Structure and properties of melt-spun Al-Zr-Ti alloys IV. Microstructure and microhardness stability at elevated temperatures

The influence of long term annealing at temperatures up to 773 K on microha rdness and the microstructure of several melt spun Al-Zr-Ti alloys Aas stud ied. The microhardness stability depends on the content of Zr and Ti in the se alloys. The best results were obtained in the Al-Al-3(Zr0.5Ti0.5) alloy where microhardness increases during annealing at 600 K with time up to 500 hours. The peak microhardness corresponds to the microstructure containing very small particles of the metastable L1(2) Al-3(ZrxTi1-x) phase formed b y discontinuous precipitation. Over-aging occurring for longer annealing ti mes at temperatures T greater than or equal to 650 K is caused by the forma tion of much coarser particles of the stable Al-3(ZrxTi1-x) phase. The diff erence between the decomposition behaviour of supersaturated Zr-rich and Ti -rich Al-Zr-Ti alloys mag: be explained as a result of extremely different diffusion rates of Zr and Ti in Al at temperatures close to 600 K.