STRUCTURAL CHANGE IN HEAT AFFECTED ZONE AND ITS INFLUENCE ON PROPERTIES OF WELDED WHITE CAST IRON

The structural change in heat affected zone(HAZ)and its influence on properties of welded white cast iron have been investigated by means of thermal cycle simulation technique.The structure of the white cast iron at peak temperature 800℃ was examined as cementite in pearlitic matrix,of which the hardness and impact toughness are the lowest,while the struc- ture after cyclic heating at high peak temperature is mainly cementite together with twin martensite,of which the hardness and impact toughness are rather higher.The phase bounda- ries in the structure of low hardness are smooth and regular as well as with fine precipitates. Both the cleavage and interphase fracture were revealed in the structure of low hardness,while the transgranular fracture was found in those areas of higher hardness.     

Effect of rare earth La_2O_3 on the microstructure and mechanical properties of TiC/W composites

In this study, La2O3 was investigated as an additive to TiC/W composites. The composites were prepared by vacuum hot pressing, and the microstructure and mechanical properties of the composites were investigated. Experimental results show that the grain size of the TiC/W composites is reduced by TiC particles. When 0.5 wt.% La2O3 is added to the composites, the grain size is reduced further. According to TEM analysis, La2O3 can alleviate the aggregation of TiC particles. With La2O3 addition, the relative density of the TiC/W composites can be improved from 95.1% to 96.5%. The hardness and elastic modulus of the TiC/W 0.5 wt.% La2O3 composite are little improved, but the flexural strength and the fracture toughness increase to 796 MPa and 10.07 MPa·m1/2 respectively, which are higher than those of the TiC/W composites.     

Effect of metallic phase content on mechanical properties of （85Cu- 15Ni）/（10NiO-NiFe2O4） cermet inert anode for aluminum electrolysis

（85Cu-15Ni）/（10NiO-NiFe2O4） cermets were prepared with Cu-Ni mixed powders as toughening metallic phase and 10NiO-NiFe2O4 as ceramic matrix. The phase composition, microstructure of composite and the effect of metallic phase content on bending strength, hardness, fracture toughness and thermal shock resistance were studied. X-ray diffraction analysis indicates the coexistence of （Cu-Ni）, NiO and NiFe2O4 phases in the cermets. Within the content range of metallic phase from 0% to 20% （mass fraction）, the maximal bending strength （176.4 MPa） and the minimal porosity （3.9%） of composite appear at the metallic phase content of 5%. The fracture toughness increases and Vickers＇ hardness decreases with increasing metal content. When the thermal shock temperature difference （At） is below 200 ℃, the loss rate of residual strength for 10NiO-NiFe2O4 ceramic is only 8%, but about 40% for （85Cu-15Ni）/（10NiO-NiFe2O4）cermets. As At is above 200 ℃, the residual strength sharply decreases for sample CN0 and falls slowly for samples CN5-CN20.     

Mechanical Properties and Microstructural Evolution of Bulk UFG Al 2014 Alloy Processed Through Cryorolling and Warm Rolling

Tensile properties,microstructural evolution and fracture toughness of A1 2014 alloy subjected to cryorolling followed by warm rolling(CR + WR)have been investigated in the present study.The solution-treated(ST)A1 2014 alloy is cryorolled followed by warm rolling process at different temperatures(110,170 and 210 ℃).The mechanical properties and microstructural features of deformed and undeformed A1 2014 alloys were characterised by optical microscopy,transmission electron microscopy(TEM)and scanning electron microscopy(SEM).The CR + WR samples at 170 ℃ showed an improved hardness(179 HV),tensile(UTS 499 MPa,YS 457 MPa)and fracture toughness(K_Q= 37.49 MPa m~(1/2),K_(ee) = 37.39 MPa m~(1/2) and J integral= 33.25 kJ/mm~2)with respect to ST alloy as measured from the tensile and fracture toughness test.The improved mechanical properties of CR + WR alloy are attributed to grain boundary strengthening,combined recovery and recrystallisation,precipitation hardening and dynamic ageing effect during the deformation.The precipitation of metastable spherical phase Al_2Cu was responsible for the improved tensile and fracture properties of finegrained A1 2014 alloy observed in the present work.     

