In spite of much effort, the effect of oxygen on the intergranular fracture (IGF) of iron has not been clarified as yet. There are at least three reasons for this; (i) the specimens were not pure enough, (ii) the irons so far used showed IGF at low temperatures even without oxygen, and (iii) the solubility of oxygen in iron is not known accurately.
We have prepared a high purity iron which in a form of smooth wire does not show IGF at 4.2 K by tensile test. Polycrystalline specimens of this iron was doped with oxygen; a thin oxide film was formed on the surface and a concentration (<several mass ppm) of oxygen was dissolved in equilibrium with the oxide. Thereafter, the oxide film was removed and tensile test was performed between 4.2 and 20 K. Results are as follows:-
1. As the oxygen concentration is increased by increasing the doping temperature (873, 973 and 1073 K), the elongation to fracture at 4.2 K is decreased.
2. Specimens, which are doped with oxygen at 973 and 1073 K and fracture along grain boundaries without any appreciable elongation, are completely ductile at 20 K.
3. If well-decarburized and not doped with oxygen, a commercial high purity iron (the Johnson-Matthey iron) is more susceptible to IGF than our high purity iron.
The above results show that oxygen segregated at grain boundaries weakens the grain boundary cohesion of iron to some extent, but the effect is not so large as considered before. Some previous researches have shown that smooth specimens fracture along grain boundaries by tension at 77 K. These results should be interpreted as due to oxide particles or to interactions between oxygen and other impurities, and not due solely to the oxygen segregated at grain boundaries.

View full abstract

In order to investigate the shape effect of the particles on the high-temperature strength of dispersion-hardened alloys, two kinds of nickel alloys with SiO₂ particles of complex and spherical shapes were made by internal oxidation and a heat-treatment after the internal oxidation, respectively. Their strengths were compared by means of hardness test at high temperatures.
The micro-Vickers hardness of the alloy with particles of complex shape was found to be always higher than that of the alloy with particles of spherical shape at all test temperatures from room temperature to 1273 K, though the difference in hardness depended on test conditions; for a short loading time only a slight difference was observed, whereas the difference at high temperatures increased gradually with the increase in loading time and approached a final constant value.
The dispersion hardening estimated from the final value was in good agreement with the Orowan stress calculated from the dispersion, that is, the final difference between the two alloys can be well explained by the difference in the particle density on a slip plane, and any shape effect on the decrease in high-temperature strength due to the local climb of dislocations around particles was not detected.
Creep behaviour of the alloys was estimated from the loading time dependence of the hardness. Both of the stress exponent and the apparent activation energy for the creep were larger in the alloy with complex particles than in the alloy with spherical particles.

View full abstract

β-phase CuSn alloys, like many other β-phase alloys of the noble metals, exhibit various “extra” reflections in diffraction patterns. In the present work, the phases which give rise to extra reflections in a quenched Cu-15 at%Sn shape memory alloy were examined by means of electron microscopy, using a dark field imaging technique. The as-quenched alloy was found to consist of a DO₃ matrix containing scattered small particles (designated as the “s”-phase) and another phase which may be identified as a surface phase or a so-called “surface martensite.” The structure of the s-phase resembles that of an omega phase. In the same alloy annealed at 373 K after quenching, coarse precipitates (designated as the “L”-phase) formed. The L-phase forms as four variants in the β-phase each of which takes the shape of a hexagonal platelet, the basal plane of which is parallel to {111}_β. After considering the structure of the L-phase it is concluded that the unit cell has the lattice parameters a=b≈\sqrt6a_β, c≈\sqrt3a_β/2, and γ=120°, where a_β is the lattice dimension of the fundamental β-phase parent unit cell.

