The flowability of particulate TiB2-containing metal matrix composite

This paper reports a study of the flow behaviour of a particulate TiB₂-containing metal matrix composite (MMC) and a SiC particulate MMC containing the same matrix alloy, both in comparison with the unreinforced matrix alloy, Al-7Si-0.4Mg.

Both the MMCs exhibited a similar extreme sensitivity to surface turbulence during casting, to the point at which it was not easily possible to produce good-quality castings using a conventional gravity-poured filling system. Further results obtained from fluidity measurements showed that the fluidity is not so different to conventional casting alloys, although the fluidity of the TiB₂ MMC was slightly less good than the Al-SiC metal matrix composite, which in turn was poorer than the matrix alloy. A surprising result was that increasing the casting temperature did not significantly improve the fluidity of either of the MMCs.

The relatively low fluidity and the high density of major defects appears to be associated with the high viscosity of the mixture and the fact that the mixture exhibits thixotropy, i.e. the apparent viscosity decreases with increasing rate of shear. Thus to assist the flowability of the mixture a number of experiments have been carried out to assess the effect of high rates of imposed shear, applied by vibration and stirring actions. Results show that shearing is only marginally effective in fluidity enhancement but it has no influence on casting quality. The only method which was eventually identified as giving a practically defect-free casting was the low-turbulence casting technique adapted from the Alcan-Kingston laboratory development.     

Fabrication and mechanical properties of in situ TiC/Ti metal matrix composites

In situ TiC particle reinforced titanium matrix composites (TMCs) were successfully fabricated by reactive sintering of Ti + Mo₂C and Ti + VC compacts. The results of the tensile tests at ambient and elevated temperatures show that the strength of the composites increases with increasing additive content (Mo₂C and VC), and decreases with increasing temperatures. Comparing the two types of TMCs, the Ti + VC composites have a lower strength than the Ti + Mo₂C composites, but can more effectively retain the strength to elevated temperatures. Microstructural analyses show that the main strengthening mechanisms of the TMCs are solid solution, grain refinement and particulate strengthening. Different dominant strengthening mechanisms in different composites are responsible for the variations of the mechanical properties. At elevated temperatures, the volume fraction of TiC particles is the main factor for increasing the strength.     

原位合成TiC和TiB增强钛基复合材料的微观结构与力学性能

利用钛与Ｂ４Ｃ之间的自蔓延高温合成反应经普通的熔钐工艺原位合成制备了ＴｉＣ、ＴｉＢ增强的钛基复合材料。光学金相、ＥＰＭＡ、ＴＥＭ和Ｘ射线衍射的研究结果表明：存在匠两种不同形状的增强体,即短纤维状ＴｉＢ晶须和等轴、近似等轴状ＴｉＣ粒子。ＴｉＢ、Ｔｉ基体界面洁净,没有明显的界面反应,而ＴｉＣ、Ｔｉ基体界面有非化学配比的ＴｉＣ过度层存在。由于增强体承受载荷,基体合金晶粒细化以及高密度位错的存在,制备钛基     

Microstructural characterization of titanium matrix composite coatings reinforced by in situ synthesized TiB + TiC fabricated on Ti6Al4V by laser cladding

Titanium-based composite coatings reinforced by in situ synthesized TiB and TiC particles between titanium and B4C were successfully fab-ricated on Ti6A14V by laser cladding. Phase constituents of the coatings were predicted by thermodynamic calculations in the Ti-B4C-Al and Ti-B-C-Al systems, respectively, and were validated well by X-ray diffraction (XRD) analysis results. Microstructural and metallographic analyses were made by scanning electron microscopy (SEM) and electron probe micro-analysis (EPMA). The results show that the coatings are mainly composed of α-Ti cellular dendrites and the eutecticum in which a large number of needle-shaped TiB and a few equiaxial TiC particles are embedded. C is enriched in α-Ti cellular dendrites and far exceeds the theoretical maximum dissolubility, owing to the extension of saturation during laser cladding. The coatings have a good metallurgical bond with the substrate due to the existence of the dilution zone, in which a great amount of lamella β-Ti grains consisting of a thin needle-shaped martensitic microstructurc are present and grow parallel to the heat flux direction; a few TiB and TiC reinforcements are observed at the boundaries of initial β-Ti grains.     

TiB2对TiB2/Al复合材料组织与氢含量的影响

采用混合盐反应法（LSM法）制备TiB2质量分数分别为5%、10%、15%、20%的TiB2/Al复合材料,利用光学显微镜、扫描电镜、X射线衍射仪和激光粒度仪等观察微观形貌及物相组成,分析TiB2质量分数对复合材料的组织的影响,同时利用氧氮氢分析仪检测TiB2质量分数对复合材料热处理前后氢离子含量的影响。结果表明,随TiB2质量分数的增加,复合材料中生成的TiB2颗粒尺寸不断增大,并且氢离子含量不断增多。TiB2质量分数为5%和10%的TiB2/Al复合材料中,生成的颗粒尺寸细小且均匀分布于基体中,而15%和20%的TiB2/Al复合材料生成的颗粒尺寸偏大。采用合适的热处理工艺可有效降低复合材料的氢离子含量,尤其对TiB2质量分数为5%和10%的TiB2/Al复合材料降低氢离子量的效果更为显著。     

