The kinetics of the phase changes in glass-forming metallic alloys is generally so fast that the transformation of metastable structures cannot be systematically investigated by conventional DSC. Flash DSC technology however enables measurements to be performed in the relevant time domain. Using a gold-based alloy as an example, we show that monotropic polymorphic modifications can exist at temperatures above the glass transition. The melting point of the metastable modification was determined.
Introduction
About 140 years ago, Otto Lehmann recognized the existence of different types of polymorphism in chemical compounds and in particular the distinction between monotropic and enantiotropic polymorphism [1]. In enantiotropic polymorphism, at least two solid polymorphic phases exist whereby one phase is stable at low temperatures and the other at higher temperatures. Below the melting point, a solid-solid phase transition occurs on heating in which the low-temperature modification becomes unstable and transforms to the high-temperature modification. The transition takes place reversibly at about the same temperature on heating and cooling.
In monotropic polymorphism, at least two solid modifications exist that in each case have an equilibrium melting point whereby the more stable modifications melt at higher temperatures. Depending on the heating rate, the transition can follow different paths [2]:
- At relatively low heating rates, the less stable modification transforms directly to the more stable modification in a solid-solid transition. The temperature of the exothermic transition increases with increasing heating rate. Finally, the stable modification melts at the equilibrium melting point. The DSC curve exhibits two endothermic peaks.
- At medium heating rates, the less stable modification reaches its melting point. A supercooled melt is formed which then crystallizes to the stable modification. This is a solid-liquid-solid transformation and the DSC curve exhibits two endothermic melting peaks. An exothermic crystallization peak appears between these two peaks.
- At high heating rates, there is not enough time for crystallization of the stable modification to occur after melting of the less stable modification. The DSC curve shows only a melting peak.
In this context, what we mean by low, medium or high heating rates is determined by the kinetics of the phase transitions. This type of polymorphism was long unknown for metal alloys. It was generally assumed that the transformations of the different modifications were enantiotropic solid-solid transitions. However, using Flash DSC measurements at heating rates of several hundred to one thousand Kelvin per second, it was recently shown that monotropic polymorphism can also occur in alloys that form metallic glasses [3, 4]. This work will be briefly discussed in this article.
Monotropic polymorphism in metallic alloys, Part 1: Flash DSC investigation of the melting of metastable phases | Thermal Analysis Application No. UC 514 | Application published in METTLER TOLEDO Thermal Analysis UserCom 51
