Motozo Hayakawa

Structural study on the tetragonal to monoclinic transformation in arc-melted ZrO2-2mol.%Y2O3—I. Experimental observations

Abstract Martensitic transformation from the tetragonal to monoclinic phase in an arc-melted ZrO 2 -2mol.%Y 2 O 3 alloy was studied by using XRD and EM. The parent phase had a so-called herringbone structure, in which the monoclinic phase was introduced either by a thinning procedure for EM specimens or by aging of bulk specimens at 573 K. The resulting monoclinic phase was in a thin-plate form with a habit approximately on the (301) m plane. The plate contained planer faults on the {101} m plane which corresponds to the twin plane of the matrix. The faults were interpreted as evidence of a lattice invariant shear. The lattice orientation was such that the b - and c -axes of the monoclinic lattice remained parallel to two of the principal axes of a tetragonal cell. The lattice parameters of both phases were also determined so that the present results can be examined in the succeeding paper on the basis of the phenomenological theory of martensitic transformation.

Structural study on the tetragonal to monoclinic transformation in arc-melted ZrO2-2mol.%Y2O3—II. Quantitative analysis

Abstract Previously obtained experimental data on the herringbone tetragonal to monoclinic transformation were subjected to a quantitative analysis based on the phenomenological theory of martensitic transformation. The basic equation of the theory was modified so that the twinned structure of the parent phase was properly taken into account. A scheme of the transformation mechanism was proposed which agreed with the experimental observations. The scheme includes slip on a {101}〈101〉 m system as the lattice invariant shear along with four non-equivalent lattice correspondences, resulting in a habit plane of the (702) m or the {702} c type in the underlying average cubic basis. A few consequences of the twinned structure of the parent lattice are also discussed.

Isothermal and athermal martensitic transformations in a zirconia–yttria alloy

Abstract The M s (martensite start temperature) of the tetragonal to monoclinic ( t – m ) transformation in the ZrO 2 –Y 2 O 3 system has been measured for bulk specimens as a function of Y 2 O 3 composition. It decreased nearly linearly with increasing Y 2 O 3 content up to 1.75 mol.%, but the transformation suddenly ceased to exist beyond 2.0 mol.% of Y 2 O 3 . The M s showed a tendency to be suppressed by rapid cooling. In fact with a small specimen containing 1.6 mol.% of Y 2 O 3 , the transformation was completely suppressed by water quenching. The highly unstable tetragonal specimen prepared by such a method was found to exhibit both an isothermal transformation at elevated temperature and an athermal transformation during subzero cooling to liquid nitrogen temperature. The C-curve for the isothermal transformation and the M s temperature for athermal transformations were obtained.

Atomic force microscope study of thin‐plate martensites in an Fe–Ni–C alloy

Morphology and accommodation of thin‐plate martensites in an Fe–Ni–C alloy have been investigated with atomic force microscopy (AFM). The quantitative observation of surface relieves made it possible to determine the kinds of thin‐plate martensite variants by combining with single‐face trace analysis. The martensites keep complementary configuration of variants in order to make the overall strain minimize. Strain distribution induced by the martensite formation in the parent phase was calculated using the height of the surface relief obtained by AFM observation and it was found that a highly strained region exists near the front of the martensite.

Low temperature aging of the freshly formed martensite in an Fe-Ni-C alloy

Aging regime, below and at room temperature, of the martensite formed in an Fe-24.67Ni-0.87C (mass pct) alloy (Ms = 160 K) is studied by using a poly crystalline X-ray diffraction technique. Parameters such as tetragonal axial ratio, unit cell volume, intensity ratios of the tetragonal doublet peaks,etc. are obtained for various degrees of aging. At temperature below ∼200 K, where the carbon atoms can shift only into their neighboring sites, the variation of the parameters indicates some discrepancies with the existing models. At room temperature, a new peak appears in between the tetragonal doublet peaks. This peak and the variation of the parameters at room temperature are explained in terms of carbon cluster formation.

