Papers by Author: Matthias Göken

Abstract: Dispersed nanoparticles are introduced from stabilized suspensions during the accumulative roll bonding process in aluminium AA1050A by air gun spraying up to a final volume fraction of 0.1 % after eight cycles. Additional strengthening caused by particle insertion is observed and strongly depends on the suspension medium and stabilizing agent as both influence interfacial bonding of the particles to the matrix. The particle insertion furthermore results in reduced peel strength of the sheets irrespective of particle material and size caused by a reduction of effective metal to metal bonding area during rolling through the presence of the particles.

Abstract: Among the well-known methods of severe plastic deformation the accumulative roll bonding (ARB) process is most promising for producing ultrafine-grained (UFG) materials with extraordinary mechanical properties at an industrial scale. Besides, it has also been shown that the ARB process can be successfully used to produce multi-component materials with tailored properties by reinforcement or grading, respectively. In this work, laminates with alternating layers of the high strength aluminium alloy AA5754 and the AA6014 alloy, well-known for good formability and high surface quality, were produced by ARB at 230 °C. Microstructural and mechanical investigations were performed after 2, 4 and 6 ARB cycles by means of light and electron microscopy, nanoindentation experiments and tensile testing. After ARB processing an ultrafine-grained microstructure is obtained. The UFG microstructure as well as the local mechanical properties alter with the layer composition. With increasing number of ARB cycles the interfaces between the layers become more and more wavy by shear band formation. Compared to the pure accumulative roll bonded AA6014 the yield and ultimate tensile strength of the multi-component laminates are considerably higher and are only slightly reduced in comparison to the high strength AA5754. In terms of elongation to failure no reduction in ductility is found. The serrated yielding effect, clearly visible in AA5754, is shifted to higher strains or fully disappears, respectively, whereas in AA5754 the magnitude of serrations increases with increasing number of ARB cycles. Combining AA5754 and AA6014 sheets by ARB results in well bonded ultrafine-grained laminates which exhibit a combination of the beneficial properties of the single-component materials: high strength of AA5754 and good surface quality of AA6014.

Abstract: The addition of rhenium and ruthenium to single crystal nickel-base superalloys improves the high-temperature properties of the alloys. In this work the applicability of the database TTNi7 (ThermoTech Ltd, UK) for developing 4th generation single crystal superalloys containing rhenium (Re) and ruthenium (Ru) was investigated. We systematically compared experimentally determined alloy properties to the predictions of ThermoCalc with the database TTNi7. The investigated properties were liquidus, solidus and ´ solvus temperature as well as incipient melting point and segregation. Calculations were based on thermodynamic principles with the assumption of either equilibrium or Scheil-Gulliver conditions, i.e. no diffusion in the solid and complete diffusion in the liquid. Furthermore the composition of the  and the  phase of a Re- and Ru-containing superalloy was measured and compared to calculations. Our results show that the database is capable of simulating general trends of 4th generation superalloys up to 6 weight percent (wt.-%) Re and 6 wt.-% Ru. The present work shows that Scheil-Gulliver calculations can only be used as a first approximation for nickel-base superalloys.

Abstract: The fatigue behaviour of aluminium-magnesium alloys has been investigated in the recrystallized CG state and in an ultrafine-grained (UFG) state after equal channel angular pressing (ECAP). A strong improvement of the fatigue behaviour up to 12 ECAP passes has been found for an AlMg0.5 alloy. The results have been interpreted in consideration of the microstructure of the different states. Additionally, for an investigation of the influence of impurities on the cyclic stability of Aluminium, 3 different AlMg alloys with Magnesium contents of 0.5, 1, and 2 wt.-% have been compared. Total strain controlled fatigue tests have shown an improvement of the cyclic stability with increasing Mg content.

Abstract: Commercial purity aluminium AA1050 and aluminium alloy AA6016 were accumulative roll bonded and subsequently friction stir welded. The microstructure of the conventional and ultrafine-grained materials produced by accumulative roll bonding is strongly affected by friction stir welding. The elongated ultrafine-grained microstructure of roll bonded sheets becomes coarser and equiaxed in the nugget region. Hydraulic bulge tests showed that higher burst pressure can be achieved for samples without friction stir welding than for the ones with friction stir welding. Localised deformation, crack initiation and propagation, as well as the final fracture occurred within the nugget. Friction stir welded AA1050 sheet showed similar achievable burst pressures and von Mises equivalent strains compared to the as-received conventionally grained sheets. On the other hand, significantly higher burst pressures and at the same time higher von Mises equivalent strains were observed for the friction stir welded ultrafine-grained material than for the friction stir welded conventionally grained material.

