Materials Science Forum Vol. 879

Abstract: Ni-based superalloy single-crystal turbine blades are widely used in gas turbines for aircraft propulsion and power generation as they can be subjected to high service temperature and show high mechanical properties due to the almost total elimination of grain boundaries. Particularly in presence of complex geometry shapes, rare grains nucleating apart from the primary grain, become a serious problem in directional solidification, when characterized by high-angle boundaries with the primary grain, extremely brittle due the elevated amount of highly segregating elements and the absence of grain boundary strengthening elements. It is of fundamental importance analyzing the physical mechanisms of formation of stray grains, to understand which thermo-physical and geometrical factors highly influence their formation and to find possible ways to reduce the impact of the problem. In this paper, constrained dendrite growth and heterogeneous grain nucleation theories have been used to model the formation of stray grains in directional solidification of Ni-base superalloys. The study allows to derive the preferred locations of stray grains formation and the role played by the most affecting factors: (i) geometrical: angle of primary grain dendrites with withdrawal direction and orientation of the primary grain with respect to the side walls, responsible for the formation of volumes where the stray grain undercooling is lower than the undercooling of the columnar dendrite tip; (ii) process and alloy: thermal gradient ahead to the solidification front and alloy composition, influencing the columnar dendrite tip undercooling; (iii) wettability of foreign substrates, on which the stray grain undercooling strongly depends.

Abstract: In this study local texture of process vessels made of carbon steel cladded by protective layers of stainless steel by submerged arc welding (SAW) were investigated by neutron diffraction using the diffractometer STRESSSPEC at FRM 2 (Garching, Germany). Different samples were prepared: as welded and as welded plus relevant industrial heat treatment. Local texture measurements with a gauge volume of 3 x 3 x 2 mm³ of the three cladding layers (at depths of 2 mm, 5 mm and 7.5 mm) for each sample were determined. Texture results indicated that there exists an annealed cube component in all the studied samples. Based on the measured pole figures at each depth and sample, the calculated orientation distribution functions data were used to calculate the Young's modulus with respect to the main welding directions. The calculated local and bulk anisotropic Young’s modulus in depth is presented and discussed.

Abstract: The ferritic stainless steels are materials used in several segments due to the excellent combination of mechanical properties and corrosion resistance. The mechanical properties of these alloys are strongly dependent on the microstructural characteristics and crystallography texture. The aim of this experimental study is to investigate the roles of the grain size of the hot rolled sample on the development of the microstructure, texture and formability of ferritic stainless steel. The main elements of chemical composition of the steel under investigation were 16.0 %Cr, 0.021 %C, 0.024 %N and 0.35 %Nb. Coarse and fine grains samples were cold rolled up to 90% thickness reduction and annealed at 880°C with soaking time of the 24 s. The texture measurements were performed by Electron Backscattered Diffraction (EBSD) in the longitudinal section. The formability was evaluated by the R-value and planar anisotropy (Δr) in tensile tests. The final microstructure after annealed was more homogenous for smaller initial grain size sample. This condition was favorable to develop γ-fiber, with sharpness intensity in 111121 components. The highest R-value and smallest planar anisotropy was obtained for a {111}/{001} ratio around 5.37. On the other hand, coarser initial grain size sample had showed a heterogeneous microstructure and texture, performing badly in mechanical tests (anisotropy).

Abstract: Graphitized steels are claimed to perform excellent in machining processes. They therefore can be considered as environmental friendly alternatives to the widely used Pb-alloyed steels. Due to liquid metal embrittlement and in-situ lubrication Pb improves machinability in a narrow tool-chip interface temperature window corresponding to low machining speeds. Although graphite inclusions are also supposed to generate in-situ lubrication, the mechanism and the corresponding optimum working zone is not very clear. The present work applies a new test methodology (including in-situ tribology, analysis of material flow and chip formation, optimum working zone analysis) to investigate the effects of graphite inclusions on turning and drilling operations. A Pb-alloyed low carbon free-cutting steel and Pb-alloyed case hardening steel were used as reference steels.

Abstract: The objective of the present work was the development of a micro-pH meter for the determination of the pH value within bioreactors with a volume of up to 200 μl in total. Two different prototypes of optodes were designed and tested. In a first approach spectroscopic analysis of bromothymol blue in a micro-sized-channel structure was carried out utilizing glass fibers, enabling measurements in sample volumes down to the range of picoliters. In a second approach a different illumination system consisting of a RGB-sensor and a LED light source was used. Phenol red was successfully applied as the pH indicator for this setup.

