Short-Flowing Preparation Methods And Several Fundamental Research Of Nanoparticle Dispersion Strengthened Copper Alloys

Two kinds of short flow technologies, in situ reaction of double beam melts-rapid solidification and simplified internal oxidation technology, for the preparation of dispersion strengthened cooper alloys, have been investigated systematically in this paper. And both Cu-TiB₂ and CU-Al₂O₃ nano dispersion strengthened cooper alloys have been prepared by these technologies respectively. The mechanical properties, electricity properties, processability and the evolving law of structure have been studied deeply. The main results can be summarized as follows:1. The investigation of thermodynamics of in-situ reaction of Cu-Ti and Cu-B melts demonstrates that, the Gibbs free energy values of both TiB₂ and TiB phases are negative, TiB₂ phase is the main strengthening phase generated in the in-situ reaction, since the energy of TiB₂ phase is lower. According to the unilateral diffusion kinetics of the in-situ reaction of double-beam melts, the penetration rate of reaction front forTiB₂ particles can be described by the equation (?). Thenucleation number Z(x) of TiB₂ particles per unit volume is givenby Z(x) =(?) and the radius of the particles is givenby r(x)=(?)2. The investigation of the flow characteristics of melts ejected by in situ reactor with flat nozzles using the turbulent theory indicates that, the current velocity ofturbulent jet melt in transverse direction satisfies the equation(1/a)(v/u_m)= (?)F'((?))-1/2F((?)) .The current velocity parallel to the axis satisfies the equation u/u_m= F'((?)). The axiscurrent velocity u_m for jet melt satisfies (u_m)/(u₀) = 2.28(?). The entrainment amount of ejected melt satisfies q/q₀= 0.62(?). The initial segment of jet melt satisfiesL₀ = 5.2(2b₀). On the basis of the above mentioned results, in-situ reactor of double beam melts has been designed. The proper spans of the thickness 2bo for the flat-shaped ejection nozzle( 0.5mm < 2b₀ < 3.0mm), ejection angleθ(40-60°) andrelative sizes of reaction cavity have been determined. The Shangguan model was extrapolated to consider the effect of cooling rate on interaction between freezing interface and front-end particles, and the interaction of particles in melts at the same time, the results indicate that only if the cooling rate V can satisfy the relationship ofV_CI < V < V_CP, TiB₂ particles synthesized by in situ reaction of Cu-Ti and Cu-B meltscan be trapped by freezing interface, and uniformly distribute in the copper matrix. At last, the device of combined in situ reaction with rapid solidification were successfully assembled, and can be used to prepare Cu-TiB2 alloys.3. According to the theory analysis and experiment results, The optimum conditions of ejecting double beam by in situ reactor are as follows: 2b₀=1.0-2.5mm,θ=50°, Cu-Ti melt temperature=1400-1500℃, Cu-B melt temperature=1300-1400℃, air pressure=0.2-0.35MPa. The corresponding properties of CU-TiB₂ alloys prepared by using these optimum conditions are as follows: Cu-0.45wt%TiB2 alloy: HV=102,σ_b =389MPa,σ_0.2=330MPa,δ=21%, relative electric conductivity=92%IACS; Cu-1.6wt%TiB₂ alloy: HV=142,σ_b=456MPa,σ_0.2=415MPa,δ=14%, relative electric conductivity=81%IACS ;Cu-2.5wt%TiB₂ alloy:HV=169,σ_b=542MPa,σ_0.2 =511MPa, 8 =12%, relative electric conductivity =70%IACS. A large number of nano TiB₂ particles can be observed in the matrix of Cu-TiB2 alloys.4.Through statistic analysizing of the sizes of TiB₂ particles and grains in the matrix of CU-TiB₂ alloys prepared under their optimum conditions, it is found that, the frequency of TiB₂ particles with the size of 50-75nm is the highest, and with increasing of solute concentration, the volume fraction of nano TiB₂ particles is also increased, yet the grain sizes decrease. On the basis of the above statistic results, both the strengthening and the conductivity mechanisms have been studied. The results show that: dispersion strengthening and fine-grained strengthening are main strengthening mechanisms for Cu-TiB₂ alloys prepared by this technology, and the strength value contributed from dispersion strengthening is higher than that of fine-grained strengthening. The difference between the calculated and measured electric conductivity values for the Cu-0.45wt%TiB2 alloy is much smaller, yet, with increasing of TiB₂ particles concentration, their differential values are also increased gradually. The influencing factors for electric conductivity of Cu-TiB₂ alloys prepared by this technology mainly include the residual amount of Ti and B solute elements, the content, size and distribution of in situ synthesized TiB₂ particles.5. Because the traditional internal oxidation process is very complicated, products are not stable enough, and their costs are also very high, the simplified internal