How the interface type manipulates the thermomechanical response of nanostructured metals : A case study on nickel

The presence of interfaces with nanoscale spacing significantly enhances the strength of materials, but also the rate controlling processes of plastic flow are subject to change. Due to the confined grain volumes, intragranular dislocation-dislocation interactions, the predominant processes at the micrometer scale, are replaced by emission of dislocations from and their subsequent accommodation at the interfaces. Both processes not only depend on the interfacial spacing, but also on the atomistic structure of the interface. Hence, a thorough understanding how these processes are affected by the interface structure is required to predict and improve the behavior of nanomaterials. The present study attempts to rationalize this effect by investigating the thermomechanical behavior of samples consisting of three different interfaces. Pure nickel samples with predominant fractions of low- and high-angle as well as twin boundaries with a similar average spacing around 150 nm are investigated using high temperature nanoindentation strain rate jump tests. Depending on the interface structure, hardness, strain rate sensitivity and apparent activation volumes evolve distinctively different with testing temperature. While in case of high-angle boundaries for all quantities a pronounced thermal dependence is found, the other two interface types behave almost athermal in the same temperature range. These differences can be rationalized based on the different interfacial diffusivity, affecting the predominant process of interfacial stress relaxation

Effect of boron doping on grain boundary cohesion in technically pure molybdenum investigated via meso-scale three-point-bending tests

Molybdenum has numerous advantageous functional and high-temperature properties. However, plastic deformation as well as structural applications are limited due to a propensity for brittle, intercrystalline failure, especially at low temperatures. It is well known that oxygen segregations have a detrimental effect, whereas it is assessed that carbon and/or boron have a beneficial effect on grain boundary cohesion. An advanced approach for the improvement of these interfaces is segregation engineering, e.g. the addition of cohesion enhancing elements segregating to the grain boundaries. To investigate early stages of crack formation, three-point bending tests on recrystallized commercially pure and boron micro-doped molybdenum were conducted between −28 \ub0C and room temperature. The tensile-loaded top surface of the specimens was examined post-mortem close to the final fracture area via scanning electron microscopy. The occurring separations of grains are investigated for distinct features and the chemical composition of the interface is complementary measured by atom probe tomography

Influence of annealing on microstructure and mechanical properties of ultrafine-grained Ti45Nb

Beta-Ti alloys have been intensively investigated in the last years because of their favorable low Young's moduli, biocompatibility and bio-inertness, making these alloys interesting candidates for implant materials. Due to their low mechanical strength, efforts are currently devoted to increasing it. A promising way to improve the strength is to tailor the microstructure using severe plastic deformation (SPD). In this investigation high pressure torsion was used to refine the microstructure of a Ti-45wt.%Nb alloy inducing a grain size of ~50 nm. The main focus of the subsequent investigations was devoted to the thermal stability of the microstructure. Isochronal heat-treatments performed for 30 min in a temperature range up to 500 °C caused an increase of hardness with a peak value at 300 °C before the hardness decreased at higher temperatures. Simultaneously, a distinct temperature-dependent variation of the Young's modulus was also measured. Tensile tests revealed an increase in strength after annealing compared to the SPD-state. Microstructural investigations showed that annealing causes the formation of α-Ti. The findings suggest that the combination of severe plastic deformation with subsequent heat treatment provides a feasible way to improve the mechanical properties of SPD-deformed β-Ti alloys making them suitable for higher strength applications

Evolution of nano-pores during annealing of technically pure molybdenum sheet produced from different sintered formats

Molybdenum is a refractory metal with no phase transformation in the solid state and a high melting point. It is therefore an excellent structural material for various high temperature applications. Especially in this field of operation, significant creep resistance is essential. To achieve this, a microstructure with grains in the range of millimeters is desired. However, as demonstrated in the present study, the onset temperature for secondary recrystallization, which would lead to a beneficial grain size, is among other things dependent on the initial dimensions of the sintered part. One possible reason for the different microstructural evolutions is the influence of residual pores in sub-micrometer size. Sheets were thus fabricated via three different production routes employing the same initial Mo powder to exclude chemical variation as an influencing factor. The samples were investigated by in-situ small-angle X-ray scattering at a synchrotron radiation source with two different heating rates. Additionally, selected annealed samples were studied ex-situ with high energy X-rays. The apparent volume fraction of pores is compared to a volatilization model for the vaporization of typical accompanying elements and the induced thermal expansion

Dynamic nanoindentation testing: is there an influence on a material’s hardness?

