| High strength and high conductivity copper alloys are important industrial materials which are widely used for microelectronics, communications, aerospace, aviation and so on, owing to their excellent conductivity combined with high strength. So far, the precipitation strengthening for these commercial copper alloys is the main enhanced strength way. For the precipitation strength alloys, the mechanical and physical properties of them are directly dependent of nearly all the crystallographic characteristics of these strength precipitates. Therefore, understanding the precipitation characteristics, such as structure, morphology, crystallographic orientation relationship and evolution, of the strengthening precipitates in these alloys is of crucial important for marking improvement in the synthetic process.In the present study, several typical commercial precipitation strengthened high strength and high conductivity copper alloys, such as Cu-Ni-Si、Cu-Ni-P and Cu-Cr-Zr, were selected to investigate a series of heat treatment and measure their corresponding mechanical properties. Follow-up high resolution transmission electron microscope (HRTEM) in association with HRTEM simulation and First-principles Energy Calculations were performed to reveal phase transformation mechanism of hardening precipitates, the following can be concluded:(1) The strengthening precipitates of Cu-Ni-Si alloy are-Ni2Si nanocrystals with orthorhombic structure, though they evolve in morphology and crystallographic orientation relationship with increasing aging time. By their orientation relationships,3-D shapes and broad interfaces (i.e. the habit plane), the δ-Ni2Si precipitates in the alloy can be classified as two types:almond-like δ-Ni2Si precipitates with their habit plane parallel to the (110)cu plane and French baguette bread-slice-like δ-Ni2Si particles with their broad interface parallel to the high-index (11119)cu plane. In the early stages of aging (including peak aging), most precipitates are small and belong to the almond-like δ-Ni2Si type, with an orientation relationship of [010]δ‖[110]cu and (001)δ‖(001)cu-In the late stages, the main precipitates are larger and become the French baguette bread-slice-like δ-Ni2Si type, with an orientation relationship of [010]δ‖[110]cu and (301)δ‖(111)Cu (with an approximate deviation of2.7°). Above transformation processes of δ-Ni2Si nanocrystals are continuous, therefore metastable δ-Ni2Si nanocrystals are also observed in the present study.(2) Morphology and orientation relationships of the δ-Ni2Si nanocrystals in Cu-Ni-Si alloy are dependent on interface energy and strain energy between the precipitates and matrix, which can be explained excellently by invariable line principle (crystallographic orientation with zero strain). When the sizes of precipitates are very small, morphology and orientation relationships are controlled mainly by interface energy. To minimize interface energy, a low-index habit plane (110)cu can be formed between the δ-Ni2Si nanocrystals and Cu matrix. However, a high-index habit plane (11119)cu containing an invariant line can occur between the very large precipitates and Cu matrix to lower the strain energy between them.(3)□Two types of precipitates with different structure are observed for Cu-Ni-P alloy, one is h-Ni2P phase with hexagonal structure; the other is t-Ni12P5phase with tetragonal structure. These nano-slice precipitates with the same habit plane (111)cu are embedded into Cu matrix. Further observation shows that only one orientation relationship exists between the Ni2P phase and Cu matrix, but four orientation relationships exist between the Ni12P5phase and Cu matrix, and these four orientation relationship can transformed to each other by rotating the normal direction of their habit plane.(4) Morphology and orientation relationship of strengthening precipitates of Cu-Ni-P alloy can be understood well by two dimensions (2D) near coincidence site lattice (NCSL) model for the interface. According to orientation relationship between the h-Ni2P particles and Cu matrix, the2D NCSL models for the habit plane of h-Ni2P particles are proposed, the habit plane between h-Ni2P phase and Cu matrix has a very high degree of matching atom, in addition, the stran energy of their habit plane is much lower, so only one orientation relationship exsits for them. Similarly, some2D NCSL models for the habit plane of t-Ni12P5particles are built according to the different orientation relationships between the t-Ni12P5phase and Cu matrix, some models reveal relatively high atomic matching degree in the habitplane, but strain of these habit planes is much larger; the other models show relatively low atomic matching degree, but the strain in the habit plane is much smaller, therefore, a variety of orientation relationships are observed between t-Ni12P5particles and Cu matrix.(5) Main strengthening precipitates of Cu-Cr-Zr alloy are rich-Cr nanocrystals with body-centred cubic structure, while the Zr element connects perfectly with Cu element and forms mainly microcrystal Cu5Zr phase with face-centred cubic structure. When the Cu-Cr-Zr alloy is treated at the atmosphere, most of the precipitates are CuCrO2phase and some are the co-precipitates that comprised by CuCrO2phase and other oxide phase. |
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