Microstructure Of AZ91 Mg Alloy Research Articles

Role of microstructure and secondary phase on corrosion behavior of heat treated AZ series magnesium alloys

Abstract Magnesium (Mg) and its alloys belong to light metals group are now attracting the attention of the manufacturing engineers as promising candidates to develop structures for energy efficient applications. Among the all available Mg alloys, AZ series is the important group of alloys which contains Aluminum (Al) and Zinc (Zn) as the main constituting alloying elements. As the solubility of Al into Mg is limited to ≈ 1 Wt. % at the room temperature, presence of secondary phases is inevitable in AZ series Mg alloys if Al content is more than 1%. In the present study, pure Mg and two AZ series Mg alloys, i) AZ31 which is with 3% Al and ii) AZ91 which is with 9% Al were selected and heat treated (pure Mg and AZ31 at 340 and AZ91 at 410°C) to alter the amount of Al dissolution and to investigate the effect of heat treatment on microstructural modification, microhardness and corrosion behavior. Interestingly, heat treatment severely affected the microstructure of AZ91 Mg alloy as clearly demonstrated by the decreased secondary phase. Hardness was observed as decreased for all the samples due to stress revealing and modified microstructure. From the immersion studies carried out in 3.5% NaCl solution, weight loss measurements showed decreased corrosion rates for all the heat treated samples which can be attributed to the microstructural modification due to heat treatment.

Study on the effect of mischmetal (La,Ce) on the micro-galvanic corrosion of AZ91 alloy using multiscale methods

The relative potential difference can be treated as an index of local corrosion of alloys. Since local corrosion is driven by the micro-galvanic coupling, which is consist of the micro-constituent phases and the matrix. They present distinct potential difference normally. In the present work, the relationship between the Volta potential and the corrosion effect of the dominated intermetallic phase in the AZ91 Mg alloy with (La, Ce) MM addition was investigated. A multiscale approach coupling global and local measurements was used to study the corrosion behavior of the alloy and its intermetallic phase. Results showed that the microstructure of AZ91 Mg alloy with (La, Ce) MM addition was complex, with the presence of Al4(La, Ce) phases in acicular or rod shape, a small number of Al8Mn4Ce, β-Mg17Al12 and α-Mg solid solution. In this case, the improved corrosion resistance of the alloy was achieved. Scanning Kelvin probe force microscopy (SKPFM) analysis revealed that all the intermetallics were noble comparing with the α-Mg solid solution. In-situ electrochemical atomic force microscopy (EC-AFM) observation for the initial stages of corrosion showed the α-Mg matrix surrounded by β-Mg17Al12 or Al8Mn4Ce was susceptible to pitting corrosion. No noticeable corrosion occurred at the α-Mg adjacent to Al4(La,Ce) phase, in spite of the relatively higher difference of Volta potential between Al4(La,Ce) phase and the matrix. Volta potential is insufficient to determine whether the phase is an effective cathode or anode.

Electrodeposition of Copper on AZ91 Mg Alloy in Cyanide Solution

Copper electrodeposition on AZ91 Mg alloy was studied in views of preferential deposition on α- or β-phases and how to achieve uniform deposition over the entire surface on α- and β-phases in a cyanide solution. The inhomogeneous microstructure of AZ91 Mg alloy, particularly α- and β-phases, was found to result in non-uniform deposition of zincate layer, preferential deposition of zincate on β-phases, which leads to non-uniform growth of copper layer during the following electrodeposition process. The preferential depositions of zincate can be attributed to higher cathodic polarizations on the β-phases. Pin-hole defects in the copper electrodeposit were observed at the center of large size β-phase particles which is ascribed to gas bubbles formed at the β-phases. The activation of AZ91 Mg alloy in hydrofluoric acid solution was used to obtain uniform growth of zincate layer on both the α- and β-phases. By choosing an optimum activation time, a uniform zincate layer was obtained on the AZ91 Mg alloy surface and thereby uniform growth of copper was obtained in a cyanide copper electroplating solution.

