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Formation and growth mechanism of TiC crystal in TiC_p/Ti composites

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中国有色金属学会会刊：英文版 2002年第6期12卷 1158-1163页

作者：金云学王宏伟曾松岩张二林 national key laboratory for precision hot processing of metal Harbin Institute of Technology

Ti C and Ti Al C alloys were prepared using gravity and directional solidification processes. Morphologies of TiC crystal were investigated by using SEM, XRD and EDX. Also, the formation and growth mechanism of TiC crystal have been analyzed on the basis of coordination polyhedron growth unit theory. During solidification of titanium alloys, the coordination polyhedron growth unit is TiC 6. TiC 6 growth units stack in a linking mode of edge to edge and form octahedral TiC crystal with {111} planes as present faces. Although the growing geometry of TiC crystal is decided by its lattice structure, the final morphology of TiC crystal depends on the effects of its growth environment. In solute concentration distribution, the super saturation of C or TiC 6 at the corners of octahedral TiC crystal is much higher than that of edges and faces of octahedral TiC crystal. At these corners the driving force for crystal growth is greater and the interface is instable which contribute to quick stacking rate of growth units at these corners and result in secondary dendrite arms along TiC crystallographic directions. TiC crystal finally grows to be dendrites.

关键词： TiC 钛合金合成物碳化钛形态学生长机制

Microstructure and mechanical properties of ultra-fine grained MoNbTaTiV refractory high-entropy alloy fabricated by spark plasma sintering

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Journal of Materials Science & Technology 2019年第11期35卷 2600-2607页

作者： Qing Liu Guofeng Wang Xiaochong Sui Yongkang Liu Xiao Li Jianlei Yang national key laboratory for precision hot processing of metals Harbin Institute of Technology

The MoNbTaTiV refractory high-entropy alloy(RHEA)with ultra-fine grains and homogeneous microstructure was successfully fabricated by mechanical alloying(MA)and spark plasma sintering(SPS).The microstructural evolutions,mechanical properties and strengthening mechanisms of the alloys were systematically *** nanocrystalline mechanically alloyed powders with simple bodycentered cubic(BCC)phase were obtained after 40 h MA ***,the powders were sintered using SPS in the temperature range from 1500℃to 1700℃.The bulk alloys were consisted of submicron scale BCC matrix and face-centered cubic(FCC)precipitation *** bulk alloy sintered at 1600℃had an average grain size of 0.58μm and an FCC precipitation phase of 0.18μm,exhibiting outstanding micro-hardness of 542 HV,compressive yield strength of 2208 MPa,fracture strength of 3238 MPa and acceptable plastic strain of 24.9%at room *** enhanced mechanical properties of the MoNbTaTiV RHEA fabricated by MA and SPS were mainly attributed to the grain boundary strengthening and the interstitial solid solution *** is expectable that the MA and SPS processes are the promising methods to synthesize ultra-fine grains and homogenous microstructural RHEA with excellent mechanical properties.

关键词： Refractory high-entropy alloy Ultra-fine grain Mechanical alloying Spark plasma sintering Mechanical properties

NUMERICAL SIMULATION OF ATOMIZATION GAS FIELDS IN VARIOUS ATOMIZING PROCESSES

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Acta metallurgica Sinica(English Letters) 2002年第3期15卷 279-284页

作者： F.Y. Cao, J.F. Sun, C.S. Cui, J. Shen and Q.C. Linational key laboratory of precision hot processing of metals, Harbin Institute of Technology, Harbin 150001,ChinaManuscript received 10 December 2001 in revised form 31 January 2002 national key laboratory of precision hot processing of metals Harbin Institute of Technology

A Computational Fluid Dynamics Software was used to calculate the atomizing gas fields generated by a self-designed atomizer and to analyze the effects of key atomizing variables such as gas pressure and protrusion length of delivery tube on the gas flow state at the tip of or inside the delivery tube. Increasing the length of delivery tube to a certain extent, the eddy flow region with positive pressure moves away from the tip of delivery tube, which is favorable to achieve the effective atomization of the melt.

