喷射成形镍基高温合金过喷粉末特征分析

对一种喷射成形镍基高温合金过喷粉末进行了分析。利用激光粒度分布仪、光学显微镜和扫描电镜对粉末形貌、粒度分布、形核方式及显微组织特征进行观察。结果表明,随着粉末颗粒尺寸的减小,颗粒表面变得光滑,形状规整;经测定本试验条件下粉末d50约为46μm;粉末以多种形核方式形核;微观组织以树枝晶为主,当颗粒尺寸小于10μm时,出现了均匀形核现象。过喷粉末的二次枝晶间距随冷却速度而改变,它们之间符合关系式λ2=0.252+0.022B(ε)-m。     

Effects of prehafnizing on morphological development of a chemical vapor deposition aluminide coating formed on single-crystal Ni-based superalloy

We examined a sequential Hf doping procedure, which consisted of (1) “prehafnizing” the surface of a single-crystal Ni-based superalloy (RENé N5) with HfCl₄ and H₂, and (2) aluminizing with AlCl₃ and H₂, as a means of incorporating Hf as a dopant in the aluminide coating matrix. The prehafnized layer on RENé N5 substrate significantly altered the growth behavior and therefore the morphology of the resulting aluminide coating. With the prehafnizing step, the coating layer became much thinner with a significant amount of Hf incorporated as Hf-rich phases (Hf₂Ni₇, Hf₃Ni₇, and/or Hf₈Ni₂₁). However, the Hf-rich phases segregated to the coating surface and retarded the inward Al diffusion required to form the β-NiAl coating matrix. The sequential Hf doping procedure provided a mechanism to incorporate a significant amount of Hf in the coating, but did not produce a uniform distribution of Hf as a dopant. The results were compared to those observed for a continuous doping procedure that was previously studied, and were discussed in the context of understanding the limitations of these procedures.     

Characterization of mechanically alloyed oxide dispersion-strengthened nickel-base superalloy MA760

The directional recrystallization behavior, the nature and stability of particles, and theγ′ precipitation behavior of a mechanically alloyed oxide dispersion-strengthened (ODS) nickelbase superalloy MA760 have been studied using a combination of microscopy, microanalysis, microhardness, and calorimetric techniques. The extruded material is in a primary recrystallized condition with a submicron grain size. Secondary recrystallization into a coarse, directional microstructure occurs by heating above 1215 °C. The stored energy driving the secondary recrystallization is estimated to be about 1 J g⁻¹. Y₂O₃, Al₂O₃-Y₂O₃ garnets, and chromium carbides have been identified in the extruded alloy. More Al₂O₃-Y₂O₃ garnets are formed during subsequent heat treatment; with continuous variation of composition, the garnets are richer in A1₂O₃ with increasing particle size. The chromium carbides present in the as-extruded material dissolve completely during heating to temperatures well below the recrystallization temperature. They can reprecipitate during cooling if the alloy is not recrystallized. The alloy contains a variety of inhomogeneities, including stringers of oxide particles and carbide depleted zones. Theγ′ precipitation and dissolution characteristics are also reported.     

Fracture toughness of gamma-base titanium aluminides

Effects of microstructures, alloying additions, and processing routes on the room- and elevated-temperature fracture toughnesses of gamma-base titanium aluminides were investi-gated. Microstructure was found to have a strong influence on the fracture toughness both at room and elevated temperatures. Lamellar microstructure materials exhibited high fracture toughness compared with duplex microstructure materials, which in turn possessed high fracture toughness compared with the equiaxed microstructure materials. Alloying additions affected the fracture toughness at elevated temperatures significantly. The addition of chromium improved the low-temperature fracture toughness (below 800 °), while the addition of niobium increased the fracture toughness above 800 °. Grain size refining, as a result of isothermal forging after casting and heat treatment, had less influence on the fracture toughness. Formerly Graduate Student, Nagaoka University of Technology     

Superplastic behavior of two-phase titanium aluminides

A two-phase Ti(57 at. pct)-Al(43 at. pct) alloy with an initial lamellar microstructure was thermomechanically processed to form an equiaxed fine-grained structure. The fine-grained (- L = 5 μm) material was superplastic in the temperature range 1000 °C to 1100 °C, exhibiting a stress exponent of about 2 with a tensile ductility of 275 pct. The rate-controlling deformation mechanism is proposed to be grain boundary sliding accommodated by slip controlled by lattice diffusion in TiAl. At room temperature, the lamellar and fine-grained materials exhibit the same compressive yield stress. The compressive strain to failure, however, for the fine-grained material was about 28 pct compared to 6 pct for the lamellar material.     

