Cyclic Deformation Behavior of A Heat-Treated Die-Cast Al-Mg-Si-Based Aluminum Alloy

The purpose of this investigation was to study the low-cycle fatigue (LCF) behavior of a newly developed high-pressure die-cast (HPDC) Al-5.5Mg-2.5Si-0.6Mn-0.2Fe (AlMgSiMnFe) alloy. The effect of heat-treatment in comparison with its as-cast counterpart was also identified. The layered (α-Al + Mg₂Si) eutectic structure plus a small amount of Al₈(Fe,Mn)₂Si phase in the as-cast condition became an in-situ Mg₂Si particulate-reinforced aluminum composite with spherical Mg₂Si particles uniformly distributed in the α-Al matrix after heat treatment. Due to the spheroidization of intermetallic phases including both Mg₂Si and Al₈(Fe,Mn)₂Si, the ductility and hardening capacity increased while the yield stress (YS) and ultimate tensile strength (UTS) decreased. Portevin-Le Chatelier effect (or serrated flow) was observed in both tensile stress-strain curves and initial hysteresis loops during cyclic deformation because of dynamic strain aging caused by strong dislocation-precipitate interactions. The alloy exhibited cyclic hardening in both as-cast and heat-treated conditions when the applied total strain amplitude was above 0.4%, below which cyclic stabilization was sustained. The heat-treated alloy displayed a larger plastic strain amplitude and a lower stress amplitude at a given total strain amplitude, demonstrating a superior fatigue resistance in the LCF regime. A simple equation based on the stress amplitude of the first and mid-life cycles ((Δσ/2)first, (Δσ/2)mid) was proposed to characterize the degree of cyclic hardening/softening (D): D=±(Δσ/2)mid - (Δσ/2)first(Δσ/2)first, where the positive sign "+" represents cyclic hardening and the negative sign "-" reflects cyclic softening.