Effect of Aging Isothermal Time on the Microstructure and Room-Temperature Impact Toughness of Fe–24.8Mn–7.3Al–1.2C Austenitic Steel with k-Carbides Precipitation

Yifan Feng,Renbo Song,Zhongzheng Pei,Renfeng Song,Guoyu Dou 대한금속·재료학회 2018 METALS AND MATERIALS International Vol.24 No.5

The microstructure and impact toughness of the as-cast Fe–24.8Mn–7.3Al–1.2C austenitic steel after solution treatmentand subsequent aging treatment were investigated in the present work. Research on the κ-carbides precipitation behaviorwas carried out by transmission electron microscope. The results show that nano-sized coherent κ-carbides were obtained inthe as-solutionized steel after aging treatment, which produced precipitation hardening. After being aging treated at 550 °Cfor 1 h, the steel with regular hexagonal grain structure exhibited a good combination of yield strength (~ 574 MPa) androom-temperature impact toughness (~ 168 J). In the present steel, the typical cube-on-cube orientation relationship betweenaustenite and κ-carbides was observed. However, due to the long aging isothermal time and high C content, the coarseintergranular κ′-carbide was formed and grew along the austenite grain boundary, which caused this orientation relationshipto be destroyed and a dramatical increase of the coherency strain energy at grain boundary. Furthermore, serious embrittlementof grain boundaries caused that cleavage cracks trend to propagate along the grain boundaries. Accordingly, theroom-temperature impact toughness decreased sharply. After aging isothermal time prolonging to 13 h, the Charpy V-notchimpact toughness was only ~ 5 J and fracture mode turned to fully brittle fracture accompanied with fl at facets, shear cracksand well-developed secondary crack.

Novel Schemes to Optimize Sampling Rate for Compressed Sensing

YiFan Zhang,Xuan Fu,Qixun Zhang,Zhiyong Feng,Xiaomin Liu 한국통신학회 2015 Journal of communications and networks Vol.17 No.5

The fast and accurate spectrum sensing over an ultra- wide bandwidth is a big challenge for the radio environment cogni- tion. Considering sparse signal feature, two novel compressed sens- ing schemes are proposed, which can reduce compressed sampling rate in contrast to the traditional scheme. One algorithm is dynam- ically adjusting compression ratio based on modulation recogni- tion and identification of symbol rate, which can reduce compres- sion ratio. Furthermore, without priori information of the modu- lation and symbol rate, another improved algorithm is proposed with the application potential in practice, which does not need to reconstruct the signals. The improved algorithm is divided into two stages, which are the approaching stage and the monitoring stage. The overall sampling rate can be dramatically reduced with- out the performance deterioration of the spectrum detection com- pared to the conventional static compressed sampling rate algo- rithm. Numerous results show that the proposed compressed sens- ing technique can reduce sampling rate by 35%, with an acceptable detection probability over 0.9.

Adaptive Importance Channel Selection for Perceptual Image Compression

( Yifan He ),( Feng Li ),( Huihui Bai ),( Yao Zhao ) 한국인터넷정보학회 2020 KSII Transactions on Internet and Information Syst Vol.14 No.9

Recently, auto-encoder has emerged as the most popular method in convolutional neural network (CNN) based image compression and has achieved impressive performance. In the traditional auto-encoder based image compression model, the encoder simply sends the features of last layer to the decoder, which cannot allocate bits over different spatial regions in an efficient way. Besides, these methods do not fully exploit the contextual information under different receptive fields for better reconstruction performance. In this paper, to solve these issues, a novel auto-encoder model is designed for image compression, which can effectively transmit the hierarchical features of the encoder to the decoder. Specifically, we first propose an adaptive bit-allocation strategy, which can adaptively select an importance channel. Then, we conduct the multiply operation on the generated importance mask and the features of the last layer in our proposed encoder to achieve efficient bit allocation. Moreover, we present an additional novel perceptual loss function for more accurate image details. Extensive experiments demonstrated that the proposed model can achieve significant superiority compared with JPEG and JPEG2000 both in both subjective and objective quality. Besides, our model shows better performance than the state-of-the-art convolutional neural network (CNN)-based image compression methods in terms of PSNR.

In-Plane Dynamics Characteristics and Multi-Objective Optimization of Negative Poisson’s Ratio Honeycomb Structure with Power Function Curve

Zhu Yifan,Xu Feng Xiang,Guan Yijie,Zou Zhen,Duan Libin,Du Zhanpeng,Ma Hongfeng 한국자동차공학회 2023 International journal of automotive technology Vol.24 No.5

As an alternative to the conventional concave hexagonal honeycomb structure (CHHS), a negative Poisson’s ratio honeycomb structure with power function curve (NHPC) was devised. The relationship between the power function exponent (PFE) and normalized power function coefficient (NPFC) of honeycomb structure and its equivalent Poisson’s ratio (EPR) was explored to identify the range of variables required for the negative Poisson’s ratio effect. To investigate the in-plane mechanical properties and energy absorption characteristics of NHPC, the deformation mode, dynamic response, and energy absorption characteristics under various impact velocities were studied by constructing an in-plane impact simulation model. The results showed that NHPC obviously exhibited a negative Poisson’s ratio effect on medium and low impact velocities, and the deformation was primarily uniform. As the NPFC increased, the honeycomb structure was less prone to stress concentration, while the peak crushing force (PCF) and the specific energy absorption (SEA) declined and the plateau stress increased. A multi-objective optimization experiment was operated with low PCF and high SEA as the targets within the range of design variables in order to generate the optimal NHPC. According to the experimental findings, the improved NHPC showed a 25.48 % reduction in PCF and a 19.29 % increase in SEA. This paper provides theoretical recommendations for improving the energy absorption and structural optimization of the honeycomb structure.

