  Melting kinetics of polycrystalline materials is analyzed on the basis of a
new model which explicitly couples homogeneous and heterogeneous melting
mechanisms. The distinct feature of this approach lies in its ability to
evaluate not only grain-size-distribution effects on the overall melting
kinetics but also competitions between the two melting mechanisms. For the
first time, we reveal the three-part structure of
temperature-time-transformation diagrams for melting of polycrystalline
materials, through which it is possible to determine a critical temperature
across which the dominant melting mechanism switches. The critical temperature
increases as the mean-grain-diameter decreases following a negative power-law.
The results are qualitatively consistent with experimental observations.
