Cr Si Alloys Research Articles

Compositional dependence of the Néel transition, structural stability, magnetic properties and electrical resistivity in Fe–Mn–Al–Cr–Si alloys

Compositional dependence of the Néel transition temperature, T N, structural stability, magnetic properties, and anomaly of resistivity, ρ, have been investigated in γ-Fe–Mn–Al–Cr–Si–C alloys. In the Fe–(20–32.5 at.%)Mn-based alloys, an increase in Mn, Al, Cr or C content will inhibit the γ→ε→α′ martensitic transformation and lower the ε martensitic transformation temperature M s or deformation-induced martensitic transformation temperature M d, while an increase in Si content promotes the γ→ε martensitic transformation and increases the M s and M d temperature. Manganese raises T N, decreases the susceptibility, χ, and slightly increases the anomaly of electrical resistivity, ρ. The temperature dependence of χ for Fe–Mn binary alloys shows a break at T N and χ becomes almost independent of temperature above T N. The effects of Cr on χ versus T and ρ versus T are similar to that of Mn but lowering T N. With increasing Al or Si content, T N decreases markedly; above T N, the magnetic state of the alloys changes from Pauli-paramagnetism to the Curie–Weiss behavior, and the anomalous ρ increases rapidly, leading to a negative temperature coefficient below T N. An empirical formula has been developed to express the compositional dependence of T N in γ-Fe–Mn–Al–Cr–Si alloys. The alloying elements that lower T N are in increasing order of severity, Cr, Al, Si and C. Manganese is the only element which both raises T N and decreases χ and M s. Adding up to 10 at.% Al to Fe–Mn based alloys strongly stabilizes the γ phase and retains antiferromagnetism down to 4 K. Increasing the Ni content of Fe–Cr–Ni steels improves their stability relative to the α′ martensitic transformation but induces a transition from the paramagnetic γ-phase to the ferromagnetic one at very low temperatures. Alloying Fe–Mn based alloys with Al and Cr exhibits an excellent combination of FCC structural stability, corrosion resistance and antiferromagnetism down to liquid helium temperature. The present results are valuable for the design of cryogenic non-magnetic steels and certain functional Fe–Mn based alloys.

The formation of diffusion coatings on some low-alloy steels and their high temperature oxidation behaviour: Part 2. Oxidation studies

The oxidation of aluminised coatings on FeNi, FeSi and FeCr alloys and of chromised coatings on FeNi and FeSi alloys were studied at temperatures in the range 700 to 1000°C and at oxygen pressures from 10 −8 to 1 atm. and related to appropriate phase diagrams. Up to 800°C Al 2O 3 or Cr 2O 3 scales were formed leading to good oxidation resistance. At higher temperatures (∼1000°C) the oxidation resistance was frequently lost due to oxide-scale failure and formation of iron oxides. Only aluminised Fe-19.2 wt%Cr alloy remained fully protective up to 1000°C in pure oxygen. Parabolic rate constants for the initial period of oxidation were a linear function of oxygen pressure. Activation energy data for the oxidation of chromised steels gave values in reasonable agreement with those obtained from the literature for the oxidation of pure Cr and FeCr alloys. The effect of substrate structure on the oxide-scale failure mechanisms was studied in detail. The cracking of the scales has been shown to be associated with substrate grain boundaries and the formation of nodular oxides has been shown to be associated with grain boundaries and in particular with the triple points of grain boundaries.

Formation and properties of Cr–Si sputtered alloys

Cr–Si metastable alloys were prepared by RF sputtering in a low-pressure argon gas. The structure of sputtered alloys results in Cr (Si) solid solution, Cr+Cr 3Si, the amorphous phase with increasing Si content for Cr–Si alloys. The dissolution hardening effect of Si increases the microhardness of Cr(Si) solid solution. The microhardness of Cr–Si sputtered alloys yields the peak around 50 at% Si, and furthermore, the microhardness of Cr–Si alloy, with Si rich content of 80 at% or more increases with Si content. These results suggest that the bonding character of Cr–Si amorphous alloys is different from the metallic-to-covalent bonding of Si-rich alloys. This bonding character difference for Cr–Si rich alloys is also seen in the Si content dependence of the crystallization temperatures for Cr–Si alloys.

Spindigtheidsgolfgedrag van Cr-Si- en Cr-Ga-allooi-enkelkristalle

The spin-density-wave (SDW) effects in Cr-Si and Cr-Ga alloy single crystals were investigated by means of thermal expansion, electrical resistivity and ultrasonic wave velocity measurements. The complete temperature-concentration and temperature-pressure magnetic phase diagrams of Cr-Ga were constructed.