Modern nanoindentation devices are capable of dynamic experimentations, which allow us to exploit instrumented hardness tests extensively. Beside the assets of recording mechanical properties continuously over displacement, there are ongoing debates whether the superimposed force alters the material’s hardness. We will show for a broad range of materials that significant hardness differences are noted between dynamic and static tests, even for large displacements. Those mainly result from a changing indentation strain-rate during the hold segment at peak load. This fact must be implicitly considered in studies using static indentation tests to guarantee comparability of obtained nanoindentation hardness values and derived quantities

Interface dominated mechanical properties of ultra-fine grained and nanoporous Au at elevated temperatures

Modern design and engineering of highly efficient devices and machines demand innovative materials to satisfy requirements such as high strength at low density. The purpose of this study was to oppose the mechanical properties and deformation behavior of ultra-fine grained Au to those of nanoporous Au, to study the influence of different types of interfaces. Microstructural investigations of the foam surrendered a ligament size of ∼100 nm which themselves consist of 70 nm grains in average, while the ultra-fine grained gold features a mean grain size of 325 nm. Nanoindentation lends itself as a convenient technique to obtain material properties at ambient as well as high temperature conditions. In this work, a substantial indentation test series was performed in order to determine hardness, Young's modulus, strain-rate sensitivity and activation volume at room and elevated temperatures up to 300 °C. On account of the small characteristic dimensions, high hardness values were noted for both materials, which rapidly drop at elevated temperature. Additionally, an enhanced strain-rate sensitivity accompanied by low activation volumes was determined at room temperature, which further increased at elevated temperatures. This behavior is associated with thermally activated interactions between dislocations and interfaces. For nanoporous Au, due to the presence of free surfaces, a considerable increase of hardness was observed upon annealing. This can be attributed to a reduced number of mobile dislocations in the material after annealing, as supported by implemented porosity maps on indent cross-sections, showing distinct differences for tests at varying temperature. © 2016 Acta Materialia Inc110101sciescopu

Addressing H-Material Interaction in Fast Diffusion Materials – A Feasibility Study on a Complex Phase Steel

Hydrogen embrittlement (HE) is one of the main limitations in the use of advanced high-strength steels in the automotive industry. To have a better understanding of the interaction between hydrogen (H) and a complex phase steel, an in-situ method with plasma charging was applied in order to provide continuous H supply during mechanical testing in order to avoid H outgassing. For such fast-H diffusion materials, only direct observation during in-situ charging allows for addressing H effects on materials. Different plasma charging conditions were analysed, yet there was not a pronounced effect on the mechanical properties. The H concentration was calculated while using a simple analytical model as well as a simulation approach, resulting in consistent low H values, below the critical concentration to produce embrittlement. However, the dimple size decreased in the presence of H and, with increasing charging time, the crack propagation rate increased. The rate dependence of flow properties of the material was also investigated, proving that the material has no strain rate sensitivity, which confirmed that the crack propagation rate increased due to H effects. Even though the H concentration was low in the experiments that are presented here, different technological alternatives can be implemented in order to increase the maximum solute concentration

Deformation mechanisms during severe plastic deformation of a Cu-Ag composite

A Cu-37 at%Ag composite was produced by high-pressure torsion processing of elemental Cu and Agpowders at room temperature. The initial micrometer-sized powder particles were compressed directlyin the high-pressure torsion tool and subsequently deformed to different strain levels. The microstructuralevolution was studied in detail by scanning and transmission electron microscopy and synchrotronX-Ray measurements, and related to the mechanical properties by microhardness and nanoindentationmeasurements. The HPT process led to an alignment of Cu and Ag into a lamellar composite microstructure.With increasing applied strain the Cu and Ag lamellae were continuously thinned andsimultaneously an ultrafine-grained microstructure was formed in the separate Cu and Ag lamellae.When the lamella spacing reached values lower than the respective grain sizes inside the lamellae, afurther lamella thinning occurred causing a significant hardness increase of the composite. At lamellaspacings below 50 nm deformation started to localize in 150e300 nm broad shear bands, which surprisinglyexhibited no softening. Instead, the steady formation of new shear bands aided to transform thelamellar structure into a nanocrystalline equi-axed microstructure and additionally rotated the lamellarmatrix towards the shear plane. This process led to an additional refinement of the alloy and a hardnessincrease until a constant hardness level was obtained. Combined analyses by synchrotron X-ray andtransmission electron microscopy measurements revealed that, after reaching the saturation microhardnesslevel, mechanical mixing of Cu and Ag occurred in the shear bands, which can be attributed tothe enormous strains accommodated in the shear bands. Due to the localized deformation by shearbands, structural and chemical homogenization of the alloy was not achieved even at very high appliedstrains. The final microstructure was composed of nanocrystalline single-phase supersaturated regionsembedded in a residual nano-lamellar matrix