oxidation process is quite needed to be studied. Through investigation, the simplified internal oxidation process is determined as follows: melting of Cu-Al master alloy→preparing powder by gas atomization→mixing of Cu-Al powder and oxidant→hot pressing(internal oxidation & proforming)→hot extrusion. Some steps in the traditional technology are saved, such as internal oxidation→crushing and screen separation→reduction→crushing and screen separation→cool isostatic compression→vacuum stintering→canning, vacuum-pumping, sealing-off and other procedures, which cuts down the production period, avoids the oxygen pollution and improves the product quality. The properties of the CU-Al₂O₃ alloys under extrusion condition fabricated by the simplified process are as follows: Cu-0.23vol%Al₂O₃ alloy: HV=85,σ_b=260MPa,σ_0.2=195MPa, 8=30%, relative electric conductivity=96.5%IACS; Cu-2.7vol%Al₂O₃ alloy: HV=145,σ_b=580MPa,σ_0.2=521MPa,δ=13%, relative electric conductivity=82%IACS.6. With increased cold rolling deformation, a work softening phenomenon can be observed in the CU-Al₂O₃ alloys. The higher concentration of Al₂O₃ particles is, the poorer work softening is. In order to explain this phenomenon, the microstructure changes of Cu-Al₂O₃alloys were analyzed by TEM as a function of deformation, and the models of interaction between dislocation and dispersion particles were also introduced. The reason for the work softening is: annihilation of unlike dislocations during the cold-rolling with large deformation amount, leads to the agglomeration and growing of adjacent dislocation cells, which results in the decrease of alloy hardness, and the appearance of work softening phenomenon. The anisotropy of unidirectional rolled CU-Al₂O₃ alloy is significant, and the strengths in transverse direction are quite lower than those in longitudinal direction. In addition, the phenomenon of stress fluctuation or steep dropping appears in its transverse tension curve. The research of metallographic and tensile fracture analysis demonstrates that, fibre structure with lower bond strength boundary is formed in the CU-Al₂O₃ alloy after unidirectionaly rolling, and leading to the splitting of fibres along their interface during the transverse tension, which are the essential reasons for the stronger anisotropy of unidirectional rolled CU-Al₂O₃ alloy. Tandem rolling can avoid the anisotropy of CU-Al₂O₃ alloy effectively.7. With the increasing of strain rate, the stresses of CU-Al₂O₃ alloys compressed in the longitudinal and transverse directions at room temperature are increased. However, the stresses in the longitudinal direction are higher than those in the transverse direction, which can be explained by the rotation model of gliding plans and glide directions, and the model of interaction between moving dislocation and dispersion particles. Through the quantitative research in the effect of strain rate and compression temperature on peak yield stress, it is found that hot compression deformation of CU-Al₂O₃ alloy is a thermal activation process; the higher Al₂O₃ particle concentration is, the higher activation energy of the alloy is. And the activation energy of Cu-Al₂O₃ alloy compressed in longitudinal direction is higher than that in the transverse direction. According to the relative material parameters obtained from the compression experiment, the deformation constitutive equations of CU-Al₂O₃ alloy describing the relationship of yield stress peak value, strain rate and temperature are given as follows: u-0.23vol%Al₂O₃ alloy:Transverse direction(?) = [sinh(0.0124836σ)]^4.39909 exp(11.65218-99.848×10³/T) Longitudinal direction(?)= [sinh(0.006078σ)]^8.86218 exp(23.22611-183.614×10³/RT)Cu-2.7vol%Al₂O₃ alloy:Transverse direction(?) = [sinh (0.007653σ)]^4.20761 exp (14.84478 -120.59×10³ / RT)Longitudinal direction(?) = [sinh(0.005638σ)]^8.52908 exp(26.31261-209.892×10³/RT)8. The observation of metallographical structure demonstrates that, with increasing of deformation temperature, fibre structure is gradually weakened, and the number of dynamic recrystallization grains appearing among the fibers or on its boundary are also increased. However, dynamic recrystallization is difficult to happen in the high concentration CU-Al₂O₃ alloy. During the compression along its longitudinal direction, because the compression direction is parallel to the arrangement orientation of fibres, the fibres are damaged seriously. The generation of crack along the fibre interface is more difficult to occur in the transverse compression than in the longitudinal direction. The observation of TEM microstructure demonstrates that, hot compression makes the subgrain sizes decrease, and the orientation difference between the adjacent subgrain increase, yet, with the increasing of strain rate, the dislocation density first increases, then followed by decrease.