Optimization of the Strength-Fracture Toughness Relation in Particulate-Reinforced Aluminum Composites via Control of the Matrix Microstructure

The article of record as published may be found at http://dx.doi.org/10.1007/s11661-998-0119-9The evolution of the microstructure and mechanical properties of a 17.5 vol. pct SiC particulatereinforced aluminum alloy 6092-matrix composite has been studied as a function of postfabrication processing and heat treatment. It is demonstrated that, by the control of particulate distribution, matrix grain, and substructure and of the matrix precipitate state, the strength-toughness combination in the composite can be optimized over a wide range of properties, without resorting to unstable, underaged (UA) matrix microstructures, which are usually deemed necessary to produce a higher fracture toughness than that displayed in the peak-aged condition. Further, it is demonstrated that, following an appropriate combination of thermomechanical processing and unconventional heat treatment, the composite may possess better stiffness, strength, and fracture toughness than a similar unreinforced alloy. In the high- and low-strength matrix microstructural conditions, the matrix grain and substructure were found to play a substantial role in determining fracture properties. However, in the intermediate- strength regime, properties appeared to be optimizable by the utilization of heat treatments only. These observations are rationalized on the basis of current understanding of the grain size dependence of fracture toughness and the detailed microstructural features resulting from thermomechanical treatments.United States Army Research OfficeArmy Research LabratoryUnited States Air Force Office of Scientific ResearchWright Materials LabratoryDWA Composite

Microstructural evolution of submicron sized ferrite in bimodal structural ultrafine grained ferrite/cementite steels by annealing below austenized temperature

The microstructural evolution of submicron sized ferrite in bimodal structural ultrafine grained ferrite/cementite steels with 0.15 pct carbon content and 0.45 pct carbon content upon annealing below the austenized temperature was investigated. The average grain sizes of the ferrites with a normal density and with a high density of cementite particles were plotted, respectively, as a function of the annealed temperature and time, and exhibited different coarsening behaviors. The average grain sizes of the ferrites with a normal density of cementite particles gradually coarsened by increasing the annealing temperature or time, while those with a high density of cementite particles hardly changed at first, and then coarsened after reaching a certain annealing condition. The coarsening of the ferrite grain size in the steel with 0.15 pet carbon content occurred much more readily than that in the steel with 0.45 pet carbon content upon annealing. The spacing and the critical spacing of cementite particle were measured and hypothetically calculated, respectively. The size and the distribution of cementite particles was one of the critical factors affecting the microstructural evolution in this type of cementite particle spherodized steels. Most of the coarsening of the ferrite grain size occurred after the cementite particle spacing reached the required critical value