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E. J. Mittemeijer - One of the best experts on this subject based on the ideXlab platform.
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microstructure and kinetics of Nitride Precipitation in a quaternary iron based model fe 2 82 at pct cr 0 13 at pct mo 0 18 at pct v alloy
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2015Co-Authors: C W Kang, Sai Ramudu Meka, R E Schacherl, E. J. MittemeijerAbstract:Internal Nitride development in iron-based quaternary Fe-Cr-Mo-V alloy, as a model alloy for 31CrMoV9 steel, was investigated by performing controlled gaseous nitriding experiments. The Nitride-Precipitation process starts with the development of nanosized platelets of, coherent, cubic NaCl-type Nitride, along {100} lattice planes of the ferrite matrix, in association with matrix-lattice dilation. The development of Nitride platelets having a NaCl-type crystal structure, satisfying the Baker–Nutting orientation relationship with the ferrite matrix, and the nitrogen content of the Nitrided zone suggest the development of a quaternary “mixed” (Cr x ,V y , Mo1−x−y)N Nitride, similar to the development of “mixed” ternary Nitrides as reported for Nitrided Fe-Cr-Al and Fe-Cr-Ti alloys. In a later stage, the Nitride platelets undergo discontinuous coarsening resulting in the development of a lamellar microstructure consisting of Nitride and ferrite lamellae. Kinetic analysis demonstrated that the thermally activated nature of growth of the diffusion zone is controlled with about equal weights, by the diffusion of nitrogen in the substrate matrix and the matrix lattice solubility of nitrogen.
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Molybdenum-Nitride Precipitation in Recrystallized and Cold-Rolled Fe-1 at. pct Mo Alloy
Metallurgical and Materials Transactions A, 2013Co-Authors: H Selg, R E Schacherl, Thomas Waldenmaier, E. Bischoff, S. R. Meka, E. J. MittemeijerAbstract:Nitriding of recrystallized and cold-rolled Fe-1 at. pct Mo alloy at 853 K (580 °C) in a NH_3/H_2 gas mixture leads to the formation of cubic nanometer-sized Mo_2N-type precipitate platelets. These platelets obey a Baker–Nutting orientation relationship with the ferrite matrix. After prolonged nitriding, micrometer-sized colonies of lamellae consisting of a hexagonal MoN-type Nitride and ferrite develop in a discontinuous Precipitation reaction. These Nitride lamellae have a Burgers-type orientation relationship with the ferrite lamellae. As compared to the recrystallized specimens, in the cold-rolled specimens, the Precipitation of the initial Mo_2N-type platelets occurs much faster and moreover leads to incoherently diffracting precipitates; upon continued nitriding, a much earlier but only partially occurring transition of Mo_2N-type to MoN-type precipitates is observed. The results indicate that incorporation of iron in the Nitrides can occur, if at all, only up till a negligible level, thereby invalidating earlier data.
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molybdenum Nitride Precipitation in recrystallized and cold rolled fe1 at pct mo alloy
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2013Co-Authors: H Selg, Sai Ramudu Meka, R E Schacherl, E. J. Mittemeijer, Ewald Bischoff, Thomas WaldenmaierAbstract:Nitriding of recrystallized and cold-rolled Fe-1 at. pct Mo alloy at 853 K (580 °C) in a NH3/H2 gas mixture leads to the formation of cubic nanometer-sized Mo2N-type precipitate platelets. These platelets obey a Baker–Nutting orientation relationship with the ferrite matrix. After prolonged nitriding, micrometer-sized colonies of lamellae consisting of a hexagonal MoN-type Nitride and ferrite develop in a discontinuous Precipitation reaction. These Nitride lamellae have a Burgers-type orientation relationship with the ferrite lamellae. As compared to the recrystallized specimens, in the cold-rolled specimens, the Precipitation of the initial Mo2N-type platelets occurs much faster and moreover leads to incoherently diffracting precipitates; upon continued nitriding, a much earlier but only partially occurring transition of Mo2N-type to MoN-type precipitates is observed. The results indicate that incorporation of iron in the Nitrides can occur, if at all, only up till a negligible level, thereby invalidating earlier data.
