The role of dynamic transmission electron microscopy on the development of next generation magnesium alloys

The need to increase fuel efficiency in automobiles due to consumption restrictions and environmental concerns has led engineers to reduce the overall weight of the vehicle over the last two decades. Magnesium (Mg), the lightest structural metal with a density of ∼1.738 g/cm³, has become an attractive alternative to steel (d_Fe ∼ 7.85 g/cm³) and aluminum (d_Al ∼ 2.7 g/cm³). There have been numerous studies for the development of creep-resistant Mg alloys since the 1990s; however, only three alloys, AJ62-(Mg-6Al-2Sr), AE44-(Mg-4Al-4RE) and AS31-(Mg-3Al-1Si), have been successfully used for powertrain components [1], [2], [3] and [4]. Further improvement in creep performance can only arise from an in depth understanding of the creep mechanisms and the related nano-structural interactions in Mg alloy systems.

The main processing routes of Mg alloy components for automotive applications yield a non-equilibrium microstructure with a supersaturated Mg matrix. This means that during long term heat exposure under creep loading, precipitation is inevitable. If the thermal stability of precipitates is low (such as the Mg₁₇Al₁₂ precipitate in Mg-Al alloys) the creep resistance decreases. Moreover, recent studies revealed the crucial importance of dynamic precipitation in the intradendritic regions to delay dislocation creep. Not only the size and distribution, but also the morphology and relative orientation of precipitates influence the resistance to creep deformation [5]. Even though thermodynamic calculations help us assess the phases that would form upon heat exposure, the only way to determine the orientation and morphology of dynamically formed precipitates is via TEM analysis conducted after the deformation is completed. In cases where multiple elements take place in precipitation, the assessment becomes more difficult. The development of next generation creep-resistant Mg alloys therefore depends on how well we can control the size, distribution, morphology and orientation of dynamic precipitates. Here, ab-initio studies on Mg systems have great importance for understanding the crystallization behavior of intermetallic compounds from the Mg matrix. Although these investigations are mostly focusing on binary alloys (Mg-X, where X is usually rare earth elements), in the near future they would shed light into multi-element Mg systems. As in all modeling studies, ab-initio calculations in multi-element systems should be supported via experimental studies. Dynamic Transmission Electron Microscopy (DTEM) is an attractive candidate technique enabling in-situ observations with high temporal resolution to understand fast chemical processes such as the nucleation and growth mechanisms of dynamic precipitates responsible for high creep resistance in Mg alloys. DTEM is a powerful technique combining a standard TEM with ultra-fast lasers in order to increase temporal resolution to 15 ns with 10 nm spatial resolution. However, the spatial resolution (at nanosecond time scale) is inferior to standard TEM due to low number of electrons per pulse [6].

Developing a new generation Mg alloys for powertrain applications necessitates a deep understanding of dynamic precipitation kinetics during creep deformation. And this can only be feasible by systematic studies based both on modelling (ab-initio) and in-situexperiments (DTEM). This will help us to avoid trial-error approaches that result in low yields, and develop commercially high quality alloys to be used by the automotive industry.