Abstract:
The microstructures, phase characteristics, mechanical and corrosion behaviour of Cu-Zn-Al alloys modified with Fe, B and Fe-B micro alloying additions was investigated in this research. This was aimed at appraising the potentials of these micro alloying additions in improving the mechanical and corrosion performance of Cu-Zn-Al alloys, which are current concerns limiting their full scale utilisation for shape memory applications. Cu-Zn-Al alloys were produced by casting method with and without the addition of the micro alloying elements. The alloys were subjected to a homogenization – cold rolling- annealing treatment schedule, before the alloys were machined to specifications for tensile test, fracture toughness and hardness measurement. Optical, scanning electron microscopy and X-ray diffraction analysis were utilized for microstructural analysis and phase characterization respectively. Tafel plot electrochemical method was adopted for the study of the corrosion behaviour of the Cu-Zn-Al alloys. From the results, a distinct difference in grain morphology was observed in the alloys produced – the unmodified alloy had predominantly needle-like lath martensite with sharp grain edges while significantly larger transverse grain size and curve edged/near elliptical grain shape was observed for the modified Cu-Zn-Al alloys. Cu-Zn with fcc structure was the predominant phase identified in the alloys while Cu-Al with bcc structure was the secondary phase observed. The hardness of the unmodified Cu-Zn-Al alloy was higher than that of the modified alloys. However, the tensile strength was significantly lower than that of the modified alloy grades. Similarly, the percentage elongation and fracture toughness of the modified alloy was higher than that of the unmodified alloy grade. The modified alloy compositions mostly exhibited fracture features indicative of a fibrous micro-mechanism to crack initiation and propagation, characterized by the prevalence of dimpled rupture. In 3.5%wt NaCl and 0.3M H2SO4 solutions, the corrosion current densities were highly dependent on the type and concentration of micro alloy additions. Essentially, the corrosion rates for the 0.1 Fe and Fe-B modified alloys were significantly lower in 3.5%wt NaCl solution; but in 0.3 M H2SO4 solution, the unmodified and Fe-B modified Cu-Zn-Al alloy grades were observed to be more susceptible to corrosion. Generally, the mechanical properties of the Cu-Zn-Al alloys improved with the addition of the micro alloying elements.
