Soldification segregation in ruthenium-containing nickel-base superalloys

Ruthenium-containing multicomponent Ni-base superalloys with large variations in refractory alloying elements (Re, Ru, Ta, and W) have been investigated with respect to solidification, segregation characteristics, and the tendency to develop grain defects during directional solidification. Phase transformation temperatures and the effects of alloy composition on the liquidus temperature were determined by differential thermal analysis (DTA). The liquidus temperatures for most Ru-containing superalloys are generally higher than those of current commercial single-crystal superalloys. The partitioning behavior of individual constituents under the influence of alloy chemistry was characterized using a quantitative segregation mapping technique combined with a Scheil-type analysis. Whereas ruthenium partitioned preferentially to the dendrite cores during soldification, segregation of Ru is much less pronounced than Re and W. A higher degree of rhenium segregation was observed in Ru-containing superalloys. For the fixed processing conditions and moderate levels of Ru+Re, single-crystal solidification occurred without freckle formation or convection-induced breakdown of the solidification front. However, with high levels of Ru (9.6 ∼ 14.1 wt pct) and Re (7.2 wt pct), grain defects or the complete breakdown of single-crystal solidification was observed. Results from segregation and DTA analyses were used to estimate the corresponding Rayleigh numbers present during solidification of the experimental alloys. The Rayleigh criterion is effective for predicting the conditions under which the grain defect formation occurs during directional solidification of Ru-containing superalloys.     

Analysis of Microstructural Changes Induced by Linear Friction Welding in a Nickel-Base Superalloy

A detailed microstructural analysis was performed on a difficult-to-weld nickel-base superalloy, IN 738, subjected to linear friction welding and Gleeble thermomechanical simulation, to understand the microstructural changes induced in the material. Correlations between the microstructures of the welded and simulated materials revealed that, in contrast to a general assumption of linear friction welding being an exclusively solid-state joining process, intergranular liquation, caused by nonequilibrium phase reaction(s), occurred during joining. However, despite a significant occurrence of liquation in the alloy, no heat-affected zone (HAZ) cracking was observed. The study showed that the manufacturing of crack-free welds by linear friction welding is not due to preclusion of grain boundary liquation, as has been commonly assumed and reported. Instead, resistance to cracking can be related to the counter-crack-formation effect of the imposed compressive stress during linear friction welding and strain-induced rapid solidification. Moreover, adequate understanding of the microstructure of the joint requires proper consideration of the concepts of nonequilibrium liquation reaction and strain-induced rapid solidification, which are carefully elucidated in this work.     

Ternary restricted-equilibrium phase diagrams—II. Practical application: Aluminium-rich corner of the AlCuMg system

Results of numerical calculations of the solidification paths of 5000 compositions in the aluminium-rich corner of the AlCuMg system are used to construct the phase diagram for restricted-equilibrium conditions in the range of 0–6 wt% Cu and 0–2 wt% Mg for a cooling rate of 1 K/s at the liquidus. Polythermal projections, temperature-composition sections and the isothermal section below the solidus are shown. The fields of microstructural constituents (primary crystals, binary eutectic mixtures and ternary eutectic) as well as the weight fractions of nonequilibrium phases are given. It is demonstrated that large deviations from equilibrium solidification exist regarding the boundaries of the phase fields and the amount of phases and microstructural constituents, while the differences for varying cooling rates in the range of 0.1 and 10 K/s are less pronounced but still significant. Together with the secondary dendrite arm spacing, the restricted-equilibrium diagram yields the information on the solidification behaviour of Al-rich AlCuMg alloys needed in practical work.     

