THE PROCESS AND MECHANISM OF TiAl－BASED ALLOY SYNTHESIZED FROM Ti AND AlP OWDERS

ＴＨＥＰＲＯＣＥＳＳＡＮＤＭＥＣＨＡＮＩＳＭＯＦＴｉＡｌ－ＢＡＳＥＤＡＬＬＯＹＳＹＮＴＨＥＳＩＺＥＤＦＲＯＭＴｉＡＮＤＡｌＰＯＷＤＥＲＳＸｉｏｎＡｘＸｉａｎｙ；ＨｕａｎｇＢａｉｙｕｎ（ＰｏｗｄｅｒＭｅｔａｌｌｕｒｇｙＲｅｓｅａｒｃｈＩｎｓｔｉｔｕｉｅ...     

Preparation of TiAl/Al_2O_3 composites by combustion synthesis

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Instability of TiC and TiAl3 compounds in Al-10Mg and Al-5Cu alloys by addition of Al-Ti-C master alloy

The performance of Al-Ti-C master alloy in refining Al-10Mg and Al-5Cu alloys was studied by using electron probe micro-analyzer （EPMA） and X-ray diffractometer （XRD） analysis. The results indicate that there are obvious fading phenomena in both Al-10Mg and Al-5Cu alloys with the addition of Al-5Ti-0.4C refiner which contains TiC and TiAl3 compounds. Mg element has no influence on the stability of TiC and TiAl3, while TiC particles in Al-10Mg alloy react with Al to form Al4C3 particles, resulting in the refinement fading. However, TiC particles are relatively stable in Al-5Cu alloy, while TiAl3 phase reacts with Al2Cu to produce a new phase Ti（Al, Cu）2, which is responsible for the refinement fading in Al-5Cu alloy. These indicate that the refinement fading will not occur only when both the TiC particles and TiAl3 compound of Al-Ti-C refiner are stable in Al alloys.     

Fabrication of in situ Ti2AlN/TiAl composites by reaction hot pressing and their properties

Ti2AIN/TiAI composites with different volume fractions of reinforcement were successfully fabricated by hot-pressing sintering method （reaction hot pressing） using Ti, Al and TiN powders as starting materials. The synthesis process includes four stages： first, the reactions between Al and Ti powers and between Al and TiN powders respectively occur and result in TiAl3 phase; secondly, AI powders in the sample are exhausted; the remaining Ti cores react with TiAl3 layer to form Ti-Al intermetallics; moreover, a few Ti2AlN particles precipitate from the TiAl3 phase; thirdly, Ti-Al intermetallics react with the remaining Ti cores to form Ti3Al and TiAl phases. TiAl phase and original TiN powers are in direct contact each other; finally, the residual TiN powers react with TiAl phase and result in a plenty of TizAIN phase. Compared with TiAl matrix, the hardness, elastic modulus and high-temperature compressive strength of Ti2AlN/TiAl composite are improved obviously and they are all enhanced with increasing the volume fraction of Ti2AlN phase.     

Al-Ti-C grain refiner made by ultrasonic levitation

An electrostrictive uniaxial ultrasonic levitation reaction system was designed to perform TiC synthesis reaction by suspending graphite powder in Al-3Ti melt and an Al-3Ti-0.15C grain refiner alloy was obtained. The results show that sound pressure node in which graphite suspends is formed in the melt between radiation block and reflecting board by ultrasonic and TiAl3 particles congregate around C powder. Meanwhile, due to ultrasonic cavitation, dissolved TiAl3 provides better surface condition for the synthesis reaction of TiC. The reaction route is that C and dissolved Ti react to form TiC. Ultrasonic cavitation has a thermal activation effect on TiC particles and Al-3Ti-0.15C refiner has excellent microstructural refining effect on α-Al.     

