MORPHOLOGY AND CRYSTALLOGRAPHY OF CR PRECIPITATES IN A CU-0.33 WT-PERCENT CR ALLOY

Lath-shaped, faceted Cr-rich precipitates in a Cu-0.33 wt% Cr alloy were investigated by transmission electron microscopy. Their orientation relationship with the matrix was identical for all precipitates observed. One set of close-packed planes in the two phases were exactly parallel, with the close packed directions enclosing an angle of 0.5-degrees, a systematic deviation of 0.5-degrees from the Kurdjumov-Sachs orientation relationship, or 5.76-degrees from Nishiyama-Wassermann. The lath axis was aligned precisely with the [561]fBAR invariant line direction, located at the intersection of the close-packed planes (111)f parallel-to (110)b with the cone of unextended lines. In agreement with previous observations the dominant interface facet was found to be (3.1 3.5.5)f or approximately 2-degrees away from (335)f. All crystallographic measurements were in excellent agreement with the predictions of an invariant line strain model assuming the Bain correspondence and a pair of parallel close packed planes, but inconsistent with the theory of martensitic transformations.