FORMATION OF NANOCRYSTALLINE CUBIC (L1(2)) TITANIUM TRIALUMINIDE BY CONTROLLED BALL-MILLING

Pre-alloyed, as-cast ingots of the Mn-modified, cubic (L1(2)) titanium trialuminide (65 at% Al, 25.6 at% Ti and 9.4 at% Mn) were homogenized (1000 degrees C/100), crushed into a coarse-particle powdered materia l and subsequently ball milled for up to 386 h under shearing mode in a controlled ball movement mill. X-ray spectra of milled powders showe d line broadening and decrease in intensity of Bragg peaks with increa sing milling time. This is associated with the formation of nanocrysta lline grains and lattice strains upon milling. Crystallite size calcul ated from peak broadening, remains relatively unchanged from 19 up to 100 h of milling (20-30 nm) and then drastically decreases reaching a saturation size of about 3 nn after 200 h of milling. Lattice strains are on the average less than 1%. Simultaneously, the ordered L1(2) cry stal structure undergoes disordering which commences after approximate ly 40 h and terminates after 160 h of milling. The microstructure of p owder particles undergoes a complex evolution. With increasing milling time the particles are formed which appear to contain a work- hardene d core. Each such a particle is surrounded by a heavily deformed, hard outer layer containing nanometer grains. After 386 h of milling all t he core/outer layer particles are transformed into uniform 'no core' o nes, characterized by approximately 3 nm crystallite size (XRD measure ments). The microhardness data for both outer layer in the powder part icles with a core, and the'no core' particles can be fitted by a Hall- Fetch dependence on the inverse root of crystallite size: HV 0.01 = 43 1.7 + 387.5d(-0.5) (kg mm(-2)) where HV 0.01 is Vickers microhardness at 0.01 kg and d is crystallite size in nm. These results are discusse d in view of the existing models of hardening of nanosized materials. (C) 1998 Elsevier Science Limited. All rights reserved.