外加Al2O3颗粒对Ti-Al-TiO2系合成产物微观结构的影响

Ti-Al-TiO2体系外加不同含量的Al2O3颗粒获得了Al2O3／TiAl基复合材料,研究了产物的相组成及微观组织。结果表明：Al2O3颗粒多分布在基体晶界处。随外加Al2O3含量的增大,晶粒变小,但增强相颗粒团聚严重。外加Al2O3量存在一个临界值。含量过高,易造成结构疏松,不致密,形成大的气孔,且由于TiO2的活化作用,会造成外加的Al2O3颗粒烧结呈板状联结分布;含量较低,颗粒与界面的结合不够紧凑,但颗粒的结晶度较好。     

Ti-C-Al-Fe_2O_3反应体系的热力学及形成产物的分析

针对Ti-C-Al-Fe2O3体系进行了热力学计算,并结合DSC及XRD分析,为研究该体系反应合成TiC,Al2O3提供了理论依据.计算结果表明：该体系在发生SHS反应时,可以生成TiC,Al2O3,Fe2Ti,TiAl3,Fe3C和Al4C3等产物,但TiC,Al2O3的热力学稳定性要远远大于Fe2Ti,TiAl3,Fe3C和Al4C3等相,因此,在1600℃的钢液引燃的SHS反应过程中,只能生成TiC,Al2O3两种产物.通过XRD分析也证明了在反应产物中只有TiC,Al2O3两种物质形成.     

Effects of HIPing pressure on microstructures and properties of TiAl alloy

ＴｉＡｌａｌｌｏｙｉｓａｃａｎｄｉｄａｔｅｍａｔｅｒｉａｌｆｏｒａｄｖａｎｃｅｄａｅｒｏｓｐａｃｅａｉｒｆｒａｍｅｂｅｃａｕｓｅｏｆｉｔｓｌｏｗｄｅｎｓｉｔｙ ,ｇｏｏｄｏｘｉｄａ ｔｉｏｎｒｅｓｉｓｔａｎｃｅａｎｄｈｉｇｈｍｅｃｈａｎｉｃａｌｓｔｒｅｎｇｔｈａｔｈｉｇｈｔｅｍ ｐｅｒａｔｕｒｅ.Ｈｏｗｅｖｅｒ,ｉｔｓｌｏｗｅｒｔｅｎｓｉｌｅｄｕｃｔｉｌｉｔｙａｔａｍｂｉｅｎｔｔｅｍｐｅｒａｔｕｒｅｉｓｈｉｎｄｅｒｉｎｇｉｔｆｒｏｍｂｅｉｎｇｗｉｄｅｌｙｕｓｅｄ[1,2 ] .Ｔｏｉｍｐｒｏｖｅｔｈｅｓｅｍｅ…     

Ti6Al4V的激光选区熔化单道成形数值模拟与实验验证

对于Ti6Al4V材料的激光选区熔化（SLM）成形工艺,基于离散元方法（DEM）建立粉末颗粒随机分布的三维介观模型.采用流体体积法（VOF）对SLM成形过程的三维自由表面进行动态追踪;考虑TC4颗粒随机分布的粉床、随温度呈非线性变化的热物性参数、熔池自由液面演化、由温度梯度引起的表面张力以及蒸发作用;通过数值模拟研究激光与粉末颗粒相互作用过程中的传热、熔化、流动、凝固等过程. 结果表明,由温度梯度及表面张力梯度产生的马兰戈尼对流是影响熔池内部传热传质和熔池三维形貌的主要因素;线能量密度（LED）与马兰戈尼对流的强度呈正相关,当LED=92.9~183.0 J/m时,单道表面质量较优. 通过单道成形实验对熔池与熔道的三维尺寸与形貌进行观察分析,有效验证了数值模拟的正确性.     

Preparation of TiAl/Ti2AlC Composites with Ti/Al/C Powders by In-situ Hot Pressing

TiAl/Ti2AlC composites were prepared by in-situ hot pressing of TilAl/C powders mixtures and sintered at different temperatures were investigated by X- ray diffraction （ XRD ） of samples. The reaction procedure of Ti-Al-C system could be divided into three stnges. Below 900℃ , Ti reacts with Al to form TiAl intermetallics ; above 900 ℃ , C reacts with remain Ti to form TiC triggered by the exothermal reaction of Ti and Al ; TiAl reacts with TiC to produce dense TiAl/Ti2AlC compasites.In the holding stage, ternary Ti2AlC develops to layered polycrystal and composites pyknosis in the meanwhile. The mechanism of synthesis and microstructure was especially discussed.     

