航空材料学报

Effects of Grain Size and Lamellar Thickness on Yield Strength of Fully Lamellar γ-TiAl Based Alloys

SU Ji-long, LIN Gao-fei, ZHENG Shu-he

2009, 29(4): 1-5.

[Abstract](1712) [PDF 1878KB](284)

Abstract:
An analytical method,based on the multiple-piled-up dislocation model,was presented to investigate the effects of the lamellar thickness(λ)and the grain size(d)on the yield strength of fully lamellar γ-TiAl based alloys.A simple formula was derived analytically to describe the relation of microstructural parameters to the yield strength,which gave a good agreement with the experimental results.It is found that yield strength is not sensitive to the grain size.Meanwhile,the numerical results show that the yield strength of fully lamellar γ-TiAl based alloys depends strongly on the lamella thickness,it increases with the decrease of the lamellar spacing(λ),and the yield strength has significantly size-dependent property.Also,Hall-Petch relation has been holden between lamellar spacing(λ)and the yield strength of the alloys within a certain scale range of lamellar spacing.Furthermore,it is worth noting that the yield strength saturates at a value of about 1000MPa when the lamellar thickness is less than the critical scale of 90nm.

Effect of Al Concentration on Microstructure and Mechanical Properties of Heat-Resistant Mg-Al-Sr Alloy

ZHOU Jian, BAI Jing, XUE Feng, SUN Yang-shan, TAO Wei-jian

2009, 29(4): 6-11.

[Abstract](1564) [PDF 1346KB](248)

Abstract:
Microstructures,tensile and creep properties of Mg-(4～7)Al-2Sr and the effect of Al concentration on microstructure and properties were investigated.The as-cast microstructure of Mg-(4～7)Al-2Sr consists of the dendritic α-Mg and grain boundary second-phases.In the alloy containing 4% Al divorced eutectic and small amounts of lamellar eutectic Al4Sr and ternary bulky τ phase(Mg-Al-Sr) are observed at grain boundaries.With the increase of Al contents all the Al4Sr phases transform to coarser lamellar eutectic,meanwhile the volume fraction of eutectic increases and that of τ phase decreases.There exists only α-Mg + Al4Sr eutectic microstructure when the Al concentration is 6%.A small amount of Mg17Al12 appears with Al content rising to 7%.The microstructure of alloys containing 4% and 5% of Al,in which Sr restrains the formation of Mg17Al12 and its discontinuous precipitation at elevated temperature,does not occur obvious changes after creep for 100h,contributing to good creep resistance of Mg-Al alloys at high temperatures.The τ phase has higher thermal stability than Al4Sr,thus,when the Al concentration is less than 6%,the increase of Al contents results in the decrease of the volume fraction of τ phase and the increase of Al4Sr phase,and further the decrease of creep resistance.When the Al concentration is greater than 6%,the discontinuous Mg17Al12 precipitates.The increase of Al contents causes increment of discontinuous precipitation Mg17Al12 and therefore,decline of creep resistance.

Effect of Drawing Rate on Microstructure Evolution of Ti-47.5Al-2Cr-2Nb-0.2B Alloys under Directional Solidification

ZHENG Yuan-bin, LI Shuang-ming, LI Zhen-xi, ZHU Peng-chao, FU Heng-zhi

2009, 29(4): 12-16.

[Abstract](1476) [PDF 600KB](260)

Abstract:
The directionally solidified samples of Ti-47.5Al-2Cr-2Nb-0.2B alloys were successfully prepared by a near-rapid directional solidification apparatus.The interface morphologies were observed as cellular interface at 2μm/s drawing rate and fully-developed dendritic interface at 100μm/s.The angle between lamellar(γ+α2) orientation and the growth direction lay exhibited 0° or 45° at 2μm/s which showed the leading phase growth in microstructures was β phase.However,at 100μm/s,the lamellar orientation was perpendicular to the growth direction and the angle reached 90°,the leading phase changed to α phase.This phase selection process occurred at 100μm/s matched with the prediction of the theoretical calculation result of 180μm/s,based on the comparison of interface growth temperatures of α and β phase at different drawing rates in TiAl-based alloys under directional solidification.

