Pore structure of reactively synthesized nanolaminate Ti3SiC2 alloyed with Al

Ti₃SiC₂ has the unique properties integrating the advantages of metals and ceramics, and good open pore structure when alloyed with Al. In this work, porous Ti₃SiC₂ compounds with different Al/Si atom ratios were prepared through the reactive synthesis of elemental powders at 1300 °C. The results indicate that the phase compositions are determined by Al element mole number, and that the pore structure can be controlled through varying Ti particle size. The MAX phase transits from Ti₃SiC₂ with Al element mole number no more than 0.6 to Ti₃AlC₂ with Al element mole number in the range of 0.8–1.2. When Al element mole number is 0.6, the porous compound has a single MAX phase of Ti₃SiC₂ with uniform microporous structure and high bending strength. Porous Ti₃SiC₂ alloyed with 0.6Al has a slow linear increase rate of 0.0083%/μm in open porosity with increasing Ti particle size, and a strict linear relationship between the maximum aperture and Ti particle size with the increase rate of 0.0342 μm/μm. The pore structure formed by the phase transition mechanism for porous MAX phase has the smallest tortuosity factor compared with that formed by the clearance mechanism and the Kirkendall effect.     

A theory of avalanche breakdown during anodic oxidation

The breakdown characteristics during anodization of valve metals are interpreted in terms of a recent model. The model quantitatively takes into account, in addition to the oxidation ionic current, the current transported by the electrolyte incorporated into the oxide and the avalanche electronic current. The primary electronic current of the avalanche is attributed to the electrons released by the electrolyte species in the oxide.     

A study on carbon concentration distribution and microstructure of P/M materials prepared by carbusintering

Carbusintering (CBS) process was used to produce powder metallurgy (P/M) parts (Fe–2Ni–1.5Cu–xC). Influence of sintering parameters such as temperature on diffusion coefficient of carbon was investigated. The specimen with the porosity of 6–13% got the carbon content of the surface ranged from 1.23 to 1.32 wt.%. After heat treatment, the hardness and impact energy were enhanced to 48 HRC (484 HV) and 13 J, respectively. The change in carbon concentration along the case depth was determined quantitatively by means of chemical analysis. The carbon concentration profile was satisfactorily modeled by classical solution derived from Fick’s second law. An undesired acicular structure was observed in the specimen with low green compact density due to the high carbon content. CBS process can provide outstanding performance in surface hardening of P/M materials. The use of CBS process was able to make process simple by taking sintering and carburizing as one step. The higher carbon content of the surface in P/M parts can be easily achieved compared to conventional carburizing.     

Enhanced oxidation resistance of Mo-12Si-8.5B alloys with ZrB2 addition at 1300 °C

Mo-12Si-8.5B and Mo-12Si-8.5B-1.0wt%ZrB₂ alloys were fabricated using mechanical alloying, followed by hot-pressing. Both alloys exhibited uniform microstructure, with the Mo₃Si and Mo₅SiB₂ phases distributing dispersedly in the α-Mo matrix. Mo-12Si-8.5B-1.0wt%ZrB₂ showed a finer-grained microstructure than Mo-12Si-8.5B alloy owing to the addition of ZrB₂. The results of isothermal oxidation tests at 1300 °C in air revealed that Mo-12Si-8.5B and Mo-12Si-8.5B-1.0wt%ZrB₂ alloys initially suffered a transient stage with high mass loss due to the volatilization of MoO₃, and then achieved a steady stage owing to the formation of a protective borosilicate scale on the alloy surface. Especially, the transient stage of Mo-12Si-8.5B-1.0wt%ZrB₂ alloy was shortened to be less than 300 s, and the mass loss of this stage was reduced by at least 88% compared with that of Mo-12Si-8.5B alloy, indicating a significant improvement in the oxidation resistance. The addition of ZrB₂ not only resulted in a continuous borosilicate scale quickly covering the entire base alloy during the transient stage, but also improved the protectiveness of the borosilicate scale of the steady stage by bringing out a large number of ZrO₂/ZrSiO₄ particles embedded discontinuously in the borosilicate scale, which effectively restricted the inward diffusion of oxygen by acting as diffusion barriers and decreased the thickness of inner oxide layers in particular.     

