An extensive study has been made on glassy alloys and crystallization of the amorphous phase during the last two decades. However, the formation of nanocrystals - that is, a crystalline morphology with an average grain size of 5 to 50nm - from the amorphous matrix was detected only a few years ago. In this case, nanophase alloys are made not directly by quenching, but rather by overquenching to produce a fully amorphous and thus glassy phase which is then partially devitrified by isothermal annealing around crystallization temperatures as in glass-ceramic systems. This annealing is more controllable than direct quenching process where a microstructural state is obtained being composed of both crystalline (not always in a nano-scale) and amorphous phases.
Among a wide range of metallic alloys that has been tested to observe this behavior, there has been a special interest in Al-based alloys which can be rapidly solidified by gas atomization, melt spinning or laser melting process, or mechanically alloyed by the solid state reaction into a fully glassy state and then annealed to give a nm-scale dispersion of Al crystallites in a glassy matrix. These materials first discovered in Japan are supposed to show excellent mechanical properties possibly together with unusual electronic and thermal properties.
Thermodynamic properties may be viewed as a necessary condition for amorphization to occur, only if a kinetic requirement is fully satisfied. Therefore, a detailed thermodynamic analysis may beam into the glass forming ability of a given system as long as kinetic requirements such as activation energy are fulfilled. Among various plausible multicomponent alloys based on Al, the ternary Al-Mg-Y and Al-Ni-Y system has been selected in this study because the thermodynamic description of all the three bounding binary alloys that constitute the ternary system is relatively available and experimental data on the glass forming range have been partially published.
The glass forming range which is often called glass forming ability to describe the composition range where metallic glasses are formed, has been systematically correlated with the thermodynamic limit for the possibility of partitionless crystallization of equilibrium phases. A thermodynamic approach has been employed to describe $T_o$ curves, which are temperature-composition locus of the equality between the free energies of the liquid and crystal phases, then $T_o$ curves have been compared with measured crystallization temperatures $T_x$ under the assumption that $T_x$ is not far from the glass transition temperature $T_g$. In the composition range of $T_o < T_g$, the partitionless crystallization is not possible because the glass formation is thermodynamically favored, while the partitionless crystallization is possible in the range of $T_o > T_g$ and thus this is outside the glass forming range.
