Microstructures And Properties Of Cu-Cr Composites Processed By Double Ball Milling And Spark Plasma Sintering

Having high electrical and thermal properties, pure Cu is widely used in electrical appliances and thermal equipments as a functional material. However, pure Cu is easy to be deformed and failed due to the low strength and hardness, which restricts its application. In this regard, Cr particle was used as reinforcement to produce Cu-Cr composites in this paper. The Cu-Cr composites with high strength and hardness were prepared by a combination of double ball milling(mechanical milling–mechanical alloying) and spark plasma sintering, aiming at broadening the applications of Cu matrix composites.Firstly, commercially available Cu and Cr powders were subjected to mechanical milling respectively. The ball milling was conducted in both dry and wet conditions. It was found that the Cu powders were coarsened after milling. The particle size of Cu powders was slightly increased after wet milling, and the powders were changed from branch-like particles to flaky ones. In contrary, the Cu powders became globular and large in size after dry milling. For Cr powders, their particle sizes were decreased in the two conditions. However, dry milling resulted in smaller Cr powders with unchanged particle shape, while the powders were changed to flaky particles after wet milling. On the other hand, nanocrystal Cu and Cr powders were produced. The crystallites could be refined more efficiently by dry milling. A viable explanation was provided to explain the differences in morphology and structure of Cu and Cr powders in dry and wet conditions.Secondly, pre-milled Cu and Cr powders were mechanically alloyed with as-received Cr and Cu powders respectively for producing Cu-8 at.% Cr composite powders. Compared with mechanical alloying without pre-milling and pre-milling of Cu, mechanical alloying of pre-milled Cr and as-received Cu mixtures could produce finer composite powders with higher powder yield, narrower range of size distribution, smaller crystallite size and higher dissolution of Cr, which will show desirable properties after subsequent consolidation and present an attractive potential for technological applications. The mechanical alloying mechanisms including morphological changes, crystallite refinement and solid solubility extension were discussed to explain the phenomena during the fabrication process.Lastly, the produced Cu-8 at.% Cr composite powders were subjected to spark plasma sintering(SPS). The microstructures of the produced materials were characterized by scanning electron microscopy(SEM) and transmission electron microscope(TEM) as well as by X-ray diffraction(XRD). The mechanical properties were evaluated by compression tests and Vickers hardness measurements. It was found that the grain size of Cu matrix was about 82 nm after SPS. The composites showed high mechanical properties whilst maintaining considerable density and electrical conductivity. The Vickers hardness, compressive yield strength and compression ratio of the composite were 327 HV, 1049 MPa and 10.4%, respectively. The excellent mechanical properties are primarily ascribed to dispersion strengthening of Cr particles, fine grain strengthening of the Cu matrix and the strong Cu/Cr interface.