Hisashi Kuwano

Average widths of grain boundaries in nanophase alloys synthesized by mechanical attrition

Many binary ferrous alloys were synthesized by mechanical attrition in a high‐energy ball mill. X‐ray diffractometry and transmission electron microscopy were used to measure grain sizes, which were as small as a few nanometers in several alloys. The nanocrystalline alloys showed new features in their Mossbauer spectra, which we associated with 57Fe atoms at and near grain boundaries. The experimental data on the fraction of 57Fe atoms at and near grain boundaries were correlated to the measured grain sizes to obtain an average width of the grain boundaries. The average grain‐boundary widths of the fcc alloys Fe–Mn and Ni–Fe were approximately 0.5 nm, but the average widths of grain boundaries in the bcc alloys Cr–Fe, Mo–Fe, and Fe–Ti were somewhat larger than 1 nm.

Mössbauer study of iron-chromium alloys irradiated by energetic protons

Abstract Compositional changes in ferritic alloys due to irradiation by 16 MeV protons have been studied using Mossbauer spectroscopy. Enrichment of Cr occurs in the peak-damage region of an Fe-4.7% Cr alloy, whereas depletion of Cr takes place in the peak-damage region of FeCr alloys with more than 7.4% Cr content. The latter is attributed to the phase separation which occurs within the miscibility gap existing in the low-temperature regions of the FeCr system.

Effects of proton irradiation on the hardening behavior of HT-9 steel

Abstract Depth profiles of microvickers hardness and X-ray diffraction line broadening, and the depth dependence of the Mossbauer spectra are measured to study the effects of 11 MeV proton irradiation on HT-9 steel and SUS410L steel. Damage peaks due to the irradiation appear about 350 μm in depth with a full width at half maximum of about 140 μm, which is much broadener than that of the calculated result using the modified E-DEP-1 code. The proton irradiation gives rise to an increase in the mean internal magnetic field of Fe in the SUS410L steel, which means Cr depletion occurs in the ferrite matrix, suggesting the possibility of radiation induced segregation.

Characterization of the spinodal decomposition of Fe−Cr alloys by Mössbauer spectroscopy

Addition of 5%Ni to an Fe−28%Cr alloy causes a transition of the aging behavior from the nucleation and growth to the spinodal decomposition. Very rapid increase in the average internal magnetic field and a broadening of the internal magnetic field distribution occur in the spinodal decomposition. Mössbauer spectra are synthesized by assuming a rectangular and a sinusoidal wave for the composition fluctuation, and the internal magnetic field distributions obtained from those are compared with the experimental results to estimate a time evolution of the amplitude of the composition wave.

The 11 MeV proton irradiation behavior of ferritic steels

Abstract Depth profiles of micro-Vickers hardness, X-ray diffraction line-width and internal magnetic field were measured on SUS410L steel and pure iron after irradiations with 11 MeV protons at ∼ 800 K and ∼ 400 K. The increases of hardness, diffraction line-width and internal magnetic field after the low temperature irradiation were similar to that after the high temperature irradiation. In the case of pure iron only a hardness increase was observed, and the increase is also similar to the hardness increase on the SUS410L steel. The temperature dependence of fracture energy derived from small punch tests provided an information on the ductile-brittle transition temperature (DBTT) of HT-9 steel by using the statistical analyses. Proton irradiation did not result in a considerable shift of the DBTT. The origin of hardness increase by the proton irradiation may be the irradiation-induced defect. Proton irradiation did not cause a severe embrittlement on HT-9 steel. Depletion of Cr from the matrix seems to be enhanced by the proton irradiation.

Enhancement of critical Pr ion concentration (xcr) in (La1−xPrx)Ba2Cu3Oz

The substitutional effect of Pr ion for La in (La1−xPrx)Ba2Cu3Oz bulk samples prepared under modified synthesis condition has been studied. Low field magnetization studies showed that superconductivity is retained up to the critical Pr ion concentration, xcr=0.40. The effective magnetic moment of Pr3+ free ion is very low in the present series of compounds. Results of X-ray absorption near-edge structure spectra for the series of (La1−xPrx)Ba2Cu3Oz compounds with 0.00<x<0.60 reveal that the doped Pr ion reduces hole concentration within the CuO2 planes, leading to a destruction of superconductivity.

Spinodal decomposition of Fe−Cr−Ni alloys studied by Mössbauer spectroscopy

SummaryA thermodynamic calculation of the Fe−Cr−Ni phase diagram suggests that the composition 30 at.%Cr-5 at.%Ni is expected to be found inside the spinodal line, whereas 16 at.%Cr-5 at.%Ni is expected outside at 723 K. These two alloys have been thermally aged for time periods up to 40 minutes at 723K. A significant broadening of the magnetic-hyperfine-field distribution indicates that Fe-30 at.%Cr-5 at.%Ni undergoes typical spinodal decomposition in agreement with the calculation. Spinodal decomposition is distinguished from nucleation-growth by a detailed analysis of the magnetic-hyperfine-field distribution.

Small-angle neutron scattering study on FeCuNbSiB nanocrystalline alloys

Abstract The small-angle neutron scattering (SANS) from Fe Cu Nb Si B nanocrystalline alloys has been measured in magnetic field. The features of nuclear scattering profiles are different from those of the magnetic scattering. The magnetic scattering shows that the distribution of local magnetization is not uniform in a BCC Fe grain. On the other hand the nuclear scattering shows that the Ornstein—Zernike-type fluctuation of concentration exits in nanocrystalline alloys.

A Mössbauer spectrometry study on the phase decomposition of cast duplex stainless steel

Cast CF3M duplex stainless steel having 25% of ferrite in volume fraction was aged at 723 K for time periods up to 10000 h. Phase decomposition of ferrite was investigated by Mössbauer spectroscopy. Ferrite decomposed initially via a spinodal process to finally yield the Fe-rich and the Cr-rich phase. The hyperfine magnetic field distribution obtained from the experimental Mössbauer spectrum was analyzed by assuming trinomial distribution of main constituent atoms Fe, Cr, Ni to determine Cr and Ni content of the Fe-rich phase. Main compositions of the phase were 84 at.% Fe, 11 at.% Cr, 5 at.% Ni.

Superconductivity in Y1-x-yPrxCayBa2Cu2.85Re0.15Oz (y=0.0 & 0.1; 0<x<0.5)

Y 1-x Pr x Ba 2 Cu 2.85 Re 0.15 O z and Y 0.9-x Ca 0.1 Pr x Ba 2 Cu 2.85 Re 0.15 O z (0<x<0.5) phases have been examined for superconductivity in an effort to understand the role of Cu (1) chain sites and T c suppression by Pr in 1-2-3 superconducting cuprates. X-ray diffraction (XRD) results showed orthorhombic to tetragonal transformation with the increase in Pr content. Superconductivity vanished for critical concentration of Pr, x cr = 0.45 which is lower than that of corresponding Re free system where x cr is 0.55. The T c of YBa 2 Cu 2.85 Re 0.15 O z is 60 K which is 30 K less than the undoped YBa 2 Cu 3 O z . It is attributed to the reduction in the amount of charge transferred from the Cu (1) chain to Cu (2) plane sites caused by Re6+ ions. Upon Ca-substitution at Y-site, Y 0.9 Ca 0.1 Ba 2 Cu 2.85 Re 0.5 O z , T c has been restored back to 90 K reveals that Ca ions generate additional holes. While the suppression of T c by Pr substitution is able to explain by AG-pair breaking model.