Groves research group
Department of Materials Science and Engineering
University of Virginia

"Fiber Damage During the Consolidation of Metal Matrix Composites"
James F. Groves

M.S. Thesis, University of Virginia, 1992.

Abstract
A continuous fiber reinforced metal matrix composite incorporating SiC fibers into a Ti-14Al-21Nb wt% matrix is being developed for high-temperature applications requiring strength and toughness. Currently a two-step plasma spray + hot isostatic press (HIP) or vacuum hot press (VHP) process is used to make multilaminate sheets and tubes. This study investigates the fracture of fibers during the consolidation process.

The study had three goals: 1) to determine when fiber fracture occurred during processing, 2) to correlate fiber fracture during HIP / VHP processing with processing variables, e.g. temperature, pressure, and pressurization rate, and 3) to identify fiber fracture mechanisms. To achieve these goals, three lines of research were pursued. First, the condition of the fibers after plasma spraying was determined by measuring fibers removed from the matrix using an acid etch. Also, the fibers' Weibull modulus was determined to see if spraying changed this distribution. Second, the number of fibers fractured by HIP / VHP processing under different conditions was calculated by measuring the fiber segments after processing. Third, partial VHP compaction runs and furnace thermal cycles identified specific fiber damage mechanisms.

The following conclusions were drawn. Fiber fracture does not occur during spraying, and the fibers' Weibull modulus is unaffected by spraying. Most fiber fractures during consolidation are bending failures caused by the foil surface roughness. As consolidation temperatures decrease, the number of fiber fractures increases. As processing pressures increase, the number of fractures increases. As the rate of pressure application increases, the number of fractures increases. Pressure applied to stacked, unconsolidated foils at room temperature can cause fracture. Spray parameter modifications can decrease the incidence of fracture.