P. Rogl and A. Grytsiv

Inst. of Materials Chemistry, University of Vienna, A-1090 Wien, Austria

Socalled G-phases (Th₆Mn₂₃-type and ordered derivatives with Mg₆Cu₁₆Si₇-type, all space group Fm-3m) constitute a large class of intermetallic compounds comprising aluminides, gallides, silicides, germanides and stannides.

Our systematic investigation (phase relations, X-ray single crystal structure analysis, neutron powder diffraction) of titanium based transition metal (TM) aluminide solid solutions Ti₆{TM,Al}_~23 however, revealed a filled variant of the Th₆Mn₂₃ type (space group Fm-3m) for the Al-rich single-phase region, whereas a novel structure variant with space group F-43m was established for the Ti-rich side of the G-phase region. The direct group-subgroup relation between space groups Fm-3m and F-43m may favour a second order phase transition.

In contrast to the various structure variants observed for titanium aluminium-based G-phases Ti₆({Fe,Co,Ni,Ru,Rh,Pd,Os,Ir,Pt}_1−xAl_x)₂₃₊₁ and Ti₆Ni_16+xSi₇, the G-phase Mn₆Ni₁₆Si₇ was confirmed to crystallize as a simple ordered variant of the Th₆Mn₂₃ structure type.

It should be noted that the ternary intermetallic G-phase silicide precipitates preferentially at grain boundaries and dislocations in both ferrite and austenite of duplex stainless steels and recently gained significant interest for ultra-high strength steels.

In view of applications, the presentation will provide a comprehensive overview on the structural chemistry of G-phases, particularly focusing on aluminides and silicides covering also stannides.