机械诱发自蔓延反应合成Ti3AlC2的机理研究

以3Ti／Al／2C粉体为原料,采用机械合金化的方法以合成Ti3AlC2材料。研究结果表明,在机械合金化过程中诱发自蔓延反应,反应会产生大量坚硬的小块体颗粒,大小约为0．2-11mm。粉体的组成相为TiC、Ti3AlC2、Ti2AlC,而块体仅含有TiC和Ti3AlC2。获得的粉体和块体产物中Ti3AlC2含量分别约为63wt％和84．8wt％。提出了一个机械诱发自蔓延反应合成Ti3AlC2的反应机制,即Ti3AlC2是从固相TiC与Ti-Al液相中形核并长大。     

Combustion Properties of Metal Particles as Components of Modified Double-Base Propellants

Metal particles such as aluminum (Al),magnesium (Mg),boron (B) and nickel (Ni),as well as Mg/Al alloy (Mg/Al =3/4) are currently the most widely used ingredients in modified double-base propellants.In this contribution,the combustion properties of the metal species are studied by means of the high-speed photography technique and the non-contact wavelet-based measurement of flame temperature distribution.The combustion process of the A1,Mg and Mg/Al sampies shows both gas phase reaction and surface oxidation,which yield volatile and nonvolatile products,corresponding to the oxide and suboxide respectively.However,the combustion of B and Ni shows only gas phase reaction,due to their high melting point as well as high enthalpy of vaporization.In addition to the experiments,a hypothetical combustion model has been proposed to clarify the combustion characteristics of metal species in modified double-base propellants.     

Al2O3-NbAl／TiAl基高温复合材料的研究

以Ti-Al-TiO2-Nb2O5为反应体系,利用热压烧结工艺原位合成了Al2O5／TiAl复合材料．通过Mo元素掺杂对体系进行增强,借助X射线衍射（XRD）、扫描电镜（SEM）等分析研究了掺入不同含量Mo时,Ti-Al-TiO2-Nb2O5体系的热压反应过程及其对体系微观组织结构、力学性能等的影响．结果表明：放热反应加速了系统的反应,从而降低了该体系的反应温度;产物体积密度随着Mo掺杂量的增大而增大;抗弯强度和断裂韧性呈峰值变化．     

Kinetics of Combustion Synthesis in Ti-C-3Ni-Al System

The Kinetics and mechanisms of the combustion reaction in theTi-C-3Ni-Al system were studied. Samples were prepared by igniting compacts of elemental Ti, C, Ni and Al powders with a heating tungsten coil under an inert argon atmosphere. The activation energies of Ti+C+50wt%(3Ni+Al)→TiC+50%Ni3Al and Ti+C+80wt%(3Ni+Al) →TiC+80%Ni3Al exothermic reactions were determined by measuring the combustion wave velocity and the combustion temperature, which are 129kJ.mol-1 and 79kJ*mol-1, respectively. The mechanism of formation of products for Ti-C-3Ni-Al system was found:metal phases (Ti-3Ni-Al) are melted in combustion process,and carbon dissolves into the liquid metal and TiC is subsequently precipitated out of solution, and Ni3Al is crystallized during the cooling process. The mechanism is fairly similar with that of Ti+C→TiC and Ti+C+Ni→TiC+Ni. There are two reactions (Ti+C→TiC, 3Ni+Al→Ni3Al) in the Ti+C+3Ni+Al system, and the wave velocity is mainly controlled by the velocity of Ti + C→TiC, but the velosity of 3Ni+Al→Ni3Al may play a significant role when Ni3Al contents are higher than 70wt%.     