Effect of Rare Earth La and Ce on Microstructure and Properties of TiAl Alloy Prepared by Mechanical Alloying

HONG Yu, WU Yu-cheng, LI Yun, WANG Feng-tao, WANG Qing-ping, LI Dong-hui

2009, 29(4): 17-20.

[Abstract](1461) [PDF 776KB](236)

Abstract:
Ti-44.7Al-xLa-yCe(at%) alloys were prepared by mechanical alloying with subsequent traditional powder metallurgy technique.The effect of the different La and Ce contents on microstructures was investigated by using scanning electron microscopy and metallographic microscope.The mechanical property of alloys was measured.The results show that the minim adding of rare earth as La and Ce into TiAl based alloy system have obvious effect on thinning the alloy by machine alloying.The bending strength of Ti-Al alloy is reinforced by the La addition,the bending strength reaches the peak as 0.5at% La addition,and later decreases with the increase in La content.But the bending strength of the alloy decreases rapidly with the increase in Ce content.Meanwhile the bending strength of the alloy with rare earth La is much higher than the addition of Ce.

Effect of Heat Treatment on Tensile Properties and Microstructure of Ti₃Al/TC11 Dual Alloy Disc

LIU Ying-ying, YAO Ze-kun, YANG Hang-hang, QIN Chun, GUO Hong-zhen

2009, 29(4): 21-26.

[Abstract](1406) [PDF 731KB](346)

Abstract:
The Ti3Al/TC11 dual alloy discs were joined and deformed by vacuum electron beam welding and near isothermal forging,respectively.Then the tensile properties and microstructure of dual alloy discs processed by different heat treatment regimes were investigated.The results show that the strength of the dual alloy disc processed by gradient heat treatment is higher.However,the plasticity is related to the degree of deformation,and the plasticity of the deformation of 40% is higher.Heat treatment regime has a significant effect on the microstructure.When the dual alloy disc is processed by gradient heat treatment,the interfaces are not obvious and the microstructures change homogeneously.But the interfaces are very obvious and the microstructures change abruptly as the dual alloy disc is processed by general heat treatment.In addition,the heat treatment regime nearly has no effect on the phase constitution of the welding seam which consist of O,MoNb,Nb3Al and TiAl3 phases besides α,α2 as well as β phase.It has only a certain effect on the content of each composition phase.

LBW/SPF Complex Forming for Multi-Sheet Structure of GH4169 Superalloy

QU Feng-sheng, ZHANG Kai-feng, LU Hong-jun

2009, 29(4): 27-32.

[Abstract](1517) [PDF 745KB](298)

Abstract:
GH4169 superalloy with multi-sheet structure was manufactured by LBW/SPF(laser beam welding/superplastic forming) complex technology.Superplasticity and microstructure of GH4169 superalloy was investigated.Test results show that GH4169 superalloy with ultra-fine grain has excellent superplasticity.Under T=940℃ and initial strain rate with =6.1×10-4s-1,the elongation of base material is 368.2% and its strain rate sensitivity m is 0.39.Superplastic forming process is favorable to improving the microstructure and mechanical properties of welding joints.Accordingly,LBW/SPF complex technology is an appropriate forming technique for GH4169 superalloy with multi-sheet structure.The welding parameters are as follows: pulse frequency 32Hz;impulse duration 3ms;peak power per pulse 4500 W;welding speed 180 mm/min;the parameters for superplastic forming are as follows: temperature Tf=960℃;pressure Pf=4.2MPa;time tf=130min.

Mechanical Properties and Microstructure of Linear Friction Welded TC4+TC17 Joint

MA Tie-jun, SHI Dong-gang, ZHANG Yong, YANG Si-qian

2009, 29(4): 33-37.

