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The impact of high temperature exposure on the mechanical damage behavior of the intermetallic TiAl alloy TNM has been investigated by means of high resolution laboratory computer tomography (CT). An in situ testing device was adapted allowing the study of cracking behavior during tensile testing of unnotched samples by CT. Accordingly, failure of exposed and unexposed samples is controlled by the fracture toughness. Unexposed samples reveal crack formation in the interior at only about 1/3 of their plastic elongation to failure. These internal cracks grow gradually with increasing nominal stress and are subjected to significant crack deflection by the microstructure until the crack growth accelerates and the samples fail. After exposure for 1 h at 800 °C cracks originate in the surface region of the samples at almost the same nominal stresses at which internal cracks first appear. However, due to higher fracture mechanical geometry factor, surface cracks become critical almost instantly after their formation resulting in a reduced ductility, which is known as environmental embrittlement in literature. It is supposed that crack formation at the surface after exposure is the result of stress concentrations which are built-up during plastic deformation due to interstitials in the surface layer.

In this research paper the microstructure of the magnesium AZ31B alloy is investigated after the application of a single monotonic compressive strain of −0.4%. A band of twinned grains is observed to form in the test bar approximately 10 mm wide, corresponding to the zone of highest plastic strain. The microstructure of this region is studied using electron backscatter diffraction and transmission electron microscopy (TEM). Twinning mode and fraction are reported along with a detailed analysis of precipitate phase and size distribution. Evidence is given for twin-precipitate interaction during the twin formation process. High resolution TEM and energy dispersive X-ray spectroscopy are used in conjunction with structural simulation software to identify the Al-Mn phase. A zinc shell is observed forming around precipitates, and this is related to zinc exclusion by the precipitating Al-Mn phase.

Microstructural investigations and mechanical fatigue tests with in situ optical strain field measurement were performed on unnotched, notched, and bending AZ31B magnesium specimens. Twinned and twin-free regions are clearly delimited and compressive strain within twinned regions is much higher compared to the strain in twin-free regions. Furthermore, initial macroscopic cracks are observed to be always located within the twinned regions. Thus, only twinned regions are considered for fatigue modeling. To cover stress concentrations, stress gradients, different types of loading, and strongly inhomogeneous distribution of strain within bands of twinned grains all together, current fatigue lifetime description methods are insufficient. Hence, this work presents a newly developed method, referred to as the method of highly strained volume. The new method bases on the principals of the method of highly stressed volume, which is suitable for modeling the fatigue limit. This method uses the size of the highly strained volume and the effective strain amplitude in the highly strained volume to describe the number of cycles to failure. Based on results of 75 fatigue tests (strain amplitude: 0.30–1.17%, strain ratio: 1.47 to −0.431) the accurate description of the fatigue lifetime by the method of highly strained volume is demonstrated.

Notched AM50 sheet metal specimens with four different stress concentration factors are tested under quasi-static and cyclic uniaxial loading, and the results are analyzed and compared with those of unnotched specimens. Special attention is given to the formation of bands of twinned grains and the inhomogeneous local strain field in the vicinity of notches, which is measured by an in situ optical strain measurement technique. It is shown that the notch sensitivity is reduced, if bands of twinned grains occur. In the range between 500 and 1000 load cycles to fracture, the endurable load is only slightly reduced by the presence of a notch for completely reversed fatigue tests. In contrast, specimens are notch sensitive under loading conditions, where no twin formation occurs.

The damage behaviour of the TiAl alloy Ti-43.5Al-4Nb-1Mo-0.1B was investigated by isothermal thermogravimetric-analysis (TGA) and strain controlled low-cycle-fatigue (LCF) experiments at high temperature. Aluminium depletion and the formation of a continuous brittle layer in the subsurface zone takes place in synthetic exhaust gas. Contrary, a continuous ductile γ-phase forms in the subsurface zone in air. Since a continuous TiN layer as diffusion barrier is not formed in the early stages of corrosion, strong hardening is observed in LCF test samples in exhaust gas and a strain amplitude of 0,6 %, leading to a significant reduction of the cycles to failure.

