Horst Vehoff

The effect of grain size on strain rate sensitivity and activation volume - from nano to ufg nickel

Abstract The strain rate sensitivity of nanocrystalline nickel was studied at different temperatures in tensile tests and with a nanoindenter in order to examine the effect of grain size on the different deformation mechanisms of nanocrystalline materials. The experiments yielded, depending on temperature and strain rate, the strain rate sensitivity, the activation volume and the creep exponents as a function of stress and grain size. From the creep experiments the transition between grain boundary sliding and dislocation climb as a function of temperature was obtained. The strain rate jump tests gave extremely small activation volumes, nearly a factor of 100 smaller than in conventional nickel as a function of grain size. To help in understanding this behaviour the strain rate sensitivity of single grains was tested with a nanoindenter. The results clearly showed that the primary interaction of dislocations with grain boundaries is the reason for the strong rate effects and small activation volumes observed.

In situ electrochemical nanoindentation of a nickel (111) single crystal : hydrogen effect on pop-in behaviour

Abstract The hydrogen effect on dislocation nucleation in Ni single crystals with (111) surface orientation has been examined with the aid of a specifically designed nanoindentation set-up for in situ electrochemical experiments. The effect of the electrochemical potential on the indent load–displacement curve, especially the unstable elastic-plastic transition (pop-in), was studied in detail. The experiments allowed the exclusion of the surface from hydrogen effects. The observations showed a pop-in load drop from an average value of 250 to 100N due to in situ hydrogen charging, which is reproducibly observed within sequential hydrogen charging and discharging. Clear evidence is provided that hydrogen atoms facilitate homogeneous dislocation nucleation.

Deformation processes at crack tips in NiAl single- and bicrystals

Pre-cracked single- and bicrystals were examined in order to understand the quasi brittle behaviour of NiAl single crystals and the strength of grain boundaries. Single crystals were tested in-situ within a modified scanning force microscope (SFM). Sequences of elastic deformation, dislocation emission, followed by discontinuous crack growth on a submicron scale were observed within the SFM. In addition the fracture toughness of bicrystals with stoichiometric and off-stoichiometric composition pre-cracked along and perpendicular to the boundary was measured. The results proved that NiAl is inherently ductile. However, depending on purity, discontinuous crack growth occurred with local crack opening displacements (COD) of less than 100 nm and slip band lengths of less than 10 μm, showing clearly that local dislocation pinning can initiate brittle crack growth after blunting. In all cases the toughness of grain boundaries was more than a factor of two lower than the toughness of the adjacent component crystals. This proves that grain boundary brittleness is an inherent property of pure NiAl. However, boundaries with a surplus in Ni were tougher. For cracks precracked perpendicular to the boundaries dislocation-boundary interaction can either increase or decrease the toughness depending on the orientation of the adjacent grains.

Novel methods for micromechanical examination of hydrogen and grain boundary effects on dislocations

Most of what is known about the local interaction of dislocations with grain boundaries and hydrogen is based on transmission electron microscopy studies, which suffer from the distinct disadvantage that only extremely thin samples can be used. Recently, micropillar compression testing has become a popular means by which investigation of the size effect is conducted. This method, in combination with orientation imaging techniques, is used here to study the interaction of dislocations with a pre-selected grain boundary during the deformation of a bicrystalline specimen. Furthermore, by utilizing a custom built electrochemical cell, the micropillar compression testing can be extended to study in situ examination of micropillars charged with hydrogen. The effects of hydrogen and grain boundary on the deformation process in this small, but still bulk-like volume are presented, and our initial results reveal the value of this new technique for investigations of hydrogen embrittlement and grain boundary strengthening.

Effect of Hydrogen and Grain Boundaries on Dislocation Nucleation and Multiplication Examined with a NI-AFM

A nanoindenting AFM(NI-AFM) with an environment chamber was constructed to study the effect of hydrogen on decohesion and dislocation nucleation and the effect of grain boundaries on dislocation nucleation and multiplication. Ultra fine grained Ni single crystals were examined. It could be clearly shown that hydrogen influences the pop in width and length. Testing single grains with grain sizes below one micron at different rates inside a NI-AFM showed that the rate dependence of ultra fine grained Ni is a result of the interaction of the growing dislocation loops with the boundary.

