Seiichi Takaki

Hydrogen-induced softening and hardening in high purity FeC alloys

Abstract The effects of hydrogen charging on the flow stress of high purity FeC alloys (containing 20–500 at.ppm C) were investigated at temperatures between 170 K and room temperature. Hydrogen causes softening when the carbon concentration is less than that corresponding to the maximum solid solution softening by carbon. In this case the flow stress in the hydrogen-softened state is independent of the carbon concentration and almost equal to the flow stress due to full softening by carbon or hydrogen only. When carbon is added beyond the concentration required for full softening, hydrogen causes hardening. The amount of the hydrogen-induced hardening is almost independent of the carbon concentration, the temperature and the hydrogen charging current density (10–40 A m −2 ). The hydrogen-induced hardening is most probably due to the interaction of CH pairs (complexes) with kinks which retards their sideward motion along screw dislocations. The formation of the pair increases the resistance of carbon atoms to the motion of the kink.

Preparation of 10 kg Ingot of Ultra-Pure Iron

A new induction melting furnace with a cold-copper crucible has been designed and constructed using ultra-high vacuum technology. The main chamber can be evacuated to 6.7 × 10—8 Pa as the base pressure. The furnace can melt high-purity iron of 10 kg, at most, within 300 s. The total pressure becomes less than 4 × 10—6 Pa during melting of iron under the best condition. As starting materials for melting, high-purity electrolytic iron containing impurities of 25.2 massppm which is determined after the analysis of 18 elements was prepared. The content of gaseous impurities such as carbon, nitrogen, oxygen, sulfur and hydrogen is 19.6 massppm in total. It is concluded that ultra-high vacuum melting of iron is quite useful to increase the purity of iron. The total content of impurities in iron is reduced from 25.2 to 12.6 massppm by melting under the atmosphere of 7.5 × 10—6 Pa. The amount of gaseous impurities can be reduced from 19.6 to 10.1 massppm in total. Especially, the amount of oxygen is reduced from 14.1 to 7.5 massppm. These amounts of gaseous impurities are almost the limit of detection by conventional methods of analysis.

Effects of Neutron Irradiation on Tensile Properties in High-Purity Fe–Cr Alloys

The tensile properties of high- and low-purity Fe–9, –18 and –30Cr alloys irradiated by neutrons up to a dose of 5 × 1024 n/m2 (E >1 MeV) at 613, 673, or 763 K have been examined. The yield strength and the ultimate strength are increased and the elongation is decreased by irradiation. The enhancement of these strengths due to the irradiation has a tendency to increase with chromium and impurity content. Large stress drops are often observed, especially at 763 K, in stress–strain curves of high-purity and high-chromium-content alloys except for Fe–9Cr alloys. Irradiation-induced precipitates, with 2% larger interplanar spacings than the α′-phase, on dislocation loops are more easily formed in the specimens of higher chromium content and higher purity. The precipitates are formed even in the irradiated Fe–9Cr alloy of high purity. The stress drop behaviour during the tensile tests is predominant in the specimens of higher chromium content and higher purity.

Resistivity recovery at low temperatures in high-purity iron charged with hydrogen

Abstract High-purity iron specimens (RRR H = 1 800 − 5 000) were electrolytically charged with hydrogen so as to avoid any observable damage in the specimens. They were quenched into liquid helium and aged at below room temperature. The change in resistivity was measured to investigate the behavior of hydrogen. The resistivity recovers in two stages, a large recovery at 110 K and a small recovery at 170 K, in specimens without silicon contamination, but at 150 K and above 200 K (with a small stage around 100 K in less contaminated specimens) in specimens contaminated with silicon during dry hydrogen treatment. The two-stage (110 K and 170 K) recovery is considered to be the representative behavior of hydrogen in iron for this range of purity. A large part of the quenched hydrogen is likely to condense at sites (most likely dislocations) in the crystal in the 110 K stage. The activation energy for the 110 K stage is measured to be about 0.2 eV, which implies that the hydrogen diffusion around 100 K is affected by impurity trapping, even in this high-purity iron. The 170 K stage seems to be due to the release of hydrogen from dislocations and its escape to the surface.

Recent progress and future R&D for high-chromium iron-base and chromium-base alloys

Attractive characteristics of high-chromium iron-base alloys and chromium-base alloys have been demonstrated compared with those of conventional ferritic and austentic stainless steels. Recent progress especially on purification techniques indicates the possibility for these alloys to be developed into engineering materials. It is argued that the purification may overcome the brittleness of drawback of these alloys.

