Immiscible bimetal materials generally possess unique combined mechanical and physical properties due to the significantly different properties of their metal components. W–Cu bimetal composites, as a typical representative of immiscible bimetallic systems, have advantages of high hardness and low thermal expansion coefficient of W and good electrical and thermal conductivities of Cu, and have been widely used in the fields of electronic packaging, heat sinks, high-voltage electric contact parts, and welding electrodes. Achieving excellent integrated properties, such as simultaneously high mechanical properties and electrical conductivity, is strongly demanded in the development of advanced bimetal composites.
To overcome these challenges, a team of researchers from Beijing University of Technology, including Professors Xiaoyan Song and Chao Hou, developed a new type of W–Cu bimetal composite that exhibited outstanding integrated mechanical properties and electric conductivity. This innovative composite is an ultrafine-grained W–Cu bimetal with spatially connected Cu and specific W islands, which consist of aggregation of ultrafine W grains. The unique microstructure was achieved through a meticulously designed powder-pretreatment process followed by rapid low-temperature sintering. As compared with other W–Cu composites reported in the literature, the present bimetal exhibited exceptional comprehensive properties, including high yield strength, large plastic strain, and excellent electrical conductivity.
A finite-element simulation, based on the actual microstructure of the experimental materials, was employed to quantitatively investigate the stress distribution, strain response of each phase, and fracture behavior of the present bimetal. It demonstrated that the contiguity of fine W grains has a significant effect on the stress distribution in each phase. Under the same load, a higher contiguity of W grains is more beneficial to hinder stress increase in the Cu phase and prevent long-range extension of stress. As a result, the specific W islands containing contacted ultrafine W grains can experience greater stress than the homogeneously distributed W phase among Cu in the conventional W–Cu bimetal. Therefore, both refinement of bimetal's microstructure and high grain contiguity in W islands contribute to the high yield strength.
Furthermore, the coordinative plastic deformation of Cu and W islands can extend without premature failure at the W/Cu phase boundaries. Thus, the unique configuration of W islands and Cu phase facilitates concurrent deformation of both phases and thereby increases the contribution of W phase to the total plasticity of the bimetal, leading to a good combination of yield strength and plastic strain in this composite. Additionally, due to the increased mean free path of Cu and reduced proportion of phase boundaries resulting from the presence of W islands, this bimetal has a high electrical conductivity.
This study proposed an innovative strategy for preparing immiscible bimetals to achieve superior mechanical properties and electrical conductivity, which is applicable to a variety of composites for outstanding multi-objective properties by manipulating configuration and microstructure of the main phases.
The paper "Synergistic Enhancement of Mechanical Properties and Electrical Conductivity of Immiscible Bimetal: A Case Study on W–Cu", authored by Qixiang Duan, Chao Hou, Tielong Han, Yurong Li, Haibin Wang, Xiaoyan Song, Zuoren Nie. Full text of the open access paper: https://doi.org/10.1016/j.eng.2024.07.024