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TiB_2-B_4C CERAMIC COMPOSITES

The microstructure and mechanical properties of TiB_2 /B_4C composites have been investi-gated.It was found that both the strength and hardness for TiB_2 greatly increase with the ad-dition of 20 to 30 wt-% B_4C,and the fracture toughness K_(IC) value remaines on the originalhigh level.The flexure strength,Vicker's hardness and fracture toughness are 782 MPa,26.2GPa and 7.2 MPam~(1/2),respectively,for the TiB_2-30 wt-% B_4C composite,compared to450 MPa,21 GPa and 7.0 MPam~(1/2) for monolithic TiB_2.The toughening and strengtheningmechanisms,have also been discussed.     

Effect of rod-like grain on properties and toughening mechanism of 3Y-TZP/Al2O3 ceramics

In-situ rod-like A1203 grain was prepared by adding CAS multiphase additives under the sintering condition of 30 MPa, 1 550℃ and 1 h. The sintering behaviors, microstructure, toughening mechanism and access of Al2O3 ceramics were investigated by SEM, EDS and WDW omnipotent electronic mechanical testing machine, etc, and the crack propagating model of cylindrical crystal/3Y-TZP composite toughening Al2O3 ceramics was established. The results show that the composite additives prompt the anisotropic growth of Al2O3 grain, which strengthens toughening effect of 3Y-TZP in 3Y-TZP/Al2O3 composite ceramics. Moreover, the experimental material density is near to theoretical density, bending strength is 556.35 MPa, and fracture toughness is 6.73 MPa.m1/2. The mechanical properties of the materials are obviously improved.     

Microstructure and high-temperature mechanical properties of Nb-silicide based in-situ composites

Nb-15W-18Si-xHf （x = 0, 5, 10 and 15, mole fraction, %） alloys were prepared by arc melting and then homogenized at 1 750 ℃ for 50 h. The microstructure and mechanical behaviors, such as Vickers hardness, fracture toughness, room- and hightemperature strength of the alloys were investigated. The microstructure of the annealed Nb-15W-18Si-xHf alloys is composed of large primary Nbss dendrite and fine eutectic mixture of Nbss and MsSi3 silicide. Both the hardness and the fracture toughness show an increase tendency at room temperature with increase of Hf content. The 0.2% compressive yield strength, σ0.2 of the alloys with 5 %Hf and 10%Hf are larger than 960 MPa at 1 200 ℃, and about 500 MPa even at 1 500 ℃.     

High-temperature wear behaviors of MoSi2 under different loads

MoSi2 was prepared by SHS, pressed at room-temperature and then vacuum sintered at 1 500 ℃ for 1 h. The tribological properties of MoSi2 against Al2O3 were investigated by using an XP-5 type High Temperature Friction and Wear Tester. Micrographs and phases of the worn surface of MoSi2 were observed by SEM with EDS and X-ray diffraction. The results show that the wearing process of MoSi2 at high temperature exists three stages： running-in, interim and steady periods. MoSi2 exhibits preferable wear resistance when the load is lower than 50 N. Adhesion and oxidation wear exists widely at elevated temperature; however besides these, with increasing the load, the main wear mechanisms of MoSi2 could be changed from adhesion, plastic forming to fatigue fracture in turn.     

Self-propagating high temperature synthesis of MoSi2 matrix composites

MoSi2 is presently regarded as the most important material for electrical heating and as one with huge potential for high temperature structural uses. MoSi2 and MoSi2 matrix composites were prepared by self-propagating high temperature synthesis （SHS）. Pure MoSi2 was obtained and a compound of MoSi2 and WSi2was synthesized in the form of predominant solid solution （Mo,W）Si2. By adding aluminum of 5.5 at.% to Mo-Si, the crystal structure of MoSi2 changed into a mixture of tetragonal Cllb MoSi2and hexagonal C40 Mo（Si,Al）2. The （Mo,W）Si2-Mo（Si,Al）2-W（Si,Al）2 composite materials were synthesized by adding aluminum of 5.5 at.% to Mo-W-Si. However, if the amount of the added aluminum was not larger than 2.5 at.%, it did not have any significant effect. SHS is an effective technology for synthesis of MoSi2 and MoSi2 matrix composites.     

Mechanical properties and structure of zirconia-mullite ceramics prepared by in-situ controlled crystallization of Si-A1-Zr-O amorphous bulk

Zirconia-mullite nano-composite ceramics were fabricated by in-situ controlled crystallization of Si-A1-Zr-O amorphous bulk, which were first treated at 900-1 000℃ for nucleation, then treated at higher temperature for crystallization to obtain ultra-fine zirconia-mullite composite ceramics. The effects of treating temperature and ZrO2 addition on mechanical properties and microstructure were analyzed. A unique structure in which there are a lot of near equiaxed t-ZrO2 grains and fine yield-cracks has been developed in the samples with 15% zirconia addition treated at 1 150℃. This specific microstructure is much more effective in toughening ceramics matrix and results in the best mechanical properties. The flexural strength and fracture toughness are 520 MPa and 5.13 MPa·m^1/2, respectively. Either higher zirconia addition or higher crystallization temperature will produce large size rod-like ZrO2 and mullite grains, which are of negative effect on mechanical properties of this new composite ceramics.     