Strain-magnetization properties and domain structure change of silicon steel sheets due to plastic stress

The effects of tensile stress and strain on magnetization and magnetic domains in silicon steel sheets were investigated. The strain-magnetization properties of plastic deformation regions under stress showed peculiar characteristics. We observed lancet domains in a sample after removing plastic stress, and found that the appearance of lancet domains was the cause of peculiar stress magnetization characteristics.

Effect of loading velocity and testing temperature on the fracture toughness of a SiCw/6061Al alloy composite

Abstract The effects of loading velocity and testing temperature on the fracture toughness of a SiC whisker reinforced 6061 aluminum alloy composite were investigated. A precracked three-point bend specimen configuration was selected for fracture toughness measurement, with tests being conducted at loading velocities of 10 −2 –10 m s −1 , and from room temperature to 473 K. The results showed that the fracture toughness increases with increasing loading velocity, but, the difference with respect to room temperature is small, because the fracture toughness decreases slowly with the increase of testing temperature. The composite material failed mainly by whisker pull-out and whisker breaking.

Experimental Study on the Mechanism of Lubrication in Hydrodynamic Porous Thrust Bearings With a Wavy Surface

Hydrodynamic fluid lubrication theory is applied to thrust bearings with three-dimensional, wave-shaped axial end surfaces. Optimizations of the wave height and surface porosity are experimentally determined using lubrication characteristics such as the friction coefficient and worn-out surface conditions. Lubrication characteristics such as the friction coefficient, temperature rise, thrust flotation, and oil consumption rate are evaluated for an estimation of life expectancy. In this research, the results of this theoretical application on thrust bearings clearly showed considerable improvement over conventional thrust bearings. Furthermore, this experimental research has clarified the lubrication conditions of various thrust-loaded axial end surfaces.

Scanning tunneling microscope study of surface relief induced by the tetragonal-to-monoclinic transformation in a zirconia-yttria alloy

Abstract Surface relief induced by the tetragonal-to-monoclinic transformation in an arc-melted zirconia-yttria alloy has been investigated quantitatively, on a nanometer scale, with the scanning tunneling microscope. It was found by optical microscopy that two types of transformation products exist, the herringbone type and the acicular type. The surface relief of the herringbone and acicular products was successfully delineated by STM. The surface relief angles of the herringbone product were measured quantitatively, and they were in good agreement with results calculated from the phenomenological theory of martensitic transformation. Thus, STM was proved to be a useful tool for studying the surface relief resulting from a shear-type transformation, particularly when a quantitative measurement is needed for comparing the result with a predicted value from the phenomenological theory.

Peak effect in critical current density induced by oxygen non-stoichiometry in cation-stoichiometric SmBa2Cu3O7-δ superconductor

Abstract Cation-stoichiometric SmBa 2 Cu 3 O 7− δ superconductor (Sm-123) samples were synthesized by a conventional (without any liquid phases involved) solid-state-reaction technique, followed by annealing in flowing O 2 gas at 350°C for 40–200 h. A “peak effect” was observed in the critical current density ( J c ) vs. applied magnetic field ( H ) characteristics measured at 77 K. Pronounced peak effect was observed in a sample annealed at 350°C for 40 h, while the peak was suppressed in a sample annealed at 350°C for 200 h. Since the average oxygen content determined by a coulometric titration method increased with decreasing J c value at the peak, certain oxygen-deficient regions induced by oxygen non-stoichiometry might have formed. They could work as the magnetic fluxon pinning centers, since transmission electron microscopy (TEM) observations revealed the presence of both the tetragonal phase and a twin-free orthorhombic phase with width of ca. 200 nm in the sample annealed in O 2 gas at 350°C for 40 h which would have contained a significant amount of oxygen deficiency. The presence of the oxygen-deficient Sm-123 phase especially close to the boundary between either or both of the orthorhombic and tetragonal phases and an ordinary orthorhombic phase containing twins could be working for magnetic flux pinning to yield the peak effect in the J c vs. H characteristics.