Factors Influencing Stage I Crack Propagation in Age-Hardened Alloys

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Abstract
This paper reviews some recent experimental results on age-hardened A1-5Zn-1Mg, A1-4Cu, and Cu-2Co alloy single crystals. Experiments were performed with unnotched specimens to study their cyclic deformation behavior and with notched specimens to investigate the crack propagation in Stage I. Since at present no quantitative statement of the formation of persistent slipbands in age-hardened alloys is available, we choose to represent this important microscopic activity indirectly in terms of the amount of cyclic hardening. It is shown that lowering the test temperature and, in the aluminum alloys, changing the type of alloy additions from zinc plus magnesium to copper have qualitatively the same influence of impeding the formation of persistent slipbands and of homogenizing their distribution. The crack propagation experiments show that the same factors decrease the velocity of and the tendency to crack propagation in Stage I. In the A1-5Zn-1Mg alloy the presence of water vapor in the environmental atmosphere, which increases the Stage I crack propagation velocity at 50 Hz, was found to prevent crack propagation in Stage I if the frequency is lowered to 5 Hz, whereas in dry nitrogen gas Stage I crack propagation is not affected by a frequency change. The experiments show that easy formation of persistent slipbands and deformation highly localized in these bands are a prerequisite for the occurrence of extended slipband (Stage I) cracking. The influence on State I crack propagation exerted by the parameters considered is attributed to their effect on the formation of the persistent slipband ahead of the crack tip in which the crack propagates.