Metals are not usually associated with the verb "to creep" in the minds of most people, in fact metal is almost the epitome of stability. Nevertheless, metal creep is a real phenomenon, and depends on the temperature of the metal in relation to its melting temperature. "It starts at about half the melting temperature," as Prof. Siegfried Schmauder, head of the Department of Multi-Scale Simulation at the University of Stuttgart’ Institute for Materials Testing, Materials Science and Strength of Materials (IMWF) explains. The higher the temperature of the metal, the more its atoms will vibrate and the stronger the vibration, the more likely it will be that atoms will change places: under a constant external stress, the metal will begin to deform.
Of course, the creep that takes place in metallic elements and alloys is not just an atomic-level phenomenon. Take aircraft turbines for example: temperatures of more than 2000 degrees Celsius are reached during the fuel combustion process inside these machines, and their components rotate at high speeds generating the corresponding centrifugal forces. The risk of wear and tear is increased if the alloys from which the turbines are made begin to creep under these conditions. Similar conditions prevail in power plant gas turbines.
Even glass can 'creep' at room temperature, but it would take many hundreds of thousands of years until the effect could be measured.
Prof. Siegfried Schmauder, head of the Department of Multi-Scale Simulation at the Institute for Materials Testing
Experiments require an enormous effort
Because wear and tear affects the cost-effectiveness of such systems as well as their sustainability, the development of robust and, above all, creep-resistant materials is a perennial task of industry. At an experimental level, this involves an enormous effort: new alloys have to be cast and tested in lengthy trials under extreme conditions. And, because it is usually not just one novel alloy that gets tested, the costs of such development processes, which are still standard in many places, increase.
Yet the still young field of virtual materials development has increasingly been making a name for itself over the past few years. Rather than analyzing novel alloys in a long series of experiments, they are first tested in the University of Stuttgart’s high-performance computer before promising alloys are actually confirmed in physical experiments.
"All this was still a pipedream when I began my doctoral thesis in the 1980s.” says Schmauder: Nobody believed it could work." Even then it took several years before researchers in Stuttgart managed to calculate the formula for a new iron and copper alloy in 2008.
Several groups of researchers, working under the auspices of the Stuttgart Center for Simulation Science Cluster of Excellence (SimTech), are currently trying to identify creep-resistant alloys. One project involves research into so-called Ni-based superalloys, the main area of application of which is in turbines. These nickel and aluminum alloys are considered to be resistant to phenomenon known as atomic dislocation.
How does temperature affect the behavior of atoms?
The researchers involved in this project are now planning to use modeling to find out how temperature influences the behavior of the atoms within this alloy and under which conditions atomic dislocations occur. This special nickel alloy is particularly interesting in this context, because, to a certain extent, it forms obstacles to the dislocation at the atomic level, thus preventing creep.
"This will help us to understand how this mechanism is affected by temperature, which will open up ways of increasing this barrier as much as possible," explains Schmauder. At the same time, the research group also wants to look into how hydrogen affects the barriers, as previous applications of the material have shown thathydrogen can have a negative influence on creep resistance.
The effects of hydrogen are also being investigated in parallel research into how the hydrogen atmosphere’s pressure and temperature affect material behavior. Theobjective is to enable the production of components that are both cost-effective and reliable, and which, for example, could be used in vehicle hydrogen tanks.
Cobalt and rhenium for particle reinforcement
In another project, in which the IMWF is collaborating closely with the TU Braunschweig’s Institute for Materials, the simulation experts are investigating a creep-resistant alloy of cobalt and rhenium reinforced with tantalum carbide, which is a compound of tantalum, a metallic element, and carbon, whereby their primary focus is on the question of how the particle amplification mechanism generated by the tantalum carbide behaves under load.
The pure cobalt-rhenium alloy is already considered to be extremely hard, but the project is based on the assumption that the alloy needs to be made even more durable for use under high temperatures and mechanical loads. "We need to find a solidification mechanism that is effective at high temperatures,” says Prof. Joachim Rösler from the TU Braunschweig: “In this project, we want to investigate the potential of metal-carbon carbides for this purpose." To this end, researchers from both universities are carrying out simulations and experiments in parallel.
Extensive preparatory work was first needed to enable such complex simulations. "Modern computers can do a lot," Prof. Schmauder explains, "but they’re still unable to model components at the atomic level.” The researchers used a trick to make the phenomena visible in the simulation using a proportionate amount of computing resources: they coupled calculation processes in series. "We calculate the input we need at the next level up at the atomic level,” Schmauder explains. At that level the researchers then obtain the results for the macroscopic level.
Editor: Jens Eber
Prof. Siegfried Schmauder, Institute for Materials Testing, Materials Science and Strength of Materials (IMWF), e-mail, phone: +49 711 685 62556