Volume 101, Issue 9

Using X-ray and neutron radiography and tomography, images of material and component inhomogeneities and their development with time can be obtained. Due to their non-destructiveness and non-invasive nature both methods give insight into the function of devices and their decay processes. Fundamentals of X-ray and neutron radiography and tomography are briefly outlined, examples for both techniques are given, their complementarities are highlighted and emerging techniques and frontiers are discussed.

Fast in-situ X-ray tomography has been used at ESRF Grenoble to characterise the microstructural evolution and the formation of strain-induced damage in various materials during different thermomechanical treatments. Materials are both light alloys (Al and Mg based alloys) and steels. The thermomechanical treatments include tensile deformation both in the solid state at room and high temperatures and in the semi-solid state together with thermal treatments above the solidus temperature. Melting and solidification of Al-alloys are also considered in this paper.

The microstructure evolution in heat-treatable Al-alloys is characterized by a complex sequence of precipitation processes. These can be either endothermic or exothermic in nature and they can be investigated by thermal analysis. The individual peaks identified in a differential scanning calorimetry (DSC) analysis can be correlated to the nucleation, growth and dissolution of certain types of precipitates. Simultaneously, these data can also be obtained by thermo-kinetic simulation based on models implemented, for instance, in the software MatCalc. The simulations make use of information stored in thermodynamic databases, including stable and metastable phases. In the present work, a thermo-kinetic computational analysis of Al–Mg–Si DSC curves is carried out. The comparison with experimentally observed DSC signals for precipitation and dissolution of metastable GP-zones, β″, β′, as well as stable β-Mg 2 Si and Si precipitates provides a quantitative insight into the kinetics and sequence of precipitation during DSC probing. The combination of thermo-kinetic and experimental DSC analysis offers new possibilities in interpretation of DSC peaks with multiple metastable phases. In the present paper, we discuss the linking of the simulated precipitation sequence with the measured DSC signal. In addition, with the proposed methodology, a consistent set of parameters to describe the non-equilibrium kinetic parameters of a specific alloy system can be obtained, which can substantially aid in alloy and process development.

The grain refinement after high pressure torsion of pure metals and alloys exhibiting fcc crystal structure was examined. The aim was to characterise the influence of the stacking fault energy and the alloying on the grain size in the saturation regime. In this regime no further hardening of the material is observed and a constant mean grain size is obtained. Pure metals such as silver, copper and nickel and different brass alloys were severely deformed using high pressure torsion. During the severe deformation the torque curves were recorded for the different metals at different hydrostatic pressures. Due to the different deformation conditions, the influence of friction on the torque in the saturation regime can be estimated. In addition tensile tests were performed and the results were compared with flow stresses determined in the high pressure torsion experiments. A clear influence of the alloying on the flow stresses and the grain sizes of the materials was found, the stacking fault energy has a significant effect on the refinement, but only a minor effect on the saturation grain size.

Silver matrix composites with short and long carbon fibres of high intrinsic thermal conductivity were fabricated either by liquid metal infiltration or electroless deposition of the Ag matrix and subsequent hot pressing. Active elements enhancing the bonding between inclusions and matrix were introduced by pre-conditioning the fibres thus to influence and improve the overall thermo-physical properties of the composites. The evolution of the composite thermal conductivity (κ c ) and the coefficient of thermal expansion (α c ) as a function of the fibre volume fraction, fibre type, fibre length (fibre orientation) and the presence of active elements such as cobalt, molybdenum, nickel, tungsten and boron are presented in detail and compared with different analytical models.

Pore-free composites combining alumina particles within a matrix of pure aluminium are produced by pressure infiltration, varying systematically the degree of the contiguity, β, of the ceramic phase by sintering prior to infiltration. The thermal expansion of these composites is characterized using repeated cycling in a dilatometer from 303 to 543 K. The initial composite thermal expansion measured immediately after changing the sign of the temperature–time function conforms with predictions from thermoelasticity. Upon further temperature excursion, upward deviations from thermoelastic predictions are found; these are attributed to the influence of matrix plasticity, the influence of which decreases as β increases. The composite thermal expansion travels across 80 % of the theoretical range of initial thermoelastic values as β increases from 0 to 0.123, leading to the conclusion that even a small level of connectivity of the ceramic phase causes significant changes in the thermal expansion of isotropic metal/ceramic composites.

Starting from the fundamental relation between the diffusion flux of point defects (vacancies, interstitials) and the gradient of the diffusion potential the diffusion equation is set up and applied to the calculation of the concentrations of both types of point defects. Thereby, all possible sources and sinks and their mutual effects are taken into consideration. These equations are coupled with the equations for the temporal derivatives of dislocation density, porosity, pore size and temperature. For the close-up region of spherical inhomogeneities (pores, hard inclusions) and a grain boundary this system of six partial differential equations is solved numerically. For simplicity a one dimensional model in space is used and uniaxial tension or pure shear load is assumed. Hardening and the simultaneous recovery are characterized by the local and temporal development of the point defect concentrations and especially of the dislocation density, which is proportional to the intrinsic strain energy. The gradients of both strain energy and lattice rotation determine the formation and growth of recrystallization nuclei. It is the aim of this work to present a new mathematical model and also to give a perceptual general view on the concerted action of the fundamental physical processes, which necessarily lead to hardening, recovery (softening) and recrystallization nucleation. Moreover it seems to be a basis of a future general theory of microstructure changes.

Various techniques to manufacture low-density metallic foams containing sub-millimetre or even micrometre-sized pores are discussed and first trial experiments presented. Three strategies are evaluated: use of an intrinsic blowing agent, foaming under high pressure and foam control by mechanical pressure manipulation. In all three cases, average pore diameters well below 1 mm could be achieved for some aluminium or zinc-based foams while keeping the relative density in a range between 20 % and 50 % of the full metal density.

