Polymorphic phase transformation in Ti50Zr50 binary alloy by mechanical alloying

The present study reports a reversible hexagonal close packed (hcp) to face centered cubic (fcc) polymorphic phase transformation in Ti50Zr50 elemental powder blend in the course of mechanical alloying at ambient temperature in a planetary ball mill. This transformation is monitored at appropriate stages by X-ray diffraction and high-resolution transmission electron microscopy. It is suggested that structural instability due to plastic strain, increasing lattice expansion, and negative (from core to boundary) hydrostatic pressure is responsible for this reversible hcp-fcc transformation. Isothermal annealing at 1000 °C leads to partial transformation of the milled product from single phase fcc to hcp. Thus, the present study confirms that Ti50Zr50 binary alloy undergoes precisely the same metastable hcp to fcc polymorphic transformation that was earlier reported in elemental Ti and Zr.