[Abstract](1241) [PDF 1125KB](394)

Abstract:
The linear friction welding(LFW) experiment of the dissimilar titanium alloys(TC4 +TC17) was carried out with homemade LFW machine(XMH-160).By means of optical microscopy(OM),scanning electron microscopy(SEM) and transmission electron microscopy(TEM),the microstructure of joint was analyzed in detail.The characteristic of microstructure was described,with particular emphasis on the discipline of phase transition in the weld zone,the near weld zone and the deformation zone.The microstructure of the weld is ultra-fine equiaxial grain,while the microstructures in the near weld zone and the deformation zone are very different from each other of two sides of the weld.Combining with the behavior of the linear friction welding,the formation mechanism of the different microstructures was analyzed respectively combining.The tensile test result shows that the tensile strength of the joint exceeds that of the parent TC4 alloy.It indicates that,therefore,using linear friction welding a sound TC4+TC17 joint can be obtained with reasonable welding parameters.

Study on Character of Electron Beam Dynamic Focus during EBW

FU Peng-fei, WANG Ya-jun, MAO Zhi-yong, GONG Shui-li, WANG Chun-ming

2009, 29(4): 38-41.

[Abstract](1618) [PDF 783KB](223)

Abstract:
The processing of electron beam welding(EBW) was studied for TC4 titanium alloy with 20mm thickness,and the character of dynamic behavior for electron beam focus was analyzed.The effects of welding speed and work distance on the dynamic behavior of the focus also were studied.The results show the critical penetrating currents increase compulsorily and the curve of the focus behavior shifts up with the enhancement of welding speed.The dynamic focus shifts down with the increase of work distance.The increase of the focus currents and the changing of focus during EBW simultaneity contribute to the change of weld shapes from nail shape to bell shape.

Microstructure Forming Mechanism of Cermet Coatings Prepared by Reactive Plasma Spraying of Fe-Ti-C Powders

WANG Hai-tao, ZHANG Shou-quan, HUANG Ji-hua, ZHU Jing-lei, ZHANG Qiang, ZHANG Hua

2009, 29(4): 42-46.

[Abstract](1524) [PDF 759KB](237)

Abstract:
TiC/Fe cermet coatings were prepared by reactive plasma spraying of Fe-Ti-C powder,which used sucrose as carbonaceous precursor and ferrotitanium as raw materials.Quenching test was employed to investigate microstructure forming mechanism of the coatings.Microstructure of the composite powders,coatings and quenched products were analyzed by XRD and SEM.It is shown that every granule of the composite powders acts as a reactive unit.Ferrotitanium melt firstly,then the whole granule.Lots of TiC particles are formed inside the granule and some agglomerate outside.These granules collide onto substrate to form the coatings with superposition of composite layers and agglomerated TiC.As flight distance increases,the outside agglomerated TiC increases and the inside TiC particles decrease,so the granule is more difficult to form layer after impinging onto the substrate.Finally,the coatings are composed of both TiC and Fe,and a number of TiC particles are uniformly distributed inside crystal grains of Fe matrix.

Effect of Magnetron Sputtered DLC/SiC/Ti Films on Friction/Wear Behaviors of Magnesium Alloy

XIA Deng-fu, XU Xiao-jing, ZHUO Liu-cheng, HAO Xin-ni, ZONG Liang

2009, 29(4): 47-51.

[Abstract](1565) [PDF 472KB](371)

Abstract:
The nano-indentation and friction/wear properties of DLC/SiC/Ti(diamond-like carbon/silicon carbide/Ti)thin films(SiC and Ti films as interlayer) deposited on Mg alloy(AZ91D) substrate using magnetron sputtering technique at room temperature was investigated.The results show that the DLC film exhibits a low nano-hardness(4.01 GPa)and Young's modulus(40.53 GPa)but the ratio of hardness to modulus(0.10)is high.When sliding against Si3N4(silicon nitride) ball using ball-on-disc frictional and wear tester under dry frictional condition,the films-substrate system exhibits a good film-substrate adhesion and a good friction/wear properties with the friction coefficient about 0.19.The special wear rate is decreased 3 orders of magnitude together with little film cracking and interface delaminating after long time worn(3.5h).The film-substrate systems greatly improve the friction/wear properties of magnesium alloy.

