Environmental Exposure Dependence of Low Growth Rate Fatigue Crack Damage in Al-Cu-Li/Mg

摘要

The objective of this research is to establish the water vapor exposure dependence of fatigue crack surface morphology and facet crystallographic orientation using an SEM-based EBSD/stereological method applied to two planar slip prone Al-Cu-Mg/Li alloys stressed in the low crack growth rate regime. Crack surface morphology depends on environmental exposure (water vapor pressure/loading frequency); indicating multiple transitions in damage mechanism, crack growth rate, and facet orientation that are similar for each alloy. Moderate to high levels of exposure eliminate {111} facets, characteristic of slip band cracking in high vacuum and suggesting that environmental hydrogen does not promote this slip-based damage process. Varied facet crystallography, typical of crack growth at high environmental exposures, is explained by evolution of several facet orientations with increasing water vapor pressure/frequency ratio. A fraction of high index facets is produced by stressing in very low pressure water vapor, due to either a hard surface film or low-concentration H embrittlement. Further increases in exposure favor a large fraction of low index fatigue facets; {100} for Al-Cu-Mg and {100} plus {110} for Al-Cu-Li, as projected for H-enhanced decohesion. Additional high index cracking is produced at higher exposures for each alloy. For this regime, FIB/TEM analysis showed subgrains formed by fatigue and within ～200 nm of the crack wake for Al-Cu-Mg stressed in high pressure water vapor, perhaps contributing to fatigue damage and high index facet orientation.