Prediction of Second Melting Temperatures Already Observed in Pure Elements by Molecular Dynamics Simulations

Abstract

A second melting temperature occurs at a temperature Tn+ higher than Tm in glass-forming melts after heating them from their glassy state. The melting entropy is reduced or increased depending on the thermal history and on the presence of antibonds or bonds up to Tn+. Recent MD simulations show full melting at Tn+ = 1.119Tm for Zr, 1.126Tm for Ag, 1.219Tm for Fe and 1.354Tm for Cu. The non-classical homogeneous nucleation model applied to liquid elements is based on the increase of the Lindemann coefficient with the heating rate. The glass transition at Tg and the nucleation temperatures TnG of glacial phases are successfully predicted below and above Tm. The glass transition temperature Tg increases with the heating rate up to Tn+. Melting and crystallization of glacial phases occur with entropy and enthalpy reductions. A universal law relating Tn+ and TnG around Tm shows that TnG cannot be higher than 1.293Tm for Tn+= 1.47Tm. The enthalpies and entropies of glacial phases have singular values, corresponding to the increase of percolation thresholds with Tg and TnG above the Scher and Zallen invariant at various heating and cooling rates. The G-phases are metastable up to Tn+ because the antibonds are broken by homogeneous nucleation of bonds.

Multiple Melting Temperatures in Glass-Forming Melts

“…The coexistence of glassy phases with crystals is known and viewed as being due to phase separation after annealing near T x , the crystallization temperature of glass-forming melts. This coexistence could result, in fact, from the presence of a glassy fraction in nanocrystallized materials having a glass transition temperature, T g , higher than T m, as already shown for liquid elements [73].…”

“…A third transition was observed with the density at T 3n+ = 1013 K in another experiment, in the absence of transformations at T 1n+ and T 2n+ [40]. With the model already developed for other liquid elements [73], these temperatures are used to calculate a Bi Lindemann coefficient equal to 0.09119, in good agreement with the value 0.095 recently found [85]. The glass transition temperature is predicted at T g = 203.5 K and the singular enthalpy coefficients ∆ε are ∆ε lg0 = 0.1907, −∆ε lg (θ g ) = 0.094065, −∆ε lg (θ 0m = −2/3) = 0.10594 and −∆ε lg (θ= −1) = 0.238375.…”

“…The nonclassical nucleation model predicted the G-phase formation equivalent to Phase 3 formation in liquid elements via first order transitions, and the second melting temperatures observed by molecular dynamics simulations at very high heating rates [73].…”

“…The reduced temperature θ x is given by Equation ( 14), with ∆ε equal to singular values of the enthalpy coefficient ∆ε lg of Phase 3. These singular values are zero, ∆ε lg0 /2, θ n+ , ∆ε lg0 , −∆ε lg (θ 0m ) or the sum of several singular values as observed in liquid elements and as expected from hidden thresholds of configurons [73]. The crystallization enthalpy at T x of the liquid phase coexisting with glassy Phase 3 is given by ( 18):…”

“…The question of their observation at low heating rates was raised, considering that the crystallization enthalpy of these elements would be reduced and recovered via liquidliquid transitions occurring above T m [73]. The singular values of the latent heats in all glass-forming melts could correspond to percolation thresholds at various temperatures.…”