Lamellar Precipitates Research Articles

The nitrogen-absorption isotherm for Fe–21.5 at. % Cr alloy: dependence of excess nitrogen uptake on precipitation morphology

Nitriding of Fe–21.5 at. % Cr alloy leads to a “discontinuously coarsened”, chromium-nitride/ferrite lamellar precipitation morphology in the nitrided zone. The nitrogen-absorption isotherm for this alloy with this precipitation morphology was determined at 560°C. To assure a constant precipitation morphology the Fe–21.5 at. % Cr specimen was first homogeneously pre-nitrided (at 580°C in an ammonia/hydrogen gas mixture of nitriding potential 0.103 atm−1/2) and then de-nitrided (at 470°C in hydrogen gas atmosphere). The amount of nitrogen remaining in the de-nitrided specimen indicated that the composition of the nitride precipitates is CrN and not (Fe, Cr)N. The measured nitrogen-absorption isotherm revealed the presence of excess nitrogen in the nitrided specimen, which is a surprise in view of the coarse, lamellar precipitation morphology. The occurrence of this excess nitrogen could be ascribed to an unexpected, minor fraction of the total chromium content in the alloy present as coherent, tiny nitride platelets within the ferrite lamellae of the “discontinuously coarsened” lamellar precipitation morphology, as evidenced by transmission electron microscopy. A possible kinetic background for this unusual phenomenon was discussed.

Influence of the microstructure on steel hardening in pulsed plasma nitriding

The plasma technologies are widely used in metal surface engineering processes. Basically, these treatments improve the mechanical, tribological, and chemical properties of the material such as wear resistance, hardness, fatigue resistance, friction, and corrosion resistance. In this work, a comprehensive study of the influence of the microstructure on hardness of AISI P20 steel treated at different temperatures and times by pulsed plasma nitriding is reported. The processes were done by using a pulsed plasma industrial system. The samples were characterized by nano-indentation (hardness), x-ray diffraction (XRD), scanning electron microscopy (SEM), and x-ray dispersion spectroscopy (EDS). At lower treatment temperatures (360 °C), a high density of small lamellar precipitates, constituted by more ε-Fe2–3N phase than γ′-Fe4N phase, is formed. At intermediate treatment temperatures (480 °C), big lamellar precipitates, constituted by more γ′-Fe4N phase than ε-Fe2−3N phase, are formed at grain boundary. At higher treatment temperatures (520 °C), the nitrided layer does not contain lamellar precipitates and it is only constituted by α-Fe phase saturated in nitrogen. Hardness depends on size, shape, and distribution of precipitates and crystalline phases (microstructure). The higher hardness values are obtained when more and smaller lamellar precipitates are presented and constituted by more ε-Fe2−3N phase.

Effect of Sn on the microstructure and compressive deformation behavior of the AZ91D aging alloy

The effect of Sn on the microstructure and compressive deformation behavior of aging AZ91D magnesium alloy was investigated. The results show that the discontinuous precipitation occurs near the grain boundary in the form of lamellar precipitates of the Mg 17Al 12 phase. These precipitates alternatively lie in the α-Mg matrix perpendicular to the grain boundaries. With an increase in the Sn content, the cellular discontinuous precipitation can be efficiently suppressed, such that no discontinuous precipitation is observed when the Sn content is increased to 2 wt.%. The AZ91D aging alloy has a higher ultimate fracture strength than the base 2%Sn alloy due to the large numbers of grains containing the cellular discontinuous precipitation forming nearby the grain boundaries. In these regions the β phase can effectively hinder the dislocation movement. Conversely, the soft behavior induced by a high fraction of twins is the main reason that the strength of 2%Sn alloy is lower than that of the AZ91D alloy.