Effectiveness of Ti-micro alloying in relation to cooling rate on corrosion of AZ91 Mg alloy

In this study, micro Ti-alloyed AZ91 Mg alloys (AZ91 + 0.5wt.%Ti) have been investigated in order to clarify effectiveness of micro alloying and/or cooling rate on their corrosion properties. Molten alloys were solidified under various cooling rates by using four stage step mold. The microstructural investigations were carried out by using scanning electron microscopy (SEM). Corrosion behaviors of the alloys were evaluated by means of immersion and electrochemical polarization tests in 3.5% NaCl solution.Results showed that the Mg17Al12 (β) intermetallic phase in the microstructure of AZ91 Mg alloy formed as a net-like structure. The Ti addition has reduced the distribution and continuity of β intermetallic phase and its morphology has emerged as fully divorced eutectic. Compared to AZ91 alloy, the effect of the cooling rate in Ti-added alloy on the grain size was less pronounced.When AZ91 and its Ti-added alloys were compared under the same cooling conditions, the Ti addition showed notably high corrosion resistance. Electrochemical test results showed that while Icorr values of AZ91 decrease with the increase in the cooling rate, the effect of the cooling rate on Icorr values was much lower in the Ti-added alloy. The corrosion resistance of AZ91 Mg alloy was sensitive towards the cooling rates while Ti-added alloy was not affected much from the cooling conditions.

Effect of Intermetallic Phases on Corrosion Initiation of AZ91 Alloy With Rare Earth Y Addition

The effect of the rare earth (RE) element Y on the microstructure of AZ91 Mg alloy is investigated. A detailed description of the procedure of identifying the intermetallic phases containing Y in the AZ91 Mg alloy with 0.9 wt% Y addition is presented. The results shows that there are two different kinds of RE phases in scanning electron microscopy (SEM) images. Energy dispersive spectrum (EDS) analysis shows that one is rich in Y element and the other is rich in Mn element. According to the element mapping of electron probe microanalysis (EPMA), the element Al can be found not only in the β-Mg17Al12 phase but also in these two RE phases. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results further confirm one of the two RE phases is Al2Y; the phase of Al10Mn2Y has just been determined by transmission electron microscopy (TEM) instead of XRD probably because it is scarce. The shapes of these two RE intermetallic phases are pretty similar. The 3D digital microscopy is employed to observe in-situ the effect of the β phase and the above two RE intermetallic phases on the corrosion initiation of the alloys. The results show that the corrosion attack of AZ91 Mg alloy without Y addition starts around the eutectic phase β-Mg17Al12. With the addition of 0.9 wt% Y, the two kinds of Y-rich intermetallic phases act as cathodic effect and α phase in the adjacent of them was activated and cathodic effect of the β-Mg17Al12 phase was inhibited.

Correlative change of corrosion behavior with the microstructure of AZ91 Mg alloy modified with Y additions

Microstructure characterization of the AZ91 magnesium alloys with or without rare earth element yttrium (Y) has been revealed by SEM, EDS and EPMA. Some Y-rich phases can be found in the magnesium alloys with Y additions. The fraction of β-Mg17Al12 phase obvious decreases and turns into granular distribution with the increase of Y addition instead of original wet distribution along grain boundaries. The results of the potentiodynamic polarization tests show that the corrosion resistance of AZ91 alloy is improved with appropriate Y additions. But an in-situ observation of 3D digital microscopy for the initial stage of corrosion of the magnesium alloy with 0.9%Y addition shows that Y-rich phases act as cathodic effect and the α phases in the vicinity of them acting as anode are corroded. So the additions of Y have a beneficial effect that can depress the overall corrosion of AZ91 alloy, whereas its harmful effect is related to Y-rich phases because they present the highest Volta potential difference from the adjacent matrix and they can drive galvanic corrosion.