Twinning and orientation relationships of the monoclinic θ-Al45(Mn,Cr)7 phase in an Al–Mg alloy containing Mn

The spherical-like θ-Al45(Mn,Cr)7 particle having a monoclinic structure is observed in an Al–Mg alloy using TEM. Two types of twins with different twin directions are found in these θ-Al45(Mn,Cr)7 particles. One type is shown to be a (1‾11) twin with (1‾11) plane serving as the twinning plane and the shear direction is η1 = [12‾3]; the other type is seen to be a (1‾10) orientation twin with (1‾10) plane acting as the coherent plane. At least six different orientation relationships between the θ-Al45(Mn,Cr)7 particles and the α-Al matrix are observed. These relatively complicated orientation relationships are attributed to the pseudo ten-fold symmetric building block of the θ phase viewed along the [101] axis. The possible formation mechanisms of these twins are discussed based on a crystallographic consideration.