Wrought magnesium alloys typically reveal strong basal textures and thus, non-symmetric sigmoidal shaped hysteresis loops within the elastoplastic load range. A detailed description of those hysteresis loops is necessary for numerical fatigue analyses. Therefore, a one-dimensional phenomenological model was developed for elastoplastic strain-controlled constant and variable amplitude loading. The phenomenological model consists of a three-component equation, which considers elastic, plastic, and pseudoelastic strain components with a set of eight material constants. Experimentally and numerically determined hysteresis loops of four different magnesium alloys were compared by means of different examples with constant and variable amplitude. Good correlation is reached and the relevant fatigue parameters like strain energy density were estimated with good accuracy. Applying an energy based fatigue parameter on modelled hysteresis loops, the fatigue life is predicted adequately for constant and variable amplitude loading including mean strain and mean stress effects.

The mechanical behavior of twin roll cast AM50 magnesium sheet metals was investigated via quasi-static and cyclic tests. Nine different stress or strain ratios were evaluated to cover the most technical relevant load range. Furthermore, the dependence of the mechanical behavior on loading direction was examined with the transverse direction showing the least tensile elongation and fatigue resistance. Due to the strong basal texture, twinning dominates the plastic deformation in compression of samples parallel to the sheet plane. Especially in stress-controlled cyclic tests, twinning leads to large compressive deformations. In quasi-static and strain-controlled cyclic tests, twin bands were observed and were found to be responsible for local plastic deformation. Special focus is given on the shape of hysteresis loops as well as the cyclic behavior of stress, strain, and energy density to track twinning and detwinning effects. Different possible fatigue parameters are compared concerning their evolution as a function of the fatigue life. An energy based fatigue model with weighted elastic strain energy density shows the best correlation between the lifetime prediction and the experiments.

The deformation behavior and the fatigue life of AM50 magnesium alloy sheet metals under variable amplitude loading were determined experimentally via strain-controlled tests. Hysteresis loops were recorded precisely to evaluate its shapes and to calculate the fatigue life. A strong basal texture is responsible for non-symmetric and sigmoidal shaped hysteresis loops. Thus, the shape of inner hysteresis loops in variable amplitude tests highly depends on the mean stress. To evaluate the influence of the mean strain on the fatigue life, tests with different load sequences were carried out. For a fatigue calculation, the Smith-Watson-Topper and two energy based fatigue models were compared, where all in all the energy based fatigue models achieved the best results.

In cyclic stress and strain controlled tests, the fatigue life was determined for twin roll cast AM50 magnesium alloy sheet metals. Different stress and strain ratios were applied to cover the technical relevant load range. The investigated sheet metal shows a strong basal texture which leads to twinning in compression parallel to the sheet plane direction. Accordingly, at low cycles to failure Nf the cyclic plastic deformation is dominated by twinning and detwinning. In this study an energy based approach for lifetime estimation is presented and compared with the Smith-Watson-Topper damage model as well as with experimental test results. The energy based approach shows a comparatively high correlation at all investigated stress and strain ratios in a wide cycle range (20< Nf

The present work investigates the deformation behaviour of strongly textured AZ31B wrought magnesium sheets under monotonic as well as cyclic mechanical loading. Of special focus is the distribution of strain, which is investigated employing optical strain field measurements on the specimen surfaces. Microstructural investigations of the material after loading provide detailed information on the underlying deformation characteristics. The investigations show that bands of twinned grains (BTGs) form up once twinning occurs. These BTGs consist of solely { 10 1 ‾ 2 } tension twins and are clearly delimited to the adjacent absolutely twin-free regions. Most importantly, strain fields are strongly inhomogeneous and discontinuous due to these BTGs with considerably higher compressive strain inside BTGs, both after monotonic and cyclic loading. During the first few load cycles, BTGs are observed to grow and the formation of new BTGs can occur. Additionally, a significant hardening is found during the first load cycles, most likely caused by residual twins and unretwinnable twins. However, material behaviour stabilizes after a few load cycles and remains approximately constant for most of a specimen's lifetime. The presented specific deformation characteristics are important for understanding and describing the material. Modelling the investigated material behaviour is not yet completely feasible with phenomenological constitutive models, which is why important modelling aspects are discussed.

