Iron-nitrogen Martensite Research Articles

Nanostructured iron-nitrogen martensite synthesized from Fe79Mo10B10Cu1 magnetic nanocomposite alloy

A Fe79Mo10B10Cu1 nanocomposite alloy was prepared by rapid solidification processing and subsequent isothermal annealing of ribbons with 6 nm, α-Fe grains embedded in an amorphous matrix. The nanocrystalline alloy was then nitrided in a gas mixture of ammonia and hydrogen at 650 and 700 °C for varied time between 300 and 7200 s followed by quenching. A majority (~55 vol%) of nitrogen austenite γ-(Fe,N) with a grain size of ~20 nm was produced when nitriding conditions were at a temperature of 700 °C in 12% NH3–88% H2 for 3600 s. The nitrided alloy has a magnetization of 50 ± 20 A·m2/kg at an applied field of 1.19 MA/m. Retained α-Fe, the only ferromagnetic phase in the sample, is responsible for the magnetization. And paramagnetic phases of γ-(Fe,N) and η-Fe3Mo3N account for ~80% of the material by volume. After the high-temperature nitridation, the alloy was soaked in liquid nitrogen and then tempered at 117 °C. Nanostructured iron-nitrogen martensite (α′) was transformed from the nitrogen austenite phase in the nitrided Fe79Mo10B10Cu1 alloy, and was characterized to have an amount of ~20 vol% and a grain size of ~12 nm. This iron-nitrogen martensite nanocomposite has a magnetization of 65 ± 8 A m2/kg at an applied field of 1.19 MA/m and coercivity of 14 ± 6 kA/m.

Inhibition of Hydrogen Entry into Pure Iron By Formation of Nitrogen Solid Solution Layer in the Surface

Hydrogen embrittlement is a serious problem for steels such as high strength steels. In order to prevent hydrogen embrittlement, the inhibition of hydrogen entry into the steel matrix is one of important subjects. The formation of surface layers with low hydrogen diffusivity is thought to be effective in inhibiting the hydrogen entry into the steel matrix. In this study, we formed nitrogen solid solution layers containing iron-nitrogen martensite and retained austenite phases on the surface of pure iron specimens by plasma-nitriding and subsequent quenching, and investigated the hydrogen permeation behavior. Pure iron sheets of 1 mm in thickness were used as the specimens. Prior to the nitriding treatment, the specimen surfaces were polished using a diamond past down to 1 μm. The plasma nitriding treatments were carried out in a nitrogen-hydrogen atmosphere at 370 Pa for 600 s. The temperature of the specimen surface was kept at 873 K by controlling the supplied DC voltage. After the plasma nitriding, the specimens were quenched from 1073 or 1273 K for 180 s. In the case of the quenching from 1273 K, an iron-nitrogen martensite phase was formed in the surface. The thickness of the surface layer was over 100 µm. Iron-nitrogen martensite and retained austenite phases were formed in the surface of the specimen quenched from 1073 K. The thickness was ca. 50 µm. To investigate the effect of the nitrogen solid solution layers on the hydrogen permeation behavior through the specimens, a hydrogen permeation test using a conventional Devanathan-Stachurski cell was carried out for 3.6 ks. The nitrided surface was arranged in the hydrogen entry side. Pd of 200 nm in thickness was deposited on the specimen in the hydrogen detection side. The hydrogen detection side was polarized at 0.2 V vs. SHE in 0.1 M NaOH fully deaerated by pure Ar gas. The specimen in the hydrogen entry side was polarized at −0.8 V vs. SHE in 0.1 M H2SO4. The hydrogen permeation tests reveal that the permeation current of the specimen quenched from 1273 K was smaller than that of as-polished one. The surface martensite phase seems to suppress the hydrogen permeation. In addition, no permeation current of the specimen quenched at 1073 K was detected. This is because the hydrogen diffusivity in the austenite phase is much lower than the ferrite phase due to high solubility limit in the austenite phase. The surface layer containing the austenite phase is found to be effective in inhibiting the hydrogen entry into the matrix.

純鉄の浸窒焼入れ組織

Austenite formation on the surface of pure iron during nitriding in a mixture of NH3 and H2 gases and surface hardening due to formation of iron-nitrogen martensite by subsequent quenching were investigated. In nitriding at 973 K at a partial pressure of NH3(fNH3) of 5% and 10%, austenite layer is formed at the surface while double-layered structures consisting e-Fe2~3N and austenite are developed at fNH3 of 20%. In addition, at these high fNH3, voids are formed in e-Fe2~3N nitride as well as austenite with high nitrogen content. At fNH3 of 5% and 10%, the growth rates of austenite accords with parabolic growth law and become faster with increasing fNH3. On the other hand, after prolonged treatment at fNH3 of 20%, thickness of the nitrided zone is less than that for 10% due to substantial formation of voids and resultant emission of nitrogen from the surface. The hardness of the nitrided zone is nearly the constant at 800HV in the specimen nitrided at fNH3 of 5% whereas softening near the surface occurs in the specimens nitrided at fNH3 of 10% and 20% due to formation of the retained austenite and the voids.

