Fe-Cr Phase Research Articles

Thermally stimulated crystalline phase transformations in metastable Fe–Cr alloy films prepared by pulsed laser deposition

Thermally induced crystalline phase formation of metastable Fe–Cr alloy films with different Cr-content (34.5, 43.5 and 48.5 at.% Cr) and thickness of 100 nm prepared by cross-beam pulsed laser deposition was examined by wide angle X-ray scattering. Therefore the annealing temperature was increased stepwise up to 1100 °C, the duration of the treatment per step was 90 min. Through a number of intermediate metastable crystalline phases, above 550 °C the phase composition did not change until the onset of reaction with Si/SiO 2 substrate at ∼900 °C. Equilibrium σ-FeCr is formed in all samples. In addition, body-centered cubic (b.c.c.) and tetragonally distorted b.c.c. Fe–Cr phases are formed in a film with nearly equiatomic composition. Reactions with substrate result in formation of Fe- and Cr-silicides.

Microstructure and magnetic properties of multilayer [(Nd2Fe14B)x/M]n films (M=Nb or Cr)

Multilayer [(Nd2Fe14B)x/M]n films (x being thickness in nm; M=Nb or Cr, with a fixed thickness 5 nm; n being the number of layers) have been deposited by ion-beam sputtering. Spacer layer was found to have substantial effects on the multilayers to exhibit different magnetic properties and microstructure. The average grain size of the multilayer films can be well controlled by both annealing temperature and thickness of the Nd2Fe14B layer. From a cross-sectional HRTEM study, the average grain size of [(Nd2Fe14B)x/M]n films is 50 nm for x=40 nm, n=10, that is smaller than the grain size of single-layer Nd2Fe14B films with 200 nm thickness. The average grain size of single-layer film is 160 nm. From selected area diffraction patterns, the structure of Nb spacer is amorphous, while that of Cr is columnar, crystalline, and with some soft magnetic Fe–Cr phase in the broken layer structure around the hard magnetic Nd2Fe14B phase. Therefore, the multilayer films with Nb spacer exhibit good hard magnetic properties. While for those with Cr spacer, random exchange-coupling effect was observed in demagnetization curves.

X-ray investigation of metastable crystalline phases in co-deposited Fe–Cr alloy nanometer films

Fe 100− x Cr x ( x=24.5–61.7, 100 at.%) alloy nanometer films prepared by crossed-beam pulsed laser deposition were examined by wide-angle X-ray scattering. On varying the Cr content, x, a change of phase content is observed starting from the known body-centered cubic phase at low Cr content. Indications for three new metastable Fe–Cr phases were found to form in body-centered tetragonal, face-centered orthorhombic and primitive orthorhombic lattice types, respectively.

パルス通電焼結法によるＴｂ‐Ｄｙ‐Ｆｅ‐Ｃｒ合金の磁歪および圧縮強度

Recently, the number of the car increases as the automotive industry develops. Therefore, the reduction of the environmental pollutant exhausted by the cars becomes a serious problem. Especially, the pollutant exhausted by the diesel engine such as NOx and particulate matter (PM) gives the life environment big damages. The generation of these environmental pollutants can be controlled by using magnetostrictive alloy for the control mechanism of the fuel injection system. Technical equipment is necessary for the industrial manufacturing though the single crystal of giant magnetostrictive alloy obtains large magnetostriction. Moreover, it is difficult for the single crystal to fabricate a compact with a complex shape. To obtain the compact of the polycrystal, the new process that mechanical alloying (MA) was combined with a pulsed current sintering(PCS) is reported to be effective. In this study, the effect of chromium addition on the magnetostriction and compressive strength of the (Tb0.5Dy0.5)(Fe1-xCrx)1.8 (x=0-0.2) compacts have been investigated. The powders of (Tb0.5Dy0.5)(Fe1-xCrx)1.8 (x=0-0.2) were synthesized by an MA of the elemental Tb, Dy, Fe and Cr powders using a planetary ball milling for 360 ks, and they were consolidated at 1273 K by PCS. The Cr addition has been very effective to improve compressive strength of the sintered alloys, which is attributed to the formation of ductile Fe-Cr phase. On the other hand, the Cr addition of x=0.2 decreased magnetostriction of the alloys due to an excess amount of Fe-Cr phase. As a result, it has been found that the optimum Cr content is x=0.05 for achieving large magnetostriction and high compressive strength simultaneously.

