Scientific Events

Due to its high diffusivity hydrogen atoms alloy with metals even at room temperature. At this temperature, the materials microstructure remains rather stable. When the system size is reduced to the nano-scale, microstructural defects as well as mechanical stress significantly affect the thermodynamics and kinetics properties of the system.[1-6] Effects will be demonstrated on Niobium-H and Palladium-H thin films.Hydrogen absorption in metal systems commonly leads to lattice expansion. The lateral expansion is hindered when the metal adheres to a rigid substrate, as for thin films. Consequently, high mechanical stresses arise upon hydrogen uptake. In theory, these stresses can reach about -10 GPa for 1 H/M. Usually, metals cannot yield such high stresses and deform plastically. Thereby, maximum compressive mechanical stress of -2 to -3 GPa is commonly measured for 100 nm Nb thin films adhered to Sapphire substrates. It will be shown that phase transformations change in the coherency state upon film thickness reduction. The coherency state affects the nucleation and growth behaviour of the hydride phase as well as the kinetics of the phase transformation.[1] It will be further demonstrated that plastic deformation can be hindered and even suppressed upon film thickness reduction. In this case the system behaves purely elastic and ultra-high stress of about -10 GPa can be experimentally reached.[2] These high mechanical stresses result in changes of the materials thermodynamics. In the case of Nb-H thin films of less than 8 nm thickness, the common phase transformation from the α-phase solid solution to the hydride phase is completely suppressed, at 300 K.[3,4,5] The experimental results go in line with the σDOS model that includes microstructural and mechanical stress effects on the chemical potential [6]. [1] V. Burlaka, K. Nörthemann, A. Pundt, „Nb-H Thin Films: On Phase Transformation Kinetics“, Def. Diff. Forum 371 (2017) 160. [2] M. Hamm, V. Burlaka, S. Wagner, A. Pundt, “Achieving reversibility of ultra-high mechanical stress by hydrogen loading of thin films”, Appl. Phys. Letters 106 (2015) 243108. [3] S. Wagner, A. Pundt, “Quasi-thermodynamic model on hydride formation in palladium-hydrogen thin films: Impact of elastic and microstructural constraints “, Int. J. Hydrog. Energy 41 (2016) 2727. [4] V. Burlaka, S. Wagner, M. Hamm, A. Pundt, “Suppression of phase transformation in Nb-H thin films below switchover-thickness”, Nano Letters 16 (2016) 6207. [5] S. Wagner, P. Klose, V. Burlaka, K: Nörthemann, M. Hamm, A. Pundt, Structural Phase Transitions in Niobium Hydrogen Thin Films: Mechanical Stress, Phase Equilibria and Critical Temperatures, Chem. Phys. Chem. 20 (2019) 1890–1904. [6] S. Wagner, A. Pundt, Hydrogen as a probe for defects in materials: Isotherms and related microstructures of palladium-hydrogen thin films, AIMS Materials Science 7 (2020), 399–419. [more]

Refractory metals and their alloys are considered a versatile group of high temperature resistant materials. Promising design approaches to extend their application range include the modification of both existing commercial materials, but also novel alloys that are still at earlier stages of development. Additionally, rapid solidification processes offer interesting possibilities in this respect. In this presentation, selected research activities in this combined area will be discussed and possible approaches for further research will be presented, while the focus is on nano-oxide composites and titanium-based alloys. [more]

