Hiromitsu Ino

Local interactions in carbon-carbon and carbon-M (M: Al, Mn, Ni) atomic pairs in FCC gamma -iron

Mossbauer spectra of Fe-8.0 at.%C, Fe-10.9 at.%Al-8.2 at.%C, Fe-1.4 at.%Mn-7.9 at.%C, Fe-2.4 at.%Mn-7.8 at%C and Fe-14.6 at.%Ni-6.2 at.%C FCC gamma -irons have been measured in order to investigate the distribution and the local interactions of C in the lattice. The local interaction energies in nearest and second-nearest C-C and M-C (M: Al, Mn, Ni) atomic pairs have been determined by a Monte Carlo method simulating the distribution obtained. The interaction energies in nearest and second-nearest C-C pairs are strongly repulsive, in contrast to the weak interaction in second-nearest N-N pairs. The results are compared with the interaction energies derived from activity data. The interaction in nearest Al-C pairs is repulsive but that in second-nearest pairs is strongly attractive, which can lead to the formation of the perovskite Fe3AlC-type ordering. The interaction between nearest Mn and C atoms is strongly attractive, while between Ni and C the interaction is very weak.

Mössbauer Effect in Iron-Carbon Martensite Structure

The Mossbauer spectra in iron-carbon martensite could be resolved into four components arising from the 1st, 2nd, (3, 4)th neighboring iron atoms for the carbon atoms and the remnant iron atoms. The carbon atoms were found to cause localized changes of electronic states of the surrounding iron atoms. The internal magnetic field, isomer shift and quadrupole splitting for the first neighbors largely deviated from those for pure iron, i.e. , 265±2 KOe, -0.03±0.05 mm/sec and 0.13±0.05 mm/sec, respectively, suggesting the covalent admixture between the iron and carbon atoms. The internal fields and isomer shifts for the 2nd and (3, 4)th neighbors also deviated slightly from those of pure iron, but with the sign opposite to those for the 1st neighbors. The quadrupole splittings for them were nearly zero, which mean that the positive charge of a carbon ion is almost screened out within a distance of the order of 2A.

A Study of the Mössbauer Effect during the Tempering of Iron-Carbon Martensite

The Mossbauer effect of 57 Fe in the iron-carbon martensite steels was measured during their tempering process, and the appearance of the e -, χ- and cementite phases and the bonding nature in them were studied. It was clarified that the χ-carbide is formed at the tempering stage III a, the formation of which had not been fully proved in previous X-ray or magnetic measurements. It was also concluded that the electronic states of the iron atoms co-ordinating the carbon atoms in the martensite and the e -carbide formed in the stage I are substantially the same, suggesting that the motive force in the stage I is mainly the relaxation of the large strain energy in tne martensite structure. The observed localized and additive nature of the iron-carbon interaction, which was concluded from the linear relation between the amount of the internal field reduction and the number of carbon co-ordination for iron atoms in the various phases, seems to indicate that the Alexander-Anderson-Moriyas idea is applicable to ...

Interaction and arrangement of nitrogen atoms in FCC γ-iron

A Mossbauer effect measurement of 57Fe has been performed for the Fe-N austenite in order to study the interaction between nitrogen atoms and the distribution among the octahedral sites of the FCC lattice. The spectrum for Fe-N austenite is decomposed into one singlet gamma 0 and two sets of doublets gamma 1 and gamma 2 which are identified as being caused by iron atoms with different configurations of nitrogen atoms at the nearest-neighbour sites. The fractional intensities for the three components are precisely determined by taking into account the effect of the absorber thickness. It is concluded from the analysis that the interaction between the first-nearest-neighbour nitrogen atoms is strongly repulsive and that between the second nearest nitrogen is weakly attractive.

Stoichiometry of Quasicrystalline Al–Mn

Electron microscopy has shown that in rapidly solidified Al–(14–20) at.%Mn alloys a new metastable phase of a quasicrystalline phase with the icosahedral symmetry, which was first discovered by Shechtman et al . (Phys. Rev. Lett. 53 (1984) 1951), is formed but with a considerable amount of fcc Al second phase in alloys with lower Mn content. From the change with Mn content of the intensity of fcc Al lines in the powder X-ray diffraction spectrum, the stoichiometry of the Al–Mu quasicrystalline phase is concluded to be nearly Al 4 Mn (∼20.5 at.%Mn), being at variance with the reported composition of Al 6 Mn.

Induced Magnetic Moment in Ferromagnetic Fe Alloys by Tetragonally Elongated Lattice Expansion

Magnetization and Mossbauer spectra were measured for Fe-C, Fe-N and Fe-Ni-C systems. The mass ratio of the tetragonal martensite in the mixed phases was determined by means of Mossbauer spectroscopy, leading to the average magnetic moment of Fe atoms in the body centered tetragonal (bct) structure. The magnetic moment of Fe atoms increases from 2.2 µ B to 2.6 µ B as the axial ratio and the volume of unit cell increase. In other words, the volume expansion and/or the tetragonal elongation seem to cause an increase in the magnetic moment of Fe in bct alloys. It is concluded that the variation of the magnetic moment of Fe in the bct structure can be explained as a superposing effect of both volume expansion and tetragonal expansion. The mechanism of these effects is discussed in terms of energy band splitting of d electrons and magnetovolume effects.

