On the origin of the high strain rate sensitivity in a superplastic Al-4.5% Mg alloy

Molecular dynamics simulations were performed to analyze nanovoid growth in single crystalline vanadium under tension. Radial distribution function at the first nearest neighbor distance was calculated to find out the critical strain rate below which the deformation of specimen was static. Then a tensile stress was exerted on both void contained box and intact box under two constraint conditions. Homogenous dislocations were nucleated in intact box at yield point; while for void contained box with void radius twice the lattice constant, 〈1 1 1〉{1 1 0} shear loops were punched out from void surface. The formation of shear loops was the result of the splitting of purely screw cores on three non-planar planes, as well as their transformations to more stable two-fold non-planar dislocations under tension. The asymmetry of loops was influenced by both strain rate and triaxiality of system. It is also found that, in lower rate cases the yield point and peak stress point coincided; however, the two points separated at higher rate due to the inadequate void growth rate. Mean square displacement of void surface atoms were given out to geometrically depict the void evolution. Moreover, simulations with different initial porosity and box size were performed respectively. It is shown that when void reduced to contain only one vacancy, dislocations can be nucleated independently of the void; when porosity was large enough, the interactions between void and its periodic images were noticeable. Also, when both the void and box were large, triangular prismatic loops on {1 1 0} planes were observed at void surface, which may be contributed to a combined effect of the intersection of shear loops and the ledges along the void surface. Finally, the results of our MD simulations agreed well with that from Lubarda equation.

Rapid quenching of Al–Mg alloys during superplastic deformation has revealed the presence of nano-scale cavities along many grain boundaries. They were observed only under deformation conditions where grain boundary sliding was the dominant mechanism. Fine-probe compositional measurements revealed that the cavity surface is enriched in Mg, and in situ heating in the transmission electron microscope demonstrated that they are not stable above 175°C. Kinetic analysis of cavity formation during a quench concludes that the cavities did not exist during deformation but were formed as the sample cooled. It is proposed that these cavities are evidence for a localized excess of vacancies during grain boundary sliding.

Recrystallization and grain growth studies of Al-Mg-X (where X is Mn, Zr or Sc) alloys have been carried out using a combination of optical and transmission electron microscopy of interrupted heating tests, changes in hardness, and in-situ observations in the transmission electron microscope. Nucleation sites have been identified as typically large (> 1 μm diameter) eutectic constituent particles, but also occasionally clusters of sub-micron dispersoids. By altering the density and size of the Mn-, Sc- and Zr-based particles with selected thermomechanical treatments, recrystallization could be suppressed up to temperatures near the melting point. These results can be explained by examination of the stored energy from cold work and the particle-substructure interactions due to the precipitate morphology. Implications of these results for grain size control in Al-Mg based alloys will be discussed.