关键词： atomization computational fluid dynamics numerical simulation powder metallurgy

Decomposition of Nb_(3)Si and mechanical-property improvement by adding appropriate amount of MgO in Nb-16Si-20Ti alloy

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Rare metals 2023年第4期42卷 1306-1315页

作者： De-Zhi Chen Qi Wang Rui-Run Chen Shu Wang Yan-Qing Su Heng-Zhi Fu national key laboratory for precision hot processing of metals Harbin Institute of TechnologyHarbin150001China

Five Nb-16Si-20Ti-xMgO alloys(x=0,0.1,0.5,1.0 and 3.0)were prepared via arc melting in this study,and the effect of MgO addition on their phase composition,microstructure evolution,and mechanical properties was *** results demonstrated that MgO reacted with the Nb-Si-Ti alloy while Mg atoms replaced Nb atoms in the Nbss *** hypoeutectic alloy was transformed into a hypereutectic alloy upon the addition of 3.0 at%MgO,and the Nb_(3)Si phases decomposed into a fine Nbss/α-Nb5Si3 eutectic *** highest fracture toughness was achieved for the Nb-16Si-20Ti-3MgO alloy,with an ambient fracture toughness value of 9.4 MPa·m^(-1/2)due to its largest Nbss phase content and optimal Nbss/α-Nb_(5)Si_(3)eutectic ***,the alloy compressive strength increased with MgO *** compressive strength of the Nb-16Si-20Ti-3MgO alloy was 2624.1 MPa,26.0%higher than that of Nb-16Si-20Ti,due to the formation of a solid solution of Mg atoms in the Nbss phase and reinforcement of a small amount of the Ti_(2)O ***,an increase in the content of the Nbss/α-Nb_(5)Si_(3)eutectic structure increased both the alloy strength and fracture strain.

关键词： Niobium silicon alloy Magnesium oxide Eutectic structure Fracture toughness

Mechanism and morphology evolution of the O phase transformation in Ti-22Al-25Nb alloy

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Journal of Materials Science & Technology 2021年第30期89卷 97-106页

作者： Zong Yingying Wang Jiwei Shao Bin Tang Wei Shan Debin national key laboratory for precision hot processing of metals Harbin Institute of TechnologyHarbin150001China

The phase transformation and microstructure in Ti-22Al-25Nb alloy are extremely *** this work,the morphology evolution of the O phase during the heating and cooling process was investigated by electron backscatter diffraction(EBSD)and first-principles *** results show that the O→α_(2)phase transformation process during the heating process is as follows:spheroidization of the O phase occurs first,then theα_(2)phase nucleates in the spheroidized O phase,grows and replaces the O phase,completing the O→α_(2)phase *** the meanwhile,the diffusion of Nb from Nb-poor O to Nb-rich B2 phases is a back-diffusion *** to first-principles calculations,the driving force of the O→α_(2)phase transformation is the difference in the free energies of formation for the two phases(0.09 eV/atom).When the Nb content is greater than 15.625%,the lattice distortion of theα_(2)phase sharply increases,and the distortion energy drives the back-diffusion of *** the cooling process,theα_(2)→O phase transformation is difficult and slow due to the difficult diffusion of Nb from the B2 toα_(2)*** holding for 60 min at 960℃,the coarseα_(2)phase gradually transforms to the O phase from the margin to the inside,forming a dispersed mixed structure of the O andα_(2)*** the B2→O transformation,the nucleation of the O phase induces a high stress region,in the range of approximately 200 nm.

关键词： Ti-22Al-25Nb alloy Phase transformation Microstructure O phase First-principles

Superplastic Properties of Al_2O_3/Ni-Mn Nanocomposite Fabricated by Electrodeposition

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Journal of Materials Science & Technology 2010年第7期26卷 625-628页

作者： Guofeng Wang Kaifeng Zhang national key laboratory of precision hot processing of metals Harbin Institute of Technology Harbin 150001 China

Ni-Mn nanocomposite reinforced by Al2O3 particles was fabricated by pulse electrodeposition. The average grain size is 60 nm and the content of Mn and Al2O3 particles is 0.3 and 0.6 wt pct, respectively. The superplastic deformation behavior was further studied at the temperatures ranging from 673 to 873 K. A maximum elongation of 530% is obtained in the tension test at a temperature of 773 K and at a strain rate of 1.67×10^-3 s^-1. The curves of the flow stress vs strain rate have features similar to the trend of conventional superplastic materials. The test temperature （773 K） equals to 0.35Tin, which means the material obtains low temperature superplasticity. The microstructures of the composite were examined and grain growth was observed during deformation.

