Effect of niobium and aluminium additions in TiAl prealloyed powders during high-energy ball milling

Mechanical alloying (MA) of TiAl/TiAl-xNb-yAl-1Cr (x = 4, 8 and 12, y = 2.5, 4 and 6.5 in at.%) was carried out with optimized ball milling parameters. MA of prealloyed TiAl powder resulted nanocrystalline structure at early stages of MA and sustained up to 50 h. TiAl-Nb-Al-Cr systems exhibited amorphous structure in lower Nb and Al content and formation of Nb2Al nanocrystalline compound with increasing Nb and Al additions. Stability of TiAl covalent bonded intermetallic compound was weakened by dissolution of Nb in the matrix and resulted amorphous structure. Formation of brittle Nb2Al and presence of TiAl compounds favoured fracturing during MA which resulted particle refinement to submicron size and spherical morphology. DTA scans exhibit no phase transformation in nanocrystalline TiAl and nanointermetallic composites, but amorphous powder exhibits a sharp exothermic peak.