A study of phase stability in invar Fe--Ni alloys obtained by non-conventional methods

It is known that thermodynamic equilibrium in Fe--Ni alloys, in the invar composition at temperatures below 450^°C, is difficult to achieve because of the slow diffusion rate at low temperatures. One of the ways in which we can study phase transformation which may be responsible for invar behavior is to investigate: (i) materials of similar composition obtained by non-conventional methods, known to allow the enhancement of diffusion at temperatures where atomic mobility is nil on the laboratory time scale; (ii) materials which have been treated for very long periods of time (geological time scale) in the same temperature range, such as in meteorites. In this context we have studied the phase stability of Fe--Ni phases in mechanically alloyed powders, in ion-beam mixed multilayers and in meteorites. This revised version was published online in August 2006 with corrections to the Cover Date.     

57Fe CEMS study on the surface damage of alloys produced by He+ ion bomabardment

Using the⁵⁷Fe conversion electron Mössbauer spectroscopy (CEMS), the structural and compositional changes at near surface of the amorphous Fe-13%B-9%Si and Fe-15%Cr-9%Ni stainless steel specimens have been clarified after the bombardment of 25kV accelerated He⁺ ions.     

Electron spectroscopic study of surface segregation and oxidation of Cu-In and Cu-Au alloys

XPS and AES techniques were employed to study the surface segregation and oxidation of Cu-1at%In alloy. Surface segregation of In has been observed with an enthalpy of segregation of about −34 kJ/mol. The surface oxidation of the Cu-In alloy at 480 K showed first a formation of Cu₂O on the alloy surface. The displacement reaction 3Cu₂O + 2In → 6Cu + In₂O₃ occurred on the alloy surface, on further heating of the oxidized surface in vacuum at 700 K. XPS and UPS techniques were employed to study the oxygen interaction with Cu-5at%Au and Cu-11at%Au alloys. At 300 K, oxygen was dissociatively chemisorbed on the Cu sites of the Cu-11% Au alloy surface, and the oxygen desorbed on heating. TDS study showed that desorption follows second order kinetics with an activation energy of desorption of about 75 kJ/mol.     

CEMS investigation of near surface structure

Conversion Electron Mössbauer Spectroscopy (CEMS) studies are reported for as-cut and laser melted surfaces of single phase crystalline Fe₂Y, Fe₂₃Y₆, Fe₂Zr, Fe₂B and FeB ingots. Disorder and the appearance of a new phase with a low value of the room temperature hyperfine field was observed for the Fe?Y and Fe₂Zr ingots even on the as-cut surfaces due to the mechanical processing. In case of these ingots surface melting by ns laser pulses resulted in the formation of amorphous alloys. In case of the Fe?B ingots the formation of amorphous phase by laser melting was observed for Fe₂B only, while in case of FeB the low temperature α-FeB modification appeared both, for mechanical processing and laser melting.     

CEMS investigation of laser melted Fe−Zr alloys

Conversion Electron Mössbauer Spectroscopy (CEMS) studies are reported for as-cut and laser melted surfaces of Fe?Zr ingots in the 25–80 at.% Zr composition range. Disorder and amorphization was observed even on the as-cut surfaces due to the mechanical processing. Besides a significant enhancement of the non-crystalline fraction, surface melting by as laser pulses also results in the appearance of new metastable phases. Solidification via an extremely high cooling rate thus produces amorphous phase in composition ranges where its formation was previously assumed to be restricted to non melt-quenching methods only.     

A Monte Carlo study of surface segregation in alloys

The Monte Carlo method has been applied to the study of surface segregation in a multi-layer, regular solution model of alloy surfaces. Three different alloy configurations have been investigated: semi-infinite slabs, thin films and small particles. The results show that the alloy component with the lowest surface energy tends to segregate to the first three or four surface atom layers and that segregation is greater in clustering alloys than in ordering alloys. Furthermore, segregation is more pronounced in low coordination surfaces, as evidenced by a comparison of {110} and {100}-oriented surfaces of fcc alloys. The degree of surface segregation in thin films and small particles (in the particle size range studied) tends to be smaller than in semi-infinite slabs, because of mass conservation constraints, and decreases with decreasing film thickness and particle size. The results obtained are contrasted with previous calculations and possible avenues for improving surface segregation models are discussed.     

