Solid State Phenomena Vol. 186

Abstract: The present work summarises the results, which were obtained from studies carried out on the structure of the nitride and nitride-oxide surface layers with use of the electron transmission microscopy. The layers were formed using glow discharge technique at relatively low temperature (300°C). It has been shown that low temperature nitriding or nitriding/oxiding process produced a thin layer ~30 nm thick. They were formed from titanium nitride as well as titanium oxides. The structure revealed that nanoparticles were surrounded by high amount of amorphous phase. Especially, electron microscopy was useful method for studying the phase boundary between the layer and the NiTi matrix. During deposition process, which was carried out at temperature above 300°C, the intermediate layer of Ni3Ti intermetallic phase appeared between titanium oxides and/or nitrides. Lowering deposition temperature down to 300°C or below resulted in absence of such sublayer. Moreover, thickness, structure of layers, absence of sublayer formed during glow discharge process, can significantly influence deformation during inducing of the shape memory or superelasticity effect.

Abstract: This paper presents the influence hydrogen effect in structure of surface layers on Ti-6Al-4V two-phase titanium alloys. It was described that hydrogen can be used as a temporary alloying element. The paper regarding the HTM - Hydrogen Treatment of Materials as an attractive method for controlling structure and improving final mechanical properties of titanium alloys. Hydrogen treatment consists of the three stages: hydrogenation in hydrogen gas atmosphere (600°C), cyclic heat hydrogen treatment (250°C), dehydrogenation in vacuum (550°C). This treatment uses purposeful, temporary introduction of hydrogen into the alloy to generate defects in its microstructure. Furthermore, it then causes recrystallization of defect microstructure during the dehydrogenation stage to grain refinement. This high-temperature treatment is very efficient for changing the microstructure and mechanical properties of two-phase titanium alloys.

Abstract: The investigated material was a commercial CW111A (CuNi2Si) copper alloy containing nominally 1.6–2.5 wt% Ni and 0.4–0.8 wt% Si. The thermo-mechanical treatment consisted of three-stage forging beginning at 900°C and immediate quenching followed by ageing at 500°C for 5 hours bringing about a balance of strength and electrical conductivity suitable for application of the alloy as electrical connectors. The main strengthening phase in the CW111C (CuNi2Si) copper alloy is the β-Ni3Si phase.

Abstract: Ferromagnetic shape memory alloys (FSMA) are relatively new smart materials group. Recently, new FSMA from NiMnX (X=Sb, Sn, In, Co+In) systems are considered as alternative to the well known NiMnGa alloys. Four alloys of the following compositions: Ni₄₃Mn₃₅Co₄In₁₈, Ni₄₁Mn₃₅Co₄In₂₀, Ni₄₂Mn₃₅Co₅In₁₈, Ni₄₀Mn₃₅Co₅In₂₀ were studied in order to determine microstructure, phase composition and martensitic transformation temperatures versus their chemical composition. Structure of the alloys was studied by optical and transmission electron microscopy (TEM). All of the studied alloys showed macrostructure consisting of radially oriented columnar grains in the direction perpendicular to the casting axis. The structure of the phases occurred in the studied alloys depended on the cobalt and indium content. For the alloys containing 20 at. % of In at room temperature only L2₁ parent phase was observed whereas for those containing 18 at. % of In either single phase 14M modulated martensite or mixture of 14M martensite and L2₁ parent phase were seen. DSC measurements showed in studied alloys single-state martensitic transformation. Decrease In content of 2 at.% caused about 80°C fall of martensitic transformation temperatures. Curie temperature T_c increases of 20°C with 1 at% rise of the cobalt content.

Abstract: Soldering of joints in electronic circuits is performed in all branches of electronic industry. At temperatures below 13.2°C, an allotropic transformation of white β-tin into gray α-tin called tin pest may occur, leading to the degradation of mechanical properties or even a total disintegration of the alloy. Presence of some chemical elements in the alloy can inhibit the transformation, while other can promote it, e.g. a significant Pb addition prevents the transformation. However, the Restriction of Hazardous Substances Directive adopted by the European Union since 2006, limits Pb amount to 0.1 wt.%, leading to tin pest phenomenon in tin rich materials. The energy dispersive X-ray spectroscopy (EDXS) in scanning electron microscope was performed on samples of tin-rich lead-free alloys subjected to accelerated low temperature stress and after months of storage at -18 °C showed the tin pest occurrence. Much higher oxygen content in EDXS spectrum was revealed at old regions of transformed α-tin than at new α-tin and non-transformed β-tin regions. The tin pest oxidation is much accelerated in comparison to β-tin and can finally lead to the consuming of the whole tin in deteriorated places by the tin oxide.

