On DTA Curves for the Melting and Freezing of Alloys

Using a heat flow model of a DTA (differential thermal analyzer) and thermal characteristics obtained by fitting experimental results for a pure metal, the response of the DTA is modeled for the melting and solidification of alloys. The enthalpy-temperature relation used for the alloy simulations is obtained by two different methods: 1) equilibrium and Scheil considerations derived solely from thermodynamic information and 2) solute diffusion micromodels coupled to the DTA heat flow equations. During the consideration of pure material melting, simple expressions are obtained for the effect of sample size and heating rate on the DTA melting onset temperature, peak temperature and peak height that assist in the proper calibration of a DTA. For alloys, the smearing effect of the DTA heat flow at different heating and cooling rates is demonstrated for various solidification path features. In particular, the DTA peak temperature during melting, which is often selected as the liquidus temperature experimentally, is shown to be significantly higher than the liquidus temperature for small freezing range alloys and/or for alloys with slow solid diffusion. DTA curves calculated for freezing with dendritic growth due to supercooling quantify the errors associated with of the determination of the liquidus temperature on cooling.