Phase formation and superconductive properties of Cu doped MgB2 sintered with milled B powder

► The formation of MgB2 phase is almost completed before Mg melting. ► MgB2 grains near the Mg-Cu liquid grow faster than others before Mg melting. ► Abnormal growth of formed MgB2 grains could occur after Mg melting. ► Jc becomes more sensitive to the applied magnetic fields. ► Improved Jc at low fields is observed due to higher density.