Abstract: Rapid solidification (RS) of Al-2Fe-2Ni-5Mg alloy and following mechanical consolidation of powders by means of powder metallurgy (PM) methods was used with success to produce a bulk RS-material. RS powders were manufactured using an inert gas atomizing of the molten alloy and the spray deposition on the rotating water-cooled copper roll. Rods of 7 mm in diameter were received by means of the cold pressing of the flakes, vacuum degassing and hot extrusion method. For comparison purposes, the conventionally casted and hot extruded Al-2Fe-2Ni-5Mg alloy was tested as well. Mechanical properties of as-extruded materials were examined at 293 K – 873 K by compression tests performed at constant true strain rate of 5·10^-3[s^-1]. It was found that relatively high strength of as-extruded RS/PM material was accompanied by the high ductility of the samples deformed by hot compression tests. It was noticed that the most effective solution strengthening due to particles refining was observed at low deformation temperatures. Rising the test temperatures above ~ 420 K, was found to result in reduction of the flow stress to the values received for the industrial material (IM).The formation of coarse primary intermetallic compounds, which is typical for IM material, was effectively reduced for RS material. However some inhomogeneity of fine precipitates distribution in RS/PM material was observed. Nevertheless, it was considered that both solid solution hardening due to Mg addition and the dispersion strengthening due to refining of intermetallic compounds substantially increase the mechanical properties of the RS/PM material.

Abstract: A new high chromium martensitic steel, VM12, was recently developed for advanced coal-fired ultra-supercritical steam power plants. A very important factor in maintaining high creep resistance during service exposure is its stable microstructure, in particular high dispersion particle strengthening, which slow down the recovery and softening processes of the matrix. Quantitative characterisation of the precipitates in the VM12 steel as received condition and after creep tests up to about 30000 h at 625 °C using TEM was carried out.

Abstract: The paper presents research on the microstructure of high chromium martensitic GX12CrMoVNbN9-1 (GP91) cast steel. The steel was subject to investigation in the as-cast state, after hardening as well as after quenching and tempering. The microstructure and morphology of precipitates in GP91 cast steel was examined in terms of changes taking place during the technological process. It has been shown that the examined cast steel is characterized by a microstructure of lath martensite (and polygonized ferrite) with numerous precipitates, such as: M3C, M23C6 and MX (NbC, VX) of diverse morphology. Tempered martensite in the cast steel consists of former austenite grains separated by high angle grain boundaries. These grains are subdivided by high angle boundaries into blocks of martensite laths. The dislocation density of the tempered martensite is high (of about 1014 m-2).

Abstract: Addition of some active elements such as yttrium, cerium, lanthanum and other rare earths elements (REE) to austenitic stainless steels helps to improve their high temperature oxidation resistance and tribological properties. The high intensity plasma pulses were used to introduce Ce and La into AISI 316L austenitic stainless steel. The plasma pulses contained both ions/atoms of Ce-La and those of the working gas. The pulse energy densities were sufficient to melt the near surface layer of the steel and introduce those elements into the surface layer. Scanning electron microscopy (SEM) as well as energy dispersion spectroscopy (EDS) was used during each one part of surface characterisation. Obtained results allowed us to make decision about directions of modified material successive investigations.

Abstract: The T24 steel belongs to a new group of bainitic steels introduced currently to the power industry. Higher requirements connected with applying higher steam parameters in power units are the reason why the low-alloy steels used so far can no longer be applied. Therefore, new T24 steel has been developed in Europe on the basis of 10CrMo9-10 (10H2M) steel, which has been used in the power industry for many years, as a result of modification in its chemical composition. This modification consisted in introducing additions and microadditions of titanium, vanadium, boron and nitrogen into the base steel. As a result of the modification the new-found T24 steel is characterized by higher mechanical properties in comparison with the base steel, which allows to use the steel for tight shields in the new supercritical power units. The material for research was low-alloy bainitic T24 steel. Samples for examination were taken from a pipe section of the following size: outside diameter 44.5mm, wall thickness 7mm. Samples were isothermally aged in the air atmosphere, at the temperature of 580°C and at times up to 12 000 hours. Changes in the microstructure were observed and recorded by means of high-resolution electron microscope, JOEL JEM 3010. Identification of the precipitates was made using carbon extraction replicas and thin foils with the SAED method. The aim of research was the analysis of precipitation processes. They are extremely important in the context of long-term service and maintaining strength parameters above the minimum level. The tests were performed on T24 steel for the as-received condition (after heat treatment) and after 12 000 hours of ageing at the temperature of 580°C. The research made it possible to determine the morphology of precipitates. It also allowed to establish the sequence of precipitation process for the examined steel.