ACOUSTIC-EMISSION MEASUREMENTS DURING ORD ER-DISORDER TRANSFORMATION AND ISOTHERMAL ORDERING PROCESS OF PD-CU ALLOYS

The acoustic emission (AE) method was applied to the determination of the transformation temperature and the level of dispersed elastic ener gy during isothermal aging of the alloy. The binary alloys of Pd-30, 4 5 and 50 mass%Cu were prepared as the specimens and were heat-treated in the following procedures; that is, the first group was homogenized at 1000-degrees-C after alloying and cooled slowly in a furnace. The o ther group of the alloys was heated up to 950-degrees-C after the same homogenization annealing as the first group and quenched into ice-wat er (solid solution heat treated). AE signals from the alloy which was placed in an electric furnace were passed through the wave guides (SUS 304 rod) to the outside of the furnace, and were detected by the AE se nsor. The signals were converted to electrical pulses by the AE sensor and amplified. Detected data showed that AE events were high at the t emperatures where the order-disorder transformation and the precipitat ion of the beta phase into the solution treated alloy structure took p lace. These results were confirmed by differential thermal analysis an d the change in hardness. Activation energy for the precipitation of t he beta phase in the disordered matrix of the solid solution-treated a lloy was estimated to be 140 kJ/mol from the Arrhenius plots on the de tected time of AE events versus isothermal aging temperature of Pd-45 mass%Cu alloys. This is consistent with the values reported previously on the measurements of electrical resistance and hardness changes for similar compositional alloys. It is suggested that the AE method is a helpful technique to investigate the mechanisms of phase transformati on of the alloy because the AE signals can be picked up in a dynamic t emperature field.