Microstructure features and mechanical/electrochemical behavior of directionally solidified Al−6wt.%Cu−5wt.%Ni alloy

The effects of the addition of 5.0 wt.%Ni to an Al−6wt.%Cu alloy on the solidification cooling rate(T),growth rate(V_(L)),length scale of the representative phase of the microstructure,morphology/distribution of intermetallic compounds(IMCs)and on the resulting properties were investigated.Corrosion and tensile properties were determined on samples solidified under a wide range of T along the length of a directionally solidified Al−6wt.%Cu−5.0wt.%Ni alloy casting.Experimental growth laws were derived relating the evolution of primary(λ_(1))and secondary(λ_(2))dendritic spacings with T and V_(L).The elongation to fracture(δ)and the ultimate tensile strength(σ_(U))were correlated with the inverse of the square root of λ_(1) along the length of the casting by Hall−Petch type experimental equations.The reinforcing effect provided by the addition of Ni in the alloy composition is shown to surpass that provided by the refinement of the dendritic microstructure.The highest corrosion resistance is associated with a microstructure formed by thin IMCs evenly distributed in the interdendritic regions,typical of samples that are solidified under higher T.

Corrosion behavior of hypoeutectic Al-Cu alloys in H_2SO_4 and NaCl solutions

The aim of this work was to evaluate the electrochemical behaviour of hypoeutectic Al-Cu alloys immersed in two different solutions containing sulphate and chloride ions, respectively. The influence of Al2Cu associated to the dendritic arm spacing on the general corrosion resistance of such alloys is analysed. The typical microstructural pattern was examined by using scanning electron microscope. The corrosion tests were performed in both 0.5 M sulphuric acid and 0.5 M NaCl solutions at 25℃ by using an electrochemical impedance spectroscopy （EIS） technique and potentiodynamic polarization curves. Equivalent circuits by using the ZView software, were also used to provide quantitative support for the discussions. It was found that as the Cu content increased （i.e., increasing the Al2Cu fraction）, a higher susceptibility to the corrosion action in the NaCl solution is detected. In contrast, the tests carried out in the H2SO4 solution resulted in similar corrosion,rates for the three different hypoeutectic alloys.

Tailoring microstructure and microhardness of Zn-1wt.%Mg-(0.5wt.%Mn, 0.5wt.%Ca) alloys by solidification cooling rate

Biodegradable Zn-based alloys, particularly Zn-Mg alloys with the addition of alloying elements, have been intensively investigated aiming to improve both mechanical properties and corrosion behavior. Since such properties are strongly dependent on the alloy microstructure, any evaluation should commence on understanding the conditions influencing its formation. In this study, the effect of the solidification cooling rate on the microstructural evolution of Zn-1 wt.%Mg-(0.5 wt.%Ca, 0.5 wt.%Mn) alloys during transient solidification was investigated. The results show that the microstructures of both alloys have three phases in common: η-Zn dendritic matrix, intermetallic compounds(IMCs) Zn11Mg2, and Zn2 Mg in the eutectic mixture. MnZn9 and two Ca-bearing phases(CaZn11 and CaZn13) are associated with Mn and Ca additions, respectively. These additions are shown to refine the dendritic matrix and the eutectic mixture as compared to the Zn-1 wt.%Mg alloy. A correlation between cooling rate, dendritic or eutectic spacings was developed, thus permitting experimental growth laws to be proposed. Additionally, hardness tests were performed to evaluate the effects of additions of Ca and Mn. Experimental correlations between Vickers microhardness and secondary dendritic spacings were proposed, showing that the microstructural refinement and characteristic Ca and Mn based IMCs induce an increase in hardness as compared to the binary alloy.

