Materials Science Forum Vols. 706-709

Abstract: It was shown that in a 10% Cr martensitic steel enriched by boron this element tends to segregate within M₂₃C₆ carbides having the film-like shape and precipitated on the boundaries of prior austenite grains (PAG), mainly. It leads to a low value of Charpy V-notch impact toughness of 6 J/cm². These carbides are highly resistant against the spheroidizing. Only the tempering at 770°C leads to the final formation of M₂₃C₆ carbides having the equiaxed shape. Concurrently, this tempering strongly decreases boron segregation. As a result, the 10% Cr martensitic steel exhibits a high value of Charpy V-notch impact toughness of 260 J/cm².

Abstract: Differential scanning calorimetry (DSC) has been used to study the phase changes in samples of as-received Zr-2.5Nb pressure tube material by continuous heating and cooling. Two different heating rates (5 and 20°C/min) were used to heat the sample up to 1050°C. After a short time hold at 1050°C, all the samples were continuously cooled to 300°C at a rate of 20°C/min. On continuous heating, the DSC signals obtained showed two endothermic transitions. The low-temperature transition, occurring between about 500 and 650°C, is attributed to a thermal decomposition of metastable niobium-stabilized β-phase. The highertemperature transition, occurring between 600 and 950°C, is due to phase transformations of hcp α-Zr to bcc β-Zr, as previously confirmed in a companion study on the same pressure-tube material that was examined in-situ by neutron diffraction. The neutron diffraction results provided a positive identification of the two phases and also a quantification of the β-phase present in the sample at different heating temperatures, and thus provided a guide to extract the volume fraction of β-phase from the DSC signals obtained in this study. The DSC signals revealed only one exothermic transition which is correlated to the reverse transformation of β-Zr to α-Zr, as previously identified in the companion neutron diffraction study of the same pressure tube material.

Abstract: A new way to melt and refine recycled silicon powders from the wafer back grinding wastes was proposed. We designed a new equipment using induction heating for the fast melting and directional solidification of the recycled silicon powders, and investigated the feasibility of utilizing them as silicon feedstock for solar cells through lab-scale experiments on the melting and refining them. Silicon particles recovered from back grinding slurry were flake like and very fine, about 3um and was covered with very thin silicon oxide. Slag was observed on the top of the melt during melting, which was composed of oxygen, silicon and carbon which was believed to come from the graphite crucible. Complete melting and subsequent feeding of powders into the melt were prevented by the slag formed during melting, which could be solved by changing the melting atmosphere. Slow growth rate made the impurities such as Al, Ti, Fe, Zr segregate to the top of the ingot purifying the lower part. Over 5N purity was obtained in the middle and lower part of the ingot by the fast melting and directional solidification.

Abstract: Our report deals with investigation of evolution of AB₂ phase in three melts of new 9% Cr heat resistant steel with different chemical compositions during creep rupture tests at 650°C in the course of 10000 hours. For each sample the following research procedure was applied: (i) identification of all second phases precipitations using differential X-ray analysis method; (ii) study of spatial distribution of second phases using scanning electron microscope and transmission electron microscope; (iii) numerical investigation of mass fraction and average size of AB₂ particles using developed algorithm of statistical processing of images from electron microscope. It was found that for all melts mass fraction-time curve has global maximum and the rate of subsequent reduction of mass fraction depends on content of alloying elements.

Abstract: Thermal mechanical fatigue (TMF) was found to accelerate microstructure transformation in martensitic-bainitic Cr-Mo-V creep-resisting steels used in power generation. Particular role in this acceleration has been ascribed as being due to configurations of dislocations generated in the compression part of the TMF cycle, which dislocations in the tensile part of the cycle appeared to assist precipitation processes while important role in this acceleration is due to Bauschinger effect. Based on this finding an accelerated creep test (ACT) on Gleeble thermal mechanical simulator was developed to gain in a short time of less than 30 hours the microstructures and properties of steels and welds similar to these appearing after the long-term exposure in power plants. Investigations by scanning electron microscopy and transmission electron microscopy confirmed similarity of microstructure transformation after the ACT with this occurring after the multi-year real creep in power generating installations, in particular in these where premature failures occurred.

Abstract: Liquid Pb–Bi eutectic (LBE) alloy has been selected as coolant and neutron spallation source for the development of MYRRHA, an accelerator driven system (ADS). The alloy has been characterized in liquid state from melting (125 °C) to 750 °C by mechanical spectroscopy, i.e. internal friction (IF) and dynamic modulus measurements. The experiments have been carried out using hollow reeds of austenitic stainless steel filled with Pb-Bi alloy and sealed at the extremities. Dynamic modulus showed a remarkable drop in the range 350-520 °C. In the same temperature range radial distribution functions (RDFs), determined from X-ray diffraction patterns, evidenced variations of the mean distance between the 1st nearest neighbour atoms. The phenomenon has been explained as a structural re-arrangement of atoms in the liquid metal.