Cross-sectional TEM investigations of the influence of P + ions implanted in the Si substrate on the atomic interdiffusion in the Cr–Si system

Cross-sectional transmission electron microscopy was used to study the effect of doped silicon substrate on the formation of CrSi 2 disilicide. A chromium film 800 Å thick was electron gun deposited onto unimplanted and phosphorus implanted Si(111) substrates. The implanted dose was 5×10 15 at. cm −2 at 30 keV. The Cr–Si samples were heat treated in vacuum at 475°C for different times. Transmission electron microscopy investigations, performed on doped and undoped Si substrates, have shown that the presence of P + ions resulted in the delay of the CrSi 2 compound growth. In addition, the nanoanalysis of the samples with P + implanted silicon has revealed the apparition of a crystalline Si–Cr alloy in the Si substrate near the CrSi 2–Si interface, the formation of an amorphous Si thin layer between the formed silicide and this alloy, and a diffusion of Si atoms towards the free surface.

Influence of a magnetic field on the magnetoelasticity of commensurate spin-density-wave Cr–Ru and Cr–Si alloy single crystals

Previously reported attempts to produce a single-Q commensurate (C) spin-density-wave (SDW) state in a dilute Cr alloy by magnetic field cooling through the Néel temperature (TN), were unsuccessful. In the present study the influence of a cooling field of 4.5 T on the magnetoelasticity of Cr+0.3 at.% Ru and Cr+1.6 at.% Si alloy single crystals, both having CSDW phases below TN, are reported. The results show that cooling the Cr+0.3 at.% Ru crystal through TN in a field of 4.5 T has no influence on the first-order commensurate–incommensurate SDW magnetic phase transition. On the other hand, cooling the Cr+1.6 at.% Si crystal in a field of 4.5 T through TN noticeably affects the elasticity in the CSDW state, suggesting a condensation of part of this crystal into a single-Q CSDW phase.

Fe-based magnetic shape memory alloy-sheet specimen

We contrived a new composite functional alloy which is compatible for both magnetization and the shape memory effect (SME). A FeMnSiCr alloy, one of the shape memory alloys (SMA) has strong magnetization in addition to SME after heating in air. It is because the specimen is composed of a ferromagnetic iron oxide Fe 3O 4 (magnetite) surface layer, the inside being SMA. The measurements were carried out on a sheet specimen.

First-order paramagnetic-to-commensurate phase transition in Cr alloys

The coexistence of spin- and charge-density waves in pure Cr and its dilute alloys continues to fascinate both experimentalists and theorists. Using a three-band model, we show that the charge-density wave (CDW) in incommensurate (I) Cr alloys is governed by the same Coulomb interaction that produces a first-order N\'eel transition in commensurate (C) alloys. Therefore, we predict that the first-order paramagnetic-to-commensurate transition observed in CrFe and CrSi alloys is preceded by the growth of a large CDW in the I phase of each alloy. We also provide a justification for the three-band model in the I phase and show that it reduces to the correct form in the C limit.

Wetting of silicon carbide by chromium containing alloys

We have studied the wetting behaviour of Cu–Cr, Cu–Ni–Cr, Cu–Si–Cr and Cu–Ni–Si–Cr alloys on sintered β-SiC. The addition of Cr to liquid Cu or Cu–Ni alloys significantly improves their wettability on SiC. The reactions between the Cu–Cr or Cu–Ni–Cr alloys and SiC lead to the formation of a duplex layer containing Cr 3C 2 and a mixture of Cu and C phases. The formation of this duplex layer can be suppressed by the addition of sufficient Si to the Cu–Cr alloys and the addition of Si does not significantly alter their behaviour. Cu–Ni–Si–Cr alloys also suppressed the formation of the duplex layer below some critical Cr concentration. A thermodynamic model developed for metal/oxide systems can be used to explain the behaviour of their alloys on SiC and also their behaviour on Si 3N 4.

Coupled spin- and charge-density waves in chromium alloys

Both the spin- and charge-density waves in Cr alloys are produced by the Coulomb attraction between electrons and holes on two nested Fermi surfaces. The Coulomb interaction responsible for the charge-density wave increases the energy splitting between quasiparticles which are strongly and weakly bound to the spin-density wave. If the splitting is sufficiently large, the paramagnetic-to-commensurate transition becomes strongly first order, as observed in CrFe and CrSi alloys.

