[en] Magnesium diboride (MgB₂) powder was mechanically alloyed by high-energy ball milling with C to a composition of Mg(B_0.95C_0.05)₂ and then sintered at 1000 deg. C in a hot isostatic press. Milling times varied from 1 to 3000 min. Full C incorporation required only 30-60 min of milling. The grain size of sintered samples decreased with increased milling time to <30 nm for 20-50 h of milling. Milling had a weak detrimental effect on the connectivity. A strong irreversibility field (H*) increase (from 13.3 to 17.2 T at 4.2 K) due to increased milling time was observed and correlated linearly with inverse grain size (1/d). As a result, the high-field J_c benefited greatly from lengthy powder milling. J_c (8 T, 4.2 K) peaked at>80 000 A cm^-2 with 1200 min of milling compared with only ∼26 000 A cm^-2 for 60 min of milling. This non-compositional performance increase is attributed to grain refinement of the unsintered powder by milling, and to the probable suppression of grain growth by milling-induced MgO nanodispersions