钛在铁水中的行为

由热力学计算结果分析了高炉冶炼含ＴｉＯ２铁矿时钛的物理化学行为,指出：有固体碳存在果,渣界面不能有纯ＴｉＮ生成,应该析出Ｔｉ（Ｃ３Ｎ）固溶体,     

Al2O3对3Ti/Si/2C体系自蔓延高温合成Ti3SiC2的影响

以3Ti/Si/2C粉体为原料,通过自蔓延高温合成技术合成了Ti3SiC2材料。研究了Al2O3助剂对自蔓延高温合成Ti3SiC2的影响。研究结果表明,3Ti/Si/2C粉体会发生自蔓延反应,产物的组成相为TiC、Ti3SiC2和Ti5Si3,产物中Ti3SiC2含量约为23%。添加适量的细粒度Al2O3可显著促进反应合成Ti3SiC2,3Ti/Si/2C/0.1Al2O3原料反应后得到的产物中Ti3SiC2含量达64%。     

The Process of TiB2-Cu Composite Phase and Structure Formation during Combustion Synthesis

The reaction process of combustion synthesis for TiB2 -Cu was investigated in detail using combustion-wave arresting experiment, X-ray diffraction （ XRD ） analysis, SEM analysis and differential thermal analysis （ DTA ）. The XRD analysis results for the different parts of the quenched specimen show that TiCux intermetallic phase firstly forms with the propagation of combustion wave, and then Ti1.87 B50 and Ti3B4 metastable phases come forth due to the diffusion of B atoms and finally the stable TiB2 phase forms because of the continuous diffusion of B atoms. The formation of TiB2 phase is uot completed by one step, but undergoes several transient processes. The process of reaction synthesis for Ti-B-Cu ternary system can be divided into three main stages： melting of Cu and Ti , and the formation of Cu- Ti melt and few TiCux , TiBx intermetallic phases ; large numbers of TiCux intermetallic phases formation and some fine TiB2 particles precipitation ; and the TiB2 particles coarsening and the stable TiB2 and Cu two phases formation in the final product.     

Preparation and Microstructural Analysis of Ti2AlC/TiAl（Nb） Composite

By using the spark plasma sintering process, Ti2AlC/TiAlcomposite with the addition of Niobium （Nb） was prepared in-situ and the microstructure of Ti2AlC/TiAl （Nb） composite was investigated by means of transmission electron microscopy （TEM） and high-resolution electron microscopy （HREM）. The results indicate that new-formed Ti2AIC particles disperse with a high degree of uniformity and well combine with the matrix. In the area of phase interface the d-spaces of Ti2AlC （100） and TiAI （110） were measured as 0.2648 nm and 0.2991 nm,respectively. The atom arrangement beside the interface was only partly corresponding, existing in semicoherent state. On the contrary, in the area of grain interface the d-spaces of TiAl （100） and TiAl （110） were measured as 0.2462 nm and 0.2631 nm,respectively and the atom arrangement beside the interface was almost corresponding, existing in coherent state.     

Dissolution-precipitation mechanism of self-propagating high-temperature synthesis of TiC-Cu cermets

The mechanism of self-propagating high-temperature synthesis (SHS) of TiC-Cu cermets was studied using a combustion front quenching method. Microstructural evolution in the quenched sample was observed using scanning electron microscope (SEM) with energy dispersive X-ray (EDX) spectrometry, and the combustion temperature was measured. The results showed that the com- bustion reaction started with local formation of Ti-Cu melt and could be described with the dissolution-precipitation mechanism, namely, Ti, Cu, and C particles dissolved into the Ti-Cu solution and TiC particles precipitated in the saturated Ti-Cu-C liquid solu- tion. The local formation of Ti-Cu melt resulted from the solid diffusion between Ti and Cu particles.     

Si和Al对机械合金化合成Ti3SiC2的影响

采用3Ti／Si／2C单质粉体为原料,进行机械合金化,以合成Ti3SiC2粉体。研究了Al和过量Si对机械合金化合成Ti3SiC2的影响。研究结果表明,机械合金化单质混合粉体,会诱发自蔓延反应。反应后产生大量坚硬的颗粒状产物。机械合金化3Ti／Si／2C粉体,会产生组成相为TiC、Ti3SiC2、TiSi2和Ti5Si3的粉体与颗粒产物。添过量Si并不会促进机械合金化反应合成Ti3SiC2。添适量Al可消除硅化物,明显促进反应合成Ti3SiC2。采用3Ti／Si／2C／0．15Al粉体作原料时,颗粒产物中Ti3SiC2含量最高,为92．8wt％;而采用3Ti／Si／2C／0．20Al粉体作原料时,粉体产物中Ti3SiC2含量最高,为61．9wt％。