Solidification of Nb-bearing superalloys: Part I. Reaction sequences

The solidification reaction sequences of experimental superalloys containing systematic variations in Fe, Nb, Si, and C were studied using differential thermal analysis (DTA) and microstructural characterization techniques. The reaction sequences responsible for microstructural development were found to be similar to those expected in the Ni-Nb-C ternary system and commercial superalloys of comparable composition. The solute-rich interdendritic liquid generally exhibited two eutectic-type reactions at the terminal stages of solidification: L → (γ+NbC) and L → (γ+Laves). The Ni-base alloys with a high C/Nb ratio represented the only exception to this general solidification sequence. This group of alloys terminated solidification with the L → (γ + NbC) reaction and did not exhibit the γ/Laves constituent. At similar levels of solute elements (Nb, Si, and C), the Fe-base alloys always formed more of the γ/Laves eutectic-type constituent than the corresponding Ni-base alloys. Silicon additions also increased the amount of the γ/Laves constituent that formed in the assolidified microstructure, while C additions promoted formation of γ/NbC. The influence of Nb was dependent on the C content of the alloy. When the C content was low, Nb additions generally promoted formation of γ/Laves, while Nb additions to alloys with high C led to formation of the γ/NbC constituent. The results of this work are combined with quantitative analyses for developing γ-Nb-C pseudoternary solidification diagrams in a companion article.     

Modelling of Manganese Sulphide Formation during Solidification,Part II: Correlation of Solidification and MnS Formation

The high temperature properties of steels depend on the solidification parameters and the formation parameters of manganese sulphide precipitates. Therefore, the occurrence of MnS precipitations in relation to primary and secondary microstructures was studied for different steel grades with a primary delta‐ferritic solidification or a primary austenitic solidification. The liquidus and solidus temperatures as well as the δ‐γ‐transformation temperature were calculated thermodynamically and measured by a DTA analysis in order to describe the solidification and transformation temperature range. The MnS formation temperature was calculated thermodynamically and compared to the results of SEM/EDX investigations on fracture surfaces of hot tensile specimens torn at different temperatures after in situ melting and controlled solidification. A special focus of these investigations was the location of MnS precipitates in relation to the primary and secondary grain boundaries. To explain the results, calculations were carried out taking into account the supersaturation of manganese and sulphur during the solidification in residual melt on the primary grain boundaries.     

On differential thermal analyzer curves for the melting and freezing of alloys

Using a heat-flow model of a differential thermal analyzer and thermal characteristics obtained by fitting experimental results for a pure metal, the response of the differential thermal analyzer is modeled for the melting and solidification of alloys. The enthalpy-temperature relation used for the alloy simulations is obtained by two different methods: (1) equilibrium and Scheil considerations derived solely from thermodynamic information and (2) solute-diffusion micromodels coupled to the differential thermal analysis (DTA) heat-flow equations. During the consideration of pure material melting, simple expressions are obtained for the effect of sample size and heating rate on the DTA melting onset temperature, peak temperature, and peak height, which assist in the proper calibration of a differential thermal analyzer. For alloys, the smearing effect of the DTA heat flow at different heating and cooling rates is demonstrated for various solidification-path features. In particular, the DTA peak temperature during melting, which is often selected as the liquidus temperature experimentally, is shown to be significantly higher than the liquidus temperature for small-freezing-range alloys and/or for alloys with slow solid diffusion. The DTA curves calculated for freezing with dendritic growth due to supercooling, quantify the errors associated with the determination of the liquidus temperature on cooling.     

A comparison of the solidification behavior of INCOLOY 909 and INCONEL 718

The solidification behavior of two commercial aerospace superalloys, INCOLOY 909 and INCONEL 718, has been examined. Specifically, differential thermal analysis (DTA) revealed that INCOLOY 909 terminates solidification with the formation of a single minor constituent at ≈1198 °C. INCONEL 718 terminates solidification with the formation of two minor constituents, at ≈1257 °C and ~1185 °C, respectively. Metallography performed on the DTA samples confirmed that a single minor constituent was present in INCOLOY 909 while two minor constituents were present in INCONEL 718. Differential thermal analysis samples were also examined by electron probe microanalysis to reveal the patterns of elemental segregation. Arc welds of these alloys were examined by transmission and analytical electron microscopy (TEM and AEM). It was observed that the arc welds of INCOLOY 909 contained only a y/Laves eutectic-like constituent, while the arc welds of INCONEL 718 contained both y/Laves and γ/MC eutectic-like constituents. Compositional analyses of these minor phases revealed that all were enriched in Nb relative to the bulk alloy. The Laves phases were also enriched in Si relative to the bulk alloy concentration. Comparisons of the observed solidification sequences in these alloys with other Nb-bearing austenitic matrix alloys are made.     