磁性材料 ＭＡ‐ＰＣＳ法で作製したＴｂ‐Ｄｙ‐Ｆｅ合金の磁歪特性および圧縮強度におよぼすＣｒ添加の効果

The powders of (Tb0.5Dy0.5)(Fe1-xCrx)1.8 (x=0-0.2) were synthesized by a mechanical alloying (MA) of the elemental Tb, Dy, Fe and Cr powders using a planetary ball milling for 360 ks, and they were consolidated at 1273 K by a pulsed current sintering (PCS). The Cr addition was very effective to improve compressive strength of the sintered alloys, which is thought to be attributed to the formation of ductile Fe-Cr phase. On the other hand, the Cr addition of x = 0.2 decreased magnetostriction of the alloys due to an excess amount of Fe-Cr phase. As a result, it was found that the optimum Cr content is x = 0.05 for achieving large magnetostriction (380 ppm at 112 kA/m) and high compressive strength (800 MPa) simultaneously.

FE-RICH PORTION OF THE FE-CR PHASE DIAGRAM: ELECTRON MICROSCOPY STUDY

Transmission electron microscopy and the corresponding selected-area electron diffraction patterns of Fe-(10;20;30;40;47) wt. pct. Cr alloys heat treated isothermally from 500°C up to 1500°C at 100°C was employed to construct the equilibrium Fe-rich portion of the Fe-Cr phase diagram. It has been found to be quite different from the generally accepted one. Two regions of phase separation are in the diagram at low and high temperatures. Two lines of the second-order phase transition “ordering-separation” are pointed also. The σ-phase region in the diagram is considered as a region containing two phases, namely, the σ-phase surface layer and the solid solution bulk.

New approximant phases in Al–Cr–Fe

Recently, new approximant phases were pointed out in the Al–Cr–Fe system, namely orthorhombic O-Al–Cr–Fe, hexagonal H-Al–Cr–Fe and monoclinic M-Al–Cr–Fe. In the corresponding analysed samples, the new approximant phases were always coexisting with metallic aluminium. We have studied the Al–Cr–Fe system within a broad composition range. In one alloy with composition Al81Cr11Fe8, two new crystalline approximants of the decagonal phase were discovered. These phases were analysed by differential thermal analysis (DTA), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and microprobe analysis. The phases were found to be isomorphic with a C3,I-phase earlier discovered by Van Tendeloo et al. in Al–Mn–Ni and an O1-phase earlier discovered by Dong et al. in Al–Cr–Cu–Fe. Moreover, we have found in the same sample the O-Al–Cr–Fe phase as well as metallic aluminium.

Structural and magnetic studies of sputtered Fe1−xCrx thin films

X-ray diffraction, magnetization, and Mössbauer effect investigations have been performed for sputtered Fe1−xCrx (0⩽x⩽0.54) thin films. The body-centered-cubic (bcc) phase appears for x<0.32, while the σ phase is formed for 0.38⩽x⩽0.44. For 0.32⩽x<0.38 and 0.44<x⩽0.54, the samples are composed of both bcc and σ phases. As the Cr concentration increases, the ferromagnetic fraction, magnetization, and magnitude of the hyperfine field decrease, whereas the magnitude of the isomer shift increases. The Fe and Cr magnetic moments, isomer shift, and hyperfine field of the bcc Fe–Cr phase have been deduced and are discussed consistently in terms of charge and spin distributions, as well as magnetic valence.

Sigma-phase equilibria and nucleation in Fe-Cr-Ni alloys at high temperature

The sigma-phase is of crucial importance because of the disadvantageous influence on mechanical properties of the commercial alloys, mainly lowering the impact toughness and reducing creep ductility. It is usually assumed that the sigma-phase forms in the iron based alloys which contain the high level Cr and the formation is not possible above 1,104 K and below about 713 K (critical temperatures for stable existence of sigma-phase in an Fe-Cr system). However, the temperature range of the thermodynamic stability and the sigma-phase nucleation cannot be derived from a binary Fe-Cr phase diagram but from the phase diagram of the system corresponding to the actual composition of the alloy. Great progress has been made in the calculations and predictions of phase diagrams. Using advanced theoretical models of the phases and programs for calculations of phase equilibria, it is possible to obtain phase diagrams for many systems including the Fe-Cr-Ni system.