Abstract: Ti-6Al-4V ELI (extra low interstitials) was processed by equal channel angular pressing in order to obtain an ultrafine-grained (UFG) microstructure which is known to enhance the fatigue behavior of metallic materials. Fatigue properties of UFG Ti-6Al-4V ELI were studied by strain and stress controlled fatigue tests. UFG Ti-6Al-4V ELI shows an improvement of the fatigue behavior compared to conventional grain (CG) size counterpart. Microstructural investigations prior to and after fatigue testing confirm a high structural stability of the UFG material. Hence, the UFG alloy has a high potential for prospective use in biomedical and engineering applications.

Abstract: The strain rate sensitivity of the aluminium alloy AA6061 has been investigated in a conventional grain sized (CG) state and in two different ultrafine grained (UFG) conditions processed by Equal Channel Angular Pressing (ECAP) for 2 and 6 passes at 100o C. Strain rate jump tests in compression were performed at different temperatures and the strain-rate sensitivity exponent m was determined. The tests were accomplished by microstructural investigations before and after compression testing in CG and UFG conditions. It is shown that all UFG microstructures exhibit strongly increased strain-rate sensitivity (SRS) compared to the CG state. The SRS increases with increasing temperature and is more pronounced for the UFG material processed using 6 ECAP passes. The microstructural investigations show a rather high stability of the grain structure for the UFG conditions up to 250o C. The results are discussed with respect to the relevant deformation mechanisms.

Abstract: Brazing is a well established repair technique for high temperature components in both industrial gas turbines and aero engines. Conventional nickel base braze alloys contain boron or silicon as melting point depressing elements. The major benefit of boron and silicon compared to other melting point depressants is its large effect on the melting point and its high diffusion coefficient in nickel base superalloys. However these elements promote precipitation of undesired brittle phases during the brazing process. To avoid these phases, transient liquid phase bonding in combination with boron and silicon free brazing alloys will be examined in this work. The influence of the brazing temperature on solidification and diffusion behaviour during transient liquid phase bonding for a single crystalline first generation and a second generation superalloy will be reported. Our experiments show that isothermal solidification without precipitation of brittle phases in the braze joint or the base material can be achieved. The brazed joint consists of fine γ/γ´ microstructure. EBSD measurements demonstrated that the single crystalline orientation of the base material was maintained throughout the joint. Electron probe micro analysis is used to characterize the diffusion behaviour. Solidification velocity will be compared with the theory of transient liquid phase bonding established by Tuah-Poku [1].

Abstract: The origin of dislocation evolution during SiC crystal growth is usually related to lattice relaxation mechanisms caused by thermal stress. In this paper we discuss dislocation generation and dislocation propagation related to doping and suppression of basal plane dislocations, the latter being of particular interest for bipolar electronic devices. We have prepared alternating p-/n-/pdoped SiC crystals using the donor nitrogen and the acceptors aluminum or boron. In addition we determined the mechanical properties of n-type and p-type SiC; in particular we measured the critical shear stress by nano-indentation on c-plane and a-plane 6H-SiC surfaces. A considerably lower basal plane dislocation density is found in aluminum as well as in boron doped p-type SiC compared to nitrogen doped n-type SiC. It is concluded that the explanation of the reduced basal plane dislocation density in p-type SiC needs the consideration of electronic as well as mechanical effects.

Abstract: The dependence of the strain rate sensitivity (SRS) of α-Fe and Al 99.5, as typical representatives of fcc- and bcc-type metals, on the testing temperature and with respect to the microstructure is investigated. In particular, the differences between conventional grain size (CG) and ultrafine grain size (UFG) are pointed out. UFG Al 99.5 generally shows an elevated SRS compared to CG Al 99.5. In case of α-Fe the SRS of the UFG state is decreased at room temperature, but increased at 200 °C, compared to the CG state. It is shown that the SRS also influences the ductility of UFG-metals in tensile tests.