The SPhTI Laboratory of High-Strength Crystals’ staff has found a way to harden multicomponent alloys that can withstand critically low temperatures close to -200 °C. Such materials are resistant to liquid media and have increased wear resistance. As a result, new modified alloys can be used to harden the bow of icebreakers, make wear-resistant cutting material, and for other purposes.
– A distinctive feature of high-entropy alloys (HEAs) is their composition. Such materials contain five or more metals, mixed in approximately equal quantities, – explains Anna Vyrodova, a laboratory staff member. – HEAs have unique mechanical properties: high strength and good plasticity while maintaining fracture toughness (slowness) up to cryogenic testing temperatures. This behavior is unusual, because in traditional structural materials (austenitic steel, Hadfield steel) an increase in strength is accompanied by an increase in the brittleness of the composite.
At present, the Laboratory of Physics of High-Strength Crystals’ staff is studying high-entropy alloys – FeNiCoCrMn and (CoCrFeNi) 94Al4Ti2.
The physicists have found a way to significantly increase the strength of the alloy (CoCrFeNi) 94Al4Ti2. They first subjected it to deformation at a temperature close to -200 °C, then held it for 4 hours at a temperature of 650 °C. After that, the strength of the HEA increased 250% both at temperatures from room temperature to -196 °C and at high temperatures up to 700 °C.
This material can be used at extremely low temperatures, for example, for the manufacture of valves on oil pipelines in the Arctic. The high-strength alloy, resistant to water, is promising for the shipping industry, for example, for strengthening individual parts of icebreakers.
As the scientists note, an increase in the strength of (CoCrFeNi) 94Al4Ti2 occurs while maintaining predominantly ductile fracture. This quality is an advantageous difference because the ductile fracture is less dangerous than the brittle one. In the case of a brittle nature of the fracture, the crack nucleates and propagates rapidly, and ductile fracture is preceded by significant preliminary elongation, slow formation, and propagation of the crack. This means that when using HEA, a defect can be noticed at the initial stage and measures can be taken before the destruction of a part or structure.
As for the strength properties of the FeNiCoCrMn alloy studied by the TSU physicists, it has other features. At low temperatures (close to the temperature of liquid nitrogen), the alloy retains high strength, its plastic deformation begins at a stress of 0.5 GPa, and at high temperatures (from room temperature and above), the alloy becomes low-strength and begins to deform at stress below 0.2 GPa.
Now the laboratory is solving the problem of increasing the strength properties of high-entropy alloys at high temperatures. This will open up the potential for their use as single crystal blades for gas turbines.