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13 results on '"Ma Tengfei"'

8. Effects of ultrasonic vibration on the microstructure and mechanical properties of high alloying TiAl

Author

Ma Tengfei, Zheng Deshuang, Guo Jingjie, Ding Hongsheng, Su Yanqing, Chen Ruirun, and Fu Hengzhi

Subjects
010302 applied physics, Multidisciplinary, Materials science, Alloy, Nucleation, 02 engineering and technology, engineering.material, 021001 nanoscience & nanotechnology, Microstructure, 01 natural sciences, Indentation hardness, Article, Compressive strength, 0103 physical sciences, engineering, Ultrasonic sensor, Lamellar structure, Composite material, 0210 nano-technology, Chemical composition
Abstract

To modify the microstructure and enhance performances, the ultrasonic vibration is applied in the mould casting of TiAl alloy. The effects and mechanism of ultrasonic vibration on the solidifying microstructure and mechanical properties are investigated and the model for predicting lamellar colony size is established. After ultrasonic vibration, the coarse microstructure is well modified and lamellar colony is refined from 534 μm to 56 μm. Most of precipitated phases are dissolved into the lamellar colony leading to a homogenous element distribution. The phase ratio of α2-Ti3Al and γ-TiAl is increased, and the chemical composition is promoted to more close to equilibrium level by weakening the influence of β-alloying elements. The microhardness and yield strength are gradually improved by 23.72% and 181.88% due to the fine grain strengthening, while the compressive strength is enhanced by 24.47% through solution strengthening. The critical ultrasonic intensity (Ib) for TiAl alloy is estimated at 220 W cm−2 and the model for average lamellar colony size is established as "Equation missing". The ultrasonic refinement efficiency exponentially increases as the ultrasonic vibration time with a theoretic limit maximum value of Elim = 88% and the dominating refinement mechanism by ultrasonic vibration is the cavitation-enhanced nucleation rather than cavitation-induced dendrite fragmentation.

Published
2017

13. The β and γ subunits play distinct functional roles in the α2βγ heterotetramer of human NAD-dependent isocitrate dehydrogenase.

Author

Ma, Tengfei, Peng, Yingjie, Huang, Wei, Liu, Yabing, and Ding, Jianping

Abstract

Human NAD-dependent isocitrate dehydrogenase existing as the α2βγ heterotetramer, catalyzes the decarboxylation of isocitrate into α-ketoglutarate in the Krebs cycle, and is allosterically regulated by citrate, ADP and ATP. To explore the functional roles of the regulatory β and γ subunits, we systematically characterized the enzymatic properties of the holoenzyme and the composing αβ and αγ heterodimers in the absence and presence of regulators. The biochemical and mutagenesis data show that αβ and αγ alone have considerable basal activity but the full activity of α2βγ requires the assembly and cooperative function of both heterodimers. α2βγ and αγ can be activated by citrate or/and ADP, whereas αβ cannot. The binding of citrate or/and ADP decreases the S0.5,isocitrate and thus enhances the catalytic efficiencies of the enzymes, and the two activators can act independently or synergistically. Moreover, ATP can activate α2βγ and αγ at low concentration and inhibit the enzymes at high concentration, but has only inhibitory effect on αβ. Furthermore, the allosteric activation of α2βγ is through the γ subunit not the β subunit. These results demonstrate that the γ subunit plays regulatory role to activate the holoenzyme, and the β subunit the structural role to facilitate the assembly of the holoenzyme. [ABSTRACT FROM AUTHOR]

Published
2017
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