The combination of niobium, tantalum, titanium, and hafnium in a new metal alloy is amazing material scientists. It’s strong and tough at hot and cold temperatures—a difficult feat. The alloy is also resilient to bending and fracture across an enormous range of conditions, and it could be the basis of materials for next-gen high-efficiency engines. The scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley, in collaboration with the groups at UC Irvine and Texas A&M, discovered the alloy’s properties. Their work was published April 11, 2024 in Science.
The alloy is from a new class of metals called refractory high or medium entropy alloys (RHEAs/RMEAs). Most metals are alloys made of one main metal mixed with small quantities of other elements, but RHEAs and RMEAs are made by mixing near-equal amounts of metallic elements with very high melting temperatures.
Most RMEAs have a fracture toughness of less than 10 MPa√m, causing them to be the most brittle metals on record. The best cryogenic steels, specially engineered to resist fracture, are 20x tougher than these materials. However, niobium, tantalum, titanium, and hafnium (Nb45Ta25Ti15Hf15) RMEA alloy was able to beat even the cryogenic steel, clocking in at over 25 times tougher than typical RMEAs at room temperature. The scientists evaluated strength and toughness at five temperatures total: -196°C (the temperature of liquid nitrogen), 25°C (room temperature), 800°C, 950°C, and 1200°C. The last temperature is about 1/5 the surface temperature of the sun.
The alloy had the highest strength in the cold and became slightly weaker as the temperature rose, but it still had impressive figures throughout the wide range. The fracture toughness was high at all temperatures.
The team analyzed the stressed samples alongside unbent and uncracked control samples, using four-dimensional scanning transmission electron microscopy (4D-STEM) and scanning transmission electron microscopy (STEM) at the National Center for Electron Microscopy, part of Berkeley Lab’s Molecular Foundry. The alloy’s unusual toughness comes from an unexpected side effect of a rare defect called a kink band. Kink bands form in a crystal when an applied force causes strips of the crystal to collapse on themselves and abruptly bend. The direction in which the crystal bends in these strips increases the force that dislocations feel, causing them to move more easily. This phenomenon causes the material to soften. The team knew that kink bands formed easily in RMEAs but assumed that the softening effect would make the material less tough by making it easier for a crack to spread through the lattice—but that did not happen.
They showed, for the first time, that in the presence of a sharp crack between atoms, kink bands resist the propagation of a crack by distributing damage away from it, preventing fracture and leading to extraordinarily high fracture toughness.
