Lattice Stabilities
The CALPHAD technique, most likely started by Johansson in 1937, but abandoned shortly thereafter, was finally taken up at MIT in the early 1950s. Professor Morris Cohen, who studied martensite, had a graduate student, Paranjpe, who had some interest in thermodynamics. He found and appreciated Johansson’s paper, published in German and containing the word “Martensit” in the title. He showed the paper to his professor, who soon engaged a new graduate student, Larry Kaufman, to study the formation of martensite in the Fe-Ni system and to relate it to the results of what we now regard as a CALPHAD assessment.
There was an important difference between the Fe-C and Fe-Ni systems because C is an interstitial solute and Ni a substitutional solute. The end-members of the bcc solution phase are bcc-Fe and an FeC3 compound for the first system but bcc-Fe and bcc-Ni for the second one. Kaufman thus had to evaluate the properties of bcc-Ni, which is not known experimentally. Whether it should be regarded as metastable or unstable is still being discussed to this day, but Kaufman had to assign it some thermodynamic properties in order to carry out his thermodynamic analysis. He was thus forced to evaluate what he later called the lattice stability of bcc-Ni. His work was published in 1955.
Larry Kaufman graduated and left for a job at a research company where he could continue on the same line of research. Thanks to considerable government contracts, he was able to work on a large number of related systems and soon realized that he should use the same lattice stability for a non-stable state of an element in whatever system it appeared. It was thus necessary to produce a set of recommended lattice stabilities for a large number of elements in the most common states. He was able to present such a list in 1967. It should be noted that there is no need for a similar list for interstitial solutions. The FeC3 compound only appears in the Fe-C system.
It thus took 30 years from the first CALPHAD assessment to the creation of the first list of lattice stabilities, which are a necessity for rational CALPHAD assessments.
This article is the third in a three-part series tracing the evolution of CALPHAD from the Gibbs Phase Rule to the first lattice stabilities.
