The equilibrium conditions between the thermodynamically separated phases or the phases which exist in the material system at different pressures, temperatures, and compositions are portrayed in a phase diagram. Thermoanalytical techniques play a key part in determining binary and ternary phase diagrams. They are based on the principle that, when a phase transition happens, the chemical, physical, and/or structural properties are altered.
Phase diagrams are usually created by utilizing either the DSC, or the older DTA technique. The method is to generate appropriate substance mixtures and utilize the measured DSC/DTA effects to establish the desired phase diagram. The results which happen in alloys, the corresponding DTA curves and the establishment of phase diagrams are examined in detail in reference 1.
Figure 1. DSC curve of a NiAl alloy for determination of the solidus and liquidus temperatures.
Determination of NiAl Alloy Composition
If the phase diagram and the original substances are known, establishing the composition using the DTA curves, i.e., the opposite way, is also possible in principle. The below example shows the technique utilizing a NiAl alloy which is used in a variety of applications – due to its strong resistance to corrosion and high temperature – like driving mechanisms and components for engines in automotive and aerospace engineering.
The liquidus temperature was established to be at 1506 °C using the DSC curve, and the solidus temperature at 1435 °C by using the peak temperature and extrapolated onset, respectively (Fig. 1). This gives an approximate composition of 66% Ni and 34% Al (Fig. 2) for the alloy examined in this example according to the phase diagram2.
Figure 2. Phase diagram from reference 2. The temperature points obtained from the DSC curve suggest a composition of Ni 66% Al33%.
The heating curve was utilized to stop any problems with supercooling of the melt in this instance. The peak temperature was also employed in performing the evaluation, whereby the influence of the cooling of the sample (time constant) is considered.
References and Further Reading
- W. J. Boettinger, U.R. Kattner, K.-W. Moon, J.H. Perepezko DTA and Heat-flux DSC Measurements of Alloy Melting and Freezing NIST, Special Publication 960-15
- L. J. Kecskes, X. Qiu, R. Liu, J. H. Graeter, S. Guo, J. Wang Combustion Synthesis Reaction Behavior of Cold-Rolled Ni/Al and Ti/Al Multilayers Army Research Laboratory, ARL-TR-5507, April 2011
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