CEMS study of silicon implanted iron

Thin layers of iron-rich Fe-Si alloys were formed by silicon implantation into iron at room temperature with different energies (100, 200, and 300 keV) and ion doses (2 × 10¹⁷ to 1×10¹⁸ cm^–2). The produced layers were investigated by⁵⁷Fe conversion electron Mössbauer spectroscopy (CEMS) to identify the phases formed by the ion implantation. Auger electron spectroscopy (AES) was used to measure the concentration depth profiles of the implanted silicon. Depending on the implantation parameters different disordered Fe-Si structures were detected. At low doses only magnetic phases were formed while at high doses a non-magnetic phase with a hitherto unknown structure appeared. Annealing of the samples resulted first in the formation of a D0₃-like short-range order and a slow decrease of the non-magnetic phase, and subsequently in the migration of Si out of the investigated depth range.     

A method for the study of surface segregation in multicomponent alloys

A simple algorithm for the determination of segregation profiles in multicomponent systems based on a mean field formalism and a quantum approximate method for the energetics is introduced. The method is described and applied to two ternary systems, concentrating on the changes in segregation patterns relative to the corresponding binary cases.     

CEMS study on aluminium implanted iron

Up to now a great deal of investigations in ion beam mixing of iron-aluminium layers are known. However, the easier way to produce such layers by direct implantation of aluminium ions in iron is less studied. In the present work aluminium implanted iron layers are studied. Iron samples were implanted with aluminium ions at 50, 100, and 200 keV, respectively, with doses between 5×10¹⁶ and 5×10¹⁷ cm⁻². Independent of energy, at doses up to 2×10¹⁷ cm⁻², besides alpha iron further magnetic fractions with a Fe₃Al-like structure are formed while at a dose of 5×10¹⁷ cm⁻² amorphous nonmagnetic components are formed.     

CEMS study of H2 annealed YIG films

We report on a CEMS investigation of the surface layer obtained in pure YIG film after an annealing of 20 h in H₂ at 480°C. We show that the surface layer has a garnet structure with the hyperfine fields coinciding with those of pure YIG. The magnetization in the surface layer is found to be parallel to that of the underlying film bulk. Furthermore we analyze the line widths of the Mössbauer peaks pertaining to such surface layer and compare the results with those obtained for a YIG film implanted with two different doses of Ne⁺, namely 7x10¹³ Ne⁺ cm^-2 at 50 keV and 2.7x10¹⁴ Ne⁺ cm^-2 at 50 keV. From this comparison it results that the peaks for the annealed film do not show any broadening while those for the implanted film are broadened. This clearly indicates that, within the experimental error, there is no damage in the surface layer obtained by annealing in H₂ and, in any case, the damage is much lower than found in the surface layers of YIG films implanted with doses of practical interest.     

A positron annihilation study of the Fermi surface in concentrated α-phase CuGa alloys

High resolution positron annihilation studies upon a sequence of copper- gallium alloys have determined the variation of the important Fermi surface features with gallium concentration. The Hume-Rothery rules do not arise from contact between the Fermi surface at the [200] Brillouin zone boundary.     

Metallographic study of the oxidation of Ni3Al-Ni3Nb alloys

Conclusions A study has been made of the microstructure of the scale formed on single- and two-phase alloys of the system Ni₃Al-Ni₃Nb and also on the compound Ni₃Nb. It was found that the scale on the alloys investigated consists of several layers, and the microhardness of each layer was measured.An X-ray phase analysis was made of each layer of the scale formed on Ni₃Nb after 100 hours oxidation at 1100 °.A scheme has been devised which shows the distribution of the oxides in the scale on the compounds Ni₃Al and Ni₃Nb and also on single- and two-phase alloys at 650 and 900 °. Some ideas concerning the mechanism of formation of the individual oxides are put forward.     

