Papers by Author: Akihiko Chiba

Abstract: This work investigated the influence of oxidation durations on the formation of oxide on the surface of wrought Co-28Cr-6Mo-1Si alloy. The iso-thermal oxidation was individually performed in air at 550°C for 4, 12 and 24 h. For comparison, the surface of the non-oxidized Co-28Cr-6Mo-1Si alloy was concurrently examined. The chemical compositions of the non-oxidized and oxidized alloys were principally analyzed via X-ray photoelectron spectroscopy (XPS). The XPS results revealed that the surface of the non-oxidized alloy enriched in Cr-oxide. After oxidation treatment, the Co-oxide, existing as Co²⁺ state was observed coexisting with two Cr-oxide states, Cr³⁺ and Cr⁴⁺. The low concentrations of Mo⁶⁺ were also observed on the oxidized alloy surface. With the increase in oxidation durations, the Co-oxide was suppressed by Cr-oxide. The XPS depth profile analysis indicated that the thickness of the oxide film increased with increasing the oxidation duration.

Abstract: The effect of Zr addition on the microstructure, cold workability and hardness of low-cost Ti-14Mn alloy was investigated. A set of alloys with1.5, 3 and6%wt. Zr were produced by arc melting. The alloys were subjected to solution heat treatment at 900 °C.Zr seems working as weak β-stabilizer in this Ti-14Mn-xZr alloy system. The effect of Zr-content on the cold workability was assessed.The cold workability of the Ti-14Mn-xZr alloys (withZr up to 6%) is higher than 90% reduction ration. The hardness slightly increased with the increase ofZr-content in Ti-14Mn-xZr alloys in both solution treated and cold rolled conditions.

Abstract: The Young’s modulus of Ti-Cr-Sn-Zr alloy varies with the composition of Cr, Sn and Zr, in which the elements act as β stabilizers. Some Ti-Cr-Sn-Zr alloys show very low Young’s modulus under 50GPa. The amount of Zr in alloys with very low Young's modulus increases with the decrease of Cr. We investigated the Young’s modulus and deformation behavior of Ti-xCr-Sn-Zr (x=0~1mass%) alloys containing a large amount of Zr. The quenched microstructure of Ti-Cr-Sn-Zr alloys changes from martensitic structure to β single-phase structure if the amounts of β stabilized elements are increased. The Ti-Cr-Sn-Zr alloys with compositions close to the transitional composition of microstructure from martensite to β phase show minimum Young’s modulus. The clear microstructural transition disappears and the minimum Young’s modulus increases if the amount of Cr becomes too small. In Ti-Cr-Sn-Zr alloys containing a large amount of Zr, Young’s modulus depends on β phase that is intermingled with martensite.

Abstract: MoSi₂–based alloys are attracting attention as ultra-high temperature structural material for super-high efficiency gas turbine power generation systems. In this study, the effects of Cr-and Zr-addition on interface migration in MoSi₂/NbSi₂ lamellar silicide were examined by phase field simulations employing the segregation energies evaluated by the first principles calculation in addition to thermodynamic free energy in order to take into account the chemically-driven interfacial segregation. The simulation results indicate that both Cr and Zr can segregate at the lamellar interface to suppress its migration, and the Zr-addition is more effective to lower the interface migration rate than the Cr-addition owing to its higher segregation energy.

Abstract: Nitrogen addition is known to effectively suppress the athermal γ (fcc) → ε (hcp) martensitic transformation of biomedical Co–Cr–Mo alloys and ultimately provides a combination of high strength and good ductility. In this work, the nanostructural evolution and its influence on dislocation slip as an elementary process in the martensitic transformation were investigated to reveal the origin of their enhanced γ phase stability due to nitrogen addition. The biomedical Co–29Cr–6Mo (wt.%) alloys containing nitrogen in different concentrations (0–0.24 wt.%) were prepared. A single phase γ matrix was attained by adding nitrogen contents higher than 0.1 wt.%. We discovered nanosized Cr₂N precipitates that form on the {111}_γ planes in the N-containing alloy specimens. It was revealed that the nanoscale inhomogeneities function as obstacles to the glide of partial dislocations and consequently significantly retard the γ → ε martensitic transformation. Since the formation of ε martensite plays a crucial role in plastic deformation and wear behavior, the developed nanostructural modification associated with nitrogen addition must be a promising strategy for highly durable orthopedic implants.

