Papers by Author: J. Martin Herrera-Ramírez

Abstract: Al-based composites were fabricated by solid-state route and were characterized by optic and scanning electron microscopy in order to follow their microstructural evolution. Composites were prepared using powder metallurgy techniques in order to obtain samples to carry out mechanical tests on hot extruded and machined samples. Microstructural characterization reveals that, by milling, a homogeneous dispersion of insoluble particles into Al matrix is obtained; this produced an important improvement on hardness and strength compared with the reference. Milling intensity and particle concentration have an important effect on the mechanical properties of composites.

Abstract: Elemental powders and carbon nanotubes (CNTs) were mixed and milled in a high energy shaker mill (SPEX-8000M), to produce 2024 aluminum (Al₂₀₂₄) matrix composites reinforced with CNTs. Milled products were consolidated by uniaxial load pressing followed by pressure-less sintering under argon atmosphere for 2 h at 773 K. The effect of CNTs concentration and milling time on Vickers microhardness (µHV) was studied. Scanning electron microscopy (SEM) micrographs show that by milling process it is possible to obtain a homogeneous dispersion of CNTs into the aluminum matrix. The mechanical properties of the composites show an important improvement with respect to reference samples. The possible strengthening mechanisms are discussed in the present work.

Abstract: Aluminum-based nanocomposites have been produced by mechanical milling, introducing silver nanoparticles within the matrix of a 7075 aluminum alloy using a high energy ball mill. The milled products were compacted by uniaxial load and pressure-less sintered under argon atmosphere, and finally hot extruded. Silver nanoparticles are well dispersed into the matrix of the powder particles as well as in the matrix of the extruded material. Transmission electron microscopy (TEM) analyses are used to corroborate and understand the hypothesis that second-phase particles finely and homogeneously dispersed in the matrix give greater strength to the material. In addition to the strengthening effect, the nanoparticles act like a process control agent (PCA) since the crystallite size of the nanocomposite is smaller at higher contents of nanoparticles.

Abstract: The behavior of two different types of ultra-high-performance polyamide (PA) 66 fibers under fatigue loading up to failure, and the correlation between the fibers (nano)structures and their structural heterogeneity with fatigue lifetimes, have been studied using scanning electron microscopy, differential scanning calorimetry, wide angle x-ray diffraction and micro-Raman spectroscopy. The role of the microstructure of the fibers in determining fatigue life is presented and the possibility of improving their resistance to fatigue or eliminating the fatigue process will be discussed.