A study of the microstructure of nanocrystalline Al–Ti alloys synthesized by ball milling in a hydrogen atmosphere and hot extrusion

Nanocrystalline Al–Ti alloy powders were produced by reactive ball milling (RBM) in a hydrogen atmosphere and its microstructure consisted of nano-sized Al and nano-sized TiH 2. Thermal analysis of as-milled powders showed that the decomposition of TiH 2 and the subsequent formation of Al 3Ti occurred at 370–480°C. The powder was consolidated by hot extrusion at 500°C. The grain size of as-extruded specimens was about 50–100 nm. The hardness of Al-5 at.%Ti specimens synthesized by RBM and subsequent hot extrusion was 25–75% higher than that of Al-8 wt.%Ti alloys produced by mechanical alloying (MA) in Ar atmosphere and hot extrusion. Room temperature and high temperature (300, 400, 500°C) tensile strength of RBM Al-5 at.%Ti alloys were superior to those of MA Al-8 wt.%Ti alloys. The strength in these alloys appeared to be related to a large extent to the very fine grain size. The ductility of RBM alloys decreased with grain refinement. It is possible that the deterioration in ductility of nanocomposite Al–Ti alloys has to be attributed to the increase of the interface area between Al and Al 3Ti and its high energy. SEM fractograph showed that fracture progressed intergranularly.