Extruded ME21 (nominal thickness 1.5 mm) and twin roll cast AZ31 sheet metals (nominal thickness 1.2 mm) are investigated via quasi-static tensile and compressive tests and cyclic stress and strain controlled fatigue tests at ambient temperature. Both materials show a strong basal texture with the c -axis being almost normal to the sheet plane in AZ31 and tilted about 20° from the normal towards the extrusion direction in ME21. Furthermore, a direction dependence of the tensile strength values is observed. The difference in tensile yield strength comparing the extrusion (rolling) to the transverse direction is about 10% in AZ31 and 25% in ME21. In compression the quasi-static stress–strain curves show negligible differences in extrusion (rolling) and transverse direction up to 10% total strain. ME21 shows comparatively low tangent modulus and pseudoelastic behavior at low stresses. Sigmoidal shape of hysteresis loops starts to appear at strain amplitudes >0.4% in both materials. The cyclic stress–strain curve is approximated by the Ramberg–Osgood equation using different parameters for tension and compression. The strain-life curve is described by the Basquin equation and the bilinear Manson–Coffin approach. In addition, the evolution of mean stress, stress amplitude, and Masing behavior are analyzed and strain- and stress-life curves of both materials are compared and discussed.

The quasi-static, static, and cyclic compressive behavior of a novel epoxy matrix cellular composite reinforced with glass foam granules is investigated. Three different grain-size fractions of the granules are used: 0.5–1, 1–2, and 2–4 mm. The density of the cellular composite varies between 0.65 and 0.82 g/cm³. The material exhibits high specific compressive strength and stiffness within the class of cellular materials; these properties can be varied using appropriate size of granules. The glass foam granules increase the stiffness of the cellular composite compared to neat epoxy foam with the same weight. The measured elastic properties are in good agreement with results obtained from analytical and numerical homogenization methods. The fatigue behavior is determined in static tests and in cyclic tests at 1 and 20 Hz on one type of cellular composite. The fatigue process for cyclic loading is a result of an interaction between static and cyclic damage. The sensitivity to static damage is found to be higher than to cyclic damage. The damage behavior is investigated by evaluation specimen’s stiffness and using scanning electron microscopy.

The response to deformation of samples of cast Ti46Al8Ta and Ti46Al8Nb alloys, which had been heat treated to produce similar refined microstructures, has been investigated; the refinement increases the ductility from about 0.2% found in cast HIPped samples to 0.5% for the Nb-containing alloy and up to 1.5% for the Ta-containing alloy. It has been shown that the tensile fracture surface of samples can show facetted regions characteristic of a brittle fracture and regions which appear ductile. Back scattered images and electron back scattered diffraction data indicates that there are large areas of similarly oriented alpha2 in the Nb-containing alloy and that the alpha2 is coarser than in the Ta-containing alloy. Examination of dislocation activity in the alpha2 and gamma phases in both alloys shows that deformation via slip and twinning within gamma is dominant and that limited slip of both c-component and a-dislocations can occur within alpha2. In some alpha2 grains, which are oriented so that slip of a-dislocations cannot relieve the applied stress, large regions, which are strongly strained elastically, can be imaged at the alpha2/gamma interfaces. In such cases, dislocation slip and twinning in gamma can appear to be transmitted across alpha2 via these elastic stresses, without any dislocation activity within the alpha2. Synchrotron observations aimed at defining the onset of plasticity in the alpha2 did not reveal any obvious difference in behaviour of the two alloys. Analysis shows that the oxygen-content of the alpha2 in the two alloys is very similar and analytical TEM shows that the site occupancy of the Nb and Ta within the alpha2 is only slightly different. These observations are discussed in terms of the tensile properties, in particular the ductilities, of these two alloys.

The low cycle fatigue (LCF) behavior of two hot extruded alloys with the compositions Fe–27.1Al and Fe–26.4Al–4Cr (at%) containing microalloying additions of Nb, Zr, C, and B was investigated at room temperature and 300 °C in air. The LCF tests were carried out in strain control with strain amplitudes in the range of ea = 0.2–0.7% with frequencies of 1 or 3 Hz. During fatigue, strong cyclic hardening was observed. No significant influence of the Cr addition on the tensile properties and the LCF behavior was noticed. Similarly, a heat treatment of 100 h at 800 °C did not change the microstructure or the LCF behavior noticeably. For cycle numbers below ∼10 000, the fatigue resistance at 300 °C tended to be higher than that at room temperature, and vice versa.

