Advanced Engineering Materials

We present in situ transmission electron microscope tensile tests on focused ion beam fabricated single and multiple slip oriented Cu tensile samples with thicknesses in the range of 100-200 nm. Both crystal orientations fail by localized shear. While failure occurs after a few percent plastic strain and limited hardening in the single slip case, the multiple slip samples exhibit extended homogenous deformation and necking due to the activation of multiple dislocation sources in conjunction with significant hardening. The hardening behavior at 1% plastic strain is even more pronounced compared to compression samples of the same orientation due to the absence of sample taper and the interface to the compression platen. Moreover, we show for the first time that the strain rate sensitivity of such FIB prepared samples is an order of magnitude higher than that of bulk Cu.

The use of microneedles for transdermal drug delivery is limited due to the risk of infection associated with formation of channels through the stratum corneum layer of the epidermis. The risk of infection associated with use of microneedles may be reduced by imparting these devices with antimicrobial properties. In this study, a photopolymerization-micromolding technique was used to fabricate microneedle arrays from a photosensitive material containing polyethylene glycol 600 diacrylate, gentamicin sulfate, and a photoinitiator. Scanning electron microscopy indicated that the photopolymerization-micromolding process produced microneedle arrays that exhibited good microneedle-to-microneedle uniformity. An agar plating assay revealed that microneedles fabricated with polyethylene glycol 600 diacrylate containing 2 mg mL(-1) gentamicin sulfate inhibited growth of Staphylococcus aureus bacteria. Scanning electron microscopy revealed no platelet aggregation on the surfaces of platelet rich plasma-exposed undoped polyethylene glycol 600 diacrylate microneedles and gentamicin-doped polyethylene glycol 600 diacrylate microneedles. These efforts will enable wider adoption of microneedles for transdermal delivery of pharmacologic agents.

Ceramic Matrix Composites (CMC), based on reinforcements of carbon fibres and matrices of silicon carbide, show superior tribological properties in comparison to grey cast iron or carbon/carbon. In combination with their low density, high thermal shock resistance and good abrasive resistance, these Si-infiltrated carbon/carbon materials, called C/SiC or C/C-SiC composites, are promising candidates for advanced friction systems. Generally, the carbon fibres lead to an improved damage tolerance in comparison to monolithic SiC, whereas the silicon carbide matrix improves the wear resistance compared to carbon/carbon. In combination with new design approaches cost-efficient manufacturing processes have been developed and have lead to successfully tested prototypes of brake pads and disks, especially for passenger cars and emergency brake systems.

The shark skin effect is the mechanism of wall friction reduction of a fluid due to a riblet structured surface. A new application for riblet surfaces may be jet engine blades. Riblet structured coatings on the blades would act as oxidation protection and additionally reduce the skin drag on the surface. In this study structuring surface areas of high temperature nickel-based alloys is investigated. These alloys are used for compressor and turbine blades near the combustion chamber. Experiments are performed by depositing titanium on a nickel base alloy through particular metal grids using magnetron sputtering. For single-digit micrometer structures, photolithography with subsequent electrodeposition of nickel as well as sputter deposition and thermal evaporation of several materials are investigated. Successfully fabricated structures are oxidized at 900–1 000 °C for up to 100 h and the resulting shape is characterized using scanning electron microscopy. The most accurate structures are obtained using photolithography and subsequent electrodeposition. The reduction of the wall shear stress was measured in an oil channel. The riblet structures prior to oxidation show a reduction of the wall shear stress of up to 4.9%.

High-temperature application of titanium alloys in aeroengines is often limited by their insufficient resistance to the aggressive environment. Magnetron-sputtered Ti–Al based coatings were developed in order to increase the maximum service temperature of conventional titanium alloys from the present 520–600 °C, the temperature limit set by the mechanical capabilities of most advanced alloys. The coatings not only demonstrated excellent oxidation resistance but also demonstrated beneficial effects on mechanical properties. Most importantly, the fatigue behavior of the substrate alloys was not degraded, a major hurdle for coating application on titanium alloys so far. Initial results on Cr-containing Ti–Al based coatings indicated significant potential for application on titanium aluminides.

Ceramic thermal barrier coatings (TBCs) offer the potential to significantly improve efficiencies of aero engines as well as stationary gas turbines for power generation. On internally cooled turbine parts temperature gradients of the order of 100 to 150 °C can be achieved. Today, state-of-the-art TBCs, typically consisting of an yttria-stabilised zirconia top coat and a metallic bond coat deposited onto a superalloy substrate, are mainly used to extend lifetime. Further efficiency improvements require TBCs being an integral part of the component which, in turn, requires reliable and predictable TBC performance. Presently, TBCs fabricated by electron beam physical vapor deposition are favoured for high performance applications. The paper highlights critical research and development needs for advanced TBC systems, such as reduced thermal conductivity, increased temperature capability, lifetime prediction modelling, process modelling, bond coat oxidation, and hot corrosion resistance as well as improved erosion behaviour.