In this work, the gigacycle fatigue response of several tool steel grades, produced by ingot metallurgy and powder metallurgy, has been studied using an ultrasonic frequency resonance testing device. In all cases the S–N curves exhibited a consistent drop up to 10 billion cycles, i. e. there is no fatigue “limit” with these materials. Surprisingly, there was virtually no effect of the composition and hardness of the materials, both for powder metallurgy and ingot metallurgy grades cold work tool steels and high speed steels exhibiting virtually the same S–N curves, however the powder metallurgy steels revealed significantly higher endurance strength levels than ingot metallurgy grades. In the ingot metallurgy tool steel grades, crack initiation started at large primary carbides or carbide clusters, while in the powder metallurgy grades, markedly smaller non-metallic inclusions turned out being the critical defects. Furthermore, it has been shown that it is very important to avoid introducing residual stresses into the specimen surfaces during preparation, since these stresses significantly affect the endurance strength levels and the location of crack initiating sites.

Powder metallurgy produced Ti-6Al-4V-1B-0.1C is a modification of the Ti-6Al-4V commercial titanium alloy. TiB whiskers precipitate in the Ti64 matrix during the solidification of the powders before consolidation. The whiskers present two size classes due to the solidification rate and satelliting effect. Scanning electron and light optical microscopy were used to characterize the morphology, distribution and damage of TiB before and after hot deformation. These results were compared with those obtained by parallel beam and magnified synchrotron holotomography, and by focused ion beam tomography. The damage produced during hot deformation was provoked by debonding and cracks at the larger TiB particles. Simulations of the compression tests using finite element analysis were carried out to identify strain and stress localizations.

In the present work structural features of near eutectic Al–Si and Al–Si–Cu alloys prepared by a normal melting and casting route are studied. It is observed that morphology of Si plays an important role in enhancing the mechanical properties of Al–Si alloys. In order to modify the Si network in the matrix of Al some immiscible elements such as Pb, Sn, Cd and Bi have been added in small quantities to the alloy. Since the structure developed after casting governs the mechanical properties, it is essential to understand the structural features before testing the mechanical properties. Taking into consideration the age hardening characteristics of the alloys, microstructural assessment and a thermal study of these alloys have been done. Thermal analysis shows that immiscible elements are homogeneously distributed in the alloys. It is observed that the addition of Pb leads to a rosette type structure, whereas in the case of Cd and Sn blunt and twisted Si needles can be seen. The structural features of the eutectic change to hypoeutectic completely on addition of Bi. The shift in the eutectic depression on addition of a ternary element causes a change in the morphology of Si. Addition of Cu to the Al–Si alloys gives precipitation hardening characteristics.

The stress corrosion cracking behavior of a commercial AZ31 magnesium alloy after T5 and T6 heat treatments under a vacuum of 10 −3 torr has been evaluated via slow strain rate tensile tests in an aqueous 3.5 wt.% NaCl solution. The experimental results show that this material displays a high sensitivity to stress corrosion cracking. The fractured surface was consistent with a significant transgranular component. Double-line cracking that resulted from twinning could be readily spotted on the cleavage plane. The presence of such deformation twins accompanied by the formation of voids is the main reason for the stress corrosion cracking susceptibility of this alloy. The disappearance of twins after artificial aging at 260 °C for 4 h significantly improves the corrosion resistance, increases the breakdown potential difference ΔΦ (thus lowering the pitting tendency), and decreases the corrosion current density in a 3.5 wt.% NaCl solution.

The effects of strain rate and temperature on the deformation behavior of hardened 5083-H111 aluminum magnesium alloy sheet were investigated by performing uniaxial tensile tests at various strain rates from 0.0083 to 0.16 s −1 and temperatures from – 100 to 300 °C. Results from the prescribed test ranges indicate that the formability of this material at cold and warm temperatures is better than at room temperature. The improvement in formability at cold temperatures is principally due to the strain hardening of the material. However, the improvement at warm temperature and low strain rate is specifically due to the high strain rate sensitivity characteristic of the material. Results indicate that this alloy should be formed at temperatures higher than 200 °C and at low strain rates.

Super heater tubes and outlet headers are subjected to thermal fatigue from alternating heating and cooling during start up and shut down, causing cracks and eventually failure. Super heater tubes are subjected to a temperature of 600 °C and steam at a temperature of 540 °C and a pressure of 160 bar. In this paper, a laboratory simulation for reproducing thermal fatigue phenomena was developed to determine the number of cycles before failure occurs in a header tube to stub welded joint. For this purpose a carbon steel tube (stub) welded to a carbon steel plate which resembles a header pipe to stub welded joint was subjected to thermal cycles from 800 °C (accelerated test) to room temperature. A tungsten inert gas arc was used as a heating source and a water bath was utilized for quenching. After 300 thermal cycles, the cross-section of the header was subjected to optical microscopy. Micro fissures were observed in the header tube. This study reveals that subsequent localized heating and cooling will cause thermal fatigue, which will initiate and propagate cracks in the tubes.

The hexagonal lyotropic liquid crystalline phases of nonionic surfactants AEO-9 were used to template directly the fabrication of nanostructured Cu/ZnO/Al 2 O 3 oxides. The reaction between metal precursors and alkaline reacting substance dissolved within the aqueous domains of the liquid crystalline templates generated nanostructured oxides. Low-angle X-ray diffraction and transmission election microscopy characterization reveals the product as a hexagonal array of cylindrical pores with pore-to-pore separation of ca. 6.3 nm. Possessing ordered nanochannels, Cu/ZnO/Al 2 O 3 oxides could find more potential application in catalysis.