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crystal structure and morphology of mixed cr1 xalxn Nitride precipitates gaseous nitriding of a fe 1 5 wt pct cr 1 5 wt pct al alloy
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2009Co-Authors: A R Clauss, R E Schacherl, E. J. Mittemeijer, Santosh S HosmaniAbstract:The crystal structure and morphology of Nitride precipitates developing in the matrix of a Fe-1.5 wt pct Cr-1.5 wt pct Al (Fe-1.6 at. pct Cr-3.1 at. pct Al) alloy upon gas nitriding were investigated. To this end, the Nitrided zone was studied using metallographic methods, X-ray diffractometry (XRD), electron probe microanalysis (EPMA), microhardness measurement, and transmission electron microscopy (TEM), including high-resolution TEM (HRTEM), and scanning TEM (STEM). Furthermore, a nitrogen-absorption isotherm was determined, for use in characterizing the nature of the Nitride-Precipitation process. It could be shown that the expected equilibrium Nitrides, cubic CrN and hexagonal AlN, do not develop. Instead, mixed Cr1–x Al x N Nitride precipitates of the cubic, rock-salt structure type develop upon nitriding the ternary alloy. These precipitates obey a Bain-type orientation relationship (OR) with the ferrite matrix and are associated with a considerable uptake of excess nitrogen and a very pronounced hardness increase.
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Nitride Precipitation and coarsening in fe 2 23 at v alloys xrd and hr tem study of coherent and incoherent diffraction effects caused by misfitting Nitride precipitates in a ferrite matrix
Acta Materialia, 2008Co-Authors: N Vives E Diaz, R E Schacherl, E. J. Mittemeijer, Santosh S HosmaniAbstract:Abstract Specimens of Fe–2.23 at.% V alloy were Nitrided in a NH 3 /H 2 gas mixture at 580 °C. The Nitrided microstructure was investigated by X-ray diffraction (XRD), and by conventional and high-resolution transmission electron microscopy ((HR)TEM). For specimens homogeneously Nitrided for relatively short times no separate VN reflections developed but instead sidebands associated with ferrite reflections, most pronouncedly for the α-Fe-2 0 0 reflection, appeared. The diffractograms measured for the different specimens were interpreted as the result of coherent diffraction of the Nitride platelets with the surrounding ferrite matrix, which is tetragonally distorted: the distorted ferrite matrix and the Nitride platelets are represented by a single body-centered tetragonal lattice, whereas the remaining part of the ferrite is described by a body-centered cubic lattice. Analysis of the microstructure of the Nitrided specimens using (HR)TEM confirmed the existence of very tiny VN platelets, coherent with the surrounding matrix. Annealing at elevated temperatures (up to 750 °C) after nitriding led to (moderate) coarsening of the Nitride precipitates. The coarsening is associated with the occurrence of local disruptions/bending of lattice planes in the VN platelet. This effect causes the VN platelets to appear segmented in the diffraction–contrast images. The specific changes in the X-ray diffractograms, as function of the stage of aging, could be consistently described as consequences of the transition from coherent to incoherent diffraction of the Nitride platelets with reference to the surrounding ferrite matrix.
John W Simmons - One of the best experts on this subject based on the ideXlab platform.
