Materials Science Forum Vols. 706-709

Abstract: The evolution of solidification structure in Incoloy800 with and without electromagnetic stirring (EMS) are studied to investigate its influence on the morphology of solidifying crystals and element segregation. The samples examination and EDX analysis show that some broken dendrite in the subsurface of the ingot were brought into the internal part of the ingot, it is proves the shear mechanism of EMS to dendrite or column crystal in the initial solidifying period, and the equiaxed grain region is formed in the internal part of ingots owing to the stirring effect of EMS. Both of them are helpful to reduce the element segregation in Incoly800 ingots.

Abstract: Over the past several years we have worked to develop tools to improve the quality of superalloy ingots produced by vacuum arc remelting (VAR) and electroslag remelting (ESR). Part of this work has focused on developing model-based process controllers that employ predictive, dynamic, low-order electrode melting and ingot solidification models to estimate important process variables. These estimated variables (some of which are not subject to measurement) are used for feedback and to evaluate the health of the processes. Modern controllers are capable of detecting and flagging various process upsets and sensor failures, and can take remedial action under some circumstances. Model-based variable estimates are continuously compared with measurements when available, and the residuals are used to correct the next generation of estimates. This technology has led to improved VAR and ESR melt rate controllers and is currently being used to develop a VAR ingot solidification controller. A first generation ingot pool depth controller has been tested on a laboratory VAR furnace and the results are very encouraging. In this test, a 152 mm diameter Alloy 718 electrode was remelted into a 216 mm diameter ingot, but the technology is easily scaled to industrial sizes. Successful development of this technology could allow for melting at higher powers without the formation of channel segregates (freckles) by stabilizing the ingot solidification zone. It may also allow for the production of larger diameter VAR superalloy ingots than is possible to produce with the current generation of VAR controllers for the same reason.

Abstract: The samples of single crystal superalloy DD6 were grit blasted and then heat treated in the temperature range of 1100-1250°C for 4h and the DD6 alloy ‘standard heat treatment’ in vacuum furnace, respectively. The results showed that cellular recrystallization occurred in the surface layer after heating at 1100°C for 4 hours. While equiaxed recrystallization grains occurred near the surface of the samples annealed at 1200°C for 4 hours, meanwhile, cellular recrystallization located between equiaxed recrystallization grains and the original region. With the improvement of the heating temperature, the size of cellular recrystallization decreased, while the size of equiaxed recrystallization grains increased, and the shape of the coarse γ′ phase in the cellular recrystallization changed from lamellar to equiaxial. Fully equiaxed recrystallization grains nucleated after standard heat treatment. Furthermore, the twins occurred in fully equiaxed recrystallization grains, and that the γ′ phase of the twin plane appeared different from that of the equiaxed recrystallization boundary. On the contrary, the twin formation was not observed in the cellular recrystallization grains. Therefore, the differences in twin behavior between the fully equiaxed recrystallization and cellular recrystallization grains were discussed.

Abstract: We have been studying the microstructure change of B2 cubic precipitates into an A2+B2 complex structure in Fe-Al-Ni alloy. In this study, we carried out detailed observation using focused ion beam (FIB) and scanning transmission electron microscopy (STEM). First, Fe-14.3at%Al-10.3at%Ni solid solution was prepared. Secondly, the specimens were heated at 1173 K, at which they formed B2 cubic precipitates (ordered bcc) dispersed in an A2 matrix (disordered bcc). After that, the B2/A2 two-phase specimen was annealed at 973 K. Then we fabricated STEM specimens using FIB, followed by high-resolution secondary electron imaging. We repeated this slice-and-observation procedure to determine the detailed microstructure of this heat-treated alloy. At the early stage of the 973 K annealing, the A2 phase appeared in the original B2 precipitates and showed a spongelike structure, whereas small nanometer-order B2 particles appeared in the A2 matrix. The A2/B2 interface at this stage showed no anisotropic morphology. Therefore, the main driving force of this process may not be strain energy, but chemical and interface energies. Further annealing at 973 K decreased the number of small B2 particles in the A2 matrix, and these particles dissolved into the matrix eventually. The annealing also changed the A2/B2 spongelike structure, which was observed in the original B2 precipitates, into simple structures such as the A2 core and B2 crust. Then the B2 phase showed ordinal coarsening behavior. When B2 precipitates, which had hollow cubic morphology, were observed to be very close to each other, the face-centered area of the B2 crust tended to dissolve and only large B2 precipitates remained.

Abstract: In the present study, the recrystallization and grain growth behavior of HR3C austenitic heat resistant steel were studied by scanning electron microscopy and electron backscatter diffraction (SEM/EBSD). The HR3C steel was forged into a plate of 12mm in thickness and then solution treated at 1150-1250°C followed by an annealing treatment at 650-1050°C for 1h. The results show that the recrystalization occurs during the annealing treatment when the steel is solution-treated at 1150°C. Increasing the solution temperature to 1200°C or above, the recrystalization will not occur during the same annealing treatment. EBSD measured results show that the solution temperature has a significant effect on grain boundary character, especially on density of the low angle grain boundaries within the austenitic grain. The mechanism of the recrystallization and effect of annealing treatment on precipitation behavior of second phase were analyzed.

