Harder composite material for nuclear conditions

Materials World magazine
,
29 Oct 2019

A new composite material with greater microhardness has been developed and can withstand harsh conditions such as those in nuclear rectors. Shardell Joseph finds out more.

A composite material has been developed that can achieve high microhardness and withstand temperatures up to 700°C, making it a suitable fit for the conditions found inside nuclear reactors.

Scientists from the National University of Science and Technology (NUST MISIS), Russia, layered a steel and vanadium alloy hybrid material to achieve three times the microhardness compared to its individual components.

Explained in the paper High thermally stable multi-layer steel/vanadium alloy hybrid material obtained by high pressure torsion, published in Materials Letters, the steel and vanadium alloy hybrid was developed to provide a solution to the structural limitations of materials, where operating at high temperatures will often be accompanied by loss of strength, along with radiation and corrosion resistance.

To do this, the scientists used a method of severe plastic deformation (SPD), which deforms in special machines under high pressure. During this process, a smaller nanocrystalline structure is formed in the material, which has rapid grain growth when the material is heated. This gives it a low thermal stability and loss of microhardness when heated. The scientists combatted this problem by using high-pressure torsion (HPT), an SPD method, which allowed the creation of a specific multilayer structure with vanadium alloy.

‘We created a sample with 0.5mm and 0.3mm steel layers, adding 0.2mm vanadium alloy in between,’ said NUST MISIS Head of Research, Stanislav Rogachev.

‘Hence, the overall thickness of the sample was 1mm. During the HPT, the sample is placed between two flat-base strikers and compressed under an applied pressure of several hectopascal (HPa). The lower striker rotates and the surface friction forces the sample to deform by shear. As a result, we obtained a thin multilayer structure.’

Testing the hybrid

The researchers described in the paper how they carried out the HPT of the three-layer sample, using a Bridgman camera type installation with a lower profiled anvil, at a pressure of 6GPa, five turns of the anvil and temperatures of 20-400°C.

For comparative measure, the team used flat samples of the vanadium (V–10Ti–5Cr) alloy and steel type AISI 439 with a diameter of 8mm and initial thickness of 1mm. This was processed by the HPT with the same conditions.

The samples obtained by HPT were heated from 100-800°C, exposed for an hour and cooled with the furnace. The single component vanadium alloy sample and the vanadium and steel alloy hybrid were then heated in a vacuum of 1.3x10-3Pa in order to eliminate interaction of the oxygen and nitrogen with the vanadium alloy temperatures above, leading to a considerable increase in hardness and embrittlement.

According to the paper, test results showed that HPT of the vanadium alloy samples at 20-400°C led to an increase of microhardness by two times, up to 4.09–4.36GPa, and HPT of the steel samples at the same temperatures led to microhardness rising 3.3-3.5 times, to 5.90–6.05GPa. However, when heated to 500-600°C, there was a monotonic decrease in the vanadium alloy sample’s microhardness value to 3.30GPa, and the steel samples decreased to 2.1-2.4GPa.

Heating of the hybrids obtained by HPT to temperatures of 400–450°C, however, did not lead to a change in the microhardness values. ‘Heating to temperatures of 500–700°C leads to a decrease in the microhardness of the outer steel layers up to values close to those for the initial steel – 2.10GPa,’ the study said.

‘At the same time, the microhardness of the middle layer of the hybrid obtained by HPT at 20°C, having an ultrathin multilayer structure, does not change up to a temperature of 700°C and is 4.4–5.2GPa at the sample mid-radius and 5.4–6.1GPa at the sample periphery.’

The team is planning to continue experiments on SPD of metal composites, specifically working on combinations of steel and zirconium, steel and copper and steel and aluminium.