AGE HARDENING IN HYDRONALIUM SHEETS

Abstract

Fundamental studies on the age hardening in hydronalium alloys 52S and 56S has been carried out to obtain some basic informations to consider the problem of intercrystalline corrosion. The ageing process at 40°C and 200°C has been observed by measuring the changes in Vickers hardness and the results thus obtained has been compared with that of binary Al-Mg alloys and 3S alloy. A small found in the ageing curves; at about ten minutes from the beginning of ageing at 200°C has been discussed and an explanations has been proposed that the slight hardening is caused by the homogenization of strain which has been heterogenously distributed after quenching.Precipitation occurs locally first, followed by general precipitation and after all over-ageing can be observed when excess coalescence of the precipitates occurs. Hardening occurs in 52S if quenched from 500°C, but the hardening was found to occur chiefly at the grain boundary region. This seem to have an intimate relation with the intercrystalline corrosion problem whether the hardening is caused by local precipitation at the grain boundaries or by the enrichment of Mg atoms at the boundary region.The slight hardening called peak as mentioned above has been discussed, paying attention to the analogy to the first in the low-temperature anneal hardening in brass which has been studied by the author and others in details. This hardening, however, seems to have no practical importance since it is completed within a month at room temperature after the cold working.