Research On The Preparation Of Nanoparticle Dispersion Strengthened Copper Alloys Cu-Nb And Relative Fundamental Theory

Preparation methods for nanoparticle dispersion strengthened copper alloys Cu-Nb and relative fundamental theory have been investigated in this paper, supported by the "863" Project of the Ministry of Science and Technology of the People's Republic of China (No.2006AA03Z517). The major results can be summarized as follows:1. The influences of milling parameters on the microstucture of Cu100-x-Nbx(x= 5,10,15,20, wt%) powder mixtures have been investigated. According to the experimental results, the optimum conditions of mechanical alloying can be obtained as follows:milling atmosphere:argon, milling medium:stainless steel, milling speed: 300 rpm, ball-to-powder weight ratio:15:1, milling time:40?100 h. Under the above optimum conditions, Cu100-x-Nbx powder mixtures tend to higher alloying lever and their crystallite sizes decrease gradually with increasing milling time. After milling for 100 h, about 10wt%Nb can be dissolved homogeneously within the Cu matrix, and the Cu crystallite sizes decrease to 7?13 nm. The maximum values of microhardness for Cu100-X-Nbx powder mixtures are about 367-490Kgf/mm2. The contributions to the high hardness can be attributed to the grain boundary strengthening referring to the Hall-Petch relation, solution strengthening, twin boundary strenghening and strain strengthening.2. The microstructure evolution and the formation of nanocrystalline grains of Cu90Nb10 alloy during mechanical alloying have been studied systematically. The results show that the main structures of the milled powders are the dislocation cell blocks and dislocation cells during the initial milling. With increasing milling time, the density of dislocation increases and the cell blocks develop into subgrains. With further milling, the subgrains further refine into nano-subgrains or nanocrystalline grains. During the intermediate stage of milling, the average Cu crystallite size decreases to about 50nm, and nano-deformation twins begin to form in some regions. With the continued increase of milling time, the number of deformation twins increases, and the deformation twinning contributes to further the refinement of the nano-grains. During the final milling stage, the Cu crystalline size reduces below 20 nm, therefore, the deformation mechanisms of dislocation slip and twinning are both suppressed. The generation and movement of partial disclination defects become critical in the nanograin refinement. Finally, the extremely fine nanocrytalline grains with random grain boundaries are formed by milling. It should be pointed out that the grain refinement and solid solubility extension mutually reinforce during milling.3. The free energy of formation for Cu and Nb solid solutions is calculated to be positive on the basis of thermodynamic theory, which is inhibited the alloying process. Otherwise, for the milled Cu-Nb alloys, the system stored mechanical energy as the surface and elastic strain energy caused by milling supplies the main driving force for alloying, and is sufficient to increase the solubility between Cu and Nb. Using the thermodynamic analysis, the solid solubility limit of Nb in Cu is estimated to be extended to 11.6 wt.%Nb. The thoeretical results are in reasonable agreement with the experimental ones. The solid solubility extension mechanisms during milling process are proposed. In the initial and intermediate stages of milling, the dislocation pump mechanism and layer structure supply fast channel for atom diffusion between elements. In the final stage of milling, the lattice rotation, trans-phase dislocation-shuffling, grain boundary sliding and structural phase transition of Nb nanoparticles are all benefit for the diffusion of Nb into Cu matrix and the formation of nonequilibrium solid solution.4. The microstructure evolution and the high thermal stability of the mechanically-alloyed Cu90Nb10 alloy during a subsequent heat treatment have been investigated. The results show that the microhardness of Cu90Nb10 alloy decreases gently with increasing annealing temperature, and can keep to be about 375Kgf/mm2 even after annealing at 900?. TEM observations show that no significant change of the microstructure of the solid solution can be detected after annealing at 300?400?. The pronounced phase separation can be detected at 700?. After annealing at 900?, the average Cu crystallite size is just about 39nm, and the sizes of the Nb precipitates mostly keep to below lOnm. Thus, the bi-nanostructured Cu-Nb alloy with Cu nano-grains and