structure, an E2, compound has a stoichiometric composition of T 3MX, where as far as intermetallic compounds are concerned T usually represents a transition metal element, and M is a non-transition metal element and X is a metalloid. Besides Co 3A1Cx, some other E21 type intermetallic compounds containing carbon KK7.8.1 Mat. Res. Soc. Synup. Proc. Vol. 552 0 1999 Materials Research Society
have also been found such as Mn 3A1C, Fe3AIC[8], Ti3AICx, Mn 3ZnCx[9], Fe 3GeCx[10]. Of all these carbides, the composition of carbon has been shown to be quite different from the stoichiometry, i.e., x is usually less than 1 in the formula T 3MCx. Actually it has been reported that x=0.59 in Co 3AICJ8], x=0.66 for Fe 3AICx[8], and x=0.45 for Fe 3GeCx[10]. Also for Ni 3A1Cx the value for x is even smaller to be 0.12[11,12], by which the compound could be regarded not as E21 , but as L12 containing carbon as an interstitial solute[ 13]. These information on the value of x in a series of T3MCx compounds implies that the real crystal structure of them would be different from the ideal E2 1 structure in many cases. In the Ti-Al-C system, it has actually be shown that Ti3AICx has a modified E2, structure at the temperature below 1073 K when x= 0.5[14]. The crystal structure consists of two E2 , sub-lattices and two L1 2 sublattices, the latter consisting only of Ti and Al atoms. In the present work, the crystal structure for Co3A1Cx is critically investigated using electron diffraction analysis in a two-phase alloy with a large volume fraction ic-phase in the primary Co solid solution matrix, a(Co). Also the crystallographic relationship between the two phases is examined to deduce the lattice misfit between the two-phases. EXPERIMENTAL An alloy (Co-18.75at%A1-9.15at%C) with a high volume fraction of *'-phase in a(Co) was prepared by a combination of powder and ingot metallurgy. Starting materials, Co and Al, were of the highest purity elements readily available. Carbon was alloyed by using a masteralloy A14C3 of 99% purity. To avoid oxidation and hydrolysis of the raw materials, powders were weighed and mixed in a high purity argon gas atmosphere. Then the mixed powder was cold-compacted in air by uniaxial load. The green compact was melted together with pieces of Co flakes which served as a molten pool in a Tammann furnace in a flowing argon atmosphere. The ingots obtained weighed about 100g. The ingot was cut into small pieces and arc-melted by using a non-consumable electrode in an argon gas atmosphere. Melting was repeated at least four times in order to assure homogeneity of the ingot. Then the ingot was sealed in an evacuated quartz tube and annealed at 1473 K and 1373 K both for 50 h, followed by air cooling. Wet chemical analysis of composition was carried after the two-step-melti
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