Volume 97, Issue 10

The residual stresses in flat coupons of sintered iron (ASC100.29) deep rolled on both surfaces were studied in the as-rolled condition and upon quasistatic or cyclic bending. In the as-rolled condition, the stresses are compressive and exhibit a clear orientation dependence with respect to the rolling direction. Also, a significant difference between the primarily and finally deep rolled surfaces is observed, which can be attributed to the effects of specimen bending occurring during the deep rolling process. The evolution of the residual stresses during fatigue is partly counter-intuitive: in transversely deep rolled specimens, an increase in compressive residual stresses in the transverse direction is observed within the very first loading cycle. Possible reasons for this unexpected effect are discussed.

The evolution of residual stress with plastic pre-strain was studied on a squeeze-cast, 2D-random short-fibre-reinforced metal matrix composite (aluminium/Saffil®). The plastic pre-strain was applied in three different modes: (1) uniaxial compression parallel to the fibre plane, (2) uniaxial tension parallel to the fibre plane, and (3) uniaxial compression normal to the fibre plane. The mean deviatoric residual stresses in the aluminium matrix were analysed using neutron diffraction. A major outcome is that the residual matrix stress is controlled by the load transfer to the fibres during deformation of the composite and reverse matrix plastic flow during unloading. The interplay between matrix and fibres depends strongly on the loading mode, and so does the evolution of the residual stresses as a function of pre-strain. This could be rationalised in a unified way using an Eshelby model.

Thermal residual stresses (TRSs) in continuous Al 2 O 3 fiber reinforced NiAl composites with and without a BN interlayer were studied by using the finite element method (FEM). The FEM model includes the effect of neighboring fibers of the composite. A minimum sample thickness and a minimum size of the surrounding composite have to be exceeded in order to obtain the correct TRS distribution in simulation. It is shown that the TRS, caused by different thermal expansion of fiber and matrix in the NiAl composite, is sufficiently large to cause fiber damage owing to twinning on different rhombohedral planes in the sapphire fiber. The introduction of a BN interlayer or a higher fiber volume fraction (40–60%) essentially reduces the TRS level in the NiAl composite and lessens the fiber damage. The residual stress in the NiAl composites was experimentally measured by nanoindentation. A good agreement between measurements and finite element analysis was achieved.

The present paper reports on the cyclic stress–strain response of body-centered cubic ultrafine-grained (UFG) interstitial-free (IF) steel severely plastically deformed at room temperature utilizing equal channel angular extrusion (ECAE). Low-cycle fatigue tests were conducted with various strain amplitudes and strain rates on samples obtained through different ECAE routes and number of ECAE passes in order to determine the optimum processing route(s) for improved fatigue response of this material. UFG IF steel is superior to its coarse grained counterpart under both monotonic and cyclic loading in terms of properties, such as stress ranges tolerated, strength levels attained, and the corresponding fatigue behavior. All UFG steels subjected to more than 4 ECAE passes exhibit stable cyclic stress–strain response. Moreover, it was shown that dynamic grain coarsening, which usually leads to cyclic softening in UFG materials, is not prevalent in the ECAE processed UFG IF steel. For representing the fatigue life of UFG IF steel, the parameter after Smith, Watson and Topper, which is an indication of energy dissipation per cycle, proved adequate while comparing materials obtained through different ECAE routes.

Interfering laser beams of a high-power pulsed laser provide the opportunity of applying a direct lateral interaction with the surface microstructure of metals in micro/nano-scale based on photo-thermal nature mechanisms. This “Laser interference metallurgy” allows the creation of periodic patterns of features with a well defined long-range order on metallic surfaces at the scale of typical microstructures (from the sub micrometer level up to micrometers). This technique is an approach to initiate metallurgical processes such as melting, recrystallization, recovery, and defect and phase formation in the lateral scale of the microstructure itself and with an additional long range order given by the interference periodicity. In this work, the laser interference theory is described and used to calculate multi-beam interference patterns. A method to calculate the numbers of laser beams as well as the geometrical arrangement of the beams to obtain a desired periodical pattern prior to experiments is presented. The formation of long-range-ordered intermetallic compounds as well as macroscopic and microscopic variations of mechanical properties on structured metallic thin films are presented as examples.

Measurements of the yield points Δσ A on reloading after ageing at various stress levels (obtained by partial unloading of the tensile specimen) have been performed with Cu-2at.% Mn polycrystals at 313K (strain rate 310 −5 s −1 ). At these temperatures solute atoms are supposed to increase the effective obstacle strength (intersections of mobile with forest dislocations) by pipe diffusion from the forest to the mobile dislocations during the waiting time for thermal activation. The stress dependence Δσ A ∝ (σ*) 1/3 , (with the effective stress σ*) expected according to Friedel's equation is observed. This supports the intersection strengthening model and, hence, the pipe diffusion mechanism for static and dynamic strain ageing in the temperature and strain rate regime of this study.

