Traditional materials often face a trade-off between strength and ductility, which limits their performance in demanding applications. This research, led by Dr Muhammad Naeem and Dr Liliana Romero and comprising a team from the universities of Birmingham, Bournemouth and Kings College London, in collaboration with ISIS scientists, addresses this challenge by demonstrating a material that significantly enhances both properties, particularly in extreme environments.
Many modern technologies, such as hydrogen storage, aerospace, superconducting magnets, and polar infrastructure, operate at very low temperature. Under these cryogenic conditions, conventional materials typically become brittle, but in this study, published in Scripta Materialia, the researchers identified a type of steel that retains excellent strength and ductility as low as 77 K, making it ideal for these applications.
The steel that was the focus of this study was the heterostructured and antimicrobial stainless steel (HS&AMSS) 316L+3Cu. The team found that it had a yield strength of 1400 MPa and 36% ductility at 77 K. They then came to ISIS to use the Engin-X beamline to find out what was happening to the structure of the steel that gives it these excellent properties.
“The unique insight gained by using neutrons in this study comes from their ability to provide real-time, in-situ analysis of the phase transformations and microstructural evolution under tensile deformation. Thanks to the custom-designed stress rig cryostat developed by the ISIS Sample Environment Group, this could even be measured at cryogenic temperatures," explains ISIS scientist and paper co-author Tung Lik Lee. “Neutron diffraction is particularly well-suited for studying bulk materials because of its deep penetration depth, which allows for the examination of internal structures and phase distributions in a non-destructive manner."
Through their ISIS experiments, the researchers gained critical insights into the mechanisms underlying the material's enhanced mechanical properties at cryogenic temperatures. They found that the improved combination of strength and ductility in HS&AMSS is driven by the synergy between dislocation-blocking capabilities of defects, which enhance strengthening, and the formation of new phases or domain regions that provide sites for dislocation accumulation, thus enhancing ductility.
This interaction between defects and phase transformations at cryogenic temperatures results in significant strengthening and strain hardening. Identifying these effects as key factors in enhancing strain hardening at low temperatures offers new pathways to engineer materials with superior mechanical properties, potentially influencing the design of future alloys and composites.
The synergistic effect between different microstructural mechanisms, such as dislocation-blocking and phase transformation, underscores the potential of heterostructured materials to revolutionise the development of metals with tailored properties for specific applications.
The full paper can be found at DOI: 10.1016/j.scriptamat.2024.116527
