“A holy grail of structural materials has long been, how do you simultaneously enhance strength and ductility? Defeating the strength-ductility trade-off will enable a new generation of lightweight, strong, damage-tolerant materials.”Easo George
Researchers from the Department of Energy's Oak Ridge National Laboratory and the University of Tennessee, Knoxville, have demonstrated how to increase both the strength and ductility of an alloy using tiny precipitates in its matrix and then tuning their size and spacing. The findings could lead to enhanced resilient structural building materials and vehicles that are lighter and more energy-efficient to manufacture and operate.
As reported in Nature [Yang et al. Nature (2021) DOI: 10.1038/s41586-021-03607-y], model alloys were designed and produced that could undergo a phase transformation from a face-centered cubic (FCC) to a body-centered cubic (BCC) crystal structure from changes in either temperature or stress. By putting nanoprecipitates into a transformable matrix and precisely controlling their attributes, the team showed control of when and how the matrix transformed, demonstratinghow microstructure control can help activate different deformation mechanisms to achieve the desired balance of strength and ductility.
The alloy contains iron, nickel, aluminum and titanium that form the matrix and precipitates, as well as carbon, zirconium and boron to limit the size of the grains. At room temperature, ultrastrong materials were developed with useful levels of tensile ductility, while at high temperatures precipitate-strengthened ferritic alloys were found to have high yield and tensile strengths while minimizing, or even eliminating, the need for expensive elements such as nickel and cobalt.
The composition of the matrix and the total amount of nanoprecipitates were identical in different samples, but precipitate sizes and spacings differed by changing the processing temperature and time. As principal investigator Easo George said “A holy grail of structural materials has long been, how do you simultaneously enhance strength and ductility? Defeating the strength-ductility trade-off will enable a new generation of lightweight, strong, damage-tolerant materials.”
How strong a material is tends to depend on the closeness of the precipitates, but although the precipitates in standard alloys work to make them very strong, it also makes them brittle. Here this was avoided since the precipitates also prevent it from transforming when immersed in water to cool the alloy to room temperature, with the result that the matrix stays in a metastable FCC state. On stretching, it progressively changes from metastable FCC to stable BCC, a phase transformation that improves strength while maintaining sufficient ductility.
The team now hope to look at further factors and deformation mechanisms for combinations that could improve the materials’ mechanical properties. The yield strength of the alloys could perhaps be increased by expanding the spatial confinement of the precipitates by a factor of two without sacrificing ductility, although to prevent this making the alloy more brittle, its composition will have to be modified.
Stress-strain curves show how mechanical properties of a phase-transformable medium-entropy alloy can be tailored by adding precipitates and tuning their sizes and spacings.