Crystallography of tempered iron-nitrogen martensite: The roles of (011) twin boundaries

Abstract The decomposition of iron—nitrogen martensite (α′) containing 5.4 at.% N, was analysed by both conventional and high-resolution transmission electron microscopy of specimens annealed at 333, 373 and 403 K. As a result of the anneals, α′′ precipitates (the α′′ unit cell has ideally a composition indicated by Fe16N2) were obtained in a ferrite (α) matrix (‘first stage of tempering’). Because of the determined specific orientation relationship between α′′ and α′-α, unambiguous proof for extensive (011)-type twinning in the original martensite was obtained. The crystallography at and near the original (011)α′ twin boundary, after the decomposition α′ ← α′′ + α was scrutinized. After decomposition three types of interface originated from the original (011)α′ twin boundary: α′′T1—α′′T2, αT1-αT2 and α′′T2-αT2, where T1 and T2 denote the original α′ crystals constituting the twin. It followed that the local lattice orientations of α′′ and α at or near the T1—T2 interface were dominated by the intrinsic p...

Crystallography of tempered iron-nitrogen martensite: the roles of (011) twin boundaries

Crystallography of tempered iron-nitrogen martensite: the roles of (011) twin boundaries

Electron crystallography study of tempered iron-nitrogen martensite and structure refinement of precipitated α″-Fe 16N 2

At the first stage of the tempering of iron-nitrogen martensitic steel, martensite α′ decomposes into tetragonal nitride α″-Fe 16N 2 and low nitrogen martensite α′ 1. The α″ crystals are plate-like in shape with wide (001)α″ surface and thin in the [001]α″ direction, while α′ 1 is mosaic. They are in parallel orientation, and make a lamellar texture. Rarely, a new nitride α″′-Fe 16N is formed coherently with α″. It is a base-centered orthorhombic crystal and is formed from α″ by removing nitrogen atoms in every two (110)α″ planes. The crystal structure of α″-Fe 16N 2 has been refined on the basis of single crystal electron diffraction intensity measurements using imaging plates and dynamical intensity calculations. In the space group 14/ mmm, Fe atoms are at the 8 h position with x = 0.220, at 4 e with z = 0.303 and at 4 d, while N atoms are at 2 a. The refined structure may contribute to the interpretation of the giant saturation magnetization of α″-Fe 16N 2.

Occurrence of incoherent α"-Fe16N2 at room temperature in iron-nitrogen martensite observed by neutron and X-ray diffraction

Aging behaviour of iron-nitrogen martensite (about 5 at.% N) at room temperature is investigated by neutron and X-ray diffraction experiments. An attempt is made to reveal the configuration of the nitrogen atoms during the aging processes especially by means of neutron diffraction, since the scattering lengths of iron and nitrogen atoms are almost the same. Two successive processes are distinguished: a redistribution of nitrogen atoms in the iron matrix and the formation of incoherent Fe16N2. During about the first 40 h of aging a decrease in the integrated intensity (10-20%) of the neutron and X-ray (002) martensite reflection is observed. This reduction is ascribed to the change in mean square displacement of iron atoms close to occupied interstices within nitrogen-enriched regions. After only about 1 d of aging, incoherent Fe16N2 has already formed. The changes in the integrated intensities of the precipitate and martensite reflections indicate that, during the precipitation process, Fe16N2 and ferrite are formed locally while the nitrogen content of the remaining martensite is unchanged.