Interrupted quenching has been used to investigate the factors influencing the nucleation of massive gamma during air cooling of Ti46Al8Ta from the alpha phase field; air cooling directly to room temperature results in virtually 100% massive gamma and water quenching results in virtually 100% retained alpha. If samples are air cooled to successively lower temperatures within the (α + γ) phase field, before being water quenched, it has been found that they contain increasing amounts of massive gamma. Measurements of the cooling rates during interrupted quenching and during air cooling to room temperature together with microstructural observations suggest that the undercooling required to nucleate massive gamma is reduced by the presence of “equilibrium” grain boundary gamma formed above T 0 . It has been shown that a {111} of the massive gamma is parallel with a {111} of the grain boundary gamma implying that the massive gamma is nucleated by the grain boundary gamma. These observations are discussed in terms of the factors which influence nucleation of massive gamma. The formation of significant amounts of grain boundary gamma above T 0 (the temperature above which massive gamma cannot form) sets a lower limit to the cooling rate which can be used to generate virtually 100% massive gamma.

Al-rich Ti – Al alloys, as compared to Ti-rich -TiAl-based alloys, offer an additional reduction in density of 20 %, better oxidation resistance and sufficient strength at high temperatures. High temperature creep of a binary Al62Ti28 alloy was studied in compression in the temperature range between 1 173 K and 1 323 K in air. It is shown that the alloy exhibits quite reasonable creep resistance at 1 173 K, especially in view of its low density of around 3.8 g cm– 3. Stress exponents calculated as the slope n = log (strain rate)/ log (stress) = 4 were found to be relatively constant for the temperature and stress regime investigated. This indicates that dislocation climb may be the rate controlling creep mechanism. The values of the activation energies for creep for the as-cast and the annealed Al62Ti38 material coincides well with those found in the literature for interdiffusion of Al in -TiAl.

We review the current development status of Mo-Si-B alloys consisting of Mo solid solution and the intermetallic phases Mo3Si and Mo5SiB2 which could take advantage of the beneficial oxidation resistance of the silicide phases and of the outstanding mechanical properties of molybdenum. For adequate low temperature toughness a continuous Mo solid solution matrix should be established in the microstructure. Besides, wrought processing of such alloys at elevated temperatures requires the presence of an ultra-fine grained (UFG) microstructure. Both the prerequisites can be fulfilled using mechanical alloying (MA) as the crucial processing step which even yields nanostructured supersaturated powders after milling. However, values for the ductile-to-brittle transition temperature (DBTT) close to room temperature are unlikely due to grain boundary embrittlement by Si segregation. The possibility of reducing this segregation tendency by various micro-alloying additions will be demonstrated. Finally, the high temperature deformation behaviour of these UFG materials will be comparatively assessed against state-of-the-art Nickelbase single-crystalline superalloys.

Mo–1.5 at.% Si alloys with additions of either Y2O3 or Zr were manufactured by mechanical alloying. The Y2O3 particles reduced the grain size and increased the room temperature strength, but did not alleviate the brittleness of previously investigated Mo–1.5 at.% Si without Y2O3. Additions of Zr, on the other hand, resulted not only in a fine grain size and an extremely high bend strength (∼2 GPa), but also in limited bend ductility at room temperature. Zr additions are seen to be beneficial for three reasons. First, Zr reduces the grain size. Second, Zr getters detrimental oxygen by forming ZrO2 particles (which in turn help to pin the grain boundaries). Third, in situ Auger analysis shows that Zr reduces the concentration of Si segregated at the grain boundaries. This is thought to enhance the grain boundary cohesive strength and thus leads to the observed ductility.

