Materials Science Forum Vols. 783-786

Abstract: The effect of long-term creep at 600°C under 137 MPa on the microstructure of a P92-type steel was investigated. The microstructure after tempering consisted of laths with an average thickness of 400 nm. Dispersion of secondary phases consists of M₂₃C₆ carbides with an average size of 85 nm located mainly on lath, block and prior austenite boundaries and MX carbonitrides with average size of 31 nm homogeniously distributed throughout. Creep with duration of 40738 hours led to coarsening of M₂₃C₆ carbides up to 182 nm. Precipitation of Laves phase with an average size of 290 nm took place in both grip and gauge portions of ruptured specimen. Vanadium-rich MX particles were replaced by particles of Z-phase with sizes of 97 and 48 nm after long-term creep and aging, respectively. The average misorientation of the lath boundaries was approximately 2° and scarcely varied during creep, while the mean lath thickness increased to 890 nm in gauge section of ruptured specimen and remained essentially unchanged in the grip section. Dislocation density decreased slightly under long-range aging and creep.

Abstract: Automotive components for injection systems are subjected to load spectra with up to 1E9 load cycles during the expected service life. However, fatigue testing with such a large number of cycles using original components is extremely time-consuming and expensive. A contribution for fatigue reliability assessment is available by the application of specimen testing and the transfer of the results to components including the verification by component spot tests.In this contribution very high cycle fatigue results in laboratory air and in ethanol fuel using notched specimens of high strength stainless steel are discussed. The influence of testing frequency was studied using ultrasonic and conventional test techniques. The validation and transfer of these accelerated testing results to components is one of the main challenges for a reliable fatigue designing.KeywordsVery High Cycle Fatigue (VHCF), automotive components, fuel injection, bio-fuels, corrosion fatigue, testing concepts, fatigue design concepts

Abstract: New aluminum alloys with unique mechanical and electrochemical properties have been developed in response to a strong demand for new technologies tailored to the expanding hydrocarbon multi-stage fracturing market. The novel alloys disclosed in this paper purposely possess an augmented degradability in water-based oilfield fluids, an unusually high hardness and high compressive strength following proper processing while withstanding thermal aging in wellbore environments. In highlighting major findings from years of research in developing new alloys, this paper overviews major characteristics of these novel alloys from microstructural development, processing, mechanical properties, and electrochemical response in oilfield fluids such as salinated water (brine), acids, and cross-linked fracturing fluids.

Abstract: Three Ni-Cr-Mo alloys with 140ksi minimum yield strength, namely 625Plus, 718, and 945X were tested in selected sour environments to evaluate and rank their environmentally-assisted cracking susceptibility. The testing revealed that none of the alloys, stressed to 90% of their actual yield point, cracked as a result of an agressive 30-day NACE level VII-modified environment (401 ± 9 ^oF, 500 psia H₂S, 500 psia CO₂ and 151,700 mg/liter chlorides). In a complementary attempt to rank the alloy cracking susceptiblity, quasi-static strain rate (SSR) tests per NACE TM0198-2004 were conducted in a modified H₂S-saturated NACE TM0177 Solution A. Following a critical analysis of parameters such as time-to-failure (TTF), percentage reduction in ductility (%ε), percentage reduction of cross-sectional area (% RA) and fractomicrographs, 625Plus was confirmed to outperform its counterparts while some evidence of hydrogen embrittlement was found on 945X.

Abstract: The use of oak sawdust as carbon precursor for SiC manufacture via reactive infiltration was studied. The effect of oak sawdust pressing parameters, temperature and pressure, on the final SiC's properties was studied. Final product's quality was evaluated through density measurement, and microstructure and pore size distribution variations were characterized with optical microscopy and mercury porosimetry. Pressed oak sawdust preforms were carbonized to obtain a carbon porous preform which was then infiltrated with melted silicon. Successful infiltration of preforms pressed at room temperatures were performed, to obtain a porous SiC. Hot pressed preforms were not satisfactory infiltrated due to a narrowing of their pores caused by SiC formation's expansiveness.

