Study On Metastable Liquid Separation Behavior And Properties For Cu-Fe Alloys

Nowdays, Cu-Fe alloy has aroused great concern for their excellent comprehensive performance and low cost. In order to correctly understand the microstructure forming with liquid separation for Cu-base alloys, and to prepare a kind of copper alloy with superior performance, the Cu-Fe alloys are studied. First, the growth and congregation of minor phase and solidification in immiscible Cu-Fe alloys are investigated. Then, the Cu-Fe alloys with different ingredients are prepared using copper mould. The tensile strength and resistivity of the Cu-Fe alloy with normalizing treatment, cold rolling and subsequent annealing treatment are explored. The obtained main results are as follows. Ⅰ. The Cu-Fe alloys are prepared by melt-fluxing in combination with cyclic superheating and melt-spinning technique, respectively. The metastable liquid separation and microstructures of the Cu-Fe alloys were discussed.1. Thermal analysis for Cu-15 wt pct Fe alloy has been conducted by Differential Scanning Calorimetry. It is found that the metastable liquid separations will occurre, once the single liquid L is undercooled into the metastable liquid gap. According to experimental results and theoretical calculations, both the number of nucleation and volume fraction of Fe-rich droplets increase with the increase of undercooling.2. When the undercooling of the Cu-25 wt pct Fe melt reaches 45 °C, the typical spherical structures are observed in the microstructures. The numerical results show that the Brown motion is strongly affected by the initial number of the separated droplets. With the increase of initial numbers of droplets, the time needed for completing the Brown congregation reduces. The effects of Stokes motion and Marangoni migration on congregation of separated droplets are enhanced as the droplets radiuses enlarge. And the calculation confirmes that increasing the radiuses of droplets will lead to the decrease of the time for congregation completed. With the increases of the undercooling and the droplet radius, the ratio of Stokes motion velocity to Marangoni motion velocity increases.3. A complicated sub-microstructure in the separated minor phase is observed when the undercooling of Cu-35 wt pct Fe is 120 °C. A layered structure(α+ε) phase in separated particles resembling pearlite in the Fe-C alloy has been discovered first. By studying the solidification process of metastable liquid separation of Cu-35 wt.%Fe alloys, the distributions of L, L1 and L2 are given. The Fe-rich phase is distinguished as γ, γ1, γ2, according to the difference of their solidification process. The solidification mechanism with the metastable liquid separation is first explained definitely. This provides an important guiding significance to correctly understand the microstructure forming with liquid separation for alloy system of Cu-Co, Cu-Fe and Cu-Cr, etc.4. With the high cooling rate, the size of the separated Fe-rich spheroids can be decreased to nanometer and the structure of the Cu-rich matrix is microcrystalline. It is revealed that grains size of Cu-Fe alloy can be refined greatly with the increase of cooling rate. Ⅱ. A Cu-Fe(-Sn) material with high strength and high conductivity is prepared using copper mold. The influence of the processing technology, such as normalizing treatment, cold rolling and subsequent annealing treatment, on microstructure and property of Cu-Fe(-Sn) alloy is explored systematically.1. The microstructures of Cu-Fe alloy are simulated by CA-FE(Cellar Automation-Finite Element) method. The simulation results showe that average grains size of casting increase with increasing size of ingots. Reducing superheating and enlarging heat transfer coefficient would contribute to the forming of equiaxed grain zone and the decrease of columnar structures.2. The bars of Cu-Fe(-Sn) alloys with different ingredients are developed by casting, normalizing treatment, cold rolling and subsequent annealing treatment. The results show that the tensile strength and resistivity of the Cu-Fe(-Sn) alloys increased with increasing content of Sn and Fe. Compared with the resistivity of as-cast alloys, the resistivities of normalized Cu-Fe(-Sn) alloys increase. And the resistivities of the Cu-Fe(-Sn) alloys after cold rolling treatment are less than that of normalized alloys. The resistivities of the alloys after the annealing treatment are improved significantly, and mechanical properties are decreased slightly. When the Cu-5wt pct Fe alloy is conducted to annealing treatment(500 °C×360min), its tensile strength, conductivity and mechanical elongation reach 552 MPa, 85.78%IACS and 28.5%, respectively. 3. Research for microstructures shows that the microstructure morphology of as-cast Cu-Fe alloy prepared by Cu mould is equiaxed crystal with the size of 10~30μm in diameters. Twin crystals are discovered in the rich-Cu substrate of Cu-5wt pct Fe alloy conducted annealing treatment(500 °C×360min). And many dispersive rich-Fe microcrystalline particles can be observed in the rich-Cu matrix. The formation of microstructures can brought about a vast improvement in performances of Copper alloys.