Tensile creep testing of an Al–Cu alloy above solidus with a dynamic mechanical analyser

A dynamic mechanical analyser (DMA) was used to perform tensile creep tests at constant load and temperature to improve the accuracy in the measurement of mechanical properties above the solidus for an aluminium alloy. The binary Al–5.78 wt%Cu alloy was chosen for the investigation and specimens having columnar dendritic or equiaxed globular microstructures were tested to assess the validity of the technique and to see how the distribution of the liquid phase can affect the mechanical properties above solidus. It was found that the equiaxed globular microstructure was more rigid than the columnar dendritic microstructure with 8–9 vol% liquid phase. This was explained by the small thickness of the dendrite arms, which make the grains behave like sponges. Failure of the specimens occurred when the strain rates exceeded a critical level. Microstructure examinations performed before and after the creep tests gave indications that liquation occurred even at coalesced grain boundaries. Grain boundary sliding was promoted by liquid films at the contact points and generated cavities which accelerated the failure. The DMA was proven to be a very sensitive and reliable tool, able to heat specimens with good temperature uniformity while providing accurate measurements of the displacement and excellent control of the applied load.