Influence of dendritic solute segregation on tensile properties of Al-4 mass% Cu alloy castings consisting of single columnar crystal

Al-4 mass% Cu alloy ingots consisting of single columnar crystal were prepared and subjected to tensile testing. Tensile properties, such as the 0.2% offset proof stress, tensile strength and elongation, were obtained in directions parallel and perpendicular to the growth direction of the columnar crystal. The tensile properties in the growth direction of the as-cast specimen decreased with increasing primary arm spacing λp, but after solution heat treatment, they became independent of λp. On the other hand, tensile properties perpendicular to the growth direction decreased with increasing λp before and after heat treatment. From these results, it was concluded that the tensile properties in the growth direction were controlled by solute segregation, but those perpendicular to the growth direction were largely influenced by casting defects, such as cavities. The as-cast yield strength σy in the growth direction was analyzed based on a continuously distributed dislocation model considering solute distribution in the primary arm, and the relationship σy=σconst+kG1/8λp−1/4 was obtained, where σconst and k are constants, and G is the temperature gradient around the dendrite tip. The calculated yield strength was compared with the 0.2% offset proof stress σ0.2 of the as-cast specimen. Also, the yield strength perpendicular to the growth direction was analyzed based on fracture mechanics, and the relationship σy=Ky (πlT/2)−1/2 was obtained, where Ky is the stress intensity factor at yielding, and lT is the length of the cavity perpendicular to both the growth and the tensile directions and is related to λp. The calculated yield strength was compared with σ0.2 of the as-cast and the heat treated specimens. The estimated value of Ky was comparable with the fracture toughness KIC reported in previous work.