Phase characterisation and mechanical behaviour of Fe-B modified Cu-Zn-Al shape memory alloys

The microstructures, phase characteristics and mechanical behaviour of Cu-Zn-Al alloys modified with Fe, B, and Fe-B mixed micro-alloying additions has been investigated. Cu-Zn-Al alloys were produced by casting with and without the addition of the microelements (Fe, B and Fe-B). The alloys were subjected to a homogenisation - 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 utilised for microstructural and phase characterisation of the alloys. A distinct difference in grain morphology was observed in the alloys produced - the unmodified alloy had predominantly needle-like lath martensite structure 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 with reductions in hardness ranging between 32.4 and 51.5%. However, the tensile strength was significantly lower than that of the modified alloy grades (28.37-52.74% increase in tensile strength was achieved with the addition of micro-alloying elements). Similarly, the percent elongation and fracture toughness (10-23% increase) 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, characterised by the prevalence of dimpled rupture.