Abstract: The aim of this study was to examine the effect of cobalt addition on the deformation and recrystallization behavior. The prepared material was Inconel 713C nickel base superalloy. In order to improve the deformability, the IN713C was modified by cobalt addition with 5 and 10wt%. Mechanical property was determined by room temperature tensile test with the strain rate of 10^-3 s^-1.The recrystallization was performed at 1473K for 300sec followed by oil quenching. The final microstructure becomes homogeneous through cobalt addition. Strength and work hardening rate were decreased during room temperature tensile test. By contrast, elongation was increased. Nucleation site was changed by cobalt addition. It was found that kinetics of recrystallization is dependent of the cobalt addition. The cobalt addition is attributed with ductility and recrystallization behavior and the latter is related to change of deformation behavior.

Abstract: This work presents a formation of ultrafine-grained microstructure (d ~ 0.2 μm) with high fraction of high-angle boundary in industrial Ti–6Al–4V alloy produced by the hot compression of a sample with the acicular α′martensite starting microstructure . It is found that heterogeneous nucleation becomes dominant in the case of the α’ starting microstructure.α

Abstract: The Plastic Deformation Behavior of a Biomedical Co–29Cr–6Mo–0.2N (wt.%) Alloy with a Fully γ (fcc) Matrix Was Studied by Compression Tests from Room Temperature to 1073 K. Serrated Stress–Strain Curves Caused by Dynamic Strain Aging (DSA) Was Clearly Observed at Temperatures of 773–973 K at a Strain Rate of 10⁻⁴ s⁻¹. Such a Flow Behavior Was Not Observed Significantly in other Conditions. Electron Backscatter Diffraction (EBSD) Analysis Revealed that Deformation Microstructures with DSA Occurrence Exhibited a Large Lattice Distortion over the Grains, while Local Strain Preferentially Increased in the Vicinity of Grain Boundaries in the Specimen Deformed at Room Temperature. Dislocations Were Dissociated into Stacking Faults (SFs) Bounded by Shockley Partial Dislocations both before and after Deformation; the DSA Observed in this Alloy Would Originate from the Interactions between Nitrogen Atoms and the Partial Dislocations/SFs.

Abstract: This work investigates the mechanical properties of Ti-Cr-Sn-Zr alloys containing large amount of Zr. We focuses on the effect of the varying alloy composition on the microstructure, the Young’s modulus, the deformation mechanism and the deformation behavior. Ti-Cr-Sn-Zr alloys show much low Young’s modulus in the narrow composition range for Cr but wide for Zr. The Young’s modulus of Ti-2Cr-6Sn-xZr (x=5~60mass%) alloy decreases with increasing Zr and shows the minimum value of 41GPa in Ti-2Cr-6Sn-45Zr alloy. The alloys with low Young’s modulus consist in meta-stable β phase. The composition of the meta-stable β phase is close to the transition where the quenched microstructure transits from martensite to meta-stable β phase. Ti-2Cr-6Sn-45Zr alloy with lowest Young’s modulus show the super-elastic property.

Abstract: Compression tests are carried out at high-temperature on Thermec-master Z, followed by gas quench. Microstructures after deformation are evaluated using SEM-EBSD. Significant grain refinement occurs by dynamic recrystallization for high temperature and low strain rate (T>1100°C, SR<0.1s^-1), and at high strain rate (SR~10s^-1). Dynamic recrystallization is discontinuous and takes place from the grain boundaries, leading to a necklace structure. The nucleation mechanism is most likely to be bulging of grain boundaries. However, recrystallization occurs also by rotation of annealing twins. Thereafter the twin boundaries can bulge as well. The modeling of mechanical behavior gives a fair quantification of flow softening due to dynamic recrystallization indicating the progress of dynamic recrystallization with deformation.