Phase equilibria and eutectic growth in ternary organic system (D)camphor–neopentylglycol–succinonitrile

Abstract

The temperature and enthalpy of transformations of organic alloys from the ternary systems (D)camphor–neopentylglycol–succinonitrile (DC–NPG–SCN) were measured by means of differential scanning calorimetry (DSC). The phase diagram of this ternary system was assessed via the CALPHAD approach using Thermo-Calc based on the thermodynamic descriptions of the constituent binary systems published earlier and the thermodynamic and phase equilibrium data for the ternary alloys measured in the present work. Proper agreements between the experimental and the calculated data were achieved. Experiments and calculations show that the DC–NPG–SCN exhibits a nonvariant eutectic reaction with the eutectic point at 297.0 K, 26.3 mol%DC and 14.8 mol%NPG. This eutectic reaction involves the ordered monoclinic crystals (OCs) of NPG and the orientationally disordered crystals (ODICs) of both, SCN (BCC_A2 phase) and DC (hexagonal phase). Unidirectional solidification experiments were performed with the nonvariant eutectic DC–NPG–SCN alloy in order to verify the phases involved in the solid–liquid equilibrium and the nature of eutectic growth. We find that in the eutectic alloy the eutectic growth is irregular: the extremely facetted monoclinic phase (NPG) grows as thin and long needle-like crystals that protrude far into the liquid and are then decorated with nonfacetted eutectic of BCC_A2 (SCN) and hexagonal (DC) lamellae, which mainly grow perpendicular to the growth direction of the monoclinic (NPG) needles. Few unidirectional solidification experiments were performed with dilute ternary alloys with composition close to the univariant eutectic reactions that extend from the constituent binary systems NPG–DC and SCN–DC, respectively. The destabilisation of planar eutectic interfaces due to constitutional supercooling were qualitatively monitored.