Cooling Curve Analysis as an Alternative to Dilatometry in Continuous Cooling Transformations
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TRODUCTION
DILATOMETRY is commonly used to quantify the progress of solid-state phase transformations during continuous heating or cooling because it gives accurate and simple to interpret results that indicate start and finish temperatures of the transformation, relative phase fraction as a function of temperature and, when combined with information about the relative densities of the two phases, it can be used to determine the absolute amount of second phase formed as a function of temperature.[1–4] One of the downsides of dilatometry is that it requires specialized equipment to make precise length/diameter measurements; this can be a limitation if one does not have access to a dilatometer or if a dilatometer is incompatible with a concurrent experiment (e.g., a tensile test or a synchrotron experiment). One proposed alternative method that has a significantly simpler experimental setup is cooling curve analysis (CCA); a technique that involves measuring the temperature of a sample as it cools and transforms, then using the transformation-induced deviations from single-phase cooling behavior to determine the phase fraction evolution.[5,6] The goal of both dilatometry and CCA is to obtain phase fraction evolution information of a material as it undergoes monotonic heating or cooling. Because of the JOHN W. GIBBS, Ph.D. Student, is with Northwestern University, Evanston, IL. Contact e-mail: [email protected] CHRISTIAN SCHLACHER, Ph.D. Student, is with the Graz University of Technology, Graz, Austria. ATA KAMYABI-GOL, Ph.D. Student, and PATRICIO F. MENDEZ, Professor, are with the University of Alberta, Edmonton, Canada. PETER MAYR, Professor, is with the Technische Universita¨t Chemnitz, Chemnitz, Germany. Manuscript submitted April 14, 2014. Article published online October 15, 2014 148—VOLUME 46A, JANUARY 2015
similarities in the results and sample conditions during the experiment, dilatometry and CCA can be seen as complimentary techniques; however, there has never been a published, quantitative comparison of the two. This work is intended to provide an initial comparison for one type of transformation. For the comparison between the two methods, an austenite to martensite transformation is used because this is a common transformation to be analyzed using dilatometry. Dilatometry and CCA techniques are well suited to this type of transformation because both methods provide information on both the formation temperature and the amount formed as a function of temperature. The properties of the transformed martensite depend on the temperature at which it forms and the properties of the alloy depend upon the properties and amount of martensite present. While this one comparison does not make for a comprehensive comparison between the two methods, the combination of the derivations of the two methods and the results should make it possible to determine which of the two methods presents a better set of compromises for almost any given transformation. Although the following methods are not the focus of this paper, it is im
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