A DISCUSSION ON THE FORMATION OF BAINITE AND OTHER PRECIPITATES IN CU-ZN AND AG-ZN ALLOYS

Various transformation products appearing in Cu-Zn and Ag-Zn alloys up on aging at temperatures in the alpha + beta and single alpha-phase ra nge were examined by optical and electron microscopy. Upon aging at re latively low temperature, bainite plates with the 9R structure are fir st formed and then retransformed to face-centered cubic (fee). At high er temperature, other transformation products with the fee structure, such as rod-shaped alpha and massive alpha, are concurrently observed with the bainite. Measurements of solute concentration in bainite were performed by using analytical electron microscopy. Bainite plates wer e found to have lower solute concentration than that of stable alpha, not only in Cu-Zn but also in Ag-Zn alloys. These results were discuss ed in terms of the free-energy change in the transformation. The first appearance of the 9R bainite in place of the stable fee phase in low- temperature aging should be attributed to the ''shear'' process which lowers the nonchemical energy and diminishes the necessary driving for ce for the transformation. The measured solute concentration of the ba inite is well reproduced theoretically on the view that the bainite is a product of shear transformation combined with the concentration cha nge.