Micron-scale mixing of magnesium and boron powders by mechanical alloying allows the MgB2 formation reaction to proceed as low as 450 C, with high-quality material formed in an hour at 600 C. At these low reaction temperatures the reaction can be performed in situ in a copper sheath, and it is found that the presence of copper enhances the superconducting properties of the MgB2. Self-field critical current densities, calculated from full critical-state simulation of magnetization hysteresis, are up to 7 x 10(5) A/cm(2) at 13.6 K and 1 x 10(5) A/cm(2) at 30 K. (C) 2004 Elsevier B.V. All rights reserved.