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effect of Nitride cr2n Precipitation on the mechanical corrosion and wear properties of austenitic stainless steel
Isij International, 1996Co-Authors: John W Simmons, B S J R Covino, Jeffrey A Hawk, J S DunningAbstract:High-nitrogen austenitic stainless steels are thermally unstable and susceptible to Nitride Precipitation (primarily Cr2N) during elevated temperature exposure. This paper describes the effect of Cr2N Precipitation on the mechanical, corrosion, and abrasive wear properties of a high-nitrogen austenitic stainless steel, nominally Fe-19Cr-5Mn-5Ni-3Mo-0.02C-0.7N. In the annealed state, Cr-rich Nitrides (Cr2N) precipitate sequentially as intergranular, cellular, and finally, intragranular precipitates. Cold working, prior to aging, increases grain boundary and intragranular Precipitation kinetics, but retards cellular phase formation. Nitride Precipitation has only a minor influence on the yield and ultimate tensile strength of annealed materials, but causes embrittlement which is enhanced by high strain rate conditions such as impact testing and by the accelerated pricipitation kinetics associated with prior deformation. Nitride Precipitation results in sensitization which is also accelerated by prior deformation. The degree of sensitization, as measured by electrochemical potentiokinetic reactivation (EPR) testing, correlates to intergranular, cellular, and intragranular Cr-depletion. Although the degree of sensitization is defined by both the Cr-minimum and width of Cr-depleted region, a Cr level below approximately 14 wt% is required for sensitization. Precipitation also results in a greater corrosion susceptibility as determined by potentiodynamic polarization scans. The wear behavior of the 0.7 wt% N alloy is little affected by the Precipitation of Cr2N since the volume fraction and size (compared to the abrasive) of the Cr2N precipitates is insufficient to alter the wear resistance of the alloy.
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overview high nitrogen alloying of stainless steels
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 1996Co-Authors: John W SimmonsAbstract:Abstract High-strength austenitic stainless steels can be produced by replacing carbon with nitrogen. Nitrogen has greater solid-solubility than carbon, is a strong austenite stabilizer, potent interstitial solid-solution strengthener, and improves pitting corrosion resistance. Although the solubility of nitrogen in liquid iron is very low, 0.045 wt.% at 1600 °C and atmospheric pressure, nitrogen levels above 1 wt.% can be obtained through alloying and specialized high-pressure melting techniques. An austenitic stainless steel should be considered “high-nitrogen” if it contains more nitrogen than can be retained in the material by processing at atmospheric pressure; for most alloys, this limit is approximately 0.4 wt.%. This article describes melting and processing technologies applicable to high-nitrogen steels and the effects of interstitial nitrogen on a variety of material properties. Thermal stability, Nitride Precipitation kinetics, and the effects of Nitride formation on mechanical properties and corrosion resistance are discussed.
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influence of Nitride cr2n Precipitation on the plastic flow behavior of high nitrogen austenitic stainless steel
Scripta Metallurgica Et Materialia, 1995Co-Authors: John W SimmonsAbstract:Aging at 700 °C results in grain boundary Nitride Precipitation only, while aging at 900 °C results in grain boundary and cellular Precipitation (40 vol%). These thermal treatments have a small but positive effect on YS, no effect on the UTS, but dramatically reduce the ability of the material to deform under localized plastic deformation (necking), leading to reduced tensile ductility. The plastic flow behavior of all annealed and aged high-nitrogen samples were modeled using the modified Ludwik relation (equation 2). Below the UTS, grain boundary Nitride Precipitation at 700 °C has no measurable effect on the plastic flow behavior of the material, and the modeling parameters n1, K1, n2, and K2 have values which do not deviate significantly from the behavior of the un-aged material. Cellular Precipitation, caused by aging at 900 °C, does significantly affect the plastic flow behavior of the material at both low and high strains. Cellular Precipitation causes both increased strengthening of the matrix in the low strain regime (0.001 < e < 0.03) and systematic decreases in the strain hardening exponent (n1) and strength coefficient (K1) with increased aging. The rate of strain hardening (dσde) measured from the σ-e plots is unaffected by isothermal aging and Nitride formation.