Examination of solidification pathways and the liquidus surface in the Nb-Ti-Al system

The solidification pathways, subsequent solid-state transformations, and the liquidus surface in the Nb-Ti-Al system have been examined as part of a larger investigation of phase equilibria in Nb-Ti-Al intermetallic alloys. Fifteen alloys ranging in composition from 15 to 40 at. pct Al, with Nb to Ti ratios of 4:1, 2:1, 1.5:1, 1:1, and 1:1.5, were prepared by arc melting and the as-cast microstructures were characterized by optical microscopy (OM), microhardness, X-ray diffraction (XRD), differential thermal analysis (DTA), backscattered electron imaging (BSEI), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). The results indicate that the range of primary β solidification is much wider than that indicated in previously reported liquidus surfaces, both experimental and calculated. Differential thermal analysis has identified the existence of a β to σ+γ transformation in three alloys where it was previously thought not to exist; confirmation was provided by high-temperature vacuum heat treatments in the single-phase β region followed by rapid quenching. The location of the boundary between the β, σ, and δ primary solidification fields has been redefined. A massive β → δ transformation, which was observed in the cast microstructure of a Nb-25Ti-25Al alloy, was repeatable through cooling following homogenization. A β → δ+σ eutectoid-like transformation in the 25 at. pct Al alloys, was detected by DTA and evaluated through microstructural analysis of heat-treated samples. Trends in the β phase with variations in composition were established for both lattice parameters and microhardness. As a result of this wider extent of the primary β solidification field, a greater possibility exists for microstructural control through thermal processing for alloys consisting of either σ+γ, β+σ, or β+δ phases. An erratum to this article is available at .     

A melting and solidification study of alloy 625

The melting and solidification behavior of Alloy 625 has been investigated with differential thermal analysis (DTA) and electron microscopy. A two-level full-factorial set of chemistries involving the elements Nb, C, and Si was studied. DTA results revealed that all alloying additions decreased the liquidus and solidus temperatures and also increased the melting temperature range. Terminal solidification reactions were observed in the Nb-bearing alloys. Solidification microstructures in gastungsten-arc welds were characterized with transmission electron microscopy (TEM) techniques. All alloys solidified to an austenitic (γ) matrix. The Nb-bearing alloys terminated solidification by forming various combinations of γ/MC(NbC), γ/Laves, and γ/M₆C eutectic-like constituents. Carbon additions (0.035 wt pct) promoted the formation of the γ/MC(NbC) constituent at the expense of the γ/Laves constituent. Silicon (0.4 wt pct) increased the formation of the yJLaves constituent and promoted formation of the γ/M₆C carbide constituent at low levels (<0.01 wt pct) of carbon. When both Si (0.4 wt pct) and C (0.035 wt pct) were present, the γ/MC(NbC) and γ/Laves constituents were observed. Regression analysis was used to develop equations for the liquidus and solidus temperatures as functions of alloy composition. Partial derivatives of these equations taken with respect to the alloying variables (Nb, C, Si) yielded the liquidus and solidus slopes t(m _L , m _S ) for these elements in the multicomponent system. Ratios of these liquidus to solidus slopes gave estimates of the distribution coefficients (k) for these same elements in Alloy 625.     