Influence of Partial Aluminum Clad on Pitting of Aluminum alloy Fatigue Samples in NSS

LIU Hui-cong, GU An, ZHU Li-qun, LIU Jian-zhong, YE Xu-bin, HU Ben-run

2009, 29(4): 52-56.

[Abstract](1514) [PDF 1306KB](334)

Abstract:
The pitting of 2024 and 7B04 aluminum alloy plate fatigue samples with partial aluminum clad in 3.5% NaCl NSS(Neutral Salt Spray Test) was investigated.It was found that the partial aluminum clad not only protected the covered area from corrosion but also decreased the pitting probability of uncovered area indirectly,because the reducing of corrosion area decreased the pitting probability of aluminum alloy.As the area reduced,the pitting expansion of 2024 aluminum alloy samples slowed and the pitting probability of 7B04 aluminum alloy samples became zero.

Study on Residual Stresses Induced by Moisture Absorption in Unidirectional Carbon Fiber/Epoxy Resin Composites

HUANG Yuan, WAN Yi-zao, HE Fang, WANG Yu-lin, GAO Zhi-fang, LI Gang

2009, 29(4): 57-62.

[Abstract](1609) [PDF 687KB](464)

Abstract:
The residual stresses induced by the moisture absorption in unidirectional carbon fiber reinforced epoxy composites(CFL/EP) were investigated.First,with the mass diffusion module of the ABAQUS package,the moisture absorption behavior of the CFL/EP was simulated both at 37℃ and 80℃ and the results coincided well with the experimental results.Also,the simulations showed that the random spaced fiber distribution model was better than the regularly spaced hexagonal model adopted in the simulations.Then,the residual stresses induced by the moisture absorption varying with time were calculated on the basis of the moisture distribution in the composites obtained through the simulations.The calculation results showed that the residual stresses induced by the absorption reached very high levels(30MPa and above).The higher stresses developed especially at the locations between neighboring fibers of smaller space and the interfaces between the fibers and the matrix,in which,the stress value at the interfaces was the maximum.Those high level of residual stresses led to the composites fatigue damage and failure possibly during the cycles of moisture absorption and desorption.

Study on Multi-Walled Carbon Nanotubes Modified with KH550 on Toughening Epoxy Resin

XIONG Lei, MA Hong-yi, WANG Ru-min, LIANG Hong-bo, GUAN Jing

2009, 29(4): 63-66.

[Abstract](1592) [PDF 759KB](445)

Abstract:
The multi-walled carbon nanotubes were modified with coupling agent KH550 and carbon nanotubes/epoxy resin nanocomposites were prepared.The effects of different MWCNTs contents on the mechanical properties of nanocomposites were studied.The results show that a certain amount of KH550 are introduced onto the surface of MWCNTs.The mechanical properties of nanocomposites are greatly enhanced by filling the proper amount of MWCNTs.When MWCNTs content is 0.5%,the mechanical properties are the best.Compared with the pure epoxy resin,the impact strength and flexural strength are increased by 85.3% and 21.6% respectively.SEM show that the fracture pattern of composites are changed from brittle fracture to ductile fracture.

Studies on Behavior of Melt Crystallization of PTFE/SGM Composites

WANG Zhi-chao, KOU Kai-chang, ZHANG Dong-na, CHAO Min, BI Hui

2009, 29(4): 67-71.

[Abstract](1546) [PDF 386KB](400)

Abstract:
Effects of SGM content in PTFE/SGM composites on behavior of melt crystallization were studied by differential scanning calorimetry(DSC).The Jeziorny equation was used to describe the non-isothermal crystallization process of PTFE/SGM composites.The results show that the crystallinity of PTFE composites reaches a maximum value as the content of SGM increases up to 15%.The SGM exhibited a remarkable heterogeneous nucleation activity and enhanced the crystallization rate of PTFE/SGM composites.The regular influence of SGM content on the crystallization rate of PTFE/SGM composites was not observed.The non-isothermal crystallization kinetics of PTFE/SGM composites was analyzed successfully with the Jeziorny method.It was found that the crystallization mechanisms of PTFE were not affected by the change of SGM content and several crystallization mechanisms were taken in the crystallization process of PTFE.

