THE BAINITE FORMATION UNDER STRESS IN Cu-Zn-Al ALLOYS

Abstract
β₁ single crystals of Cu-29.8at%Zn-6.lat%Al alloy with different orientation were heated at temperatures higher than 420°K under a tensile stress. Two variant crystals of bainite having different habit planes, for example (12 11 2) and (12 [MATH]) for [10 [MATH]] tensile direction, from that of stress-induced martensite, (12 [MATH] 11), were observed after a certain period of incubation. The variant crystal having a maximum Schmid factor with respect to the shear on the habit plane was preferably formed in the martensite case. The close-packed plane of martensite is to be transformed from (110) plane of matrix crystal by the shear of ([MATH]) [[MATH]]. On the other hand, it is found that the variant crystal of bainite having (12 [MATH]) habit plane has a larger Schmid factor with respect to the shear of ([MATH]) [[MATH]], which would also transform (110) matrix plane to the close-packed plane. The other variant having (12 11 2) habit plane has an orientation such that the fcc structure can be formed by crossing with the first one in pair. The crossed region of these two bainite crystals was observed as a pink-colored area. Electron diffraction patterns indicate that the bainite has a disordered 9R structure. It is supposed that the intrinsic transformation shear producing the close-packed structure operates preferably in the bainite case, whereas the minimization of strain energy in the transformation is dominant in the martensite case. A mechanism of the shape change associated with the stress-induced bainitic transformation is also discussed.