Additive manufactured steel breaks strength-ductility trade-off

Improving the strength of metals and alloys usually comes at the price of reduced ductility. But now an international team of researchers has found a way of breaking the strength-ductility trade-off for stainless steel [Liu et al., Materials Today (2017), doi: 10.1016/j.mattod.2017.11.004].

The researchers, from Stockholm University in Sweden, the University of Birmingham in the UK, and Zhejiang University in China, rejected conventional methods in favor of additive manufacturing which builds up components layer by layer. Selective laser melting (SLM), a type of additive manufacturing, uses a high-energy laser to fuse together layers of deposited metal particles. The deposition and lasing steps are repeated to build up three-dimensional components. 

But to their surprise, SLM produces metal parts with completely different microstructures to those found in cast or wrought steel. The team observed a unique dislocation network resembling an irregular honeycomb pattern in samples of SLM-316L stainless steel. Dislocations are concentrated in the ‘walls’ of columnar cells that are 500 nm in diameter and range in length from a few to tens of microns.

“We discovered that the dislocation network structure is quite unique - different from the dislocation cells observed in deformed metals,” says Leifeng Liu, first author of the study.

The researchers believe that the unusual dislocation network, which has also been found in one- and two-dimensional structures, is a result of the high temperature gradient and growth rate conditions that arise during SLM. It is this unusual dislocation network that is the basis of the metal’s exceptional mechanical properties.

The properties of most metals are determined by their plastic deformation behavior. This, in turn, depends on the motion of dislocations within the metal’s microstructure. Usually, metals are strengthened by introducing features that interrupt the motion of dislocations through the structure – such as different phases, grain boundaries, or other internal interfaces. But in SLM-316L stainless steel, the unusual dislocation network changes all that. 

“It regulates the motion of other dislocations, impeding dislocation motion in the beginning to strengthen the steel then, when the stress increases, slowing it down while still allowing dislocations to pass through, which stabilizes plastic deformation (increasing the ductility),” explains Liu.

The combination produces impressive results: the tensile yield strength of SLM-316L stainless steel is over twice that of wrought-annealed steel, with compression tests showing a similar trend, while the failure elongation (or ductility) is simultaneously improved. 

“We thought that the major advantage of AM was the ability to manufacture parts with complicated shapes. But now we think AM can also be used as a tool to design materials, by controlling the microstructure to achieve certain mechanical properties,” says Liu.

The ability to produce complex metal parts with outstanding mechanical properties, which cannot be achieved by conventional means, marks AM out as a disruptive technology for many different industries.