The high thermal stability of a nanocrystalline mechanically-alloyed Cu–Nb alloy was studied by means of isothermal annealing at high temperatures. The structure of the Cu matrix remains nanocrystalline with a grain size of about 50nm even after annealing at 1000°C for 10h. The copper grain growth is described with the help of kinetic analyses. The activation energy of the copper grain growth in Cu-10at.% Nb is about 0.98eV and the grain growth exponent is 4. The enhanced stability of the nanocrystalline Cu microstructure is attributed to the diffusion of Nb atoms along the Cu grain boundaries, “Zener drag” of NbO, Cu 2 O or Fe 7 Nb 6 particles, and a non-equilibrium state of Cu grain boundaries.

The microstructure of the metastable -titanium alloy Timetal®10-2-3 was changed systematically by means of thermohydrogen processing, in order to improve the mechanical properties. The thermodynamic and kinetic data-base was determined experimentally and the change in the beta transus temperature was determined quantitatively. The results were used to design a 4-step heat treatment process involving hydrogen during the first two steps. A comparison of the fatigue properties of the standard heat-treated and the thermohydrogen-processed alloy revealed that, despite the increase of the monotonic strength of the thermohydrogen-treated alloy, the fatigue limit is slightly reduced, while the fatigue crack growth threshold remains almost unaffected. From this correlation a strategy can be deduced for a further improvement of the thermohydrogen-processing procedure, in order to tailor the fatigue properties according to the requirements of the respective application.

YBa 2 Cu 3 O 7 is a high-temperature superconductor. Due to its complex lattice structure, belonging to the layered perovskite family, a strong biaxial texture of the grains is necessary to achieve technically relevant currents. In this paper, coarse-grained Ni, Ni-4at.% W, and electrolytic copper (E-Cu) obtained from swaged rods, and ultra-fine grained E-Cu produced by equal channel angular pressing, were thermo-mechanically processed. The development of the primary recrystallisation of the cube texture was studied and compared to determine the suitability of these materials as lattice-matched templates for coated conductor applications. Electron backscatter diffraction was performed to measure the texture and microstructure at various deformation levels and annealing temperatures. The texture sharpness formed in E-Cu with a coarse-grained starting microstructure proved superior, and it correlates with an anomaly observed in the temperature sensitivity of the flow stress in tensile tests, arising from crystal rotations.

The performance of advanced ceramics under cavitation loading in distilled water was studied by using a laboratory test with vibration-induced cavitation following the pattern of the ASTM standard G32–92. The hardened and tempered martensitic steel 100Cr6 was used as a reference. The aim was to identify mechanisms and the effects of important microstructural parameters on damage of polished Al 2 O 3 , Al 2 O 3 –ZrO 2 , ZrO 2 and SiC ceramics. Results showed that surface damage of brittle ceramics was mainly dominated by intergranular fracture, followed by detachment of single grains or fragments of them. Both incubation time and erosion rate were affected by the amount of initial surface cavities, grain sizes and secondary phases at grain boundaries.

The plastic deformation behavior of the Mg wrought alloy AZ31 is investigated in tension–compression tests at room temperature. It strongly depends on the loading direction and is characterized by dislocation slip as well as by the formation of deformation twins. The latter is reversible in the case of a change of the deformation direction. Because of the pronounced crystallographic texture of the material investigated, a macroscopic deformation anisotropy exists with lower flow stresses in compression than in tension. Hysteresis loops with characteristic shapes are observed when the material is cyclically plastically deformed, depending on the tension–compression loading history.

In order to obtain optimum lightweight constructions in the automobile and aircraft industry, it is important to consider the use of extruded magnesium sections. A lot of the recent research has dealt with the extrusion of AZ31. These extrudates often miss the requirements of the automobile or aircraft industry for mechanical properties. A possible solution might be an increase in the aluminum content. Therefore, this paper deals with the extrusion of AZ31, AZ61 and AZ80 and the effect of the aluminum on the extrusion process as well as on microstructure and the mechanical properties of the extrudates. Whereas the aluminum content does not have a significant influence on the texture of the extrudates, the grain size and structure and, thereby, the mechanical properties can be improved by adding more aluminum to the Mg wrought alloys.

Fatigue behaviour and mechanical properties of peak-aged AA6063T6 with ultra-fine grain size, produced by equal channel angular extrusion, were evaluated with special emphasis on the microstructure before and after cyclic loading. The strength significantly increased with grain size reduction and is described by an exponential power-law constitutive relationship. A remarkable enhancement of fatigue life compared to commercial AA6063T6 with coarse grains was found in the high-cycle regime after the first two extrusions. Further extrusions eliminated this improvement. It is shown that the optimum fatigue performance correlates very well with the minimum tensile ductility. Electron backscatter diffraction revealed that the material behaviour can basically be attributed to the grain boundary characteristics. Low grain boundary misorientation angles yield the best fatigue performance in the ultrafine-grained microstructure.

