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.
Highlights
Glass transition temperatures are observed at a temperature T = Tg during heating of quenched melts
The non-classical homogeneous nucleation (NCHM) model predicts the temperatures of glasses, stable and ultrastable glasses [13,14,15,16,17,18,19,20,21,22,23,24,25], and glacial phases [26,27,28,29,30,31,32,33,34,35] showing that a new phase called Phase 3 appears after heating the quenched liquids through Tg with an enthalpy equal to the difference ∆εlg between those of liquids 1 and 2
The conclusions of this chapter are: (i) the full crystallization of liquid elements can occur with reduced entropy and enthalpy at temperatures weaker than Tm in glassforming melts after heating the material from the glassy state or in the presence of a glacial phase resulting from a high undercooling rate (ii) enthalpy and entropy reductions are expected for crystallization of quenched glass-forming melts at the well-known temperature called TX < Tm [92] which could reveal the existence of a second melting temperature at Tn+
Summary
The formation of stable and ultrastable glasses by vapor deposition and glacial phases by heating or annealing glass-forming melts above Tg induces new liquid states having higher glass transition temperatures. This idea was relaunched in glass-forming melts accompanied by predictions of their melting temperatures Tn+ > Tm using the non-classical model of homogeneous nucleation [1,33,34,51] confirmed by experimental observations [52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68] These predictions had established that the medium-range order persists in liquids from Tg up to Tn+ due to residual bonds producing endothermic enthalpy or due to antibonds producing exothermic enthalpy at Tn+ where the homogeneous state of liquids appears. New values of Lindemann constants for each element [77] are used to calculate the glass transition temperature of elements at low and high heating and cooling rates, the transition temperatures of glacial phases and the various melting temperatures of Ag, Cu, Zr, Ta, Re, Ni and Co compared with those deduced from MD simulations or from crystallization enthalpy of highly supercooled liquid elements
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