How  DSC Measurements help with Phase Diagrams – A Possibility for Determining the Composition of a NiAl Alloy

How DSC Measurements help with Phase Diagrams – A Possibility for Determining the Composition of a NiAl Alloy

E. Post, J. Blumm

A phase diagram portrays the equilibrium conditions between the thermodynamically separated phases or it shows which phases are present in the material system at different temperatures, pressures and compositions. Thermoanalytical methods play an important role in establishing binary and ternary phase diagrams. They are based on the principle that, when a phase transition occurs, the physical, chemical and/or structural properties change.

A phase diagram portrays the equilibrium conditions between the thermodynamically separated phases or it shows which phases are present in the material system at different temperatures, pressures and compositions. Thermoanalytical methods play an important role in establishing binary and ternary phase diagrams. They are based on the principle that, when a phase transition occurs, the physical, chemical and/or structural properties change. Phase diagrams are often established using the DSC or the older DTA method. The approach is to produce appropriate substance mixtures and use the measured DSC/DTA effects to establish the desired phase diagram. The effects occurring in alloys, the corresponding DTA curves and the establishment of phase diagrams are discussed in detail in [1].
Fig. 1. DSC curve of a NiAl alloy for determination of the solidus and liquidus temperatures
Fig. 1. DSC curve of a NiAl alloy for determination of the solidus and liquidus temperatures
The opposite way – i.e., determining the composition on the basis of DTA curves – is, of course, also possible in principle, if the phase diagram and the original substances are known. The following example illustrates the approach using a NiAl alloy which – due to its strong resistance to corrosion and high temperature – is used in a variety of applications, such as components for engines and driving mechanisms in aerospace and automotive engineering. With the help of the DSC curve, the liquidus temperature was determined to be at 1506°C  and the solidus temperature at 1435°C by means of the peak temperature and extrapolated onset, respectively (fig. 1). According to the phase diagram [2], this yields an approximate composition of 66% Ni and 34% Al (fig. 2) for the alloy investigated here.
Fig. 2 Phase diagram from [2]. The temperature points obtained from the DSC curve suggest a composition of Ni 66% Al33%.
Fig. 2 Phase diagram from [2]. The temperature points obtained from the DSC curve suggest a composition of Ni 66% Al33%.
In the present case, the heating curve was used to eliminate any problems with supercooling of the melt. The peak temperature was also used in carrying out the evaluation, whereby the influence of the cooling of the sample (time constant) is largely taken into account. [1] 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 [2] 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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with the present heat rate 20K/min, is the alloy sample reach equilibrium state? how to connect with the continuous heating process to the real phase equilibrium? And, the definition of liquidus temperature and solidus temperature at Fig 1, I don’t understand. In my mind, I think the present case is the special example that the two peak corresponding to liquidus temperature and solidus temperature were overlapped with each other. I am sincerely hope you consider my comment and help me to better understand this part content, thank you!