Hydrogen diffusivity in the ingot of aluminum-copper alloys

Hydrogen diffusivity D(m2s-1) in the ingot of Al-(0∼50 mass%)Cu alloys were measured for ND-samples solidified unidirectionally in pure nitrogen gas flow. The Arrhenius type dependence of D on temperature is realized above about 400degC, but an upward departure is noted below 400degC. In the region of α-solid solution (Cu<5.7 mass%), D decreases with the addition of copper. In the coexistent region of primary and eutectic crystals (5.7<Cu mass% <52.6), D is controlled by the following two factors, (1) an enhanced diffusion of hydrogen along the interface between primary and/or eutectic crystals and (2) a suppressed diffusion by hydrogen trapping at eutectic-lamellae interfaces. The value of D for θ-phase is a little greater than that for α-solid solution with 5.7 mass% Cu. For AM samples melted in air, D is clearly smaller than that of ND or VM samples melted in vacuum with no voids. The Arrhenius type plots D=D0 exp (-Q/RT) above 400degC have yielded a linear relationship between log D0(m2s-1) and Q(kJ·mol-1), namely D0=2.22 x 10-8 exp (0.138Q). (author)