Effects of ZrC Content on Microstructure and Properties of 2D C/SiC-ZrC Composites via Precursor Infiltration and Pyrolysis

WANG Qi-kun, HU Hai-feng, CHEN Zhao-hui

2009, 29(4): 72-76.

[Abstract](1528) [PDF 433KB](379)

Abstract:
According to the working requirements of hypersonic spacecrafts,reusable space vehicles and next generation rocket engines operating at ultra-high temperature condition,a proposal to improve anti-ablative property of 2D C/SiC composites by introduction of ZrC filler was presented.2D C/SiC-ZrC composites with different ZrC contents were prepared via precursor infiltration and pyrolysis(PIP) process,and acclordingly the influence of ZrC contents upon the mechanical,anti-ablative,structural properties and compositions of 2D C/SiC-ZrC composites were investigated.The results were that flexural strength and flexural modulus of composites decreased with ZrC content increase,but the anti-ablative properties were greatly improved.The sample S-50 with the highest ZrC content(33.3 vol%) has the lowest flexural strength of 192.3 MPa and flexural modulus of 27.5GPa.After ablation for 60 seconds in oxyacetylene torch environment,the surface temperature of sample S-50 was as high as 2123℃.The sample S-50 showed the best anti-ablative property,and mass loss rate and linear recession rate were as low as 0.006g/s and 0.004mm/s,respectively.

Processin of 2D C/SiC-ZrB₂ Composites by Slurry Infiltration and Reactive Melt Infiltration

TONG Chang-qing, CHENG Lai-fei, YIN Xiao-wei, LIU Yong-sheng, ZHANG Li-tong

2009, 29(4): 77-80.

[Abstract](1492) [PDF 803KB](264)

Abstract:
2D C/SiC-ZrB2 composites were prepared by combining slurry infiltration technology with reactive melt infiltration method.The phase identification and microstructure of the composites were analyzed by XRD,SEM and EDS.Mechanical properties were evaluated by three-point flexural test.The results show that when ZrB2 powder and phenolic resin is infiltrated in turn under single-oriented pressure,they can be satisfactorily infiltrated into inter-bundles.But during the reactive melt infiltration precess,Si can not infiltrate into the samples efficiently.The open porosity and flexural strength of the as-fabricted specimens are 18.3% and 110MPa,respectively.When ZrB2 slurry is infiltrated under vacuum and the carbon matrix is infiltrated by chemical vapor infiltration method,the open porosity and flexural strength of as-fabricted composite are 5.5% and 230MPa,respectively.

Preparation of Nano-Hydroxyapatite/Chitosan Bio-Coatings on Carbon/Carbon Composites

LI Ying-Hua, CAO Li-Yun, HUANG Jian-Feng, ZENG Xie-Rong

2009, 29(4): 81-84.

[Abstract](1457) [PDF 887KB](368)

Abstract:
Hydroxyapatite/Chitosan(HAp/CS) bio-coatings were prepared by a novel hydrothermal electrophoretic deposition on CS modified C/C composites in isopropyl alcohol,using HAp nanoparticles prepared by sonochemical process as raw material.The influences of hydrothermal condition and deposition voltage on microstructures and morphologies of the as-prepared coatings were investigated.The as-prepared HAp/CS coatings were characterized by X-ray diffraction(XRD),transmission election microscopy(TEM),fourier transform infrared(FTIR) spectrum and scanning electron microscope(SEM) analyses.Results show that with the increase of deposition voltage,density and homogeneity of the as-prepared HAp/CS composite coatings are well improved.Due to the growth of HAp nanoparticles in the hydrothermal condition,the subsequent heat treatment of the HAp/CS coatings is not needed.

Effects of Poly(methylphenylsiloxane) Coating on High Silica Glass Fiber/Phosphate Composites

WU Li-na, HUANG Yu-dong, LIU Li

2009, 29(4): 85-89.

