Emmanuelle S. Freitas

Inter-relation of Microstructural Features and Dry Sliding Wear Behavior of Monotectic Al–Bi and Al–Pb Alloys

Abstract Immiscible Al-based alloys of monotectic composition have a particular feature of minority phases embedded into the Al-rich matrix. The disseminated particles may act as in situ self-lubricating agents due to their lower hardnesses compared with that of the Al-rich matrix, favoring good tribological behavior. There is a lack of systematic fundamental studies on the microstructural evolution of monotectic alloys connected to application properties. In the present investigation, the monotectic Al-1.2wt%Pb and Al-3.2wt%Bi alloys have been chosen to permit the effect of microstructural parameters on the wear behavior to be analyzed. Directional solidification experiments were carried out under transient heat flow conditions allowing a large range of cooling rates to be experienced, permitting a representative variation on the scale of the microstructure to be examined. Samples of the monotectic alloys having different interphase spacing, λ, have been subjected to microadhesive wear tests, and experimental laws correlating the wear volume with the microstructural interphase spacing and test time are proposed. It was found that microstructural features such as the interphase spacing and the morphology of the minority phase play a significant role on the wear process and that for the alloys examined λ exhibits opposite effects on the corresponding wear volume.

Electrochemical Behavior of an Al-Fe-Ni Alloy Affected by Nano-Sized Intermetallic Particles

The aim of this study is to determine the effect of the resulting microstructural array of an as-cast Al-1 wt% Fe-1 wt% Ni alloy on its electrochemical corrosion behavior in a stagnant and naturally aerated 0.5 M NaCl solution at room temperature. Electrochemical impedance spectroscopy (EIS) plots, potentio-dynamic polarization curves, and equivalent circuits are used to evaluate the corrosion response. It was found that nano-sized Al9FeNi intermetallic (IMC) particles have an important role on the corrosion resistance, which significantly affect the anode/cathode area ratio and as consequence, the examined electrochemical parameters. A mixture of interconnected spheroids and fiber-like IMC particles induces a drastic and rapid increase in the current density, while intermittent IMC particles reduce the corrosion action.

Electrochemical Impedance Spectroscopy and Potentiodynamic Polarization Studies Affected by the Microstructure Array of a Monotectic Al-Pb Alloy in a NaCl Solution

In this study, electrochemical impedance spectroscopy (EIS) plots and potentiodynamic polarization curves of tests carried out with monotectic Al-Pb alloy samples in a 0.5 Molar sodium chloride (NaCl) solution at 25°C are evaluated. It is shown that for a microstructure characterized by immiscible Pb droplets disseminated into an Al matrix, the microstructural array and segregation pattern are the corrosion driving forces. Microstructures characterized by Pb droplets of higher diameter and more homogeneously distributed into the Al-matrix, typical of positions close to the casting surface, have lower corrosion resistance. This has been attributed to a higher anode/cathode area that characterizes such microstructures and that, as a consequence, induces a higher number of galvanic couples that are formed between the Pb droplets and the Al matrix.

Growth of tertiary dendritic arms during the transient directional solidification of hypoeutectic Pb–Sb alloys

Despite the importance of a complete characterization of dendritic patterns in castings, the availability of studies on the development of tertiary dendrite arms is scarce in the literature. In the present study, the tip cooling rate, local solidification time, primary and tertiary dendrite arm spacings have been determined in Pb–Sb alloys castings directionally solidified under unsteady-state heat flow conditions. The alloys compositions experimentally examined are widely used in the as-cast condition for the manufacture of positive and negative grids of lead-acid batteries. The initial growth of tertiary dendritic arms from the secondary branches was found to occur only for a Pb–3.5 wt% Sb alloy at cooling rates in the range 0.4–0.2 K/s, with no evidence of this spacing pattern for Pb–Sb alloys having lower solute content. Tertiary dendritic branches have been observed along the entire casting lengths for alloys of the Pb–Sb hypoeutectic range having compositions higher than 4.0 wt% Sb. It is shown that a power function experimental law with a characteristic −0.55 exponent is able to characterize the tertiary spacing evolution with the solidification cooling rate for alloys compositions ≥4.0 wt% Sb. The only exception was the Pb–3.5 wt% Sb alloy for which λ 3 exhibited significant lower values when compared with the experimental values obtained for the other Pb–Sb alloys for a same solidification cooling rate.

Electrochemical Corrosion Behavior of as-cast Zn-rich Zn-Mg Alloys in a 0.06M NaCl Solution

The electrochemical corrosion behavior of as-cast samples of Zn1.2 wt.% and Zn2.0 wt.% Mg alloys, solidified under similar cooling rates, is investigated in the present study. A stagnant and naturally aerated 0.06 M NaCl solution at 25 o C was used in the corrosion tests. In order to evaluate the corrosion resistance, electrochemical impedance spectroscopy (EIS) plots, potentiodynamic polarization curves and an equivalent circuit are used. It is found that the increase in the alloy Mg content (from 1.2wt.% to 2.0wt.%) refines both the Zn-rich dendritic matrix and the eutectic mixture and decreases the volume fraction of the Zn-rich phase. Consequently, this is shown to affect the cathode-to-anode area ratio, which decreases affecting the corrosion behavior. The experimental corrosion parameters demonstrated that the Mg content is associated with susceptibility to pitting corrosion.