Phase stability and the martensitic transformation in Cu-Zn alloys

The critical resolved stress tau M for the martensitic transformation from beta to orthorhombic 9 R has been measured at liquid air temperature for Cu-Zn alloys between 40.5 and 43 at.% Zn. Whereas alloys with a zinc content below 41 at.% Zn also transform spontaneously on cooling, those for zinc concentrations between 41 and 43 at.% transform only when stresses are applied. At 42.5 and 43 at.% Zn the alloys can deform plastically in the beta phase prior to the transformation, depending on the orientation of the stress axis. Measurements have been performed which permit extrapolation of tau M to the undeformed state. The composition dependence of tau M between 41 and 43 at.% Zn agrees within 20% with that predicted from data obtained for alloys with a zinc concentration below 40 at.%. This suggests that the same mechanism controls the relative phase stability of beta and 9 R in the whole composition range, and that the energy from the monoclinic distortion of 9 R martensite plays only a minor role.