Nanoscale metal-graphene nanolayered composites are known to have ultra-high strength due to the ability of graphene to effectively block dislocations from penetrating through the metal-graphene interface. The same graphene interface can deflect generated cracks, thereby serving as a toughening mechanism. In this talk, the role of graphene interfaces in strengthening and toughening the Cu-graphene nanolayered composite will be discussed. In-situ TEM tensile testing of Cu-graphene showed that the dislocation plasticity was strongly confined by the graphene interfaces and the grain boundaries. The weak interfacial bonding between Cu-graphene induced an interesting stress decoupling effect, which resulted in independent deformation of each Cu layer. MD simulations confirmed such independent deformation of each Cu layer and also showed that the graphene interfaces effectively block crack propagation as delamination occurs at the Cu- graphene interfaces to allow for elastic strain energy dissipation. Bending fatigue testing was also conducted on Cu-graphene nanolayered composites that indicated ~5 times enhancement in robustness against fatigue-induced damage in comparison to the conventional Cu only thin film. Such an enhancement in reliability under cyclic bending was found to be due to the ability of the graphene interface to stop fatigue-induced crack propagations through thickness of the thin film, which is contrary to how a metal only thin film fails under cyclic loadings. [more]

In order to clarify the deformation and fracture mechanism in oxides such as Al2O3 and STO, TEM in situ nanoindentation experiments were conducted for their single crystals and bicrystals. We successfully observed the dynamic behavior of twin formation, twin-GB interaction, pile-up dislocation, jog and kink formation and jog drag dynamics and so on. The mechanism of each dynamic behavior will be discussed in detail in this presentation. GB migration plays an important role in considering the high temperature mechanical properties. Recently, we have found that GB migration behavior in Al2O3 can be precisely controlled by the aid of the high-energy electron beam irradiation. This technique was applied to directly visualize the atomistic GB migration. It was revealed that the GB migration is processed by a cooperative shuffling of atoms in GB ledges along specific routes. References [1] S. Kondo, T. Mitsuma, N. Shibata, Y. Ikuhara, Sci. Adv., 2[11], e1501926(2016). [2] S. Kondo, A. Ishihara, E. Tochigi, N. Shibata, and Y. Ikuhara, Nat. Commun., 10, 2112 (2019). [3] J.Wei, B.Feng, R.Ishikawa, T.Yokoi, K.Matsunaga, N.Shibata and Y.Ikuhara, Nat. Mater.,20 (7), 951 [4] J.Wei, B.Feng, E.Tochigi, N.Shibata and Y.Ikuhara, Nat. Commun., 13(1), 1455, (2022) [more]

Polycrystalline Ni-based superalloys are vital materials for disks in the hot section of aerospace and land-based turbine engines due to their exceptional microstructural stability and strength at high temperatures. In order to increase operating temperatures and hold times in these engines, hence increasing engine efficiency and reduction of carbon emissions, creep properties of these alloys becomes increasingly important. Microtwinning and stacking fault shearing through the strengthening g’ precipitates are important operative mechanisms in the critical 600-800°C temperature range. Atomic-scale chemical and structural analyses indicate that local phase transformations (LPT) occur commonly during creep of superalloys. Furthermore, the important deformation modes can be modulated by LPT formation, enabling a new path for improving high temperature properties. [more]

The persistent demand for green, strong and ductile advanced high strength steels, with a reduced climate footprint, calls for novel and improved multi-phase microstructures. The development of these new steels requires an in-depth understanding of the governing plasticity mechanisms at the micron scale. In order to address this challenge, novel numerical-experimental methods are called for that account for the discreteness, statistics and the intrinsic role of interfaces. This lecture sheds light on recent and innovative developments unravelling metal plasticity at the micron scale. Multi-phase through-thickness samples allow for a full characterization of the underlying microstructure. Using computational crystallographic insights, a slip system based local identification method has been developed, which provides full-field crystallographic slip system activity maps. The resulting deformation maps are directly used to assess the model predictions. Heterogeneous spatial variations are introduced by sampling the slip system properties of individual atomic slip planes from a probability density function. This allows to recover naturally localized slip patterns with a high resolution. It is demonstrated that this discrete slip plane model adequately replicates the diversity of active slip systems in the corresponding experiment, which cannot be achieved with standard crystal plasticity models. Recent experimental observations on dual-phase steels demonstrate substructure boundary sliding parallel to the habit plane in lath martensite, for which a habit-plane slip enriched laminate model is developed. This model adequately captures the role of the substructure boundary sliding on the deformation of the martensite aggregate. [more]

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