View full abstract

ESCA data have been obtained on the incorporation of oxygen into nickel surfaces at room temperature. The surfaces were cleaned by argon ion bombardment, and the Ni 2p 3/2 line was observed as a function of the exposure to oxygen. The development of NiO-like components indicated that oxygen was being incorporated into the surface lattice. At low oxygen exposures a linear kinetic law was observed, with W=1.65×10⁻³L+0.226, where W is the thickness of the incorporated layer in nm and the exposure, L, is tp^1⁄2, t being the exposure time in seconds and p the oxygen pressure in Pa. At high exposures, which were defined approximately as L>250, the thickness followed the logarithmic tarnishing law, with W=1.44×10⁻²lnL+0.533. We have postulated that the initial stage involves the incorporation of oxygen into the surface lattice via a cooperative displacement of the surface nickel atoms by a process of thermal activation assisted by a surface electric field induced by a charge transfer from a chemisorbed layer. The second stage appears to require the diffusion of oxygen into the bulk by means of thermal activation assisted by a strong electric field across the oxidized layer maintained by electron tunneling from the valence band of the bulk nickel.

View full abstract

In order to obtain a better fundamental understanding of factors responsible for an excellent improving effect of rare earths addition on the high temperature oxidation resistance of Ni–Cr alloys, the electrical conductivities of sintered Cr₂O₃ doped with various amounts of La₂O₃, Y₂O₃ and NiO were measured in the temperature range from 773 to 1473 K at near atmospheric oxygen pressures.
The electrical conductivities of Cr₂O₃ doped with La₂O₃ and Y₂O₃ up to 0.5 mass% gradually increased. The increment in the electrical conductivity by the addition of La₂O₃ was larger than that caused by the addition of Y₂O₃. The electrical conductivities of Cr₂O₃ undoped and doped with La₂O₃ and Y₂O₃ decreased with decreasing partial pressure of oxygen. This result showed that they were p-type semiconductors under the conditions investigated. The pressure dependence of the electrical conductivity of Cr₂O₃ slightly changed with the addition of La₂O₃ and Y₂O₃. The electrical conductivity of Cr₂O₃ markedly increased with increasing NiO content up to 1 mass%. In contrast, the temperature and pressure dependence of the electrical conductivity of Cr₂O₃ doped with NiO abruptly changed with the addition of more than 0.2 mass%. The electrical conductivity of Cr₂O₃ doped with NiO more than 0.2% was independent of the partial pressure of oxygen at all temperatures investigated. Both the magnitude and the temperature and pressure dependence of the electrical conductivities of Cr₂O₃-1%NiO-1%R₂O₃ (R: La and Y) were very close to those of Cr₂O₃-1%NiO. This result suggested that there was little interaction between NiO and R₂O₃ in Cr₂O₃

View full abstract

The lattice and grain boundary tracer diffusion coefficients D_PL^* and P_P^* of P in bcc iron, Fe–P binary alloys and Fe-0.1 at%P-M (M: Cr, Si, Mn, Mo and Ni) alloys were measured. There was a break in each of the D_PL^*−1⁄T and P_P^*−1⁄T lines at temperatures near and below the Curie temperature, respectively. It is considered that the anomalous breaks were caused by magnetic transformation in the bulk and grain boundary. The effects of Cr, Si and Ni on D_PL^* and P_P^* were small. Alloying with Mn increased D_PL^* and P_P^*, but alloying with Mo decreased them.

View full abstract

The glass-coated melt spinning of Pb alloy was investigated to produce a superconducting long filament with T_c more than 4.2 K.
Continuous smooth filaments less than 60 μm in diameter of Pb, and Pb–Bi, Pb–In and Pb–Sn system alloys, and of Pb_84.4Au_15.6 and In₂Bi alloys were obtained from a molten state at 1450 K with various spinning speeds ranging from 0.95 m/s to 7.95 m/s. The Pb system alloy filament obtained was a ductile material with the tensile strength of 40 MPa and elongation of 2.0%.
The T_c of the Pb alloy filament did not depend on the sample current density (J_s) less than 3×10⁷ A/m², whereas the T_c of the Pb filament depended on the J_s more than 2×10⁶ A/m². The maximum T_c at 8.0 K was observed for the Pb_60.2Bi_39.8 filament.
The enhancement of superconductivity was observed for the Pb₇₀Sn₃₀ and Pb₆₅Sn₃₅ filaments.

View full abstract