Failure and fracture analysis of a high-alloy Ni-Al bronze chain connector of a tube drawing machine

摘要

The investigation of a fractured Ni-Al bronze alloy connector, from a chain assembly, was performed in order to determine the potential cause of the observed failure. It is shown that the fracture occurred by fatigue due to the combined result of poor machining quality and the brittle nature of the as-cast component. This synergistic effect facilitated intergranular crack propagation through the "microstructurally sensitized" interdendritic regions. Various irregular and coarse-shaped intermetallic phases, corresponding to kappa-type phases, were detected and their detrimental role on crack propagation was evaluated. Thermodynamic and kinetic simulation of solidification and phase transformation was elaborated in order to verify qualitatively and quantitatively the presence of the microscopically detected phases. Thermodynamic results were in agreement with metallographic observations. Kinetic results showed that increased cooling rate affects solidification particularly at high content of substitutional elements, such as Fe, Ni and Mn, since their diffusion is restricted and the completion of solidification is decelerated. Moreover, the high content of these elements favors the appearance of kappa(I) precipitates. The formation of shaped kappa(I)-phase cannot be eliminated, however its fraction and size can be suppressed by increasing cooling rate. The alteration of chemical composition of cast component could provide promising results as it shows a reducing tendency of kappa(I)-phase intermetallics' formation and a higher homogeneity, even at moderate cooling rates.