過剰Si型Al-Mg-Si合金のSCC過程における水素脆化の関与

抄録

For underaged Al-Mg-Si alloys with excess Si, No. 5 (Al-0.7mass%Mg-1.1mass%Si) and No. C5 with 0.2mass%Cr addition, SSRT tests were carried out to reveal the SCC process and relation to hydrogen embrittlement at strain rate 6.9×10^-7/s under three environments; (1) dry nitrogen gas, (2) wet air with 90% relative humidity and (3) an acid sodium chloride (ISO) solution. Under env. (2), alloy No. 5 with coarse grains shows a decrease in elongation comparing with that under the inert env. (1), while alloy No. C5 with finer gains exhibits rather an increase. On the ruptured specimen, the small areas of intergranular- and transgranular-facet with a trace of wavy slip exist bounded on the specimen surface, respectively, which are regarded as an evidence of hydrogen-enhanced localized plasticity. Under env. (3), alloy No. 5 shows a high susceptibility to SCC, while alloy No. C5 exhibits a low that improved through Cr addition. On the SCC fracture surface of alloy No. 5, three modes of intergranular facets with crystallographic pits and with fine ledges and transgranular facet with an appearance of quasi-cleavage are presented, which support the existence of a mechanism of plastic deformation localization induced by anodic dissolution. There is a participation of hydrogen embrittlement in the SCC process, still the effect is small.