Volume 101, Issue 8

This article presents some modifications of the solid state sintering model summarised in Riedel and Blug (2001) and shows its application for the simulation of distortions due to co-sintering of composite structures. The densification due to rearrangement in the early stage of sintering is taken into account by introducing an extra term in the viscosity. Detailed analyses of a layered thin structure are conducted with the finite element method and the results are compared to various analytical approximations for the case of a thin layer on a thick substrate. Two more examples show the applicability of the model to cases of practical interest.

In this paper the thermomechanical fatigue properties of 1.4849 cast steel, which is used for exhaust manifolds and turbochargers, are investigated and a fracture mechanics based approach is used for fatigue life prediction. Isothermal low-cycle fatigue tests and thermomechanical fatigue tests are conducted in the temperature range from room temperature up to 1 000 °C. Fractographic investigations show that fracture occurs predominantly intergranularly at 600 °C, whereas mixed transgranular and intergranular crack growth is found otherwise. The methodology for fatigue life prediction is based on a time and temperature dependent cyclic plasticity model, which describes the transient stresses and strains, and on a law for time and temperature dependent microcrack growth. The crack growth law assumes that the increment in crack length in each cycle, d a /d N , is correlated with the cyclic crack-tip opening displacement, δCTOD . An analytical fracture mechanics based estimate of δCTOD is used, which is derived for non-isothermal loadings. The fatigue lives of the low-cycle and the thermomechanical fatigue tests are predicted well with the model. Only predictions for the low-cycle fatigue tests at 600 °C, where integranular fracture is predominant, are non-conservative.

Current 3D shot peening simulation models proposed in literature do not take into account coverage as a process parameter influencing the residual state after shot peening. In this study a classic approach, using an ordered dimple pattern, and a new approach, using a stochastic dimple pattern were tested to describe the correlation between coverage, the surface topography and the residual stress state. Model verification was conducted based on X-ray and confocal white light microscopy measurements on shot peened test specimens. The test material was age hardened IN718. Simulations showed that the dimple pattern and the impact order of the shots can have a strong influence on the calculated macroscopic residual stress state. The stochastic approach enabled a realistic prediction of the surface topography and the residual stress state for arbitrary values of coverage while the classic approach strongly underestimated the number of shot impacts needed to achieve a certain value of coverage.

The objective of this work is the characterisation and modelling of the influence of triaxiality and deformation history on the damage behaviour of components manufactured by deep drawing. An evaluation chain including material characterisation, numerical simulation and verification was conducted. The influence of triaxiality and pre-deformation on damage behaviour of a TRIP steel was characterised with different specimen tests, such as shear, uniaxial and biaxial tension. Phenomenological and micromechanical damage models taking into account shear fracture and fracture induced by void nucleation, growth and coalescence (dimple rupture) were developed. Both damage models describe the influence of triaxiality on damage development under a broad range of loading conditions. Validation tests on an automobile component under a loading close to reality were performed and successfully simulated with pre-strains and pre-damage mapped from a forming model to the crash model.

Reactive air brazing offers economically and technologically advantageous joining of ceramics to metals. Solid oxide fuel cells and membranes for oxyfuel combustion are recent fields of application. However, it remains a problem that strong metallurgical reactions between brazes and base materials occur. These reactions were analysed by differential scanning calorimetry tests to get a better understanding. Therefore, three braze alloys (Ag8Cu, Ag8Cu0.5Ti and Ag4Cu4Ni) and five base materials (alumina, 3YSZ partially stabilised zirconia, BSCF perovskite ceramic, X1CrTiLa22 and X15CrNiSi25-20) were investigated. The reaction peaks correlate with the formation of reaction layers, which were observed in metallographic analysis of brazed specimens. The results help to explain the reaction mechanisms and allow optimised selection of filler metals and brazing temperature.

The lubrication mechanisms in ultrathin solid gold films confined between two rough nickel surfaces have been investigated employing classical molecular dynamics with a second moment tight-binding potential. Three types of nickel surfaces are considered: Ni(111), Ni(001) single- and an Ni(001) –(111) bicrystal. In all three systems, gold layers that have been quenched from the melt organise in (111) layers parallel to the nickel interfaces. The relative sliding of the two single crystal nickel tribopartners requires a shear stress of around 170 MPa – a value that is almost one order of magnitude lower than the ideal plastic shear stress of single crystal bulk gold. This reduced stress can be explained by a misfit dislocation mechanism in a single plane close to the Ni/Au interface. In the case of the Ni(001) – (111) bicrystal, the nickel grain boundaries induce grain boundaries in the quenched gold film which vanish during sliding. During subsequent sliding the nickel grain boundaries act as nucleation centres for dislocation loops leading to an increased shear stress of 490 MPa. The same is observed for an embedded hydrocarbon impurity. Also here dislocation loops are emitted on (111) planes that are tilted with respect to the sliding plane.