Phase transformations during long-term ageing of NiFeCr alloys in the temperature range 450–600 °C

Phase equilibria in a selected phase region of the ternary NiCrFe system were investigated at temperatures between 450 and 600 °C. The experimental study, based on microstructural observation and electrical resistivity measurements, focused on the phase transformations occurring in a series of ternary alloys with 30%–67% Ni, 1%–51% Fe and 17%–32% Cr during ageing for up to 32 000 h. Those materials, which take on short-range order during earlier stages of ageing, undergo three metallurgical changes at very slow rates: long-range order based on Ni 2Cr, occurring at lower temperatures; the precipitation of the embrittling b.c.c. α′-Cr phase and, possibly, of the tetragonal σ-(FeCr) phase, occurring especially at higher temperatures. The degree of order and the ordering rate, which decrease with increasing temperature, fall with Cr concentration and are markedly reduced by increasing amounts of Fe. The amount and size of grain boundary precipitates increase with time and/or temperature of ageing, and with the Fe concentration.

Revision of thermodynamic descriptions of the Fe-Cr & Fe-Ni liquid phases

New thermodynamic descriptions of the Fe-Cr and Fe-Ni liquid phase have been obtained, which give better agreement with experimental data on the bcc/liquid or fcc/liquid tie-lines and liquidus temperatures in the Fe-rich region of the Fe-Cr-C and Fe-Cr-Ni ternary systems than in the previous assessments. The effect of the modification on the calculated phase equilibria in the other alloy systems including Fe and Cr or Fe and Ni were inspected and confirmed to keep the good agreements already obtained.

New Hardmetals based on TiB 2

Sintering of TiB 2-hardmetals with ferrous binders results in the formation of parasitic Fe 2B which consumes a good part of the ductile binder. Besides Fe 2B, Ti 2O 3 is formed during sintering. Thermodynamic considerations dilatometry and X-ray analysis show that the impurities in commercially available TiB 2-powders are responsible for the development of these phases. It is possible to avoid Fe 2B by adding metals with a higher affinity to boron than iron. Based on these investigations, TiB 2-hardmetals with single phase FeCrNi binders or two phase Fe 2BFeCrNi binders were produced. The binder content was varied between 5 and 20 vol.%. These TiB 2-hardmetals show better hardness-fracture toughness combinations than conventional WC-Co-hardmetals or Ti(C,N)-cermets and excellent hot hardness. These properties are very promising at the present state of development, and it appears that the full potential of the system has probably not yet been reached. Further progress seems to be possible, when fine grained TiB 2-powders with low O and C content and little excess boron become available. Such powders are expected to improve sintering behaviour, microstructure and mechanical properties of TiB 2-hardmetals.

Diffusion chromizing of ferrous alloys

The results of an extensive study of the mechanism of formation of chromium diffusion coatings on ferrous alloys are described. The effects of the significant coating variables on the composition, thickness and microstructure of the coatings are discussed. These variables include the pack geometry, substrate composition, type of halide activator, type of “inert filler”, time, temperature and the type of the chromium source material. The diffusion of chromium and chromium plus vanadium are considered. The chromium concentration profiles are seen to deviate from those predicted from the Fe-Cr phase diagram. Mechanisms for the coating formation are described.

Some Properties of Sintered Chromium Steel

In order to find the best way of adding Cr to sintered ferrous material, the authors investigated the fundamentals of the manufacturing process and the properties of the sintered chromium steel of the system Fe-Cr-C. Three kinds of chromium containing powders, metallic chromium, ferro-chromium and σ (sigma)-phase Fe-Cr powders, were used as a source of chromium for Fe-Cr-C alloys. The mixtures of iron powder, chromium-containing powder and carbon were compacted at 5t/cm2, pre-sintered for lhr at 750°C and sintered for lhr from 1100 to 1320°C. The results obtained are as follows.(1) Sintered chromium steels made from the σ-phase powder have the best properties of those from three kinds of chromium-containing powders. For example, the Fe-5% Cr-0.5%C alloy from σ-phase powder which was sintered at 1300°C has sintered density of 7.45g/cm3, tensile strength of 95kg/mm2, hardness of 110 HRB and elongation of 1.0%. The metallographical investigation showed that this alloy has a homogenized martensitic microstructure.(2) The properties of sintered chromium steel made from σ-phase powders, containing 1.5-10.5% chromium and 0-1.0% carbon are as follows: the sintered densities range from 7.36 to 7.50g/cm3, the tensile strength varies from 50 to 110kg/mm2, and the elongation from 0.6 to 120%.Microstructures of the as-sintered alloys depend on the Cr-content. The martensitic structures are found for more than 5% Cr. This phenomenon can be attributed to the self-hardenability of the Cr element. The properties the above mentioned alloys are in good agreement with those of ordinary cast wrought chromium steel.