Anomalous coarsening behavior of small volume fractions of Ni3Al precipitates in binary NiAl alloys

The kinetics of coarsening of γ′ precipitates in binary NiAl alloys containing nominally 5.72, 5.74 and 5.78 wt% Al and aged at 630°C were investigated by transmission electron microscopy and magnetic analysis. The alloys were each pre-aged at a temperature just below its solvus prior to re-aging at 630°C. The volume fractions of γ′ were low between 0.02 and 0.03. the pre-aging treatment produced microstructures containing precipitates that nucleated heterogeneously on dislocations and grain boundaries, leaving empty matrix areas in which coherent γ′ precipitates subsequently nucleated and coarsened. Measurements were made only on these precipitates. The coarsening kinetics are anomalous in that the rate constant decrease with increasing volume fraction, in contradiction with all the theories of this process. Furthermore, the increase is quite large, exceeding a factor of four over a range of volume fractions that increased by less than 40%. Additionally, the rate constants exceed the values expected from the literature by factors of 10–40. The distributions of particles sizes were measured, and with few exceptions were found to agree with those of earlier investigations. It is suggested that elastic interactions among the γ′ precipitates play a pivotal role in decelerating the kinetics of coarsening, overpowering the expected accelerating effect of increasing volume fraction.     

Influence of reaction of Al2O3 and carbonaceous materials in Al2O3–C refractories on aluminum and carbon pick-up of iron

Carbon-containing Al2O3 refractory crucibles and pure Al2O3 refractory crucible were fabricated to study the effect of carbon-containing Al2O3-C refractories on aluminum and carbon pick-up of iron.Refractory crucibles with pure iron powder were placed in a vacuum induction furnace and heated at 1600℃ for regular time under flowing argon atmosphere.The Al and C contents of iron samples were analyzed,and iron samples were also investigated using a field scanning electron microscope equipped with energy-dispersive spectroscope.The results showed that carbon materials did affect the interac?tion of(Al2O3-C)/Fe system,carbothermal reduction in alumina occurred in the reaction system and the extent of reactions depended on the kinds of carbon materials.     

Dissolution rate of Al2O3 into molten CaO–Al2O3–CaF2 flux

Abstract

Dissolution of Al₂O₃ into molten CaO–Al₂O₃–CaF₂, a base system of mould flux for continuous casting of high Al steel, has been investigated by employing a rotating cylinder method. The dissolution rate of an alumina rod into molten CaO–Al₂O₃–CaF₂ flux increased with increase in rotating speed and temperature. The apparent activation energy for mass transport of flux C was calculated to be 255·6 kJ mol^?1. The rate controlling step during the dissolution process of the alumina rod into molten CaO–Al₂O₃–CaF₂ flux was found to be the diffusion of the solute in the flux boundary layer. The dissolution rate of alumina into molten CaO–Al₂O₃–CaF₂ flux increased with increasing CaO/Al₂O₃, and it may be caused by the increase in thermodynamic driving force and the decrease in the viscosity of the flux. When the Al₂O₃ rod was immersed into molten flux, an intermediate compound of CaO.2Al₂O₃ formed firstly and then dissolved into molten flux.     

Effects of Y2O3 on Thermal Shock of Al2O3／TiCN Composites

Advancedstructuralceramics ,suchasoxides ,nitridesandcarbides ,havebecomepreferredrefracto rymaterialsforaviation ,spaceflight ,energyandmet allurgybecauseoftheirchemicalstability ,creepresis tanceandhightemperaturestrength .Buttheyareaf fectedgreatlybyte…     

伪二元合金Zr_3Al和Ti_3Al的机械性能

研究了Zr_3Al-Ti_3Al合金在77～1073K时的抗张性能。Zr_3Al中加入钛可提高合金的屈服应力及其对温度的依赖性,这种作用是由于Zr_3Al颗粒的应力集中所致。从X射线分析看,Zr_3Al中添加钛的极限为10～15at％,超过此限则合金韧度降低,但屈服应力增大。由在空气和真空度低于1.3×10~(-2)     