金属空心球的研究进展

综述了金属空心球的制备方法和应用方面的一些进展,并对今后的发展方向进行了分析.     

Fabrication and characterization of Li3TaO4 ceramic pebbles by wet process

Lithium-containing ceramics have long been recognized as the tritium breeding materials in the fusion–fission or fusion reactor blanket. Li₃TaO₄ (lithium orthotantalate) pebbles, with high melting point (～1406 °C), good thermal stability, and high thermal conductivity, were fabricated by wet process (freeze–drying) as a new potential candidate of tritium breeder. The diameter of ceramic pebbles is 0.7–1.0 mm, density is over 90% (TD), pore diameter is 1.86 μm (a.v), grain size is 15 μm (a.v), crush load is up to 46.7 N (a.v).     

Sedimentation and liquid fluidization of solid particles of different sizes and densities

The sedimentation and fluidization of particles of mixed sizes and different densities were studied. A new concept—the apparent porosity of suspensions—was introduced, on the basis of which equations have been developed that are capable of predicting the settling velocities of individual types of particles in a suspension containing mixtures of particles. The validity of these equations was tested against experimental data in both sedimentation and fluidization.     

惰气熔融-红外吸收法测定钽钨合金中氢

通过对助熔剂、称样量、分析功率和积分时间的研究,建立了惰气熔融-红外吸收法测定钽钨合金中氢含量的方法。实验结果表明,称取0.15 g钽钨合金样品,放入镍囊,投入脱气后的石墨套坩埚中,设置分析功率为4 000 W,积分时间为60 s,可实现惰气熔融-红外吸收法对钽钨合金中氢含量的测定。方法检出限为0.000 03%,定量限为0.000 1%。将实验方法应用于钽钨合金样品中氢含量的测定,结果的相对标准偏差(RSD,n=11)为16.5%。按照实验方法对另一钽钨合金样品中氢含量进行测定,并加入适量钛标样GBW(E)020187进行加标回收试验,得加标回收率为94%~101%。     

一种新型近β型Ti-5.5Mo-6V-7Cr-4Al-2Sn-1Fe合金热变形行为

采用Gleeble-3800型热模拟试验机对一种新型近β型Ti-5.5Mo-6V-7Cr-4Al-2Sn-1Fe(质量分数/%)钛合金进行等温恒应变速率压缩实验。变形温度范围为:655~855℃,应变速率范围为:0.001~10s^-1 ,最大真应变为0.8。根据实验数据,建立了该合金的高温流变应力模型,计算出热变形激活能约为255kJ/mol,并绘制出热加工图。结合热加工图与材料的显微组织分析可知,在高应变速率(1~10s^-1 )条件下变形时,在热加工图上表现为材料的功率耗散值(η)低,为失稳区域,易产生绝热剪切带与局部塑性流动、开裂等现象。在应变速率小于0.01s^-1 和相变点( T β)温度以下(655~755℃)进行热变形时,组织变化主要以动态回复为主;在应变速率小于0.01s^-1 和 T β以上(755~855℃)进行热变形时,组织发生动态再结晶,且随着温度的升高,新产生的再结晶晶粒逐渐长大。在相变点附近(755~770℃),变形速率为0.001~0.003s^-1 区域内变形时,功率耗散值达到最大值,组织发生动态再结晶,该区域为合金热变形的“安全区”。     

The formation of Ti3Al within TiAl during the deformation of XD™ titanium aluminide

A mechanism involving the formation of TiAl_SF can explain how Ti₃Al layers and TiAl_T can be formed within TiAl during the deformation of TiAl. This mechanism is similar to the previously suggested for the formation of TiAl_M/TiAl_T laths which initiate at Ti₃Al_SF during the ordering transformation of Ti₃Al (3) and can be applied to the formation of TiAl_M/TiAl_T during the deformation processing of Ti₃Al.