CEMS study of defect annealing in Fe implanted AlN

An AlN thin film grown on sapphire substrate was implanted with 45 keV ⁵⁷Fe and ⁵⁶Fe ions at several energies to achieve a homogeneous concentration profile of approximately 2.6 at.%. in the AlN film. Conversion electron Mössbauer Spectroscopy data were collected after annealing the sample up to 900 °C. The spectra were fitted with three components, a single line attributed to small Fe clusters, and two quadrupole split doublets attributed to Fe substituting Al in the wurtzite AlN lattice and to Fe located in implantation induced lattice damage. The damage component shows significant decrease on annealing up to 900 °C, accompanied by corresponding increases in the singlet component and the substitutional Fe.     

CEMS study of a nitrogen implanted CrNi 18.9 steel

Conversion electron Mössbauer spectroscopy (CEMS) has been used to study a commercial X10CrNiTi 18.9 steel implanted with varying nitrogen doses and at varying implantation temperatures. At low nitrogen doses the fcc γ-phase transformed to the bcc structure. With increasing implantation dosages we have observed the precipitation of Fe-nitrides and a reverse (α→γ) transformation. High implantation temperatures lowered the nitrogen content of the Fe-nitrides and favoured the formation of Cr-nitrides.     

Surface oxidation behavior of MgNd alloys

The oxidation kinetics of MgNd alloys oxidized in pure O₂ at high temperatures has been investigated. The results revealed two stages of the reaction: A fast initial oxidation was followed by a slow oxide growth with a parabolic kinetics. For MgNd alloys (Nd = 25 wt.%), the oxidation rate increased with the enhancement of the oxidation temperature. A sudden ignition was found for this alloys oxidized at 873 K up to about 80 min. Moreover, the increase of the Nd content would harm the oxidation resistance of the MgNd alloys. By Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis, it was found that a triplex structure of oxide film formed. The outer layer was composed of MgO, Nd₂O₃ and Nd(OH)₃, the middle layer mainly consisted of MgO and Nd₂O₃, and the inner layer was the transitional layer made of MgO, Nd₂O₃ and the content of the substrate. The protective oxidation was associated with the formation of the dense Nd₂O₃/MgO layer during isothermal oxidation process. The oxidation mechanisms for the formation of oxide film are discussed.     

A calorimetric study of lead-thallium alloys

Heat capacity is studied in alloys of the system lead-thallium over the concentration range from pure lead to pure thallium. The alloys were studied after various thermal processing. On the basis of calorimetric and physicochemical properties it is established that in this system there are formed inter metallic compounds with compositions Tl₇Pb, Tl₃Pb, Tl₅Pb₃.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 11–14, January, 1976.     

Spectroscopic study of the surface oxidation of mechanically activated sulphides

Surfaces of chalcopyrite CuFeS₂, sphalerite ZnS, pyrite FeS₂ and galena PbS modified by mechanical activation have been studied by means of infrared (IR) spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray photoelectron spectroscopy (XPS). The different oxidation species were identified on mechanically disordered surfaces of the minerals. The mechanochemical surface oxidation increases with the time of mechanical activation. The sulphides under study are partially covered with oxidised sulphates and carbonate species. They are distinguished by an affinity to the formation of surface compounds. According to the affinity of sulphides for mechanochemical S²⁻→S⁶⁺ surface oxidation the decreasing rate was observed in the following order: FeS₂>PbS>CuFeS₂>ZnS. The ratio between bivalent and hexavalent form of sulphur depends on the degree of mechanochemical surface oxidation of the sample under study.