Microstructural transitions during containerless processing of undercooled Fe-Ni alloys

The microstructural development associated with solidification in undercooled Fe-Ni alloys has been reported in different studies to follow various pathways, with apparent dissimilarities existing as a function of sample size and processing conditions. In order to identify the possible hierarchy of microstructural pathways and transitions, a systematic evaluation of the microstructural evolution in undercooled Fe-Ni alloys was performed on uniformly processed samples covering seven orders of magnitude in volume. At appropriate undercooling levels, alternate solidification pathways become thermodynamically possible and metastable product structures can result from the operation of competitive solidification kinetics. For thermal history evaluation, a heat flow analysis was applied and tested with large Fe-Ni alloy particles (1 to 3 mm) to assess undercooling potential. Alloy powders (10 to 150 μm), with large liquid undercoolings, were studied under the same composition and processing conditions to evaluate the solidification kinetics and microstructural evolution, including face-centered cubic (fcc)/body centered cubic (bcc) phase selection and the thermal stability of a retained metastable bcc phase. The identification of microstructural transitions with controlled variations in sample size and composition during containerless solidification processing was used to develop a microstructure map which delineates regimes of structural evolutions and provides a unified analysis of experimental observations in the Fe-Ni system.     

On Peritectic Reactions and Transformations in Low-Alloy Steels

Differential thermal analysis (DTA) experiments on low-alloy steels with varying C, Si, Cr, and Mo contents indicated an increase in the difference between the liquidus and peritectic temperatures during solidification with the decrease in C and increase in Mo contents. In a number of the quenched samples, massive transformations of ferrite to austenite were observed. Electron microprobe analysis of the diffusion across a massive transformation front, along with the high growth rates estimated, gives strong reason to believe that these growths are uncontrolled by diffusion. As ferrite transforms to austenite during the peritectic reaction, shrinkage in volume occurs, causing elastic straining at the interface separating the two phases. It was shown through thermodynamic analysis of the equilibrium at the triple point that the increase in energy of the two phases due to this strain can result in undercooling below the equilibrium peritectic temperature and decreases in the equilibrium peritectic concentrations.     

Effects of liquidus and solidus curvature in solidification modeling of binary systems with constant partition ratio

A microsegregation model is used to investigate the effect of approximating liquidus and solidus lines in binary phase diagrams by straight lines during solidification modeling. Even if repartitioning of solute can be described by a constant partition coefficient, the curvature of the phase boundary lines exerts an influence on results of microsegregation calculations. Deviations of liquids and solidus lines from linearity have a distinct influence on microstructural parameters predicted for a wide range of cooling conditions, owing to the effect of pronounced changes of the solidified fraction at a given temperature. Results obtained with simplified phase diagrams should therefore be considered with care.     

Solidification of M2 high speed steel

The freezing process in AISI type M2 high speed tool steel (6 pct W, 5 pct Mo, 4 pct Cr, 2 pct V, 0.8 pct C) was studied by metallographic and thermal analysis techniques. Unidirectional solidification of small laboratory melts in a modified crystal growing apparatus was employed to provide metallographic sections of known macroscopic growth direction. Also cooling curves were obtained on 40 g specimens solidified in thimble crucibles. X-ray microradiography, electron probe scanning techniques, and quantitative microanalysis of dendrites and interdendritic carbides were extensively used to supplement conventional metallography. Carbon and vanadium contents of M2 were varied in order to observe the effect of an austenite and ferrite stabilizer on the thermal analysis curves and microstructure. The nonequilibrium freezing process in M2 includes three major liquid-solid reactions: 1) Liquid → Ferrite, 1435°C; 2) Liquid + Ferrite → Austenite, 1330°C; 3) Liquid → Austenite + M₆C + MC, 1240°C. These reactions account for the as-cast structure of the commercial alloy. The addition of carbon depresses the liquidus (1) and solidus temperatures (3) and narrows the gap between the liquidus (1) and peritectic transformation (2). This gap is eliminated at > 1.39 wt pct C, where the initial freezing reaction is the crystallization of austenite. The accompanying microstructural change is the elimination of σ eutectoid dendrite cores. The addition of vanadium promotes ferrite formation by strongly depressing the peritectic reaction and thus widening the gap between the liquidus and the peritectic.     

