Tuning Ductility
Fusion reactors and turbine engines contain components made of metals that are ductile at high temperatures but become brittle and prone to cracking at room temperature. This brittleness can lead to machine failures that are both dangerous and expensive to fix. New theoretical calculations now show that an unexpected route to making certain alloyed metals more ductile at room temperature is to tune their density of conduction electrons.
A brittle metal tends to crack under an applied force, while ductile metals incur a permanent stretch. Which property dominates depends both on the intrinsic crystalline arrangement of the atoms and the presence of defects: brittle materials tend to keep their crystalline symmetry until the moment that they fail; ductile materials instead change from one type of crystalline symmetry to another before structural failure.
Liang Qi and Daryl Chrzan at the University of California, Berkeley, conducted quantum-mechanical modeling of defect-free metals to study how electronic structure affects the brittleness or ductility of certain alloys—specifically those with degenerate electron energy levels. As they report in Physical Review Letters, Qi and Chrzan found that as the Fermi level of the alloy passes through the degenerate energy levels, the crystalline structure distorts and the energy levels split as a result of the distortion. Since symmetry breaking is associated with ductility, tweaking an alloy’s composition so as to shift the density of conduction electrons—and hence the Fermi level—is a way to engineer substances that are ductile at room temperature. As a proof of principle, the authors showed (with calculations) that their strategy works for molybdenum-niobium, an alloy currently being considered for nuclear fuel rods. – Katherine Kornei