Kinetics of phase layer growth during aluminide coating of nickel

The diffusion coating of nickel with aluminum was studied by a two-step aluminizing pack process involving initially an influx of aluminum at the surface (step 1) and later a partial honiogenization of the aluminum-rich region under conditions of zero surface flux (step 2). The process was studied in the temperature range from 870 to 1000°C. Step 1 was characterized mainly as the rapid, parabolic growth (after an initial transient period) of the Ni₂Al₃ phase (γ) as a surface layer with concurrent growth of a thinner NiAl (δ) layer. Step 2 was characterized mainly as the rapid loss of the aluminum gradient in the γ layer followed by parabolic growth of the δ layer primarily by the solution of they phase. Mathematical models were developed, in which numerical methods and computer techniques as well as closed-form solutions were utilized. The models yielded growth rate predictions in agreement with the experimental data and were used to define the critical parameters controlling growth kinetics for the aluminide layers formed during this process.     

Kinetics of phase layer growth during aluminide coating of nickel

The diffusion coating of nickel with aluminum was studied by a two-step aluminizing pack process involving initially an influx of aluminum at the surface (step 1) and later a partial honiogenization of the aluminum-rich region under conditions of zero surface flux (step 2). The process was studied in the temperature range from 870 to 1000°C. Step 1 was characterized mainly as the rapid, parabolic growth (after an initial transient period) of the Ni₂Al₃ phase (γ) as a surface layer with concurrent growth of a thinner NiAl (δ) layer. Step 2 was characterized mainly as the rapid loss of the aluminum gradient in the γ layer followed by parabolic growth of the δ layer primarily by the solution of they phase. Mathematical models were developed, in which numerical methods and computer techniques as well as closed-form solutions were utilized. The models yielded growth rate predictions in agreement with the experimental data and were used to define the critical parameters controlling growth kinetics for the aluminide layers formed during this process.     

The effect of δ- ferrite formation on the post-solidification homogenization of alloy steels

The profound influence of the δ-γ transition on the rate of alloy steel homogenization is demonstrated using a controlled unidirectional solidification technique. The effect is related to the high rates of diffusion of substitutional solutes in the γ-phase: As shown in this work, for example, the diffusion coefficient of nickel in 5-ferrite is two orders of magnitude greater than the comparable value for austenite.     

不同温度时效对GH220合金渗Al层的影响

研究了时效温度及时间对渗Al层组织的影响。结果表明,时效渗Al层主要由外、内两层组成,外层包括Al₂O₃膜区,β—NiAl区及贫化区;内层为富C区及针状相区。重点讨论了渗Al层蜕化机制及针状相形成原因。     

The deposition of aluminide and silicide coatings on γ-TiAl using the halide-activated pack cementation method

The halide-activated pack cementation method (HAPC) was utilized to deposit aluminide and silicide coatings on nominally stoichiometric γ-TiAl. The deposition temperature was 1000°C and deposition times ranged from 2 to 12 hours. The growth rates of the coatings were diffusion controlled, with the rate of aluminide growth being about a factor of 2 greater than that of silicide growth. The aluminide coating was inward growing and consisted of a thick, uniform outer layer of TiAl₃ and a thin inner layer of TiAl₂, with the rate-controlling step being the diffusion of aluminum from the pack into the substrate. Annealing experiments at 1100 °C showed that the interdiffusion between the aluminide coating and the γ-TiAl substrate was rapid. In contrast to the aluminide coating, the silicide coating was nonuniform and porous, consisting primarily of TiSi₂, TiSi, and Ti₅Si₄, with the rate-controlling step for the coating growth believed to be the diffusion of aluminum into the γ-TiAl ahead of the silicide/γ-TiAl interface. The microstructural evolution of the aluminide and silicide coating structures is discussed qualitatively.     

Kinetics of interlayer formation on polycrystallineα-Al2O3/copper-titanium alloy interface

Copper-titanium alloys are known to wet an alumina surface. Traditionally, contact angles are used as measurable parameters to monitor the progress of wetting. Contact angle is a measure of the thermodynamic equilibrium at the interface. For reactive wetting systems, it is well known that the interfacial properties vary as a function of time. In the present study, kinetics of reaction were studied by monitoring the rate of interfacial phase formation. An immersion apparatus was built for this purpose. The extent of reaction was measured both by a simple surface compositional analysis and by interfacial reaction layer thickness measurements. The reaction layer exhibited a parabolic growth with an associated activation energy of 180 to 230 kJ/mole. A speculative growth mechanism is proposed based on the experimental observations and the information available in the literature.     

中低碳齿轮钢中合金元素的偏析行为及其对带状组织的影响

带状组织是影响中低碳齿轮钢内在质量的主要缺陷之一.本研究利用光学显微镜观察到了20CrMnTiH和SAE8620H齿轮钢工业样品的带状组织,电子探针分析表明两钢种样品中均存在着Cr、Mn、Si等合金元素的带状偏聚.分析结果表明,元素偏聚只是产生带状组织的必要和前提条件,而非充分条件.除了微观偏析之外,合金元素对γ→α转变温度A_r³的影响趋势、在钢液中的溶解度或含量、对C活度的影响趋势、以及对CCT曲线的影响趋势等均对带状组织的形成或消除具有重要影响.减弱或消除带状组织的形成,一方面需要获得细小的铸坯二次枝晶间距,促进元素均匀分布,另一方面即便是在存在元素偏聚的条件下,若控制适当的轧制冷却工艺及合适的奥氏体晶粒尺寸,也有可能从根本上消除带状组织.     

