No abstract

The simple method for evaluation of fracture toughness in Mg alloys was examined by AE. In this study, fracture toughness (K IAE ) in AZ31 (Mg 9Al 1Zn), AZ91 (Mg 9Al 1Zn), and LA141 (Mg 14Li 1Al) was investigated. Specimens (L S, T S) were machined from as rolled AZ31, LA141, and as cast AZ91. Four point bending tests were conducted according to ASTM E 399 with AE measurement, and the crack pass was observed by microscope during the testing. K IC was supposed to be the value of K I when the crack initiates from the tip of the pre crack. K IAE was defined as the value of K I when the RMS voltage of AE indicated the maximum value before ultimate load. The crack propagation was observed by microscope at the time of RMS peak. The values of K IAE were 12.4 15.0 (AZ31), 11.5 11.7 (AZ91), and 24.

In order to consider the effect of cooling conditions on microstructure formation during casting of Cu-Ni-Si alloy (Corson alloy), the microstructures of unidirectional solidification of the alloy were evaluated. Ingots of Corson alloy with several cooling conditions were prepared by Mizuta method, and microstructural observations and analysis of the distribution of added elements were performed. The primary and secondary dendrite arm spacings (P-DAS and S-DAS) became smaller with increasing cooling rate, and it was found that P-DAS and S-DAS can be accurately approximated by multiplier of cooling rate times temperature gradient, and by that of cooling rate, respectively. Between secondary dendrites, Si was enriched from the initial solidification zone to the final solidification zone, and Ni was almost constant around the initial solidification zone and enriched slightly around the final solidification zone. The Si enrichment was larger under the condition of lower cooling rate, which is expected to be closer to the equilibrium state. In this study, it was found for Corson alloy that DAS became bigger and Si enrichment between secondary dendrites was larger under the condition of lower cooling rate, and Ni enrichment was not affected by cooling rate.