Synthesis of TiAl-Based Intermetallic Compound by Electro-Pressure Sintering

Abstract Elemental powder metallurgy is an efficient processing technique to synthesize intermetallics of high melting points. TiAl-based intermetallic compound was produced by electro-pressure sintering (EPS) consolidation techniques. Through a balanced control of the heating scheme, sintering temperature and pressure, near full density state of TiAl-based intermetallic compound was obtained, in which a high hardness and absence of porosity, respectively, were realized. An optimum condition of EPS was determined to be 980°C/90MPa/120s. Aided by interstitial carbon atoms, the intermetallic compound showed attractive compressive strength at room temperature. The roles of carbon atoms were refinement of the lamellar microstructure and precipitation hardening as effective in the hot extruded materials.

A Numerical Model for the Description of Massive and Lamellar Microstructure Formation in Gamma-TiAl

A phenomenological modeling approach has been developed to describe the massive transformation and the formation of lamellar microstructures during cooling in gamma titanium aluminides. The modeling approach is based on a combination of nucleation and growth laws which take into account the specific mechanisms of each phase transformation. Nucleation of both massive and lamellar γ is described with classical nucleation theory, accounting for the fact that nuclei are formed predominantly at α/α grain boundaries. Growth of the massive γ grains is calculated with a mathematical expression for interface-controlled reactions. A modified Zener model is used to calculate the thickening rate of the γ lamellar precipitates. The model incorporates the effect of particle impingement and rapid coverage of the nucleation sites due to growth. The driving pressures of the phase transformations are obtained form Thermo-Calc based on actual temperature and matrix composition. The model permitted investigating the influence of alloy chemistry, cooling rate and average α grain size upon the amount of massive γ and the average thickness and spacing of the lamellae. Calculated CCT diagrams were compared with experimental data collected from the literature and showed good agreement.

高窒素オーステナイトステンレス鋼の組織と機械的性質に及ぼす時効の影響

The effect of aging on microstructures and mechanical properties of 1%N-23%Cr-4%Ni-2%Mo austenitic stainless steel aged at 600∼800°C for up to 1000 h were investigated with electron microscope, analysis of extracted residues, ferrite scope, and mechanical tests such as hardness, tensile, and Charpy impact. Chromium nitrides precipitated on grain boundary at the aging temperature below 650°C, while lamellar precipitates, which were composed of plate like Cr2N and martensite, were formed above 700°C. σ phase precipitated at 700°C for 1000 h and above 750°C for longer than 100 h. Martensite was also observed around σ phase, and the total amount of martensite was 40-50% after aging at 700-800°C for 1000 h. Age hardening was observed above 700°C and hardness, which increased with aging time and temperature, reached about Hv 500 after aging at 800°C for 1000 h. Increase in tensile strength by aging was higher than that in proof strength, because of pronounced increase in work hardening rate by aging. Strengthening by aging depended mainly on the amount of lamellar precipitates and additionally on σ phase and its surrounding martensite at higher temperature. Tensile ductility and impact toughness were decreased by aging due to lamellar precipitates and σ phase.

Microstructure and Growth Kinetics of Nitrided Zone in Plasma-nitrided Fe–Cr Alloys

Microstructure and growth kinetics of the nitrided zone have been examined in various Fe-Cr binary alloys ranging from 1 to 30 mass% Cr that had been plasma-nitrided at temperatures between 743 and 943 K. CrN precipitates form under all the nitriding conditions. With increasing Cr content or decreasing nitriding temperature, the morphology of CrN precipitates changes drastically from continuous precipitation with a fine disk-shaped morphology to CrN lamellar precipitation. In addition, small amounts of Cr 2 N form when the Cr content and the nitriding temperature are high. In spite of the different manners of Cr-nitride precipitation, the thickness of the nitrided zone is approximately proportional to the square root of the nitriding period. Furthermore, in most of the alloys the activation energies for the overall growth kinetics of the nitrided zone are nearly constant.

