Kazumi Hirano

Ultra-high temperature compressive creep behavior of an in-situ Al2O3 single-crystal/YAG eutectic composite

Abstract Compressive creep tests were conducted for an in-situ single-crystal alumina/yttrium aluminum garnet [Al 2 O 3 /Y 3 Al 5 O 12 (YAG)] eutectic composite for a specimen with a compressive axis with 0 or 90° to the solidified direction at temperatures between 1723 and 1923 K under stress ranges of 140–450 MPa in air environments. The single-crystal Al 2 O 3 /YAG eutectic exhibited a stress exponent of 5.4–10, indicative of compressive creep behavior characterized by a dislocation mechanism. Activation energies for creep deformation were 810–1024 kJ/mol, close to that for 42° from c -axis single-crystal Al 2 O 3 or single-crystal YAG, in agreement with that of self-diffusion for c -axis single-crystal Al 2 O 3 . The 0° specimen exhibited a slight decrease in creep rate by about half for a 90° specimen corresponding to results from crystal anisotropy in single-crystal Al 2 O 3 . The Larson–Miller method provided an effective means of comparing creep resistance of oxides on the basis of results obtained under different stresses and temperatures.

LSD1 Mediates Neuronal Differentiation of Human Fetal Neural Stem Cells by Controlling The Expression of a Novel Target Gene, HEYL

Histone‐modifying enzymes dynamically regulate the chromatin status and have been implicated in the fate specification of stem cells, including neural stem cells (NSCs), which differentiate into three major cell types: neurons, astrocytes, and oligodendrocytes. Lysine‐specific demethylase 1 (LSD1, also known as KDM1A) catalyzes the demethylation of H3K4me1/2 and H3K9me1/2, and it was recently suggested that functional disruption of LSD1 links to various human diseases. However, the mechanism by which LSD1 regulates human neural development remains unclear. Here, we present evidence that specific inhibition of LSD1 suppresses the neurogenesis of cultured human fetal NSCs (hfNSCs) isolated from the human fetal neocortex. Notably, we found that LSD1 directly associates with the promoter of the HEYL gene, and controls the demethylation of H3K4me2, which is accompanied by repression of HEYL expression during hfNSC neuronal differentiation. Furthermore, we also showed that HEYL expression is sufficient to inhibit the neuronal differentiation of hfNSCs. This mechanism seems to be primate‐specific because mouse NSCs do not exhibit the LSD1 inhibitor‐induced upregulation of Heyl. Our findings suggest that LSD1 plays an important role in primate neurogenesis and may contribute to the characterization of an evolved primate brain. Stem Cells 2016;34:1872–1882

AFM/MFM hybrid nanocharacterization of martensitic transformation and degradation for Fe-Pd shape memory alloy

Martensitic transformation and degradation characteristics for Fe-Pd ferromagnetic shape memory alloy were investigated by the developed AFM (Atomic Force Microscope)/MFM (Magnetic Force Microscope) hybrid nano-characterization technique. In AFM martensitic transformation was detected by the changes of surface topography of martensite plates. In MFM martensitic transformation was detected by the changes of magnetic domain structures. This technique has an advantage that martensitic transformation characteristics such as martensitic transformation temperature and reverse transformation temperature can be measured at microscopic and nanoscopic small area. Degradation characteristics of martensitic transformation under cyclic loading were also detected by the changes of AFM and MFM images. In AFM images surface topography of martensite plates became flat and in MFM images the morphology of magnetic domain structures became unfocused under cyclic loading. Then it was found that the hybrid nano-characterization was very high sensitive technique to evaluate degradation for Fe-Pd ferromagnetic shape memory alloy.

New insight into LSD1 function in human cortical neurogenesis

ABSTRACT The cerebral cortex of primates has evolved massively and intricately in comparison to that of other species. Accumulating evidence indicates that this is caused by changes in cell biological features of neural stem cells (NSCs), which differentiate into neurons and glial cells during development. The fate of NSCs during rodent cortical development is stringently regulated by epigenetic factors, such as histone modification enzymes, but the role of these factors in human corticogenesis is largely unknown. We have recently discovered that a lysine-specific demethylase 1 (LSD1), which catalyzes the demethylation of methyl groups in the histone tail, plays a unique role in human fetal NSCs (hfNSCs). We show that, unlike the role previously reported in mice, LSD1 in hfNSCs is necessary for neuronal differentiation and controls the expression of HEYL, one of the NOTCH target genes, by modulating the methylation level of histones on its promoter region. Interestingly, LSD1-regulation of Heyl expression is not observed in mouse NSCs. Furthermore, we first demonstrated that HEYL is able to maintain the undifferentiated state of hfNSCs. Our findings provide a new insight indicating that LSD1 may be a key player in the development and characterization of the evolved cerebral cortex.