Deoxidation of titanium aluminide by Ca-Al alloy under controlled aluminum activity

Removal of oxygen in titanium aluminide (TiAl) by chemically active calcium-aluminum (Ca-AI) alloy was carried out around 1373 K with the purpose of obtaining extra-low-oxygen TiAl. The deoxidation experiments were preceded by an investigation of the phase equilibria of the system Ti-Al-Ca at 1273 and 1373 K. The compositions of the Ca-AI alloy deoxidant, which equilibrates with TiAl, and the experimental conditions suitable for the deoxidation were of particular interest. In experiments in which Ti-Al samples were submerged in liquid Ca-AI alloys at 1373 K, the surfaces of the samples severely deteriorated and became nodular. When TiAl powders were mixed with CaO and the deoxidant was supplied in vapor form, powders which initially contained 510, 1100, and 4200 ppm O were deoxidized to about 160, 490, and 670 ppm O after deoxidation at 1373 K in 86.4 ks (1 day). Among many conditions tested, the use of TiAl powders mixed with CaCl₂ was most effective for deoxidation at 1373 K. CaCl₂ was used as a flux to facilitate the deoxidation by decreasing the activity of the deoxidation product CaO. In the case that TiAl powders mixed with CaCl₂ and reacted with Ca-AI vapor at 1373 K for 86.4 ks, the powders initially containing 510, 1100, and 4200 mass ppm O were deoxidized to a level of 62, 140, and 190 mass ppm O, respectively. No significant change in morphology of the particle after deoxidation was observed. The titanium and nitrogen concentrations in the powders remained constant, whereas calcium, which was present only in trace amounts initially, increased up to 160 mass ppm after the deoxidation treatment.     

Analysis of grain growth in a two-phase gamma titanium aluminide alloy

Microstructure evolution during annealing of a wrought near-gamma titanium aluminide alloy, Ti-45.5Al-2Nb-2Cr (at. pct), in the temperature range 1200 °C to 1320 °C was investigated. The mean grain size of the alpha phase as well as the volume fraction and size of the gamma particles were evaluated as a function of annealing temperature and time. Isothermal annealing at temperatures above the alpha transus, T _α=1300 °C, led to rapid grain growth of the alpha phase, the kinetics of which could be described by a simple power-law type expression with a grain growth exponent p=2.3. Alpha grain growth was significantly retarded during annealing at subtransus temperatures (1200 °C≤T≤1300 °C) by the pinning influence of gamma-phase particles. Limiting grain size values predicted by computer simulation models applicable for high-volume fractions of precipitates/particles were in good agreement with experimental findings. The kinetics of alpha grain growth in the presence of gamma particles were analyzed, and the results showed that a grain growth exponent of p≈2.6 could satisfactorily account for the experimental results.     

Kinetics of low-temperature homogenization in fine powder systems

Translated from Poroshkovaya Metallurgiya, No. 1(325), pp. 16–19, January, 1990.     

The chemical and mechanical processes of thermal fatigue degradation of an aluminide coating

The influence of strain history on the oxidation and mechanical degradation of an aluminide coating was examined by induction heating of stepped-disk specimens. The coating was applied to a single-crystal Ni-base superalloy (RENè N4) by pack aluminization. The anisotropic elasticity of the single-crystal substrate allowed simultaneously subjecting the aluminide coating to different strain amplitudes. Two distinct modes of coating degradation were observed for tests performed in air between temperature limits of 520 °C and 1080 °C: scalloping (spatially periodic surface oxidation and roughening) and cracking. The degree of scalloping became more severe as the compressive strain imposed on the coating was increased. Six thousand cycles between peak strains of -0.20 and 0.007 pct produced uniform surface oxidation, without scalloping, whereas 6000 cycles between peak strains of -0.56 and 0.01 pct gave oxidation and scalloping to 80 pct of the coating thickness. Cracks along coating grain boundaries were observed after 6000 cycles between peak strains of -0.45 and 0.16 pct. The depth of scalloping was found to correlate approximately with peak compressive substrate strain. Based on this correlation, a mechanism for scallop initiation and growth involving cyclic breakdown of the surface oxide and irreversible cyclic creep of the coating is proposed. Cracking along coating grain boundaries is attributed to tensile strains applied below the transition temperature of the coating. The results obtained from this study indicate that cyclic strain history is an important variable which should be included when determining the oxidation rate of coatings and alloys.     

Influence of microstructure on intrinsic and extrinsic toughening in an alpha-two titanium aluminide alloy

The toughening mechanisms in the Ti-24A1-11Nb (Ti-24-11) alloy have been identified previously to include crack-tip blunting, bridging, and deflection by the ductileβ phase, while the fracture mechanisms involve the nucleation, growth, and linkage of microcracks with the main crack. By performing appropriate theoretical analyses and critical experiments, the relative contributions of intrinsic and extrinsic toughening mechanisms, including microcrack shielding, crack-tip blunting, bridging, and deflection by theβ phase, to the initiation and crack growth toughness values of the Ti-24-11 alloy have been studied for three microstructures. The results indicate that the microstructure affects not only the amount of toughness enhancement, but also the type of toughening mechanisms present in the Ti-24-11 alloy. The initiation toughness in Ti-24-11 arises from the matrix toughness, crack-tip blunting, and, occasionally, from crack deflection by the ductile phase. As a result, theK _IC values increase with the volume fraction of the ductile phase. In contrast, the resistance curve behavior originates from (1) a change of crack-tip singularity, which occurs when the blunted crack extends into the plastic zone, (2) crack bridging by ductile phase and shear ligaments, and (3) microcrack shielding, which occurs mostly at elevated temperatures.