The effect of serrated yielding on the determination of r-values in aluminium alloys and yield locus calibration

This study investigates the effect serrated yielding (Portevin-Le Châtelier effect) has on the determination of the plastic strain ratio (r-value) for the 2024 aluminium alloy. The aluminium alloy specimens were initially in the fully annealed O-condition. The specimens experienced a softening mechanism through a solution heat treatment (SHT) process, followed by an oil quench. The specimens were permitted to recover for periods ranging between 0.5 and 12 h by room temperature (RT) natural ageing before undergoing a uniaxial tensile test along the longitudinal axis to determine the r-value as a function of natural ageing time. The current study reveals the irregular variation in the transverse strain affects the gradient of the longitudinal–transverse strain plot and consequently the r-value. It is demonstrated that the serrated yielding directly influences the Hill 1948 yield locus when calibrated using two r-values and one yield strength for the 2024 aluminium alloy. Serrated yielding results in an underestimation of the plane stress (rolling) of 0.5 ≤ plane stress rolling (MPa) ≤ 5.6, plane stress (transverse) of 1.0 ≤ plane stress transverse (MPa) ≤ 9.2, and biaxial stress of 1.4 ≤ biaxial stress (MPa) ≤ 7.4. It is also shown that serrated yielding occurs after a critical strain of 0.07 ≤ plastic strain ≤ 0.13 and is reduced by natural age hardening.

Research highlights▶ The r-value is affected by serrated yielding as shown by the collinear 0–0.05 strain range plots. ▶ The irregular variation caused by serrations affects the gradient of the strain plot. ▶ Discontinuous yielding occurred within a critical strain range of 0.07 ≤ plastic strain ≤ 0.13. ▶ The Hill 1948 yield criterion is affected by calibration from r-values subject to serrated yielding.