Search

International audience

International audience; Titanium aluminide (TiAl) alloys have attracted considerable attention due to their unique combination of properties such as high specific strength and stiffness, good creep properties and resistance against oxidation and corrosion, which make them suitable candidate materials for High Temperature (HT) applications [1]. TiAl alloys are brittle at Room Temperature (RT), i.e. below their brittle-to-ductile transition temperature which lies between 800°C and 1000°C [2]. Despite intense research, literature suffers from a lack of understanding of their elementary deformation mechanisms and the precise role of microstructures [2].The investigation presented here focuses on the study of RT deformation mechanisms at the mesoscopic scale. With an original combination of experiments, we investigate the evolution of deformation microstructures at RT in the γ phase of a dual-phase bulk TiAl alloy. Because of the RT brittleness of this material, plastic deformation is induced by nanoindentation. The evolution of the microstructures is characterized by accurate Electron Channeling Contrast Imaging (aECCI) before and after deformation [3]. aECCI is a non-destructive powerful method offering the ability to provide, inside a SEM, TEM-like diffraction contrast imaging of sub-surface defects (at a depth of about one hundred of nanometers) on centimetric bulk specimen [4]. Defects, such as dislocations, can be characterized by applying the TEM extinction criteria [5]. Accommodation of the deformation are reported and a scenario is proposed [3]. Références[1] – Y. Kim, D. Dimiduk, JOM 43, (1991).[2] – C. Zambaldi, Thèse, Aachen (2010).[3] – A. Guitton, H. Kriaa, E. Bouzy, J. Guyon, N. Maloufi, Materials 11, 2 (2018)[4] – H. Kriaa, A. Guitton, N. Maloufi, Scientific reports 7, (2017).[5] – H. Kriaa, A. Guitton, N. Maloufi, Materials 12 (2019)

International audience; Developing new materials and understanding how they deform is the main challenge of engineers in order to follow and predict the fast evolutions of our society. For instance, in a framework of energetic cost reductions, titanium aluminide (TiAl) alloys have attracted considerable attention due to their unique combination of properties such as high specific strength and stiffness, good creep properties and resistance against oxidation and corrosion, which make them suitable candidate materials for high temperature applications [1]. However, TiAl alloys are brittle at Room Temperature (RT), i.e. below their brittle-to-ductile transition temperature, which lies between 800°C and 1000°C [2]. Furthermore, their complex microstructures (multiphase, different types of microstructures, specific dislocation mechanisms…) with several impacts at different scales are puzzling the materials science community. Despite intense research, literature suffers from a lack of understanding of their elementary deformation mechanisms and the precise role of microstructures [2]. In order to address these questions, we report here, an original and an innovative approach bringing the necessary information, thus allowing linking the multiscale aspects of the mechanical behavior of TiAl alloys at RT. Particularly, we bring new breakthrough on the evolution of deformation microstructures at RT in the vicinity of interfaces in γ phase of a dual-phase bulk TiAl alloy. Plastic deformation is induced locally by µN-nanoindentation. The evolution of the microstructures is characterized comprehensively by accurate Electron Channeling Contrast Imaging (aECCI) before and after deformation [3]. aECCI is a non-destructive groundbreaking procedure offering the ability to provide, inside a SEM, TEM-like diffraction contrast imaging of sub-surface defects (at a depth of about one hundred of nanometers) on centimetric bulk specimen with still unsurpassed resolutions [4]. Defects, such as dislocations, can be characterized by applying the TEM extinction criteria [5]. All features help to explain the poor ductility of the TiAl-based alloys at RT. Accommodation of the deformation is reported and a scenario is proposed [3].References[1] – Y. Kim, D. Dimiduk, JOM 43, (1991).[2] – C. Zambaldi, Thèse, Aachen (2010).[3] – A. Guitton, H. Kriaa, E. Bouzy, J. Guyon, N. Maloufi, Materials 11, 2 (2018)[4] – H. Kriaa, A. Guitton, N. Maloufi, Scientific reports 7, (2017).[5] – H. Kriaa, A. Guitton, N. Maloufi, Materials 11 (2018)

