Zero Gravity Helps Create Homogeneous Material Structure

The mathematical model of the UrFU scientists helps to simulate the solidification process of an alloy. Credit: unsplash.com / Dan Cristian Pădureț

In space, due to the absence of gravity, metal hardens more homogeneously than on Earth. This was discovered by physicists who calculated the solidification process of aluminum-nickel metal alloys. Alloys were not chosen by chance, as they are one of the most common and account for 20% of all metalworking in the world. The model was built based on experimental data: the results obtained for alloy samples in microgravity on board the International Space Station were compared with the results of samples processed in terrestrial conditions. The results of experiments and modeling are presented in the journal Acta Materialia.

All aluminum-based materials are produced from the liquid phase, which is the initial phase. The solidification process and the conditions present at the moment of solidification determine the microstructural state of the final part, the scientists explain. The model considers the effects of crystallization rate and supercooling on the formation of alloy structure and properties, and allows the correct prediction of the microstructure and the required mechanical and electrical properties of the alloy.

"Our model will allow us to simulate the solidification process of an alloy without the time- and energy-consuming trial-and-error tests. We first processed data from the International Space Station and found out what factors affect the microstructure of samples obtained under quasi-weightlessness or microgravity conditions. Processing an alloy with the same percentage of metals in terrestrial and space conditions leads to the fact that their structure differs significantly from each other: in weightlessness, alloys get more homogeneous structures," explains Pyotr Galenko, Head Specialist at the Laboratory of Multi-Scale Mathematical Modeling at UrFU.

As the researchers explain, in zero gravity, container less conditions are created, meaning that there is no contact between the metal and the walls of crucibles in which alloys are contained on Earth. Scientists have also identified two other important factors that are influenced by the presence or absence of gravity during volumetric crystallization. The first factor is the convection of the liquid phase. In convective heat transfer, internal energy is transferred by the jets and flows of the substance itself. It occurs when the alloy begins to crystallize, to solidify into the desired shape, and convection determines how well the metals blend when the part is formed. The second important factor is supercooling of the melt. Crystallization changes the density of the alloyed sample as it cools and moves from a liquid state to a solid crystalline state. As the scientists explain, usually the rate of crystallization always increases with increasing supercooling of the liquid, but the state of weightlessness creates an anomalous effect - at some point the dependence of the rate on supercooling begins to fall.

"The dependence of crystallization rate on supercooling is important for the formation of the alloy structure. For example, the formation of the so-called intermetallic phase is observed in an alloy with an equal ratio of aluminum and nickel - fifty to fifty percent - which at low supercooling is formed as an ordered superlattice crystal with the appearance of large dendritic (tree-like) crystallites. Such structure provides improved electrical conductivity of the material. At higher supercooling there is a significant increase in crystallization rate, there is an effect of entrapment of disorder from the liquid. In this case the superlattice does not have time to form, resulting in disordered micro- and mesostructure of crystallites. They provide enhanced mechanical properties such as microhardness and resistance to mechanical fracture of samples crystallized at high rates. Taking into account the anomaly, in which the supercooling rate can decrease, will allow modeling the microstructure of materials," adds Dmitry Aleksandrov, Head of the Laboratory of Multi-Scale Mathematical Modeling at the Ural Federal University.

The work was supported by the Russian Science Foundation (project No. 21-19-00279). The researchers note that this is only the first stage of work to study the effect of gravity on the microstructure of alloys. Many other factors, such as the alloy's contact with the crucible walls, will also need to be taken into account when transferring laboratory studies to an industrial scale.

Reference

All alloys were processed in an electromagnetic levitation facility on Earth under reduced gravity and under quasi-weightlessness or microgravity conditions, which occurs in the levitators installed on space devices on the International Space Station. Electromagnetic levitation (EML) is a method for directly observing the solidification process of electrically conductive samples and estimating the thermophysical parameters of the liquid phase using pyrometers and high-speed digital cameras. The EML method is used to study the rate and morphology of solidification of metallic materials as a function of supercooling. In this setup, the samples are free to levitate in the ultrapure medium throughout the experiment, thus avoiding the influence of contact with the crucible walls and foreign impurities.

Conditions of quasi-weightlessness occur in levitators mounted on sounding rockets such as Texus; airbuses providing parabolic flights; on the European Columbus bay of the International Space Station.

Source/Credit: Ural Federal University

phy110822_01