H.-J. Christ

Microstructurally short fatigue crack initiation and growth in Ti-6.8Mo-4.5Fe-1.5Al

Abstract Microstructurally short fatigue crack initiation and growth was studied in single-phase titanium alloy Ti-6.8Mo-4.5Fe-1.5Al (TIMETAL®LCB) by means of the electron back-scatter diffraction (EBSD) technique. The evolution of surface cracks was traced by interrupting fatigue testing to obtain the details of the crack initiation and growth process. Cracks were found to initiate preferentially either at slip bands or grain boundaries (GBs) during cyclic loading, both of these two types of cracking being usually associated with GB constraints. EBSD examination showed that high-misorientation-angle conditions are favorable for crack nucleation. An elastic–plastic incompatibility mechanism is proposed to account for the crack initiation behavior. Furthermore, short crack growth behavior was found to be closely related to the misorientation between the grains involved, the GB direction and the loading direction with respect to the crack plane. The most favorable conditions for the transmission of a short crack from one grain to another were: (i) the operative slip plane in the next grain lies at a low angle with respect to the crack plane; (ii) the angle between the surface trace of the operative slip plane (or GB) in the expected cracking grain and the loading axis is close to 90°. In addition, the crack growth behavior was found to be influenced by the interaction between short cracks.

Internal Nitridation of Nickel-Base Alloys. Part I. Behavior of Binary and Ternary Alloys of the Ni-Cr-Al-Ti System

The internal-nitriding behavior of several modelalloys of the Ni-Cr-Al-Ti system in an oxygen-freenitrogen atmosphere at 800-1100°C was studied.Thermogravimetry as well as various metallographic techniques (SEM and TEM) were used. It wasshown that both the nitrogen solubility and the nitrogendiffusion coefficient are strongly affected by the Crcontent of the Ni alloy. Hence, in Ni-Cr-Ti alloys a higher chromium content leads to an increaseddepth of the internal precipitation of TiN. Nitridationof the alloying element Cr takes place only at highconcentrations of Cr. In general, the nitridation rate was found to obey Wagners parabolic ratelaw of internal oxidation. Changes in the parabolic rateconstant with alloy composition can be understood bymeans of thermodynamic calculations in combination with microstructural observations.

Thermomechanical fatigue - Damage mechanisms and mechanism-based life prediction methods

An existing extensive database on the isothermal and thermomechanical fatigue behaviour of high-temperature titanium alloy EVII 834 and dispersoid-strengthened aluminum alloy X8019 in SiC particle-reinforced as well as unreinv conditions was used to evaluate both the adaptability of fracture mechanics approaches to TMF and the resulting predictive capabilities of determining material life by crack propagation consideration. Selection of the correct microstructural concepts was emphasised and these concepts were, then adjusted by using data from independent experiments in order to avoid any sort of fitting. It is shown that the cyclic /-integral (δJeff concept) is suitable to predict the cyclic lifetime for conditions where the total crack propagation rate is approximately identical to pure fatigue crack growth velocity. In the case that crack propagation is strongly affected by creep, the creep-fatigue damage parameter δCF introduced by Riedel can be successfully applied. If environmental effects are very pronounced, the accelerating influence of corrosion on fatigue crack propagation can no longer implicitly be taken into account in the fatigue crack growth law. Instead, a linear combination of the crack growth rate contributions from plain fatigue (determined in vacuum) and from environmental attack is assumed and found to yield a satisfactory prediction, if the relevant corrosion process is taken into account.

