Ti-Al系电子结构及Mn掺杂对TiAl_3室温脆性的影响

以Ti-Al的3个化合物相(Ti3Al、TiAl和TiAl3)及Ti3Al8Mn为对象,采用密度泛函的赝势平面波法,在优化驰豫的基础上计算其电子结构和弹性模量,系统分析成分对各相电子结构的变化及脆性的影响。结果表明:Al含量逐步增多导致Al2p—Ti3d成键并抑制Ti—Ti键,使共价键以及成键的各向异性增强,因而使合金脆性增大;Mn替代Al位掺杂后,可减少Al—Al共价键,抑制Al2p—Ti3d成键并增强Mn与Ti的3d电子层杂化程度,降低由Al—Al共价键和Al2p—Ti3d杂化键形成所带来的键的空间各向异性和高位错能垒,进而改善合金的室温脆性。     

Formation of Fe3N,Fe4N and Fe16N2 on the surface of iron

In order to clarify the phenomenon of nitride formation on the surface of iron, highly polished specimens of well refined and coarsened iron grains have nitrided in flowing H2 + NH3 gas. The morphology and the conditions for formation of Fe₃N are clarified; it forms only on the surface of {lll}_α or near {lll}_α grains and grows in {112}_α directions during nitriding treatment at temperatures between 450 and 550°C. Fe₁₆N₂ and Fe₄N are also formed preferentially on the surfaces of {00l}_α and {210}_α grains, respectively. It is suggested that these iron surfaces are those satisfying the coherency relationships between nitrides and iron matrices. The morphologies and the formation temperature regions of Fe₁₆N₂ and Fe₄N on the surface of iron are quite different to those observed in iron. In particular, Fe₁₆N₂, which has been generally accepted as metastable in bulk iron below 200°C, can exist even at temperatures from 450 to 500°C when it is formed on the surface of iron.     

Reactions in the systems MoSi3N4 and NiSi3N4

The reaction products, formed during annealing of porous powder mixtures of Si₃N₄ with non-nitride forming metals like Ni or Mo, will depend on the partial pressure of N₂ in the atmosphere. In a diffusion couple, however, nitrogen has to be released at the Si₃N₄-interface during the formation of a metal silicide. It cannot escape easily and builds up a higher pressure of nitrogen at this interface. Therefore, the reaction products are different from those in porous pellets. This has been verified for NiSi₃N₄ and MoSi₃N₄ couples. The role of traces of oxygen on these reactions will be discussed.     

Low-cycle fatigue of an Ni3Al-Based VKNA-25 alloy at room temperature

The life of 〈001〉, 〈011〉, and 〈111〉 single crystals made of a heterophase (γ’ + γ) structural cast γ’-Ni₃Al-based superalloy having an ordered crystal structure and optimally alloyed with refractory metals is studied under low-cycle fatigue testing conditions at axial pulsed stresses on smooth specimens at room temperature. A correlation is revealed between the type of fracture surface and the structure of the heterophase alloy. The alloy behaves like a composite material in which only ductile structural constituent γ, which represents precipitates of an fcc disordered nickel-based solid solution, has a sufficient ductility margin. Real plastic deformation during tests at 20°C develops only in this structural constituent, which is indicated by slightly elongated Γ layer regions tensioned during fracture and the propagation of a main crack and secondary cracks deep into a specimen.     

Fatigue behaviour of Ni3Al(B) alloys at room temperature

The high cycle fatigue behaviour of high boron polycrystalline Ni₃Al alloys is studied. A single phase Ni₃Al alloy with a mole fraction of 0.6% B and a Ni₃Al alloy containing a mole fraction of 1.0% B with a small amount of the boride eutectic at the grain boundaries are selected for investigation. High cycle fatigue tests at room temperature with R (minimum stress/maximum stress) 0.1 are conducted in air and at 30 Hz. The results show that the Ni₃Al(0.6% B) alloy is better than the Ni₃Al(1.0% B) alloy in the fatigue resistance, although, the latter is much higher than the former in static strengths. The fatigue fracture surfaces are observed by SEM.     

Stability of free and complexed human immunodeficiency virus type 1 antigen at 4 degrees C and at room temperature

Free and immune-complex-dissociated (ICD) human immunodeficiency virus type 1 (HIV-1) antigenemias in serum specimens stored at room temperature (RT) and 4 degrees C for 1 to 35 days were evaluated. At all time points examined, there was no significant loss in detectable levels of ICD HIV-1 antigen at either RT or 4 degrees C. Free HIV-1 antigen was not stable in serum samples stored at RT for more than 2 days but was stable in samples stored at 4 degrees C for up to 4 days. Loss of free antigen occurred more rapidly in samples with high antigen content at baseline. Use of the ICD antigen assay allowed accurate quantitation of antigen in samples stored at RT or 4 degrees C for as long as 1 month.     

