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]