High-temperature creep of a binary Al 60 Ti 40 (at.%) alloy in the as-cast state and after annealing at 1223 K for 200 h which produced nearly lamellar γ-TiAl + r-Al 2 Ti microstructure was studied utilizing creep compression tests in a temperature range between 1173 and 1323 K in air. The material was manufactured by centrifugal casting. Microstructural characterization was carried out employing light-optical scanning (SEM) and transmission electron microscopy (TEM) as well as X-ray diffraction (XRD) analyses. It is shown that the alloy exhibits reasonable creep resistance at 1173 K, especially in relation to its low density of around 3.8 g/cm 3 . Stress exponents calculated as n = Δlog (strain rate)/Δlog (stress) = 4 were found to be relatively constant for the temperature and stress regime investigated. This indicates that dislocation climb may be the rate controlling creep mechanism. The assessment of creep tests conducted at identical stress levels and varying temperatures yielded activation energies for creep of around Q = 457 kJ/mol in the as-cast condition. This value is significantly higher than those found in literature for interdiffusion of Al or Ti in γ-TiAl. It is concluded that the difference is a due to the instability of the microstructure of the as-cast multi-phase alloy.

The present exploratory study considers microstructures of two potential high-temperature alloy systems, based on the high-melting-point base alloys Co–Re–Cr and Mo–Si/Mo–Si–B. Flat Co–Re–Cr samples were produced by arc-melting (ingot metallurgy) whereas the Mo–Si and Mo–Si–B samples were produced following a powder metallurgical processing route involving consolidation and aging under hot isostatic pressure. Transmission electron microscopy was carried out using a FEI Tecnai F20 to characterize the microstructures of a number of specific alloys: Co–17Re–23Cr–2.6C, Co–17Re–23Cr–2.6C–1.2Ta, Co–31Re–23Cr, Mo–1.5Si, Mo–1.5Si–1Zr and Mo–9Si–8B–1Zr (numbers indicate at.%). For the Co–Re–Cr samples with the lower Re-content, electron diffraction analysis reveals a hexagonal ɛ(Co,Re,Cr) matrix with plate-like precipitates of type M23C6. A closer inspection of the diffraction patterns of precipitates and matrix reveals orientation-relationships of types (1 1 1) M23C6 || (0 0 0 1)hex and [ 0 1 ¯ 1 ] M23C6 || [1 0 0]hex. Ta additions lead to the formation of tiny TaC precipitates. In the microstructure of the Re-rich alloy Co–31Re–23Cr two phases are detected: the ɛ(Co,Re,Cr) phase and a phase which is rich in Cr and Re with lattice parameters similar to those in Cr2Re3. The Mo–0.5Si alloy consists of a bcc matrix phase with Si in solid solution. Adding Zr leads to the formation of small Zr-rich precipitates as was shown using energy dispersive analysis of X-rays. These precipitates are also found in the Mo-phase of the Mo–9Si–8B–1Zr alloy. The results obtained in the present study will serve as a reference for further work which will focus on the evolution of microstructure during creep.

Fully massively transformed samples of Ti46Al8Nb and Ti46Al8Ta have been HIPped (hot isostatically pressed) in the (α + γ) phase field in order to generate a fine convoluted microstructure and their tensile properties compared with those of samples with coarse lamellar microstructure. It has been found that the yield strengths and ductilities of the microstructurally refined samples were significantly improved with respect to those with coarse microstructures. The proof stresses of the microstructurally refined samples of the Ta- and Nb-containing alloys were almost identical (550 MPa) but the ductility in the Ta-containing alloy is about double that of the Nb-containing alloy, reaching values of up to 1.1% plastic strain. However, there is significant scatter in the elongation in both alloys, which in the case of the Ta-containing alloy has been shown to be associated with segregation which hinders the massive transformation and leads to the formation of some large grains. These observations are discussed in terms of the practicality of using massively transformed and heat-treated cast alloys in engineering components and in terms of the factors controlling the tensile behaviour.

Mechanical alloying (MA) followed by cold isostatic pressing (CIPing), sintering and hot isostatic pressing (HIPing) was used to prepare Mo-base silicide alloys consisting of a Mo(Si) solid solution and the intermetallic phases Mo 3 Si and Mo 5 SiB 2 . The microstructural development during milling of both binary Mo–Si and Mo–B and ternary Mo–Si–B powder mixtures was evaluated. While obtaining a supersaturated solid solution requires rather long milling times, domain size (which correlates with the grain size) is reduced to below 100 A after about 10 h of milling. After heat treatment and consolidation the microstructure reveals a continuous α-Mo matrix with embedded, uniformly distributed intermetallic particles. The establishment of such a microstructure is beneficial for the mechanical properties since, as compared to a discontinuous α-Mo matrix, it decreases the brittle-to-ductile transition temperature (BDTT) by about 150 K.

