The effect of H upon the macroscopic flow stress of metals was considered. Strain localization occurred when the flow stress in the active slip planes was less than the flow

stress in the bulk of the specimen. This could result from a H-enhanced dislocation velocity and H distributions, or from a lowering of the barriers to dislocation motion via cutting. Macroscopic hardening always resulted from strain localization unless the softening effects of H upon dislocation behavior were sufficiently great. Increases in the tensile flow stress did not always indicate a decreased dislocation mobility due to H, but could also be due to slip localization caused by H. However, a decrease in the tensile flow stress required that dislocation mobility be increased by the presence of H.

H.K.Birnbaum: Scripta Metallurgica et Materialia, 1994, 31[2], 149-53