Yoshinobu Motohashi

Potentiodynamic study of the corrosion protection of aluminium by plasma-polymerized coatings

Abstract The corrosion-protective performance of plasma-polymerized (PP) coatings on aluminium substrates has been investigated by potentiodynamic (cathodic) polarization curve measurement, X-ray photoelectron spectroscopy (XPS) and temperature-programmed photoelectron emission (TPPE) measurement. PP films from tetraethoxysilane (TEOS)/O 2 , hexamethyldisiloxane/O 2 and cyclohexane were deposited on previously argon plasma treated aluminium substrates by a 13.56 MHz radiofrequency generator. The weight loss rate obtained from the cathodic polarization curve for the PP film samples utilized as the working electrode was employed to evaluate the corrosion-protective performance. The PP films of TEOS/O 2 mixtures gave the best protective performance. The corrosion potential for all the PP films became more negative than that for the substrate only. The XPS analysis indicated that TEOS films have a chemical structure like SiO 2 . The TPPE analysis revealed that the argon plasma pretreatment of the metal substrate enhances greatly the ability of the surface to emit electrons.

Superplastic Phenomenon and Electric Properties of Sc2O3 Doped Zirconia-Based Ceramics

Scandia-doped zirconia has been found to be a very promising material for an electrolyte of solid oxide fuel cells (SOFC). If superplasticity occurs in the Sc2O3 doped ZrO2 polycrystals, then they can be formed into suitable shapes for the SOFC by plastic working. There have been, however, few studies on the superplasticity and its influence on the electric properties. In this study, five types of Sc2O3 doped ZrO2 specimens were prepared and were first deformed in bending and then in tension at elevated temperatures to examine whether superplasticity could occur. The flexural tests revealed that all specimens could show m-values larger than 0.5. The 4 mol% Sc2O3 doped ZrO2 showed elongation no less than 200% in tension, while other specimens including 10Sc2O3-1CeO2-89ZrO2 showed limited elongations. The effect of the deformations on the ionic conductivity of the specimens was studied by the AC impedance method. It was found that the deformations had only a little effect on the conductivity.

Comparative study of porosity in 3Y-TZP superplastic ceramics by ultra-small-angle neutron scattering and scanning electron microscopy image analysis

Recent measurements of cavitation in superplastically strained 3Y-TZP ceramics by scanning electron microscopy and ultra small-angle neutron scattering (USANS) led to systematic differences in volume fractions of cavities determined by both methods. We show that the scattering curves measured by double crystal USANS diffractometers can be calculated directly from scanning electron micrographs taken from a single section of the material, without any assumptions of cavity shape or distribution. This technique was used to compare the USANS measurements with electron micrographs and showed that the calculated and measured curves agree well for medium scattering vectors. This result confirms that the electron micrographs represent well the microstructure of small cavities up to about 0.6 μm in radius. On the contrary, the calculations resulted in systematically larger intensities for small scattering vectors. This indicates that large cavities are responsible for the observed differences in volume fraction for the samples with nominal strain above 100%. An explanation by an inhomogeneous distribution of large cavities generated at high strains in the bulk of the tensile specimen and/or by surface artifacts is suggested.

Cavitation Behaviors in a Tetragonal Zirconia Polycrystal Subjected to Superplastic Deformations Measured by SANS Method

3Y-TZP specimens were pulled at temperatures ranging from 1623 to 1723 K with strain rates ranging from 3.3×10 to 6.7×10 s to various nominal strains. Characterization of cavities was conducted for gauge section of deformed specimens by SANS method and conventional ones including density measurement method and scanning electron microscopy analysis. The volume fraction of cavities and the evolution of cavity shapes were evaluated from the SANS results as a function of nominal strain for deformations carried out under the same superplastic condition. These behaviors were also discussed with the change in deformation condition. It is found that the SANS method is the most excellent technique for cavity characterization: the SANS method can measure the characteristics of cavities with better statistics and can measure flat cavities or fine defects which are quite difficult to identify by the conventional methods.

The Effect of Strain on the Development of Ultra-Fine Grain Structure in 7055 Aluminium Alloy Processed by Equal Channel Angular Extrusion

Grain refinement taking place in a commercial 7055 aluminum alloy under equal channel angular extrusion at a temperature of 250°C, was examined. The material was deformed up to a total strain, ε, of ~12. At ε≈1, the development of subgrain bands was found. Upon further straining the average misorientation of deformation-induced boundaries increases; low-angle grain boundaries (LAGBs) gradually convert into true high-angle grain boundaries (≥15°) (HAGBs). At ε≈4, a structure consisting of boundaries with low and high angle misorientations was observed. At ε≈12, a structure with an average grain size of ∼0.7 µm was formed. This size is roughly similar to that for subgrains developed at preceding strains. It was shown that the formation of submicrocrystalline grains occurs through continuous dynamic recrystallization both along initial boundaries and within interiors of original grains as well.