Abstract: Over the past three decades a lot of effort was made to optimize the chemical compositionof 9% Cr martensitic steels, aiming to increase the operating temperature up to 923K and thus im-proving the efficiency of thermal power plants. Under these service conditions (high temperature andstress exposure), the creep strength of such steels is closely related to the long term stability of theirmicrostructure. The time to rupture can also be understood as an equivalent to the time of microstruc-ture deterioration. Optimization of the initial microstructure and understanding of the microstructureevolution during creep exposure are therefore decisive to improve the creep behavior of 9% Cr steels.Selected chemical compositions of MarBN steels (Martensitic 9% Cr steels strengthened by Car-bides, Nitrides and Boron) were subjected to different heat treatments to produce an optimized mi-crostructure to improve the creep rupture time. The initial microstructure before creep exposure wasinvestigated using optical microscopy, SEM and EBSD. Short term creep rupture tests at 923K and150MPa were performed, followed by systematic microstructure investigations.Comparative EBSD investigations confirm an optimized microstructure for creep exposure, pro-duced by an appropriate heat treatment. From comparative creep test results, it can be concluded thatadvanced microstructures increase the time to rupture of the selected MarBN steels by more than 10percent, without reduction of the ductility.

Abstract: A hybrid model of microstructural evolution of a coupled multi–field system that is subjected to ion irradiation is presented. Materials exposed to low energy ion irradiation experience a wide range of radiation effects, e.g. surface roughening and chemical segregation. The hybrid model combines Monte Carlo methods and a phase field model to simulate the kinetic and radiation-induced processes that lead to radiation induced chemical segregation with associated phase transformations of a binary system by differential diffusivity.

Abstract: An experimental approach has been used to investigate the charateristics of silicon type powders using heat source of thermal plasma. In this paper, laboratory scale furnace (0.0529m³) using non-transferred hollow thermal plasma torch (150kW) is applied for the rapid melting and the impurities components volatilization. The components and the crystallization behavior of silicon type raw material for complete melting using in-flight melting technique with continuous powder feeding (500~1000 g min^-1) into the furnace are investigated by changing the melting atmosphere. The determination of impurities absorption and erodability of the furnace caused by the extremely high localized tempeartures of plasma heat source and molten material is also followed.

Abstract: The relation between fragility and cooperativity of atomic motion in bulk metallic glass forming liquids is studied based on the bond strength-coordination number fluctuation model. The model describes the temperature dependence of the viscosity in terms of the mean values of the bond strength, coordination number and their fluctuations of the structural units that form the melt. According to the model, the cooperativity increases with the increase of fragility. The model estimates that the magnitude of the cooperativity N_B extends approximately from 7 to 60 structural units, depending on the material. The temperature dependence of N_B for different metallic glass forming systems reveals that N_B increases with the decrease of temperature. The relation between N_B and diffusivity of atoms is discussed briefly.

Abstract: The effect of B₂O₃ fluxing on the glass-forming ability (GFA), the structure and the soft magnetic properties of Fe (-Co)-B-Si-Nb bulk metallic glasses (BMGs) has been investigated. The large Fe-Co-B-Si-Nb BMG specimens with diameters up to 7.7 mm, which is approximately 1.5 times as large as that of the maximum diameter of the copper mold cast one (= 5 mm), were prepared by the fluxing and water quenching. Thus the GFA of the Fe-Co-B-Si-Nb BMG are improved by the fluxing. It was confirmed that the soft magnetic properties of the Fe-Co-B-Si-Nb BMG are also improved by the fluxing. On the other hand, it was found for the Co-free Fe-B-Si-Nb BMG that the B₂O₃ fluxing promotes the precipitation of the α-Fe (-Si) and Fe₂B phases in the central region of the specimens; i.e., the GFA of the Fe-B-Si-Nb BMG is decreased by the fluxing. The Fe-B-Si-Nb BMG specimens show a flat hysteresis loop, indicating a good linear relationship between the magnetic induction and the applied magnetic field. These results of the Fe-B-Si-Nb BMG show that it is possible to develop a new soft magnetic material that exhibits constant permeability, which is necessary for producing inductors and choke coils.