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sensitization of high nitrogen austenitic stainless steels by dichromium Nitride Precipitation
Corrosion, 1994Co-Authors: John W Simmons, D G Atteridge, J C RawersAbstract:Abstract High-nitrogen (N) stainless steels (SS) are receiving increased attention because of their strength advantages over carbon (C)-alloyed materials, but they have been found susceptible to dichromium Nitride (Cr2N) Precipitation during thermal exposure between ∼ 600°C and 1,050°C. Sensitization susceptibility of a high-N, low-C austenitic SS by Cr2N Precipitation at 700°C and 900°C was determined using the single-loop electrochemical potentiokinetic reactivation (EPR) test. High-N SS was found susceptible to sensitization caused by grain boundary (GB) Precipitation of Cr2N, with the degree of sensitization increasing systematically with aging time at 700°C. Sensitization of high-N materials did not require the concomitant Precipitation of chromium (Cr)-rich metal carbide (M23C6). Materials aged at 900°C were not sensitized, although the rate of Precipitation was greater than at 700°C. This indicated the minimum Cr level in the Cr-depleted zone of the matrix associated with Nitride Precipitation at 9...
Andras Mucsi - One of the best experts on this subject based on the ideXlab platform.
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analysis of interaction between recrystallisation and Nitride Precipitation in cold rolled al killed low carbon steel
International Journal of Microstructure and Materials Properties, 2015Co-Authors: I Felde, Andras MucsiAbstract:In this paper the conditions of interaction between Nitride Precipitation and recrystallisation in a low carbon steel have been investigated. On cold rolled steel specimens thermoelectric power tests and microstructural investigations have been performed in order to analyse the conditions of the so–called retardation phenomenon occurring during recrystallisation. On the basis of experimental investigations it was concluded that the precipitated Nitrides affect recrystallisation only in that cases, if the precipitated fraction is more than 45–50% regardless to the treatment temperature. Retardation in recrystallisation during isothermal heat treatments was observed only if the precipitated fraction of Nitrides was between 50% and 90% independently of the temperature of heat treatment. The rate of recrystallisation process is also strongly affected by the Nitride particles.
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effect of hot rolled grain size on the Precipitation kinetics of Nitrides in low carbon al killed steel
Journal of Materials Processing Technology, 2014Co-Authors: Andras MucsiAbstract:Abstract The Precipitation of Nitrides plays a general role in the industrial processing of deep drawing quality Al-killed low carbon steels. In this paper, the effect of hot rolled grain size on the Precipitation of Nitrides has been analysed. To evaluate the effect of grain size on the Nitride Precipitation kinetics, thermoelectric power based investigations have been performed on hot and cold rolled specimens. In the hot rolled state, the Precipitation of Nitrides occurs more intensively in the fine grain size microstructure (average grain size = 9 μm) than in the large grain size microstructure (average grain size = 23 μm) until the precipitated fraction of Nitrides reaches about 70%. In the cold rolled state the effect of grain size is much less significant; probably the Precipitation process occurs simultaneously at the grain boundaries and along dislocations. According to the simulation results, significant differences can be found between the precipitated fraction of Nitrides in fine and large grain size sheets coiled in the temperature range 550–650 °C. In this interval, the precipitated Nitride fraction is about two times larger in a fine grain microstructure (9 μm) than in sheets with 23 μm average grain size. The local position in the coil also affects significantly the precipitated fraction of Nitrides. In the outer ring of the coil, less than 20% precipitated fraction is predicted in coiling temperature range 550–700 °C. However, in the middle ring of a hot rolled coil, the precipitated fraction changes from 5% to 85% with increasing coiling temperature from 550 to 700 °C.
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thermoelectric power study of Nitride Precipitation and recrystallization in continuously heated low carbon al killed steels
2014Co-Authors: Andras MucsiAbstract:This paper deals with the metallurgical phenomena occurring in aluminium- killed low carbon steels during industrial batch annealing process. The formability of these steels is strongly influenced by the Nitride Precipitation - recrystallization interaction during the batch annealing stage of the production technology. The accurate Precipitation kinetics of Nitrides is not clearly described yet because of the difficult evaluation of precipitated fraction of Nitrides in cold rolled state steels. The aim of this study is to present a methodology for measuring the precipitated Nitride fraction in cold rolled state, moreover to investigate the Precipitation - recrystallization sequence during batch annealing process. Another purpose of this study is to give the conditions of the development of good deep-drawable microstructure. The Nitride Precipitation process in cold rolled and annealed state is measured using a special thermoelectric power test based methodology. On the basis of the experimental work, it is concluded that good formable microstructure develops if the precipitated Nitride fraction reaches at least ~40% at the beginning of the recrystallization. This condition can be satisfied if the heating rate is held between 30 and 45 °C/h during the industrial batch annealing process.
