Microstructural evolution of an Al67Mn8Ti24Nb1 alloy during mechanical milling and subsequent annealing process

Microstructural evolution of the ordered L12+DO22 multiphase Al67Mn8Ti24Nb1 alloy during mechanical milling and subsequent annealing were investigated by X-ray diffractometer (XRD), transmission electron microscope (TEM) and hot-stage TEM. The transitions induced by severe plastic deformation during mechanical milling with increase of milling time were as follows: L12+DO22→disordered fcc supersaturated solid solution→amorphous structure. The amorphization of the Al67Mn8Ti24Nb1 alloy resulted from the increase of the free energy by introducing high dense nano-crystalline grain boundaries and the large internal lattice strain caused by the super-saturated dissolving of the Al3(TiNb) second phase in fcc solid solution. Transitions during subsequent annealing of the as-milled powders were amorphous-fcc crystalline transition at 250 °C, disorder fcc transforming into the L12 ordered structure at 300 °C and DO22 structured second phase precipitating in the L12 matrix around 500–650 °C, meanwhile grains growing up to the sub-micron.