Microstructural Morphologies and Experimental Growth Laws during Solidification of Monotectic and Hypermonotectic Al-Pb Alloys

AI-Pb alloys with monotectic and hypermonotectic compositions were directionally solidified under unsteady- state heat flow conditions. The cooling curves recorded during solidification allowed solidification thermal parameters such as the cooling rate （T）, growth rate （v） and thermal gradient （G） to be experimentally determined. Different microstructural patterns have been associated with the alloy solute content, i.e., AI-1.2 and 2.1 wt% Pb. A sequence of morphologies from the bottom to the top of the A1-1.2 wt% Pb alloy casting （monotectic） can be observed： Pb-rich droplets in the aluminum-rich matrix, followed by a region of microstructural transition formed by droplets and fibers and finally by a mixture of fibers and strings of pearls. A completely fibrous structure （without transition） has been observed along the entire AI-2.1 wt% Pb alloy casting （hypermonotectic）. The interphase spacing （λ） was measured along the casting length, and experimental correlations between λand experimental solidification thermal parameters have been established. Power laws with a -2.2 exponent expressing λ as a function of the growth rate, v, were found to better represent the fibrous growth of both AI-Pb alloys. Moreover, a single experimental law expressing λ as a function of both G and v was found to describe the fibrous growth of both the monotectic and the hypermonotectic alloys experimentally examined.

Horizontally Solidified Al-3 wt%Cu-(0.5 wt%Mg)Alloys:Tailoring Thermal Parameters,Microstructure,Microhardness,and Corrosion Behavior

In the present experimental investigation,Al–3 wt%Cu and Al–3 wt%Cu–0.5 wt%Mg alloys castings are produced by a horizontal solidification technique with a view to examining the interrelationship among growth rate(GR),cooling rate(CR),secondary dendrite arm spacing(λ2),Vickers microhardness(HV),and corrosion behavior in a 0.5 M NaCl solution.The intermetallic phases of the as-solidified microstructures,that is,h-Al2Cu,S–Al2CuMg,and x-Al7Cu2 Fe,are subjected to a comprehensive characterization by using calculations provided by computational thermodynamics software,optical microscopy,and scanning electron microscopy/energy-dispersive spectroscopy.Moreover,electrochemical impedance spectroscopy and potentiodynamic polarization tests have been applied to analyze the corrosion performance of samples of both alloys castings.Hall–Petch-type equations are proposed to represent the HV dependence onλ2.It is shown that the addition of Mg to the Al–Cu alloy has led to a considerable increase in HV;however,the Al–Cu binary alloy is shown to have lower corrosion current density(icorr)as well as higher polarization resistance as compared to the corresponding results of the Al–Cu–Mg ternary alloy.

Dendritic Growth, Eutectic Features and Their Effects on Hardness of a Ternary Sn–Zn–Cu Solder Alloy

The present investigation is based on the results of a directionally solidified （DS） Sn-9 wt%Zn-2 wt%Cu alloy, including primary/secondary/tertiary dendrite arm spacings of the Sn-rich matrix, the morphologies of the eutectic mixture and the corresponding interphase spacing, the nature and proportion of the Cu-Zn intermetallic compound （IMC）. The main purpose is to establish interrelations of these microstructure features with experimental solidification thermal parameters, such as cooling rates and growth rates （v）, macrosegregation and hardness. Such interrelations are interesting for both industry and academy since they represent a tool permitting the preprogramming of final properties based on the design of the microstructure. In the case of Sn-Zn-Cu alloys, hardly anything is known about the combined effects of the length scale of the microstructure and fraction and distribution of the primary IMC on hardness. The alloy microstructure is composed of a β-Sn dendritic region, surrounded by a eutectic mixture of α-Zn and β-Sn phases and the γ-Cu5Zn8 IMC. The eutectic interphase spacing varies in the range 1.2-3.6 μm, with the α-Zn phase having a globular morphology for ν 〉 0.5 mm/s and a needle-like morphology for ν 〈 0.3 mm/s. A modified Hall-Petch-type experimental expression relating hardness to the interphase spacing is proposed.