Materials Science Forum Vols. 706-709

Abstract: Paper removed at author request

Abstract: The effect of electric current on the recrystallization texture evolution with the rolling direction both parallel and perpendicular to the current flow during electric current pulses (ECP) treatment was investigated. The results showed that the exerted current direction played a great role on the formation of recrystallization texture {111}<112> and Goss texture {011}<100> at the primary stage of recrytallization induced by ECP treatment. However, with the current density increasing, the effect of current direction on texture evolution almost could be ignored and the final texture components in the two cases all are Goss texture.

Abstract: Effects of magnetic field intensity on carbon diffusion in pure iron in the single-phase austenite region were investigated. Specimens of high purity iron (99.99%) were buried in an air-proof melting pot filled up with cementation agent, and respectively subjected to isothermal annealing at 930° for 25 min with a heating rate of 5°C /min, and then cooled in the furnace. A magnetic field with different intensity was applied during the whole heating, isothermal holding and cooling processes. The results showed that the magnetic field annealing obviously hinders the carbon diffusion in the direction perpendicular to the magnetic field direction and this effect increased with the enhancement of magnetic filed intensity.

Abstract: The yield strength and impact energy properties for martensitic steel fabricated by vacuum induction melting, is investigated. It is found that the addition of Ti can improve the yield strength property of the martensitic steel after reheat quenching process, which can be attributed to increase in precipitation hardening from formation of TiC precipitates in the martensitic matrix and a superfine sized (~8μm) grains in the martensitic structure. Moreover, the yield strength can be further enhanced by Mo addition, which can be ascribed to a large amount of freshly nano-sized (1-10nm) precipitates in the final martensitic structure. The experimental and theoretical results on the contribution of TiC precipitates to hardening of the martensitic steel are in excellent agreement. In addition, the impact toughness also has been improved along with yield strength followed by the heat treatment, which can be attributed to the grain refinement and high ratio of high-angle grain boundaries after Mo addition.

Abstract: Precipitation strengthening by fine Nb-rich particles represents an important element on the design of low carbon high strength steels. This is typically obtained on steel strips by thermal holding at temperatures above 600°C following the austenite to ferrite transformation. These conditions are beneficial to obtain a large precipitation of small Nb-rich precipitates. On the other hand, precipitation at lower temperatures, in a phase already hard, such as bainite, has been scarcely studied. In this work, the precipitation phenomena occurring during isothermal treatments following the austenite to bainite transformation at 450°C are investigated. For this purpose, two Nb-alloyed low carbon steels with and without silicon are studied and the evolution of the microstructure is determined by the use of transmission electron microscopy and followed by hardness measurements. The presence of a hardness peak is not detected until long isothermal times (150h). Preliminary atom probe tomography (APT) characterization provides insight on the possible presence of fine NbC precipitates at the peak hardness treatment. A comparison with a Nb-free alloy indicates a significant hardening effect of niobium on the bainitic structure.

Abstract: Inconel 718 is a nickel-chromium-iron superalloy which presents excellent mechanical properties at high temperatures, as well as good corrosion resistance and weldability. These characteristics can be optimized with an appropriate control of microstructural features such as grain size and precipitation. Precipitates of different nature can form in these alloys, i.e. γ’’ (a metastable metallic compound Ni₃Nb), γ’ (Ni₃(Ti, Al), carbides and/or δ phase (intermetallic Ni₃Nb). Aging treatments are usually designed to obtain the precipitation required in order to optimize mechanical properties. However, precipitation can also appear induced by deformation and therefore interfere with hot forming operations, such as forging. Under these conditions, precipitation may lead to an increase of the loads required to carry out the process. The aim of the work was the characterization of precipitation kinetics for Inconel 718. With this purpose, stress relaxation tests were carried out at temperatures ranging from 950°C to 800°C. Moreover, different amounts of deformation were applied to the samples, prior to stress relaxation, to evaluate the effect of this variable on inducing precipitation. Some samples were quenched at different relaxation times for metallographic evaluation. The results obtained through mechanical testing, together with a proper characterization of precipitation by Scanning Electron Microscopy, were the basis for obtaining precipitation-time-temperature (PTT) diagrams after different deformation conditions.

