A phenomenon related to the invar effect — which enables magnetic materials such as nickel-iron (Ni-Fe) alloys to keep from expanding with increasing temperature — has been discovered in paramagnetic, or weakly magnetized, high-temperature alloys.
The research, which includes a general theory explaining the new invar effect, promises to advance the design of high-temperature alloys with exceptional mechanical stability. Short for “invariant,” invar plasticity enables magnetically disordered Ni-Fe alloys to show practically invariant deformation behavior over a wide temperature range, making them ideal for turbines and other mechanical uses in extremely high temperatures.
The invar effect, however, has never been fully understood. The new findings help explain the peculiar high-temperature properties of special alloys used in jet engines such as nickel-based superalloys. Invar has two known effects: thermal expansion and elasticity (the ability to spring back after bending). Because both of these effects are linked with the interplay between temperature and magnetic order, they are considered to be specific to magnetically ordered alloys.
Using first-principles quantum mechanical modeling, the researchers identified how invariant plasticity also occurs in non-magnetic alloys when a structural balance exists at the atomic level between cubic and hexagonal close-packed structures. The new finding broadens the palette of invar phenomena and material compositions, with clear implications for new applications.