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The phase components, structures, and magnetic properties of Pr1-xGdxCo5 (x=0-1) powders synthesized by mechanical alloying and subsequent annealing have been investigated systematically. The optimal magnetic properties, with a coercivity of 12.5 kOe, a remanence ratio of 0.72, and a maximum energy product of 12.7 MGOe, have been obtained from PrCo5 powders milled for 5 h and annealed at 700 degreesC for 2 min. The remanence and maximum energy product were decreased monotonically with increasing Gd content, whereas the coercivity was increased, reaching a maximum of 23.7 kOe in Pr0.2Gd0.8Co5 powders. X-ray diffraction and transmission electron microscopy observations reveal that an uniform (Pr,Gd)Co-5/(Pr,Gd)(2)Co-17 nanostructure with an average grain size of 20-30 nm forms in the powders annealing at 700 degreesC. The obtained magnetic hardening apparently originates from the high anisotropy field of the hard (Pr,Gd)Co-5 phase and the uniform nanostructure developed by mechanical alloying and subsequent annealing. (C) 2003 American Institute of Physics.