| Popis: |
The kinetics of coarsening of γ′ precipitates in binary NiAl alloys containing nominally 5.72, 5.74 and 5.78 wt% Al and aged at 630°C were investigated by transmission electron microscopy and magnetic analysis. The alloys were each pre-aged at a temperature just below its solvus prior to re-aging at 630°C. The volume fractions of γ′ were low between 0.02 and 0.03. the pre-aging treatment produced microstructures containing precipitates that nucleated heterogeneously on dislocations and grain boundaries, leaving empty matrix areas in which coherent γ′ precipitates subsequently nucleated and coarsened. Measurements were made only on these precipitates. The coarsening kinetics are anomalous in that the rate constant decrease with increasing volume fraction, in contradiction with all the theories of this process. Furthermore, the increase is quite large, exceeding a factor of four over a range of volume fractions that increased by less than 40%. Additionally, the rate constants exceed the values expected from the literature by factors of 10–40. The distributions of particles sizes were measured, and with few exceptions were found to agree with those of earlier investigations. It is suggested that elastic interactions among the γ′ precipitates play a pivotal role in decelerating the kinetics of coarsening, overpowering the expected accelerating effect of increasing volume fraction. |