Field-activated combustion synthesis of titanium aluminides

The feasibility of synthesizing the titanium aluminides Ti₃Al and TiAl through field-activated, self-propagating combustion synthesis is demonstrated. A self-sustaining combustion wave can be initiated only when the imposed field is above a threshold value for each of these two aluminides. At the threshold values, wave propagation resulted in an incomplete reaction between the metals and the products, which contained several phases in addition to the desired one. As the field strength was increased, the reaction approached completion and the amounts of the secondary phases decreased. At a sufficiently high field, a single-phase product was obtained in the case of Ti₃Al, but, in the case of TiAl, the product contained Ti₃Al as a secondary phase even with the highest imposed field. The effect of reactant compact density was investigated for the case of Ti₃Al synthesis. At a fixed value of imposed field, the degree of reaction completion and the conversion to the desired phase increased as the relative density decreased. These observations are discussed in light of the role of the electric field in activating the self-propagating combustion synthesis reactions and the effect of relative density on this activation. The results show that the synthesis by self-propagating high-temperature synthesis (SHS) can be optimized by the combination of field strength and relative density.     

Development of iron aluminides containing carbon

In the last decade iron aluminide composites containing carbon have been developed through ingot metallurgy routes. Work on Fe-Al alloys containing 15 to 50 atom% Al and up to 16 atom% carbon is reviewed here. Presence of carbon in the alloy may lead to savings in the processing and material costs. It also improves the machinability in ordered iron aluminides. Carbide formation in the iron aluminide matrix results in significant improvements in strength and creep resistance. The work so far has concentrated on alloys with low (<35 atom%) Al-contents based on disordered α-phase and on Fe₃Al because at Al-contents of ～37 atom% or above graphite formation may occur in Fe-Al-C alloys. Alloys containing carbide as the major phase were found to be very brittle. The presence of carbon may limit the role of substitution alloying elements such as Cr, Mo or Ti. It may also affect hydrogen mobility in these alloys which are susceptible to hydrogen induced cracking. These factors are analysed. An assessment of the progress made in development of carbon containing iron aluminides is presented along with possible directions for future work.     

The combustion synthesis of copper aluminides

The formation of Cu-Al alloys by the thermal explosion mode of the combustion synthesis method was investigated for compositions ranging from 25 to 50 at. pct Al. Through X-ray, differential thermal analysis (DTA), and scanning electron microscopy (SEM) analyses, the nature of the reaction between powders of the two metals was determined. Solid-state diffusional reactions preceded the main combustion reaction which, in turn, was triggered by the appearance of a liquid phase. The relative amounts of phases formed through solid-state and liquid-phase reactions depended on heating rate. A significant amount of volume growth is observed during the solid-state reaction stage, and it is concluded that it is caused by pore formation by the Kirkendall effect.     

The effect of molybdenum addition on properties of iron aluminides

The effect of the addition of up to 10 pct molybdenum on several metallurgical properties of Fe-28Al (at. pct) to which 1 pct TiB₂ was added for grain refinement has been studied. It was determined that the addition of molybdenum results in a decrease in grain size, an increase in the recrystallization temperature, and an increase in the DO₃ to B2 ordering transformation temperature. The solubility limit of molybdenum in the matrix of the base alloy was found to be about 6 pct. At this concentration, another 1 pct is dissolved in the TiB₂ precipitates. Tensile strengths were increased slightly by adding up to 2 pct Mo, but ductility decreased, even though grain sizes were reduced. The fracture mode in tension did not change with addition of molybdenum up to 2 pct.     

Characterization and modeling of quenching-induced residual stresses in the nickel-based superalloy IN718

The residual stress fields in pieces of quenched IN718 superalloy have been characterized by neutron diffraction. The samples were in the form of cylindrical rods of length sufficient to ensure that steady-state conditions prevail at the midsection. Quenching the samples in air, water, and oil generated various residual stress fields. The interfacial heat-transfer coefficients were estimated using an inverse-modeling technique. The findings were rationalized with an elastic-plastic finite-element model that included temperature-dependent properties. The hoop and axial stresses are the most significant components of the stress field and arise from the plastic deformation occurring at the periphery of the cylindrical sections, the extent of which depends strongly upon the severity of the quench. The model is used to examine the residual stress fields to be expected in a turbine-disc forging of idealized geometry.     

