Mechanically induced cyclic metastable phase transformations ofZr2Nialloys

Abstract

Cyclic crystalline-glassy-crystalline phase transformations have been investigated during high-energy ball milling of tetragonal-${\mathrm{Zr}}_{2}\mathrm{Ni}$ alloy powders using mechanical disordering (MD) method. The results show that the metastable $\mathrm{fcc}\text{\ensuremath{-}}{\mathrm{Zr}}_{2}\mathrm{Ni}$ alloy powders which is obtained after $173\phantom{\rule{0.3em}{0ex}}\mathrm{ks}$ of MD time, transforms into a new metastable phase of nanocrystalline big cube-${\mathrm{Zr}}_{2}\mathrm{Ni}$ upon milling for $360\phantom{\rule{0.3em}{0ex}}\mathrm{ks}$. The lattice parameters ${a}_{0}$ for the metastable-fcc and big-cube phases were calculated and found to be $0.451\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and $1.226\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, respectively. The obtained big-cube phase subsequently transforms to a single glassy phase after $540\phantom{\rule{0.3em}{0ex}}\mathrm{ks}$ of MD time. This obtained glassy phase, however, devitrificated into the same metastable big-cube phase upon increasing the MD time to $720\phantom{\rule{0.3em}{0ex}}\mathrm{ks}$. As the MD time increases $(806--900\phantom{\rule{0.3em}{0ex}}\mathrm{ks})$, the obtained nanocrystalline big-cube phase transforms into the pervious metastable $\mathrm{fcc}\text{\ensuremath{-}}{\mathrm{Zr}}_{2}\mathrm{Ni}$ phase, indicating a cyclic-phase transformations between the three obtained metastable phases. Such cyclic phase transformations are observed several times during the milling procedure. On the basis of our results, the destabilizing effect of the defects created by the milling media (balls) is responsible for such mechanically induced solid-state devitrification that leads to the cyclic transformations.