Amorphous-i phase-big cube-amorphous cyclic phase transformation of mechanically alloyed Zr75Ni20Al5 system

Cyclic glassy-metastable-glassy phase transformations were investigated during mechanical alloying of Zr75Ni20Al5 powders, using high-energy ball milling technique. After 20 h of continuous milling, a single glassy phase was obtained. The synthesized glassy powder, which is homogeneous in structure and composition, showed a glass transition temperature lied at 510 °C and a large supercooled liquid region extended to 86.5 °C. This glassy phase, which crystallized through a single sharp exothermic peak at 596.5 °C, tended to transformed into icosahedral (i-phase) nano-quasicrystalline grains upon further milling to 30 h. This i-phase showed serious instability against the mechanical deformations generated by the milling tools, and hence transformed into big cube phase with increasing the milling time to 45 h. In a cyclic phase transformation the obtained big cube phase subsequently transformed into the same glassy phase after 60 h of milling. This observed cyclic phase transformation was observed twice upon further milling to 90 h without serious compositional changes. The iron contamination contents in the ball-milled powders had constant values of about 0.16 wt.% and did not change with changing the structure of the powders. The present work shows that this phenomenon does not have any obvious analogues with the periodic redox reactions or with diffusive-reactive phenomena known in chemistry. On the basis of our results, the destabilizing effect of the defects created by the milling media (balls), which leads to the cyclic transformations, depends on the milling time.