Metal alloys made up of nanoscale crystals are far stronger and harder compared to their larger counterparts. The benefits of these ‘nano alloys’ have been limited by the fact that their nanocrystalline structure breaks down at relatively high temperatures. The EU-funded TheSBIE project developed an iron-gold alloy that retains its nanocrystalline structure at temperatures above 1 000 °C – close to the melting point of gold.
Big problems with tiny crystals
Metals comprising crystallites are usually unstable at high temperatures that are routinely employed in metal fabrication and processing. Crystallites can grow and merge together, causing the metals to soften which is undesirable. “Neighbouring crystallites have different orientations, leading to the formation of a grain boundary between them. This interface is disordered especially at high temperatures, causing large areas of comparatively open structure between the crystallites,” explains Dor Amram, TheSBIE coordinator. Therefore, the atoms that occupy the grain boundary are in a higher than usual energy state – ideally, they would prefer to squeeze inside crystallites rather than in their interfaces. This excess energy stems from the crystal striving to expel the grain boundaries. The result is that grains tend to grow, ideally up to the size of a single crystal.
A novel thermodynamic approach to reduce excess energy
To overcome fundamental stability issues of nanocrystalline alloys at high temperatures, project researchers employed an approach called ‘thermodynamic stabilisation by interface engineering’ that relies on segregation. Amram explains the principle behind the concept: “Segregation of a solute to grain boundaries produces a section of material with a discrete composition and its own set of properties that can positively or negatively impact on the overall material properties. Importantly, these ‘zones’ with an increased (or decreased) concentration of solute can reduce the excess energy of the grain boundaries so that the crystal no longer ‘desires’ to remove them. The driving force for grain growth becomes zero, and the nanocrystalline alloy is thermally stable.” “Our results challenge textbook wisdom that associates grain growth acceleration and strength reduction of the nanocrystalline alloy with rising temperatures. Grain sizes that decrease at elevated temperatures open up a world of possibilities in material design, especially for applications that require high-strength materials at high temperatures,” adds Amram.
A better alternative to kinetic stabilisation
The mechanism governing thermal stability of nanocrystalline materials is not solely a thermodynamic one, but also a kinetic one which is old news. The kinetic stabilisation of nanocrystallines involves scattering secondary particles of different phase in the alloy. These particles serve as ‘anchors’, retarding or impeding grain boundary motion. This approach offers little benefits as it does not tackle the driving force of the system for grain boundary growth. “It is just like throwing sticks under the wheels. The material will ultimately find its way to increase the grain boundary size,” says Amram. Project results challenge fundamental research on grain boundaries in nanocrystalline materials. “Grain boundaries having zero (or negative) energy is a polarising and contentious topic in the materials science community. Theoretical work and simulations may run counter to our results, but we experimentally demonstrated this is indeed possible,” concludes Amram.
TheSBIE, high temperatures, grain boundaries, crystallites, nanocrystalline alloy, metal, iron-gold, segregation, thermodynamic stabilisation