High Temperature Deformation Behavior of a Beta Titanium Alloy for Biomedical Application

The deformation characteristics of a beta-type Ti-29%Nb-13%Ta-5%Zr alloy, developed for biomedical application, and their relation with the microstructure are investigated. The cold-rolled specimen is subjected to a tensile test at high temperatures ranging from 700 to 800°C under a constant cross-head speed ranging from 1×10-4 to 1×10-2s-1. The elongations tested at different temperatures are compared with that of Ti-15%V-3%Cr-3%Sn-3%Al, a typical beta titanium alloy. The deformation mechanism is characterized from the parameter of the strain rate sensitivity. The microstructures before and after the tensile test are observed with optical microscope and the correspondent grain sizes are measured. The grain growth during the deformation is also described.

Influence of Stress State on Superplastic Deformation Behavior in a Zn-Al Eutectoid Alloy

A Zn-Al eutectoid alloy known as a fine-grained superplastic material is tested both in tension and compression under superplastic conditions, and the influence of the stress state on superplastic deformation behavior, such as flow stress, is investigated. In the compression test, deformation is interrupted and turning is performed to remove a barreled portion so that the gauge section becomes uniform, and then the deformation is continued. Microstructural change during the deformation is also investigated. As a result, flow stress in the compression becomes higher than that in tension even after the barreled portion is removed. After deformation, grain growth is observed both in the tension and compression, and growth rate in the compression is larger than in the tension.

Superplastic Behaviour and Microstructure Evolution in a Commercial Ultra-Fine Grained Al-Mg-Sc Alloy

An Al-6%Mg -0.3%Sc-0.3%Mn alloy was subjected to equal-channel angular extrusion (ECAE) at 325 o C to a total strain of about 16 that resulted in an average grain size of about 1 μm. Superplastic properties and microstructural evolution of the alloy were studied in tension at strain rates ranging from 1.4x10 -5 to 1.4 s -1 in the temperature interval from 250 to 500 o C. It was shown that this alloy exhibits superior superplastic properties in the wide temperature range 250-500°C at strain rates over 10 -2 s -1 . The highest elongation to failure of 2000% is attained at a temperature of 450 o C and an initial strain rate of 5.6x10 -2 s -1 with the corresponding strain rate sensitivity coefficient of about 0.4. Two different fracture mechanisms were revealed in high strain rate superplasticity. At temperatures higher than 300 o C and/or strain rates less than 10 -1 s -1 , failure took place in brittle manner practically without necking and cavitation played a major role in the failure. In contrast, at low temperatures and/or high strain rates, fracture occurred in a ductile manner suggesting the fracture of samples by localized necking. At these conditions, evidence of very limited cavitation was found in the samples.

Achievement of Low Temperature Superplasticity in a Commercial Aluminium Alloy Processed by Equal-Channel Angular Extrusion

A commercial Al-4.1%Mg-2.0%Li-0.16%Sc-0.07%Zr alloy was used as a starting material. The submicrocrystalline structure with an average grain size of about 0.8 μm was developed by equal-channel angular extrusion. Superplastic behavior of this alloy was examined in the temperature range 150-250 o C at strain rates ranging from 1.4x10 -5 to 5.6x10 -2 s -1 . A maximum elongation-to-failure of 440% was recorded at 175 o C (0.5Tm, where Tm is the melting point) and a strain rate of 2.8x10 -5 s -1 . The strain rate sensitivity was measured as 0.32 at these conditions. This temperature is exceptionally low for superplasticity in Al-based alloys, as it has been reported to date. Features of superplastic deformation at such the low temperature are considered.

Recrystallized Grain Size in Cold-Rolled and Annealed AZ31 Wrought Magnesium Alloys Affected by Rolling Direction

A convenient grain refinement method has been developed in wrought magnesium alloys. Commercial hot-rolled sheet and extruded bar of AZ31 alloys were subjected to further cold rolling and subsequent annealing. The influence of the angle, , between the original working direction and the cold-rolling direction on the grain size was also investigated. The present process was found to be effective to refine the grain size. Grain refinement became more marked when increased. Yield strength and hardness were confirmed to increase with decreasing grain size.