Kyosuke Yoshimi

Oxidation behavior of Mo5SiB2-based alloy at elevated temperatures

Abstract A Mo 5 SiB 2 -based alloy having composition of Mo–12.3 mol% Si–24.9 mol% B was produced by arc-melting in an Ar atmosphere, and its oxidation behavior was investigated at temperature between 973 and 1673 K. At and above 1273 K, transient and steady state oxidation stages were clearly observed. The occurrence of the transient and steady state oxidation is interpreted in terms of rapid volatilization of MoO 3 and B 2 O 3 under ambient O 2 pressure at the initial stage and the passive oxidation after completely sealing the substrate by silicate glass. Development of two layers onto the substrate, i.e. SiO 2 glass scale and Mo solid solution interlayer including SiO 2 dispersions, strongly supports the interpretation. Dissolution of B into the SiO 2 scale was not confirmed because of low B concentration that was under a detectable limit of EPMA and TEM-EDS. It is suggested that the SiO 2 glass scale formed on the Mo 5 SiB 2 -based alloy is more protective than as expected.

Yielding and plastic flow behavior of B2-type Fe-39.5 mol.% A1 single crystals in compression

Yielding and plastic flow behavior of B2-type Fe-39.5 mol.% Al were investigated by deforming single crystals in the temperature range from room temperature to 1073 K. Yield stress exhibits a distinct positive temperature dependence followed by a peak for all the orientations examined. The temperatures of the anomalous peak are located between 823 and 873 K for all the orientations except the near-[111] orientation. Only for the near-[111] orientation the peak temperature is located between 773 and 823 K. The slip transition from 〈111〉 direction at intermediate temperatures to 〈100〉 at high temperatures occurs at the peak temperatures. The yield stress at 773 K exhibits a strong orientation dependence and has a good correlation with respect to non-glide stress component. Specimens having compression axes of χ ⪖ 0° exhibit serrations in stress-strain curves below the peak temperatures, whereas the serrations are not observed in those of χ < 0°. In addition, a yield drop is observed around the peak temperatures for all the orientations. Below the peak temperatures, even as low as at room temperature, the yield stress hardly depends on the applied strain rate. This indicates that the motion of 〈111〉-type superdislocations has very small strain-rate sensitivity in the temperature range. On the other hand, there is a strong strain-rate dependence at the peak temperature and above, indicating that the motion of 〈100〉-type dislocations is strongly rate sensitive. The positive temperature dependence of yield stress in B2 FeAl is discussed on the basis of the present results.

Thermal expansion, strength and oxidation resistance of Mo/Mo5SiB2 in-situ composites at elevated temperatures

Abstract Four Mo/Mo5SiB2 in-situ composites with different volume fractions of Mo solid solution, VfMo, were produced by ingot metallurgy and their microstructure, thermal expansion, compressive strength and oxidation behavior were investigated relating to VfMo. Mo-8.5Si-13.2B annealed at 2073 K for 24 h has nearly eutectic microstructure consisting of Mo solid solution and Mo5SiB2. For higher VfMo, there are micro-cracks in Mo5SiB2 phase probably induced by thermal stress during the cooling process. Mo-8.5Si-13.2B shows the highest yield stress among the four Mo/Mo5SiB2 in-situ composites, and about 1 GPa even at 1773 K. For lower VfMo than that of Mo-8.5Si-13.2B, the steady state oxidation behavior is observed at temperatures between 1173 and 1473 K, indicating that the oxidation resistance is relatively good, whereas the oxidation resistance is catastrophic for higher VfMo than that of Mo-8.5Si-13.2B.

Texture and grain boundary character distribution (GBCD) in rapidly solidified and annealed Fe–6·5 mass% Si ribbons

Abstract The type and frequency of grain boundaries, the so-called grain boundary character distribution (GBCD), has been determined in rapidly solidified and subsequently annealed Fe-6·5 mass% Si alloy ribbon by the scanning electron microscopy-electron channelling pattern (SEM-ECP) technique. High frequencies of low-angle boundaries and coincidence boundaries with Σ3, Σ9, Σ11, Σ17 and Σ19 were observed in a fully annealed ribbon with well defined {110} texture. The total frequency of low-angle boundaries and coincidence boundaries is almost one-half of all grain boundaries. The coincidence boundaries which occurred more frequently are exactly those predicted theoretically from the coincidence orientations for 〈110〉 rotation in cubic crystals, similar to those observed previously in {100} textured ribbons of the same alloy produced by the same processing method. The presence of a close relationship between the type of texture and GBCD has been confirmed by experiment on differently textured ribbons of th...

