Researchers at IMDEA materials Institute are investigating the in-situ deformation of metallic materials using a tensile testing micromachine coupled to a scanning electron microscope (Fig. 1a). This device allows to analyze in real time, the deformation, recrystallization and fracture mechanisms operative in these materials in different conditions of temperature and strain rate. Thus, it renders very valuable information regarding the kinetics of all these mechanisms, a difficult task to accomplish using ex-situ observations.
In particular, the in-situ deformation of magnesium alloys, very light metals that are increasingly being introduced in transportation vehicles, has been investigated at various temperatures. It has been concluded that grain boundary sliding, a mechanism consisting on the sliding of grains along its interfaces without significant deformation of the grain interiors, predominates at intermediate temperatures during quasi-static straining. At lower and higher temperatures, deformation is mostly controlled by crystallographic slip and twinning. Fig. 1b illustrates the surface of the AZ31 Mg alloy after deformation at 250°C. The white lines inside the grains are an indication of the presence of crystallographic slip. The waviness of the boundaries suggests, additionally, the presence of dynamic recrystallization.
The deformation and fracture mechanisms of gamma TiAl alloys, a promising material that is currently being used to build low pressure turbines for aircraft, have also been investigated using this technique. These materials are known for their high strength and low oxidation resistance, which makes them suitable for applications under extreme service conditions. It has been observed that, during creep at 700°C, colony sliding is a dominant mechanism during deformation. As a consequence, cracks nucleate at colony boundaries and then propagate along the colony interfaces until catastrophic failure ensues. Figures 2 a-c illustrate a sequence of crack nucleation and propagation. Thus, if the behaviour of these alloys under service conditions is to be optimized, special care must be taken in improving the resistance of colony boundaries to sliding.
Fig. 1. (a) Tensile testing micromachine coupled to an scanning electron microscopy; (b) MgAZ31 alloy deformed at quasi-static strain rates at 250°C.
Fig. 2. Nucleation of a crack in a gamma TiAl alloy creep deformed at 700ºC and propagation of that crack along the colony boundaries with increasing testing time (a) 7.6h; (b) 22.4h; (c) 23.1h.
 R. Muñoz-Moreno, E. M. Ruiz, M. T. Pérez-Prado, C. Boehlert. An in-situ SEM evaluation of the creep deformation behaviour of a gamma TiAl alloy. Aerodays 2011, Madrid.
By Dr. Teresa Pérez-Prado
Head of Metal Physics Group