Effects of Re and Ru on the Solidification Characteristics of Nickel-Base Single-Crystal Superalloys

The solidification characteristics, such as the phase transformation temperature and partition coefficient, were investigated in four Ni-base single-crystal superalloys with varied contents of Re and Ru. Differential scanning calorimetry (DSC) analysis reveals that the liquidus and solidus temperatures decrease with increasing levels of Re additions. However, the Ru only has negligible influences on the liquidus and solidus temperatures. The partition coefficient of constituent elements is investigated by using the directional solidification followed quenching (DSQ) technique. Re is found to reduce the segregation of Re and W, whereas Ru only has negligible effects on the segregation of Re and W. For comparison, the matrix scanning is performed on the representative dendritic structures. With increasing withdrawal rate, Re and Ru additions show the stronger influences on segregation behavior of alloying elements, particularly for Al and Ta.     

The Effect of Secondary Gamma-Prime on the Primary Creep Behavior of Single-Crystal Nickel-Base Superalloys

Some second-, third-, and fourth-generation single-crystal Ni-base superalloys (i.e., Re-containing alloys) have demonstrated the propensity for excessive primary creep at intermediate temperatures. This behavior has been attributed to the presence of secondary gamma-prime precipitates in the gamma channels as well as on the Re content of the alloys. This investigation examined creep behavior for a common first-generation alloy, PWA 1480, a common second-generation alloy, PWA 1484, as well as a modified first-generation alloy, PWA 1480, with 3 wt pct rhenium added. In addition, two different aging heat treatments were given to each alloy to either precipitate or prevent the formation of fine (nanometer-scale) secondary gamma-prime in the gamma channels. The intermediate creep properties and tensile properties of the alloys were determined for both conditions. The microstructures of these samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD), and then the role of the fine-scale microstructure and the alloy composition on the primary creep deformation was determined.     

Effects of a High Magnetic Field on the Microstructure of Ni-Based Single-Crystal Superalloys During Directional Solidification

High magnetic fields are widely used to improve the microstructure and properties of materials during the solidification process. During the preparation of single-crystal turbine blades, the microstructure of the superalloy is the main factor that determines its mechanical properties. In this work, the effects of a high magnetic field on the microstructure of Ni-based single-crystal superalloys PWA1483 and CMSX-4 during directional solidification were investigated experimentally. The results showed that the magnetic field modified the primary dendrite arm spacing, γ′ phase size, and microsegregation of the superalloys. In addition, the size and volume fractions of γ/γ′ eutectic and the microporosity were decreased in a high magnetic field. Analysis of variance (ANOVA) results showed that the effect of a high magnetic field on the microstructure during directional solidification was significant (p < 0.05). Based on both experimental results and theoretical analysis, the modification of microstructure was attributed to thermoelectric magnetic convection occurring in the interdendritic regions under a high magnetic field. The present work provides a new method to optimize the microstructure of Ni-based single-crystal superalloy blades by applying a high magnetic field.     

Solidification Map of a Nickel-Base Alloy

The solidification behavior of the advanced nickel-base alloys, such as Inconel^® Alloy 690, is important for understanding their microstructure, properties, and eventual service behavior in nuclear power plant components. Here, an experimental and theoretical program of research is undertaken with the aim of developing a quantitative understanding of the solidification behavior under a wide range of temperature gradients and solidification growth rates. The temperature gradient and solidification rates vary spatially by several orders of magnitude during keyhole mode laser welding. Therefore, the solidification structure is experimentally characterized from microscopic examinations of the resulting fusion zones and correlated with fundamental solidification parameters to provide a widely applicable solidification map that can be employed for a broad range of solidification processes. The cell and secondary dendrite arm spacings are quantitatively correlated with cooling rates. An Alloy 690 solidification map, which illustrates the effect of temperature gradient and solidification rate on the morphology and scale of the solidification structures, is also presented.     

振动激发金属液形核对锌凝固组织的影响

为提高铸坯的等轴晶率和凝固组织的均匀性,以金属锌为研究对象进行了大量的振动激发金属液形核实验。结果表明,当一种带有冷却结构且进行高频振动的晶核发射器插入锌液内时,锌液在晶核发射器表面迅速形核且长大,在高频振动作用下表面形成的晶粒将被折断剥离,从而连续不断地弹射到锌液中,成为凝固过程中形成大量等轴晶的晶核来源;采用带有水冷和气冷结构的晶核发射器对锌液进行振动激发形核处理后,锌凝固组织中的等轴晶率可分别提高到65%和80%。     

Solidification Characteristics and Segregation Behavior of Nickel-Base Superalloys in Dependence on Different Rhenium and Ruthenium Contents

The influence of Re and Ru on phase diagram, as-cast microstructure, and segregation behavior has been investigated using a series of 12 experimental nickel-base superalloys derived from the parent alloy CSMX-4. Up to 2 at. pct Re and 4 at. pct Ru were added at the expense of nickel, while all other alloying elements were kept constant. The Re is found to increase the liquidus temperature and to decrease the γ′-solvus. The Re additions also strongly increase the amount of eutectic and the tendency for microsegregation. The Ru, on the other hand, displays much weaker effects or even no effect at all. The effects of Re on the eutectic formation and on segregation are shown to be related. Calculations based on ThermoCalc software were carried out and compared with the experimental results. ThermoCalc is found to be able to predict the changes in the phase diagram rather well. However, the calculation of the eutectic fraction with the Scheil–Gulliver model is not satisfying, which is due to the one-dimensionality of the model.     