The preparation of aluminides of the refractory metals

Summary A study of the conditions of preparation of aluminides showed that the latter can be produced by direct synthesis, using the hot pressing method; however, this process can only be employed when a high purity of the materials is not essential.High-purity aluminides can be obtained by arc or vacuum melting, using very pure starting materials. In this case, in view of the volatibility of aluminum and its higher vapor pressure compared with that of the transition metals, an excess of aluminum must be provided.Aluminides produced by arc or induction melting should be subjected to grinding, followed by hot pressing for the preparation of dense specimens.     

Q235碳钢在西沙大气暴晒的锈层特征

在西沙群岛高温、高湿的海洋大气环境下对Q235碳钢进行了3个月的暴晒实验,利用电子探针、激光拉曼等观察分析了暴晒后样品的锈层特征.结果表明:Q235碳钢暴晒1个月后迅速形成较厚的锈层,锈层疏松多孔,多裂纹;当暴晒3个月时,锈层却明显减薄.Q235碳钢在西沙暴晒1个月后形成的外层腐蚀产物主要是γ-FeOOH、β-FeOOH、α-FeOOH及少量Fe₃O₄等,而锈层内部主要为Fe₃O₄、γ-Fe₂O₃等铁的氧化物.暴晒3个月后,疏松锈层的内部电解液蒸发加剧,内层还原后的锈层重新被氧化,生成较多的FeOOH,同时部分γ-FeOOH转化成为α-FeOOH.Q235碳钢在西沙大气环境下的暴晒过程(也就是锈层的氧化还原反应的交替过程)中,钢基体不断地被腐蚀.     

Characterization of carbonitrided layers formed on stainless steel by conversion electron Mössbauer spectrometry

Austenitic stainless steel was carbonitrided by the tufftride process, and the hardened layers formed on the surface were investigated by conversion electron Mössbauer spectrometry (CEMS) and grazing angle X-ray diffractometry (GXRD). It was found that carbides such as M₇C₃ (M = Fe, Cr), chromium nitride (CrN),ε-nitride (M₂N, M = Fe, Cr), andε-carbonitride º_2+x (C,N), M = Fe, Ni} were precipitated on the outermost surface at the initial stages of carbonitriding. By the increase of treatment time up to 20 and 30 minutes,∈ M_2+x (C,N) became a main component, while M₇C₃ and CrN disappeared in the outermost surface. After 60 minutes, M₇C₃ and CrN were observed again, and theγ nitride, the oxide of iron and chromium (FeCr₂O₄), was formed on the outermost surface for the first time. Cross-sectional micrographs of surface layers using a scanning electron microscope (SEM) after etching the hardened layers with Marble reagent revealed the presence of black and white layers. The former layer mainly consisted of∈ M_2+x (C,N),∈ M₂N, CrN, and M₇C₃, and the latter layer did not contain nitrogen, although carbon was detected in both layers. The Vickers hardnesses of the black and white layers were HmV(0.l) 1000 to 1200 and HmV(0.l) 500 to 600, respectively. It was said that both layers were harder compared with HmV(0.1)200 of bulk. The white layer was far superior to the black one in the corrosion resistance proved by anodic polarization curve measurements in 5 vol pct H₂SO₄ solution. The white layer formed on carbonitrided stainless steel beneath the black layer has possibilities as an excellent corrosion and wear resistive layer.     

Tribological properties of detonation coatings based on titanium aluminides and aluminum titanate

Operation of coatings based on tialite, γ-TiAl, titanium aluminides with discrete inclusions of TiN and also alloys based on cobalt, nickel, and titanium in friction pairs with polyethylene grade “Chirulen” and stainless steel 12Kh18N10T is studied. It is established that tialite coatings formed from nanostructure powders of Ti-Al by detonation deposition are the best for operation under dry friction conditions in a pair with stainless steel 12Kh18N10T (minimum friction coefficient and minimum wear of the contact surfaces). It is shown that in the sliding rate range selected effective operation of material in friction pairs with polymer is provided due to transfer it to the contact surface and formation between rubbing surfaces of a thin film fulfilling the function of a solid lubricant. It is established that stable operation with the minimum wear in a friction pair with polyethylene grade “ Chirulen” is provided by a coating of γ-TiAl and Al₂TiO₅, and also coatings based on titanium aluminide with fine TiN inclusions.