Fiber damage prediction for the consolidation of titanium-6aluminum-4vanadium (PVD) metallized alumina fibers.

摘要

A predictive model designed to simulate the fracture of the fibers during the high temperature consolidation cycle necessary to densify this metal matrix composite system was developed and compared to experimental results. The model attempts to describe and simulate the mechanical processes active during high temperature densification (bending fibers, viscoplastic matrix response, concurrent grain growth effects, matrix flow along fibers) by solving a system of 37 coupled, ordinary differential equations.; The matrix alloy was PVD processed and was not expected to exhibit the high temperature flow behavior normally observed in conventionally processed Ti-6Al-4V. To incorporate the viscoplastic behavior of this matrix into the process model, constant load creep tests were conducted on tensile test specimens fabricated from PVD'ed Ti-6Al-4V sheet. It was found that the as-deposited material was nanocrystalline and "coarsened" into the sub-micron grain size range when heated to creep test temperatures (600{dollar}spcirc{dollar}-900{dollar}spcirc{dollar}C). At test temperatures less than 680{dollar}spcirc{dollar}C the alloy was predominantly single phase (HCP {dollar}alpha{dollar}) and exhibited exceptionally low creep resistance. Above 760{dollar}spcirc{dollar}C enhanced superplastic behavior, facilitated by the formation of an intergranular {dollar}beta{dollar}-phase, was observed. A detailed analysis of the creep deformation behavior (including TEM analysis of strained gauge sections) was conducted and conventional creep models based on the grain boundary sliding mechanism (GBS), dislocation creep and diffusional flow were used to predict the true stress-true strain rates measured experimentally. The creep model predictions were improved significantly when the effects of phase distribution and concurrent grain growth were considered. The grain size dependent, constitutive response of the alloy was then used in the process model to simulate the viscoplastic response of the matrix.; The simulation method predicts fiber damage experimentally observed to occur early in the consolidation cycle and other trends.