Study the effects of nitrogen annealing on oxygen precipitation in fast neutron-irradiated Czochralski silicon

The effects of nitrogen on the annealing behavior of fast-neutron irradiation CZ-Si were studied using Fourier transform infrared spectrometry (FTIR). Compared with annealing in argon atmosphere, concentration of interstitial oxygen decreased greatly after annealing in nitrogen atmosphere. Some new absorption peaks at 801 and 1030 cm −1were found using low-temperature (20 K) FTIR which were connected with N–O complexes. The IR absorption peaks at 1078, 1090 and 1224 cm −1 are instead connected with oxygen precipitates. Among these, the absorption at 1224 cm −1 not be found in normal CZ-Si and is probably associated to lamellar oxygen precipitation.

Monte-Carlo simulation study of the self-organization of nanometric amorphous precipitates in regular arrays during ion irradiation

High-dose ion implantation of materials that undergo drastic density change upon amorphization at certain implantation conditions results in periodically arranged, self-organized, nanometric configurations of the amorphous phase. A simple model explaining the phenomenon is introduced and implemented in a Monte-Carlo simulation code. Through simulation conditions for observing lamellar precipitates are specified and additional information about the compositional and structural state during the ordering process is gained.

Precipitation hardening of a Cu-free Au–Ag–Pd–In dental alloy

The hardening mechanism and related microstructural changes of the Cu-free dental casting alloy composed of Au–Ag–Pd–In was examined by means of hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations and electron probe microanalysis (EPMA). The Au–Ag–Pd–In alloy showed apparent age-hardenability. In the SEM photograph, three phases were observed in the solution-treated specimen, that is, the Au–Ag based phase with small amounts of In and Pd as matrix, the InPd phase as particle-like structures, and the Pd-rich phase as lamellar precipitates. By aging the specimen, the very fine Pd-rich inter-granular precipitates grew toward the grain interior as lamellar structure, and finally the coarsened Pd-rich precipitates covered a large part of the Au–Ag based matrix. The hardness increase in the early stage of the age-hardening process was assumed to be caused by the diffusion and aggregation of Pd atoms from the Au–Ag based matrix. The hardness decrease in the later stage of age-hardening process was caused by coarsening of the lamellar precipitates composed of the Pd-rich phase.

Microstructural characterization of a carbonitrided heat resisting alloy using focused ion beam-based techniques

An austenitic Fe–25Cr–20Ni (wt.%) alloy was first nitrided in N2–H2 then carburised at 1000°C. The nitridation produced lamellar internal precipitates of Cr2N which grew by discontinuous precipitation at a recrystallised austenite boundary formed at the precipitation front. Subsequent carburisation converted the Cr2N to a mixture of chromium-rich M7C3+CrN, and released nitrogen into the austenite matrix. The submicron CrN precipitates were stabilized by a matrix supersaturated with nitrogen and exhibited a strong orientation relationship with the surrounding austenite. The rejected nitrogen diffused deeper into the alloy to form new, more finely spaced Cr2N lamellae. Carbon diffusion overtook the nitride structure and precipitated finely spaced, lamellar M23C6 by a discontinuous precipitation process. This process is available to the carbides only when a prior boundary is present. This phenomenon, and the ability of nitridation to form a boundary and the inability of carburisation to do so, are interpreted using the energetics of nucleation.

Hardening mechanism of an Ag–Pd–Cu–Au dental casting alloy

Age-hardening behaviour and the related microstructural changes were studied to elucidate the hardening mechanism of an Ag–Pd–Cu–Au dental casting alloy by means of hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations and electron probe microanalysis (EPMA). By considering hardness test and XRD results together, it was revealed that the hardness increased during the early stage of phase transformation of α into α 1. In the SEM photographs, two phases of matrix and particle-like structures were observed, and the precipitation of element from the matrix progressed during isothermal aging. By SEM observations and EPMA analysis, it could be supposed that the increase in hardness was caused by the diffusion and aggregation of Cu atoms from the Ag-rich α matrix containing Au and Cu in the early stage of age-hardening process, and that the decrease in hardness was caused by the progress of coarsening of Cu-rich lamellar precipitates in the later stage of the age-hardening process. The changes in the Ag-rich matrix caused both the increase and decrease in hardness, and the CuPd phase containing small amounts of Zn and Sn did not contribute to the hardness changes.