Si3N4(p)/SiC(w)协同复合MoSi2材料的强韧化及机理研究

采用X射线衍射(XRD)、扫描电子显微镜(SEM)、维氏硬度计、电子万能材料试验机研究MoSi_2-Si_3N_(4(p))/SiC_(w)复合材料的结构、形貌、硬度、断裂韧性,并对SiC晶须和Si_3N_4颗粒复合强韧化MoSi_2的机理进行了探讨.结果表明,SiC晶须和Si_3N_4颗粒对MoSi_2具有协同强韧化作用,MoSi_2-20%Si_3N_(4(p))-20%SiC_(w)(体积分数,下同)复合材料的抗弯强度达427 MPa,室温断裂韧性达到10.4 MPa·m~(1/2),均高于单一强韧化剂的强韧化效果.MoSi_2-20%Si_3N_(4(p))-20%SiC_(w)复合材料的强化机理为细晶强化和弥散强化;韧化机制为细晶韧化、裂纹偏转与分支和微桥接韧化.     

热循环对SiCp／MoSi_2复合材料性能的影响

研究了热循环对ＳｉＣｐ／ＭｏＳｉ_２复合材料抗弯强度和断裂韧度的影响，并测定了材料的宏观残余应力。实验材料是用热压方法制备的ＭｏＳｉ_２和不同体积百分数（１０，２０，３０ｖｏｌ％）ＳｉＣ_ｐ增强ＭｏＳｉ_２复合材料。实验结果表明，复合材料的抗弯强度和断裂韧度都随ＳｉＣ含量的增加而增加，经过热循环以后，四种材料的抗弯强度都有不同程度的增加，而断裂韧度则下降约２０％左右。这是由于材料经过热循环以后，造成ＳｉＣｐ和ＭｏＳｉ２界面结合过强和基体晶界过弱。尽管ＳｉＣ_ｐ和ＭｏＳｉ_２热膨胀系数相差很大，但在复合材料中未发现由此而产生的裂纹和宏观残余应力。     

原位反应热压复合SiCP/MoSi2的显微结构与力学性能

以Mo粉、Si粉和C粉为原料，采用湿法混合和原位反应高温热压一次复合工艺制备了不同配比的SiCp/MoSi2复合材料，研究了该种工艺原位生成的SiC颗粒对MoSi2基体显微结构和室温力学性能的影响。结果表明：原位反应生成的适量SiC颗粒可以细化基体晶粒，改善其力学性能，与同样工艺下制备的纯MoSi2相比，含40vol%SiCp的SiCp/MoSi2复合材料室温抗弯强度是其3.4倍，含50vol%SiCp的SiCp/MoSi2复合材料室温断裂韧性是纯MoSi2的1.5倍；该种工艺的强化机制为细晶强化和弥散强化，韧化机制为细晶韧化。     

MoSi2和WSi2的价电子结构及性能分析

根据固体与分子经验电子理论，对MoSi2和wSi2的价电子结构进行了定量的分析，通过键距差方法计算了MoSi2和WSi2晶体中各键上的共价电子数．结果表明：在MoSi2和WSi2晶体中，沿（331）位向分布的Mo-Si和W-Si原子键最强，这些键上的共价电子数和键能分别影响化合物的硬度和熔点．晶体中晶格电子数影响其导电性和塑性，MoSi2晶体中含有较高密度的晶格电子，因此MoSi2的导电性和塑性比WSi2好．并从键络分布的不均匀性解释了MoSi2和WSi2脆性产生的原因．     

ZrO2+SiC颗粒强韧化MoSi2复合材料的显微组织和性能

通过显微组织观察和力学性能测试 ,初步探讨了ZrO2 SiC颗粒对MoSi2 基体材料的强韧化效果和机制。结果表明 ,材料复合具有较好的强韧化协同效应 ,复合材料中ZrO2 相和少量SiC颗粒在基体的间层作用 ,可抑制MoSi2 晶粒长大 ;断口呈现晶粒细小、裂纹扩展曲折和沿晶与穿晶混合型断裂等特征 ;ZrO2 SiC颗粒通过弥散强化和细化晶粒使复合材料强度提高 ,通过晶粒细化、裂纹偏转和分支、微裂纹形成等机制的综合作用使复合材料增韧     

