Impact of thermal exposure on the microstructure and mechanical properties of a twin-roll cast Al-Mn-Fe-Si strip

Abstract Al-Mn-Fe-Si strips have been fabricated via both the twin-roll casting (TRC) and the more conventional route, direct-chill casting (DC). The two types of strips prepared were subjected to thermal exposure at a series of temperatures. Uniaxial tensile tests after the thermal exposure show that while the DC strip presents a ∼74% decrease in the yield strength and ∼35% decrease in the ultimate tensile strength (UTS) after being exposed to 350 °C for 12 h, the TRC strip, in contrast, maintains its strength at temperatures as high as ∼460 °C for the same time. Systematic microstructure characterization reveals that the different thermal stability in the strength of the two types of strips arises from their distinct evolution in grain morphology and second phase particles during the thermal exposure. The calculation based on Cahn-Lucke-Stuwe (CLS) model suggests that due to the highly supersaturated solute atoms, at the beginning of the thermal exposure, the TRC strip presents a strong solute drag which decreases the grain boundary migrating velocity by 2 orders of magnitude compared to that in the DC strip. With the progress of the thermal exposure, the solute atoms precipitate out, forming densely distributed second phase particles. For one thing, these particles stabilize the grain structure by inducing Zener pinning pressure which could be 10 times higher than that in the DC strip, depending on the temperature. For another, they act as dislocation obstacles and compensate for the strength loss owing to decreasing solution hardening. Both effects contribute to the TRC strip's fairly stable strength regarding thermal exposure below 460 °C. The present work could guide the direct application of the TRC strips in the industry. The results should also be helpful for the development of a fundamental framework for designing advanced TRC Al strips with improved mechanical properties at elevated temperatures.