Ai Serizawa

The characterization of dislocation-nanocluster interactions in Al-Mg-Si(-Cu/Ag) alloys

The quantification of the interaction between nanoclusters and dislocation motion has received relatively little experimental or theoretical research. In this work, the relationship between nanoclusters and dislocations was investigated by conducting tensile tests at different temperatures for a variety of nanoclusters in Al–Mg–Si alloys. Further, the nanoclusters were characterized by 3D atom probe. The normalized energy required for a dislocation to shear through a nanocluster, go , was estimated by using the results from the tensile tests and thermal activation theory. It was possible to characterize differences in nanoclusters for different ageing times as well as changes due to the addition of Cu or Ag. Specifically, it was found that the nanoclusters that formed at 293 K could be differentiated from those formed at 393 K, even after correcting for the nanocluster size. Finally, it was found that the addition of small amounts of Cu or Ag fundamentally altered the dislocation–nanocluster interaction.

Influence of Analysis Parameters on the Microstructural Characterization of Nanoscale Precipitates

A series of simulated microstructures containing nanometer-scale precipitates was created with an atom probe simulator. These data were then analyzed with the proximity histogram and the maximum separation method to determine the influence of the particular analysis method. For simulated 2-nm-radius spherical precipitates, the optimized voxel size and delocalization were found to be 0.5-0.6 nm and 1.0-1.5 nm, respectively. Under optimum analysis parameters, the voxelization/delocalization process only slightly degrades the interface width determined from the proximity histogram to {approx}0.15 {+-} 0.04 nm.

Control of Oriented Extracellular Matrix Similar to Anisotropic Bone Microstructure

Bone microstructure is dominantly composed of anisotropic extracellular matrix (ECM) in which collagen fibers and epitaxially-oriented biological apatite (BAp) crystals are preferentially aligned depending on the bone anatomical position, resulting in exerting appropriate mechanical function. The regenerative bone in bony defects is however produced without the preferential alignment of collagen fibers and the c-axis of BAp crystals, and subsequently reproduced to recover toward intact alignment. Thus, it is necessary to produce the anisotropic bone-mimetic tissue for the quick recovery of original bone tissue and the related mechanical ability in the early stage of bone regeneration. Our group is focusing on the methodology for regulating the arrangement of bone cells, the following secretion of collagen and the self-assembled mineralization by oriented BAp crystallites. Cyclic stretching in vitro to bone cells, principal-stress loading in vivo on scaffolds, step formation by slip traces on Ti single crystal, surface modification by laser induced periodic surface structure (LIPSS), anisotropic collagen substrate with the different degree of orientation, etc. can dominate bone cell arrangement and lead to the construction of the oriented ECM similar to the bone tissue architecture. This suggests that stress/strain loading, surface topography and chemical anisotropy are useful to produce bone-like microstructure in order to promote the regeneration of anisotropic bone tissue and to understand the controlling parameters for anisotropic osteogenesis induction.

Radius dependence of solute concentration estimates of simulated ultrafine precipitates

Estimates of the radii and solute concentrations of simulated microstructures containing ultrafine spherical precipitates were determined from isoconcentration surfaces and proximity histograms. The accuracy of the estimates of the solute concentrations and the radii of precipitates was found to depend on the size of precipitates. Optimized parameters for analyzing 0.5‐ to 2‐nm‐radius precipitates are proposed. The solute content of 0.5‐nm‐radius precipitates was not estimated correctly by this method. The accuracy of the estimates of the solute concentration and the radius of precipitates were primarily influenced by the solute concentrations of the precipitates. The ranges of error of the solute concentration in the precipitates, which are associated with the analytical limitations of the ultrafine precipitates, were determined, and the results indicated a limitation of the estimates. Microsc. Res. Tech. 76:1196–1203, 2013.

Single crystal growth and its microstructure in Co-Cr-Mo alloys for biomedical applications

Co-Cr-Mo based alloys have been widely employed as heat resistant materials and as biomaterials for implants because of their high strength and superior wear resistance. In general, the alloys exhibit a very complicated composition-dependent microstructure containing stacking faults and related mechanical properties. Thus, the essential properties must be clarified by using not only polycrystals but also single crystals. To our knowledge, single crystals and related properties have not been reported elsewhere. Thus, Co-Cr-Mo single crystals were grown and used to analyze the microstructure and the related properties. Single crystals with a composition Co-27 mass% Cr-6 mass% Mo alloy defined by ASTM F75 were grown by two single crystal apparatuses: the optical floating zone and the Bridgman methods. The single crystals with the smooth-surface shape were successfully obtained in the Bridgman method under an Ar gas atmosphere at a crystal growth rate of 5.0 or 2.5 mm/h. A portion of the crystals contain Al as Al2O3 precipitates from the crucible. Since the Al2O3 precipitate induces martensitic phase transformation from fcc (γ) phase to hcp (ε) phase, the single crystals were separated into two parts (a) containing Al2O3 precipitate and (b) in the absence of the clear precipitate. The microstructure was significantly altered by the martensitic phase transformation from the γ to ε phase induced by stress field or heating. In addition, variant formation of ε phase has a large influence on the mechanical functions of these Co-Cr-Mo alloys. Novel findings were preliminary obtained in the single crystals.

