THE INFLUENCE OF REINFORCEMENT HOMOGENEITY ON THE DEFORMATION AND FRACTURE OF A DISCONTINUOUSLY REINFORCED ALUMINUM MATRIX COMPOSITE
Open Access
- Author:
- Wilks, Garth Barrett
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 06, 2007
- Committee Members:
- Donald Albert Koss, Committee Chair/Co-Chair
Paul Raymond Howell, Committee Chair/Co-Chair
Christopher L Muhlstein, Committee Member
Clifford Jesse Lissenden Iii, Committee Member
Jonathan E Spowart, Committee Member - Keywords:
- Homogeneity
Deformation
Fracture
Aluminum
Metal Matrix Composite
DRA
Clustering - Abstract:
- Deformation processing (extrusion) has been used to homogenize reinforcement distribution in a discontinuously reinforced aluminum matrix composite (DRA 6092/SiC/25p). Reinforcement distribution after three conditions of varying extrusion reduction has been quantified using the homogenous length scale technique. Results indicate that deformation processing positively influences reinforcement homogeneity¡ªincreasing deformation asymptotically leads to an increasingly uniform reinforcement distribution. After heat treatment to peak age, the compressive flow behavior both along and transverse to the extrusion axis has been determined for each condition. Using chevron notch short rods, the fracture toughness behavior in several orientations has also been assesed. Variations in flow behavior with deformation processing are mainly rationalized in terms of matrix texture with the combined effect of particle alignment along the extrusion axis and reinforcement homogenization being relatively small. Regardless of orientation, toughness is shown to increase with reinforcement homogeneity; in a particular transverse orientation, chevron notch fracture toughness (KIv) demonstrates a nearly two-fold increase (10.8 MPa¡Ìm to 19.3 MPa¡Ìm) as a result of reinforcement cluster breakdown. Qualitative fractography indicates a generally ductile fracture process. Quantitative fractography indicates a strong positive relationship between fracture surface average roughness (RA¡ªa measure of out-of-plane crack deflection) and toughness (KIv), while a fractal technique used to characterize the fracture surface indicates a decrease in fractal dimension with increasing toughness. Using a simple model, the trend of increasing average roughness is interpreted as an increase in the fracture process length scale, while the decrease in fractal dimension is interpreted as a subtle decrease in the mean angle of crack deflection¡ªa result expected with reinforcement homogenization. The increase in toughness is rationalized as a combined effect of decreasing crack deflection (leading to slower strain accumulation during crack blunting) in addition to a nominal improvement in plastic strain that can be supported by the interparticle ligament after reinforcement homogenization. Such a change in local ductility is qualitatively validated with SEM fractography.