Promising CuNi&.sbnd;CrSi alloy for first wall ITER applications

Precipitation-hardened CuNiCrSi alloy, a promising material for ITER applications, is considered. Available commercial products, chemical composition, physical and mechanical properties are presented. Embrittlement of CuNiCrSi alloy at 250–300°C is observed. Mechanical properties of CuNiCrSi alloy neutron irradiated to a dose of ∼0.2 dpa at 293°C are investigated. Embrittlement of CuNiCrSi alloy can be avoided by annealing.

Thermodynamic modelling of wetting at silicon nitride/NiCrSi alloy interfaces

Through thermodynamic modelling of the reaction at ceramic-metal interface, wetting of a sintered silicon nitride was investigated with a series of Ni-based brazing alloys without addition of active elements. The thermodynamic calculations showed that NiCrSi alloys with Ni/Cr ratio of 3.5 and X Si < 0.25 will react with and wet Si 3N 4 ceramics at 1220 °C (1493 K) and P N 2 = 15 Pa. Environment verifications using sessile drop method confirmed the validity of the calculations and the assumptions made in the wetting model.

Spin-density-wave antiferromagnetism in chromium alloys

The account given here of the macroscopic and microscopic physical properties of antiferromagnetic (AFM) chromium alloys supplements the previous review of spin-density-wave (SDW) antiferromagnetism in pure Cr. The existence of an incommensurate spin-density wave results from Fermi-surface nesting, which changes with electron concentration as atoms of different valence are introduced into Cr. This gives rise for most impurities to a prototypical composition-temperature ($x\ensuremath{-}T$) magnetic phase diagram, which is described quite well by a model of nesting electron-hole octahedra and electron reservoir, with changes in the amplitude of the spin-density wave and its wave vector corresponding to the changes with impurity concentration of the N\'eel temperature. There are, however, numerous cases of idiosyncratic behavior in the $x\ensuremath{-}T$ phase diagram, some of which appear to correspond to unusual features in the pressure-temperature ($p\ensuremath{-}T$) phase diagram, e.g., in the CrFe, CrAl, and CrSi alloy systems. With these exceptions and a few others, the effect of impurities in the same group of the periodic table is fairly similar, so that this classification is used for the description of both the $x\ensuremath{-}T$ and $p\ensuremath{-}T$ phase diagrams. The properties of the alloy systems for each group of impurities are first summarized in the context of the $x\ensuremath{-}T$ phase diagram. The general features of the various physical properties are then considered: (1) magnetic susceptibility, whose temperature dependence signals the existence of local moments in some cases; (2) transport properties, in particular, electrical resistivity and thermopower, which show characteristic temperature dependence corresponding to the occurrence of spin fluctuations around the N\'eel temperature ${T}_{N}$, and in some cases well into the paramagnetic phase, followed below ${T}_{N}$ by effects due to the electron-hole condensation in the ordered AFM phase, and in some cases at lower temperatures to the formation of local impurity states; and (3) magnetoelastic properties, which show large effects associated with the spin-density wave and with SDW fluctuations in Cr alloys just as in pure Cr. Analysis of magnetic anomalies in the thermal expansion and bulk modulus in terms of magnetic Gr\"uneisen parameters is employed for some Cr alloy systems to describe the large body of experimental data. The empirical correspondence between increasing volume and increasing electron concentration is illustrated for several systems. The study of dilute Cr alloys provides insight into the reason for pure Cr having a weak first-order N\'eel transition, and provides examples of other phase transitions, including the incommensurate-commensurate SDW, the spin-glass, and AFM-superconducting phase transitions. The similarities between the incommensurate SDW fluctuations in Cr and Cr alloys and in the high-temperature superconducting cuprates strongly motivates, in particular, the examination of the inelastic neutron-scattering results. Optical properties provide an interesting picture of the relation between the magnitude of the energy gap at the Fermi surface and the N\'eel temperature, which is quite different in the incommensurate and commensurate SDW alloys. Local probes providing microscopic information about the environment of an impurity are clearly of great potential value in Cr alloy systems. The M\"ossbauer effect has, however, yielded disappointingly little information, but other probes such as perturbed angular correlation and nuclear magnetic resonance have provided interesting results.

High-temperature phase equilibria in Cr-Cr3Si two-phase alloys

Phase equilibria in two-phase Cr-Cr3Si alloys have been investigated using metallography, XRD, and electron microprobe analysis. Cr-Si alloys with compositions of up to 15.0% Si were directionally solidified using cold crucible Czochralski crystal growth and equilibrated chemically at either 1200,1400, or 1600 °C. Measured Si concentrations of the Cr and the Cr3Si phases were found to be significantly lower than those given by the most recent assessment of the Cr-Si phase diagram, particularly at temperatures above 1200 °C. This explains why the volume fraction of Cr3Si in the directionally solidified eutectic is lower than that predicted by the assessed phase diagram. X-ray lattice parameter data for both Cr and Cr3Si are also reported.