电脉冲对细化金属凝固组织的影响现状

综合论述了电脉冲在细化金属凝固组织方面的应用和研究状况，从不遇角度对其作用机理做了分析探讨。同时指出电脉冲这一新技术应用前景广阔，但仍需克服多方面的困难，进行更系统和深入的研究。     

Improved Castability of Directionally Solidified,Ni-Based Superalloy by the Liquid Metal Cooling Process

The hot-cracking susceptibility of directionally solidified, Ni-based superalloys obtained by different casting technologies, such as high-rate solidification (HRS) and liquid metal cooling (LMC), are compared in this article. Refined microstructures and the improved castability of LMC castings has been found. The improved hot-cracking resistance in castings using the LMC process can be attributed to the decreased length of the uncompensated zone and to the increased grain boundary cohesion in this zone.     

弛豫—析出—控制相变技术中冷却速度对组织的影响

利用热模拟实验,对在非再结晶温度变形后弛豫一段时间,再以不同冷速冷却的低碳贝氏体钢的相变组织进行了研究,并与同等条件不弛豫的试样组织进行了对比.给出了弛豫和冷速对中温转变组织类型及组织细化程度的影响.实验结果表明,弛豫及冷却速度对变形奥氏体的相变组织是有影响的.低冷速下主要得到边界及取向不清晰的粒状贝氏体,这时弛豫时间对细化程度影响不明显,在10℃/S以上冷速下得到的是以板条贝氏体为主的组织,与未弛豫试样比较,其组织更细,板条形状更清晰,弛豫试样组织中残余奥氏体或M/A岛的形状更细长,弛豫有利于在同等冷却条件下得到板条组织,并且在高冷速下,弛豫试样中M/A量较未弛豫试样中的要少.     

Technology and Mills with Four-Roll Passes for Rolling Small Section Nickel-Base Superalloys

Metallurgist - The results are provided for research and also development of mill technology and construction with fourroll passes for rolling small section nickel-base superalloys. The design of...     

Sintering Time Dependent Supersolidus Liquid Phase Sintering Behaviour of a Metal Injection Molded Nickel-Base Superalloy

Metallurgical and Materials Transactions A - Supersolidus Liquid Phase Sintering of a high refractory metal Ni-based superalloy revealed that the liquid fraction and associated grain growth...     

The Application of Differential Scanning Calorimetry to Investigate Precipitation Behavior in Nickel-Base Superalloys Under Continuous Cooling and Heating Conditions

Metallurgical and Materials Transactions A - A suite of experimental tools and fast-acting, numerical-simulation techniques was used to quantify the precipitation behavior of three nickel-base...     

Refinement of the Solidification Structure of Austenitic Fe-Mn-C-Al TWIP Steel

Metallurgical and Materials Transactions B - The solidification structure of austenitic Fe-22Mn-0.6C-1.6Al TWIP steel with and without Ce inoculation is compared on the macro- and microscale. The...     

The Effect of Processing Conditions on Heat Transfer During Directional Solidification via the Bridgman and Liquid Metal Cooling Processes

Finite-element-based solidification modeling was used to investigate the thermal characteristics of the Bridgman and liquid metal cooling (LMC) directional solidification (DS) processes. Physically representative boundary conditions were implemented within a finite-element model to test its applicability to a broad range of processing conditions. The dominant heat-transfer step for each case was identified. Relationships between the thermal gradient and the solid–liquid interface position relative to the transition region of the furnace were developed. The solidification rate, the local velocity of the solid–liquid interface, and the cooling rate as a function of withdrawal rate were analyzed. The curvature of the solid–liquid interface varies with the processing conditions and influences the local thermal condition and, therefore, the morphological development of dendritic structure during solidification. An extensive sensitivity analysis of process conditions was conducted for both the Bridgman and LMC techniques. The relative importance of process parameters on the resulting thermal conditions during solidification was identified. A protocol for determination of preferred process conditions was defined. The maximum axial thermal gradient at the surface of the casting occurs when the solid–liquid interface is just above the baffle for both the Bridgman and LMC DS processes, independent of casting geometry or mold-heater temperature.