Papers by Keyword: S-Phase

Abstract: We present a novel attempt to combine in-situ and ex-situ measurements -- here exemplary for aluminum alloys. For this research we have chosen an Al-1.7Cu-1.3Mg (at.\%) alloy, which has been cast from high purity elements (5N5 Al, 4N Cu and 4N Mg). DSC (differential scanning calorimetry) serves as a basic method, which is employed to determine different precipitation states towards the final S-phase formation: before the formation starts, at the maximum of the exothermal peak, and after the end of the exothermal reaction. By an abrupt truncation of the heating ramp (5 K/min), i.e. cooling quickly to room temperature, the current sample state is frozen-in for a defined temperature. After truncation all samples have been measured without further preparation by X-ray diffraction (XRD), positron annihilation lifetime spectroscopy (PALS). By this treatment we could correlate exactly different sample states, which is impossible by conventional experiments, i.e. heating to a defined temperature and holding for a certain time. This attempt opens new possibilities to investigate defined and comparable sample states by methods, which require extensive sample preparation, like TEM or 3DAP, and in-situ methods like DSC or XRD / EXAFS / SAXS at synchrotron beam lines.

Abstract: The effect of gas content and treatment temperature on the surface characteristics of hardened layers of low temperature plasma nitrided 316L Austenitic Stainless Steel was investigated. The process was performed at fixed processing time at 15Hr and changing the N₂ content from 10% to 25%, changing the treatment temperature from 370 °C to 430 °C. The surface hardness and the thickness of expanded austenite (S-Phase) layer increase with increasing both temperature and nitrogen content. At 430°C processing temperature and 25% N₂ content, the S-phase layer thickness increased up to 13 μm and surface hardness increased about 3.5 times of that of untreated sample which is 880 HV_0.1 (before treatment 250HV_0.1 hardness). However, the XRD pattern of the this sample shows the presence of γ'-(Fe,Cr)₄N precipitation on the surface which is the reason for the increased corrosion current density that results in increased corrosion rate. Thus the corrosion resistance degrades and presents even worst behavior comparing that of before treatment. Therefore, for increasing the corrosion behavior, further research was conducted by fixing the N₂ content at 10% with changing the CH₄ content from 0% to 5%. The best results were found when treated at 430°C with 5% CH₄ content. At this condition the S-phase thickness increase to around 17μm and surface hardness (980 HV_0.1) is improved about 3.9 times of the hardness of untreated one. This sample also showed superior corrosion resistance than the other treated samples and the untreated one due to much higher pitting potential.

Abstract: The development of SiAlON-based ceramics has shown great impact in the field of cutting/drilling tool industry and other engineering applications. It is highly desirable to cut-down the cost of the cutting tools by increasing their service lifetime. Potential ways to improve tool life is by preparing these SiAlON-based ceramics adopting non-conventional synthesis routes and by using different precursors. The present study reports the results of synthesis of SiAlON-based nano-ceramics via spark plasma sintering (SPS) technique. Generally, metal nitride and metal oxide precursors are used for synthesizing self-reinforced SiAlON ceramics. In this work, nano-sized metallic precursors including amorphous-Si₃N₄ and crystalline β-Si₃N₄, SiO₂, AlN and Al₂O₃ were used, which could be a novel way to synthesize SiAlONs at low temperatures with enhanced performance. The properties of these SiAlONs are tailored by optimizing the synthesis parameters. The synthesized samples were characterized by X-ray diffraction and field emission scanning electron microscopy to study the effect of processing parameters on microstructure, density and hardness.

Abstract: This paper presents results of investigations on gas nitride layers of stainless steel. Steel with ferritic structure, containing different chromium and nickel content was used in the experiments in order to compare the influence of these two main alloying elements on layer growth kinetics and layer structure. Particular attention was paid to formation of Sphase and expanded martensite which usually are formed during lowtemperature nitriding of stainless steel. It was observed, that chromium and nickel content does not have a significant influence on layer growth kinetics but the treatment temperature does. It was also stated that Sphase formation is possible both in nickel alloyed and nickelfree steel depending on treatment parameters.

