Stronger steel with better ductility

Dr. Neil Canter, Contributing Editor | TLT Tech Beat July 2014

A twisting process strengthens twinning-induced plasticity steels without compromising ductility. 

KEY CONCEPTS
• The inverse relationship between strength and ductility of steel is a barrier hindering development of high-performance alloys.
• A twisting process is used to strengthen TWIP steel through the development of a gradient structure between the hardened surface and the softer core.
• The presence of a gradient also acts to reduce the possibility of material failure due to a high level of ductility.

THE DRIVE TO DEVELOP BETTER PERFORMING MATERIALS that can function under more extreme operating conditions for longer time periods is ongoing. The lubricant industry is heavily involved in this trend as end-users are seeking to work with synthetic types that are more effective.

One area where this is actively ongoing is the development of materials that are both lighter and stronger as a means to meet upcoming regulations for better automobile fuel economy. In a previous TLT article, research discussing the preparation of third-generation advanced high-strength steel is described (1). Ultrafine grain ferrite and austenite are combined into a duplex microstructure that displays superior tensile strength and fracture toughness.

One barrier to producing high-performance steel is the inverse relationship between strength and ductility. This has been seen in twinning-induced plasticity (TWIP) steels that contain a high concentration of manganese.

Huajian Gao, Walter H. Annenberg Professor of Engineering at Brown University in Providence, R.I., says, “TWIP steels contain twin boundaries which are a special type of grain boundaries bordering two crystals that are mirror images of each other. This structure enhances the strength of the steel by hindering the propagation of dislocations and other defects.”

Attempts to further strengthen TWIP steels using common methods were successful but also resulted in reduced ductility. Gao says, “Techniques such as grain refinement and cold working have been used to manipulate the microstructure by refining grains. The problem is that as grains are refined, the resulting TWIP steel loses ductility to the point of becoming brittle.”

If a process could be found to improve steel strength without sacrificing ductility, then the resulting TWIP steel can be used in more high-performance applications. Such a process has now been developed.

GRADIENT STRUCTURE
Gao and his collaborators strengthened TWIP steel without reducing ductility by applying torsion to one-centimeter-long cylindrical samples. The resulting steel has a hardened surface but retains a ductile core.

Gao says, “We have achieved this result by creating a gradient structure between the hardened surface and the softer core. The gradient is sufficiently spread out to minimize any incompatibilities between the hardened surface layer and the interior of the steel.”

The twisting process applied a torsion that deforms the atoms on the surface of the TWIP steel cylinder to a much larger extent than those present in the core which are left virtually unchanged. A good analogy is to think of the steel as a running track where the runners on the outside lanes have to cover more ground than those on the inside.

Figure 2 shows scanning electron microscope images of the TWIP samples taken at different radial positions from the core. The photo in the upper left designated as (a) shows the core microstructure. Photo (b) is an image of the microstructure halfway to the surface and photo (c) is an image of the microstructure at the surface. Electron backscatter diffraction was taken on photo (d) to highlight the twin grain boundaries seen in the metal.


Figure 2. Electron microscope images of the TWIP steel samples show the changes in the gradient structure in progressing from the core (a) to the surface (c). An electron backscatter diffraction of (c) more clearly shows the twin grain boundaries seen in the metal (d). (Courtesy of Brown University)

Electron backscatter diffraction shows the density of twin boundaries increase with distance from the core.

Simulation studies were conducted to determine why the combination of the gradients and the twin boundaries leads to an increase in the strength of the material without losing ductility. Gao says, “We found that the theoretical studies validated our empirical work showing the gradient structure in the TWIP steel will increase strength without reducing ductility. In fact, we have found that the strength of the steel can be doubled from 300 to 600 megapascals.”

The establishment of a gradient also acts as an important safety valve to prevent material failure due to a high level of ductility. Gao says, “The concern with ductility loss is that it is closely linked to the mechanical instability of steel by not giving steel sufficient room to deform under stress. Such features as necking and fracture can occur. The presence of twin boundaries in the steel hinders but does not fully suppress the motion of internal defects, thereby raising the strength without sacrificing ductility.”

A similar twisting process was applied to 6061 aluminum to determine if this non-ferrous metal could be strengthened in the same manner without reducing ductility. While metal strength increased, the ductility of the aluminum declined. Gao explains, “Aluminum does not contain the twin boundaries present in TWIP steel. The result is no twinning was present to create the gradient nanotwinned microstructure needed to maintain ductility while the strength of the metal was increased.”

Future work will focus on determining if this effect can be found in other metals and even materials such as ceramics. One other interesting material that Gao will be studying is aragonite which is found in nature.

Aragonite is a crystalline form of calcium carbonate that is found in mollusks. Gao says, “It will be interesting to see if this twinning effect can be found in a material such as aragonite. There may be some potential commercial applications for all kinds of materials containing twin boundaries.”

Additional information on this research can be found in a recent paper (2) or by contacting Gao at huajian_gao@brown.edu.

REFERENCES
1. Canter. N. (2013), “3rd generation advanced high-strength steel,” TLT, 69 (2), pp. 10-11
2. Wei, Y., Li, Y., Zhu, L., Liu, Y., Lei, X., Wang, G., Wu, Y., Mi, Z., Liu, J., Wang. H. and Gao, H. (2014), “Evading the strength-ductility trade-off dilemma in steel through gradient hierarchical nanotwins,” Nature Communications, 5 (3580), DOI: 10.1038/ncomms4580.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.