Minoru Doi

The effect of elastic interaction energy on the morphology of γ′precipitates in nickel-based alloys

During the coarsening of coherent γ′ precipitates in nickel-based alloys, the cuboidal γ′ precipitate particle sometimes splits into eight small cuboids (i.e. an ogdoad) or a pair of parallel plates (i.e. a doublet). The extra-ordinary phenomenon of splitting can be explained as follows: the elastic interaction betwen the ogdoad or double γ′ particles formed by the split reduces the total energy by overcoming the marked increase in surface energy due to the split. This assumption can be justified by calculation of the energies before and after the split from a microelasticity theory. The above comments suggest that the elastic interaction definitely governs both the shape of the individual γ′ precipitate and the distribution of γ′ precipitates. Such effects can be described well by introducing a parameter Δ∗ equal to the ratio of the γ-γ′ lattice misfit to the interfacial energy density of the γ′ precipitate.

Microglia Activated with the Toll-Like Receptor 9 Ligand CpG Attenuate Oligomeric Amyloid β Neurotoxicity in in Vitro and in Vivo Models of Alzheimer’s Disease

Soluble oligomeric amyloid beta (oAbeta) 1-42 causes synaptic dysfunction and neuronal injury in Alzheimers disease (AD). Although accumulation of microglia around senile plaques is a hallmark of AD pathology, the role of microglia in oAbeta1-42 neurotoxicity is not fully understood. Here, we showed that oAbeta but not fibrillar Abeta was neurotoxic, and microglia activated with unmethylated DNA CpG motif (CpG), a ligand for Toll-like receptor 9, attenuated oAbeta1-42 neurotoxicity in primary neuron-microglia co-cultures. CpG enhanced microglial clearance of oAbeta1-42 and induced higher levels of the antioxidant enzyme heme oxygenase-1 in microglia without producing neurotoxic molecules such as nitric oxide and glutamate. Among subclasses of CpGs, class B and class C activated microglia to promote neuroprotection. Moreover, intracerebroventricular administration of CpG ameliorated both the cognitive impairments induced by oAbeta1-42 and the impairment of associative learning in Tg2576 mouse model of AD. We propose that CpG may be an effective therapeutic strategy for limiting oAbeta1-42 neurotoxicity in AD.

The effects of elastic interaction energy on the γ′ precipitate morphology of continuously cooled nickel-base alloys

Abstract The morphological change of γ′ precipitates in some nickel-base alloys continuously cooled across the γ′ solvus line was investigated by means of transmission electron microscopy. In slowly cooled (e.g. a cooling rate of about 10−2 K s−1) NiAl , NiSi and Nimonic 115 alloys a single γ′ particle split into eight small cuboids (i.e. an ogdoad) and the unit assemblies of particles, each of which consisted of eight cuboids, were closely distributed in the γ matrix. For quickly cooled (e.g. a cooling rate of about 10 K s−1) alloys, splitting did not occur and a large number of small γ′ particles were closely aligned along 〈100〉 directions. In NiAl alloys, γ′ precipitate ogdoads were formed during slow continuos cooling, whereas γ′ precipitate doublets were formed during the isothermal aging just below the γ′ solvus line. Such morphologies were introduced by the strong influence of the elastic interaction energies.

Crystallization of amorphous germanium in an Al/a-Ge bilayer film deposited on a SiO2 substrate

The crystallization of amorphous Ge(a-Ge) in an Al (134 nm) and a-Ge (108 nm) thin-film bilayer deposited on a SiO2 substrate has been examined by a cross section transmission electron microscope technique. When crystallization of a-Ge begins at 125 °C, amorphous AlGe (a-AlGe) alloy is formed in the Ge layer. Then, the a-AlGe alloy layer also appeared at the surface of the bilayer. After complete crystallization, those amorphous layers disappeared and the bilayer film has been converted to a polycrystalline film. We discussed the crystallization of a-Ge and proposed the mechanism of the diffusion of Ge atoms from the inner a-Ge layer through the outer Al layer to the topmost surface that involves the formation of the metastable a-AlGe alloy in the Ge layer, followed by the crystallization of this alloy by the pseudo-eutectic reaction, leading to the decomposition into an equilibrium Al and Ge crystal mixture and a-Ge. Then, Ge atoms is released to the Al layer for the compensation of the Al diffusion down...

