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 Title: Mechanism of subcritical crack growth in the alumina reinforced aluminum-copper composite: Effects of interface and matrix microstructure Author(s): Liu, Gang Doctoral Committee Chair(s): Shang, Jian Ku Department / Program: Engineering, Materials Science Discipline: Engineering, Materials Science Degree Granting Institution: University of Illinois at Urbana-Champaign Degree: Ph.D. Genre: Dissertation Subject(s): Engineering, Materials Science Abstract: Micromechanisms of fracture and fatigue crack growth were examined in an $\rm Al\sb2O\sb3/Al$-Cu composite and its matrix alloy, with particular emphases on effects of matrix and interfacial microstructure. The matrix and interfacial microstructures were controlled by aging the composite to underaging (UA), peak-aging (PA), slight overaging (OA-1) and extensive overaging (OA-150) conditions. The interface between the matrix and $\rm Al\sb2O\sb3$ was free of interfacial precipitates in the UA condition, but covered with small and coarse precipitates in the PA and OA conditions, respectively. Slight overaging was chosen to produce a yield strength similar to that of the UA condition so that the effects of the matrix microstructure and the interface microstructure on the fracture and fatigue crack growth behavior could be separated. Fracture studies indicated that failure mechanisms changed from particle cracking in UA composite to a near-interface fracture in PA and interface failure in OA composites. The fracture toughness in the composite was found to be proportional to shear strength of the $\rm Al\sb2O\sb3/Al$-Cu interface. Fatigue cracks in the composite propagated primarily in the matrix by avoiding particles. In the near-threshold regime, crack growth rate of the composite was controlled by matrix properties and was insensitive to interfacial microstructure. High threshold in the PA composite is related to high yield strength, high crack closure level, and small crack-tip opening displacement of the composite. At high stress intensities, crack growth was strongly dependent on interface microstructure. Fast crack growth in the OA composite resulted from the low fracture toughness and interfacial decohesion. At elevated temperatures, fatigue crack growth was accelerated by cyclic-deformation induced crack-tip overaging at 150$\sp\circ$C and by softening of the composite at 300$\sp\circ$C. Issue Date: 1995 Type: Text Language: English URI: http://hdl.handle.net/2142/23639 Rights Information: Copyright 1995 Liu, Gang Date Available in IDEALS: 2011-05-07 Identifier in Online Catalog: AAI9522141 OCLC Identifier: (UMI)AAI9522141
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