抄録
In order to obtain the characteristics of humid gas stress corrosion cracking (HG-SCC) , two types of tests have been performed to measure the crack growth rate, da⁄dt, under load (P)-increasing and displacement (D)-constant conditions for high and low da⁄dt regions, respectively. The tests are aimed to investigate the effects of chemical composition on the HG-SCC characteristics of standard alloys using specially prepared alloys. The P-increasing test in humid air revealed that da⁄dt is increased by excess Si content from the balanced composition of Mg2Si without Cu addition in 6000 series alloys, while by Cu addition, da⁄dt is reduced and the HG-SCC resistance is improved. The D-constant tests exhibit similar trend of crack growth and small values of the threshold stress intensity factor for HG-SCC,KISCC , for the excess-Si alloys without Cu. Observations by scanning electron microscope (SEM) demonstrate that the intergranular fracture surface is developed for the alloys having excess Si without Cu, while the dimple fracture surface becomes predominant for the other alloys. HG-SCC characteristics obtained for the standard alloys of 6000 series are consistent with those of the special alloys. 7075 alloy exhibits poor HG-SCC resistance and different crack growth mechanisms as compared with the 6000 series. The results in dry air revealed that da⁄dt decreased and KISCC increased remarkably by reducing the humidity for the alloys that have poor HG-SCC resistance. HG-SCC characteristics are independent of plastic pre-strain expected in the autofrettage process.