Andre Paulo Tschiptschin - One of the best experts on this subject based on the ideXlab platform.
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cavitation erosion resistance of low temperature plasma Nitrided martensitic stainless steel
Wear, 2013Co-Authors: L A Espitia, Luis Bernardo Varela, Carlos Eduardo Pinedo, Andre Paulo TschiptschinAbstract:Abstract The cavitation erosion resistance of non-Nitrided and low plasma Nitrided AISI 410 martensitic stainless steel was evaluated according to ASTM G32 standard. Plasma nitriding was carried out in a hot wall DC-pulsed plasma reactor at 400 °C in a mixture of 75% of nitrogen and 25% of hydrogen during 20 h. The ASTM A 743 grade CA6NM stainless steel was used for comparison purposes. The microstructure of the steels was characterized by optical and scanning electron microscopy, as well as by X-ray diffraction. Expanded martensite and iron Nitrides were formed at the surface of the martensitic AISI 410 stainless steel. Curves of mass loss, erosion rate and roughness parameters were plotted as a function of exposure time. The 25 μm thick Nitride layer showed two distinct regions: a first 5 μm thick layer just beneath the surface containing precipitated e Fe 3 N Nitrides and expanded martensite and the rest of the layer constituted solely by expanded martensite. Iron Nitride Precipitation drastically reduced the incubation period, allowing detachment of entire grains due to the impact of shock-waves over the surface. Despite this, after removal of the first 5 μm thick layer, the cavitation erosion resistance improved significantly. The relationship between microstructure and time-variation curves and wear mechanisms are discussed.
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new high temperature gas nitriding cycle that enhances the wear resistance of duplex stainless steels
Journal of Materials Science, 2004Co-Authors: Carlos Mario Garzon, Andre Paulo TschiptschinAbstract:Interstitially dissolved nitrogen improves the corrosion and wear resistance as well as the mechanical properties of stainless steels (SS) [1–5]. Production routes of High Nitrogen Stainless Steels (HNSS) by alloying, pressure metallurgy, powder metallurgy, and solid-state diffusion have been studied [6–10]. In the production route, which involves solid-state diffusion, the steel surface and near surface regions are alloyed with nitrogen through chemical, implantation, plasma, or laser techniques [9, 10]. Recently, a chemical solid-state nitrogen alloying technique was developed [10–14], consisting in annealing SS in a N2-containing gas atmosphere in the range 1273–1473 K. In this High Temperature Gas Nitriding treatment (HTGN), atomic nitrogen is absorbed at the surface of the steel and then diffuses into the near surface region. Case-depths from 0.5 to 2.0 mm and nitrogen contents in solid solution at the surface from 0.5–1.0 wt% can be obtained after 18 to 45 ks heat-treatments. HTGN has been successfully used to improve the surface properties of martensitic, austenitic, ferriticaustenitic and martensitic-ferritic SS [7, 10–13, 15]. Particularly, when ferritic-austenitic duplex stainless steels (DSS) are Nitrided austenitic cases of higher wear and corrosion resistances are formed, on high strength DSS ferritic-austenitic cores. Due to both high temperatures and long nitriding times: (i) the austenitic cases grow forming coarse columnar grains [10–13], and (ii) the maximum attainable nitrogen content in precipitatefree cases corresponds to the nitrogen solubility limit at that temperature. The solubility limit in austenite, relative to Nitride Precipitation, increases with temperature; however the amount of ferrite in the dual-phase non-Nitrided core increases with temperature too. Thus the optimum nitriding temperature for DSS is between 1423 and 1448 K. In the present work, a novel nitriding cycle that avoids formation of coarse grains in the austenitic case, inhibits Nitride Precipitation and leads to sharp textures is proposed. It consists on cycling the specimen between two different N2 partial pressures, PN2, (Fig. 1): a high-pressure stage (sorption stage) and a vacuum one (desorption stage). The high nitrogen pressure stage is a long term one where nitrogen is introduced in the specimen. It is followed by a short vacuum period (PN2 ∼ 0) where nitrogen desorption occurs and ferrite
R E Schacherl - One of the best experts on this subject based on the ideXlab platform.