Thermodynamics and Kinetics of Graphene Growth on Ni(111) and the Origin of Triangular Shaped Graphene Islands

Wang, Danxia,Liu, Yifan,Sun, Deyan,Yuan, Qinghong,Ding, Feng American Chemical Society 2018 JOURNAL OF PHYSICAL CHEMISTRY C - Vol.122 No.6

<P>To understand the origin of the triangular shaped graphene, we systematically investigated the thermodynamics and kinetics of graphene growth on Ni(111) surface. It was found that the fcc staking of graphene on the substrate is more energetically favorable than other stacking sequences. Under the near thermoequilibrium condition, a graphene island will present a truncated triangular shape with alternative zigzag (ZZ) and ZZ-Klein edges; its growth is either on the top of the terrace (on-top mode) or embedded into the metal lattice (inlay mode). If the growth process is controlled by kinetics of carbon atom incorporation, the shape of a graphene island will be triangular because of the significant growth rate difference between the ZZ and ZZ-Klein edges. This study reveals the atomic details of graphene growth on Ni(111) surface, and the deep insights into the mechanism of graphene CVD growth may lead to the rational design of experiments for the growth of desired graphene and other 2D materials.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2018/jpccck.2018.122.issue-6/acs.jpcc.7b09814/production/images/medium/jp-2017-09814u_0001.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp7b09814'>ACS Electronic Supporting Info</A></P>

The short-term impact of fine particulate matter pollution on the incidence of cardiovascular diseases in Beijing, China

Yuxia MA,Haoran JIAO,Yifan ZHANG,Bowen CHENG,Fengliu FENG 대한환경공학회 2019 대한환경공학회 학술발표논문집 Vol.2019 No.-

Influence of Martensite and Bainite Microstructure on Local Mechanical Properties of a Bainitic and Martensitic Multiphase Cast Steel

Zhongzheng Pei,Renbo Song,Yifan Feng,Jie Xu 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.11

Here we utilized two cooling methods, forced air cooling and furnace cooling, to obtain two different triplex microstructureswhich were martensite-based and bainite-based, respectively. Compared with furnace cooled samples, the forced air cooledsamples presented about 5% higher ultimate tensile strength, 10% higher hardness and 43% lower Charpy impact energyvalue. Nano-indentation test was conducted to reveal the local mechanical properties. The nano-hardness of martensite,bainite and the mixture microstructure is 5.633 GPa, 4.200 GPa and 5.082 GPa, respectively. Mn, S, C and Si segregationat the grain boundaries in martensite are severely detrimental to the toughness. High rotation angle grain boundaries andprecipitates of rare earth element enhanced the toughness of bainite. Martensite with high density dislocations in fine lathsexhibited higher nano-hardness than bainite with lower density dislocations in wider sized laths. The interaction of the twophases with martensite twinning sub structure between bainite laths showed intermediate nano-hardness.

CO2 Adsorption on the B12N12 Nanocage Encapsulated with Alkali Metals: A Density Functional Study

Haiyan Zhu,Qiyan Zhang,Qinfu Zhao,He Zhao,Yifan Feng,Bingbing Suo,Huixian Han,Qi Song,Yawei Li,Wenli Zou,Haiyan Zhu 성균관대학교(자연과학캠퍼스) 성균나노과학기술원 2019 NANO Vol.14 No.3

Density functional theory (DFT) calculations have been carried out to study the capacity of the B12N12 nanocage encapsulated with alkali metals (Li, Na, K) for the CO2 adsorption and activation. It is found that after encapsulating alkali metals, the alkali metal atoms are closer to one side of clusters instead of exactly lying at the center, and a considerable charge transfers from the inner alkali metal atoms to the B12N12 cage. Besides, the HOMO–LUMO gap (HLG) values of Li@B12N12, Na@B12N12 and K@B12N12 are decreased to about 6 eV, being much smaller than that of the pristine B12N12. Although the geometry structure parameters and the energy differences of M06-2X are slightly different from the ones of ωB97X-D, some identical results of two kinds of functional can be obtained. CO2 can be adsorbed chemically and physically on majority bonds of all the clusters, except for some bonds with large change in bond length and bond indices. The encapsulation of alkali-metal atoms may enhance the physical and chemical adsorption of CO2 on the surface of the clusters, in which Na@B12N12 and K@B12N12 are the most powerful physical and chemical adsorbent for CO2, respectively.