Experimental Investigations and Computer Simulation of the Liquid Fraction Evolution during the Solidification of Alloys Suitable for Semi‐Solid Processing

One important parameter for the processing of materials by semi‐solid forming is the actual distribution of the solid and liquid phases in the semi‐solid range. This parameter defines the process stability for the forming step. Therefore it is necessary to obtain information about the materials behaviour in the semi‐solid state for different materials grades. This kind of information can be obtained by experimental studies in the interesting temperature range or by calculations with simulation programs using thermodynamic data validated by experiments. This work shows the results of experimental studies and thermodynamic calculations of the solidification and heat treatment behaviour of the aluminium alloy A319 and the steel X210CrW12. The experimental studies of solidification and heat treatment of these alloys were carried out using a differential thermal analysis system (DTA). The theoretical fraction of liquid content was calculated from the DTA signal by using a software module called Corrdsc. The experimental data obtained were used to validate the thermodynamic simulations of the solidification of semi‐solid alloys. The simulations of the solidification process were carried out for equilibrium conditions, with the Scheil‐Gulliver model as well as with diffusion calculations. The equilibrium and Scheil‐Gulliver calculations were performed by the program Thermo‐Calc, and the diffusion by the program DICTRA. The required thermodynamic and mobility data for multicomponent systems were taken from the data bases COST 507 light alloys, TCFE2000 Steel/Alloys and MOB2 mobility and from newly added data. The comparison of calculated phase transformations and fractions of liquid content with experimental data revealed a good agreement.     

Modeling freckle formation in three dimensions during solidification of multicomponent alloys

The formation of macrosegregation defects known as “freckles” was simulated using a three-dimensional finite element model that calculates the thermosolutal convection and macrosegregation during the dendritic solidification of multicomponent alloys. A recently introduced algorithm was used to calculate the complicated solidification path of alloys of many components, which can accommodate liquidus temperatures that are general functions of liquid concentrations. The calculations are started from an all-liquid state, and the growth of the mushy zone is followed in time. Simulations of a Ni-Al-Ta-W alloy were performed on a rectangular cylinder until complete solidification. The results reveal details of the formation of freckles not previously observed in two-dimensional simulations. Liquid plumes in the form of chimney convection emanate from channels within the mushy zone, with similar qualitative features previously observed in transparent systems. Associated with the formation of channels, there is a complex three-dimensional flow produced by the interaction of the different solutal buoyancies of the alloy solutes. Regions of enhanced solid growth develop around the channel mouths, which are visualized as volcanoes on top of the mushy zone. The prediction of volcanoes differs from our previous calculations with multicomponent alloys in two dimensions, in which the volcanoes were not nearly as apparent. These and other features of freckle formation phenomena are illustrated.     

The effect of cooling rate on the solidification of INCONEL 718

The superalloy INCONEL 718 (IN718) is a commonly used material in aerospace and turbine components. The advantage of this type of material with sluggish precipitation-hardening kinetics is that IN718 is readily weldable. Both wrought and cast parts are used and welded together. While the alloy has been studied previously, new production processes such as laser treatment demand better knowledge of the solidification process in IN718, especially at high cooling rates. In this investigation, the solidification process was studied over a wide range of cooling rates by three different experimental techniques: differential thermal analysis (DTA), mirror furnace (MF), and levitation casting. The solidification sequence and the reaction temperatures were identified. The microstructure and the change in growth morphology were also studied. Segregation measurements were performed, and the distribution of Nb was analyzed in detail for the different types of samples, because of its strong impact on the solidification sequence and microstructure. New observations are that the latent heat decreases and the effective partition coefficient increases with increasing cooling rate. The diffusion rate also seems to be enhanced in the first part of primary solidified dendrites. It is suggested that the new observations can be explained by an increased number of lattice defects formed in the solid as the cooling rate increases.     

