Volume 103, Issue 1

High-strength materials on the basis of aluminum are attractive candidates for use in applications such as safety components that require high strength, high quality and integrity of various properties. In this paper, we discuss recent improvements in terms of property optimization made in the fields of (1) ultrafine-grained aluminum alloys prepared by equal-channel angular pressing, and (2) aluminum matrix composites with particle reinforcement. We discuss microstructural aspects and mechanical properties, as well as technological (processing parameters), wear and corrosion behavior. Our results highlight recent — and potential for further — improvements and for future applications of high-strength, aluminum-based materials.

The local deformation behavior of fine grained high manganese steel featuring the twinning-induced plasticity (TWIP) effect was investigated at different temperatures. A thorough set of mechanical, optical and electron optical experiments was conducted employing a miniature in-situ load frame, confocal laser scanning microscopy, digital image correlation and electron backscatter diffraction. The combination of the techniques used allowed for establishing a solid relationship between local texture, deformation and surface evolution. The results obtained show that on the microscopic scale the deformation is inhomogeneous even in the fine grained TWIP steel featuring the highest strains and the most significant surface evolution observed in regions with a local {111}-texture. Since grain orientation has an effect on both slip and twinning, the degree of heterogeneity of deformation is very similar in the temperature range studied, irrespective of the rate of twin evolution.

The eutectic NiAl-9Mo (at.%) alloy was directionally solidified using an industrial scale Bridgman furnace and using a liquid metal cooling Bridgman furnace to produce in-situ composites of aligned Mo fibers in an NiAl matrix. In the first part of the experiment, an investment casting shell mold system, produced for NiAl alloys, was used and investigated for the NiAl-9Mo alloy in the industrial scale furnace. Due to the lower temperature gradient of ∼2.8 K mm −1 , the microstructure of the samples was dendritic. In the second part of the experiment, samples with well aligned Mo fibers were produced in the laboratory Bridgman furnace using a temperature gradient of ∼11 K mm −1 and growth rates of 0.33–0.66 mm min −1 .

The influence of silicon on the oxidation behaviour of Co—Re—Cr-alloys has been studied at 1 000°C and 1 100°C. Consideration was given to the synergetic effects between chromium and silicon with respect to the development of a protective Cr 2 O 3 layer. The Si addition to the Co—Re-alloys produces a significant decrease in the evaporation rate of Re oxides. Moreover, the beneficial influence in the transient oxidation period results in a rapid formation of Cr 2 O 3 scale. While the addition of 1 and 2 at.% Si to the ternary Co-17Re-23Cr alloy was insufficient to form a continuous Cr 2 O 3 scale, the addition of 3 at.% silicon caused a change in the oxidation mode resulting in the formation of a nearly continuous Cr 2 O 3 scale. On the oxide/alloy interface of the alloy Co-17Re-30Cr-2Si, a continuous and dense Cr 2 O 3 scale was observed, which remained stable after 100 h exposure protecting the metallic substrate.

Silicon oxycarbide-based ceramic nanocomposites (SiOC, SiZrOC and SiHfOC) were prepared by means of hot pressing techniques and their behavior upon hydrothermal corrosion at moderate temperatures (up to 250°C) was investigated. The results indicated linear corrosion behavior for all samples. The corrosion rates of the SiOC ceramic materials were found to be remarkably lower than those of silicon carbide and comparable to values reported for silicon nitride. Furthermore, SiZrOC and SiHfOC were found to show improved resistance with respect to the non-modified SiOC, due to a unique synergistic effect: whereas zirconia/hafnia act as “reinforcing” phases with respect to hydrothermal corrosion (due to their extremely low solubility in water under the testing conditions), the silicon oxycarbide matrix protects the MO 2 phase from a corrosion-induced t-MO 2 → m-MO 2 phase transformation. Consequently, the prepared silicon oxycarbide-based materials exhibit high potential for applications which require high resistance in corrosive media at moderate temperatures.

