Materials Science Forum Vols. 706-709

Abstract: The well-aligned growth structures which derive from directional solidification of ceramic eutectics are of great interests due to their potential use in electronic devices and as structural materials at high temperatures. Because of the complexity of the component system and very high melting points, the solidification behavior on the oxide ceramic eutectic is still unclear up to date. In the presented paper, the Al₂O₃-Y₂O₃-ZrO₂ ternary eutectic ceramic is remelted by a DTA apparatus. The maximal heating temperature is 1950 °C. The melting and solidification behavior are investigated by the DTA analysis. The solidification microstructure is investigated by scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and X-ray diffraction (XRD). The results show that solidus temperature and the liquidus temperature are 1738.4 °C and 1750.1 °C, respectively. The formation path of eutectic phase is discussed. The microstructure of as-solidified eutectic ceramic shows a divorced ternary eutectic structure consisting of Al₂O₃, YAG and ZrO₂ phases with a random distribution. Furthermore, the microstructural comparison with directionally solidified ternary eutectic ceramic is presented and discussed.

Abstract: ATI 718Plus^® alloy is a new, cast and wrought, Ni-base superalloy with a maximum use temperature approximately 55°C higher than alloy 718. The mechanical properties have been well characterized by turbine engine OEM’s and the alloy has been specified for use as static components and blades in gas turbine engines. Broader use of ATI 718Plus alloy in engine disk applications requires detailed understanding of the damage tolerance under creep and cyclic loading conditions. The results of a large testing program to evaluate the crack growth behavior of ATI 718Plus alloy at temperatures between 649°C and 704°C under conditions of fatigue, dwell-fatigue, and creep are presented. Crack growth rates in ATI 718Plus alloy in this temperature range are lower than alloy 718 and comparable to Waspaloy under non-dwell-fatigue conditions, and comparable to alloy 720 in dwell-fatigue tests.

Abstract: The broad application of platinum and platinum alloys results from their excellent high temperature mechanical properties as well as their near-perfect oxidation resistance and their outstanding resistance to corrosive attack by metal oxide melts such as fluxing agents. However, a few basic rules must be adhered to if platinum devices are to give satisfactory service. Platinum is susceptible to “platinum poisons” and other impurities. These “poisons”, the reasons for their corrosive effects, and techniques for minimizing or eliminating their influence are outlined.

Abstract: The efficiency of gas turbine engines can improved by an increase of the working temperature. As a consequence Allvac® 718Plus™ was developed to enhance the high temperature properties. Since the performance of this alloy is strongly related to the microstructure the knowledge of the softening processes is important to develop precise microstructure evolution models. Specimens were deformed at different temperatures (950°-1050°C) and strain rate (0.1s^-1 – 10s^-1) to strains of 0.2-1.5. The microstructures obtained were analyzed by electron backscatter diffraction (EBSD) in the scanning electron microscope to investigate the softening mechanisms at the respective forming conditions.

Abstract: Mo-Si-B alloys have been received an attention due to the high temperature strength and phase stability. However, the nature of poor oxidation resistance often limits the application of the alloy system. In order to resolve the poor oxidation resistance of the alloy system, in this study, the oxidation behaviors of Si diffusion coated Mo-Si-B alloys have been investigated in order to identify the underlying mechanism for the effect of the constituent of the phase combination of Mo-Si-B alloys. The oxidation tests performed at 1100 °C show that the produced MoSi₂ phase, as a result of the coatings, give an excellent oxidation resistance at prolonged high temperature exposure in air. The oxidation behaviors of uncoated and Si coated Mo-Si-B alloys have been discussed in terms of microstructural observations during oxidation tests.

Abstract: A so called multiphase separation method (MPSM) is proposed to quantitatively separate precipitated phases from their surrounding matrix phase in chemistry for bulk alloy/steel samples under EPMA/SEM-EDS measurement conditions. Applied examples to comparisons of the results through the MPSM with the values either cited or obtained via other analytical means relevant are indicative of the feasibility, accuracy as well as applicability of the MPSM, which deal with chemistry, amount, lattice parameter, elemental partitioning, atomic-site occupancy and stability of precipitated phases of either superalloy or heat-resistant steel samples analyzed. Successful applications of the MPSM not only show a significant improvement for difficulties in accurate quantification in phase chemistry under the EPMA/SEM-EDS measurement conditions but also provide with a useful and helpful tool to determine some other important physical quantities in alloys and steels, which make it possible to quantitatively and more widely evaluate structure-property relationships of the materials investigated through analyzing their bulk samples under EPMA/SEM-EDS measurement conditions.