The effect of hydrogen on the mechanical behaviour of steels is twofold: it affects the local yield strength and it accelerates material damage. On the other hand, the diffusion behaviour is influenced by the hydrostatic stress, the plastic deformation and the strain rate. This requires a coupled model of deformation, damage, hydrogen sorption and diffusion. The deformation behaviour is described by von Mises plasticity with isotropic hardening, and crack extension is simulated by a cohesive zone model. The local hydrogen concentration, which is obtained from the sorption and diffusion analysis, causes a reduction in the yield strength and the cohesive strength. Crack extension in a C(T) specimen of a ferritic steel under hydrogen charging is simulated by fully coupled finite element analyses of hydrogen kinetics and mechanical behaviour. The simulation results are compared with test results.

Formation of precipitate-free zones due to the growth of intergranular precipitates on account of intragranular precipitates in the nearest vicinity of the grain boundary represents a significant degradation of the material. The loss of strength of the material in the precipitate-free zone allows propagation of cracks in the zones and causes embrittlement of the material. Based on application of the thermodynamic extremal principle a robust model is derived, which allows the description of the kinetics of formation of the precipitate-free zone. Several simulations based on the model are performed and the results are discussed.

The influence of temperature on the plastic deformation of high-purity tungsten single crystals with an orientation favouring single slip on the (110) or (112) planes, has been studied using tensile tests in the temperature range 26 K to 800 K with emphasis on temperatures below 400 K. The crystals were deformed with an initial applied plastic strain rate of γ a = 8.5 · 10 −4 s −1 either isothermally until fracture or necking terminated the experiment (called isothermal straining tests) or in small straining intervals at different decreasing temperatures (called isothermal-straining – temperature- lowering tests). After initial plastic deformation at high enough temperatures (T ≥ 650 K; γ ≍ 0.08) tests at lower temperatures (T ≤ 400 K) reveal that the workhardening behaviour changes completely if compared with work-hardening behaviour of tests at the lower temperatures. A combination of tests suggests that work hardening of tungsten cannot be interpreted solely in terms of an increase in the athermal component of the flow stress or an exhaustion of non-screw dislocations. Rather, the results suggest that the different mobilities of screw and non-screw dislocations and their dependence on temperature and strain rate contribute essentially to work hardening at the beginning of plastic flow. Additionally, this is strongly influenced by changes of the slip systems towards the low-temperature regime below about 230 K. Consequences of these features for work hardening and its dependence on temperature, flow stress, critical resolved shear stress are analysed and discussed.

Nanoindentation testing was used to investigate the mechanical properties of PDMS focusing on two aspects: Firstly, the results give a comprehensive overview of the mechanical behavior of PDMS Sylgard 184 which guides the property tailoring for many different application fields, where the mechanical behavior needs to be considered, but is not readily accessible. Secondly, the ability to measure the slight differences in the mechanical properties of these PDMS samples emphasized the high sensitivity of this technique and underlined its reliability for application on very compliant materials. The indentation results obtained in this study did not deviate from complementary tensile test data more than 5%. It was found that the elastic moduli of differently crosslinked samples cover a range of 2 orders of magnitude; for one and the same PDMS the modulus may increase 10-fold as a function of curing time.

Magnesium aluminate spinels were irradiated by electron beams with energy of 1.7 MeV and fluence from 10 13 to 10 17 electrons cm −2 . After irradiation, the samples were annealed at different temperatures from 100 to 500 °C. Spectroscopic ellipsometry and absorption were used to study the effects of optical properties in the spinel produced by irradiation and subsequent annealing. The results obtained by absorption spectra showed that many F-type absorption centers were produced in the spinels. With increasing electron fluence, we observed an increase in optical absorption, especially in the vicinity of the absorption band at 5.3 eV. The concentration of F-type absorption centers increased as the electron fluence increased. The F-type absorption centers can be destroyed by isochronal annealing. The refractive index obtained by analyzing the ellipsometric spectra changed as the irradiation fluence and subsequent annealing temperature changed. Based on our findings, an attempt has been made to explain the above mentioned phenomena.

