Grain refinements of Al-Mg alloy by hydrogen heat-treatments

The present paper describes the grain size refinements of an Al-7.8 mass%Mg alloy by hydrogen heat-treatment of so-called Hydrogenation-Disproportionation-Desorption-Recombination (HDDR) process. Upon hydrogenation of the alloys, a disproportionation reaction occurred in forming of MgH ₂ embedded in Al matrix phases. In the subsequent hydrogen-desorption of the alloys, MgH ₂ was decomposed and was resolved into Al matrix phase, in resulting in the original solid solution of the alloys. This means that the HDDR phenomena take place in the Al-7.8 mass%Mg alloy. It is rather surprising that the grain size of the alloy turned into a several lOnm after HDDR treatments such as heat-treatment at 350: C under hydrogen pressure of 7.5 MPa for 72 h, in following by the hydrogen desorption treatment at 350 C for 4 h in vacuum. This could be the first report that HDDR treatments are effective in producing the fine grain size of the order of nm in Al-based alloys.