In situ synchrotron X-ray diffraction experiments on Al-15%BN mechanically alloyed powder: Observation of AlN nanoparticles precipitation and enhanced thermal stability of nanostructured Al matrix

Two different in situ experiments using high energy X-ray diffraction from synchrotron source were performed in order to understand carefully the phase transformation in nanostructured Al–15%BN mechanically alloyed powder. After milling at room temperature for 10 h, a solid solution of Al, B and N was achieved. During the heating, the formation and the evolution of the metastable trigonal Al2B3 and a very fine precipitation of hexagonal AlN (d < 8 nm) within Al grains were detected quantitatively. We found a stabilization of Al2B3 between 225 ◦C and 550 ◦C and, only around 600 ◦C the hexagonal AlB2 starts to form. A detectable decreasing of the Al crystallite size between 420 ◦C and 470 ◦C was attributed to the precipitation of AlN nanoparticles that reduce the Al volume fraction and, at the same time, hinder the grain boundary propagation. The powder loses the nanostructure above 600 ◦C exhibiting an exceptional thermal stability at temperatures close to 0.9Tm.