Exploring the Microstructural Evolution and Strengthening Mechanisms in Cu-Be Alloys under Non-Equilibrium Solidification

Cu-Be alloys have long been recognized for their exceptional mechanical properties, high electrical and thermal conductivities, corrosion resistance, wear resistance, and non-magnetic characteristics. These remarkable attributes make Cu-Be alloys highly sought after in various engineering applications. However, the microstructure of these alloys, particularly the coarse precipitates that form during equilibrium solidification, often limits their full potential. To address this limitation, researchers have employed solution and aging treatments, resulting in the formation of fine coherent or semi-coherent metastable phases that significantly enhance mechanical and electrical properties. Moreover, the addition of alloying elements such as Co, Ni, Si, and Mg has been explored to tailor the microstructure and properties of Cu-Be alloys. While much research has focused on equilibrium solidification, there has been limited exploration of Cu-Be alloys under non-equilibrium solidification conditions. In non-equilibrium solidification, metastable intermetallic compounds may form directly, altering the properties of the alloys. In a new study, led by Professor Zhenyu Hong and graduate student Bowen Zhang, along with Na Yan, Liang Hu from Northwestern Polytechnical University, and Professor Hongliang Zhao from Zhengzhou University, discussed the non-equilibrium solidification of a Cu-0.2Be-1.6Co-1.6Ni alloy using electromagnetic levitation (EML). The research team investigated microstructures, precipitation characteristics, application properties, and strengthening mechanisms under different undercooling conditions, shedding light on the behavior of Cu-Be alloys in non-equilibrium solidification environments.

The researchers’ experimental setup involved the preparation of master alloy samples with a composition matching the target Cu-0.2Be-1.6Co-1.6Ni alloy. These master alloy samples were subjected to both conventional electromagnetic induction melting (EIM) and EML to achieve different undercooling conditions. A special EML setup was designed to handle toxic and volatile metal materials safely. Samples were then characterized using a variety of techniques, including optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), microhardness testing, and electrical conductivity testing.

The authors found distinct microstructural differences between the master alloy and the EML alloy samples with varying levels of undercooling. The master alloy exhibited coarse dendritic microstructures with large precipitates primarily located at interdendritic gaps and within dendrite trunks. In contrast, the EML alloy displayed two types of microstructures, consisting of small dendrites within fine grains in central areas and fine equiaxed grains in edge areas. The average spacing between secondary dendrites and grain sizes decreased significantly with increasing undercooling. Furthermore, the precipitates in both the master and EML alloys appeared as slender strips. However, the precipitates in the master alloy were considerably larger, with average lengths exceeding 1 μm and average widths of approximately 30 nm. In contrast, the EML alloy precipitates were smaller, with average lengths decreasing to around 500 nm and widths decreasing to approximately 8 nm with increasing undercooling. The phase compositions were analyzed using XRD. The master alloy showed only peaks related to α(Cu) with a face-centered cubic (FCC) crystal structure. In contrast, the EML alloy exhibited additional peaks in the 30-40° range, corresponding to the metastable γ’ phases with a body-centered tetragonal (BCT) crystal structure, alongside α(Cu) peaks.

The authors conducted TEM analyses provided insight into the characteristics of the slender-strip precipitates in both the master and EML alloys. In the master alloy, the α(Co) phase comprised coarse slender strips, maintaining a coherent relationship with the surrounding α(Cu) matrix. In contrast, the EML alloy with an undercooling of 162 K displayed γ’ precipitates in a BCT structure, maintaining a coherent relationship with the α(Cu) matrix. Further analyses confirmed that the γ’ precipitated phase in the EML alloy had a BCT structure, with specific lattice parameters. The orientation relationships between the γ’ precipitated phase and the surrounding α(Cu) matrix were determined, underscoring the maintained coherent relationship. To investigate composition distributions under various solidification conditions, energy-dispersive X-ray spectroscopy (EDS) analyses were conducted. These analyses revealed that under deep undercooling conditions, the EML alloy exhibited significantly higher concentrations of Co and Ni in the precipitated phase compared to the master alloy, reflecting substantial differences in composition distributions. They found the Vickers microhardness of the EML alloy samples increased linearly with increasing undercooling. With undercoolings of 76, 162, and 315 K, the hardness values reached 120, 132, and 154 HV, respectively. In comparison, the near-equilibrium solidified alloy displayed a considerably lower microhardness of 87 HV. The electrical conductivity of the EML alloy samples also exhibited a slight increase with rising undercooling, ranging from 23% to 26% IACS (International Annealed Copper Standard). In contrast, the master alloy displayed lower electrical conductivity, approximately 19% IACS, which was about 24% lower than that of the EML alloy. The near-equilibrium solidified alloy exhibited an electrical conductivity of around 22% IACS.

The microstructural differences observed between the master and EML alloys can be attributed to the undercooling conditions during solidification. Enhanced undercooling led to finer microstructures, with dendrite remelting-induced fragmentation playing a significant role. The reduction in the total amount of solutes in each dendrite and equiaxed grain resulted in decreased precipitate sizes with increasing undercooling.

The mechanical properties of Cu-Be alloys are primarily enhanced through solid solution strengthening, precipitation strengthening, and grain refinement. The solid solution strengthening in the master alloy was prominent due to its low undercooling, while precipitation strengthening was negligible. In contrast, the EML alloy benefited from both solid solution and precipitation strengthening. As undercooling increased, the solid solution strengthening weakened, while precipitation strengthening became more pronounced, leading to an overall improvement in hardness. The study also quantified the contributions of different strengthening mechanisms to the total microhardness of the alloy. Solid solution strengthening, precipitation strengthening, and grain boundary strengthening were considered. Their results showed that solid solution strengthening played a crucial role in the master alloy, while precipitation strengthening became increasingly significant in the EML alloy with higher undercooling. This explained the substantial improvement in hardness observed with deep undercooling.

In conclusion, Professor Zhenyu Hong and colleagues investigated the microstructural evolution, precipitation characteristics, and strengthening mechanisms in a Cu-0.2Be-1.6Co-1.6Ni alloy under various undercooling conditions. The results demonstrated that non-equilibrium solidification, achieved through EML, significantly influenced the microstructure and properties of the alloy. Under deep undercooling conditions, fine equiaxed grains and coherent γ’ precipitates formed, leading to enhanced microhardness and electrical conductivity.