"Yolk-and-white strategy” makes metals both strong and flexible

In the world of metals, it has long been considered common knowledge that achieving both strength and ductility (the ability to stretch without breaking) at the same time is nearly impossible. When a metal is made stronger, it typically becomes brittle, and when it is made more ductile, it usually loses strength—like a seesaw, where one property increases as the other decreases. However, a research team led by Professor Hyoung Seop Kim at POSTECH (Pohang University of Science and Technology) has recently overturned this conventional wisdom.

Previously, so-called high-entropy alloys (HEAs)—created by mixing multiple metallic elements in nearly equal proportions—have gained much attention. Yet, most HEAs developed so far feature uniform structures, which limit improving both strength and ductility simultaneously. Large grains in a metal enhance ductility by allowing it to stretch, while small grains increase strength by making the metal harder. But incorporating both large and small grains within a single metal has remained a significant challenge.

A new approach has emerged in which metals are deliberately designed with heterogeneous grain structures, combining grains of different sizes. This strategy has shown promise as a solution, but realizing it typically requires powder metallurgy, a process in which metal powders are pressed and then consolidated at high temperatures. Although this technique can deliver high performance, it is complex and costly, making it difficult to adopt for large-scale industrial applications.

The POSTECH research team developed a unique “core–shell” structure inside a nickel (Ni)-based high-entropy alloy by applying hot rolling—a process of rolling the metal at high temperatures—together with precise heat treatment.

In this structure, the core corresponds to the original large grains, like the yolk of an egg, while the shell consists of newly formed smaller grains surrounding them, resembling the egg white. During this process, fine nanoscale B2 precipitates formed within the alloy, preventing the small grains from growing excessively and thereby preserving structural stability. In the final heat-treatment step, these precipitates selectively formed along grain boundaries. As a result, when external stress is applied, the shell acts like a shield that blocks dislocation motion and enhances strength, while the core serves as a cushion that absorbs impact and mitigates cracking. Together, these effects enable the material to remain both strong and resistant to fracture.

The newly developed alloy demonstrated outstanding performance, recording a yield strength of 1,029 MPa, a tensile strength of 1,271 MPa, and an elongation of 31.1%. In other words, the alloy is not only much stronger than conventional metals but can also stretch more than 30% without breaking. What makes this achievement even more significant is that it was achieved solely through casting (melting and pouring into a mold) and heat treatment, without requiring complex processing.

Professor Hyoung Seop Kim, who led the research, stated:

“This study marks the first case of achieving a complex core–shell structure using only casting and heat treatment, without relying on powder metallurgy. By precisely controlling the nanoscale precipitates, we were able to secure both structural stability and deformation capability, opening possibilities for developing of structural materials that can withstand extreme environments.”

This research, conducted by Professor Hyoung Seop Kim of the Graduate Institute of Ferrous & Eco Materials Technology and the Department of Materials Science and Engineering at POSTECH, together with integrated M.S./Ph.D. student Hyojin Park and their team, was recently published in the international journal Journal of Materials Science & Technology. The work was supported by the Nano and Materials Technology Development Program of the Ministry of Science and ICT and by research funding from Hyundai Motor Group.