A comparative study of the embrittlement of monel 400 at room temperature by hydrogen and by mercury

Slow strain rate tensile tests were performed at room temperature on Monel 400 specimens of grain sizes 35 to 500 μm, in the environments of air, mercury, and electrolytically generated hydrogen. Specimens of grain size 250 μm were tested at a range of strain rates in the three environments. It was found that cracks initiated easiest in hydrogen but propagated easiest in mercury; consequently the embrittlement was usually more severe in mercury. The embrittlement decreased with increasing strain rate, and with increasing grain size in hydrogen. Embrittlement in mercury was a maximum at intermediate grain sizes. A fracture sequence of intergranular to transgranular to microvoid coalescence was common. The intergranular and transgranular fractures are interpreted in terms of the reduced cohesive stress and enhanced shear models of embrittlement, respectively.     

Effect of mechanical activation on the formation of Si3N4-SiC composite powders

The paper examines how the mechanical activation of an initial silicon-carbon powder mixture and the temperature of its subsequent treatment influence the specific surface and phase composition of the end products. It is established that the preliminary mechanical activation of Si-C mixtures significantly increases the nitration rate of silicon and decreases the temperatures of formation of silicon carbide and silicon nitride (1200–1300 °C). The products resulting from the nitration of mechanically activated mixtures consist of α-Si₃N₄ whiskers with an aspect ratio of 10–100, silicon, carbide, and silicon nitride particles to 100 nm in size, and their agglomerates.     

室温HPT制备SiC_p/Al复合材料的硬度分布特征

将纯Al颗粒和SiC颗粒混合,室温下采用高压扭转变形(HPT)制备试样,测定不同工艺参数下试样表面的显微硬度,绘制显微硬度—扭转半径曲线图,分析各试样的硬度分布特征。结果表明:由于高压扭转变形应变量的影响,SiCp/Al复合材料经HPT变形后,各试样表面硬度沿径向呈递增分布,边缘硬度值受飞边的影响较大;单一尺寸SiC颗粒强化的试样通过增大加载压力、增大扭转圈数或提高SiC颗粒体积分数可以提高试样的显微硬度值;采用双尺寸SiC颗粒最佳粒径配比或最佳体积配比更有利于提高试样硬度,通过优化双尺寸SiC颗粒粒径配比或体积配比,可以获得更高的试样硬度,且硬度分布更加均匀。     

8407模具钢表面渗铝后的常温硬质阳极氧化

为了防止模具钢表面产生热熔损失效,对渗铝后的8407钢表面进行常温硬质阳极氧化实验,使试样表面形成了氧化膜.着重研究了电流密度和氧化时间对氧化膜厚度和硬度的影响.实验结果确定适宜的氧化条件是:电流密度为2.5 A/dm2左右,氧化时间在60 min左右,氧化温度在25 ℃左右.     

Mechanisms of fatigue in mill-annealed Ti-6Al-4V at room temperature and 600°F

The mechanisms of the fatigue of mill-annealed Ti-6A1-4V were studied at 600°F and room temperature. Early crack initiationN ₀/N _f< 0.14) was found to occur in hcpα-grains by a slip-band mechanism under all but the least severe conditions of cyclic stress. Under cyclic stresses near the fatigue limit at room temperature, fatigue cracks began much laterN ₀/N _f ∼ 0.4) at the interface between hcpα and bccβ grains without detectable slip. Under all conditions, Stage I fatigue crack growth occupied 50 to 80 pct of the total life. Although mechanical twins were produced in profusion near the growing Stage II fatigue cracks, they appeared to play no role in crack initiation or Stage I crack growth; nor did they facilitate Stage II growth. None of the observations could be interpreted as evidence for a metallurgical instability or strain-induced phase transformation which might be harmful to the fatigue resistance of the alloy. J. C. GROSSKREUTZ, formerly with Midwest Research Institute     

Deformation of two C36 laves phases by microhardness indentation at room temperature

Microhardness indentation and electron microscopy were used to study the room-temperature deformation behavior of two Laves phases, ZrFe₂ and MgNi₂, in two-phase alloys. Evidence of extensive plastic deformation was found around the indentations. Activation of basal and nonbasal slip systems forms a distinctive cell structure in both Laves phases. Slip on planes nearly normal to the basal plane occurs in the lamellar form of ZrFe₂. Burgers vectors of the dislocations and resulting shear displacements are studied using high-resolution transmission electron microscopy (TEM). Curved or zigzag nonbasal slip surfaces in atomic scale were observed. Each slip plane can be considered to be composed of a series of small facets of high atomic density. A common Burgers vector and displacement vector component of 1/4 [0001] was found on many different slip planes. The “zonal glide” concept for slip on prismatic planes is discussed.