Physicists Proposed Theory of Solidification of Nickel and Iron Alloys

Nickel-iron alloy is used when high dimensional stability of finished parts is required.
Photo: unsplash.com / Laura Ockel

Physicists at Ural Federal University have created a theory for the solidification of a nickel-iron alloy (invar). They determined that an important role in the technology of creating products from invar, namely in the solidification process, is played by the oncoming flow: when the alloy cools, the liquid layer flows on top of the solidified layer. If you regulate this process, you can control the characteristics of the alloys, obtain a more homogeneous structure, thereby improving the properties of the final product.

The work of scientists is extremely important because nickel and iron alloys are used in creating high-precision devices: clocks, seismic sensors, substrates for chips, valves and engines in aircraft structures, and instruments for telescopes. The calculations will help to create an alloy with the desired structure, which will affect the quality of the finished products. Description of the model and behavior of melts, as well as analytical calculations, scientists have published in the journal Scientific Reports. The research was supported by the Russian Science Foundation (Project No. 21-79-10012).

"Let me explain the work with an analogy. When water freezes, it pushes out all the dirt. So, you can put a piece of ice in your mouth, it will be clean. This is roughly what happens to melts when they cool. The only difference is that they do not push out all the impurities, but some of them. Some of the impurities leak out, and some of the impurities stay in the melt. What remains in the melt fills the gaps between the crystals, which solidify, and the voids, which remain. As a result, the alloys are heterogeneous: one tiny piece is enriched and the neighboring piece is not. This affects the properties of the finished product," says Dmitry Aleksandrov, Head of the Ural Federal University's Laboratory of Multi-Scale Mathematical Modeling.

The main thing the scientists showed was the processes in the two-phase layer: the layer that contains both the solid and liquid phases, inside it there is a transformation from liquid to solid state.

"This layer completely changes the crystallization scenario. For example, the temperature at each point of this layer is lower than the crystallization temperature, and crystallites and dendrites release the heat of phase transformation and thus partially compensate for supercooling. In addition, the growing solid phase displaces dissolved impurity, which lowers the crystallization temperature. These processes lead to the formation of complex branched solid phase structures, the gaps between which are filled with a liquid with a higher concentration of impurities," explained the essence of the process Lyubov Toropova, senior researcher at the Laboratory of Mathematical Modeling of Physical and Chemical Processes.

Reference:

An invar is based on the word "unchangeable" because the alloy almost does not expand or contract when the temperature changes. The first invar alloy was discovered by Swiss scientist Charles Édouard Guillaume in 1896. In 1920 he received the Nobel Prize in Physics for this discovery.

The alloy of nickel and iron is used when a serious dimensional stability of finished parts is required: in precision instruments, clocks, valves, etc. One of the first applications of invar is rods for pendulum clocks. At the time the pendulum clock was invented, it was the most precise chronometer in the world. Today, invar is also used in astronomy, as components that support the size-sensitive optics of astronomical telescopes.

Source/Credit: Ural Federal University

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