International audience; Developing new materials and understanding how they deform is the main challenge of engineers in order to follow and predict the fast evolutions of our society. For instance, in a framework of energetic cost reductions, titanium aluminide (TiAl) alloys have attracted considerable attention due to their unique combination of properties such as high specific strength and stiffness, good creep properties and resistance against oxidation and corrosion, which make them suitable candidate materials for high temperature applications [1]. However, TiAl alloys are brittle at Room Temperature (RT), i.e. below their brittle-to-ductile transition temperature, which lies between 800°C and 1000°C [2]. Furthermore, their complex microstructures (multiphase, different types of microstructures, specific dislocation mechanisms…) with several impacts at different scales are puzzling the materials science community. Despite intense research, literature suffers from a lack of understanding of their elementary deformation mechanisms and the precise role of microstructures [2]. In order to address these questions, we report here, an original and an innovative approach bringing the necessary information, thus allowing linking the multiscale aspects of the mechanical behavior of TiAl alloys at RT. Particularly, we bring new breakthrough on the evolution of deformation microstructures at RT in the vicinity of interfaces in γ phase of a dual-phase bulk TiAl alloy. Plastic deformation is induced locally by µN-nanoindentation. The evolution of the microstructures is characterized comprehensively by accurate Electron Channeling Contrast Imaging (aECCI) before and after deformation [3]. aECCI is a non-destructive groundbreaking procedure offering the ability to provide, inside a SEM, TEM-like diffraction contrast imaging of sub-surface defects (at a depth of about one hundred of nanometers) on centimetric bulk specimen with still unsurpassed resolutions [4]. Defects, such as dislocations, can be characterized by applying the TEM extinction criteria [5]. All features help to explain the poor ductility of the TiAl-based alloys at RT. Accommodation of the deformation is reported and a scenario is proposed [3].References[1] – Y. Kim, D. Dimiduk, JOM 43, (1991).[2] – C. Zambaldi, Thèse, Aachen (2010).[3] – A. Guitton, H. Kriaa, E. Bouzy, J. Guyon, N. Maloufi, Materials 11, 2 (2018)[4] – H. Kriaa, A. Guitton, N. Maloufi, Scientific reports 7, (2017).[5] – H. Kriaa, A. Guitton, N. Maloufi, Materials 11 (2018)

International audience

International audience; Nanocrystalline (nc) materials, i.e. polycrystalline structures with grain sizes below 100 nm exhibit extraordinary properties strength. As a first assumption, such property derives from the short paradigm “smaller is stronger”. For grain sizes below 50 nm deformation mechanisms usually involve a quasi-stationary balance between dislocation slip and grain boundary mediated mechanisms. But there is still an ongoing debate, which one of these mechanisms governs the deformation behavior of nc metals [1,2]. Therefore determination of grain size, analysis of the local texture and characterization of grain boundaries in nanocrystalline materials are crucial. Different techniques have been tested, such as automated crystal orientation mapping in Transmission Electron Microscopes (TEM) [3]. But they suffer a lack of accuracy due to the nanocrystalline nature of tested specimen. Grain overlapping, for instance, trends to observe smaller grains. By x-ray diffraction, it is also possible to determine grain size, but measurements provide size of coherent domains only that we consider equal to grains.In this framework, a new technique based of Transmission Kikuchi Diffraction (TKD) has been been recently introduced as a SEM based method capable of giving orientation maps as the EBSD method but with a spatial resolution improved by up to one order of magnitude [4]. The technique requires a specimen thin enough to be transparent to the electron beam. In the current configuration, it uses hardware and software developed for the EBSD technique. We proposed a new configuration of the TKD where the detector is disposed horizontally on the axis of the microscope instead of being vertically positioned as in the conventional configuration (see Figure). This achieves better spatial resolution and angular resolution than the ones of the current TKD configuration [5]. Moreover, acquisition times are shorter than in the conventional technique, because the intensity of the forward scattered electrons is much higher than that of the large angle scattered electrons.Firstly, technological challenges such as technique design will be presented. Secondly first results on electrodeposited nanocrystalline nickel will be discussed and they clearly show the full potentialities of this original characterization set-up. References[1] Z. Sun, S. Van Petegem, A Cervellino, K. Durst, W. Blum and H. Van Swygenhoven Dynamic recovery in nanocrystalline Ni, Acta Materialia, 91, (2015) p. 91-100[2] T. Shanmugasundaram, E. Bouzy, A.H. Chokshi Strengthening and weakening by repeated dynamic impact in microcrystals and nanocrystals Materials Science and Engineering A, 639, (2015), p. 97-102.[3] J. Lohmiller, M. Grewer, C. Braun, A. Kobler, C. Kübel, K. Schüler, V. Honkimäki, H. Hahn, O. Kraft, R. Birringer and P. Gruber, Untangling dislocation and grain boundary mediated plasticity in nanocrystalline nickel, Acta Materialia, 65 (2014) p.295-307[4] R.R. Keller, R.H. Geiss, Transmission EBSD from 10 nm domains in a scanning electron microscope J. Microsc. 245 (2012) 245-251.[5] J.J. Fundenberger, E. Bouzy, D. Goran, J. Guyon, H. Yuan, A. Morawiec. Orientation mapping by transmission-SEM with an on-axis detector Ultramicroscopy 161 (2016) 17-22.

National audience

International audience; The microstructure and texture of Cu-8 at.% Al alloy single crystal with (112)[11 (1) over bar] orientation plane strain compressed at 77 K were characterized by scanning electron microscope and transmission electron microscope orientation mappings in order to investigate the influence of twins and shear bands on slip propagation across a structure of twin-matrix layers and the resulting texture evolution. It was shown that the strong, initial texture changes are due to deformation twinning at low deformations. At larger deformations, twin-matrix bending within some narrow areas led to the formation of kink-type bands, which became the precursors of shear bands. It is shown with scanning electron microscope and transmission electron microscope orientation mappings how the structure of twins and matrix was incorporated into the shear band, and what kind of the dislocation mechanism was responsible for strain accommodation at the macroscale. It was found that the reorientation of the main (111) twinning planes towards the shear band plane facilitates further dislocation slip in the shear direction.

International audience