The History of Education in Computational Thermodynamics – the KTH Experience

Computational thermodynamics is a rapidly developing field at the forefront of materials design. But did you know that the field is already over 40 years old? This year at the TMS Annual Meeting in San Antonio, Texas, John Ågren, one of the original developers of Thermo-Calc, gave a presentation on the history of computational thermodynamics at the Royal Institute of Technology (KTH) in Stockholm, Sweden, one of the earliest schools to teach computational thermodynamics. In his fun and fascinating presentation, he discusses how the education of computational thermodynamics started, which issues arose and how they were solved.

A Nord 10 16-bit mini computer
A Nord-10 mini computer similar to the once used by the Materials Science and Engineering (MSE) department at KTH in the 1970s.

It all started in the late 1970s when the Materials Science and Engineering (MSE) department at KTH got a minicomputer which was used in both research and education. But in the early days they came across some unexpected issues. It appeared that most students disliked computers and couldn’t do much coding themselves, which resulted in teachers spending a lot of time debugging codes. Besides that, there were also technical issues. About 30 students were working on the same computer at the same time, which made the response times very long. These difficulties made it hard for the students to understand the point of the computational exercises. As an attempt to solve these problems, teachers prepared codes for the students and handed out some written material about the underlying physics. Despite this, most time was spent on making correct inputs, which made the students not likely to understand the role of computers in materials science.

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Thermo-Calc Used to Predict Phase Fractions when Partially Substituting Cobalt

Cobalt symbol on blue background with the words Thermo-Calc used to predict phase fractions when Partially Substituting Cobalt

A new publication in Acta Materialia investigates the possibility to partially substitute Cobalt with entropic alloys, focusing on the relative stabilities of the fcc and hcp structures.

Cobalt-base alloys are important for high temperature applications due to their possibility to form duplex fcc + hcp structures and their low stacking fault energy. However, there is an interest in substituting cobalt for economical, ethical and health reasons.

Both the thermodynamic and kinetic calculations in this publication were performed with Thermo-Calc software and the Diffusion module (DICTRA) together with the thermodynamic database TCHEA and the kinetic database MOBNI. Thermo-Calc was used to predict the phase fractions of fcc and hcp which were compared with experimental results. In the article, it is stated that the calculated thermodynamic values correlate relatively well with the experimental values. The authors concluded that designing duplex fcc + hcp Co-based alloys with computational tools is feasible.

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Thermo-Calc used to Estimate Critical Temperatures in Type 410 Steels

A person welding with the words Thermo-Calc used to estimate critical temperatures in type 410 steels written on top

In the paper Alloy Composition and Critical Temperatures in Type 410 Steel Welds, Thermo-Calc was used to evaluate the effect of alloy composition on critical temperatures (A1 and A3) in Type 410 steels.

Type 410 steels are typically welded by using consumables with matching composition. However, this type of steel has shown to have poor weldability which is related to formation of hard and brittle martensite in the weld zone, hydrogen-included cracking or retention of δ-ferrite which affects the toughness. One theory that explains the inconsistent toughness is that the wide composition ranges of the base metal results in wide variations of the A1-temperature. In the paper, this theory was investigated with the design of experiment (DoE) approach using Thermo-Calc to perform thermodynamic simulations. Thermo-Calc together with the TCFE8 database was used to predict A1 and A3 temperatures for various compositions.

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Paper Investigates Carbon Behavior in the Heat-Affected Zone of Grade 91 Steel using DICTRA

A macro view of the weld metal, heat affected zone and base metal from section with the words "investigation of carbon behavior in the heat-affected zone of grade 91 steel using DICTRA".
A macro view of the weld metal, heat affected zone and base metal from section.

In a paper out of Ohio State University, the mechanism of δ-ferrite retention in the coarse-grained heat-affected zone is studied.

The efficiency of fossil-fired and nuclear power plants has caused raised operating temperatures, which requires use of creep-resistant stainless steels in the hottest regions of the plant. Grade 91 steels are used in the lower-temperature heat recovery steam generators. To be able to join the high- and low-temperature sections, dissimilar metal welds (DMWs) are necessary. The problem with using DMWs is that it often results in extensive carbon diffusion near the fusion boundary which creates brittle and large carbides that make the welds weaker.

Creep strength-enhanced ferritic (CSEF) steels are today welded with Ni-based filler metals to reduce the carbon diffusion between the dissimilar steels, which reduces the formation of hard and soft zones that negatively affects the creep strength. However, the high concentration of carbide forming elements in Ni-based alloys still creates a driving force for carbon diffusion toward the filler metal.

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