Tensile properties of B2-type Fe-39mol%Al single crystals at elevated temperatures

Abstract Tensile properties of B2-type Fe-39mol%Al single crystals have been investigated between room temperature and 923 K. Positive temperature dependence of yield stress was observed in all the orientations examined. Orientation-dependent phenomena in the temperature range where temperature dependence of yield stress is positive, viz., orientation dependences of yield stress and critical resolved shear stress (CRSS), peak stress and peak temperature, and serrated flow behavior, shows trends opposite to those obtained in compression by our previous work. Based on the present results, it is concluded that those orientation-dependent phenomena represent intrinsic characteristics of deformation behavior associated with the positive temperature dependence of yield stress.

Microstructure and oxidation resistance of a plasma sprayed Mo–Si–B multiphase alloy coating

Abstract A multiphase Mo–Si–B alloy coating can be successfully deposited onto the Mo–ZrC composite using a low pressure plasma spraying method. Microstructure of the plasma sprayed Mo–Si–B coating consists of Mo5Si3, Mo3Si and Mo5SiB2 phases, which are the same constituents as Mo–Si–B alloy powder. Nanostructure is formed in the Mo–Si–B coating with the eutectic composition owing to the rapid quenching effect by plasma spraying. The oxidation behavior of the Mo–Si–B coating shows rapid mass loss followed by slow mass gain at 1673 K in Ar–20%O2. Dense and adherent borosilicate glass formation passivates the coating against further oxidation at 1673 K.

Yielding and flow behavior of Mo5Si3 single crystals

Abstract Deformation behavior of Mo 5 Si 3 was studied using single crystals under compression. It was found that yielding and flow behavior were strongly dependent on temperature, strain rate and crystal orientation. A stress exponent and an activation enthalpy of lower yield stresses were estimated to be ≈6 and 4.5 eV, respectively.

Effect of APB type on tensile properties of Cr added Fe3Al with D03 structure

Abstract Fe25 mol.%Al2mol.%Cr, Fe28mol.%Al and Fe28mol.%Al2mol.%Cr alloys were prepared in order to investigate the effect of Cr addition on antiphase boundary (APB) types and tensile properties of D03 single-phase Fe3Al polycrystals. The highest elongation and the lowest work-hardening rate are obtained for Fe28Al2Cr. Fe25Al2Cr has the lowest elongation and the highest work-hardening rate. In addition to thermal APBs, many uncoupled superpartial dislocations dragging extended APBs can be seen in deformation microstructures for all the alloys. In Fe25Al2Cr B-type APBs are frequently observed with a small amount of D03-type APBs, whereas in Fe28Al and Fe28Al2Cr all of thermal or deformation-induced APBs are of D03-type. Uncoupled superpartial dislocations in deformed Fe25Al2Cr are blocked at thermal or deformation-induced APBs. By considering coordination configurations of neighbouring atoms at an APB, the blocking force against dislocation motions is shown to be higher for the B2-type APB than for the D03-type APB. This is related to the highest work-hardening rate of Fe25Al2Cr. In contrast, cross-slipping of uncoupled superpartials is more frequently observed in Fe28Al2Cr than in Fe28Al. The work-hardening rate of Fe28Al2Cr would be reduced by the occurrence of cross-slipping. Ductility improvement by the Cr addition is discussed in terms of an intrinsic change in the work-hardening rate of D03 Fe3Al alloys.

Transmission electron microscopic observation of thermally introduced planar faults in Fe-35 mol.% Al alloys

Abstract Thermally introduced planar faults are investigated both in an Fe-35 mol.%Al binary alloy and in B, Cr, Pd and W added ternary alloys using transmission electron microscopy. Air cooling of the alloys from 1273 K followed by annealing at 698 K for 120 h introduces two types of planar faults. One type is the antiphase boundary (APB), which is observed in all of the alloys. The other is the complex planar fault having both APB and stacking fault characters, which is observed in only the B-added alloy. The formation of the APBs is envisaged from a collapse of the lattice owing to vacancy condensation on triple layers of {111} planes. The size, distribution and planarity of the APBs are affected by ternary element addition.

Determination of density and vacancy concentration in rapidly solidified FeAl ribbons

Abstract In order to observe an anomalously large number of thermal vacancies retained in B2-type FeAl, an FeAl alloy is rapidly solidified into ribbons by a conventional melt-spinning technique, and the density and vacancy concentration of these ribbons are determined by an Archimedes water immersion technique and x-ray diffraction method. The lattice constant of as-spun ribbon, which has already B2-type ordered structure regardless of the rapid solidification by melt-spinning, is 2.8992×10 −10 m. This is smaller than that of water-quenched FeAl alloys previously reported, indicating lattice relaxation due to the existence of numerous thermal vacancies. The total vacancy concentration of as-spun ribbons is estimated to be approximately 1.38±0.15%, which is several hundred times as high as that of pure metals and disordered alloys near the melting point.