Fracture and toughening of high volume fraction ceramic particle reinforced metals

This work contributes to the fundamental understanding of fracture properties of Particle Reinforced Metal Matrix Composites (PRMMCs), by identifying the key microstructural parameters that control fracture. To this end, PRMMCs with a high volume fraction of ceramic reinforcement (40-60 vol.%) are produced by gas-pressure infiltration. These composites are considered as model ductile/brittle twophase materials in that (i): the particles are homogeneously distributed in the matrix, (ii): the matrix microstructure is kept simple, and (iii) the composites are free of processing defects. The reinforcements used are alumina (Al2O3) particles of various shape (angular, polygonal) and size (5 to 60 µm), and boron carbide (B4C) particles (5 to 60 µm). The matrix materials are (i): pure Al, (ii): Al-Cu2% alloy, and (iii): Al-Cu4.5% alloy, all being chosen in order to obtain a single-phase matrix (Cu in solid-solution for the Al-Cu alloys), and to minimize chemical reactivity with the reinforcement. Pure Al matrix composites exhibit marked R-curve behaviour; they are characterized by J-integral fracture testing. The fracture toughness increases with the interparticle distance. At a given particle size, polygonal Al2O3 particle composites are the toughest, followed by B4C particle composites, and by angular Al2O3 particle composites. Al-Cu matrix composites feature a flatter R-curve, and are tested by a Linear Elastic Fracture Mechanics (LEFM) method: the chevron-notch test. Again, polygonal particles yield tougher composites than angular ones. In the as-cast condition, coarse intermetallics formed at the interface matrix/reinforcement during solidification are strongly detrimental to the toughness. After heat-treatment, on the other hand, toughness of the alloyed matrix composites is improved and increases as the matrix is strengthened by raising the Cu content in the matrix. Using an arrested-crack technique, it is found that the dominant micromechanisms of fracture of pure Al matrix composites are strongly dependent on the particle type, shape and size: the stronger the reinforcement, the more the crack tends to propagate by a ductile mechanism of nucleation, growth, and coalescence of micro-cavities. With weaker particles, cracking of the composite is promoted by premature particle cracking. A stereoscopic method coupled with Scanning Electron Microscopy (SEM) imaging is used to reconstruct the fracture surfaces in 3D. The final dimple size (diameter, depth) is found to depend on the microstructural length scale of the composites, i.e. the interparticle distance. Data obtained from two types of measurement (quantitative metallography, dimple depth) are used to estimate the local energy necessary to create the fracture profile, by using simple micromechanical models. At the global scale, surface strain fields are revealed by photoelasticity. The observed crack-tip strain fields are fully confirmed by 3D Finite Element (FE) computations. Although most of the fracture energy is spent in the plastic zone, it is shown that toughness is controlled by the local fracture energy that is dissipated in the crack-tip process zone: the macroscopic fracture toughness is an "amplification" of the local fracture energy. This simple and linear correlation breaks down when, for a given ceramic particle type and size, a transition in the dominant micromechanism of fracture occurs as the matrix is strengthened. The local/global correlation is discussed in more detail, using a simplified approach based on the Cohesive Zone Model (CZM) for ductile fracture: the fundamental parameters allowing to achieve attractive toughness are identified as: (i) the intrinsic particle strength, and (ii) the high local stress triaxiality between the closely spaced particles, made possible by the strong interfacial bonds between matrix and reinforcement. Overall, the composites feature very high toughness for materials made of up to 60 vol.% of brittle phase. The toughest pure Al matrix composites feature a KJeq as high as 40 MPa·m1/2. For Al-Cu matrix composites, KIv (the plane-strain chevron notch fracture toughness) exceeds 30 MPa·m1/2 (a value, to our knowledge, never reported for this class of materials) together with a Young's modulus of 180 GPa, a yield strength of 400 MPa and an ultimate tensile strength approaching 500 MPa. This combination of values gives an interesting potential for these composites as engineering materials.