关键词： Electrodeposition Nanocomposites Low temperature superplasticity Microstructure

Microstructure and mechanical properties of surface layers of 30CrMnSiA steel plasma nitrocarburized with rare earth addition

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Journal of Rare Earths 2012年第12期30卷 1281-1286页

作者：唐丽娜闫牧夫 national key laboratory for precision hot processing of metals School of Materials Science and EngineeringHarbin Institute of Technology

The pulse plasma nitrocarburizing for 30CrMnSiA steel was conducted at 560 ℃ for 8 h in mixed gases of N2：3H2 and different flow rates of rare earths （RE） addition. Effects of rare earths （RE） addition in the carrier gas on the surface morphology, phase structure and mechanical properties of the nitrocarburized layer were characterized by optical microscopy （OM）, X-ray diffraction （XRD）, scanning electron microscopy （SEM）, microhardness testing and wear testing, respectively. The results showed that the surface phase structures changed from dual phases ε-Fe2.3N（C） and γ′-Fe4N（C） to phase Fe3C and incipient nitrides, and the nitrocarburized surface hardness value decreased slightly from 756 to 681 HV0.1 with the RE addition increasing in the cartier gas, and the corresponding morphology of the nitrocarburized surface was granular nitride group （diameter 0.8-1.5 μm） and compact-fine Fe3C stick and patch （mean size 100-300 nm）, respectively. The wear resistance of the experimental steel could be improved remarkably by plasma RE nitrocarburizing. The nitrocarburized layer with Fe3C phase formed in the mixed gases of N2：3H2 and flow rate of 0.5 L/min RE addition showed the lowest friction coefficient and the narrowest wear track.

关键词： 30CrMnSiA steel plasma nitrocarburizing rare earths surface morphology wear resistance

An innovative method for the microstructural modification of TiAl alloy solidified via direct electric current application

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Journal of Materials Science & Technology 2019年第1期35卷 23-28页

作者： Zhanxing Chen Hongsheng Ding Ruirun Chen Shiqiu Liu Jingjie Guo Hengzhi Fu national key laboratory for precision hot processing of metals School of Materials Science and Engineering Harbin Institute of Technology

Ti-48Al-2Cr-2Nb alloy solidified with the application of direct electric current has a refined and homogeneous microstructure without segregation. We observed an initial decrease followed by a subsequent increase in grain size and lamellar spacing, with the increase in current density. Similar trend can also be obtained by varying the amount of α2-phase(Ti_3Al). Using a directional solidification processing method,the columnar crystal microstructure transforms into an equiaxed crystal microstructure at a current density of 32–64 m A/mm^2. High dislocation density is also introduced with a minimum cross-sectional grain size of 460 μm at a current density of 64 mA/mm^2. The application of electric current alters the free energy of the critical nucleus and temperature via joule heating, causing a transformation from a columnar grain microstructure into an equiaxed grain microstructure. The increase in current density leads to a rise of the nucleation rate, and a resulting undercooling combined with temperature gradient contribute to growth of the primary phase, which finally results in grain coarsening at a critical current density of 96 mA/mm^*** climb and cross-slip of dislocation and the migration of grain boundary ultimately create variable lamellae spacing of TiAl alloy.

关键词： TiAl Solidification Direct electric current Microstructure

Microstructure and Wear Resistance of in situ Formed Duplex Coating Fabricated by Plasma Nitriding Ti Coated 2024 Al Alloy

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Journal of Materials Science & Technology 2014年第12期30卷 1278-1283页

作者： Fanyong Zhang Mufu Yan national key laboratory for precision hot processing of metals School of Materials Science and EngineeringHarbin Institute of Technology

In this study, plasma nitriding was carried out on pure titanium film coated 2024 Al alloy to improve its surface mechanical property. Ti film with the thickness of 3.0 mm was firstly fabricated by means of magnetron sputtering method. Then, the Ti coated specimen was subjected to plasma atmosphere comprising 40% N2e60% H2 at 430 C for 8 h. The microstructures of the nitrided specimens were characterized by X-ray diffraction and scanning electron microscopy. Microhardness tester and pin-on-disc tribometer were used to test the mechanical properties of the untreated and nitrided specimen. The results showed that the surface of the nitrided specimen was composed of three layers（i.e. the outside nitride Ti N0.3layer, the middle Al3 Ti layer and the inside Al18Ti2Mg3 layer）. The surface hardness and wear resistance of 2024 Al alloy were increased simultaneously by duplex treatment. The untreated specimen exhibited severe adhesive wear while the nitrided one behaved in middle abrasive wear.

关键词： Aluminum alloy Plasma nitriding Wear behavior Diffusion coating Wear