D5 Thermal Analysis (DSC / Dilatometry)

Topic

Differential scanning calorimetry (DSC)—the first of the two experimental methods described by this model—heats or cools a sample of the metal at a controlled rate while continuously measuring the heat flow exchanged between the sample and an inert reference. When the metal undergoes a phase transformation, that transformation absorbs or releases additional heat; this additional heat appears in the test record as a heat flow peak. Because the metal undergoes a phase transformation during the test, the recorded heat flow deviates at that moment from the baseline followed by the rest of the record, thereby forming that peak.

It is the integral of this heat flow peak—calculated over the entire duration or temperature range of the event—that yields the enthalpy of the phase transformation: the total amount of heat absorbed or released by the transformation, over and above the heat exchanged during the same interval by a sample not undergoing transformation. If the phase transformation involves a greater number of atoms within the sample, the integral of the heat flow peak—and the resulting enthalpy of transformation—will also be greater.

Dilatometry—the second experimental method described by this model—records how the dimensions of a metal sample change as its temperature varies. Based on the resulting expansion or contraction curve, it calculates the metal's instantaneous coefficient of thermal expansion at each temperature—representing the local slope of the curve rather than a single coefficient averaged over an entire temperature range. If the sample expands more for a given temperature increase, the instantaneous coefficient of thermal expansion calculated at that point will be higher. Thus, this model—utilizing the differential scanning calorimetry and dilatometry with which it was introduced—reemploys the same pair of techniques already used for similar purposes by a model for a different set of materials (specifically, one that thermally characterizes a ceramic); it also reemploys, in particular, the same dilatometry used by a model for yet another set of materials—one that determines the glass transition temperature of a glass via the intersection of slopes on that same expansion curve. The fact that both the transformation enthalpy measured by this differential scanning calorimetry and the instantaneous thermal expansion coefficient measured by this dilatometry depend, in both cases, on a curve measured as a function of temperature means that both techniques share the same underlying experimental logic, regardless of the material to which they are applied.

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