Microstructure Design For Copper Boron Alloy Thin Films And Its Mechanical Properties

Metal materials have good ductility and malleability,and are widely used in fields such as aerospace,electronic information,and transportation.They are an important support for national economic construction and defense projects.However,the hardness and strength of metal materials is generally low,which can easily lead to wear and deformation in actual application scenarios,especially for common soft metal materials such as copper and aluminum.Therefore,strengthening of soft metal materials has always been a focus of metal material workers.Nowadays nanocrystalline metals have been widely studied due to their inherent high strength,but their internal high grain boundary proportion exposes structural weaknesses at the interface,leading to a drastic decrease in stability.Thus,enhancing the strength and increasing the structural stability of nano-crystalline metals has become one of the key challenges in the development of metal materials.Grain boundary engineering is a versatile tool for strengthening materials by tuning the composition and bonding structure at the interface of neighboring crystallites,and this method holds special significance for materials composed of small nanograins where the ultimate strength is dominated by grain boundary instead of dislocation motion.In this work,we selected the non-miscible Cu-B system and used grain boundary engineering to design its microstructure to achieve strengthening of typical soft metal Cu by introducing B element to regulate grain boundary structure.Specifically,we prepared a series of Cu-B alloy films with different B concentrations using magnetron co-sputtering,to explore the effect of B element on the film growth,structure and mechanical properties.First,B element affects the nucleation stage of CuB alloy thin films,increasing the nucleation density.Additionally,since B and Cu do not dissolve in each other,the B atom obstructs the diffusion of Cu atoms on the surface,while also reducing the energy of the grain boundary,which encourages grain refinement.Secondly,due to the imbalance characteristics of magnetron sputtering,a small amount of B atoms may be solidified into the Cu lattice,but with the increase of B content,B atoms tend to deviate to the grain boundary and gradually form a segregation phase to regulate the grain boundary structure.Finally,the hardness of CuB alloy films increases with the increase of B content and presents a rising trend first and then a descending trend.When the B content reaches 26.5 at.%,the nano indentation hardness reaches the maximum value of 10.8 GPa,which is higher than that of all the binary Cu alloys and most of the Cu-based composites reported so far,and exceeds the highest value reported in the literature.Meanwhile,the sample also has the best thermal stability and can maintain the columnar growth structure and grain size dimensions after vacuum annealing at 200℃,as well as a relatively high hardness.In order to explore the source of high hardness and verify the idea of structure construction,we carried out more detailed structure and performance characterization of the Cu-26.5 at.% B thin film,and compared it with samples with other types of structures to analyze its strengthening mechanism.The results showed that the sample had a bamboo-like dual-phase nanostructure.The Cu grains with only 10 nm diameter but a long length that almost spans the entire film embed in the amorphous B-columnar skeleton with an average thickness of 2 nm.This structure strengthens and stabilizes the entire film under indentation while preserving some toughness and plasticity.After analysis,the extra high hardness of dual-phase Cu-B nanocomposite film mainly stems from the following three factors:(i)an indentation induced grain refinement,(ii)a strong support of the thick grain boundaries consisting of an amorphous boron framework,(iii)an enhanced stress response of the nanocolumnar copper structure constrained by the thick grain boundaries.We believe that these findings will open a new avenue for strengthening metals via construction of dual-phase nanocomposites comprising metal nanograins embedded in a strong and confining light-element grain boundary framework.