Formation of transitory bainite as a precursor of α-phase during tempering of martensitic, pp. 267-285
Authors: Leandru-Gheorghe Bujoreanu
Abstract: The paper assumes that, in the case of a martensitic Cu-Zn-Al shape memory alloy subjected to heating above 770 K, the endothermic peaks revealed by differential scanning calorimetry (DSC) and differential thermal analysis (DTA) and located around 630 K would correspond to the transitory formation of α1 bainite. This endothermic phenomenon has been regarded as transitory since heating proceeding causes exothermic precipitation of equilibrium α-phase which is copper-rich and therefore its formation augments both Zn and Al amounts in β2 austenitic matrix. Considering that the formation of tempering bainite has been completed at 670 K, specimens which were water-quenched from this temperature were analyzed by optical and transmission electron microscopy (OM and TEM, respectively) and bainite formation was ascertained by its plate-like morphology with irregularly distributed internal striations having microtwin-appearance and by its selected-area diffraction (SAD) pattern with orthorhombic unit cell having 9R atomic ordering. In order to reveal the irreversible character of tempering-martensite formation, induced by its diffusion controlled growth, specimens water-quenched from 770 K were subjected to DSC-scans during heating. In this case no martensite reversion to austenite was detected and the accumulation of irreversibly formed α-phase caused a gradual change of chemical composition of β2 matrix accompanied by a diminution in the intensity of its order-disorder transition.