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microstructure and kinetics of Nitride Precipitation in a quaternary iron based model fe 2 82 at pct cr 0 13 at pct mo 0 18 at pct v alloy
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2015Co-Authors: C W Kang, Sai Ramudu Meka, R E Schacherl, E. J. MittemeijerAbstract:Internal Nitride development in iron-based quaternary Fe-Cr-Mo-V alloy, as a model alloy for 31CrMoV9 steel, was investigated by performing controlled gaseous nitriding experiments. The Nitride-Precipitation process starts with the development of nanosized platelets of, coherent, cubic NaCl-type Nitride, along {100} lattice planes of the ferrite matrix, in association with matrix-lattice dilation. The development of Nitride platelets having a NaCl-type crystal structure, satisfying the Baker–Nutting orientation relationship with the ferrite matrix, and the nitrogen content of the Nitrided zone suggest the development of a quaternary “mixed” (Cr x ,V y , Mo1−x−y)N Nitride, similar to the development of “mixed” ternary Nitrides as reported for Nitrided Fe-Cr-Al and Fe-Cr-Ti alloys. In a later stage, the Nitride platelets undergo discontinuous coarsening resulting in the development of a lamellar microstructure consisting of Nitride and ferrite lamellae. Kinetic analysis demonstrated that the thermally activated nature of growth of the diffusion zone is controlled with about equal weights, by the diffusion of nitrogen in the substrate matrix and the matrix lattice solubility of nitrogen.
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Molybdenum-Nitride Precipitation in Recrystallized and Cold-Rolled Fe-1 at. pct Mo Alloy
Metallurgical and Materials Transactions A, 2013Co-Authors: H Selg, R E Schacherl, Thomas Waldenmaier, E. Bischoff, S. R. Meka, E. J. MittemeijerAbstract:Nitriding of recrystallized and cold-rolled Fe-1 at. pct Mo alloy at 853 K (580 °C) in a NH_3/H_2 gas mixture leads to the formation of cubic nanometer-sized Mo_2N-type precipitate platelets. These platelets obey a Baker–Nutting orientation relationship with the ferrite matrix. After prolonged nitriding, micrometer-sized colonies of lamellae consisting of a hexagonal MoN-type Nitride and ferrite develop in a discontinuous Precipitation reaction. These Nitride lamellae have a Burgers-type orientation relationship with the ferrite lamellae. As compared to the recrystallized specimens, in the cold-rolled specimens, the Precipitation of the initial Mo_2N-type platelets occurs much faster and moreover leads to incoherently diffracting precipitates; upon continued nitriding, a much earlier but only partially occurring transition of Mo_2N-type to MoN-type precipitates is observed. The results indicate that incorporation of iron in the Nitrides can occur, if at all, only up till a negligible level, thereby invalidating earlier data.
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molybdenum Nitride Precipitation in recrystallized and cold rolled fe1 at pct mo alloy
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2013Co-Authors: H Selg, Sai Ramudu Meka, R E Schacherl, E. J. Mittemeijer, Ewald Bischoff, Thomas WaldenmaierAbstract:Nitriding of recrystallized and cold-rolled Fe-1 at. pct Mo alloy at 853 K (580 °C) in a NH3/H2 gas mixture leads to the formation of cubic nanometer-sized Mo2N-type precipitate platelets. These platelets obey a Baker–Nutting orientation relationship with the ferrite matrix. After prolonged nitriding, micrometer-sized colonies of lamellae consisting of a hexagonal MoN-type Nitride and ferrite develop in a discontinuous Precipitation reaction. These Nitride lamellae have a Burgers-type orientation relationship with the ferrite lamellae. As compared to the recrystallized specimens, in the cold-rolled specimens, the Precipitation of the initial Mo2N-type platelets occurs much faster and moreover leads to incoherently diffracting precipitates; upon continued nitriding, a much earlier but only partially occurring transition of Mo2N-type to MoN-type precipitates is observed. The results indicate that incorporation of iron in the Nitrides can occur, if at all, only up till a negligible level, thereby invalidating earlier data.
