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 high 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 illustrates 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. 2. Nucleation of a crack in a γ-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.

The results of these studies have been published in prestigious international journals in the field of materials science and engineering, such as Acta Materialia, Metallurgical Transactions and presented at various international conferences and congresses as Euromat (plenary lecture) and Aerodays (prize to the best poster of graduate students).

This promising advanced characterization technique has been developed in the framework of the ESTRUMAT programme funded by the Madrid Government and of the R+D projects ALTIVA and MAGNO funded by the MICINN and CDTI respectively. In addition to the funding already mentioned, the NSF (National Science Foundation, US), in its call "Materials World Network", in collaboration with the Spanish Ministry of Economy and Competitiveness, has awarded an international project consortium (IMDEA Materials, UPM and Michigan State University) for further research into the analysis of the micorestructural evolution and the mechanical properties of Mg alloys with rare earths (MAGMAN).

Author: Dr. Teresa Pérez Prado
Metal Physics Group of IMDEA Materials Institute

References:

[1] 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, Spain, 2011.

[2] R. Muñoz-Moreno, C. Boehlert, M. T. Pérez Prado, E. M. Ruiz-Navas, J. Llorca. In situ observations of the deformation behavior and fracture mechanisms of Ti-45Al-2Nb-2Mn+0.8v%TiB2. Metallurgical Transactions, 2011, in press.

[3] C. Boehlert, Z. Chen, I. Gutiérrez-Urrutia, J. Llorca, M. T. Pérez Prado. In-situ analysis of the tensile and tensile-creep deformation mechanisms in rolled AZ31. Acta Materialia, 2011, in press.

[4] M.T. Pérez-Prado, C. Boehlert, J. Llorca, I. Gutiérrez-Urrutia. In-situ analysis of deformation and recrystallization mechanisms. Euromat 2011, Montpellier, France, 2011.

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