Ａｌ合金の静ＳＣＣ及び動ＳＣＣき裂進展に及ぼす温度の影響

Abstract

The temperature dependence of static- and dynamic-SCC crack growth was examined on high-strength aluminum alloy sensitive to active path corrosion (APC) type SCC in 3.5% NaCl aqueous solution. Subcritical crack growth of static SCC in region II was thermally activated with an apparent activation energy of 78∼83kJ/mol, and the rate controlling step would be an anodic dissolution at grain boundaries. The threshold stress intensity factor KISCC, however, showed no dependence on temperature. Dynamic SCC at every temperature caused not only faster cracking in region II than that of static SCC, but also a decrease of the threshold value KDSCC from KISCC. The values of KDSCC were also independent of temperature. However, the thermally activated region II crack growth had two apparent activation energies. The apparent activation energy at low temperatures below approximately 318K was 21∼29kJ/mol, and this seemed to be related to alternation of two fracture mechanisms of anodic dissolution at grain boundaries and transgranular hydrogen embrittlement in a ligament zone. On the other hand, 84kJ/mol was obtained at high temperature, which indicated that anodic dissolution dominates over crack growth kinetics. The crack growth component by anodic dissolution [da/dt]AD and the one by hydrogen embrittlement [da/ dt]HE were obtained by applying a summation model taking accounts of fracture area fraction to dynamic SCC crack growth behavior. The results show that [da/dt]HE prevailes over [da/dt]AD at low temperatures, whilst at high temperatures the latter prevailes over the former.