Nb nanoparticles is formed after annealing, furthermore, the bi-nanostructure has a high thermal stability. As the solute atoms will hinder the migration of fcc Cu grain boundaries, no significant grain growth can be detected before large amount of Nb atoms precipitates from Cu matrix, and also the decreases of the internal strain and the density of dislocation are slow. Furthermore, the Nb nanoparticles can also help reduce the Cu grains growth through precipitate pinning effect. Therefore, the mechanically alloyed nanocrystalline Cu-Nb alloys have a high thermal stability.5. Based on the experimental results, the optimum conditions of consolidation process for mechanical alloyed Cu-Nb alloys are as follows:hydrogen-annealing of milled Cu-Nb powders at 560?for lh and then vacuum-hot pressing sintering under 30MPa pressure and 850?for 2h. Using these optimal conditions, the bi-structured Cu-Nb bulk with relative density over 98%and free oxygen content below 12ppm is successfully produced. For the consolidated Cu90Nb10 alloy, its microhardness can reach as high as 364Kgf/mm2 and electrical conductivity is about 52%IACS. Therefore, the alloy has a good combination of mechanical and electrical properties.6. The statistic analysizing of the sizes of Nb particles and Cu grains in the bi-structured Cu90Nb10 bulk is performed. On the basis of the above statisitc results, both the strengthening and the conductivity mechanisms have been studied. The results show that the grain boundary strengthening and dispersion strengthening are the main strengthen mechanisms. Calculations of yield strength show that the contribution in the yield strength by former mechanism is larger than the one in the yield strength by the latter. The high strength of Cu-Nb alloy is related with the bi-nanostructured, i.e. the nanocrystalline grains produce the grain boundary strengthen, while the Nb nanoparticles produce the dispersion strengthen. The grain boundaries and the nanoparticles are also found to influence considerably the electrical resistivity of the alloy.7. The nanoparticle dispersion strengthened copper alloy with low content niobium is prepared by mechanical alloying, and its annealing characteristics are also studied. The results show that Cu99.5Nbo.5 alloy can obtain a much higher hardness level (151?160Kgf/ mm2) compared to oxygen-free copper. And its hardness decreases slowly with the increase of annealing temperature, which indicates mainly recovery occurs in the alloy. With annealing temperature increasing, the relative conductivity of Cu99.5Nbo.5 alloy increases at first and reaches its maximum 89%-92%IACS at 400?; however, it decreases with the further increase of annealing temperature. TEM observation shows that the dislocation density in the Cu matrix after cold rolling is very high. The subgrain structure appears in some regions after annealed at 500?. After annealed at 900?, the density of the dislocation decreases greatly, some fine recrystal grains appear in several regions; however, as the Nb nanoparticles are the obstacles to dislocation and grain boundaries motion, the subgrain structure is retained in most regions. Therefore, Cu99.5Nbo.5 alloy has a high resistance to softening at elevated temperature. This alloy has been applied in a microwave tube.8. The interface and size effects on the structural phase transition of Nb nanoparticles embedded in Cu matrix are investigated. It is found that higher coherency of the Cu/Nb interface benefits the occurrence of phase transition in Nb NPs with larger sizes. The sufficient conditions for the transition are:(1) the size of Nb NPs should be smaller than 8 nm; (2) the Cu/Nb interfaces should be semi-coherent or coherent. The experimental results are consistent with the predictions of Bond Energy model.9. TEM observation shows that the three deformation twinning mechanisms in the milled nanocrystalline Cu-Nb alloys are heterogeneous twins nucleated from Shockley partial dislocations emitted from grain boundaries, heterogeneous twins nucleated via the dissociation and migration of grain boundaries and homogeneous nucleated by the overlapping of stacking faults, respectively. Furthermore, an analytical model is developed based on the dislocation theory to explain the nucleation and growth of the deformation twins in Cu nanograins. It is found that once a twin is nucleated, it is not likely to shrink due to the higher shrinking stress than the twinning stress. The deformation twins can grow under much lower stresses via the emission of twinning partials from the grain boundaries on slip planes adjacent to the twin boundary.