Fracture Toughness of Single Crystal Al2O3/YAG Eutectic Composite up to 1973K

This paper reports on fracture toughness successfully determined f or single crystal Al 2O3 /YAG eutectic composite at ultra-high temperatures under pure mode I, combined mode I and mode II, and pure mode II loadings according to the chevron-notched diametrical-compr ession test. It was found that pure mode I fracture toughness is nearly constant and there i s no reduction in air up to 1973K. It was also shown that pure mode II fracture toughness is twice more than pure mode I. Then, empirical fracture criterion was applied to predict mixed-mode fracture toug hness. Introduction Recently, in-situ single crystal oxide ceramic eutectic compos ites, for example Al 2O3/Y3Al 5O12 (YAG), Al2O3/GdAlO3 (GAP) and Al 2O3/Er3Al 5O12 (EAG) have been researched and developed [1]. They have many potentialities that the tensile strength is maint ained right below the melting point temperature and increases with decreasing of a characteristic dimension of the micro-structural network without the reduction of creep resistance, and plastically deform ed at ultra-high temperature ranges. Also, the machinability and productivity of complex structural components ar good because of the solidification process in comparison with conventional sintered eng ineering ceramics. However very low fracture toughness and thermal shock resistance ori ginally resulted from the oxide/oxide composite are critical problems for a wide practical use. We have already reported both fracture toughness and fatigue crack growth resistance and their orientation depe ndency [2, 3]. It has also reported that there is much difference between compressive and tensile creep behavior, and high water vapor pressure accelerated tensile creep rate at ultra-high temper ature ranges [4~8]. In this paper, chevron-notched diametrical-compression test method [9] is firstly a pplied in air up to 1973K for single crystal Al 2O3/YAG eutectic composite and then fracture toughness are successfully determined under pure mode I, combined mode I and mode II, and pur e mode II loadings. Temperature dependency of pure mode I fracture toughness is discussed i n details and also compared with pure mode II fracture toughness. Finally, some fracture mechani cs criterion was applied to predict mixed-mode fracture toughness. Materials and Experimental Procedures Materials and Fabrication of Test Specimen The Al 2O3/YAG eutectic composite was fabricated by melting and directionally solidifying in a molybdenum crucible using a modified Bridgman technique with dimensions of approximately 40 mm diameter and 70 mm long [1]. The composition of the binary eutectic composite was Al 2O3/YAG =67/33 wt.%. After the solidification, all samples Key Engineering Materials Online: 2003-07-15 ISSN: 1662-9795, Vols. 243-244, pp 33-38 doi:10.4028/www.scientific.net/KEM.243-244.33

FAD influx enhances neuronal differentiation of human neural stem cells by facilitating nuclear localization of LSD1

Flavin adenine dinucleotide (FAD), synthesized from riboflavin, is redox cofactor in energy production and plays an important role in cell survival. More recently, riboflavin deficiency has been linked to developmental disorders, but its role in stem cell differentiation remains unclear. Here, we show that FAD treatment, using DMSO as a solvent, enabled an increase in the amount of intracellular FAD and promoted neuronal differentiation of human neural stem cells (NSCs) derived not only from fetal brain, but also from induced pluripotent stem cells. Depression of FAD‐dependent histone demethylase, lysine‐specific demethylase‐1 (LSD1), prevented FAD‐induced neuronal differentiation. Furthermore, FAD influx facilitated nuclear localization of LSD1 and its enzymatic activity. Together, these findings led us to propose that FAD contributes to proper neuronal production from NSCs in the human fetal brain during development.

Application of Ductile Metallic Phase Toughening Mechanism to Ceramics/Metals Functionally Gradient Materials.

It is still essential for high strength engineering ceramics to improve fracture toughness for a wide use in primary structural applications. The purposes of this paper are to apply the ductile metallic phase toughening mechanism proposed previously to ceramics/metals functionally gradient materials (FGMs) and to confirm the possibility of improvement of fracture toughness without the reduction of strength. Fracture characteristics were investigated for symmetrically step by step graded FGMs (Cr3C2/Ni, TiC/Ni) on the basis of fracture mechanics and fractography. It was found that the ductile metallic phase toughening mechanism has a good potential for ceramics/metals FGMs.

Fracture Analysis of Whisker-Reinforced Aluminum Alloys.

The fracture process of whisker-reinforced aluminum alloys is studied. First, tensile tests are carried out on specimens of various orientations. Dimple fracture occurs at the edge of the whisker fiber due to the stress concentration. It is also found that delamination occurs at the whisker matrix interface in the T-specimen. Then the fracture process is simulated by FEM. For the constitutive equation, Gursons yield fuction is used. Void volume fraction is used as a fracture parameter and a conventional fracture analysis is carried out. Numerical results qualitatively agree with those of experiments. The delamination effect between the base metal and the whisker fiber is considered. It is concluded that the effect of the delamination is very small before fracture of the specimen.

Toughening Mechanism for Ceramic by a Ductile Metallic Phase.

The basic objective of this study is to establish the material design concepts for the coexistence of good mechanical properties with super heat-resistance in the systems of ceramic/metal and ceramic/ceramic FGMs. Flexural and fracture toughness tests were conducted on gas-pressure combustion sintered Cr3C2 ceramic and its composites with Ni and TiC (Cr3C2/Ni, Cr3C2/TiC) in a vacuum environment up to 1200°C using a newly developed materials testing system. Fracture characteristics were investigated on the basis of fracture mechanics and fractography. The toughness of Cr3C2 can be improved by the addition of metallic particles. It is found that the principal mechanism of toughness improvements in these composites (ductile-phase reinforced ceramic matrix composites) is attributed to crack tip blunting by a ductile metallic phase.

FRACTURE TOUGHNESS AND FATIGUE CRACK GROWTH CHARACTERISTICS OF CRYOGENIC STRUCTURAL MATERIALS

ABSTRACT The elastic plastic fracture toughness and the fatigue crack growth characteristics of the structural material A286 steel for the superconducting generator are investigated at the cryogenic temperature. Elastic plastic fracture toughness JIc and tearing modulus TJ depend on the specimen orientation and the test temperature. The JIc and TJ for L-C orientation are larger than those for C-L orientation, and they increase with decreasing the test temperature. It is also found that the fatigue crack growth characteristics at the cryogenic temperature is almost consistent with that at a room temperature except for the lower δK region near the threshold.