Microstructure and properties of in situ A356/TiB2 composite

In situ A356/TiB2 composites were successfully fabricated via in-melt reaction among aluminium alloy, K2TiF6 and KBF4 compounds. The composite was examined by using XRD, SEM and EDX techniques. The experimental results reveal that TiB2 are dispersed homogeneously into the aluminium alloy matrix. The mechanical properties of the composites increase significantly with the addition of reinforcement, and the tensile fractography of the composite exhibits to be ductile though the elongation of the composites decreases compared with the unreinforced matrix alloy.     

热处理对铸造TiB+La_2O_3增强钛基复合材料微观组织和拉伸性能的影响

利用Ti与La B6之间的化学反应经普通的熔铸工艺原位合成了Ti B和La2O3增强的钛基复合材料,而后复合材料浇铸成20 mm的棒材。研究了α+β和β热处理对复合材料的组织和拉伸性能的影响,利用光学显微镜和扫描电镜研究热处理后材料的微观组织和拉伸试样断面上增强体的增强机制。结果表明,α+β热处理后复合材料为α+β片层与少量近等轴α的混合组织,β热处理后复合材料为网篮组织。β热处理后复合材料的室温抗拉强度为1190 MPa,比α+β热处理的提高了31 MPa;β热处理后复合材料在600℃的抗拉强度为793 MPa,比α+β热处理的提高了48 MPa。复合材料在室温和600℃的断裂机制为Ti B纤维的承载断裂,在650℃和700℃的断裂机制为Ti B纤维的承载断裂和少量脱粘。     

(TiB+La2O3)/IMI834复合材料铸造性能

通过钛基复合材料螺旋线流动性实验,探索钛基复合材料的流动停止机理。材料在真空自耗电弧炉中熔炼成母合金锭,在真空自耗电极凝壳炉中进行螺旋线浇铸,采用金相显微镜进行了螺旋线不同部位的组织和增强体观察。复合材料流动性试样的根部组织粗大,具有较大的原始β晶粒和较宽的α片层,试样中部组织变得细小,流动性试样的头部具有最细小的组织。增强体TiB均匀的分布在基体中。增强体的加入使复合材料的凝固结晶范围变宽,钛基复合材料的流动停止机制为金属液中部的等轴晶不断长大,当堵塞区的孔径减小到一定值时,金属液和复合材料等轴晶间的摩擦力使金属液停止流动。     

原位TiB2颗粒增强7075铝基复合材料的热压缩变形行为和显微组织演变

采用等温热压缩实验研究不同变形条件下(变形温度300～450℃、应变速率0.001～1 s?1)原位TiB2颗粒增强7075铝基复合材料的热成形行为、损伤机制和显微组织演变.结果表明,复合材料在低温和高应变速率下的主要损伤机制是颗粒断裂和界面脱粘,而在高温和低应变速率下主要是基体的韧窝断裂.此外,复合材料在高温、低应变...     

Tribological properties of Ti-aluminide reinforced Al-based in situ metal matrix composite

For structural application of moving components, the tribological properties (friction and wear) are considered to be one of the major factors controlling the performance. In recent years, lightweight metal matrix composites (MMC) have received wider attention for their technological application, such as automotive parts etc. This paper reports the tribological behavior of Al based composites reinforced with in situ Ti_xAl_y and Al₂O₃ particles. The wear experiments were performed on a newly designed fretting tribometer to evaluate the role of intermetallic particulates on the wear performance of in situ composites against bearing steel under the ambient conditions of temperature (22–25 °C) and humidity (50–55% RH). Based on the topographical observation of the worn surfaces the plausible wear mechanisms are discussed. An important result is that Al-based composites with 20 vol% reinforcement exhibit an extremely low coefficient of friction of 0.2 under unlubricated conditions. Also, around five times lower wear volume is measured with 20 vol% composites when compared to unreinforced Al.     

微量增强体对TC18钛基复合材料微观组织的影响

使用真空自耗炉制备出多种微量TiB和TiC增强的Ti-5Al-5Mo-5V-1Fe-1Cr钛基复合材料,增强体在热处理过程中可大幅细化β相,但是细化幅度却随着增强体含量的增加而变缓,文中揭示出这和增强体的Zener拖曳力变化有关,增强体含量增加而拖曳力增幅变缓。此外还研究了增强体对α相球化的作用,α球化率随着增强体含量增加而小幅波动,然而在其作用下,等温压缩过程中魏氏体群结构中的板条状α明显球化。等温压缩过程中,增强体对α球化的作用不同于热处理过程,微量增强体可以通过促进界面转变而加速球化。     

