Mechanisms and models of metal-matrix coated fiber densification

The hot isostatic/vacuum hot pressing and high temperature roll bonding consolidation of silicon carbide monofilaments coated with nanocrystalline Ti-6Al-4V has been studied and modeled. Systematic experimental studies relating process conditions to the specimens' shape change during hot isostatic press was accomplished though the use of an in situ eddy current sensor, during vacuum hot pressing by monitoring the ram displacement with a strain gauge and by post-process metallographic analysis for the roll bonded specimens. Surprisingly high densification rates were observed for each densification process, even at processing temperatures and pressures well below those used for processing conventional Ti-6Al-4V. The experiments used a variety of temperature/pressure cycles that resulted in both fully and partially consolidated composites. From the cross-sections of the partially consolidated specimens the evolution of coated fiber-fiber contacts and void shapes were determined. The pores were found to be cusp-shaped throughout the consolidation process indicative of a densification rate that greatly exceeded the (pore rounding) rate of sintering. In the VHP experiments, columns of coated fibers were observed to form which resulted in regions of locally high fiber volume fraction. The roll bonding experiments demonstrated that significant densification could be achieved in a continuous rolling process. The microstructural evolution of the initially nanocrystalline coating was characterized and found to be similar to that reported by Warren et al for sputter deposited Ti-6Al-4V.; Micromechanical models were developed for the HIP and VHP consolidation processes. The initial densification was based upon a micromechanical contact analysis and utilized recent finite element results for the contact mechanics of metal coated fibers. A potential method was used to generalize these results for non-isostatic stress states. Final densification was analyzed by modifying the Qian et al strain rate potential for a power law creeping body containing isolated cusp-shaped pores. Simulations of both the HIP and VHP experiments were performed using these models and compared well with the experimental density data. A grain size dependent constitutive response coupled to the experimentally determined microstructural evolution relationship was used to account for the process path dependence of the matrix creep rate. Analysis of the simulations revealed that the unusually high densification rate originated in the enhanced superplasticity of the initially nanocrystalline matrix. This same phenomenon also facilitated the roll bonding process.