Effect of Manganese on Microstructure and Corrosion Behavior of the Mg-3Al Alloys

Abstract

Microstructure and corrosion behavior of the Mg-3Al-xMn (x = 0, 0.12, 0.21, 0.36, 0.45) (hereafter in wt.%) alloys were experimentally investigated by electron probe microanalysis (EPMA), scanning electron microscope equipped with energy dispersive X-ray spectroscopy (SEM/EDX), X-ray diffraction (XRD), electrochemical, and hydrogen evolution tests. A new self-constructed Mg-Al-Mn-Fe thermodynamic database was used to predict the solidification paths of the alloys. The addition of Mn showed no grain refinement in the cast Mg-3Al alloys. According to the microstructure observation, Al-Fe phases were observed in the non-Mn-added alloy, while Al8Mn5(LT) (Al8Mn5 in low temperature) became the main intermetallic phase in the Mn-added alloys, and the amount increased gradually with the Mn addition. The τ–Al0.89Mn1.11 phase with lower Al/(Fe + Mn) ratio was observed in the alloys with 0.36 and 0.45 wt.% Mn content. According to the electrochemical tests, all five alloys showed localized corrosion characteristics in 3.5 wt.% NaCl solution. Compared with the Mg-3Al alloy, the corrosion resistance of Mn-added alloys were significantly improved and increased gradually with the Mn addition, which was due to the variation of Al-containing intermetallic compounds. The present experimental investigations and thermodynamic calculations confirmed the mechanism that the increasing amount of Al8Mn5(LT) with Mn addition could encapsulate the B2-Al(Mn,Fe) phase with higher Fe. Therefore, it could prevent this detrimental phase from contacting magnesium matrix, thus suppressing micro-galvanic corrosion and improving corrosion resistance gradually.

Eutectic intermetallic formation during solidification of a Mg-Sn-Al-Zn-Mn alloy

“…4d-f), which are the same metastable Al-Mn particles found in Refs. [45][46][47][48]. A small amount of polyhedral Al-Mn IMCs, with Al/Mn ratio close to 1.44 ~ 1.51, as shown in Fig.…”

Al-Mn Intermetallics in High Pressure Die Cast AZ91 and Direct Chill Cast AZ80

“…Lattice planes and directions were indexed on the Kikuchi pattern by OIM software and the selected ones are labelled. Past work [25][26][27] has reported metastable τ-Al 0.89 Mn 1.11 in Mg-Al alloys with lower Al content (often in AZ31 [25,27]) and it was never found in our laboratory cast AZ91 cooled at 0.1 K/s or 1 K/s. It seems that the high in-cavity cooling rate of HPDC plays an important role in enabling the formation of the metastable τ-Al 0.89 Mn 1.11 in AZ91.…”

“…In recent years, there have been extensive studies [22][23][24][25][26][27][28][29][30][31][32][33][34] that have advanced the understanding of the fundamentals of Al-Mn IMC behaviour in magnesium alloys based on controlled laboratory experiments. However, it is unclear how these findings relate to Al-Mn IMC formation in industrial casting processes.…”

Microstructure, Mechanical Properties, and Corrosion Behavior of Ultra-Low Carbon Bainite Steel with Different Niobium Content

“…Generally, a large capacitive arc diameter represents a large charge transfer resistance (R t ). R t describes the interfacial resistance between the steel and the corrosion product layer, which is the final barrier that prevents chlorine ions from reaching the metal surface [ 32 ]. A large capacitive arc diameter means that corrosion products have a strong protective ability, and the degree of metal dissolution reaction is weak [ 19 , 33 , 34 , 35 ].…”