The effect of γ′ content on the densities of Ni-based superalloys

The γ′ phase increases the mechanical strength of Ni-based superalloys. Equations have been derived to calculate the atomic percent of γ′ phase from the Al content of the alloy. It is shown that increased γ′ phase content results in an increase in density (of the solid) and a decrease in the thermal expansion coefficient. Equations based on the Al content of the alloy have been derived to calculate the magnitude of these effects. The coarsening of the γ′ phase from 800 °C to 1000 °C was found to be accompanied by an increase in the temperature dependence of the thermal expansion coefficient. Densities for the entire temperature range of the solid alloys calculated with the equations are within 2 pct of measured values. The strong bonding between the Ni and Al was maintained in the liquid and resulted in increased densities. Equations were derived to quantify the effect on density (of the liquid) and the calculated density values for Ni-based superalloys were within 2 pct of the measured values. These uncertainties are commensurate with the experimental uncertainty in the measurements.     

Measurement of the density and surface tension of Ni-based superalloys in the liquid and mushy states

The densities of three Ni-based superalloys have been measured in both liquid and mushy states by both a modified sessile drop method (MSDM) and a modified pycnometric method (MPM) for alloys CMSX-4 and CM186LC, and for CMSX-10 alloy by MSDM only. The surface tensions of liquid CMSX-4, CM186LC, and CMSX-10 superalloys were measured using the sessile drop method. All measurements were carried out in a highly purified argon atmosphere with the oxygen partial pressure of less than 10⁻¹⁹ MPa in the gas outlet. The densities of all superalloys in both liquid and mushy states were found to decrease with increasing temperature. The volume thermal expansion of each superalloy in the mushy state was found to be higher than that in the liquid state. The densities determined by different methods have been critically assessed and recommended values in both liquid and mushy states are given as a linear function of temperature for the three Ni-based superalloys. The surface tension of liquid CMSX-4 superalloy was found to decrease with increasing temperature, while that of liquid CMSX-10 superalloy increases with increasing temperature. The wetting behavior of liquid CM186LC on the alumina substrate was found (1) to differ significantly from that of CMSX-4 and CMSX-10 and (2) to vary with time. A HfO₂-rich layer was found in the contact area of CM186LC with the alumina substrate, which could lead to some uncertainty in the value obtained for the surface tension determined for CM186LC.     

镍基高温合金真空感应熔炼脱氧

研究了使用CaO坩埚真空感应熔炼Ni-6Cr-2MO-6W-5Co合金时C与Al的脱氧作用.从热力学上计算了Al-O反应的平衡值,计算结果表明,Al-O反应在1773 K能使氧的质量分数降至6×10-6以下.计算了Ni-Al-C-O平衡,结果表明,C与Al2O3的反应在真空感应熔炼条件下可以进行,从而导致合金液精炼期C含量下降值大于脱氧所需值.     

Serrated grain boundary formation potential of Ni-based superalloys and its implications

The serrated grain boundary formation potential of a large number of conventionally forged, powder processed, and investment cast Ni-based superalloys is reviewed. A mechanism of serrated grain boundary formation by which grain boundaryγ′ particles move and displace the local grain boundary segment is discussed and the prerequisite conditions for its occurrence are highlighted. The practical implications of the serrated grain boundary formation are also discussed. It is suggested that modifying the existing heat-treatment cycles in some investment cast and powder processed Ni-based superalloys would improve their properties. The possibility of minimizing weld cracking in superalloys by creating serrated grain boundaries in the base metal and the heat affected zone is also discussed.     

Effect of W on formation and properties of precipitates in Ni-based superalloys

The formation and properties of precipitates in wrought Ni-based superalloys with different W contents during long-term exposure to high temperatures were inves...     

Pycnometric-manometric measurement of liquid densities

A method of measuring the density of high-temperature liquids is described.     

Stabilization of thermosolutal convective instabilities in Ni-based single-crystal superalloys: Carbide precipitation and rayleigh numbers

The influence of carbide precipitation on grain-defect formation during unidirectional solidification of experimental single-crystal Ni-based superalloys has been assessed over a wide range of compositions with large variations in Re, W, and Ta. In all instances, carbon additions of up to 0.15 wt pct were determined to be statistically significant with respect to stabilizing against the formation of grain defects, such as freckle chains, during solidification. Assessment of the segregation behavior of the constituent alloying additions via a Scheil-type analysis enabled estimation of critical Rayleigh numbers denoting the onset of thermosolutal convection. Precipitation of Ta-rich MC carbides near the liquidus temperature of the alloy was found to interact strongly with the mechanisms associated with freckle formation. Segregation analyses and phase-transformation temperature measurements were used to assess the corresponding Rayleigh numbers for the experimental alloys and to modify the Rayleigh criterion to account for carbide precipitation. Mechanisms pertaining to the interaction of carbides with the onset of thermosolutal convection are discussed.     

