Plasticity of solid solution Al-Mn and Al-Mn-Si alloys

The influence of Mn and Si on the mechanical properties of solid solution Al-Mn and Al-Mn-Si alloys have been studied at 4 K, 78 K, and 298 K. The solute strengthening potency of the yield strength increases with decreasing temperature. The addition of Si stimulates the precipitation of Mn-containing dispersoids and decreases the solubility of Mn in the Al matrix, reducing the strengthening effect. The alloys exhibit Stage III of declining work hardening rate with the flow stress during the plastic flow at three temperatures. The mean slip length Λ in the work hardening model of Nabarro, Basinski, and Holt (NBH) evaluated from σ Θ curves correlates with the scale of the microstructure. The strain rate sensitivity (SRS) measurements conducted at 78 K show positive SRS of Al and the alloys. At 298 K, one observes a change in SRS due to the change of the mechanism from solute-dislocation to dislocation-dislocation dominated interactions. Mn and Si decrease SRS at 298 K. Al-Mn-Si alloys exhibit negative SRS and jerk flow at a sufficiently low strain rate at 298 K. Analysis of the apparent activation volume, distance, and work has been conducted at 78 K and 298 K to understand the rate-controlling mechanisms. The results indicate that dislocation intersections are rate-limiting processes, but the atomistic mechanism of dislocation cutting varies vastly depending on the temperature and alloy composition. It is suggested that at 78 K, forest obstacles are passed by the formation of jogs and sequential activation of individual partials without making constriction to the perfect defect. At 298 K, the cutting involves constriction, the creation of jogs, and the creation of point defects at each intersection site. The calculated activation work and distance provide signatures of these processes.