Quantifying the beta phase precipitation at low annealing temperatures and stress corrosion cracking behavior in Al-Mg alloys

Al 5000 series alloys are being used in ship structural and automotive applications. However, these alloys become sensitized in the low temperature range of 50-80 degrees C due to the formation of beta phase at grain boundaries. The estimation of beta phase in Al 5000 series alloys aged at low temperatures is critical as it significantly affects the stress corrosion intensity factor, K1scc, which was observed to decrease with aging and reduces to 90% of its original value in fully sensitized condition. Here, we utilize transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, stress corrosion cracking studies and nitric acid mass loss testing to address this issue. In particular, we demonstrate the beta phase formation at relatively low temperatures by analyzing the peak shift of Al (Mg) solid solution during aging with respect to the as-received condition using high-resolution XRD, and compared these results with TEM studies. For complete sensitization, we determine the observed change in lattice parameter is 0.05% and 0.1% at 70 and 75 degrees C, respectively, with respect to the lattice parameter, corresponding to the as received condition. Knowing the exact supersaturation level from the peak positions, we estimate the thickness of beta phase at grain boundaries applying the parabolic growth law. Utilizing this method, one can estimate the beta phase fraction at the early stage in the low temperature range, which would be beneficial to increase the life cycle predictability of a ship.