[Abstract](1375) [PDF 918KB](313)

Abstract:
Organic-inorganic hybrid coating was used to modify the surface of HSGF.Surface characteristics of fiber before and after coated were confirmed by X-ray photo electron spectroscopy(XPS) and atomic force microscopy(AFM),respectively.The effect of coating on acid-resistance of fiber was investigated by using immersion method.The interface bonding of composites was evaluated by interfacial shear strength test.The variety of mechanical properties of fiber and composites were measured using electronic universal-testing machine.Result shows that poly(methylphenylsiloxane) coating can effectively protect high silica glass fiber by preventing the harmful interface reaction and decreasing corrosion rate of phosphate to fiber.The flexural strength value of the composites with modified sizing is 32% higher than that of the untreated ones.

Preparation of PAXD Method and Investigation of Properties on TiB/TiAl Matrix Composites

LI Yun-ming, WANG Fen, ZHU Jian-feng

2009, 29(4): 90-93.

[Abstract](1584) [PDF 734KB](175)

Abstract:
In situ exothermic reaction of Al-Ti-B system was adopted to fabricate TiB/TiAl composites by pressure-assisted exothermic dispersion method(PAXD).The relationship of TiB/TiAl composites of microstructure and properties were analyzed by XRD,SEM and mechanical properties tests.The reinforced mechanism of TiB in TiAl intermetallic was investigated.The results showed TiB was reinforcement phase,TiAl and Ti3Al were matrix.The reinforcing phase TiB with particle shape,plate shape and needle shape was distributed in the TiAl matrix uniformly,and the size of TiB was within sub-micron scope.As to the binary TiAl intermetallic,the bending strength and fracture toughness of TiB/TiAl composites improved effectively.The main mechanism of the in-situ composite TiB/TiAl was dispersion reinforcing,thermal expansion rational mismatch reinforcing,whisker(crystal-bar) reinforcing.

Study of new Lead-Free Piezoelectric Ceramics of Bi_0.5(Na_0.82K_0.18)_0.5Ti_O3-LiNbO₃

ZHOU Chang-rong, LIU Xin-yu, YANG Gui-hua, YUAN Chang-lai, YANG Hua-bin

2009, 29(4): 94-97.

[Abstract](1549) [PDF 462KB](265)

Abstract:
A newBi0.5(Na0.82K0.18)0.5TiO3-based lead-free piezoelectric ceramic,(1-x)Bi0.5(Na0.82K0.18)0.5TiO3-xLiNbO3(BNKT-LNx) was prepared by a conventional ceramic sintering technique.The effect of LiNbO3 on crystal structure,microstructure and piezoelectric properties was investigated.X-ray diffraction patterns indicate that the ceramics studied can form pure perovskite structure solutions.The grain size increases with trace LiNbO3 addition.There are two dielectric abnormal peaks in εr-T curves,and the low abnormal peak shifts to low temperature and εm decreases and the diffuse phase transition characteristics becomes more evident with increasing content of LiNbO3.The optimum piezoelectric properties of d33=195pC/N,kp=0.336 were obtained at x=0.01.

Creep Behavior of Single Crystal Nickel-Base Superalloy Containing Element Re

WANG Ming-gang, TIAN Su-gui, YU Hui-chen, YU Xing-fu, QIAN Ben-jiang

2009, 29(4): 98-102.

[Abstract](1523) [PDF 1566KB](181)

Abstract:
By means of measuring creep curves,internal friction stress and microstructure observation,an investigation has been made into the creep behaviors of a single crystal nickel-based superalloy containing element Re.Results show that,during the steady state creep,the values of the internal friction stress(σ0) are enhanced with the increase of the applied stress,and reduced obviously with the elevated temperature.The cubic γ' phase transforms into the N type rafted structure along the direction vertical to the applied stress axis during creep.The rafted γ' phase coarsens with the creep going on and it coarsens and twists severely with the distance to fracture reducing.Thereinto,the fact that the regions near fracture endure the biggest effective stress due to the shrunken neck is thought to be a main reason of the rafted γ' phase coarsen and twisted.In the tertiary creep stage,the deformation feature of the alloy is the dislocations shearing into the rafted γ' phase.

2009 Vol. 29, No. 4