Investigation of the role of grain boundary on the mechanical properties of metals

Compression testing of micropillars was used to investigate the gain boundary effect on the strength of metals which is especially interesting in ultra fine grained and nanocrystalline metals. Single and bicrystal micropillars of different sizes and crystallographic orientations were fabricated using a focused ion beam system and the compression test was performed with a nanoindenter. A reduction of the pillar size as well as the introduction of a grain boundary results in an increase in the yield strength. The results show that the size and the orientation of different adjoining crystals in bicrystalline pillars have an obvious effect on dislocation nucleation and multiplication.

Size effects resulting from local strain hardening; microstructural evaluation of Fe-3% Si and Cu deformed in tension and deep drawing using orientation gradient mapping (OGM)

Abstract Mesoscale strain hardening phenomena and the effect on the macroscale behavior (yield under tension, earing in deep drawing) are investigated in Fe-3 % Si and Cu. Fe-3 % Si shows a smooth upper and lower yield point depending on grain size and sheet thickness which can be attributed to a lack of mobile dislocations. A free surface effect (reduced strain hardening of near-surface regions) is found for sheets of 300 μm and 960 μm thickness, but limited to small and medium strains. By means of the new orientation gradient mapping, which is correlated with local strain hardening, the local deformation microstructure is investigated in incremental tensile tests and incremental deep drawing for Fe-3 % Si and Cu. The orientation gradient distribution significantly depends on the material. Based on these results phenomenological descriptions of the microstructure at the mesoscale are proposed. From orientation gradient mapping, geometrically necessary and statistical orientation gradients are evaluated which agree well with results of transmission electron microscopy investigations of geometrically necessary boundaries and incidental dislocation boundaries. A constitutive equation correlating orientation gradient mapping and local dislocation density is proposed.

Overview of the Grain Size Effects on the Mechanical and Deformation Behaviour of Electrodeposited Nanocrystalline Nickel − From Nanoindentation to High Pressure Torsion

A summary of experimental results from nanoindentation, strain rate-controlled tension, in-situ bending and high pressure torsion on bulk electrodeposited nanocrystalline nickel, focusing on the effects of grain size on the mechanical behaviour and deformation mechanisms is presented. The interaction between dislocations and grain boundaries was locally examined by studying the dependence of nanohardness on grain size and indentation size; this is done by always performing nanoindents in the center of individual grains and by varying the grain size and indentation depth systematically. The grain size effects on the different deformation mechanisms of nanocrystalline nickel were revealed by strain rate-controlled tension and nanoindentation experiments, which show that with decreasing grain size the strain rate sensitivity increases and the activation volume decreases, indicating increased grain boundary mediated deformation processes in nanocrystalline nickel. Creep experiments at room temperature revealed that in nanocrystalline nickel grain boundary sliding or diffusion along the interface may dominate at lower stress levels, but with increasing stresses the deformation process is mainly controlled by dislocation creep. In-situ bending experiments in an atomic force microscope revealed directly that grain boundary mediated deformation processes play a significant role in nanocrystalline nickel, which is also supported by the observation of grain coarsening and softening of nanocrystalline nickel caused by high pressure torsion.

Microcracks in superalloys: From local in-situ measurements to lifetime prediction

Abstract The established K-concept with the Paris law as the central equation often used for life-time prediction and dimensioning does not fit with the micromechanical crack tip opening displacement (CTOD) concept. While the K-concept is based on continuum mechanics, the CTOD is influenced by the local microstructure. Consequently, the CTOD-concept is checked in situ in the scanning electron microscope and is found to be the appropriate micromechanical description of the crack propagation mechanism. However, it is shown that during low cycle fatigue of turbine materials, even under extreme conditions short cracks behave less harmfully than expected. Additionally a new technique of artificial crack initiation by operating with a focused ion beam is used to investigate special crack problems such as the interaction of cracks with microstructural barriers in detail, which was not previously possible in this systematic way.

Computation of the fracture stress in notched NiAl-polycrystals

Abstract Computer simulations were performed in order to investigate the influence of the geometry and the microstructure of macroscopic specimens on the brittle-to-ductile transition of NiAl. A model is proposed which permits the bridging of the gap between the length scales of mesoscopic and macroscopic modeling. The strength of the macroscopic specimen is described by conventional J 2 flow theory with a power law hardening material, while the toughness is treated on the mesoscopic length scale assuming preexisting microcracks that are shielded by the emission of dislocations. The temperature dependence of the yield stress and the activation energy of the dislocation rate were obtained from measurements on NiAl poly- and single crystals, respectively. Cleavage fracture of the macroscopic specimen occurs when the simulation on the mesoscopic scale computes unstable crack advance. The considered microcrack of finite length is loaded in mixed mode I and II.