UC Berkeley

In recent years it has been shown that it is possible to design materials with strengths approaching their theoretical ideal limit. This is an intriguing development; materials typically fail at stresses that are several orders of magnitude below their theoretical limits of strength. The development of engineering alloys with usable strengths near the ideal limit would have profound technological implications.

The most common approach used to increase a material's strength, is grain refinement. This method has been used to produce nanograined hollow nanospheres of CdS. Under nanoindentation these spheres show remarkable strength and deformation properties. The stresses and strains in the shells are studied with linear elastic finite element analyses and from this a failure criterion is developed. The stresses predicted by the failure criteria are 2.2 GPa, which is very large for an inherently brittle material. We compare the failure stress to the calculated ideal strength for CdS, calculated using density functional theory. Comparing the stress predicted by the failure criteria to the ideal strength shows that the hallow nanospheres approach 70% of their ideal strength.

In 2003 a new Ti, Nb based alloy "Gum Metal" was introduced by Toyota Research Corp. This alloy has strength approaching the ideal limit even in bulk form. Moreover, the material deforms in a novel fashion without the obvious participation of dislocations. A Ti-V alloy has been chosen to study the properties of this type of alloy. The BCC ideal yield surface is examined as a function of composition. Dislocation core structures are also examined as a function of composition. The results explain some experimental observations in this novel system.