Mg is the lightest structural metal (about two-thirds and one-fourth the densities of Al and Fe, respectively), abundant in Earth, recyclable, and biocompatible. These properties make Mg attractive for automotive, aerospace, and biomedical applications. However, Mg has low ductility, making it difficult to process at room temperature and preventing its use in many applications. The poor properties of Mg are connected to its hexagonal close-packed (hcp) crystal structure.

… Metallurgical strategies (alloying and thermomechanical processing) aim to increase ductility by engineering grain size, randomizing texture away from the unfavorable strong basal texture in sheet forming, strengthening basal slip, or activating prism slip or twinning. Rare earth (RE) solutes (Y, Tb, Dy, Ho, Er, Ce, and Gd) at very low concentrations [~0.03 to 1.0 atomic % (at %)] stand out as yielding good room-temperature ductility even at fairly large grain sizes and moderately strong basal texture. However, the mechanistic origins of the enhanced ductility are unknown, so the creation of new ductile non-RE Mg alloys is largely empirical.

We present a physical mechanism and associated theory for achieving enhanced ductility in Mg alloys.

—Wu et al.

The researchers at EPFL’s Laboratory for Multiscale Mechanics Modelling developed a model to predict how the metal behaves when mixed with different elements in order to determine which type of alloy provides the deformation capacity needed for industrial applications.

Magnesium becomes much more malleable if you add a few atoms of rare-earth metals, calcium, or manganese We wanted to understand what’s going on in these alloys at an atomic level, so that we can identify which elements to add and in what amounts to make the metal pliable.

—William Curtin, a professor at EPFL’s School of Engineering

These researchers previously identified the physical properties that make pure magnesium hard to shape. It was well known that adding certain elements—such as rare earth elements—can make it more malleable. But researchers don’t have a good grasp of the physical mechanisms taking place—meaning they have a hard time predicting what the best alloys would be.

Even with new alloys of steel and aluminum, the factors affecting an alloy’s ductility remain a mystery and many materials are still developed experimentally, said Curtin.

The EPFL researchers studied the interactions between magnesium atoms and the atoms of the elements added to make the alloys. They found that certain atoms trigger a process that “cancels out” the mechanism that makes magnesium hard to shape.

Magnesium’s low ductility is due to its low number of moveable dislocations, which are the linear defects that make metals flow plastically and that make it less likely to break when it’s deformed. The researchers found that adding certain elements substantially increases the number of moveable dislocations and therefore enhances the metal’s deformation capacity. They then spent several months using EPFL’s High Performance Computing system to calculate via quantum mechanics which combinations of atoms result in the highest ductility.

The two figures show the initial and final atomic configurations of the “cross-slip” process in the presence of two Yttrium atoms. Blue atoms are Mg atoms that are nearly in the perfect Mg crystal environment, yellow atoms are Mg atoms that are far from the perfect Mg crystal environment, and so indicate the structure and atoms involved in the “dislocation” defect. Red atoms are two Y solutes. Source: EPFL. Click to enlarge.

A quantitative theory establishes the conditions for ductility as a function of alloy composition in very good agreement with experiments on many existing magnesium alloys, and the solute-enhanced cross-slip mechanism is confirmed by transmission electron microscopy observations in magnesium-yttrium. The mechanistic theory can quickly screen for alloy compositions favoring conditions for high ductility and may help in the development of high-formability magnesium alloys.

—Wu et al.

For now the alloys are still in the modelling stage. The next step will be fabrication in the lab to see if they have the right properties for industrial use and can be manufactured on a large scale.

Resources

• Zhaoxuan Wu, Rasool Ahmad, Binglun Yin, Stefanie Sandlöbes, W. A. Curtin (2018) “Mechanistic origin and prediction of enhanced ductility in magnesium alloys” Science Vol. 359, Issue 6374, pp. 447-452 doi: 10.1126/science.aap8716