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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2 thoughts on “How DSC Measurements help with Phase Diagrams – A Possibility for Determining the Composition of a NiAl Alloy”

  1. 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!

    • Thank you for your constructive feedback.

      DSC is, of course, a dynamic method and therefore, equilibrium must be kept in mind. As we use heating curves, the solid-to-liquid phase transition is hereby less critical. Using cooling curves, one has to “fight” against supercooling effects.

      The NIST publication [lit 1] recommends for some cases to apply different heating rates for the approximation to heating rate zero. This works for heating cycles, but often during cooling, supercooling is still a problem and the values are scattering.

      The solidus curve can be determined with the DSC as extrapolated onset of the peak. This is possible because the temperature calibration of the DSC instrument is carried out in the same way by a melting standard. This means, time constants from the experiment arrangement should be negligible, at least for melting. The time constant for the liquidus temperature gives more uncertainties. Corrections can be done, e.g., by approximation to heating rate zero (as mentioned before) or a correction of the raw data by a special correction software. In this application example, we used the rough approximation taking the peak temperature. Using the extrapolated endpoint would not consider the time constant of the sample and would lead to a too high value.

      Ekkehard Post, NETZSCH Lab

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