Molybdenum base silicide alloys exhibit promising oxidation resistance in addition to the inherent high temperature strength of refractory metals. However, alloys with sufficient oxidation resistance are effectively brittle up to temperatures above 816°C (1500°F). Recent progress in alloy and process development, utilising a PM manufacturing route with mechanical alloying as a crucial step, has allowed significant improvement of both oxidation resistance and mechanical properties via micro-alloying additions including nano-dispersed second phase oxide particles.

Mo–Si alloys containing up to 1 wt.% Si were fabricated by powder-metallurgical processing and their lattice parameters, elastic constants, densities, grain sizes, strengths, ductilities, and fracture toughness values were measured. The yield strength was insensitive to the grain size, i.e., a Hall–Petch relationship was not observed. Generally, Si additions caused pronounced solid solution strengthening. However, for small Si concentrations (≤0.1 wt.%) solid solution softening was observed at room temperature and below. With increasing Si concentration, the room temperature ductility and fracture toughness dropped precipitously. This is attributed to the increase in strength and a transition from transgranular to intergranular fracture.

A novel method to manufacture a refractory metal three phase Mo–2.7%Nb–8.9%Si–7.7%B at.% silicide alloy with mechanical alloying as the crucial processing step is presented. After consolidation a triplex ultra-fine grained microstructure with the average sizes of the Mo solid solution and both intermetallic compounds Mo3Si and Mo5SiB2 (T2 phase), respectively, ranging clearly in the sub-micrometer range was obtained. Tensile tests revealed structural superplasticity with strain to failures as high as 400% and temperatures as low as 1300 °C. Due to the extraordinary thermal stability of the microstructure isothermal annealing at 1700 °C for 10 h was needed for significant coarsening of the microstructure. A comparative assessment of the compressive creep properties of both microstructures in the temperature range between 1050 and 1315 °C showed an increase of the creep resistance of the coarsened microstructure by more than one order of magnitude in strain rate compared with the material in the as-processed condition.

Mechanical properties of a T6-AlSi6Cu4 cylinder-head alloy are investigated at 4 stress levels up to 545 K through tensile and compressive test to assess creep damage. The effect of over-ageing for 200 h at comparable temperatures was studied through RT hardness tests. Creep behaviour was characterised by a minimum creep rate and an extended steady state was not observed. No threshold stresses were apparent from minimum creep rate vs. stress plots at any of the temperatures studied here. Apparent stress exponents were both temperature and stress dependent and the apparent creep activation energy was 2–3 times the lattice selfdiffusion activation energy for aluminium. The Norton plot was best described by an exponential relation. The total strain to failure exhibited a maximum for a particular applied stress and decreases for both higher and lower applied stress. Creep life is well described by the Monkman– Grant relation at the highest temperature, but exhibits deviations from the idealised behaviour at the lower test temperatures.

Mo-Si-B alloys consisting of a Mo solid solution and the intermetallic phases Mo{sub 3}Si and Mo{sub 5}SiB{sub 2} were synthesized by mechanical alloying followed by sintering and hot isostatic pressing to field an ultra-fine (triplex) microstructure with all three phases being in the sub-micron range. at temperatures as low as 1300C the material exhibits superplasticity, with tensile strain to failures of 200-400% at strain rates as high as 10{sup -3}s{sup -1}.

The intermetallic trialuminide Al67Ti25Cr8 was strengthened by nano-scaled dispersions of up to 3 vol.% yttrium and aluminum oxide via mechanical milling. Subsequent powder consolidation, heat treatment, and thermo-mechani-cal treatment yielded grain growth to an increasing degree, resulting in grain sizes ranging from sub-μm level to about 0.5 mm. Consequently, the room-temperature yield strength σ 0 as determined by constant compression rate tests, increases due to three different hardening contributions, namely dispersion strengthening σ OR due to Orowan bypassing, fine-grain strengthening σ HP due to Hall-Petch relation, and, finally, as virtually always present, some solid-solution strengthening σ s. The analysis of the particle parameters was done using TEM as well as focused ion beam (FIB) imaging. The latter provides the opportunity to directly determine the mean radius r s of planar intersections with spherical particles and their nearest-neighbour spacing L since it displays a plane section through the 3-dimensional arrangement of dispersoids. The analysis of the strengthening contributions indicates that a Pythagorean superposition permits an appropriate modeling of the measured yield strength σ 0.