碳化硅颗粒增强铝基复合粉末等通道转角挤扭的致密行为

对碳化硅颗粒增强铝基复合材料进行等通道挤扭实验和模拟,研究粉体在变形过程中的致密行为。结果表明,SiCp/Al复合粉末材料在等通道转角挤扭工艺中通过3个剪切面和7个变形区能得到有效致密。ECAPT过程中材料所处的球应力和切应力的叠加状态,是材料孔隙得到有效致密的关键。变形过程中,SiC颗粒被剪切细化,促进了团簇颗粒的分散和孔隙的致密。TE通道的剪切作用能使SiC颗粒进一步细化,但材料的整体致密度略有下降。     

一种非晶增强铝基复合材料的制备工艺及组织性能

利用搅拌摩擦加工技术成功制备了一种新型非晶增强铝基复合材料,分析其显微组织、硬度以及元素成分分布.试验结果表明,复合材料主要由母材和非晶带交替形成的层状结构组成,显微硬度有明显提高.复合材料中的非晶带产生了一定的晶化,摩擦热、机械搅拌力以及轴肩压力的综合作用可能是导致非晶带晶化的主要原因.     

Ti/Cu复合材料的界面组织及性能

采用热压扩散焊接法制备了Ti/Cu层状复合材料,并通过SEM、EDS、四电子探针及万能材料试验机等分析测试手段对样品的微观结构和性能进行表征。结果表明：热压扩散焊接法在焊接温度800 ℃、压力3.5 MPa时,随保温时间的增加,扩散层厚度逐渐增厚;不同保温时间下均生成4个亚层,各亚层金属间化合物依次为Cu₄Ti、Cu₄Ti₃、CuTi、CuTi₂;在保温时间为60 min时,复合材料有最小的电阻率(3.634×10^-8 Ω·m),仅为纯钛的8.65%;复合材料的抗弯曲性能随扩散层厚度的增加而增加。     

In situ formation of TiAl–TiB2 composite by SHS

The preparation of TiAl intermetallic compounds with or without the reinforcement phase TiB₂ was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts in this study. Effects of initial sample density, preheating temperature, particle size of the reactants, and TiB₂ content on the combustion characteristics, as well as on the composition and morphology of final products were studied. In this investigation, preheating was found to be required for the samples without boron addition to achieve self-sustained combustion, while boron-added compacts showed no need of prior heating, primarily due to the additional reaction heat liberated from the in situ formation of TiB₂. With the addition of boron, the reaction temperature and flame-front propagation velocity were correspondingly increased. Due to the melting of synthesized products caused by high-combustion temperatures, the TiAl–TiB₂ composite was contracted in dimension during the reaction. On the contrary, substantial volume expansion was observed in the sample without boron addition. This means that the in situ formed TiB₂ plays an important role not only in improving the mechanical property, but also in enhancing the densification of final products. XRD analysis of burned products identifies the in situ formed TiB₂ a reinforcing phase, and TiAl the dominant intermetallic phase. In addition to TiAl, another intermetallic compound Ti₃Al known as a major secondary phase in the Ti–Al reaction was detected in all end products of this study.     

调制周期对Ti/TiB_2多层膜的结构和力学性能的影响

采用磁控溅射法制备了Ti/TiB_2周期性(T=2,3,4,6,12)多层膜,利用X射线衍射仪、场发射扫描电子显微镜分析了薄膜的相结构和表面(断面)形貌,采用纳米压痕仪、多功能摩擦摩损试验机和显微硬度计研究了多层膜的纳米硬度、弹性模量、膜基结合力以及断裂韧性。结果表明:Ti/TiB_2多层膜具有清晰的纳米层状结构,薄膜表面致密平整,与基体保持着良好的物理结合。多层膜的硬度和弹性模量随着调制周期的增加而增大,在周期T=12时,多层膜的硬度和弹性模量达到最大值,分别为35.8和349 GPa;多层膜的断裂韧性随着周期的增加呈现出先增大而后减小的趋势,当周期T=6时多层膜的断裂韧性最好,其断裂韧度为2.17 MPa·m~(1/2)。分析认为多层膜中的Ti子层可使裂纹尖端产生钝化作用,从而引起裂纹扩展路径发生偏转,提高了多层膜的断裂韧性。     

激光原位制备TiB/Ti-6Al-4V复合涂层中TiB管状结构的形成机制

利用透射电镜(TEM)、电子背散射衍射(EBSD)技术,系统观察、分析了激光原位制备TiB/Ti-6Al-4V复合涂层中TiB生长形态、分布以及晶体学取向,以探讨管状结构TiB的形成机制。结果表明:涂层中TiB的形态主要有棒状和针状2种,受控于生长时间和元素浓度2个因素。棒状TiB形核于过共晶成分区域,针状TiB形核于共晶成分和亚共晶成分区域。当TiB的直径和生长速度之积超过临界值时,(010)_(B27)和(001)_(Bf)晶面的生长将不再稳定。界面边缘区域更易获得硼原子,生长速度快于中心区域,而边缘区域对硼元素的消耗进一步抑制了中心区域的生长,从而产生了管状结构。随着TiB直径的增加,管状结构出现的概率增加。