A fractal explanation for creep in aluminum

The phenomenon of creep in solid metals, creating slowly formed defects over time while under the influence of stress or strain, is simple in principle but complex in reality. The behavior is governed by various factors that are difficult to quantify, which in turn adds to the complexity of the time dependent, creep-induced microstructure evolution.

Creep evolution is divided into several stages, of which the secondary steady-state regime is best understood. A new Journal of Applied Physics study demonstrates that the evolution of the power law stress exponent in this regime is best explained in a fractal mesoscale model of the microstructure, recognizing the fractal nature of dislocation structures.

The current work involves creep experiments with an aluminum alloy containing 3.85 percent magnesium (Al-3.85% Mg) and previous data on pure Al-99.8% and ingot AA6061 alloy. For each, the authors modeled the evolution with the mesoscale fractal description using the SSTC (solid state transformation creep) model, recently developed by researchers to explain primary and secondary creep in aluminum alloys. Although some alloys, including Al-Mg, are thought to have different deformation mechanisms than pure aluminum, the model fits well with data for all these materials.

They also note that the model predicts a natural creep exponent of 3, which agrees best with available data (although different values have previously been associated with different deformation mechanisms). In fact, the apparent variation in the stress exponent is shown to arise from dislocation substructure evolution, related to the material’s fractal dimension.

By examining creep strain evolution in a fractal microstructure on the mesoscopic scale, the authors achieved an explanation of the stress exponent variation that encompasses the creep phenomenon over the entire range of stress and temperature conditions in aluminum alloys. The work contributes to making creep both easier to understand and possibly easier to predict.

Source: “Fractal nature of aluminum alloys substructures under creep and its implications,” by R. Fernández, G. Bruno, and G. González-Doncel, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5012035 .