Ordered bcc phases in a Cu-Pd-Mg hydrogen separation membrane alloy

► Two ordered B2 phases have been identified in the Cu-43Pd-5.6Mg alloy below 680 °C: CuPd and PdMg. ► Mg addition to the binary Cu-48Pd alloy increases the transition temperature from bcc to fcc significantly. ► Expanding the B2 phase field of the Cu-Pd alloys by adding Mg may improve the performance as hydrogen separation membranes. ► A local compositional modulation in the B2 phases was identified.