Abstract: Thermally sprayed tungsten and chromium carbide-based coatings are widely applied in several industrial applications and in harsh environments where the tribological resistance is required, such as in many components for oil &gas or petrochemical industries. The microstructural stability of the coatings is a key feature for improving the working life of components exposed at high temperature during service. While WC and Cr₃C₂-based coatings are known to be suitable materials for low and high temperature, respectively, there is the need to know their behavior at intermediate temperature to make a proper selection in these cases. In the present study two different coatings, WC-10Co-4Cr and Cr₃C₂-25NiCr, were deposited by HVOF and analyzed comparing the as received conditions with the aged conditions after isothermal treatments. The effect of aging at temperatures in the range 250-400°C was evaluated focusing on hardness behaviour in function of aging time and temperature. Furthermore, wear tests measurements were performed, before and after aging, for evaluating the exposure effect on the tribological properties of both coatings.

Abstract: A precise description of the hot deformation behavior as well as determination of the stability conditions as influenced by temperature and strain rate is fundamental for the simulation of metal forming processes. In this work, a revision of various stability criteria of magnesium alloy WE54 is conducted. The study corresponds to own work and that of Lentz et al. and is based on compression tests at high temperature and high strain rates. Stability and processing maps were obtained using a variety of stability criteria, some based on the efficiency parameter η and others on the strain rate sensitivity parameter, m. This parameter is usually determined by fitting the curves strain rate, ε, versus stress, σ, by means of a potential equation named “power law” or by a polynomial of second or third degree, and calculating the slope of the logarithmic curve at each point using successive derivatives. This procedure is compared with one developed by us where all experimental points are fitted to a single hyperbolic sine equation of Garofalo type and then m and η are calculated for each ε and T using this equation. The maps obtained by one or the other method differ considerably. The predictions of these maps were contrasted with microstructural observations and conclusions on the deformation behavior of the alloy are reached.

Abstract: In the present work, 1 wt.% of (Ti,Nb)C carbide particles prepared by self-propagating high temperature synthesis (SHS) were introduced into a melt of a conventional P23 steel to obtain a reinforced material with improved creep properties. The as-cast material showed a eutectic type microstucture, indicating partial dissolution of these carbides in the melt. Inside the dendritic regions, a bainitic/martensitic structure similar to that of the unreinforced material was present. A significant refinement of the prior austenitic grain size was revealed in the reinforced material. Brinell hardeness measurements reveal an increase of hardness in the reinforeced material due to the addition of the carbides. High strain rate compression tests were perfomed at temperatures in the range 950 and 1250oC to determine the optimum forming conditions. Stability maps for a wide range of temperatures and strain rates were drawn. The optimum temperature for the reinforced steel is about 77 K higher than for the non-reinforced steel.

Abstract: Al-Li alloys have higher mechanical properties and more lightweight than other conventional aluminum alloys. Therefore , it is focused as a good material for weight reduction of industrial fields. However, since the Al-Li alloy are highly active and hard to cast, there has been limited research on casting. In this study, age-hardening behavior of Al-2.5mass%Li alloys cast into sand and metal mold were investigated. All alloys cast into Y-block shape sand mold, and then artificial aged after solution treated at 743K for 36ks. Because of difference in quantity of precipitation by metastable δ’(Al₃Li) phase, peak hardness of metal mold casting is higher than that of sand molds castings.

Abstract: Dynamic recrystallization in Ti-1100 was investigated. Ti-1100 is one of near α titanium alloys and contains Si for improving high temperature mechanical properties. Ti-1100 exhibits martensitic transformation by quenching into iced brine after solid solution treatment. Hereafter specimens subjected to quenching into iced brine and to cooling in air after solid solution treatment are called IBQ specimen and AC specimen, respectively. After tensile test at high temperature, IBQ specimen exhibits morphological change from lath structure to equiaxed structure, but AC specimen does not. It is indicated that dynamic recrystallization occurs during the tensile test of IBQ specimen. Effect of silicide on the dynamic recrystallization was investigated using two specimens: one included more silicide precipitates and the other less. The former specimen shows smaller recrystallized grains than the latter. It is indicated that the specimen including more silicides exhibits smaller recrystallized grains.