The influence of roughness profile shape, roughness depth, bond coat creep strength and pre-oxidation on the thermal cycling damage evolution and lifetime of a plasma-sprayed ZrO 2 thermal barrier coating system was investigated. A simplified model system was used where FeCrAlY substrates simulated the bond coat. Substrate creep was varied by using the oxide dispersoid strengthened alloy MA956 and the conventional material Fecralloy. Stochastic 3- and periodic 2-dimensional roughness profiles were produced by sand blasting and high speed turning. Damage evolution is significantly influenced by substrate creep with a trend to higher lifetimes for the fast creeping substrate. Pre-oxidation has no influence. Lifetimes of the periodically profiled samples are up to 100 times lower than these of stochastically profiled samples. In the case of periodically profiled samples, the highest lifetime was reached for the highest roughness depth combined with local undercuttings in the roughness profile. For stochastically profiled samples the influence of roughness depth could not be determined due to the wide lifetime scatter.

The lifetime of thermal barrier coating systems is limited by cracks close to the interfaces, causing delamination. To study the failure mechanisms, a simplified model system is analysed which consists of a bond-coat bulk material, a thermally grown oxide, and an yttria-stabilised zirconia topcoat. The stresses in the model system are calculated using a finite element model which covers the simulation of full thermal cycles, creep in all layers, and the anisotropic oxidation during dwelling. Creep in the oxide and the thermal barrier coating is varied with the use of different creep parameter sets. The influence of creep in the bondcoat is analysed by using two different bond-coat materials: fast creeping Fecralloy and slow creeping oxide dispersion strengthened MA956. It is shown that creep in the bondcoat influences the lifetime of the coatings. Furthermore, a fast creeping thermally grown oxide benefits the lifetime of the coating system.

Steels with different content of cementite (Armco iron, Ck45 and C80) as well as four different heat treatment conditions of 42CrMoS4 are examined under both uniaxial and four-point bending loading with load reversal regarding the residual stress distribution due to straightening processes. It was found that increasing cementite content and a fine lamellar microstructure increase the developing residual stresses. While uniaxial loading causes only micro stresses, bending loading causes additional macro residual stresses. Due to this, at reversed load residual stresses decrease faster under bending loading than under uniaxial loading. Furthermore, residual stresses originating from bending loading decrease in the outer planes faster than in the inner ones when load is reversed. In general it turned out that every straightening process causes inevitable and characteristic residual stresses.

The residual stress state induced by shot peening should be taken into account in the dimensioning of turbine components. Understanding the changes in the residual stress state caused by the application of quasi-static and cyclic loads is a prerequisite. In order to describe the residual stress state after quasi-static loading, several different shot peened Inconel 718 specimens were loaded isothermally up to specific tensile loadings. To analyze the residual stress state after cyclic loading, isothermal low cycle fatigue tests were performed. These tests were stopped after a defined number of cycles. Finally, after the specimens had been subjected to different loads, the surface residual stresses and — for special loadings — the residual stress depth distributions were determined experimentally by using X-ray diffraction. The surface — core model was adapted so that the complete residual stress depth distribution after quasi-static and cyclic loading can now be described.

The objective of the current investigation consists of testing the interchangeability of polymer solutions used during induction hardening with water – air spraying regarding compressive residual stresses. Gearwheels made of 42CrMo4 steel were induction hardened and subsequently quenched using water–air spray cooling. The effect of different cooling parameters on the surface's residual stresses in the gearwheel's tooth flanks was analysed by means of X-ray diffraction. The microstructure was characterised by means of hardness measurements and light-microscopy. Residual stresses could be specified after using water – air spray cooling and are comparable to those using polymer quenching. Quench parameters leading to maximum compressive residual stresses in tooth flanks were determined.

