Structure and phase-composition of Ti-doped gas atomized Raney-type Ni catalyst precursor alloys

Raney-type Ni precursor alloys containing 75 at.% Al and doped with 0, 0.75, 1.5 and 3.0 at.% Ti have been produced by a gas atomization process. The resulting powders have been classified by size fraction with subsequent investigation by powder XRD, SEM and EDX analysis. The undoped powders contain, as expected, the phases Ni₂Al₃, NiAl₃ and an Al-eutectic. The Ti-doped powders contain an additional phase with the TiAl₃ DO₂₂ crystal structure. However, quantitative analysis of the XRD results indicate a far greater fraction of the TiAl₃ phase is present than could be accounted for by a simple mass balance on Ti. This appears to be a (Ti_xNi_1−x)Al₃ phase in which higher cooling rates favour small x (low Ti-site occupancy by Ti atoms). SEM and EDX analysis reveal that virtually all the available Ti is contained within the TiAl₃ phase, with negligible Ti dissolved in either the Ni₂Al₃ or NiAl₃ phases.     

Solidification morphology and phase selection in drop-tube processed Ni–Fe–Si intermetallics

Drop-tube processing was used to rapidly solidify droplets of Ni_64.7Fe₁₀Si_25.3 and Ni_59.7Fe₁₅Si_25.3 alloys. In the larger droplets, and therefore at low cooling rates, only two phases, γ-Ni₃₁Si₁₂ and β₁-Ni₃Si were observed. Conversely, in the smaller droplets, and therefore at higher cooling rates, the metastable phase Ni₂₅Si₉ was also observed. The critical cooling rate for the formation of Ni₂₅Si₉ was estimated as 5 × 10³ K s⁻¹. SEM and TEM analysis reveals three typical microstructures: (I) a regular structure, comprising single-phase γ-Ni₃₁Si₁₂ and a eutectic structure between γ-Ni₃₁Si₁₂ and β₁-Ni₃Si; (II) a refined lamellar structure with a lamellar spacing <50 nm comprising γ-Ni₃₁Si₁₂ and β₁-Ni₃Si; (III) an anomalous structure with a matrix of Ni₂₅Si₉ and only a very small proportion of a second, and as yet unidentified, phase. These results indicate that there is an extended stability field for Ni₂₅Si₉ in the Ni-rich part of the Ni–Fe–Si ternary system in comparison to the Ni–Si binary system. With an increase of cooling rate, an increasing fraction of small droplets experience high undercoolings and, therefore, can be undercooled into the Ni₂₅Si₉ stability field forming droplets consisting of only the anomalous structure (III). The Fe atoms are found to occupy different substitutional sites in different phase, i.e. Fe substitutes for Ni in the γ phase and Si in the L1₂ (β₁) phase respectively.     

Mathematical modelling and numerical simulation of ordered porosity metal materials formation

Ordered porosity metal materials belong to a relatively new class of porous materials named gasars. This paper presents a mathematical model of the complex physical phenomena in the production of gasars. Analyses for heat transfer, solidification kinetics and gas diffusion were coupled to describe the formation of unique gasar structure. Several criterial functions were introduced to provide significant quantitative information about the relationship between the operating technological parameters and the final structure. The computational outcomes of the numerical simulation were compared with the characteristics of real gasar ingots. The model was applied to determine the boundary conditions that would provide approximately constant physical conditions on the solidification front. The structure sensitiveness of gasars with respect to the different technological parameters is discussed.     

Étude du refroidissement avec solidification d'un fluide pseudoplastique thermodépendant

Heat transfer of highly consistent shear-thinning fluids flowing in a horizontal cylindrical channel has been investigated numerically and experimentally when the outer channel wall is subjected to a constant convective heat exchange. The case considered being cooling, a possible ice deposit must be taken into account. The variation of the rheological properties with temperature must be considered as well.     

Dynamics of Solidification in 2% Corn Starch-Water Mixtures: Effect of Variations in Freezing Rate on Product Homogeneity

Directional solidification studies of gelatinized corn starch-water mixtures were undertaken in order to examine, in situ, the freezing behavior of this food model. The solidification rate was controlled by varying sample cell velocity as it moved from a 25°C hot stage to a -25°C cold stage. While freezing at cell velocities ≤7.5 μm/sec, starch granules were alternately pushed or entrapped by the advancing solid-liquid interface producing a segregated structure consisting of alternating high-starch and low-starch bands. At a cell velocity of 10 μm/sec, the frozen product was homogeneous. The relationship between the solid-liquid interface velocity and segregation behavior was quantified and compared to an existing model of particle-interface interactions.     

Influence of Billet Size on Flow, Solidification and Solute Transport in Continuous Casting

ListofSymbol　　B———Buoyancy ,m·s- 2 ;　　c———Concentrationofsoluteelement ;　　Cμ———Turbulentconstant;　　D———Diffusivityofsoluteelement ,m2 ·s- 1 ;　　fl,fs———Liquidandsolidfraction ;　　fμ———Turbulentcoefficient ;　　h———Enthalpy ,J·kg- 1 ;　　k———Turbulentkineticenergy ,m2 ·s- 2 ;　　kp———Equilibriumpartitioncoefficient;　　Kp———Permeabilityofmushyzone ,m2 ;　　K0 ———Permeabilitycoefficient;　　p———Pressure ,Pa ;　　Pr———Prandtlnumber ;…     

Structure of rapidly solidified aluminum alloys

This paper is a summary of an extensive research program carried out by the authors on the structure of rapidly solidified aluminum alloys; and a comparison with the work of others also involved in this field. The paper discusses the changes in the dendritic and non-dendritic structure of the matrix at cooling rates from 10^–3 to 10¹⁰ K/s and discusses the hetergeneity of the structure caused by interdendritic-segretion during solidification.     

单晶高温合金DD3亚结构超细化定向凝固工艺研究EI

利用ＺＭ定向凝固装置研究了镍基单晶高温合金ＤＤ３在高温度梯度（７００～１２００Ｋ／ｃｍ）定向凝固条件下的微观组织演化特征及其对工艺参数的响应函数，结果表明：该合金在高界面温度梯度条件下，柱晶生长速度范围向高速区扩展，从而使单晶亚结构细化成为可能。所得单晶棒中亚结构一次间距可比传统ＨＲＳ法定向凝固籽晶中相应尺度减小５～１０倍。本文还对其组织演化机制进行了分析。     

全20CrMnTi表面激光重熔的组织与性能研究

本文对激光重熔20CrMnTi材料表明改性层显微组织分布特征、表面硬度分布规律以及残余应力状态作了研究,结果表明,表面改性层由熔融层、相变硬化层及过渡层组成,且材料表面显微硬度得到大幅度提高,表面硬度达到HV841,约提高4倍。硬化层深度约1mm。残余应力测试得出最大残余应力出现在熔凝带中心,表现为压应力。随着离中心距离的增大,残余应力逐步降低,到熔化带边缘,表现为低幅值压应力;而熔池的外边缘应力在热影响区则转变为拉应力状态。     

The Problem of Estimating the Uncertainty of the Fixed Point Temperature Due to the Presence of Impurities

The main problems in estimating the uncertainty due to the presence of impurities in the material: the absence in the documents of a unique definition of the quantity being measured – the fixed point temperature, the lack of agreed methods of realizing the fixed points and the insufficient justification of the equations used for the estimate, are analyzed. The results of a measurement of the melting point of gallium are presented, which differ in the conditions in which they were realized – with one and two liquid-solid interfaces and different solidification conditions, are presented. The reasons for the considerable difference between the values of the temperatures obtained in these experiments are discussed.