Structural transition with loss of symmetry in Ti–M–Al based G-phases (MFe and Co)

The extended ternary solid solution for the G-phase in the Ti–Co–Al system, Ti6+xCo7+yAl17−x−y, has been investigated by neutron powder diffraction of the as-cast alloys in order to elucidate the precise atom-site distribution. A concentration-dependent structural transformation has been observed between two crystal structures. Whilst the aluminium-rich G-phase crystallizes with the centrosymmetric, filled variant of the Th6Mn23-type (SG Fm3¯m), Ti-rich alloys adopt a new non-centrosymmetric structure (SG F4¯3m) for compositions with more than 34 at.% Ti. Three states of high atomic order were denoted for the G-phase. The unit cells containing 24Ti–28Co–68Al atoms (x=0, y  =0, SG Fm3¯m) and 56Ti–32Co–32Al atoms (x=8, y  =1, SG F4¯3m) represent fully ordered structures. The unit cell containing 40Ti–30Co–50Al atoms (x=4, y  =0.5, SG F4¯3m) shows partial atom order with a statistical distribution of atoms in the 4b site, whilst all other atom sites are completely ordered.A corresponding transition from centrosymmetric SG Fm3¯m to non-centrosymmetric SG F4¯3m was furthermore established in the Ti–Fe–Al ternary system. Al-rich compositions of the G-phase crystallise in the filled variant of the Th6Mn23-type (SG Fm3¯m). The crystal structure of the Ti-rich G-phase adopts the non-centrosymmetric structure (SG F4¯3m).