Abstract: Thin films of manganese oxides have been prepared by pulsed laser deposition (PLD) process on silicon and stainless steel substrates at different substrate temperatures and oxygen gas pressures. By proper selection of temperature and oxygen pressure during the PLD process, pure phases of Mn₂O₃, Mn₃O₄ as well as an amorphous phase of MnO_x were successfully fabricated and characterized by X-ray diffraction. The pseudo-capacitance behaviors of those manganese oxides of different phases have also been evaluated by the electrochemical cyclic voltammetry in 0.1 M Na₂SO₄ aqueous electrolyte. Their specific current and capacitance determined at different scan rates were calculated and compared. The results show that polycrystalline Mn₂O₃ phase has the highest specific current and capacitance, while the values for polycrystalline Mn₃O₄ films are the lowest. The amorphous phase MnO_x films have the values sitting in between those of Mn₂O₃ and Mn₃O₄. The specific capacitance of Mn₂O₃ films reaches 200 F/g at 1 mV/sec scan with excellent stability and cyclic durability. This work has demonstrated that PLD is a very promising technique for supercapacitor material research due to its excellent flexibility and capability of controlling microstructures and phases of various materials.

Abstract: Renewal of overhaul period of power plants equipment, especially superheater tubes, is important problem in energy sector of every country. Investigation of processes on tube’s surfaces is necessary to obtain information of equivalent temperature during maintenance. In this work the austenitic chrome-manganese steel 10Cr13Mn12Si2Ni2Cu2Nb (DI59) and chrome-nickel steel 12Cr18Ni12Ti were studied. Application of these steels provides the desired heating resource of the steam superheater (100000 h) and reliable exploitation of the boilers at high temperature (~650°C). Between the oxide layer and the base metal layers with different crystal structure compared with matrix are observed. Qualitative and quantitative electron microscopic techniques, optical metallography and X-ray phase analysis are employed to investigate the morphological evolution, elemental redistribution in the alloy system and formation and growth of discovered layers. In steel DI59 the discovered layer had a ferrite structure. In steel 12Cr18Ni12Ti it was first shown that this layer is consist of different sulfides which disposed in FeNi₃ matrix. This intermetallic phase and ferrite layer are ferromagnetic. Changes in the structure of steel on its surfaces are caused by high exploitation temperature and type of fuel which is used in power plants (combination of natural gas and mazut). This opens the possibility to use connection between thermal irregularity in metal and thickness of discovered layers in order to search most damaged part of tubes with ferrite meter.

Abstract: The main characteristic of cathodic copper is the concentration of impurities because it depends on mechanical characteristics, i.e. ductility, of the derived copper wires. The standard mechanical tests that evaluate their ductility, are quick elongation and spiral elongation tests. The results of these essais shown that there is not a clear correlation among the impurities content in cathodes and wire ductility. Also, the two mechanical tests actually used are not able to discriminate differences on the copper ductility associated to variations in impurities concentration, at ppm level. In this work, a design of new specimen for traction test, with a reduced gage length of 10 mm that clearly discriminate differences on copper ductility associated to variations in impurities concentration is exposed. In addition, from traction tests to copper wires and observation of their fracture surfaces by means of SEM and EDS it is concluded that the principal impurity affecting the ductility of the copper wires is the oxygen, mainly incorporated during the melting of the cathodes and casting of the rods. Also, the traditional annealing to the samples previous to the tensile tests must to be avoid since produces a dispersion on the matrix of the Cu₂O oxides and so the deleterious effect of the oxygen on the copper ductility cannot be detected.

Abstract: Based on previous results of both an increase of nearly 40% in static tensile strain by shortening fiber length from commercial 6.4 mm to 0.44 mm in an unsaturated polyester/styrene-butadiene GFRP-BMC composite containing 20 mass% short E-glass fibers and their acoustic emissions (AE), the fracture resistance mechanics of sub-mm length fiber dispersion reinforcement is proposed. Since the 40% strain increase acts to improve strength and toughness, the mechanics is useful. This paper aims to present the mechanism of strain-driven improvement where microcracks are prevented from propagating beyond the critical crack size (2a_c) for thermoset polymers, resulting in an increased and more dispersed total microcrack surface area as recorded by AE raising fracture strain.