Investigation of oxidation characteristics and atomic partitioning in platinum and ruthenium bearing single-crystal Ni-based superalloys

The elemental partitioning characteristics of advanced single-crystal Ni-base superalloys containing additions of both Pt and Ru have been investigated using atom probe tomography. Detailed microanalysis revealed Ru additions partitioned preferentially to the disordered γ matrix, whereas Pt additions tended to partition to the ordered intermetallic γ′ precipitates. The stability of the γ′ precipitates at elevated temperatures coupled with the subtle changes in elemental partitioning associated with the additions of Cr and Ru were found to significantly influence the bare metal cyclic oxidation behavior of the experimental single-crystal alloys. Although the presence of Pt was observed to greatly enhance oxidation characteristics, these beneficial gains were impacted by Cr and Ru additions as they both increased the tendency for oxide spallation during cyclic thermal exposures.     

Thermodynamics of the superalloys

Using the parameters of Kaufman and Nesor, we have calculated the activities of Al, Ti, Cr, Fe, Co, Ni, Mo, and W in 17 superalloys. These components behave in three distinctly different ways: Al and Ti have very.small activity coefficients; Fe, Co, and Ni have activity coefficients close to unity; and Cr, Mo, and W have activity coefficients of approximately 10. The calculated free energy of formation of the gamma prime in these alloys gives values similar to that for pure Ni3Al. The calculations of the free energy change for the formation of a Cr-Co sigma phase gives a lower estimate of the potential for sigma formation. These results show that the presently available thermodynamic representations and parameters are capable of providing useful information on very complex systems.     

Prediction of the coordination numbers of liquid metals

In this communication, the coordination numbers of liquid metals can be predicted by the modified coordination equation with their atomic parameters, molar volumes, melting points, and enthalpies. The results show that the predicted values are in agreement with experimental data, and the average percentage deviations is ±6.44 pct, which is less than those of both the Hines and Cahoon methods. This finding shows that the prediction method developed in this work is more accurate and convenient than either of the other methods.     

Hydrogen-enhanced cracking of superalloys

Internal hydrogen embrittlement was studied in two nickel-base superalloys (IN718 and IN625) and one iron-base superalloy (A286). Subcritical crack growth (SCG) measurements were made on uniformly precharged specimens containing up to 50 weight parts per million (wt ppm) hydrogen, and the behavior was correlated with metallographic observations. For intermediate hydrogen concentrations, three-stage SCG ratevs stress intensity behavior was observed in IN718 and IN625 but not in A286. For all alloys, the threshold stress intensity decreased with increasing hydrogen concentration. Cracking in the nickel alloys was transgranular, and there was a transition from dimpled to faceted failure as the hydrogen concentration increased. Failure in A286 was mainly by intergranular microvoid coalescence at high hydrogen concentrations. Enhanced localization of plasticity and void pressurization due to hydrogen are proposed to explain the observed hydrogen embrittlement of these alloys. The effects of hydrogen on the stacking fault energy, trapping sites, microstructure, and cracking ahead of the main crack front are discussed with reference to the above alloys and their hydrogen embrittlement. Formerly Research Assistant, Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign     

Prediction of solute diffusion coefficients in liquid metals

Based on the fluidity model of Hildebrand, an explicit formulation has been proposed which allowsa priori predictions of solute diffusion coefficients in molten metals. However, only the size effects have been accounted for, and no consideration has been given to the interactions between the solute and the solvent. Despite these limitations the predicted values are in reasonable agreement with the experimental values of diffusion coefficients for a variety of solutes in a range of solvents over wide ranges of temperature. The equation suggested herein has no adjustable parameter.     

Application of immiscible magnetic liquids of different densities as the separation medium for magnetic liquid separation

Results of an investigation into a new method of magnetic liquid separation, which differs from known ones by the presence of a bilayer separation medium in the form of immiscible ferromagnetic liquids of different densities, are presented. During the separation in a bilayer medium, the equilibrium condition of a particle on a flat surface, which is written according to the Young law and Neumann rule, should be supplemented by the linear tension of a curve-phase interface surface. The linear tension is the force and energy barrier, which prevents fastening of small particles of precious metals at the phase interface, and the cause of their effective separation into a heavy fraction. The method of magnetic liquid separation is tested for concentrates containing platinum group metals. It is established that, during the separation in the bilayer medium, the recovery of platinum group metals into the heavy fraction is 25.89%, while that for the waterbased ferromagnetic liquid is 19.73%. The quality of the heavy fraction makes gives the opportunity to direct it to the hydrometallurgical stage bypassing the copper plant, which increases the recovery of precious metals by 5.0 abs %.     