复合材料Al/Al4C3/Al2O3的组织结构与力学性能

采用机械合金化（ＭＡ）球磨和热压烧结工艺制备了复合材料Ａｌ／Ａｌ４Ｃ３／Ａｌ２Ｏ３，对其组织结构和力学性能进行了研究。结果表明，发育良好的Ａｌ４Ｃ３棒状单晶体和等轴状γＡｌ２Ｏ３均匀分布在铝晶界或晶粒内部，二者体积含量约为６６ｖ％。Ａｌ／Ａｌ４Ｃ３和Ａｌ／Ａｌ２Ｏ３界面洁净，为直接的原子结合，但不存在确定的位向关系。复合材料的室温、高温强度及刚度比粉末冶金纯铝（Ｐ／ＭＡｌ）显著提高。     

Effect of Rare Earth Metals on Microstructure of Directional Solidified Al－3Li Alloy

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Oxidation of Al2O3-30%TiCN-0.2%Y2O3 Composite

Thecuttingtoolmaterialshavegoodoxidationre sistance ,buttheyareoxidizedathightemperature(about 10 0 0℃ )duringoperation .Themechanicalpropertyofthecuttingtoolmaterialswillbechange d[1,2 ] .Therefore ,researchonoxidationbehaviorofcuttingtoolsmaterialsathi…     

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Influence of solution treatment temperature on microstructure and high- temperature tensile properties of Ni3Al- based alloy sheet was investigated. The results show that volume fraction of ?? phase reduces and volume fraction of ?á?+?? region increases with increasing heat treatment temperature up to 1280??. When the temperature reaches to 1310??, volume fraction of ?á?+?? region keeps at the same level but that of ?? phase increases. Due to precipitation of ?? phase along grain boundary with the temperature raising, grains begin to grow up until the temperature reaches 1280??. The grain sizes are different for different directions at the solution treatment temperature below 1280??. Crack initiates at the interface of ?? and ?á? envelope during 1000?? tensile test. The increase of volume fraction of ?á?+?? region can increase the strength of the alloy. The alloy has excellent comprehensive mechanical properties after heating at 1280?? for 16min and AC.     

Nb_3Al超导材料

日本原子能研究所与住友电工共同在世界上首次确立了 Nb_3Al 超导材料的工业生产技术。这种新超导材料在12忒斯拉的高磁场中成功地通过了4万安培的电流,具有超过实用超导线 Nb_3Sn的性能。Nb_3Al 的应力应变强,是一种被视为有发展     

Investigation on Cracking in Surfacing Layer of Ni_3Al Base Alloy

ＩｎｖｅｓｔｉｇａｔｉｏｎｏｎＣｒａｃｋｉｎｇｉｎＳｕｒｆａｃｉｎｇＬａｙｅｒｏｆＮｉ＿３ＡＬＢａｓｅＡｌｌｏｙＦＥＮＧＤｉ；ＨＡＮＧｕａｎｇＷｅｉａｎｄＹＥＷｕＪｕｎＩｎｖｅｓｔｉｇａｔｉｏｎｏｎＣｒａｃｋｉｎｇｉｎＳｕｒｆａｃｉｎｇＬａｙｅｒｏｆ...     

Dynamic embrittlement of boron-doped Ni3Al alloys at 600°C

Boron-doped Ni₃Al alloys, with and without 0.5at.%Hf, were tensile tested in vacuum and in oxidizing environments at 600°C. Tensile ductility was found to be strongly dependent on test environment, with much lower ductilities observed in air than in vacuum. The loss in ductility is accompanied by a change in fracture mode from transgranular to intergranular. The severity of this environmental effect on elevated-temperature ductility is also affected by preoxidation in air as well as by the aluminum content of the aluminide. Tests of preoxidized specimens indicate that the embrittlement is due to a dynamic effect simultaneously involving localized stress concentrations, elevated temperature, and gaseous oxygen. The oxygen embrittlement becomes less severe with a decrease in aluminum concentration from 24 to 21 at.%.     

Out-diffusion of the noble component during the initial stage of the oxidation of γ′-Ni3Al

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies of the oxidation behaviour of single crystalline γ′-Ni₃Al revealed that Ni diffuses through the oxide layer to the surface of the material during oxidation at low oxygen partial pressure. Small pure nickel particles (diameter ∼ 50 nm) form on top of the oxide scale. In this paper, this observation is explained on the basis of a micromechanical model. The model assumes that at intermediate temperatures the prevailing transport mechanism is the diffusion along the oxide/metal interface caused by compressive stresses generated during oxide growth. Relaxation of internal stresses as well as the formation of the small Ni-particles on top of the oxide surface are controlled by interface diffusivity. Other possible stress relief mechanisms can be excluded by theoretical estimates.