High Temperature Deformation Mechanism of a High-Purity Fe–50 mass% Cr Alloy

The deformation mechanism of a high-purity Fe–50 mass% Cr alloy and a floating-zone refined Fe–50 mass% Cr alloy was investigated by tensile testing between 873 and 1073 K and microstructural observation. It is concluded from the present experimental research that: 1. A stress-drop appears after yielding in both alloys between 873 and 1073 K. The stress-drop is the result of grain boundary sliding and is related to the formation of dislocation sources at the beginning of deformation. 2. Intergranular cracking occurs in the high-purity Fe–50 mass% Cr alloy and also in the fine-grained zone-refined alloy and the cracking is strongly influenced by grain size. The mechanism is not grain boundary decohesion caused by segregated sulfur. 3. The stress-drop is also observed in the Fe–50 mass% Cr–8 mass% W alloy, but grain boundary cracks are seldom observed in this alloy. 4. The deformation behavior after the stress-drop is determined by a competition between increasing dislocation density and dislocation annihilation at high temperatures. No twinning is seen in this temperature range, not even in the W-doped alloy.

Effects of oxygen and substitutional impurities on the mechanical properties of high purity iron

Abstract The present paper shows that oxygen is removed from commercial high purity iron by electron beam floating-zone melting under ultrahigh vacuum, and that hydrogen treatment of α-Fe is also effective in removing oxygen if the iron is pure enough with respect to substitutional impurities. The mechanical properties already reported by the research group of the present authors on high purity iron prepared by the same technique as for the present iron are the inherent properties of iron. These findings are in contradiction to the argument by Maruyama et al. that the high purity iron described above may contain a substantial amount of oxygen and the reported properties are not inherent in iron. Also discussed in the present paper are the effects of substitutional impurities and of grain structure on the athermal yield stress and the appearance of a hump in the yield stress vs. temperature curve.

Purification of Ti–Al Alloy by Cold‐Crucible Induction Melting in Ultrahigh Vacuum

In order to investigate the effect of ultrahigh vacuum melting on the purification of Ti-Al alloy, a 1 kg ultrahigh-purity Ti-45 mol% Al alloy ingot of more than 99.9940 mass% purity after analysis of 40 elements was prepared by cold-crucible induction melting (CCIM) in argon and in ultrahigh vacuum. High-purity aluminium of 99.9986 mass% purity and high-purity titanium of 99.985 mass% purity were used as starting materials. The impurity analysis was done using quadrupole residual-gas analysis, chemical analysis, and glow-discharge mass spectrometry (GDMS) analysis. The concentration of gaseous impurities in the ingot was 51 mass ppm (O = 35, N = 11, C = 5, S < 0.5). As a result of GDMS analysis of 36 metallic elements in the ingot, the total amount of metallic impurities was below 8.7 mass ppm, exclusive of 23 elements below their detection limits. The cold-crucible induction melting in UHV is effective in preparing an ultrahigh-purity Ti-Al alloy ingot containing low concentrations of gaseous and metallic impurities.

Orientation dependence of quasi-three-stage work hardening in high purity iron single crystals

Abstract Quasi-three-stage hardening is characterized with the existence of stage 0, and the temperature dependence of the length of stage I, which is opposite to that in the three-stage hardening in f.c.c. metals. The orientation dependence of these characteristics has been investigated for high purity iron single crystals. As the initial orientation of the specimen axis moves away from the [001]–[011] symmetry line, stage 0 becomes smaller and stage I becomes longer. As it approaches the [011]–[111] boundary, stage 0 becomes smaller and stage I becomes shorter. These results are interpreted using the mechanism of the quasi-three-stage hardening proposed by us recently and consideration of the slip plane dependence of the direction of the motion of screw dislocations in the primary and secondary slip systems.

Volume and Grain-Boundary Self-Diffusion in a High-Purity Fe–50 mass% Cr Alloy

Volume and grain-boundary self-diffusion coefficients of 51 Cr and 59 Fe in a high-purity Fe-49.7 mass% Cr alloy containing 13 mass ppm carbon and 6 mass ppm nitrogen have been determined in the temperature range between 923 and 1453 K by use of the sputter-microsectioning technique. Linear Arrhenius lines for volume self-diffusion coefficients of both tracers have been observed across the α-σ phase-transformation temperature 1103 K, which is consistent with the suppression of the formation of the σ-phase by purification. The Arrhenius equations can be expressed by D Fe = 1.2 x 10 -4 exp(-256 kJ mol -1 /RT) m 2 s -1 and D Cr = 1.3 × 10 -4 exp (-259 kJ mol -1 /RT) m 2 s -1 . Downward curved Arrhenius lines for the grain boundary diffusivity aδD gb (s segregation factor, δ width of grain boundary, and D gb grain boundary diffusion coefficient) of both tracers have been observed. This is attributed to the formation of very thin layer of the (Cr, Fe) 23 C 6 phase at the grain boundary near the surface of the specimens.