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the process of tungsten Nitride Precipitation upon nitriding ferritic fe 0 5 at w alloy
Defect and Diffusion Forum, 2013Co-Authors: Benjamin Schwarz, R E Schacherl, Eric J. Mittemeijer, Regina E HorthAbstract:The Precipitation of tungsten Nitride upon internal nitriding of ferritic Fe-0.5 at.% W alloy was investigated at 610°C in a flowing NH3/H2 gas mixture. Different tungsten Nitrides developed successively; the thermodynamically stable hexagonal δ-WN could not be detected. The state of deformation of the surface plays an important role for the development of tungsten Nitride at the surface. The morphologies of the tungsten Nitrides developed at the surface and those precipitated at some depth in the specimen are different. The Nitride particles at the surface exhibit mostly an equiaxed morphology (with the size of the order 0.5 µm) and have a crystal structure which can be described as a superstructure derived from hexagonal δ-WN. These Nitride particles show a strong preferred orientation with respect to the specimen frame of reference but have no relation with the crystal orientation of the surrounding ferrite matrix. In the bulk, nanosized and finely dispersed platelet-like precipitates grow preferentially along {100}α-Fe. It is unclear whether these precipitates consist of binary iron Nitride α´´-Fe16N2 or of a ternary Fe-W-N. Additionally to the finely dispersed particles, bigger Nitrides at ferrite grain boundaries develop exhibiting platelet-type morphology and possessing a crystal structure which can be also described as a superstructure derived from hexagonal δ-WN. Upon prolonged nitriding assumed discontinuous Precipitation of the initially precipitated finely dispersed Nitrides starts from the ferrite-grain boundaries resulting in lamellas consisting of alternate ferrite and hexagonal Nitride lamellas, whereas the Nitride lamellas having a Pitsch-Schrader orientation relationship with the surrounding ferrite matrix. The Nitrides precipitated upon nitriding in the bulk were found to be unstable during H2 reduction at 470°C. Remarkably, upon such low temperature dissolution of the Nitrides took place but only the nitrogen from the Nitride particles could diffuse out of the Nitride platelets and the specimen, leaving W-rich regions (W-clusters) at the location of the original precipitates.
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crystal structure and morphology of mixed cr1 xalxn Nitride precipitates gaseous nitriding of a fe 1 5 wt pct cr 1 5 wt pct al alloy
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2009Co-Authors: A R Clauss, R E Schacherl, E. J. Mittemeijer, Santosh S HosmaniAbstract:The crystal structure and morphology of Nitride precipitates developing in the matrix of a Fe-1.5 wt pct Cr-1.5 wt pct Al (Fe-1.6 at. pct Cr-3.1 at. pct Al) alloy upon gas nitriding were investigated. To this end, the Nitrided zone was studied using metallographic methods, X-ray diffractometry (XRD), electron probe microanalysis (EPMA), microhardness measurement, and transmission electron microscopy (TEM), including high-resolution TEM (HRTEM), and scanning TEM (STEM). Furthermore, a nitrogen-absorption isotherm was determined, for use in characterizing the nature of the Nitride-Precipitation process. It could be shown that the expected equilibrium Nitrides, cubic CrN and hexagonal AlN, do not develop. Instead, mixed Cr1–x Al x N Nitride precipitates of the cubic, rock-salt structure type develop upon nitriding the ternary alloy. These precipitates obey a Bain-type orientation relationship (OR) with the ferrite matrix and are associated with a considerable uptake of excess nitrogen and a very pronounced hardness increase.