Magnetoelasticity of dilute Cr-Si alloy single crystals

Measurements are reported of the temperature dependence of the elastic constants and thermal expansion of dilute Cr-Si alloy single crystals containing 0.5, 1.2, 1.6 and 3.0 at.% Si. Well defined anomalies in all elastic constants, and in the thermal expansion, were observed at the Neel points (TN) and at the commensurate-incommensurate spin density wave transition temperatures (TIC) of the alloys. Elastic anomalies were also observed at the longitudinal-transverse incommensurate spin density wave transition temperature of the Cr+0.5 at.% Si crystal, but not in the thermal expansion thereof. The transition at the Neel point of the Cr+0.5 at.% Si crystal is continuous, that of the Cr+1.2 at.% Si and Cr+1.6 at.% Si crystals of first order, while the order of the transition in the Cr+3.0 at.% Si crystal could not be determined unambiguously. For the Cr+1.2 at.% Si and Cr+1.6 at.% Si crystals the discontinuous transitions at TN show hysteresis behaviour. In the 1.2 at.% Si crystal it is also accompanied by structure effects that were absent for 1.6 at.% Si. Hysteresis, accompanied by structure effects, was also observed at TIC for Cr+1.2 at.% Si. The structure effects are ascribed to the possibility of mixed incommensurate-commensurate spin density wave states in the dilute Cr-Si magnetic phase diagram. The magnetovolume and magnetic contributions to the bulk modulus have been found to fit the equation a+bT2+cT4, predicted by theory, rather well up to temperatures (T) close to the Neel points of Cr+0.5 at.% Si and Cr+1.6 at.% Si.

Large unit cell crystal approximant in TiCrSi alloys

We report the discovery of a new, apparently stable, large unit cell bcc structure in slowly cooled TiCrSi alloys. This bcc phase, which is also found in TiMnSi and TiVSi alloys, is likely the crystal approximant related to i-phase formation in the Ti-3d transition metal alloys. Single phase samples of this material were obtained by prolonged annealing at 1200°C. We report the results of a Rietveld structural analysis performed on X-ray powder data obtained from these single phase samples which indicate that this phase is closely related to the α(AlMnSi) phase, a crystal approximant related to the icosahedral phase in AlMnSi systems.

Characteristics of point defects and their clustering processes in FeMnCr and FeMnSiCr alloys

Evolution of microstructure in Fe18.5wt.%Mn7.5wt%Cr and Fe17wt.%SiCr alloys under electron irradiation at elevated temperatures was examined with a high-voltage electron microscope. In both alloys interstitial type dislocation loops were formed, but those in the FeMnSiCr alloy transformed into Si-rich precipitates by the heavy irradiation at high temperatures. From the temperature dependence of the interstitial loop density, the migration energy of the interstitials was estimated to be 0.7 eV and 1.1 eV for the FeMnCr and FeMnSiCr alloys, respectively. The FeMnSiCr alloy showed very strong swelling resistance.

Silicon's role in determining swelling in neutron-irradiated FeNiCrSi alloys

Two silicon-modified alloy series, one based on Fe15Cr20Ni and another based on Fe15Cr25Ni were irradiated at target temperatures between 399 and 649°C in EBR-II. The influence of silicon on swelling is more complex than previously envisioned and indicates that silicon plays two or more competing roles while in solution. Radiation-induced formation of γ'(Ni 3Si), precipitates is dependent on silicon and nickel content, as well as temperature. Precipitation of γ' appears to play only a minor role in void formation.

Note on mixed magnetic states and critical behaviour in Cr-Si alloys

Using an interferometric technique accurate measurements of the thermal expansion of two Cr-Si alloys containing 1.42 and 1.79 at.% Si in the temperature range 80 K ≤ T < 300 K are presented. The incommensurate-commensurate transition in the 1.42% alloy displays hysteresis indicating the existence of mixed IC states. The previously found step-like transitions in the thermal expansion coefficient of both alloys at the Néel temperature have widths of approximately 3 K, which preclude reliable determinations of the critical exponent near the transitions.

High pressure study of the antiferromagnetic state of chromium alloys

We have studied the pressure dependence of the Neel temperature (TN) in Cr-Si, Cr-Ge, and Cr-Sn alloys. In all alloys we have found a transition to the commensurate antiferromagnetic spin density w...