A study of interstitial atom configuration in fresh and aged iron-carbon martensite by mossbauer spectroscopy

Mossbauer spectra of Fe-1.5 ∼ 7.2 at %C alloys without and with a 5T longitudinal external magnetic field were measured to clarify the structure of iron carbon martensite. The spectra of freshly quenched martensite are decomposed into three magnetic hyperfine patterns having different fields. The 1st component with a hyperfine field of H ∼- 27 T is identified as the absorption caused by 1st neighboring iron atoms of octahedral carbon atoms. The 2nd component (H ∼- 31.5 T). the intensity of which rapidly decreases during aging, is due to 1st neighboring iron atoms of tetrahedral carbon or possibly 2nd nearest neighbors of octahedral carbon atoms. The 3rd main component (H ∼- 34T) is due to iron atoms not strongly perturbed by more distant carbon neighbors. The appearance of three additional components (H ∼- 37. 26.5 and 18T) in the aged martensite indicates the formation of the ordered non-stoichiometric Fe4Cx(x < 1) phase. The Mossbauer spectra of the epsilon carbide formed after aging at 413 K are decomposed into three hyperfine components corresponding to different iron environments in the carbide, which suggests the existence of vacant sites of carbon in the Fe2C structure.

Crystallization process and magnetic properties of Fe100−xBx(10≤x≤35) amorphous alloys and supersaturated state of boron in α-Fe

Amorphous specimens of Fe100−xBx were prepared in the range 10 ≦× ≦ 35 at % B by a single-roller method. The crystallization process and the boron concentration dependence of the Curie temperature were examined by differential scanning calorimetry, X-ray diffraction, Mössbauer spectroscopy and magnetic measurements. Two-step crystallization was observed in specimens with× < 17: amorphous → amorphous + boron-supersaturated b c c phase (α-Fe(B)) → t-Fe3B +α-Fe. A singleα-Fe(B) phase was not observed. The transition temperature from t-Fe3B to stable (α-Fe + t-Fe2B) sensitively depends on the boron content in the alloys. The crystallization temperature (Tx) of the amorphous alloys was almost unchanged for 17 ≦× ≦ 31, but increased remarkably at high boron concentrations of× ≧ 33, where the decomposition products consisted of t-Fe2B and o-FeB. The Curie temperature (Tc) of the amorphous phase was as low as 480 K at× = 10, increased with increasing boron content up to 820 K and then decreased in the high boron concentration alloys of× > 28. A singleα-Fe(B) phase was not detected in the as-quenched specimens of× = 8 and 10. The phase coexisted with the o-Fe3B and amorphous phases. The lattice parameter of the phase was 0.28610 nm which was smaller than that of pure iron by 2/1000, indicating the substitutional occupation of boron atoms in the b c c lattice.

Structure and Stability of Quasicrystalline Al–Mn Alloys

The quasicrystalline (qc.) phase with the icosahedral symmetry in melt-spun Al 100- x Mn x ( x =14, 17, 20) has been investigated by X-ray diffraction, TEM and DSC. Nearly single phase quasicrystal is obtainable for Al 80 Mn 20 when the quenching rate is sufficiently fast, while slower quenching rate produces an alternative metastable phase called T phase. On heating, the qc. phase in Al 80 Mn 20 changes to T phase at around 300°C and then at around 600°C to the stable hexagonal Al 4 Mn, whereas the qc. phase in Al 86 Mn 14 crystallizes directly to the stable orthorhombic Al 6 Mn. Heat evolution of the latter reaction is 0.40 kcal/mol, while those of the qc. phase to T phase and T phase to the hex.-Al 4 Mn in Al 80 Mn 20 are 0.12 and 0.17 kcal/mol, respectively. Atomic model of the qc. phase with a stoichiometric composition of Al 78.35 Mn 21.65 is proposed.

A pairwise interaction model for decomposition and ordering processes in B.C.C. binary alloys and its application to the Fe-Be system

Abstract Decomposition and ordering processes in b.c.c. binary alloys are calculated by a pairwise interaction model. The interaction energy includes two terms due to a negative interchange energy V between the 1st neighbouring atoms which represents the ordering tendency and due to a positive interchange energy U between the 2nd ones representing the decomposition tendency. Both phase decomposition and B2-type ordering occur, simultaneously, as indicated by the calculated phase diagram. When U is larger than the absolute value of V , the spinodal decomposition region covers the large part of the ordered region, but not the disordered region. The DO 3 -type superlattice is not stable in any composition and temperature. The B2-type ordering and decomposition occur in the same stage during ageing and do not separate in two different stages. The growth of one accelerates that of the other. Results of electron microscope and Mossbauer effect studies in the Fe-Be alloy made by Sumitomo et al . and by Yagisawa are excellently explained by the present model. A phase diagram of the Fe-Be system is proposed and the values of interchange energies in this system are determined.