On The Yield and Fracture of Cast A205

Abstract

A205 is an Al-Cu-Ag-Ti-Mg-TiB2 age-hardenable alloy developed to improve castability in long freezing range alloys and results in a unique two-region microstructure consisting of TiB2 rich intergranular regions (IGRs) and grain interiors. The present work uses traditional and three-dimensional characterization techniques, together with mechanical testing, digital image correlation, and damage and fracture analysis to advance the understanding of the structure-property relationships of A205 under simple stress states. The IGRs contain a 17-19% local volume fraction of TiB2 particles and Cu-rich particles, the latter of which do not completely dissolve after a solutionizing treatment. Furthermore, the hardness response after ageing is reduced in these regions. The added TiB2 does not appear to contribute meaningfully to yield strength and precipitation remains the main strengthening agent in A205 like traditional age-hardenable aluminum alloys, contributing on average 265.5 MPa to the 0.2% offset yield strength from the as-quenched to the 13-hour aged state. Over the same ageing conditions, the plastic strain to failure decreases from approximately 14% to 1.5-3%. Digital image correlation through scanning electron microscopy reveals highly heterogeneous plasticity, with strain localization between the grain interior and the IGRs, and between particles within IGRs. Effective shear strains exceeding 15% occur between particles in the IGR in the 13-hour aged state at an average strain of approximately 3.5%. Damage observations reveal that almost all damage events occur within the IGRs, regardless of material condition. Particle cracking, particle-matrix decohesion, and matrix cracking in particle clusters are observed together with local coalescence. Such events are first observed on the surface at an applied stress of approximately 300-350 MPa regardless of condition. Damage growth occurs until blunted by the grain interiors. Cracks on the order of the grain size form and trigger catastrophic failure at a critical stress dictated by the local K1C of the IGRs, typically between 450-500 MPa. Such an applied stress is attained at far lower strain in the stronger aged states. Fracture surface observations reveal ductile dimpled fracture involving the IGRs.