Abstract: The paper presents results of research on nitrided layers on Sandvik Nanoflex^TM precipitation hardened stainless steel. The influence of process parameters on nitriding kinetics and structure of the layers was investigated. The gas nitriding process was conducted in a mixture of ammonia 50% and products of its dissociation, as well as in 100% ammonia atmosphere at temperature range 400500°C and time between 2 and 8 h. The obtained diffusion layers were examined using the following methods: light and scanning electron microscopy, XRD phase analysis and EDS chemical analysis. Mechanical properties were tested with hardness measurements. It was found that kinetics depends on treatment temperature and nitrogen potential of the atmosphere. Moreover, treatment conditions affecting Sphase formation and expanded martensite in nitrided layers are discussed.

Abstract: The paper presents results of research on nitrided layers on precipitation hardened stainless steel, known also as 1RK91 (Sandvik NanoflexTM). Samples were subjected to low temperature nitriding. The influence of nitriding parameters on nitriding kinetics was investigated. The nitriding process was carried out in a mixture of NH3 50% and products of its dissociation as well as in 100% ammonia atmosphere at temperature range 425-475°C. To investigate the kinetics of nitrided layer formation, the nitriding time changes between 2 and 8 h. The obtained diffusion layers were examined using the following methods: light and scanning electron microscopy, XRD phase analysis. The distribution profiles of selected chemical elements were acquired using optical spectrometry GDOES.

Abstract: The results concerning gas nitriding of sintered stainless steel are presented in the paper. The samples made of 316L steel were gas nitrided at temperatures between 400-550°C. The microstructure of the layer was investigated by means of light and atomic force microscopy. The phase composition was identified using X-ray diffraction. Moreover, tribological and corrosion properties of the samples were evaluated. It was demonstrated that in case of gas nitriding it is possible to obtain nitrided layers also inside open pores, which can be beneficial for corrosion response of nitrided sintered austenitic stainless steel applied in corrosive environments.

Abstract: In this work, the influence of multi-step solution (MSS) treatments on the constituent particle dissolution, overheating and associated quench cracking behaviour in room temperature water-quenched 7150 Al alloy has been investigated. For comparison, the microstructure and quench cracking behaviour of single step solution treated samples water-quenched from 505°C were also investigated. Based on optical microscopy of differently quenched samples, the quench cracking mode and the influence of overheating of constituents on the quench cracking behaviour have been demonstrated. The results reveal that the constituent particles can be effectively dissolved in the MSS-505°C samples. When the quench temperature of MSS-505°C samples is equal to or higher than 485°C , macro quench cracks can be clearly observed. Moreover, the density and length of the quench cracks increase with increasing quench temperature. Etched microstructures indicate that the quench crack propagation mode is intergranular. However, for samples directly heated to 505°C , typical overheating can be observed at the triple junctions and these regions preferentially act as crack propagation routes.

Abstract: In this paper, (C,N) alloyed austenitic stainless steel coatings (or hybrid S-phase with both carbon and nitrogen) were produced by a magnetron sputtering deposition process combined with ion implantation (CMSII). This technique involves a periodical high energy ion bombardment of the coating during its growth, which has a beneficial effect on the structure and properties of the deposited coating. The influence of the nitrogen and carbon addition to the deposition atmosphere on the structure, chemical composition and coating morphology was investigated using Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), and Glow Discharge Optical Spectrometry (GDOS) techniques. Wear tests were carried out and the results were compared with low temperature plasma nitrided austenitic stainless steel.

Abstract: In this study, the plasma nitrocarburizing has been used to treat AISI 316 austenitic and AISI 410 martensitic stainless steels. Treated specimens were characterized by means of morphological analysis, surface microhardness measurement, and resistivity measurement. Plasma nitrocarburizing at low temperature (420°C) produced a single phase nitrided layer of nitrogen and carbon expanded austenite (S phase) on the specimen surface, which considerably improved the resistivity property of AISI 316 austenitic stainless steel.