Mechanical and kinetic effects of cathodic hydrogen charging of copper and of CuAl alloys

Abstract

The hydrogen-defect interaction depends strongly on the stacking fault energy in the CuAl binary system. Plastic deformation gradients produced by indentation were surveyed by both microhardness and (much more sensitively) by positron annihilation measurements. The cathodic charging of hydrogen into copper with an existing gradient of plastic deformation causes a general increase in both the positron “peak-to-wings” parameter P/W and the microhardness. This is evidently due to a predominance of new defect generation (on charging) over proton pinning of dislocations since the latter also results in screening effects of the dislocations from positrons and a decrease in P/W. Aluminum additions to copper rapidly lower the stacking fault energy and, for these alloys, similar gradients of plastic deformation, when hydrogen charged, show increases in P/W in the lightly deformed region but either no increase or a decrease in P/W close to the indentation center although the microhardness increases in all regions. This is explained by increased resistance to new defect formation in the already highly deformed region together with enhanced proton screening of dislocations in that region. If the sequence of events is reversed, i.e. if deformation follows charging, even higher P/W values result. This is thought to be due to the interaction of gliding dislocations with charging generated dislocation loops. Determinations of the parameter R indicate that in both sequences of events the main defect type is the dislocation. Annealing studies of copper and of copper alloys with 4 or 8 wt.% Al indicate that hydrogen trapping started around 60°C and up to about 100°C and that detrapping occurred from 110°C and above, independent of composition; this suggested a similar hydrogen-defect binding energy for all three materials.