Ulrich Krupp

The effect of grain-boundary-engineering-type processing on oxygen-induced cracking of IN718

Abstract Polycrystalline Ni-base superalloys are prone to time-dependent intergranular failure when the loading temperatures exceeds about 873 K. The fracture process is then controlled by oxygen grain-boundary diffusion followed by decohesion of the respective boundaries. It is shown that this kind of cracking for IN718 at 923 K in air can be reduced significantly by successive steps of deformation and annealing, which is known as grain-boundary-engineering processing. This illustrates the importance of the grain-boundary-character distribution with regard to this mode of failure, which is known as ‘dynamic embrittlement.’

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.

High-temperature oxidation of pure Fe and the ferritic steel 2.25Cr1Mo

The global pressure for recycling and ecological energy production increases steadily in combination with the demand of cost-effective application of materials. However, some severe corrosion problems, associated with the high internal/intergranular corrosion rates in boiler components need to be avoid. Some commercial boiler materials contain a Cr content of 0.55 (wt. (%)) - 2.25 (wt. (%)). This Cr concentration in the alloys is not sufficient for the formation of a complete dense Cr2O3 scale. Hence, high oxidation kinetics may result. In this study, pure Fe and the steel 2.25Cr1Mo were oxidized in laboratory air at 550 °C using a thermobalance system. The surface as well as the cross section of oxidized specimens were analysed using scanning electron microscopy in order to quantify several factors (e.g. surface finishing, cold working and grain size) on the overall oxidation kinetics. For alloys with low Cr content, a decreasing in the grain size leads to an acceleration of the oxidation rate by facilitating the oxygen diffusion along alloy grain boundaries leading to an inward oxide layer formation. The study of effects of surface finish and cold working yielded results revealing that the oxidation process is complex and comparison of results from different laboratories is difficult and should be done.

Effect of alloy grain size on the high-temperature oxidation behavior of the austenitic steel TP 347

Generally, oxide scales formed on high Cr steels are multi-layered and the kinetics are strongly influenced by the alloy grain boundaries. In the present study, the oxidation behaviour of an austenite steel TP347 with different grain sizes was studied to identify the role of grain-boundaries in the oxidation process. Heat treatment in an inert gas atmosphere at 1050 °C was applied to modify the grain size of the steel TP347. The mass gain during subsequent oxidation was measured using a microbalance with a resolution of 10-5 g. The scale morphology was examined using SEM in combination with energy-dispersive X-ray spectroscopy (EDS). Oxidation of TP347 with a grain size of 4 µm at 750 °C in air follows a parabolic rate law. For a larger grain size (65 µm), complex kinetics is observed with a fast initial oxidation followed by several different parabolic oxidation stages. SEM examinations indicated that the scale formed on specimens with smaller grain size was predominantly Cr2O3, with some FeCr2O4 at localized sites. For specimens with larger grain size the main oxide is iron oxide. It can be concluded that protective Cr2O3 formation is promoted by a high density of fast grain-boundary diffusion paths which is the case for fine-grained materials.

Influence of carbon concentration on martensitic transformation in metastable austenitic steels under cyclic loading conditions

Metastable austenitic steels reveal a partial martensitic transformation during monotonic and cyclic loading. The volume fraction of martensite formed depends strongly on the amplitude and frequency of plastic strain, specimen geometry, the temperature, grain size, and the chemical composition. Particularly the latter has been varied by a carburization/decarburization treatment applied to the three austenitic steels AISI 304, AISI 304L, and AISI 301 (corresponding German designations: X5 CrNi 18 9, X2 CrNi 18 11, X12 CrNi 17 7) in the present study. Fatigue testing of the carbon-modified alloys showed that the martensite formation manifests itself by cyclic hardening which increases clearly by lowering the carbon concentration and vice versa. Since the martensite content within a specimen can be measured easily by determining the magnetic volume fraction, in this study the possibility is evaluated to apply thin foils of austenitic steel on cyclically loaded components, so that the martensite content monitors the damage history of such a component.

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.

Mechanisms of short-fatigue-crack initiation and propagation in a β-Ti alloy

Abstract The microstructurally short-crack initiation and early propagation were studied on the metastable β-Ti alloy TimetalĽB in the solution heat-treated bcc β microstructure under symmetrical pull-push fatigue testing. By means of a finite-element treatment in combination with local displacement measurements applying a laser interferometric strain-displacement gauge (ISDG), it was shown that elastic anisotropy gives rise to high mechanical stresses at certain grain boundaries (GBs). Large-angle GBs were observed to be preferred sites for short-crack initiation. Two modes of fatigue crack initiation were found: one is crack formation along slip bands, often resulting in transgranular crack propagation; the second is intergranular cracking of GBs. Using electron back-scattered diffraction (EBSD), local crystallographic orientations were determined and hence the role of GB types in the process of short crack initiation and growth could be taken into account. On the basis of the experimental observations and measurements, the preferred crystallographic conditions for short crack initiation and growth were revealed. The ISDG system was applied to measure the local crack opening displacements of short cracks in order to characterize the dependence of the short-crack closure phenomena on the applied load.

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.

Strain-induced martensite formation in metastable austenitic steels with varying carbon content

Abstract Fatigue tests are performed on cylindrical, sheet and foil specimens to investigate the strain-induced martensitic transformation of austenatic steels. The tests set a variety of strain amplitudes and temperatures as well as various carbon concentrations for the investigated materaals in order to broaden the scope of the martensitic transformation and thereby its characterization. It is confirmed that there is a threshold value of plastic strain amplitude for the investigated materials above which the incremental increase in martensite volume fraction in the specimen under continued cyclic loading is of significance. Block-loading tests indicate that cycles at amplitudes below this threshold do not contribute to the austenite transformation. Thermodynamic calculations are applied in order to provide information on the stability of the material with regard to carbon content. It is shown that carbon concentration can be established by gas charging of the materials so that the driving force for tran...