Design of quaternary Ir-Nb-Ni-Al refractory superalloys

We propose a method for developing new quaternary Ir-Nb-Ni-Al refractory superalloys for ultra-high-temperature uses, by mixing two types of binary alloys, Ir-20 at. pct Nb and Ni-16.8 at. pct Al, which contain fcc/L1₂ two-phase coherent structures. For alloys of various Ir-Nb/Ni-Al compositions, we analyzed the microstructure and measured the compressive strengths. Phase analysis indicated that three-phase equilibria—fcc, Ir₃Nb-L1₂, and Ni₃Al-L1₂—existed in Ir-5Nb-62.4Ni-12.6Al (at. pct) (alloy A), Ir-10Nb-41.6Ni-8.4Al (at. pct) (alloy B), and Ir-15Nb-20.8Ni-4.2Al (at. pct) (alloy C) at 1400 °C; at 1300 °C, three phase equilibria—fcc, Ir₃Nb, and Ni₃Al—existed in alloys A and C and four-phase equilibria—fcc, Ir₃Nb, Ni₃Al, and IrAl-B2—existed in alloy B. The fcc/L1₂ coherent structure was examined by using transmission electron microscopy (TEM). At a temperature of 1200 °C, the compressive strength of these quaternary alloys was between 130 and 350 MPa, which was higher than that of commercial Ni-based superalloys, such as MarM247 (50 MPa), and lower than that of Ir-based binary alloys (500 MPa). Compared to Ir-based alloys, the compressive strain of these quaternary alloys was greatly improved. The potential of the quaternary alloys for ultra-high-temperature use is also discussed.     

Modeling the spread of hot-rolled Ni-based alloy plates

The width spread of Ni-based alloy plates in the rolling process is studied.An equation for describing the rolling spread of Ni-based alloy plate is proposed based on production data and the Bachtinow equation,which cannot fully account for the compositional variability of Ni-based alloys.To address this,a new coefficient for alloying is added to the equation based on production data.By adding alloying coefficients,it is possible to improve the prediction accuracy for the rolling spread of Ni-based alloy plates and thus better control the width of the rolling spread of different steel grades.     

On the nature of freckles in nickel base superalloys

Freckles are chains of equiaxed grains that have previously been observed in macroetched ingots of both iron and nickel base alloys formed by the consumable arc process. These defects have been observed in nickel base superalloys in both directionally solidified polycrystalline and single crystal ingots. Such ingots exhibit a relatively periodic distribution of vertical freckle trails around their circumference which are particularly suitable for study. This paper investigates the nature of “Freckles,” examines the conditions under which they occur and specifies a set of consistent observations which any successful model for freckling must explain. Visual, chemical, electron probe, metallographic, and X-ray diffraction data are presented which establish the appearance, composition and orientation differences between freckled and normal regions in single crystals of the nickel base superalloys Udimet 700 and Mar-M200. It is shown that the freckle lines are linear assemblies of small random equiaxed grains which are enriched in all but the inversely segregated solute species. Excessive interdendritic porosity and feeding shrinkage are observed in the vicinity of a freckle line. Some correlations of the tendency toward freckling and the geometric distribution of freckle lines are made with growth rate, thermal gradient, and interface shape. The dependence of freckling tendency on composition is examined in the Ni-Al system.     

Solidification of undercooled molten Ni-based alloys

The effect of undercooling on grain structure is investigated in pure nickel, Ni₇₅Cu₂₅, and DD3 singlecrystal superalloy by employing the method of molten salt denucleating combined with thermal cycling. Meanwhile, a comparison of factors that may be related to structure formation is performed and the difference in the refined structure between Ni₇₅Cu₂₅ alloy and DD3 single-crystal superalloy is explained. Only one grain refinement occurs at the critical undercooling in pure nickel, whereas two take place at both low and high undercoolings in Ni₇₅Cu₂₅ and DD3 single-crystal superalloy melts. The first grain refinement at low undercoolings mainly originates from dendrite remelting driven by the chemical superheating produced in recalescence, and the second one at high undercoolings is due to the recrystallization process as a result of the high stress provided in the rapid solidification after high undercooling. Dislocation morphology evolution in as-solidified structure is also provided by the transmission electron microscopy (TEM) technique to further verify the recrystallization mechanism.     

转炉出钢后钢包内钢水温度预测模型

采用多元回归建立了转炉出钢终点钢水温度的预测模型,实现了对转炉出钢过程钢水温度的预测,并探讨了影响该过程钢水温度变化的主要因素,为生产合格钢水及钢水温度的动态控制提供了理论依据。本模型能较好的预测该过程的钢水温度变化,预测误差在±10℃以内的正确率达到90%以上,对现场生产实践具有一定指导意义。