Applications of pressure cycling on metal matrix composite processing

Metal matrix composites (MMC's) have become very attractive as high temperature structural materials due to their low density and good specific properties such as high strength and modulus. However, they exhibit very poor tensile ductility and formability at both room and elevated temperature due to the large volume fractions of brittle ceramic reinforcement, which also makes them quite difficult to machine. Thus, there is great interest in developing superplastic metal matrix composites. Previous experimental work in this field has been studied in thermal cycling creep of MMC's, however, the fundamentally time-consuming problem during thermal cycling process still limits the practical applications.; Furthermore, MMC's with large ceramic volume fractions are also quite difficult to fabricate by powder metallurgy methods. As ceramic volume fraction increases, larger compaction stresses are needed typically and the resulting green pressed material becomes weaker, making secondary processing more difficult. Superplastic effects based on the thermal cycling process have previously been shown to aid powder consolidation as well, but problems become complicated such as coarsening, or the loss of metastable structures, or permitting the reaction of the matrix and reinforcement. Many of the benefits of powder-processed metals can be lost as a result.; The goals of this dissertation are two-fold. The first is to develop a new superplastic technique on MMC's--pressure cycling process. The second is to apply this technique on the consolidation of composite powders to enhance densification and uniformity. In this dissertation, the fundamental theory of mismatch induced superplasticity (MISP) are described and several novel and detailed experiments are carried out. This dissertation is organized as follows:; In Part A, the superplastic behavior of MMC's is investigated in the uniaxial tension test under static and cyclic pressure conditions at elevated temperature to demonstrate the hypothesis of MISP theory. The material used is 20% (in volume) SiC particulate-reinforced 6061Al metal matrix composite (6061Al-20%SiC{dollar}rmsb{lcub}p{rcub}).{dollar} The experimental results between the static and cyclic pressure isothermal conditions are presented. Furthermore, modeling of MISP based on the Daehn and Gonzalez (D-G) model is presented to predict the creep behavior of 6061Al-20%SiC{dollar}rmsb{lcub}p{rcub}{dollar} metal matrix composites. Comparison between the predictions and experimental data shows good agreement.; In Part B, an application of pressure cycling on powder metallurgy processing (P/M) is studied. Mixed powders of lead (Pb) and various amounts of alumina (Al{dollar}rmsb2Osb3){dollar} are consolidated under static and cyclic pressure at room temperature in constrained 'uniaxial' consolidation experiments. Several interesting experimental results are presented. In addition, another six composite powder systems and a pure zinc powder system which shows anisotropic properties are also investigated to confirm the MISP effect on powder densification behavior. Moreover, modeling on densification of composite powder compacts in pressure cycling is presented. This modeling is based on the theory of MISP and void collapse by plastic deformation. The modeling results are in agreement with the experimental data.