Abstract: Throughout history, a substantial or often leap technological progress, was determined by the availability of new engineering materials, which usually stimulated the improvement in the quality of life. The development of engineering materials constitutes one of the most significant elements in the field of science and technology, and innovative policy in Poland and Europe as regards the knowledge-based economy (KBE). The enhancement of the product functional properties, required by the new strategies of engineering materials development, as well as materials processing technologies is frequently connected with appropriate formation of the engineering materials surface layer structure and properties. Functional properties of many products depend not only on the possibility of transferring the mechanical load across the whole element section or its physicochemical properties, but, most often, they depend mainly on the surface layer structure and properties. The application of foresight methodology in the assessment of the state and prospects regarding the development of technology related to the formation of the materials surface structure and properties provides the possibility to manage knowledge in this field. It can be achieved through a formalization of the explicit and tacit knowledge gathering and use in order to increase production competitiveness, effectiveness, and innovation, improve the quality of life and achieve sustainable development. The objectives accomplished in such a way contribute to the increase in intellectual capital by enhancing the effectiveness of using the existing knowledge, creating new knowledge, as well as increasing the level of understanding and disseminating knowledge. The implemented foresight project will result in identifying the priority innovative technologies and strategic research trends in the scope of surface structure and properties formation of engineering materials and biomaterials whose development in the country will be of key importance within next 20 years.

Abstract: Increasing the demand for cold rolled ultra thin strip as feedstock for miniaturized products has encouraged researchers to investigate the ways to increase the quality of such products, especially those related to strip surface roughness. Surface is known as quality factor in most of manufacturing processes. In this paper, the effect of the rolling parameters on the surface roughness transformation during metal rolling has been studied. The experimental results demonstrate that the surface roughness transformation during the metal rolling is highly affected by the designation of the processing parameters such as finishing temperature, reduction, rolling passes and lubrication. The results have been discussed to verify the validity of the new findings.

Abstract: To increase the shelf-life qualities of dairy products, a heat treatment is usually done. However, heat treatments induce physico-chemical modifications of the products. Some of them lead to the expected product but an unwanted consequence of this process is the formation of a fouling deposit on the surfaces in contact with the processed fluid. To eliminate fouling, cleaning processes have to be done once a day. It increases the processing and maintenance costs. To control and to decrease the fouling are the main problems in food industries and an active research is carried out on efficient antifouling surface treatments. In the present study, a 316L 2B stainless steel was submitted to different surface treatments (Flame and plasma pre-treatments, Plasma Enhanced Chemical Vapour Deposition, hydrophobic coatings, mechanical polishing ...) to try to establish correlations between different surface parameters (roughness, hydrophobicity, nanostructuration, surface energy, ...) onto the fouling in heat exchangers. All the treated plates were then submitted to a fouling test using an aqueous solution of β-lactoglobulin at 1% (p/p) with a final calcium concentration of 910 mg/L and compared to a bare steel plate. The results obtained imply different influences of each parameter depending on the surface roughness: the effect of a non organized micrometric roughness is preponderant compared to the surface energy: the fouling comes from a mechanical effect mainly due to rubbing. However, when the surface is nanostructured, fouling decreases. When the roughness reaches the nanometer scale (between 100 and 400 nm), it is the surface energy and the polar/apolar components which become preponderant compared to the roughness. Fouling is this time mainly due to the hydrophilicity of the surface and to the adsorption of the β-lactoglobulin on acido-basic sites. Finally, when the roughness reaches less than 50 nm, polar/apolar components show no effect anymore, the preponderant parameter is the hydrophobicity of the surface.

Abstract: The effect of dopants such as zirconium and nitrogen on the releasability of Y₂O₃-based ceramics from molds was investigated for integrated circuit packaging using epoxy molding compounds (EMCs). Co-doping of these elements was carried out by annealing the surfaces of 5mol% ZrO₂-Y₂O₃ samples under a N₂ flow at 1100-1300 °C, resulting in concentration of nitrogen near the surfaces of the samples. The adhesion strength was minimized by exposure at about 1200-1250 °C, which was less than half the value for the undoped Y₂O₃. The co-doping remarkably decreased the polar part of the surface energy and consequently hydrophobicity of the ceramic surfaces increased. The excellent releasability characteristics were likely related to the depression of dissociative adsorption of water molecules, which are considered to act as active sites for the adhesion of EMCs.

Abstract: Recently, a dual-cooled fuel (i.e. annular fuel) which is compatible with current operating PWR plants has been proposed in order to increase both power densities and safety margins. Due to the design concept that is compatible with current PWR plants, however, when compared with a current solid nuclear fuel it shows a narrow gap between fuel rods and needs to modify spacer grid shapes and their positions. Because a flow-induced vibration by fast primary coolant is inevitable phenomenon, it is necessary to examine the fretting wear behavior between an annular fuel and designed spacer grids. In this study, fretting wear has been performed to evaluate the wear resistance of the annular fuel by using specially designed spring and dimple of spacer grids that have a cantilever type and a hemispherical shape, respectively. At the spring specimen with relatively small stiffness value, fretting wear was initiated at both end regions and then proceeded gradually to center region. Based on the test results, the fretting wear behavior of annular fuel was compared with the current solid nuclear fuel and a comparative factor of its reliability was proposed.