High temperature oxidation of mechanically alloyed Mo–Si–B alloys

Abstract Nowadays, even fourth generation nickel base superalloys are approaching their fundamental limitation, the melting point. Hence, a further increase in efficiency, i.e. of jet engines, can only be realised by developing new materials for the use at temperatures beyond 1200°C. A new alloy concept using the Mo–Si–B system for ultrahigh temperature applications is discussed. Those alloys have melting points ∼2000°C, while retaining good mechanical properties and oxidation resistance in the desired temperature range. A three phase Mo–9Si–8B alloy (composition in at.-%) consisting of α-Mo, Mo3Si and Mo5SiB2 (T2) was produced by powder metallurgical processing route. At temperatures higher than 1000°C in laboratory air, a protective SiO2/B2O3 glass layer develops on the alloy surface giving excellent oxidation resistance. However, in the temperature range between 700 and 900°C, non-protective and highly volatile molybdenum oxide cause the disintegration of the material (the so called pesting phenomenon). Additions of Zr and La2O3 to the Mo–Si–B alloy systems were investigated to improve the performance of the alloys in the pesting temperature range. The oxidation kinetics was determined by means of thermogravimetric analysis and discontinuous oxidation experiments. Microstructural examinations were performed by means of optical and scanning electron microscopy in combination with energy dispersive X-ray spectroscopy. The microstructural observations were compared with the theoretical prediction of phase stability using computational thermodynamic calculations. A significant improvement of the alloys during oxidation in the pesting temperature range was found. The rate of formation of molybdenum oxides could be drastically reduced at intermediate temperature range. At high temperatures (>1000°C), a homogeneous and protective SiO2 oxide layer was formed on the alloy surface leading to a slow growing oxide scale.

Formation and compensation of internal stresses during internal nitridation of nickel-base alloys

Abstract The interactions between the internal nitridation behavior of Ni-base alloys, the precipitation-induced internal stresses in the near-surface area and the formation of metallic surface protrusions were investigated by exposing several alloys of the system Ni–Cr–Al–Ti to oxygen-free nitrogen atmosphere at 800–1100°C. Depending on alloy composition, exposure time and temperature, internal Cr, Al, and Ti nitrides precipitate, which are of a high specific volume as compared with the base metal. As a consequence of this volume increase, internal compressive stresses are generated giving rise to an outward diffusional flux of those metallic elements that do not form nitrides. As an explanation of the mechanism of this metal diffusion, which relieves the internal stresses and leads to the formation of surface protrusions, Nabarro–Herring creep as well as dislocation pipe diffusion controlled creep are discussed.

Effect of hydrogen on mechanical properties of β -titanium alloys

Conflicting opinions exist in the literature on the manner in which hydrogen influences the mechanical properties ofβ-titanium alloys. This can be attributed to theβ-stabilizing effect of hydrogen in these materials leading to major changes in the microstructure as a result of hydrogen charging. The resulting (extrinsic) effect of hydrogen on the mechanical properties can possibly cover up the direct (intrinsic) influences.On the basis of experimentally determined thermodynamic and kinetic data regarding the interaction of hydrogen withβ-titanium alloys, hydrogen concentrations of up to 8 at.% were established in three commercial alloys by means of hydrogen charging from the gas phase. In order to separate intrinsic and extrinsic effects the charging was carried out during one step of the two-step heat treatment typical of metastableβ-titanium alloys, while the other step was performed in vacuum.The results on the single-phaseβ condition represent the intrinsic hydrogen effect. Monotonic and cyclic strength increase at the expense of ductility with increasing hydrogen concentration. The brittle to ductile transition temperature shifts to higher values and the fatigue crack propagation threshold value decreases. The microstructure of the metastable, usually two-phaseβ-titanium alloys is strongly affected by hydrogen, although the extent of this effect depends not only on the hydrogen concentration but also on the temperature of charging. This microstructural influence (extrinsic effect) changes the mechanical properties in the opposite direction as compared to the intrinsic hydrogen effect.

Contribution of Lattice Distortion to Solid Solution Strengthening in a Series of Refractory High Entropy Alloys

We present an experimental approach for revealing the impact of lattice distortion on solid solution strengthening in a series of body-centered-cubic (bcc) Al-containing, refractory high entropy alloys (HEAs) from the Nb-Mo-Cr-Ti-Al system. By systematically varying the Nb and Cr content, a wide range of atomic size difference as a common measure for the lattice distortion was obtained. Single-phase, bcc solid solutions were achieved by arc melting and homogenization as well as verified by means of scanning electron microscopy and X-ray diffraction. The atomic radii of the alloying elements for determination of atomic size difference were recalculated on the basis of the mean atomic radii in and the chemical compositions of the solid solutions. Microhardness (μH) at room temperature correlates well with the deduced atomic size difference. Nevertheless, the mechanisms of microscopic slip lead to pronounced temperature dependence of mechanical strength. In order to account for this particular feature, we present a combined approach, using μH, nanoindentation, and compression tests. The athermal proportion to the yield stress of the investigated equimolar alloys is revealed. These parameters support the universality of this aforementioned correlation. Hence, the pertinence of lattice distortion for solid solution strengthening in bcc HEAs is proven.

