| A major goal in today’s development of advanced copper-based materials is to provide an optimal combination of high mechanical strength, high electrical conductivity together with superior softening resistance. These materials are generally classified into two categories, based on the different enhancing mechanisms, precipitation strengthened alloys and dispersion strengthened ones. Cu–Cr alloys, as representative precipitation hardened alloys, resulting from the decomposition of the supersaturated solid solutions, possess good low-temperature strength, but poor at high temperature. The particles in dispersion strengthened alloys are usually thermally stable ceramic particles. Ceramic nano-particle-reinforced copper alloys have excellent high-temperature properties; however, its reinforcement effect at ambient temperature is inferior to precipitation strengthening.Yttria(Y2O3) and many rare earth oxides are potentially attractive as dispersions for copper alloys owing to their excellent thermodynamic stability. Besides, these oxide-forming elements exhibit a low solubility and diffusivity in the metallic matrix, which would enhance the microstructural stability against coarsening. More importantly, all these oxides have fluosite-related structures and more likely inherently form different interfaces with copper than those developed by spinel-type γ-Al2O3, and therefore have different strengthening mechanisms. Thus, it offers feasible opportunities to explore the roles of the particle-dislocation interaction in the deformation behavior of those materials.In-situ reaction at liquidus temperature, a novel preparation for oxide dispersion strengthened copper matrix composites, is conducted by in-situ oxidating the solute atoms at molten state. The preparation method solves the material design problem caused by the low solubility and segregation of yttrium. In this paper, Cu–Y2O3 composites were prepared by in-situ reaction at liquidue temperature. The distribution state of yttrium in copper and its effects on the microstructures and properties have been studied. The microstructures and properties of Cu–Y2O3 composites, the in-situ oxidation mechanism and strengthening mechanism of the Y2O3 particles have alsobeen investigated emphatically. The main results show as follows:1. The solubility of yttrium in copper matrix is very low. Most of the yttrium atoms segregate at the grain boundary with the form of Cu7 Y. The grains of Cu–Y alloys are refined by adding yttrium elements. The grain size decreases from 500 μm to 80 μm when the yttrium dosage increases from 0.05% to 1.5%. The electrical conductivity of copper is improved by adding less than 0.05% yttrium, the maximum value of electrical conductivity is 102%IACS. However, the electrical conductivity declines when the content of yttrium is more than 0.05%, which may be contributed to the combined influence with the decontamination, refinement and intermetallic compound. With the results of the grain-boundary and intermediate phase hardening, the microhardness of Cu–Y alloys increases as the increasement of yttrium.2. The in-situ oxidation of Cu–Y alloys is the preferential oxidation of yttrium elements, and the key factors of that are temperature and oxygen partial pressure. The upper and low limit of 2OP can be determined by 73.14/16300lg2TP+-=O and 00.13/17830lg2TP+-=O. According to the kinetics of in-situ oxidation,The nucleation number Z of Y2O3 particles per unit volume at the front of oxidation can be described by the equation , and the radius of oxide particles is gven by. To gain nano-yttria particles by in-situ oxidation at liquidus temperature, oxygen atoms need to be contacted fully with Cu–Y melts to form larger reaction interface.3. During the in-situ reaction at liquidus temperature, when no oxygen atoms enter the Cu–Y melts, the melts cannot crystallize due to the dynamic equilibrium between solidification and melting. However, when the oxygen atoms in atmosphere diffuse into the melts, yttrium is oxidized firstly and form Y2O3 particles. As a result, the local content of yttrium in the surrounding regions of Y2O3 particles will gradually tend to zero, and therefore there is a supercooling region around Y2O3 particles. Besides, the yttria particles could act as nucleators, which can further promote the solidification. The combination of these twoeffects will cause the isothermal solidification of Cu–Y melts. Under the condition of isothermal solidification, the growth rate of Y2O3 particles decreases drastically and finally yields the nano-scale Y2O3 dispersions strengthened copper-based composites.4. The TEM observation results show that fine oxide particles are uniformly distributed in copper matrix, the morphology of particles are most globular and axiolitic. The particle size is less than 14 nm and has a unimodal distribution. The particle size decreases as the increasement of yttrium content. When the dosage of yttrium are 0.05/0.2/0.4/1.0%, the average size of yttria particles are 6.1 nm, 4.9 nm, 4.7 nm, 3.8 nm, respectively; the average space between particles are 24.0 nm, 18.0 nm, 20.0 nm, 16.5 nm, respectively.5. HRTEM observation indicates that the lattice plane of Y2O3 particles has a same crystallographic orientation relationship with copper matrix. The misfit δ is calculated to be 0.036, therefore it is concluded that the boundary between the yttria dispersion and copper matrix is coherent. In addition, annealing treatment will not change the interface relationship between Cu and Y2O3 because of the high thermodynamic stability of yttria. The coherent relationship could impede the coarsening of particles; therefore, the yttria is still nano-scale after high temperature heat treatment. The electrical conductivity of Cu–Y2O3 composites decreases as the increasement of the volume content of yttria, but the electrical conductivity is still higher than 90 %IACS. The electrical conductivity of Cu–0.9vol%Y2O3 composites is 98 %IACS, which is higher than Cu–Al2O3 composites prepared by SCM company. The electrical conductivity of Cu–Y2O3 composites decreases as the increasement of the deformation, however, the decrease is not notable.6. The microhardness of Cu–Y2O3 composites increases as the increasement of the volume content of yttria. The microhardness of Cu–3.34vol%Y2O3 composites is 93 HV, which is 65% more than pure copper. With the 55% deformation, the microhardness of Cu–0.9vol%Y2O3 composites is increased by 62% from 74 HV to 120 HV, which shows good work hardening capacity. With increasing the annealing temperature, the microhardness of Cu–0.9vol%Y2O3 compositesdecreases slowly and the average softening rate is about 1 HV/100 oC. The softening temperature of Cu–Y2O3 composites is about 800 oC and pure copper is about 400 oC.7. The tensile strengths of Cu–0.11/0.45/0.9/2.24vol%Y2O3 composites prepared by in-situ reaction at liquidus temperature are 356 MPa, 425 MPa, 548 MPa and 623 MPa, respectively. The elongation of Cu–Y2O3 composites is more than 30% due to the low volume content of Y2O3. The fracture appearance of Cu–Y2O3 composites is typical ductile fracture. Based on the study of strengthening mechanism, the strengthening mode of Cu–Y2O3 composites is mainly dispersion strengthening. The good agreement between the calculated strength and the experimental value indicates that Orowan and shearing strengthening mechanism are coexisted in Cu–Y2O3 composites. The maximum size of coherent precipitate that can be sheared by dislocations is 2.4 nm. The particles larger than 2.4 nm strengthen the matrix by Orowan mechanism. The analysis of threshold stress indicates that the Cu–Y2O3 composites have superior softening resistance. |
PDF Full Text Request