Mechanical fatigue at frequencies in the GHz regime in submicron metal thin films is governed by size and frequency effects. The cyclic load can lead to damage formation also known as acoustomigration and determines the reliability of micro electro mechanical systems and surface acoustic wave devices. The size and frequency effects dramatically change the fatigue behavior compared to bulk material. The resulting damage structure is similar to electromigration experiments where void and extrusion formation lead to failure. Here, a dislocation based mechanism is presented which explains the damage formation. This mechanism is induced by gradients in cyclic shear stress which is induced by the short acoustic wave length at frequencies in the GHz regime. Discrete dislocation dynamic simulations are presented that reflect the dislocation behavior at these ultra high frequencies.

The employment of ferritic nodular cast iron for components subjected to high stress requires a detailed fracture mechanics evaluation of crack growth under static and cyclic loading. During operation, for instance, in wind power plants, such component parts are subjected to loading of variable amplitude, which influences their lifetime considerably. For the evaluation of crack propagation and the remaining service life in this case, the calculation methods currently well-established in practice cannot be employed for cast iron with nodular graphite, since overloads lead to microstructure-related and material-specific load history effects in terms of crack growth acceleration. In this work, investigations of crack growth under constant and variable amplitude loading as well as static fracture toughness investigations and strain-controlled cyclic deformation experiments are presented.

Materials for turbine blades experience in service a combined loading of low and high cycle fatigue at high temperatures. In order to understand the failure behaviour under these loading conditions, systematic investigations were carried out. Low cycle fatigue, high cycle fatigue and combined low and high cycle fatigue tests were realised on MAR-M247 LC at 650 °C in an air environment under total strain control. Surface damage and fracture surfaces were analysed. Under combined low and high cycle fatigue, the lifetime is reduced if the low cycle fatigue leads to a degradation of the high cycle fatigue strength caused by crack initiation and crack growth. By analysing the fracture surface, the crack growth rate under combined cycle fatigue loading could be determined and it was significantly higher than under pure low cycle fatigue loading. The accelerated crack growth mainly causes the lifetime reduction.

To analyse the fatigue behaviour of steels in the very high cycle fatigue regime, an ultrasonic testing facility was developed that allows describing the cyclic deformation behaviour at ultrasonic frequencies by measuring characteristic fatigue data at a sufficiently high frequency. In correlation with scanning electron micrographs, fatigue tests prove that the sensitive measuring techniques indicate fatigue induced microstructural changes by a significant rise in the process parameters more than 10 6 cycles before final failure occurs. By further analysis of the attenuation behaviour of the ultrasonic resonance system, the logarithmic decrement can be used as a reasonable physical value to indicate changes in the microstructure with a very high sensitivity. This allows an improved understanding of the fatigue mechanisms in the very high cycle fatigue regime.

The strength analysis of continuous fibre reinforced plastics has to consider the reactions of the heterogeneous composite material. Ample uncertainties exist in the evaluation of fibre parallel compressive load, because no computation model has yet accomplished acceptance which on the one hand is physically fundamentally feasible and on the other hand works on the macroscopical level of the common design process. This paper deals with the combination of a physically based failure model and a suitable modelling of the complex elastic properties. The results of this model combination are overall plausible. Additionally, experimental data confirm that moderate fibre transverse compression leads to an increase in the fibre parallel compressive strength.

The mechanical properties of injection moulded plastic parts depend on the morphology, the degree of crystallinity and the molecular orientation of the formed microstructure. In order to take the variation of the microstructure into account in a structural analysis, a novel multi-scale, integrated simulation approach is presented here. At first, a coupled mould filling and heat transfer analysis is achieved at the macroscale and its temperature field is transferred to the micromodel. Based on the concept of cellular automata, a 3-D microstructure evolution model is developed. It specifies the nucleation of the spherulite germs and describes their expansion rate. To evaluate the effective mechanical properties of the simulated microstructures, the homogenisation method is applied directly to the spherulites, assembled in few classes according to their crystallinity degree. These local properties are then introduced into a new multilinear material model for structural analysis of thermoplastics. Finally, the influence of the microstructure on macroscopic behaviour is outlined for a polypropylene tensile bar, extracted from an injection moulded plate.