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

Eutectic Solidification in Mg-9al-0.7zn: From Divorced to Coupled Growth

“…While the lack of a preferred OR in most regions is generally consistent with past work on Mg-Al alloys that found no OR [21,23], we additionally discovered a small number of eutectic regions with the Burgers OR. The reason this was found here and not in references [21,23] seems to be that a larger area of microstructure was analysed here, increasing the chances of finding the small number of regions with an OR.…”

“…9, there are traces of a nanoscale phase between adjacent or impinging cells that are bright in backscatter electron images. These may be the Zn-richer phase ϕ-Mg 21 (Al,Zn) 17 or, possibly τ-Mg 32 (Al,Zn) 49 which is not predicted by the Scheil model but has been reported to form instead of ϕ-Mg 21 (Al,Zn) 17 in other Mg-Al-Zn alloys [23]. The details of this phase were not characterised further here.…”

“…At least 13 orientation relationships (ORs) have been reported between α-Mg and Mg 17 Al 12 [60][61][62][63][64][65][66][67][68][69][70][71][72], mostly after solid state precipitation. To test for any reproducible orientation relationships (ORs) between α-Mg and Mg 17 Al 12 in partially divorced eutectic growth, an approach similar to Deng et al [23] was applied which searches for reproducible angle(s) that exist between predefined axes of the two crystals. The minimum angle between < 0001 > α-Mg and < 100 > Mg17Al12 and between < 0001 > α-Mg and < 110 > Mg17Al12 was calculated for each region of contacting α-Mg and Mg 17 Al 12 using the Euler angles extracted from EBSD maps such as those in Fig.…”

“…In partially divorced eutectic growth, some eutectic α-Mg grows on the surrounding α-Mg dendrites and some grows with Mg 17 Al 12 , resulting in islands of α-Mg within Mg 17 Al 12 in 2D cross-sections [21,22]. In partially and fully divorced eutectic microstructures of gravity cast samples, prior work found no preferred ORs between α-Mg and Mg 17 Al 12 [21,23] and the eutectic α-Mg was found to have the same orientation as the surrounding primary α-Mg dendrite [24].…”

“…They discussed this in terms of: (i) Zn solute generating more highly branched α-Mg dendrites that reduce the size of the interdendritic spaces and (ii) the effects of Zn solute on the equilibrium and non-equilibrium phase boundaries. Higher Zn additions result in ϕ-Mg 21 (Al,Zn) 17 and/or τ-Mg 32 (Al,Zn) 49 ternary compounds [23,36,37] at the end of solidification which commonly share an interface and sometimes an OR with Mg 17 Al 12 [23]. However, for the Zn content typical of AZ91, these phases are usually only present in trace amounts or are not detected.…”