The quasi-static and fatigue behavior after impact of the TiAl alloy TNBV3B produced via three different processing routes—cast, forged and extruded—has been studied on flat and airfoil-like shaped specimens making use of ballistic impact experiments. For impacts resulting in cracks the behaviour can be described using a linear-elastic fracture mechanics approach. The residual strength is described on the basis of the fracture toughness. The residual fatigue strength of impact-cracked specimens is estimated on the basis of the threshold for crack growth of the TNBV3B alloys. However, when there is no visible crack or when the crack length is below the size of the deformed impact area, residual stresses and micro-damage play a dominating role making the linear-elastic fracture mechanics approach invalid. The deformation hardened zone in TiAl has been studied making use of micro-hardness tests showing their extension and the degrees of deformation for different impact energies.

Co-based melts embedded by glass flux in a crucible can be undercooled nearly as strong as samples processed containerlessly by electromagnetic levitation. This allows to measure magnetic susceptibility on undercooled melts while combining the Faraday method with undercooling technique. The magnetic susceptibility of Co and Co-based alloys will be analysed as a function of temperature from superheated to undercooled state. This will be performed by measuring the change of magnetic force F_Z on a sample in a constant magnetic field H₀ and an additional gradient field by means of a Faraday balance. When liquid Co-based alloys are undercooled the magnetisation steeply rises if temperature approaches the Curie temperature. The magnetisation is measured for some Co-based alloys in liquid state as function of alloy system and its concentration. The results show that the magnetic susceptibility of the liquid undercooled samples follows a Curie-Weiss behaviour, from which Curie temperature is inferred.

The effect of the microstructure of Ti-matrix composites on the creep behavior of SiC-fiber reinforced Ti-alloys was discussed. The creep properties in fiber direction were investigated at 600 °C for the composite with the high temperature Ti-alloy Timetal 834 and the standard Ti-alloy Ti-6-4 as matrix. Due to the production process of the composites the reinforced Ti-alloy had a much finer grain than the Ti-alloy in the delivered condition. The relaxation experiments of the alloys with two different microstructures demonstrated that the microstructure had a significant influence on the creep behavior of the alloy.

By using electromagnetic levitation, liquid Cu–Co alloys can be undercooled below their liquidus temperature into the metastable miscibility gap, leading to a phase separation into a cobalt-rich L1 phase and a cobalt-poor L2 phase. This paper reports on experimental and theoretical investigations into the properties of this system, including equilibrium shape, surface, and interfacial tension, phase separation, as well as solidification behavior. Solidification experiments were performed in microgravity in order to minimize the effect of convection on the resulting microstructure.

Grain boundary segregations were investigated by Atom Probe Tomography in an Al-Mg alloy, a carbon steel and Armco\trademark Fe processed by severe plastic deformation (SPD). In the non-deformed state, the GBs of the aluminium alloy are Mg depleted, but after SPD some local enrichment up to 20 at.% was detected. In the Fe-based alloys, large carbon concentrations were also exhibited along GBs after SPD. These experimental observations are attributed to the specific structure of GBs often described as "non-equilibrum" in ultra fine grained materials processed by SPD. The grain boundary segregation mechanisms are discussed and compared in the case of substitutional (Mg in fcc Al) and interstitial (C in bcc Fe) solute atoms.

A FePd alloy was nanostructured by severe plastic deformation following two different routes: ordered and disordered states were processed by high pressure torsion (HPT). A grain size in a range of 50 to 150 nm is obtained in both cases. Severe plastic deformation induces some significant disordering of the long range ordered L10 phase. However, Transmission Electron Microscopy (TEM) data clearly show that few ordered nanocrystals remain in the deformed state. The deformed materials were annealed to achieve nanostructured long range ordered alloys. The transformation proceeds via a first order transition characterized by the nucleation of numerous ordered domains along grain boundaries. The influence of the annealing conditions (temperature and time) on the coercivity was studied for both routes. It is demonstrated that starting with the disorder state prior to HPT and annealing at low temperature (400\degree C) leads to the highest coercivity (about 1.8 kOe).

The microsegregation and microstructural features of directionally solidified AlSi and AlSiMg alloys were discussed. Solute distributions were characterized by microanalysis and were found to be modified by solid-state diffusion during cooling after solidification. The presence of multiphase final eutectics with iron-rich phases was observed in the technical alloy. The comparison of binary and ternary technical AlSi alloys processed under the same solidification conditions revealed differences in their microstructural features that could not be simply explained with well-known steady-state growth theories.