Asymmetrical rolling was carried out in an interstitial-free steel with different velocities of upper and lower rolls. In order to analyze the effect of change in strain histories on the evolution of textures during asymmetrical cold rolling and subsequent annealing, asymmetrical rolling was performed either reversibly or unidirectionally. The two different asymmetrical rolling routes were accompanied by the formation of different cold rolling textures. The strain state during asymmetrical rolling was determined by the finite element method and verified by macro-texture analysis and texture modeling. Weakening and randomization of textures occurred during recrystallization annealing, which was interpreted by the absence of preferred nucleation in specific orientations.

Silica surface was modified with four aliphatic acyclic amines differing in the number of nitrogen donor atoms and the chain length. The structures of the four systems were confirmed by Fourier transform infrared spectroscopy. Three of the systems studied were found to show excellent scavenging of copper ions and relatively weaker of nickel ions from the water solutions. The complexing abilities were monitored by ultraviolet and visible measurements. Standard colorimetric methods of element determination were used to observe the absorbance decrease at the maximum wavelength in the presence of the compounds studied. The influence of pH of the aqueous solution, the amount of modified silica, the stirring time and the selectivity with respect to other metal ions was investigated.

In this work, the synthesis of the negative thermal expansion ceramic Y 2 W 3 O 12 within a Co-based superalloy was analyzed in detail. On the basis of thermal analysis of the synthesis from the feedstock powders Y 2 O 3 and WO 3 further experiments were conducted. Thermal behavior and stability of different mixtures of the powders Y 2 O 3 , WO 3 and the Co-based superalloy were investigated. The qualitative and quantitative results of the synthesis separately and within the superalloy are presented as achieved by energy dispersive X-ray analysis, X-ray powder diffraction, differential thermal analysis and thermal gravimetric analysis. It was found that the synthesis of yttrium tungstate inside the Co-based superalloy is possible with a defined temperature program and a simple basic production process of powder mixtures.

Nanoparticles of zinc sulfide doped with iron were synthesized by controlled co-precipitation and used as a photocatalyst in the photodegradation of congo red as color pollutant. The photoreactivity of doped zinc sulfide was studied with varying mole fraction of Fe 3+ to zinc ion, pH of solution, dosage of photocatalyst and concentration of dye. The characterization of nanoparticles was done using ultraviolet– visible spectra, X-ray diffraction pattern and scanning electron microscopy and transmission electron microscopy images. The size range of nanoparticles was determined to be 10–40 nm. In the presence of 0.8 g/L of ZnS:Fe (0.5%) nanophotocatalyst, 12.0 mg/L of congo red and pH 6, the degradation efficiency was 92% under UV and 98% under sunlight irradiations in the time of 120 min and 12 h, respectively. The pseudo-first order rate constant kinetic was obtained 2.12 × 10 −2 min −1 and 2.9 × 10 −2 h −1 under UV and sunlight irradiation. The reproducibility of photoreactivity of the proposed photocatalyst was 92–90%degradation in four cycles of photodegradation.

The supplementary waste barite aggregates deposit in Osmaniye, southern Turkey, has been estimated at around 500000000 tons based on 2007 records. The aim of the present study is to investigate the durability of concrete incorporating waste barite as coarse and river sand (RS), granule blast furnace slag (GBFS), granule basaltic pumice (GBP) and ≤4 mm granule barite (B) as fine aggregates. The properties of the fresh concrete determined included the air content, slump, slump loss and setting time. They also included the compressive strength, flexural and splitting tensile strengths and Young's modulus of elasticity, resistance to abrasion and sulphate resistance of hardened concrete. Besides these, control mortars were prepared with crushed limestone aggregates. The influence of waste barite as coarse aggregates and RS, GBFS, GBP and B as fine aggregates on the durability of the concretes was evaluated. The mass attenuation coefficients were calculated at photon energies of 1 keV to 100 GeV using XCOM and the obtained results were compared with the measurements at 0.66 and 1.25 MeV. The results showed the possibility of using these waste barite aggregates in the production of heavy concretes. In several cases, some of these properties have been improved. Durability of the concrete made with these waste aggregates was improved. Thus, these materials should be preferably used as aggregates in heavyweight concrete production.