The influence of gas atmosphere including vacuum on the fatigue crack growth rate da/dN has been investigated. da/dN decreased monotonically with decreasing partial pressure of water vapor pH2O. This was attributed to hydrogen embrittlement due to the residual water vapor. Although oxidation is inhibited in a harder vacuum, the crack rate decreased slightly. The fatigue fracture toughness was minimum in moist air and increased monotonically with decreasing pH2O. The fracture surface obtained in both moist or room air showed mostly a cleavage-like pattern and the facets of uneven crack surface were close to {100} orientation. In dry air, Ar and a vacuum of 10-3Pa, clearly defined striations were observed, whilst in a harder vacuum striation-like patterns, extrusions and flat planes appeared. In dry air, Ar and vacuum, {100} crack planes with <110> crack tip direction were observed on almost 50% of the fracture surface. These results suggest the occurance of crack extension by an alternating shear proposed by Pelloux.
