Advanced Materials Research Vols. 26-28

Abstract: Precipitation phenomena in an austenitic stainless steel, SUS316L cold-rolled with various reduction rates were studied by transmission electron microscopy and synchrotron radiation diffractmetry. After the aging at 573 K for 15000 h, two of precipitates were observed, which were identified as M7C3 and M23C6 by SR diffraction and electro diffraction measurements. The precipitates M7C3 were formed at both innergranular and grain boundary, while the precipitate M23C6 was formed at innergranular. The precipitation was promoted with increasing cold rolling reduction. Also segregation of phosphorous was detected along grain boundaries. Besides, the residual stresses were measured with side inclination method using a synchrotron radiation facility, SPring-8. The residual stresses were increased with increasing the cold rolling reduction rate.

Abstract: The high pressure simple hexagonal structure of silicon, which has not been synthesized, is quenched using femtosecond laser-driven shock wave. Any high-pressure phases of silicon do not remain after the pressure release in the case of the hydrostatic and conventional shock compression methods. We found the existence of the simple hexagonal structure after the intense femtosecond laser irradiation to silicon by analyzing the crystalline structures using a synchrotron grazing-incidence XRD method. Femtosecond laser-driven shock wave is a useful tool for the synthesis of non-equilibrium high-pressure phases.

Abstract: α-phase forming on the surface layer and precipitation of α-phase inside a Ti-Mo alloy during annealing in air has been investigated thermodynamically. α-phase layer is increased rapidly when annealing temperature exceeds 700°C. When the 850°C solution-treated specimen annealed at 400°C for 1 hour, there is no α-phase precipitates. As annealing temperature increased to 800°C, the amount of α-phase precipitates first increased to a maximum at 600°C, then decreased.

Abstract: The strain aging of heavily drawn pearlitic steel wires has been studied by means of electrical resistivity mearsurement. The kinetic of the early stage of strain aging after being drawn to true strain 2.89 is described by a Johnson-Mehl equation. The apparent activation energy ΔH is estimated as 31.7kJ/mol. The results indicate that the mechanism of the early stage of strain aging of heavily drawn pearlitic steel correlates with the formation of interstitial solute atoms-vacancy pairs.

Abstract: This study examined the phase changes, nitride precipitation and variations in hardness of Fe-18Cr-1Mo-0.2Ti (436L) ferritic stainless steels after a nitrogen permeation heat treatment at temperatures ranging from 1050°C to 1150°C. The strong affinity between nitrogen and Ti/Cr enabled the permeation of nitrogen into the 436L ferritic stainless steels. The nitrogen content of the surface layers ranged from 0.40% to 0.87%, depending on the nitrogen permeation temperatures. The nitrogen-permeated surface layers changed into martensite plus retained austenite(RA) with rod type M2N and square type TiN precipitates. Ups and downs of hardness with increasing depth below the surface was observed, depending on the volume fraction of the RA, nitrogen content and quantity of precipitates. The maximum hardness of the nitrogen permeated surface layer was 700Hv.

Abstract: It is generally accepted that Si promotes kinetics of polygonal ferrite due to thermodynamic factors such as Ae3 and maximum amount of ferrite formed. However, in this study, it was found that the difference between the measured rates of ferrite formation in C-Mn steel and Si added steel was much larger than that expected considering only thermodynamic factors. The classical nucleation theory with pillbox model was adopted to figure out what is the most controlling factor in formation of ferrite. The volume free energy change was calculated by use of the dilute solution model. The diffusivity of carbon (DC) was formulated as functions of C, Mn and Si by using experimental data. It was found that the volume free energy change was still predominant but the kinetic factors such as interfacial energy and the diffusivity of carbon by addition of Si were not negligible at lower undercooling. However, with increasing undercooling, the diffusivity of C was the most effective on the ferrite kinetics, though the ambiguity of treating interfacial energy was not yet clear.

Abstract: In Ni-13.0at%Si-3.1at%Fe alloy, when γ/γ’ two-phase microstructure formed at 1123 K is isothermally heated at 923 K which is lower than the temperature where the initial γ/γ’ microstructure forms, the phase-separation of γ/γ’ precipitate phase occurs and γ particles newly appear in each cuboidal γ’ precipitate. While in Ni-10.2at%Al-10.8at%Fe alloy, when γ/γ’ two-phase microstructure formed at 1023 K is isothermally heated at 1123 K which is higher than the temperature where the initial γ/γ’ microstructure forms, the phase-separation of γ’ precipitate phase takes place and γ particles newly appear in each cuboidal γ’ precipitate. Such appearance of new γ particles in γ’ precipitates can be explained by the difference in the volume fraction of γ phase that should exist in the γ/γ’ two-phase system depending on the heating temperature.

Abstract: A fatigue crack growth test was conducted in a polycrystalline copper. Dislocation structure formed near an intergranular fatigue crack was investigated by electron channelling contrast imaging (ECCI) method. The ECCI method enables us to observe dislocations lying under surface using a scanning electron microscope. The fatigue crack in the copper specimen was grown along both grain boundaries and slip bands inside grain. The ECCI observations revealed that both the vein dislocation structure and the cell structures were formed near the grain boundaries. The formations of different dislocation structures near boundaries could be interpreted in terms of the plastic strain incompatibility.

Abstract: Microstructures of Ni/Cu and Ni-Co/Cu multilayers were investigated by X-ray diffraction analysis. These multilayered structures were fabricated on copper substrates using electrodeposition technique. At an as-deposited Ni/Cu multilayer with the layer thickness of h=5nm, a single diffraction peak appeared, although the multilayer of h=100nm exhibited the diffractions splitting into two peaks which resulted from both the Ni and Cu layers. In the Ni-Co/Cu multilayers, it was found that composition of the Ni-Co layer depended on an electric potential applied during deposition. The fcc and hcp structures were detected at the Ni-rich and the Co-rich deposits, respectively. The Vickers hardness of the Co-Ni/Cu multilayer was higher than that of the Ni/Cu multilayer.

Abstract: High velocity oxy-fuel (HVOF) spray coating of WC base cermet has been studied as a promising candidate in some hard coating area for a replacement of the long traditional electrolytic hard chrome (EHC) plating, which has been raising public health and environmental problems. Micron size WC-Co (mWC-Co) has been coated on Inconel 718 substrate by HVOF thermal spray coating process. Surface properties of coating layer and friction behavior have been investigated for durability improvement coating on the sliding machine component surface.