Precipitation hardening mechanisms in lead-cadmium-calcium-tin alloys for battery grids

Precipitation hardening in lead-cadmium-calcium alloys is a two stage process: the first stage is a discontinuous hardening transformation without precipitation, the second stage is a discontinuous lamellar precipitation of Pb3 Ca. In order to increase the mechanical and electrochemical properties of these alloys, we have studied the influence of minor additions of tin on the precipitation hardening behaviour of PbCaCd alloys, as this element is used to improve the electrochemical properties of positive battery grids. The objective was to investigate in a systematic manner the ageing and overageing behaviour of PbCdCaSn alloys in the case of two different structural states: as-cast condition, rehomogenized state.

Recrystallization behavior of a heavily cold-rolled Ni3Al(B,Zr) alloy

The present article describes the microstructural changes during recrystallization annealing of a 73 pct cold-rolled Ni3Al(B,Zr) alloy along with a study of the recrystallization kinetics. The deformed γ regions, mostly within and near the shear bands, appear to recrystallize first. The recrystallization front leaves behind a lamellar discontinuous precipitation within the newly formed strain-free γ grains, when annealing is done at lower temperatures. At higher annealing temperatures, the precipitates within γ assume a globular morphology. This precipitate is presumably made up of γ′ particles. When γ recrystallization is nearly complete, the γ′ regions start to recrystallize. The two-stage recrystallization process is also corroborated from the kinetics results, which show that the activation energy up to 50 pct recrystallization of the material is only 117 kJ/mole, whereas beyond 50 pct until the completion of recrystallization, an activation energy of ∼274 kJ/mole is obtained.

X-ray analysis of microstructure in Au–Fe melt spun alloys

Supersaturated Au80Fe20 and Au70Fe30 solid solutions have been obtained by rapid solidification. The lattice constant a0 of the as-quenched FCC phase was of 4.025 and 3.989Å for Au80Fe20 and Au70Fe30, respectively. Thermal treatments have been performed in the temperature range between 320°C and 550°C. A precipitation reaction has been observed in Au80Fe20 only after prolonged annealing at low temperature (320°C/168h). In Au70Fe30 an increase in the lattice constant has been observed after thermal treatments, giving values ranging from 3.998 to 4.044Å. Precipitation occurs at grain boundaries and into grains. Low-temperature annealings give lamellar precipitates, which grow and transform into globules at higher temperatures.A Williamson–Hall plot for the FCC phase shows a significant scatter due to occurrence of planar defects. A detailed analysis of microstructure have been obtained by Rietveld analysis of peak broadening. Precipitation reaction leaves a Fe-depleted matrix, which shows a cristallite size of about 200Å and an r.m.s. microstrain of 6×10−3. Compound fault probability has been estimated from asymmetric peak profile.

Carbon redistribution during sigma phase precipitation in high purity 25Cr-10Ni-0.35C-Si stainless steels

The role of carbon and chromium carbide during sigma phase precipitation in austenitic 310 stainless steels can be summarised in three main points: (a) carbide is a chromium source for sigma, (b) carbon is not soluble into sigma, (c) sigma precipitation can occur when (i) the carbon content in the matrix is lower than a critical value and (ii) the chromium-equivalent is higher than a critical value. These contradictory conditions ask the question of the distribution of carbon during sigma precipitation and chromium carbide dissolution which releases carbon insoluble into sigma. Several intermediate phases have been described in industrial alloys which contained carbide formers such as niobium or titanium but the redistribution of carbon has not been discussed when these elements are absent. In the present work, model alloys (25 Cr, 10 Ni, 0.35 C) containing only chromium as carbide former have been prepared and heat treated in the interval 750-850°C. Several phenomena occurring simultaneously have been observed: intergranular and transgranular precipitation of chromium carbide, in parallel with massive and lamellar sigma phase precipitation. The distribution of carbon between different kinds of carbide is discussed.

