Influence of microstructures on the tensile and low-cycle fatigue damage behaviors of cast Al12Si4Cu3NiMg alloy

Abstract

The high-performance Al12Si4Cu3NiMg alloy with massive transition elements has been developed recently for piston of diesel engines. The influences of microstructures on the tensile and low-cycle fatigue (LCF) behaviors were investigated with conventional and in situ testing technologies in this study. Upon the tensile loading, the microcracks initiate from the primary Si near the surface when the tensile stress reaches approximately 280 MPa (close to the tensile strength); then, the microcracks grow rapidly through eutectic Si and intermetallics (Al3CuNi) for the gravity casting (GC) alloy. With regard to the LCF, the microcracks initiate from the shrinkage pores regularly because of the localization of fatigue damage in the GC alloy. The shrinkage pores, usually accompanied by the brittle intermetallics (Al3CuNi), typically exhibit more complex geometry. The lamellar Al3CuNi provides a preferred path for the crack propagation after fatigue crack initiation. The ultrasonic melt treatment (UT) was used to optimize the microstructures and reduce casting defects (shrinkage pores), thereby resulting in improvement in tensile and LCF properties. For the UT alloy (pore-free), the microcrack initiates from some coarser primary Si rather than from the shrinkage pores because of the accumulation of micro-scale plastic deformation. Furthermore, higher resistance of crack propagation can also be realized with refinement of microstructures in the UT alloy. Increase in crack initiation and propagation resistances is the main reason for improvement in the LCF life of the UT alloy.