Lattice changes of iron-nitrogen martensite on aging at room temperature

Lattice changes of iron-nitrogen martensite on aging at room temperature

Analysis of nonisothermal transformation kinetics; tempering of iron-carbon and iron-nitrogen martensites

A powerful method for the analysis of solid-state transformation kinetics can be based on non-isothermal analysis where the temperature corresponding to a fixed stage of transformation is measured as a function of heating rate. Adoption of a specific kinetic model is not required for valid application of this method. This removes constraints imposed unnecessarily on so-called Kissinger-like procedures. The method was applied to study and to compare the tempering kinetics of iron-carbon martensite and iron-nitrogen martensite (about 1.1 wt pct interstitials). Dilatometry and differential scanning calorimetry were employed. This combination of experimental techniques allowed distinction between preprecipitation processes as segregation and clustering of interstitials. In contrast with iron-carbon martensite, no indications were obtained for clustering of interstitials in iron-nitrogen martensite. The preprecipitation stages have activation energies of 75 to 85 kJ/mole, which are ascribed to volume diffusion of interstitials. The precipitation of the transition carbide/ nitride (first stage of tempering) occurs with an activation energy of 110 to 125 kJ/mole, which is ascribed to pipe diffusion of iron. The precipitation of the equilibrium carbide/nitride (third stage of tempering) is associated with an activation energy of 195 to 205 kJ/mole which is ascribed to combined pipe and volume diffusion of iron.

Analysis of transformation kinetics by nonisothermal dilatometry

Nonisothermal dilatometry was shown to provide a powerful method for the analysis of solid-state transformation kinetics by measuring the shifts of the temperatures of inflection points on dilatometric curves as a function of heating rate. The method was applied to iron-nitrogen martensite specimens. To that end iron was nitrided in a NH3/H2 gas mixture and subsequently quenched. The experiments allowed the assessment of residual terms in the theoretical development. To demonstrate the high sensitivity of the method, the activation energy was determined for the initial stage of tempering iron-nitrogen martensite, involving a minute decrease of specific volume. The method developed is equally applicable to methods where other physical properties of the material under investigation(e.g., electrical resistivity, magnetization) are measured on isochronal annealing.

Microstructural investigations of iron implanted with nitrogen ions at the temperature of liquid nitrogen: Part II: Implantation in the dose range 1×1017 to 1×1018 N+-ions/cm2

Surface compositions and microstructures of iron implanted with nitrogen ions to high fluences between 1×10 17 and 1×10 18 N + -ions/cm 2 at the temperature of liquid nitrogen have been examined by Auger electron spectroscopy, high-voltage electron microscopy and transmission high-energy electron diffraction. In part I of this paper [1], we have reported on the microstructure of nitrogen-implanted iron up to a dose of 1×10 17 N + -ions/cm 2 . The nitrogen distribution profiles are very different from the distribution predicted by the LSS-theory and from the profiles after implantation at room temperature. The nitrogen ion implantation at the temperature of liquid nitrogen leads to α′-iron nitrogen martensite, α″-Fe 16 N 2 and ∈-Fe 2 (C, N) 1− x . The structure and the habit of these phases and solid solutions are discussed as function of the implantation dose. The implantation-induced microstructure after implantation at the temperature of liquid nitrogen differs from that of implantation at room temperature. The phase formation is described by a model of implatation-induced transformation.

Crystallography and tempering behavior of iron-nitrogen martensite

Iron-nitrogen martensite was prepared by the gaseous nitrogenization of iron in the austenite-phase field, followed by quenching. Low- and high-nitrogen contents (lathvs plate morphology) were investigated. Optical microscopy, transmission electron microscopy and diffraction, and differential thermal analysis were employed to study the crystallography and the tempering behavior of the martensites. The orientation relation between austenite and martensite, the habit plane and types of (post-) transformation twins were determined. Tempering-induced changes in hardness and release of heat were related to structural changes as revealed by electron diffraction. Differences with analogous iron-carbon martensites were discussed. Aging at room temperature leads to nitrogen diffusion-controlled precipitation of coherentα″-Fe16N2 platelets, which process is completed after about one day. Aging at higher temperatures (up to about 475 K) results in incoherentα″particles, which, at temperatures above 460 K, exhibit small deviations (≃18 deg) from the usual {001}α′/α habit plane.

Mössbauer Effect in Iron-Nitrogen Alloys and Compounds

Mossbauer effects of 57 Fe in iron-nitrogen martensite and its tempering stages were measured. The spectra were resolved into the components corresponding to the 1st, 2nd, 3rd, 4th and farther n n Fe for each interstitial nitrogen atom. The analysis showed the localized perturbations by the nitrogen atoms on a host iron lattice similar to the case of iron-carbon martensite. 2) The principal axis of electric field gradient at the 2nd n n Fe in the α''-Fe 16 N 2 were determined to be directed to the nitrogen atom, not being parallel to the c -axis. The number of s and d electrons for the 1st n n Fe in some iron-nitrogen compounds were determined. The reduction of internal field, larger for interstitial alloys and compounds relative to substitutional ones, was proportional to the coordination number, n , to the neighboring nonmetal atoms. While, the variation of isomer shift depended upon both n and nonmetal concentration c .