EFFECT OF ORDERING ON THE YIELD STRESS OF MARTENSITIC CUZNAL ALLOYS

The structures of the martensite of four CuZnAI alloys with Al contents ranging from 9 to 20.8 at. pct were investigated using transmission electron microscopy. It was found that the structure of the martensite changed from 9R (having formed from a parent phase with B2 order) to 18R (DO3 parent) as the Al content was increased from 9 to 13.6 at. pct. The 0.2 pct proof stress of polycrystalline specimens initially decreased with increasing Al, corresponding to the change of order, and then increased. It is argued that the mechanism of yielding is the formation of mechanical twins which reorient the martensite, and an explanation of the change of yield stress with composition is given in terms of changes in the nature and extent of ordering affecting the twin boundary energy. © 1979 American Society for Metals and the Metallurgical Society of AIME.