TEM sample preparation using Focused Ion Beam (FIB) methods becomes more and more interesting for microscopists because the technique allows for reliable and very efficient sample preparation. The first application of TEM sample preparation by FIB-cutting was reported more than 10 years ago. Meanwhile, a lot of experience has been gathered that allows one to discuss the capabilities and limits of the FIB technique in detail. Several TEM sample preparation techniques are known that all include FIB-cutting but differ in sample pre-preparation, sample handling, etc. This paper focuses on the actual FIB process, FIB tools are closely related to Scanning Electron Microscopes, but instead of an electron beam an ion beam (mostly Ga+ ions) is used to remove and deposit material.

Thin film and interface characterization are necessary in semiconductor industry to ensure high yields and the required reliability of the products. Particularly, the analysis of layer stacks is a challenging task. Transmission electron microscopy (TEM) essentially contributes to layer stack analysis in microelectronic products. Capabilities and limits of analytical TEM techniques as well as advanced TEM sample preparation techniques are discussed for front-end (MOS transistor) and back-end (interconnect) structures. Typical examples for advanced microprocessor devices are shown.

TEM imaging combined with electron diffraction and analytical methods like EDS and EELS becomes increasingly important in cross-sectional on-product characterization and failure analysis in semiconductor industry, mainly driven by the ongoing scaling-down of device structures. A new analytical technique - energy filtering transmission electron microscopy (EFTEM) - is used more and more in physical failure analysis. Improved TEM sample preparation techniques like the FIB-cut technique allow the reliable target preparation of defects even in the case that a defect has already been prepared for SEM analysis. The application of analytical TEM for failure analysis is demonstrated for particle issues, open electrical contacts in tungsten LI and degradation of AlCul interconnects.

Recently, it has been shown that the addition of a certain amount of ternary alloying elements like Cr transforms the tetragonal D022-structure of Al3Ti to the cubic L12-structure with higher crystal symmetry. However, this attempt of overcoming its brittleness at ambient temperature results in relatively poor creep strength at high temperatures. Therefore, the methods of mechanical alloying and high energy ball milling were used to prepare the ternary (Al,Cr)3Ti L12-intermetallic compound additionally strengthened by small volume fractions of incoherent Y2O3 dispersoids. The development of the microstructure and the phase formation were investigated by X-ray diffraction and transmission electron microscopy (TEM). First results on consolidated bulk samples are presented, indicating the possibility of producing oxide dispersion strengthened (ODS) intermetallics from mechanically milled powders.

We review the current development status of molybdenum borosilicide (Mo-Si-B) alloys for ultra-high temperature applications in air. The assessment of several ingot and powder metallurgy approaches revealed that (i) the presence of a continuous Mo solid solution matrix is mandatory for adequate low temperature toughness and (ii) wrought processing of such alloys at temperatures established for refractory metals requires the presence of an ultrafine (sub-micron) microstructure. Both prerequisites could be fulfilled using mechanical alloying (MA) as the crucial processing step , however, values for the ductile-to-brittle transition temperature (DBTT) below 800°C could not be obtained due to grain boundary embrittlement by Si segregation. First results on the effect of different microalloying additions on a reduction of this segregation exemplified for the Mo solid solution will be presented and discussed.

Extended abstract of a paper presented at MC 2007, 33rd DGE Conference in Saarbrücken, Germany, September 2 – September 7, 2007

The effect of different oxide particles on the microstructure and the high temperature creep behavior of ODS ternary L12-(Al,Cr)3Ti intermetallic compounds, which were prepared by high energy ball milling, was analyzed. Particular attention was paid to the development of the dispersoid size and distribution after the milling process, the consolidation and the heat treatment.X-ray diffraction (XRD) and transmission electron microscopy (TEM) of the as-milled powders indicate that the ordered matrix can nearly be amorphizied during milling. Moreover, independent of the type of oxide the dispersoids with a mean diameter ≤ 20 nm are homogeneously distributed in the metallic matrix. Using hot pressing under protective atmosphere the powders were compacted to nearly full density, followed by heat treatment. This results in grain sizes varying between 200 nanometers and several micrometers. However, due to the nature of the dispersoids used different particle diameters were found after consolidation and annealing. Although known to be the thermodynamically most stable oxide the initial Y2O3 powder has formed a mixed yttrium-aluminum oxide during manufacturing and, hence, significant particle growth has been observed. While CeO2 shows a similar behavior, the line compound A12O3 remains stable, but the average particle size of this oxide also increases during the heat treatment. First preliminary mechanical tests supported by TEM observations show the significant influence of the grain size on the creep resistance at high temperatures.

