The low-temperature a-modification is formed by a distortion of the hexagonal @-NbC. The distorted modification may be thought of as being formed by compressing the hexagonal parent cell perpendicular to one of the principal prism planes, thus effecting an approximately equal (but opposite in sign) length change of the base plane vectors. This behavior is immediately reflected by comparing the hexagonal pseudoaxes of alH = 3.092 A and = 3.145 A in the orthorhombic structure with the base length of U H = 3.118 A in the hexagonal subcell of @-NbC. The dimensions of the orthorhombic type are related to the above pseudoaxes by ao = 4 a l~. bo = 2aw 4 3 , Co = CH For a-V?C, the corresponding dimensions are alN = 2.872 A, and aZH = 2.904 A, compared to a H = 2.884 A for the hexagonal high-temperature modification. The distortion of the hexagonal modification in vanadium carbide (1.17,) is somewhat less than in a-NbC (1.7%).A problem not yet fully understood concerns the effect of composition on the order-disorder reaction as well as the type of order itself. 'While there is little doubt that the highertemperature transformations in Ta2C and N b C proceed as homogeneous reactions, and reversibility of the low-temperature transitions also appears established, the question whether the latter transitions proceed as homogeneous or heterogeneous reactions is not yet completely resolved.Present indications, however, favor a two-phased process since intermediate states, showing the coexistence of the a-and &modification, could be produced by insufficient heat treatments of the alloy specimens. Supporting this interpretation is the observation that the reaction generally proceeded much more rapidly in dense than in porous alloys, suggesting a nucleation and growth mechanism.Other observations,14 such as the much higher reaction rates obtained in excess metal compared to excess monocarbidecontaining alloys, are probably related to epitaxial dissolution-and-growth phenomena associated with the pronounced oriented metal precipitations in the subcarbide which could promote the formation of the new phase. Similarly, epitaxial growth phenomena varying with the stoichiometry of the phases probably are responsible for the observed slight differences in the structures formed in the transformation processes.
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