EPM method of synthesizing intermetallics based on Ti

Elemental powder metallurgy (EPM) is a powerful means of synthesizing intermetallics of high melting points. In this study, two intermetallic compounds based on Ti were investigated using different consolidation techniques: TiAl–Mn–Mo–C by hot extrusion and Ti 5Si 3–Nb–C by electro-pressure sintering (EPS). Full density compounds were obtainable in both cases. Aided by interstitial carbon atoms, each intermetallic compound showed attractive mechanical properties such as high tensile yield strength and creep resistance in TiAl–Mn–Mo–C and high fracture toughness and transverse rupture strength in Ti 5Si 3–Nb–C. The roles of carbon atoms, being less than 1 at.% in each material system, were, refinement of the lamellar microstructure and precipitation hardening in the former, and a possible modification of the crystal structure towards easier slip and less thermal anisotropy in the latter.

Fatigue Crack Initiation Behavior and Notch Sensitivity of AZ92A Magnesium Alloy.

Plane bending fatigue tests were carried out for sand cast magnesium alloy AZ 92 A-T 6 in air at room temperature, in order to investigate fatigue crack initiation behavior and notch sensitivity. Successive observations on the surface have shown that the fatigue crack initiates along the slip bands in an α grain and starts to grow into the neighboring grain or lamellar precipitate phase. The notch sensitivity was higher than that in pure titanium that has hcp crystal structure. However, no non-propagating crack exists for a sharp notch (ρ=0.1 mm). Fractography using SEM has shown that the crack in which the micro pores in the vicinity of the surface became an origin, propagated and often led to final failure in the region of life Nf≥5× 105. In this case, the initial stress intensity factor; ΔKI ini was from 0.52 to 0.73 MPa ·√m, when the micro pore was considered to be a crack.

Influence of Thermal and Mechanical Processing on Room Temperature Mechanical Properties of Nickel Free High Nitrogen Austenitic Stainless Steels.

Nickel free high nitrogen austenitic stainless steels with a base composition around 18%Cr–18%Mn and varying in C and N contents and processing conditions were evaluated for their mechanical behaviour. A range of mechanical properties from high strengths to ultra high strength levels could be obtained. The steels develop ultra high strength levels in the as-hot worked condition, hot work1cold work condition and solution treated1cold worked conditions. They show high strength levels with very good ductility and impact toughness levels in the solution annealed condition. The Hall–Petch parameters and DBTT obtained in the solution annealed condition were found to be influenced by the carbon content of the steel. The work hardening behaviour of the steels have been examined at various processing conditions. Nitrogen steels with high carbon content show good mechanical properties in the solution treated and subsequently cold rolled conditions. In the as-formed condition, the high carbon steels show inferior ductility due to grain boundary lamellar nitride precipitation.

Growth of ordered lamellar precipitates during nitridation of Nb–10 at.% Ti at 1300°C

Growth of three morphologically distinct layers occurs during the parabolic nitridation of Nb–10 at.% Ti at 1300°C under 0.3 bar nitrogen atmosphere. The outermost layer is composed of δ-(Nb,Ti)N, the intermediate layer of β-(Nb,Ti)2N and the innermost layer corresponds to an internal precipitation zone with ordered lamellar precipitates of β-(Nb,Ti)2N+β-(Nb,Ti). The internal precipitation front advances with parabolic kinetics and the three layers grow by nitrogen inward diffusion. The interlamellar spacing at the precipitation front increases as the reaction time increases. The formation of the lamellar microstructure can be described as a discontinuous precipitation process and the interlamellar spacing can be considered as a measure of the distance over which lateral Nb–Ti interdiffusion occurs in the alloy.