Orthorhombic titanium aluminides represent the youngest class of alloys emerging out of the group of titanium aluminides. These new materials are based on the ordered orthorhombic phase Ti2AlNb, which was discovered for the first time in the late 1980s as a constituent in a Ti3Al-base alloy. In the 1990s primarily simple ternary Ti–Al–Nb orthorhombic alloys were investigated in countries such as the US, UK, India, France, Japan, and Germany. The drive was mainly provided by jet engine manufacturers and related research labs looking for a damage-tolerant, high-temperature, light-weight material. This follows the aim of further extending the use of lower density titanium-base materials in temperature regimes, where heavy nickel-base superalloys are the only alternative today. The present understanding of microstructure–property relationships for orthorhombic titanium aluminides reveals an attractive combination of low and high temperature loading capabilities. These involve high room-temperature ductility and good formability, high specific elevated temperature tensile and fatigue strength, reasonable room-temperature fracture toughness and crack growth behavior, good creep, oxidation, and ignition resistance combined with a low thermal expansion coefficient. This article reviews the aspects of composition–microstructure–property relationships in comparison to near-α titanium, TiAl, and nickel-base alloys. Special emphasis is also placed on the environmental degradation of the mechanical properties.

Solidification behavior of industrially relevant materials such as Ni- and Al based alloys was investigated under conditions of containerless processing on Earth and reduced gravity using atomization and levitation experiments. A spray of small droplets is solidified in atomization processing during falling through a gas atmosphere with a large heat extraction rate. Levitation experiments allow for quantitative measurements of complete temperature-time- profiles and growth velocity of dendrites as a function of undercooling. Solidification of the undercooled melts take place in two steps as soon as nucleation sets in dendrites are formed and propagate rapidly through the undercooled melt under non-equilibrium conditions. Due to the rapid release of the heat of crystallization the undercooled melt is reheated during recalescence and the remaining interdendritic liquid solidifies under near equilibrium conditions during post recalescence period.

A carbon fiber reinforced silicon carbide (SiC) ceramic with increased fracture toughness was developed by performing a liquid silicon infiltration of fiber reinforced carbon materials by the German Aerospace Center. The systematic fabrication of the potential of wood based composites (SiSiC) as precursors for dense reaction infiltrated silicon carbide ceramics was also demonstrated. Bulk density and resin content were the two important parameters for the fabrication of the wood based composites for their use as SiSiC precursors. The phase composition of the ceramic was determined by the bulk density, structure and infiltration behavior of the preforms.

The interaction of ceramic particles of Ta2O5 with a dendritic solidification front was investigated by solidification of Ni 98Ta2 melt undercooled by electromagnetic levitation technique. Ta2O5 particles are stable at temperatures far above the liquidus temperature of the alloy and can act as heterogeneous nucleation site, but by the containerless processing under high purity conditions, the metal matrix-ceramic composite samples in diameter of 6-7 mm could be undercooled by about 100K below their liquidus temperature. The engulfment of particles by a dendritic solidification front was observed only under conditions of very much reduced melt convection in reduced microgravity. It was also observed that forced convection with fluid flow velocities larger than the velocity of a growing dendrites remove particles from the dendrite tip before they can be engulfed by the respective dendrite.

In this study, metal foams made by the Slip Reaction Foam Sintering (SRFS)-process are investigated concerning their thermophysical and permeability properties. Since the foam is to be applied as a functional and structural element in the effusion air cooling system of a stationary gas turbine combustion chamber, these properties are of major interest for the calculation of the temperature distribution inside the combustion chamber walls, which may be critical for the employed materials. Experimental set-ups are presented, which are used to determine permeability, the volumetric heat transfer coefficient and the effective thermal conductivity. The results are presented for a wide range of foam materials. Porosity as well as the basic metal powder and the manufacturing parameters are varied. The influence of these parameters on the measured quantities is discussed. Thermal conductivity data are determined at temperatures of up to 1200 K. The obtained volumetric heat transfer coefficients are transferred to Nusselt-Reynolds plots, which allow generalization to the high temperature and high pressure regime. Correlations between the heat transfer properties and the permeability data are made. Using the acquired experimental data, a proposal is made for the calculation of the inner surface temperature of the combustion chamber as well as the temperature distribution inside the chamber wall, which consists of a structural element, the metal foam and a thermal barrier coating, equipped with laser drilled micro-holes.