Abstract: The effect of recrystallization on the low cycle fatigue life of DZ4 directionally solidified superalloy was investigated for specimens with three different recrystallized layers, which were generated by shot peening (0.1MPa, 0.3MPa and 0.5MPa respectively) and a subsequent annealing heat treatment. The fatigue life showed a decrease for recrystallized specimens with shot-peening of 0.1 MPa and 0.3 MPa, and an unusual increase for that of 0.5MPa, in comparison with the original DZ4 specimen. In-situ SEM observations were performed on the short crack growth behaviors for both original and recrystallized specimens, which revealed the fracture mechanism and the interaction with microstructure. Quantitative analysis of fatigue crack growth rates rationalized the influence of recrystallization on the low-cycle fatigue life of DZ4.

Abstract: In order to evaluate the microstructure and mechanical properties of cross roll rolled Ni-30Cr alloy, comparing with conventionally rolled material, this work was carried out. The materials were cold rolled to 90% in thickness reduction by conventional rolling and cross roll rolling methods and subsequently annealed at 700°C for 30 min. After this work, in order to evaluate the grain boundary character distributions of the materials, electron back-scattered diffraction technique was introduced. The application of cross roll rolling was more effective to develop the microstructure and mechanical properties than those of conventional rolling. As a result, the grain size was significantly refined to 1.3 μm in conventional rolling and 0.6 μm in cross roll rolling, compared to initial material (30 μm), respectively. Also, these grain refinements directly affected an increase in mechanical properties. In the present study, we systematically discussed the relationship between grain size and mechanical properties in terms of an increase in effective strain.

Abstract: The goal of relating a local fatigue life approach with different microstructures requires the consideration of the main forging process dependent influence factors and their effect on grain size, grain shape, grain contiguity and others. The presented methodology shows the generation and use of a microstructural based evaluation method to link the grain-shape based texture and morphology to the low-cycle-fatigue behavior of superalloy 718. The developed microstructural based energy approach supports an alternative description of the microstructure and grain shape texture. Both, the morphology and the statistical distribution, although covering as-large-as grains, are assessed by two independent numerical parameters. A parametric link to local fatigue parameters was established using this alternative microstructural characterization technique. A prediction of the microstructural evolution during the forging process is already available at design stage for hot-forging of this nickel-base superalloy. In this regard the value of the first parameter e correlates with the mean grain size; the value of the second parameter b is affected by the local forging process history. This enables the lifetime assessment of local forging process at design stage using advanced forging simulation tools. This holistic approach of establishment an experimental based methodology from specimen tests, extensive companying metallographic evaluations, linking them with local forging parameters, implementing the supported microstructural parameters to forging simulation codes and calculating the local component lifetime closes the simulation chain for superalloy 718.

Abstract: By means of calculating stacking fault energy (SFE), measuring creep properties and contrast analysis of dislocation configuration, an investigation has been made into the influence of the stacking fault energy on the creep mechanism of the single crystal nickel-based superalloy. Results show that the alloy at 760¡æ has a lower stacking fault energy (SFE), and the SFE of the alloy increases with the temperatures. The deformed mechanism of the alloy during creep at 760¡æ is the cubical γ′ phase sheared by <110> super-dislocation which may be decomposed to form the configuration of (1/3)<112> super-Shockley partials dislocation plus the superlattice intrinsic stacking fault (SISF). The deformed mechanism of the alloy which possesses the higher SFE at 1070¡æ is the screw or edge super-dislocation shearing into the rafted γ′ phase. The SFE of the alloy at 980¡æ is intervenient between the ones of 760¡æ and 1070¡æ, the deformation mechanism of the alloy during creep is the rafted γ′ phase sheared by <110> screw and edge super-dislocations which may be decomposed into the configuration of (1/2)<110> partial dislocation plus APB.