Microstructural development during solidification of stainless steel alloys

The microstructures that develop during the solidification of stainless steel alloys are related to the solidification conditions and the specific alloy composition. The solidification conditions are determined by the processing method,i.e., casting, welding, or rapid solidification, and by parametric variations within each of these techniques. One variable that has been used to characterize the effects of different processing conditions is the cooling rate. This factor and the chemical composition of the alloy both influence (1) the primary mode of solidification, (2) solute redistribution and second-phase formation during solidification, and (3) the nucleation and growth behavior of the ferrite-to-austenite phase transformation during cooling. Consequently, the residual ferrite content and the microstructural morphology depend on the cooling rate and are governed by the solidification process. This paper investigates the influence of cooling rate on the microstructure of stainless steel alloys and describes the conditions that lead to the many microstructural morphologies that develop during solidification. Experiments were performed on a series of seven high-purity Fe-Ni-Cr alloys that spanned the line of twofold saturation along the 59 wt pct Fe isopleth of the ternary alloy system. High-speed electron-beam surface-glazing was used to melt and resolidify these alloys at scan speeds up to 5 m/s. The resulting cooling rates were shown to vary from 7°C/s to 7.5×10⁶°C/s, and the resolidified melts were analyzed by optical metallographic methods. Five primary modes of solidification and 12 microstructural morphologies were characterized in the resolidified alloys, and these features appear to be a complete “set” of the possible microstructures for 300-series stainless steel alloys. The results of this study were used to create electron-beam scan speedvs composition diagrams, which can be used to predict the primary mode of solidification and the microstructural morphology for different processing conditions. Furthermore, changes in the primary solidification mode were observed in alloys that lie on the chromium-rich side of the line of twofold saturation when they are cooled at high rates. These changes were explained by the presence of metastable austenite, which grows epitaxially and can dominate the solidification microstructure throughout the resolidified zone at high cooling rates. J. W. ELMER, formerly Graduate Student at the Massachusetts Institute of Technology     

Microsegregation and Secondary Phase Formation During Directional Solidification of the Single-Crystal Ni-Based Superalloy LEK94

A multicomponent phase-field method coupled to thermodynamic calculations according to the CALPHAD method was used to simulate microstructural evolution during directional solidification of the LEK94 commercial single-crystal Ni-based superalloy using a two-dimensional unit cell approximation. We demonstrate quantitative agreement of calculated microsegregation profiles and profiles determined from casting experiments as well as calculated fraction solid curves with those determined in differential thermal analysis (DTA) measurements. Finally, the role of solidification rate on dendrite morphology and precipitation of the secondary phases is investigated and a new measure of the dendrite morphology is presented to quantify the effect of back diffusion on the amount of secondary phases.     

急冷对Ti_(0.32)Cr_(0.345)V_(0.25)Fe_(0.03)Mn_(0.055)合金储氢性能的影响

为了研究急冷对储氢合金残余氢量的影响,利用真空电弧熔炼炉和铜模喷铸制备了Ti_(0.32)Cr_(0.345)V_(0.25)Fe_(0.03)Mn_(0.055)合金,采用XRD、PCT(压力-容量-温度)、TG/DTA等手段分析了急冷对储氢合金吸放氢性能的影响。结果表明,铸态合金和急冷合金均由BCC固溶体主相和Laves第二相组成;急冷对首次吸氢动力学行为影响较大,由铸态时的化学反应控制变为急冷时的新相晶核形成长大控制;急冷后,合金吸放氢平台压得到提高,且吸氢起始点左移,但吸放氢滞后性增大。TG/DTA曲线表明,急冷并没有改变合金的残余氢量,但氢化物放氢温度升高。