Ti-Si coatings were deposited on γ-TiAl based material, and their protection capability was investigated. The surfaces were ground using SiC paper up to 4000 grit, polished and cleaned. No oxidation was observed during the annealing process at 1000°C. The oxidation resistance of the pre-treated coated samples and of bare γ-TiAl was tested in air at 900°C. The high increase of mass gain after 10 cycles was probably caused by rapid oxidation of the Ti5Si4. SEM examinations of cross-sections of vaccum annealed specimens which were exposed to air for 100 cycles revealed an oxide scale of about 10 μm thickness consisting of a porous mixture of TiO 2 and SiO2 followed by grain boundary oxidation in the Ti5Si3 layer. It was observed that the titanium silicides formed in the pre-oxidized coating transformed further into the Ti 5Si3 phase during oxidation testing, resulting in a low oxidation state.

In this study the degradation of the oxidation resistance of γ-TiAl alloys caused by the high affinity of titanium for oxygen was mitigated by forming thermodynamically stable titanium silicides on the surface. A Si-25at%Ti-coating was deposited on Ti-45Al-8Nb (at%) substrate by means of magnetron sputtering. To form surface layers of thermodynamically stable titanium silicides an optimized pre-treatment process at 1000°C for 100h at 10^-6mbar was used applying slow heating and cooling rates. This annealing process resulted in the formation of a Ti₅Si₃ phase and a Ti₅Si₄ layer on top, whereas titanium silicides with higher silicon content (Ti₅Si₄, TiSi and TiSi₂) formed when the coating system was pre-treated in air at 750°C for 100h. During latter pre-treatment, a mixed oxide scale grew on top of the coating. The oxidation behavior of the pre-treated coatings on γ-TiAl was studied by cyclic testing at 900°C in air. Phase formation and microstructure evolution of the coated samples were investigated after 10, 100 and 1000 cycles using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis. All specimens exhibited a mixed oxide scale on top after cyclic testing and exceeded a lifetime of 1000 cycles at minimum. Uncoated γ-TiAl failed after 520 cycles at 900°C by severe scale spallation.

IntroductionFabrication ProcessesProperties Strength and StiffnessCreep PropertiesFatigue PropertiesAnisotropy of TMCsThermal Residual Stresses Influence of the Fiber Distribution on Residual StressesResidual Stresses and FatigueDimensioning and Design with TMCsMaterial ModelingApplicationsSummary and OutlookReferenced Literature and Further Reading Strength and StiffnessCreep PropertiesFatigue PropertiesAnisotropy of TMCsThermal Residual Stresses Influence of the Fiber Distribution on Residual StressesResidual Stresses and Fatigue Influence of the Fiber Distribution on Residual StressesResidual Stresses and Fatigue

Fiber reinforced titanium matrix composites (TMCs) are attractive materials for future aerospace applications. Reinforcement of Ti alloys with SiC fibers leads to a significant increase on strength and stiffness, especially under high stress loading or at elevated temperatures. Mechanical properties of TMCs are strongly influenced by thermal residual stresses (TRS). A reduction of TRS leads to an increase in fully reversed high cycle fatigue resistance of TMCs at room temperature. Reduction of TRS can be easily obtained by prestraining during single tension loading of TMCs in the as processed condition.

A multi-particle 2D finite element model of a 20% particulate reinforced metal-matrix composite was developed on a statistical basis taking into account the correlations between the position, size and orientation of the ceramic particles in the matrix. The stress–strain curves in tension and compression given by the clustered multi-particle model are compared with the curves obtained from one-particle unit cell simulations. It is shown that clustering of particles increases the plastic strain accumulated in the matrix leading to a higher strain hardening and thus to a higher flow stress. The size of the representative volume element (RVE) should be at least equal to the correlation length of the geometrically relevant correlation functions, which was ∼2.4 times larger than the average interparticle distance for the experimentally studied case. Reasonable agreement is obtained between computed residual strains and data available in the literature.

The current work extends the well established approach of Kocks and Mecking by a more realistic description of strain-hardening using an original dislocation density law with a revisited physical understanding of dynamic recovery, without new material parameters and keeping only one internal variable. The current work extends the well established approach of Kocks and Mecking by a more realistic description of strain-hardening using an original dislocation density law with a revisited physical understanding of dynamic recovery, without new material parameters and keeping only one internal variable. First validation shows that the new approach is more consistent with experiments than the common Kocks-Mecking modeling with the same number of parameters.

A new type of architectured materials, namely « monofilament entangled materials », were studied in order to have a better understanding of their behavior under compressive loading and damping. The materials studied in this paper were made of an entanglement of a single steel wire. Their complex internal architecture was investigated using X-ray computed tomography. The evolution of the number of contacts per unit of volume, as well as of the density profile, were followed during the compression test in order to compare it to the mechanical results. Dynamical Mechanical Analysis (DMA) was performed to characterize the evolution of the loss factor of this material with the frequency and the volume fraction. It was shown that this material present an interesting strength/loss factor ratio. A discrete element model was proposed to model the mechanical properties of this material.