The formation of metastableC40–MoSi2andC49–TiSi2during the mechanical alloying of metal/silicon powder mixtures has been investigated by x-ray diffraction. The mechanical alloying of a Si-rich Mo/Si powder mixture results in the formation of bothC11b–MoSi2andC40–MoSi2.TheC40–MoSi2phase is metastable and transforms toC11b–MoSi2after annealing at 850 °C. The mechanical alloying of a stoichiometric Ti/Si powder mixture leads to the incipient formation ofC49–TiSi2and the subsequent polymorphic transformation toC54–TiSi2.The lattice parameters ofC49–TiSi2,as determined from a mechanically alloyed Ti/Si powder sample that has been annealed at 700 °C, are as follows:a0=3.56 Å,b0=13.57 Å,andc0=3.55 Å.©1997 American Institute of Physics.