Influence of Cu Addition on the Microstructure, Mechanical, and Corrosion Properties of Extruded Mg-2%Zn Alloy

Effects of different Cu concentrations on the microstructure, mechanical, and corrosion properties of the extruded Mg-2%Zn alloy were studied by the use of x-ray diffraction, optical microscopy, scanning electron microscopy, energy-dispersive spectroscopy, tensile testing, polarization, and electrochemical impedance spectroscopy (EIS) measurements. The hot-extruded Mg-2%Zn comprised an equiaxed grain structure with the average size of about 13 μm. By adding 5 wt.% Cu, the average grain size decreased to 4 μm, due to the grain refinement behavior of Cu addition. Microstructural observations confirmed the existence of the α-Mg matrix, Mg(Zn,Cu), and Mg(Zn,Cu)₂ intermetallics. The results obtained from mechanical testing revealed that Cu addition along with applying hot extrusion increased the hardness significantly from 80 HBN, for Cu-free alloy, to 108 HBN for the extruded Mg-2%Zn-5%Cu alloy. By the addition of 0.5 wt.% Cu, the values of ultimate tensile strength (UTS), elongation, and ultimate compressive strength (UCS) changed from about 233 MPa, 16.8%, and 294 MPa to 260 MPa, 21.5%, and 335 MPa, respectively, as optimum levels. However, more Cu addition (i.e., 5 wt.%) decreased UTS, elongation, and UCS values to 242 MPa, 15.9%, and 272 MPa, respectively, by introducing a high volume fraction of the second phase. On the other hand, tensile yield strength and compressive yield strength increased gradually at higher Cu additions from 128 to 105 MPa in the case of the Cu-free extruded alloy to 165 and 156 MPa for the alloy containing 5 wt.% Cu, respectively. The polarization and EIS results indicated that the extrusion process eliminates the protective film from the Mg-2%Zn alloy surface. The Mg-2%Zn alloy exhibited the best anti-corrosion property among the studied alloys, as further Cu addition increased the intermetallic volume fractions and enhanced the corrosion rate, due to the galvanic couple effect.