Prehistory effects on the VHCF behaviour of engineering metallic materials with different strengthening mechanisms

Engineering materials often undergo a plastic deformation during manufacturing, hence the effect of a predeformation on the subsequent fatigue behaviour has to be considered. The effect of a prestrain on the microstructure is strongly influenced by the strengthening mechanism. Different mechanisms are relevant in the materials applied in this study: a solid-solution hardened and a precipitation-hardened nickel-base alloy and a martensite-forming metastable austenitic steel. Prehistory effects become very important, when fatigue failure at very high number of cycles (N > 107) is considered, since damage mechanisms occur different to those observed in the range of conventional fatigue limit. With the global strain amplitude being well below the static elastic limit, only inhomogeneously distributed local plastic deformation takes place in the very high cycle fatigue (VHCF) region. The dislocation motion during cyclic loading thus depends on the effective flow stress, which is defined by the global cyclic stress-strain relation and the local stress distribution as a consequence of the interaction between dislocations and precipitates, grain boundaries, martensite phases and micro-notches. As a consequence, no significant prehistory effect was observed for the VHCF behaviour of the solid-solution hardening alloy, while the precipitation-hardening alloy shows a perceptible prehistory dependence. In the case of the austenitic steel, strain-hardening and the volume fraction of the deformation-induced martensite dominate the fatigue behaviour.

Effect of shot peening on high temperature oxidation behaviour of boiler steel: experimental results and simulation

Abstract A prediction of the service life of boiler steels in aggressive atmospheres requires a full understanding of the degradation mechanisms of the material due to high temperature oxidation. The developed model is a useful tool to simulate such degradation processes under complex conditions and hence, to contribute to new mechanism based life prediction methods. In the present study the effect of shot peening on the oxidation behaviour is studied and given importance to model the behaviour and simulate the effect. Thermogravimetric measurements were carried out by using a microbalance with a resolution of 10–5 g at 750°C. The scale morphology was examined by using scanning electron microscopy (SEM) in combination with energy dispersive X-ray (EDX) and X-ray diffraction (XRD). The oxidation of the alloys was modelled by a mechanism based computer simulation that has been developed in order to predict the corrosion rate of alloys depending on the chemical composition and microstructure. The computer simulation is based on the numerical Crank–Nicholson solution for the diffusion differential equation in combination with the powerful thermodynamic subroutine ChemApp. The program deals with the grain boundary diffusion and bulk diffusion in a different way and calculates the individual localised equilibrium state using ChemApp with the help of parallel processing units. Considering the effect of the grain size of the substrate, the developed model gives the distribution of alloying elements, oxide phases, the oxidation kinetics and the concentration of penetrating oxygen in a two-dimensional manner. The numerical description of oxide formation is in agreement with experimental observations.

Effect of dynamic embrittlement on high temperature fatigue crack propagation in IN718 – experimental characterisation and mechanism-based modelling

At elevated temperatures, the nickel-base superalloy IN718 is prone to the failure type ‘dynamic embrittlement’. Dynamic embrittlement is assumed to be driven by tensile stress-controlled oxygen grain boundary diffusion. Oxygen embrittles the grain boundaries in front of a crack and results in a fast brittle intercrystalline crack propagation. In order to reveal the mechanism of dynamic embrittlement, high temperature fatigue crack propagation tests were carried out at 650 °C in vacuum and air applying various dwell times and testing frequencies. The crack growth was monitored by the ACPD technique and a far-field microscope. The observations show that at low stress intensity factor ranges, crack propagation mainly occurs in the unloading and loading parts of the cycle and only minor crack propagation takes place during the dwell time. With increasing dwell time, the contribution of the crack propagation at constant stress increases. A mechanism-based model was developed on the basis of these findings which allows for a quantitative description of the effect of dynamic embrittlement on fatigue crack propagation rate. The results of respective simulations correspond very satisfactorily to the experimental data. Hence, the model is suitable for lifetime assessment.