Existence of hexagonal tabular β-phase in Al-Mg-Si alloys containing noble metal elements

The hexagonal tabular ß-Mg₂Si phase (hex-ß) co-existed with conventional truncated octagonal shaped ß-Mg₂Si (con-ß) in Al-1.0 mass% Mg₂Si alloy containing a small amount of noble metal elements (NMEs; Cu, Pt, Ag, Pd or Au) at the over-aged condition of 673 K. As this hex-ß laid on the {111} plane of the Al matrix, and its six edges were parallel to <110>_ß, its orientation relationship with the Al-matrix is as follows:{111_Al //{111}_ß and <110>_Al //<110>_β was the same as our previous report of (hex-ß) in an ingot of Al-1.0 mass%Mg2Si- 0.5 mass% Cu alloy just homogenized at 723 K for 4 days, without cold-rolling, solution heat treatment, and aging. There is no significant difference in the lattice parameter of ß-Mg₂Si by added NMEs. The formation of hex-ß was suggested to be related to defects in the Al-matrix, for example, by segregation of NMEs at the interface, altering the stacking fault energy, or introducing partial dislocations.