Properties and thermal stabilization of Nanocrystalline copper via solute additions of niobium

Abstract

Nanocrystalline materials, those with grain size <100 nm, were found to have superior mechanical properties compared to their coarse-grained (CG) counterparts. However, these materials are not thermally stable because of having a high density of grain boundaries which increases their Gibbs free energy. Consequently, the nano-grains grow back to their original size in order to release this energy, which results in losing their exceptional properties. Hence, finding ways to stabilize these nano-grains is an utmost importance. Introducing solute to such systems was found to solve the instability issue. Two approaches can describe the mechanism in which the solute atoms prevent or minimize the grain growth, Kinetic Approach which is concerned with reducing the mobility of the grain boundaries by pinning them; and Thermodynamic Approach that works on reducing the energy of the system, hence eliminating the need for the grains to grow. Nanocrystalline (nc) Cu and Cu-1 at.% Nb are prepared via mechanical ball milling under argon. The microstructure and the properties of the as-milled and the annealed samples are characterized using XRD, TEM, Vickers Microhardness, Tensile tests, SEM, and Four-Point Probe technique. Only one atomic percent of Nb is found to enough to thermally stabilize the nanostructure of copper up to 1073 K, which represents 80% of its melting point. Solute drag and Zener pinning are found to be the main kinetic stabilization mechanisms that succeeded to keep the grain size in the nanoscale. In addition, the solute atoms substantially enhanced the strength and hardness of the nc Cu, along with maintaining a better ductility. Moreover, such small amount of Nb did not sacrifice the excellent electrical conductivity of copper. This approach of synthesizing and improving the thermal stability of nc materials is not necessarily limited to Cu and could be applied to other metals and alloys. This shall make a leap forward in the production of thermally-stable and ultra-tough nanocrystalline materials for many industrial applications, without affecting their inherent properties.