Abstract: An unprecedented investigation consisting of the association of X-Ray tomography and Scanning Electron Microscopy combined with Focus Ion Beam (SEM-FIB) is conducted to perform a 3D reconstruction imaging. These techniques are applied to study the non-isothermal creep behavior of close (111) oriented samples of MC2 nickel base superalloys single crystal. The issue here is to develop a strategy to come out with the 3D rafting of γ’ particles and its interaction whether with dislocation structures or/and with the preexisting voids. This characterization is uncommonly performed away from the conventional studied orientation [001] in order to feed the viscoplastic modeling leading to its improvement by taking into account the crystal anisotropy. The creep tests were performed at two different conditions: classical isothermal tests at 1050°C under 140 MPa and a non isothermal creep test consisting of one overheating at 1200°C and 30 seconds dwell time during the isothermal creep life. The X-Ray tomography shows a great deformation heterogeneity that is pronounced for the non-isothermal tested samples. This deformation localization seems to be linked to the preexisting voids. Nevertheless, for both tested samples, the voids coalescence is the precursor of the observed damage leading to failure. SEM-FIB investigation by means of slice and view technique gives 3D views of the rafted γ’ particles and shows that γ corridors evolution seems to be the main creep rate controlling parameter.

Abstract: Waspaloy is a polycrystalline nickel-base superalloy used in disc rotors for gas turbine engines. Waspaloy, like other superalloys, is strengthened through the formation of the γ’ precipitate phase. As this precipitate phase evolves with processing and thermal exposure, it is desirable to non-destructively monitor the precipitate microstructural evolution. Electrical resistivity was used as such a non-destructive monitoring technique for aging temperatures ranging from 600°C to 800°C and aging times ranging between 2min and 263.5h. In the nucleation regime, a Johnson-Mehl-Avrami type equation was fit to the electrical response. For the growth and coarsening regimes, a volume distribution of precipitates was fit to the measured electrical resistivity. These fitting techniques were facilitated by microstructural data obtained from SEM imaging, X-ray diffraction, and small angle neutron scattering (SANS) measurements. For both cases, the models showed an excellent fit to the measured electrical data, implying that electrical resistivity is a viable technique for non-destructively monitoring the precipitate phase in Waspaloy.

Abstract: Thermal Barrier Coatings are the main type of coatings used for protecting turbine blade surfaces and the surface of modern jet engines combustion chamber parts. Depending on the type of engine element, the coatings are produced as plasma-sprayed MeCrAlY bond-coats with a ceramic outer layer or as Pt-modified aluminide coatings with a ceramic EB-PVD-deposited layer. Currently, research is being conducted on the deposition of a new type of coatings consisting of bond-coats with mulitlayer structure. In the article, the results of the study on the obtainment of TBC's with multilayer structure are presented. To obtain the metallic bond-coat, the process of atmospheric plasma spraying and the out of pack aluminizing (VPA) method were combined. The coatings were deposited on the surface of Rene 80 nickel superalloy. The first layer of the coating was a plasma-sprayed MCrAlY bond-coat, on which a diffusion aluminide layer was deposited with out of pack method. On the bond-coat, a standard ceramic zirconium oxide (ZrO₂*20Y₂O₃) layer was deposited. The microstructure analysis, was conducted, using light and SEM microscopy. The phase and chemical composition analyses were done using EDS and XRD methods.

Abstract: The tensile properties of DD6 alloy with twist low angle boundaries were investigated at 760°C, 850°C and 980°C, and the misorientation angle of twist low angle boundaries of the specimens of the alloy varies from 0º to 12º. The results showed that when the angle of low angle boundaries is relatively small, it has a little effect on the tensile properties of the alloy. It is notable that effects of low angle boundaries on the tensile strength are weaker than those on the tensile elongation. The characteristic of quasi-cleavage, intergranular and dimple fracture were observed on the tensile rupture surfaces of the specimens, and the tensile fracture mechanism was discussed.