Ordinary Casting And Rapid Solidification Of Cu-15ni-8sn-xsi Alloys Organizational Structure And Performance

As the Cu-15Ni-8Sn alloy showed a series of good characters such as high strength, high conductivity and high thermal stability, it is a new type of Cu-based elastic material with great potential of developing. But severe dendritic segregation and macroscopic negative segregation of Sn often happen in the course of solidification for the alloy prepared by conventionally casting method because of high content of Sn. In addition, discontinuous precipitation often occurs in the later stage during aging, which caused the sharp decrease of many properties including strength and corrosion resistance. Considering these problem, rapid solidification technique was introduced to prepare the alloy, at the same time, the method adding the forth element of Si was adopted to suppress discontinuous precipitation.Microstructure, phase composition, phase transformation process and properties of Cu-15Ni-8Sn-XSi alloys prepared by conventionally casting and rapid solidification methods were investigated in this paper, moreover, the effects of rapid solidification and addition of Si on microstructure and properties and their mechanisms were analyzed and discussed. The major results obtained are as follows:1. Compared with conventionally casting method, rapid solidification produced a good grain refinement effect in the course of preparing Cu-15Ni-8Sn-XSi alloys, moreover, dendritic segregation and macroscopic negative segregation of Sn were suppressed, these made structures of these alloys optimized.2. Addition of Si obtained effects of grain refinement in Cu-15Ni-8Sn-XSi alloys prepared both by conventionally casting and rapid solidification methods. These were because doped-Si combined with Ni and formed Ni₃1Si₁2 and Ni₃Si phase. Ni₃Si phase had same structure with matrix, and their lattice parameters were also close, so Ni₃Si phase had the function of modificator, grains hence refined. Compared with conventionally casting method, the effect of grain refinement of Si was weaker in Cu-15Ni-8Sn-XSi alloys prepared by rapid solidification methods, this was because very quick cooling rate caused more Si solutionized in matrix and the amount of Ni-Sicompounds decreased, moreover, very quick cooling rate caused a majorityphase solutionized in matrix, so the amount of NisSi phase acting as modificator wasdecreased greatly.3. v phase existed in casting alloys prepared by conventionally casting and rapid solidification methods could be dissolved by 850°CX2h solution treatment. NiaSi phase could be solutionized in matrix through solution treatment, while M31 Sin phase was very stable, it could not be solutionized even through 850 °C solution treatment. For Cu-15Ni-8Sn-XSi alloys prepared by conventionally casting method, when the amount of Si was just 0.3, the tendency of grain growth was markedly suppressed because Si combined with Ni and produced a great amount of fine Ni3iSii2 phase, while for alloys prepared by rapid solidification, very quick cooling rate caused more Si solutionized in matrix as the form of alloying agent, the amount of Ni-Si compound was relatively small, so when the amount of Si increased to 0.6, this evident effect of suppressing grain growth can be attained.4. Hardness value of rapid solidification alloys were more a little high than that of conventionally casting alloys as a whole, these were cause by the strengthening effect of fine grains in the course of rapid solidification.5. For the alloys prepared by conventionally casting and rapid solidification methods added Si, discontinuous precipitation was suppressed during aging, the reason was that doped-Si combined with Ni and formed Ni3iSii2 phase and Ni3Si phase, Ni3iSii2 phase distributed in grain boundary occupied the nucleation sites of discontinuous precipitation, Ni3Si phase precipitated in grain boundary during aging also had the suppressing function. For the alloys prepared by conventionally casting, when the amount of Si was 0.3, this suppressing effect was strongest, the most high peak hardness value was obtained. Increasing amount of Si continually, this effect was instead weakened, this was because the amount of Si was too big to form a great amount of indissoluble Ni3iSii2 phase which distributed not only in grain boundary but also in intracrystalline , Ni3iSii2 phase distributed in intracrystalline instead supplied the sites for the nucleation of discontinuous precipitation, discontinuous precipitation was so accelerated. While for the alloys prepared by rapid solidification,when the amount of Si increased to 0.6, this suppressing effect was strongest, this was because very quick cooling rate caused more Si solutionized in matrix, and the amount of Ni3iSii2 and NiaSi phase was relatively small.