Samples of vacuum melted commercially pure iron were fatigued in the reverse bending mode in different environments including ultrahigh vacuum, oxygen, water vapor, and their combinations. Reverse bending alternating stress in ultrahigh vacuum produced a rumpled surface (without any prominent slip lines) with cracks initiated along grain boundaries. In the presence of oxygen, such fatigue generated well-developed slip lines. Fatigue cracks were observed more along slip lines than grain boundaries. When the same sample was fatigued in both ultrahigh vacuum and oxygen in succession, the resulting surface structure was dependent on the order in which two environments were employed. In water vapor, there was a reduction in surface deformation at low pressures (~ 1 x 10-4 torr (1.33 x 10-2 Pa)) and coarse, prominent slip lines at high pressures (0.17 to 0.7 torr (9.31 to 93.1 Pa)). It also caused grain sliding and void formation. In an environment of water vapor and oxygen, fatigue caused grain boundary cracking and reduced plastic deformation inside grains surrounded by those cracks. Possible reasons for these observations are discussed. Suggestions are also made for utilizing some of these findings to improve the fatigue performance of metals.
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