UC Santa Barbara

Magnesium alloys have garnered much interest in automotive and aerospace industries, where simultaneously light-weight and strong components are needed to improve energy efficiency without sacrificing structural stability. However, the widespread application of magnesium has been limited due to its poor room temperature formability, which renders it incompatible with modern industrial processing methods, such as rolling, forming, die punching and extrusion techniques. Unlike traditional structural metals that deform primarily by dislocation slip, the deformation of magnesium alloys is carried out by a combination of dislocations motion and the development of deformation twins. Twinning deformation occurs on the grain scale and can strongly influence the mechanical properties of the material, such as ductility, strength, and stability. Thus, a fundamental understanding of how dislocations and twins interact with each other and with other common microstructural features found in magnesium alloys is critical in the design of useful magnesium components for industrial applications.

In this work, an elasto-viscoplastic fast-Fourier-transform (EVP-FFT) computation model was used in order to investigate the interaction between twins and two commonly encountered microstructural features: precipitates and free surfaces. First, we consider a newly propagated twin that intersects a Mg17Al12 basal precipitates in AZ91. It is shown that these precipitates can impede the propagation and thickening of twins, however, the impingement of a twin against one side of a precipitate can cause local stress concentrations to develop along the precipitate-matrix boundary, where a new twin of the same or different twin variant can nucleate from. Both the relative thickness and length of the precipitate, in relation to the thickness of the twin, are important factors that determine how twins can continue to develop. The size of the precipitate is a key factor in controlling twinning deformation during the early stages of development and determining the twinning pathways that emerge.

Next, a dislocation-density based hardening law was incorporated into EVP-FFT framework in order to reveal the evolution of the spatial distribution of dislocation densities in the microstructure. Here, defect content in addition to adequate stresses are used as indicators for the determining how twins will continue developing. By considering twinning from the early stages of deformation, a two-step twin growth mechanism is proposed: starting with an initial twin propagation front that impinges upon a precipitate, resulting in a stress concentration that develops on the other side of the precipitate that prompts the formation of a second twin. Subsequently, the backstresses around the first twin are relaxed by the propagation of a second twin, allowing the first twin to grow at the twin-precipitate junction and eventually engulf the precipitate. This mechanism suggests that twin growth can be achieved locally with minimal additional external forces, explaining how relatively large twin domains can develop even in the presence of arrays of precipitates, despite contradictory reports that precipitates harden against twin growth. The precipitate spacing is shown to affect the stresses and dislocation generated near the twin-precipitate interaction site that support twin growth.