Nanotechnology

Deformation mechanism

Stacking-fault energy

Ultimate tensile strength

Creep

Strain hardening exponent

Nanocrystalline material

Strain rate

Dislocation

Composite material

Flow stress

Metallurgy

Materials science

Ductility (Earth science)

Grain boundary

Microstructure

Nature

Metals with nanocrystalline grains have ultrahigh strengths approaching two gigapascals. However, such extreme grain-boundary strengthening results in the loss of almost all tensile ductility, even when the metal has a face-centred-cubic structure—the most ductile of all crystal structures1–3. Here we demonstrate that nanocrystalline nickel–cobalt solid solutions, although still a face-centred-cubic single phase, show tensile strengths of about 2.3 gigapascals with a respectable ductility of about 16 per cent elongation to failure. This unusual&nbsp;combination of tensile strength and ductility is achieved by compositional undulation in a highly concentrated solid&nbsp;solution. The undulation renders the stacking fault energy and the lattice strains spatially varying over length scales in the range of one to ten nanometres, such that the motion of dislocations&nbsp;is thus&nbsp;significantly affected. The motion of dislocations becomes sluggish, promoting their interaction, interlocking and accumulation, despite the severely limited space inside the nanocrystalline grains. As a result, the flow stress is increased, and the dislocation storage&nbsp;is promoted at the same time, which increases the strain hardening and hence the ductility. Meanwhile, the segment detrapping along the dislocation line entails a small activation volume and hence an&nbsp;increased strain-rate sensitivity, which also stabilizes the tensile flow. As such, an undulating landscape resisting dislocation propagation provides a strengthening mechanism that preserves tensile ductility&nbsp;at high flow stresses. A nanocrystalline metallic alloy with ultrahigh tensile strength and good ductility is achieved by introducing compositional undulation in a highly concentrated solid&nbsp;solution.

Uniting tensile ductility with ultrahigh strength via composition undulation