Mo-Si-B materials consisting of a Mo(Si) solid solution and the intermetallic phases Mo3Si and Mo5SiB2 (T2) were prepared by mechanical alloying (MA) as the crucial step of a powdermetallurgical process. After consolidation via an industrial processing route (cold isostatic pressing, sintering, hot isostatic pressing) the resulting microstructures of Mo-Si-B alloys up to 45% of intermetallic phases reveal a continuous α-Mo matrix with embedded, homogeneously distributed intermetallic particles. Clearly, increasing the amount of Mo solid solution reduces the BDTT (demonstrated by three point bending tests between room temperature and 1200°C), however, values below 900°C could not be obtained due to grain boundary embrittlement caused by Si segregation. Alloying with Zr was proven by Auger analysis in Mo-Si solid solutions to reduce this segregation. Therefore, in a second trial Zr as a (micro-) alloying element was added. The influence of microalloying on ductility and strength is comparatively discussed with reference compositions Mo-6Si-5B and Mo-9Si-8B.

Compared to Ti-rich γ-TiAl-based alloys Al-rich Ti-Al alloys offer an additional reduction of in density and a better oxidation resistance which are both due to the increased Al content. Polycrystalline material was manufactured by centrifugal casting. Microstructural characterization was carried out employing light-optical, scanning and transmission electron microscopy and XRD analyses. The high temperature creep of two binary alloys, namely Al60Ti40 and Al62Ti38 was comparatively assessed with compression tests at constant true stress in a temperature range between 1173 and 1323 K in air. The alloys were tested in the cast condition (containing various amounts of the metastable phases Al5Ti3 and h-Al2Ti) and after annealing at 1223 K for 200 h which produced (thermodynamically stable) lamellar γ-TiAl + r-Al2Ti microstructures. In general, already the as-cast alloys exhibit a reasonable creep resistance at 1173 K. Compared with Al60Ti40, both, the as-cast and the annealed Al62Ti38 alloy exhibit better creep resistance up to 1323 K which can be rationalized by the reduced lamella spacing. The assessment of creep tests conducted at identical stress levels and varying temperatures yielded apparent activation energies for creep of Q = 430 kJ/mol for the annealed Al60Ti40 alloy and of Q = 383 kJ/mol for the annealed Al62Ti38 material. The latter coincides well with that of Al diffusion in γ-TiAl, whereas the former can be rationalized by the instability of the microstructure containing metastable phases.

A 3-phase Mo-Si-B alloy consisting of Mo solid solution and the intermetallic phases Mo3Si and Mo5SiB2 (T2) was manufactured employing mechanical alloying (MA) as the crucial processing step. After consolidation via cold compaction, sintering in hydrogen atmosphere and final hot isostatic pressing (HIP) at 1500°C, one obtains an ultra-fine microstructure with a nearly continuous Mo(ss) matrix and the sizes of all phases being in the 1 micron range. Tensile tests were carried out in vacuum at initial strain rates ranging from 10-4 to 10-2 s-1 and the temperature varied between n1200 an 1400 °C. With a stress exponent of about 2 and the activation energy being close to that of Mo-self diffusion the material exhibits superplasticity at temperatures as low as 1300°C and tensile strain to failures up to 400%, thus, making sound wrought processing on industrial-scale facilities at temperatures typical for refractory metals and alloys feasible. To enhance creep resistance at high temperatures the alloys were annealed at 1700°C for 10h for a coarsening of the microstructure. While, still, the average sizes of all phases were below 10 microns, a considerable reduction in minimum creep rate was noted. This finding also demonstrates the extraordinary high thermal stability of this 3-phase Mo-silicide alloy.