While steel may be the benchmark for high-strength metals, materials science and engineering is now able to engineer materials that surpass this standard. Through processes such as grain refinement, metals that are typically thought of as much weaker take on unprecedented properties that rival those of traditional and even high-strength steels. This process of grain refinement strengthens metallic materials by reducing the average grain size to the nanoscale. While this may improve the strength of the material it is not without its drawbacks. Metals that are often very weak and ductile such as copper or aluminum experience a significant increase in strength when processed to have nanosized grains. However, they often become very brittle, which leads to cracking and premature failure. The origin of this brittleness, which is seen in tension, is attributed to localized strains occurring at the grain boundaries. This localization of strain in the nano-grains results in void formation and intergranular cracking.

One solution to this problem of tensile brittleness has been reported by K. Lu of Shenyang National Laboratory for Materials Science in Shenyang, China, in the September 19 issue of Science (DOI: 10.1126/science.1255940; p. 1455).Lu studied gradient microstructures in copper to resolve the problem of increased tensile brittleness that accompanies nano-grained metals. Gradient microstructures are categorized by a gradual increase in grain size, starting from nano-scale grains at the surface to a more coarse-grained microstructure near the center. The researchers were able to induce this unique type of microstructure by generating a strain gradient in a coarse-grained metal so as to cause increased deformation near the surface.

This novel approach to microstructural engineering also leads to the enhancement of other mechanical properties. Microstructural plastic deformation under tension, which typically occurs prior to failure in these materials, occurs simultaneously throughout materials with a very narrow grain-size distribution. In materials with a grain-size gradient in contrast, plastic deformation begins first in the coarse-grained region before transmitting into areas of finer and finer grains as the applied load is increased. This gradient effect helps to relieve strain localization, which leads to an increased yield strength (the onset of irreversible deformation) without sacri-ficing ductility. The researchers also found that this exterior gradient of finer grains helps to improve fatigue resistance and suppress detrimental surface cracking. This work has helped to explore gradient grained materials that retain the ductility of metals with coarse microstructures, while still benefiting from the enhanced strength of nanoscale grains.

Illustration of the tradeoff between strength and ductility that typically accompanies the shift from a coarse-grained (CG) to nano-grained (NG) microstructure (lower curve). This is compared to the synergy achieved with gradient nano-grained (GNG) structures (upper curve). Credit: K. Lu.