The mechanical properties of a unidirectional carbon/carbon (C/C) composite with a mainly highly textured matrix are investigated in tensile tests for the as-received state and after heat treatment from 1600°C to 2500°C. In order to interpret the composite behaviour single fibre tests, analyses of fracture surfaces and tests on infiltrated fibre felts were conducted. The fibre–matrix shear strength was determined by push-out indentation tests. Strength and ductility only change slightly although the strength of the fibres decreases significantly with increasing heat treatment temperature (HTT). The reason is a reduced bonding strength between fibre and matrix with increasing HTT. The Young's modulus of C/C composites increases with HTT. The best mechanical properties of the composite are reached by finding a compromise between sufficient fibre strength and weakened fibre-matrix strength at a HTT of 1600°C.

X-ray diffraction measurements on large components with complex geometry are needed in industrial applications requiring the non-destructive characterization of the near-surface material condition in terms of residual stresses, work hardening, phase transformation and formation of reaction compounds. Because many regions of interest on these components are not accessible with conventional laboratory or even mobile X-ray diffractometers, a novel center-free diffractometer with two cooperating robots named “Charon XRD” has been developed at MTU Aero Engines. Two six-axis robots are synchronized using a special optical measuring system to achieve the highest positioning accuracies. This paper describes the actual design and operating mode of Charon XRD, its current and potential functionality, and presents calibration and reference measurements, along with results on aero-engine high-technology components like bladed integrated disks.

Flat samples of AISI 1080 carbon steel and commercially pure tungsten were surface-treated using a reciprocating table-type grinding machine. Cross sections of the subsurface microstructure induced by this mechanical finishing process were prepared and imaged by focussed ion beam methods. Depth profiles of the residual normal and shear stresses were analysed using synchrotron X-ray diffraction and correlated to the observed microstructural gradients. The stress partitioning between the hard cementite and the soft ferrite phases in the ground steel samples is discussed in detail.

This investigation has considered the influence of heat treatment, environment and residual stress level on the response of titanium alloys to mechanical surface treatment. It has been shown that the high cycle fatigue benefits to be gained by mechanical surface treatment will depend upon all of these variables. For example, shot peening of Ti–6Al–7Nb a/b solution treated, water quenched and aged condition results in a substantial increase in the high cycle fatigue performance in laboratory air. In contrast, shot peening of Ti–6Al–7Nb a/b solution treated, air cooled and aged condition has only a relatively small impact on the high cycle fatigue performance. This study also shows that high cycle fatigue crack nucleation of mechanically surface treated titanium alloys within the low stress, high cycle regime is associated with subsurface fatigue crack nucleation. Further, that this phenomenon may be related to the presence of a process-induced residual tensile stress necessarily present below the mechanically treated surface and required to balance the outer process-induced residual compressive stresses.

Possible processes for shortening the heat treatment of roller-bearing steels in the lower bainitic range were investigated by computer assisted dilatometric experiments and by practical heat treatment process simulations. Several processes were evaluated and used on the steel 100Cr6 (SAE 52100). The microstructures achieved have been investigated by metallography, X-ray analysis and electron microscopy. Influences on hardness, as well as on the fatigue strength were examined. No loss in hardness was achieved and the fatigue strength, determined by rotating bar fatigue tests, was enhanced significantly by the shortened treatments, particularly by the use of the two-step bainitic treatment. The results indicated that small amounts of retained austenite, homogeneously dispersed on a sub-microscopic scale, as obtained during various treatment cycles, have influenced the fatigue strength. By means of the processes evaluated the heat treatment time can be shortened to about 10–25% of the duration of complete bainitisation in the lower range.

Fracture mechanics methods are usually applied if a crack-like flaw is detected in a structural component and a decision has to be taken with respect to limited further service, or if a failure case has to be assessed. The present contribution demonstrates various options for the inclusion of fracture mechanics in the design process. A number of fracture mechanics based concepts are available; these include classical damage tolerance, proof testing, and periodic removal of cracks through grinding. Until now, less attention has been paid to application in the design stage, where, however, consistent inclusion of fracture mechanics in design promises substantial potential in designing against failure.

The relation between flow stress, strain rate and temperature is described by the same model over wide ranges of strain rates and temperatures. The model is developed on the basis of experimental data for flow curves of TiAl6V4, AA7075 (AlZnMgCu1.5) and steel Ck45N at temperatures varied between room temperature and 500°C, 900°C or 1000°C, respectively. The strain rate values cover the range between 10 −3 and 8×10 +3 s −1 . The model considers three simultaneously acting deformation mechanisms: creep deformation at high temperatures and low strain rates, thermally activated glide at low temperatures, and moderate to high strain rates and damping controlled deformation at very high strain rates. The model provides the data required for the simulation of high speed metal cutting and forming processes.