TOUGHENING OF IN SITU SYNTHESIZED MoSi_2-SiC COMPOSITE AT ROOM TEMPERATURE

二．InttodttCt1OllMoslZ meets thebaslc requirements for ahlgh temperature structuralmaterlal．凡reXampk,nhas a hi吵一king pdntd2030”C,rd扒IV*y high re幻s枯nCe扣high比m－perature。ldatlon,alow density of6．249/cm’,ahlgh modulus,adequate strength at1200－     

La2O3-Mo5Si3/MoSi2复合材料的力学性能和高温氧化行为

通过自蔓延高温合成了稀土协同Mo5Si3复合强韧化MoSi2的复合粉末,研究了La2O3-Mo5Si3/MoSi2复合材料的室温力学性能和高温氧化特性。结果表明:与纯MoSi2相比,稀土和Mo5Si3细化了材料的晶粒,提高材料的室温弯曲强度和断裂韧性,其强化机制为细晶强化,韧化机制为细晶韧化、裂纹偏转、裂纹分支和微桥接;当Mo5Si3含量不超过30%(摩尔分数)时,随着Mo5Si3含量的增加,材料的抗氧化性能降低,而RE-40%Mo5Si3/MoSi2(摩尔分数)复合材料出现粉化现象;RE-Mo5Si3/MoSi2复合材料抗氧化性的降低,主要是由于Mo5Si3较差的抗氧化性、材料致密度的降低以及晶粒细化的结果;0.8%稀土(质量分数)协同5%Mo5Si3(摩尔分数)的RE-Mo5Si3/MoSi2复合材料具有较好的综合力学性能和高温抗氧化特性。     

烧结温度对WC-Co-Ti3SiC2硬质合金微观组织与力学性能的影响

以掺杂3.0%Ti₃SiC₂(质量分数)的WC-Co-Ti₃SiC₂硬质合金为对象,研究了烧结温度(1350～1470℃)对WC-Co-Ti₃SiC₂硬质合金中的Ti₃SiC₂分解产物和比例、微观组织及力学性能的影响规律与机制。结果表明：烧结温度的升高促进了WC-Co-Ti₃Si C₂硬质合金中Ti₃Si C₂的分解以及(W,Ti)C和WSi₂相的生成,同时导致WC晶粒尺寸逐渐增大。硬质合金的硬度随烧结温度的升高呈现出先增大后降低的趋势,而断裂韧性则逐渐下降。当烧结温度为1410℃时,WC-Co-Ti₃Si C₂硬质合金的致密性最佳(孔隙率仅为0.47%),其力学性能也较为优异,硬度与断裂韧性分别为20 348.328 MPa和10.15 MPa·m^1...     

MoSi_2-WSi_2复合体系的自蔓延燃烧合成

利用自蔓延燃烧合成法制备了 Mo Si2 - WSi2 复合粉体 ,并对合成产物进行了 X射线衍射分析。研究表明 ,通过自蔓延燃烧合成反应既可实现 Mo Si2 与 WSi2 的复合 ,又可通过调整反应物 Mo,W,Si的比例 ,在 Mo Si2 中引入 WSi2的同时还能引入足够量的 Mo5 Si3- W5 Si3相。W量的增加使体系的绝热温度降低 ,对自蔓延燃烧合成反应产生影响。研究还表明 Mo Si2 - WSi2 和 Mo5 Si3- W5 Si3均以固溶体的形式存在。     

MoSi2—WSi2复合体系的自蔓延燃烧合成

自蔓延燃烧合成法制备了ＭｏＳｉ２－ＷＳｉ２ 事粉体,并对合成产物进行了Ｘ射线衍射分析。研究表明,通过自蔓延燃烧合成反应既可实现ＭｏＳｉ２与ＷＳｉ２的复合,又可通过调整尖物Ｍｏ,Ｗ,Ｓｕｉ的比例,在ＭｏＳｉ２中引入ＷＳｉ２的同时还能引入足够量的Ｍｏ５Ｓｉ３－Ｗ５Ｓｉ３相。Ｗ量的境加使体系的绝热温度降低,对自蔓延燃烧合成反应产生影响,研究还表明ＭｏＳｉ２－ＷＳｉ２和Ｍｏ５Ｓｉ３－Ｗ５Ｓｉ３均以固溶体的