Effect of annealing on the microstructure, ordering and microhardness of ball milled cubic (Ll(2)) titanium trialuminide intermetallic powder

Ball milled powders of cubic (L1(2)) titanium trialuminide modified with Mn , possessing nanocrystalline structure, were annealed at 600 degrees C and 1000-1100 degrees C. The best results for the calculation of the nanocrysta lline grain size upon annealing from the X-ray diffraction (XRD) patterns, were obtained using the Cauchy/Gaussian approximation for both the instrume ntal broadening and nanocrystallite size/lattice strain separation. The nan ocrystallite size increased upon annealing from 1 to 240 min at 600 degrees C, from the initial several nanometers for the as-milled powders, to 30-14 0 nm for the annealed powders. This nanocrystalline grain growth is accompa nied by a continuous increase of the long-range order (LRO) parameter, from zero to similar to 0.8-0.9 after annealing at 600 degrees C for 240 min. H owever, a phenomenal thermal stability of nanocrystalline grains is manifes ted in the fact that only very few powder particles exhibited the formation of micrometer-sized grains after annealing at the 1000-1100 degrees C rang e. The observed differential thermal analysis (DTA) exothermic peaks around 410-430 degrees C (peak I) and 570 degrees C (peak II) are interpreted as the atomic re-ordering and the phase restoration peak, respectively. The ob served hardening of the "outer layer" and "no core" particles upon annealin g at 600 degrees C is discussed in terms of nanograin boundaries age-harden ing mechanism due to the pick-up of interstitials (carbon and/or nitrogen) and their preferential segregation at the nanograin boundaries. The reversa l of the process, i.e, desegregation, might be responsible for the observed softening of the "outer layer" and "no core" particles upon annealing at t he 1000-1100 degrees C range, without any apparent microstructural changes observable under optical/scanning microscope. (C) 1999 Elsevier Science Ltd . All rights reserved.