Effect of annealing on the microstructure,ordering and microhardness of ball milled cubic (L12) titanium trialuminide intermetallic powder

Ball milled powders of cubic (L1₂) titanium trialuminide modified with Mn, possessing nanocrystalline structure, were annealed at 600°C and 1000–1100°C. The best results for the calculation of the nanocrystalline grain size upon annealing from the X-ray diffraction (XRD) patterns, were obtained using the Cauchy/Gaussian approximation for both the instrumental broadening and nanocrystallite size/lattice strain separation. The nanocrystallite size increased upon annealing from 1 to 240 min at 600°C, from the initial several nanometers for the as-milled powders, to 30–140 nm for the annealed powders. This nanocrystalline grain growth is accompanied by a continuous increase of the long-range order (LRO) parameter, from zero to ∼0.8–0.9 after annealing at 600°C for 240 min. However, a phenomenal thermal stability of nanocrystalline grains is manifested in the fact that only very few powder particles exhibited the formation of micrometer-sized grains after annealing at the 1000–1100°C range. The observed differential thermal analysis (DTA) exothermic peaks around 410–430°C (peak I) and 570°C (peak II) are interpreted as the atomic re-ordering and the phase restoration peak, respectively. The observed hardening of the “outer layer” and “no core” particles upon annealing at 600°C is discussed in terms of nanograin boundaries age-hardening mechanism due to the pick-up of interstitials (carbon and/or nitrogen) and their preferential segregation at the nanograin boundaries. The reversal of the process, i.e. desegregation, might be responsible for the observed softening of the “outer layer” and “no core” particles upon annealing at the 1000–1100°C range, without any apparent microstructural changes observable under optical/scanning microscope.