Calderón-Arteaga, Hermes E.

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    Effect of cyclic high loading rates on the fatigue strength of aluminum-based composites
    (2009) Calderón-Arteaga, Hermes E.; Suárez, O. Marcelo; College of Engineering; Godoy, Luis A.; López, Ricardo R.; Suárez, Luis E.; Shafiq, Basir; Department of Civil Engineering; Pabón, Carlos U.
    The study of fatigue under high loading rates is of great interest in the complete characterization of a new series of composites with Al-Cu-Mg matrix reinforced with AlB2 dispersoids. Homogeneous and functionally graded composites were prepared via gravity and centrifugal casting, respectively. Through centrifugal casting a gradual variation of the volume fraction of reinforcing particles along the cross section was obtained. In specific fabrication conditions, even complete segregation of the reinforcement particles was achieved. Charpy impact tests as well as hardness tests were conducted to assess the composite strength as a function of the weight percent of boron. The tensile properties of gravity cast samples were obtained. Then for both casting conditions, simple edge-notched bend SE(B) specimens were tested under fatigue conditions (three-point bending). The results from impact and hardness tests allowed identifying an interaction between the Mg dissolved in the matrix and the diborides. This interaction, which has never been reported before, was responsible for the strength reduction observed. It was assumed that a substitutional diffusion of Al by Mg atoms in the hp3 structure of diboride was causing the strength reduction, and three approaches were developed to estimate the amount of Mg depleted from the matrix by the diborides during the composite processing. Gravity cast samples were more sensitive to monotonic damage due to fatigue loads where compared with functionally-graded composites. Contrary to the centrifugal cast samples, gravity samples were also affected by the loading rate. The Mg-AlB2 interaction was also responsible for the reduction in the fatigue resistance as the weight percent of boron increased in both types of composites; regression models were obtained to predict the crack growth curve slope change as function of the boron level. The particle distribution showed to affect the crack growth behavior of the FGMs, decreasing the fatigue resistance when the crack tip traveled through higher particle density zones. The fracture surface of these composites exhibited quasicleavage and the roughness was associated with the microstructure phases. Finally, the effect of the Mg-AlB2 interaction could be corrected if it is taken into account in the mass balance prior to the casting process.
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    Efectos de la deformación mecánica sobre compuestos de matriz de aluminio tratados térmicamente
    (2004) Calderón-Arteaga, Hermes E.; Suárez, O. Marcelo; College of Engineering; Pérez, Nestor; Sundaram, Paul; Department of Mechanical Engineering; Mina, Nairmen
    A novel aluminum matrix composite reinforced with ceramic particles of AlB2 obtained through a cast process at temperatures below 800°C using commercial alloys of aluminum-boron (5 wt%. B) and aluminum-copper (33.2 wt%. Cu) was characterized. The effects of cold work prior to aging on precipitation hardening were studied using a mixture design experiment combined with split-split plot experiment. Statistical models for the hardness of the composite were produced showing an increase in hardness of the composite due to cold work and aging process as well as higher levels of boron and copper. The aging treatment effect is proportional to the amount of copper present in the material, confirming that hardness and mechanical strength can be adjusted by a combination of cold work and heat treatments. The microstructural analysis showed a good distribution of reinforcements and revealed a pattern of grain subdivision by deformation. Tensile tests were performed to corroborate the results and a dynamic strain aging behavior was observed in a solutionized sample. Finally the fracture surface was analyzed using SEM and optical microscopy, which showed that the predominant failure mode is a combination of brittle and ductile fracture.