Stability bifurcations in the coarsening of precipitates in elastically constrained systems

Abstract Characteristic coarsening behaviours of precipitates in elastically constrained alloy systems are comprehensively represented. The important results obtained are as follows: (a) splitting of a precipitate into small particles during coarsening, (b) extremely slow coarsening of precipitates in high solute content alloys, (c) deceleration of the coarsening rate for large precipitates and (d) changes in the distribution function of particle size with progress of aging. These phenomena cannot be explained by the conventional Lifshitz-Slyozov-Wagner or modified Lifshitz-Slyozov-Wagner theories of Ostwald ripening, where only the interfacial energy between the particle and matrix is taken into account. The bifurcation diagrams of the particle stability can explain such extraordinary phenomena observed in elastically constrained alloy systems.

γ′ Precipitate morphology formed under the influence of elastic interaction energies in nickel-base alloys

Abstract The morphological change of γ′ precipitates in some nickel-base alloys was investigated by means of transmission electron microscopy. In NiSi, NiAl and Nimonic 115 alloys, each of the γ′ cuboids which were closely aligned along 〈100〉 directions changed into a plate during coarsening. In an NiSiAl alloy, however, the γ′ particles were not aligned along a certain crystallographic direction, and they remained spherical during coarsening. The γ′ spheres in an NiSiAl alloy had dissolved uniformly into the γ matrix during reversion, and some spheres were left in a sparse and random distribution throughout the matrix. In NiSi, NiAl and NiAlTi alloys, however, the cuboidal or plate-like γ′ particles dissolved very locally into the γ matrix during reversion, and some particles were left in groups in the matrix. The elastic interaction energies between the γ′ precipitates played an essential role in the development of such types of precipitate morphology as the above.

Phase equilibria in iron-rich Fe - Al - V ternary alloy system

Abstract Phase separation and phase diagram in the Fe-rich corner of the Fe–Al–V alloy system were investigated by means of transmission electron microscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Phase separation into disordered A2 and ordered L21 phases was found in a wide composition range in the Fe-rich corner at 923 K, 973 K and 1023 K, and their tie-lines were almost parallel to the direction connecting the Fe corner with the center of the Gibbs triangle. In addition to the A2 + L21 phase separation, phase separation into B2 and L21 phases was also observed at 1023 K at the side of Fe–Al binary system.

The phase separations of Fe-Al-Co ordering alloys

Abstract The phase diagram for the Fe-rich Fe-Al-Co ordering system has been determined by observing phase separation by means of electron microscopy and magnetization-measurements. Two types of phase separation, into B2 and B2∗ (Al-rich B2 and Co-rich B2∗) and into α-Fe (A2) and B2, exist. The A2 + B2 phase separation occurs in a band shaped region connecting Fe-22–25 at.% Al with Fe-25–30 at.% Co. The B2 + B2∗ separation is localized in a wide island shaped region, which centres approximately at 25 at.% Al and 25 at.% Co. This island has a steep slope with temperature on the Co side. The microstructure of a B2 + B2∗ mixture is finely periodic along the 〈100〉 directions, i.e. a so-called [100] modulated structure. The B2 + B2∗ phase separation has been calculated thermodynamically by taking into account the ferro-magnetic and ordering excess free energies. The magnetic effect accounts for the shape of the B2 + B2∗ phase separation island.

Effect of coherent strain energy on γ/γ′ phase equilibria in NiAlTi alloys

The characteristic influences of elastic strain energy on the phase equilibria of the [gamma] + [gamma][prime] two phases in Ni-Al-Ti alloys have experimentally been investigated by means of analytical transmission electron microscopy. The results obtained are as follows; the tie-line ends of phase decomposition do not always coincide with the equilibrium phase boundaries and move inside the miscibility gap when the precipitates are coherent with the matrix, but move both the right hand for the incoherent precipitates and are finally fixed for the large incoherent precipitates on the equilibrium compositions of the phase diagram. These experimental results are theoretically rationalized on the basis of the free energy of the microstructure proposed by the authors.

On the spinodal decomposition in zirconia-yttria (ZrO2-Y2O3) alloys

Abstract Structural changes in arc-melted zirconia alloys containing 4 and 5mol.% yttria (Y-PSZ) during ageing inside the two-phase region of cubic (c) and tetragonal (t) phases were investigated with transmission electron microscopy. The arc-melted and auenched Y-PSZs consist of only tetragonal t′ phase containing many domains. Ageing inside the two-phase region of t + c sometimes produces fine modulated structures consisting of two different tetragonal phases, i.e. t 1 and t 2 These two-phase structures exhibit satellites in the electron diffraction pattern. It was discovered for the first time that in the early stage of ageing at lower temperatures, Y-PSZ alloy exhibits a period when the wavelength λm of its modulated structure remains practically constant. Furthermore, there is a linear relation between the sauare of the wave-number β2 m (βm = 2π/λm) and the ageing temperature T The above experimental results strongly indicate the occurrence of spinodal decomposition (t′→t 1 + t 2) in Y-PSZ alloy res...