Effect of Mg or Ag addition on the evaporation field of Al.

It is known that the distribution of the charge-states as well as the evaporation field shift to higher values as the specimen temperature is decreased at a constant rate of evaporation. This study has explored the effect of Mg or Ag addition on the evaporation field of Al in terms of the charge state distribution of the field evaporated Al ions. The fractional abundance of Al(2+) ions with respect to the total Al ions in Al-Mg alloy is lower than that in pure Al, whereas it shows higher level in the Al-Ag alloy at lower temperatures. The temperature dependence of the fractional abundance of Al(2+) ions has been also confirmed, suggesting that Al atoms in the Al-Mg alloy need lower evaporation field, while higher field is necessary to evaporate Al atoms in the Al-Ag alloy, compared with pure Al. This tendency is in agreement with that of the evaporation fields estimated theoretically by means of measurements of the work function and calculations of the binding energy of the pure Al, Al-Mg and Al-Ag alloys.

Effect of Vapor Pressure During the Steam Coating Treatment on Structure and Corrosion Resistance of the Mg(OH)2/Mg-Al LDH Composite Film Formed on Mg Alloy AZ61

Corrosion resistant films with almost the same film thickness were prepared on the magnesium alloy AZ61 by steam coating at different vapor pressure and treatment times. The effect of the vapor pressure on the structures and the corrosion resistance of the films was investigated by using FE-SEM, SEM-EDX, GAXRD, and potentiodynamic polarization curve measurements in a 3.5 mass percentage NaCl aqueous solution. These studies clarified that the interlayers of Mg-Al Layered Double Hydroxide (LDHs) increased and its structure became non-uniform with an increase in the vapor pressure. The corrosion current density slightly increased with an increase in the vapor pressure during the treatment, but pitting corrosion occurred at both low and high vapor pressures. These results indicate that water molecules were pushed into an interlayer of Mg-Al LDHs by high vapor pressure. Consequently, the interlayer distance of Mg-Al LDH was widened and the cracks were generated in the anti-corrosive film. On the other hand, the Mg-Al LDH with an insufficiently large interlayer distance could not fill the cracks in the Mg(OH)2 crystallites and caused pitting corrosion when the vapor pressure was low.

Preparation of Corrosion-Resistant Films on Magnesium Alloys by Steam Coating

This chapter introduces a novel, chemical-free “steam coating” method for preparing films on magnesium (Mg) alloys and assesses their effectiveness in improving the corrosion resistance of two different Mg alloys. A film composed of crystalline Mg(OH) 2 and Mg-Al layered double hydroxide (LDH) was successfully formed on AZ31 Mg alloy, and its corrosion resistance was evaluated through electrochemical measurements and immersion tests in an aqueous solution containing 5 wt.% NaCl. An anticorrosive film was also formed on Ca-added flame-resistant AM60 (AMCa602) Mg alloy via the same steam coating method and found to be composed of crystalline Mg(OH) 2 and Mg-Al layered double hydroxide (LDH). Its corrosion resistance was also investigated, and the effectiveness of the steam coating method for improving the corrosion resistance of Mg was fully explored.

Preparation of corrosion resistant film on Mg-6Al-1Zn-2Ca alloy by steam coating

Corrosion resistant films were prepared on flame-resistant Mg–6Al–1Zn–2Ca (in mass%) alloy by steam coating using ultra pure water as steam source. The prepared films were characterized using SEM, XRD, and FT-IR. Corrosion resistance of the film was estimated by polarization curve measurement in 5 mass% NaCl aqueous solution. All XRD patterns of the films coated Mg–6Al–1Zn–2Ca alloy revealed that all films were composed of crystalline Mg(OH)2 and Mg–Al layered double hydroxide (LDH). FT-IR spectra showed that the Mg–Al LDH had carbonateand nitrate-ions in the interlayer. Potentiodynamic polarization curves of the film prepared at 433 K for 6 h indicated that the corrosion current density decreased by more than four orders of magnitude as compared to that of the uncoated Mg–6Al–1Zn–2Ca alloy. (Received May 22, 2017 Accepted July 17, 2017)

Heterogeneous Formation of Nanoclusters in a Cold Rolled 6000 Series Aluminum Alloy

The effect of high densities of dislocations on the formation behavior of nanoscale clusters (nanoclusters), which are formed during natural aging at room temperature and the pre-aging at ~373 K in a 6000 series aluminum alloy, was investigated by atom probe tomography. Cold rolling was applied to modify the formation behavior and/or the characteristics of the two types of nanoclusters and also the precipitation sequence, which involve a strengthening phase (˝) to improve the bake-hardening (BH) response for auto body panels. Cold rolling to a 5% deformation accelerates the preferential formation of nanocluster at 373K along dislocations in addition to homogeneous nucleation in the crystal grains. Atom probe tomography demonstrate the heterogeneous formation of nanocluster during pre-aging at 373K. The ˝ phase shows the high growth rate at the same level of the as-quenched specimen in the cold rolled alloy on the aging curve at 443K. Cold rolling before pre-aging is an effective process to improve the BH response of the alloys.