Numerical simulation of solidification and liquation behavior during welding of low-expansion superalloys

Low-expansion superalloys are susceptible to weld solidification cracks and heat-affected zone (HAZ) microfissures. To predict solidification cracking, QBasic procedures were developed and solidification reaction sequence, type, and amount of eutectic product were calculated. As manifested, primary solidification is followed by L → (γ+NbC) and L → (γ+Laves) eutectic reaction sequentially for GH903 and GH907; hence, the terminal eutectic constituents are made up of γ/NbC and γ/Laves. While for GH909, only reaction L → (γ+Laves) occurs and more γ/Laves eutectic forms. Therefore, GH909 is more sensitive to solidification cracking. To predict HAZ liquation, cracking Visual FORTRAN procedures were developed, and constitutional liquation of NbC was simulated. As shown, solid dissolution of NbC prior to liquation decreases, and initial liquid film increases with the rate of thermal cycle. Higher rate of thermal cycle promotes the melting of the matrix adjacent to the liquid film and postpones the solidification of the liquid by the liquid-to-γ mode. Thus, more residual liquid film remains at the eutectic point, which will promote HAZ microfissuring. The increase in original grain size and peak temperature also promotes liquation. Finally, these conclusions were verified indirectly by hot ductility tests.     

Effect of Holding Time on Microstructure and Mechanical Properties of Dissimilar γ'-Strengthened Superalloys TLP Joint

To achieve efficient connectivity of dissimilar γ'-strengthened superalloy, transient liquid-phase (TLP) diffusion bonding of 718Plus and Ni₃Al-based superalloys is carried out using BNi-2 interlayer at a constant temperature of 1100 °C with holding times of 3, 15, 30, and 60 min. It is shown in the results that when the holding time is insufficient, isothermal solidification transforms into athermal solidification. Due to the enrichment of B and Cr elements in liquid phase, ternary eutectic consisting of γ, Ni₃B, and CrB is formed at the joint center. Ternary eutectic with higher hardness (≈600 Hv) is poorly bonded to the matrix, which can easily become crack initiation location and significantly deteriorated the tensile properties. In addition, microhardness of the joint becomes more uniform and tensile strength gradually increases due to the reduction of eutectic with holding time increasing. Isothermal solidification completes and a joint without eutectic structure is formed at 1100 °C for 60 min. Diffusion of Al atoms leads to the formation of γ' with gradient size and tensile strength is comparable to that of base metal which is about 690.93 MPa.     

Oxygen enhanced crack growth in nickel-based superalloys and materials damage prognosis

This paper summarizes the results from a comprehensive multidisciplinary study to better understand the role of niobium and other strengthening elements in enhancing crack growth by oxygen in nickel-based superalloys at high temperatures, and considers its importance for materials damage prognosis and life cycle engineering in high temperature service. Three γ′ strengthened powder metallurgy (P/M) alloys, with 0, 2.5 and 5 wt pct Nb and comparable volume fractions (about 53 vol pct) of γ′′ precipitates, were specially designed for this study. Coordinated crack growth, microstructural and surface chemistry studies were conducted on the alloys. They were complemented by oxidation studies of Nb, Ni₃Nb, NbC, Ni₃Al and Ni₃Ti, and analyses of fracture surfaces of interrupted crack growth specimens by X-ray photoelectron spectroscopy (XPS). The findings taken in toto show that oxygen enhancement of crack growth is the result of the formation of a brittle film of surface oxides along grain boundaries and interfaces ahead of the crack tip by the preferential oxidation of Nb, Ti and Al in the Nb-rich carbides and Ni₃Al, Ni₃Ti and Ni₃Nb (in Inconel 718) precipitates. The results also showed that the oxidation of Nb-rich carbides alone can significantly enhance crack growth in oxygen. The findings are discussed in relation to the previously proposed crack growth mechanisms, and their applications.     

A high-resolution-electron-microscopy study of γ/γ′-α directionally solidified eutectic alloy

A master alloy with eutectic compositions of Ni-30.26Mo-6.08Al-1.43 V (wt%) has been directionally solidified (DS) into γ/γ′-α alloy. The microstructural as-ageing treatment was studied by means of high resolution electron microscopy (HREM). A majority of α fibres still display the Bain orientation relationship with the γ′/γ matrix. In a few cases, however, the so-called Nishiyama-Wasserman (NW) orientation relationship is found in specimens aged at 850 °C for 2000 h. Different microdomain structures of the γ phase, corresponding to different ageing temperatures, were revealed. Orthorhombic Ni₃Mo phase, with a size of tens of nanometres, was found to precipitate inside α fibres after ageing at both 850 and 650 °C. Occasionally, an γ′-Ni₃Al phase with lamellar twin structure was also found to coexist with Ni₃Mo precipitate inside the α fibres. The orientation relationships between the precipitates and the α fibres were determined. Energy dispersive X-ray analysis (EDX) showed that the precipitation of Ni₃Mo and Ni₃Al is due to solid solution of Ni and Al in the α fibres.     

Microstructure and properties of Ti–Nb–V–Mo-alloyed high chromium cast iron

The correlations of microstructure, hardness and fracture toughness of high chromium cast iron with the addition of alloys (titanium, vanadium, niobium and molybdenum) were investigated. The results indicated that the as-cast microstructure changed from hypereutectic, eutectic to hypoeutectic with the increase of alloy contents. Mo dissolved in austenite and increased the hardness by solid solution strengthening. TiC and NbC mainly existed in austenite and impeded the austenite dendrite development. V existed in multicomponent systems in forms of V alloy compounds (VCrFe₈ and VCr₂C₂). With the increase of alloy additions, carbides size changed gradually from refinement to coarseness, hardness and impact toughness were increased and then decreased. Compared with the fracture toughness (6 J/cm²) and hardness (50·8HRC) without any alloy addition, the toughness and hardness at 0·60 V–0·60Ti–0·60Nb–0·35Mo (wt%) additions were improved and achieved to 11 J/cm² and 58·9HRC, respectively. The synergistic roles of Ti, Nb, V and Mo influenced the solidification behaviour of alloy. The refinement of microstructure and improvement of carbides morphologies, size and distribution improved the impact toughness.     

Study of grain refinement and eutectic transformation of undercooled IN718 superalloy

Abstract

A substantial undercooling up to 250 K was produced in the IN718 superalloy melt by employing the method of molten salt denucleating, and the microstructure evolution with undercooling was investigated. Within the achieved undercooling, 0–250 K, the solidification microstructure of IN718 undergoes two grain refinements: the first grain refinement occurs in a lower range of undercooling, which results from the ripening and remelting of the primary dendrite, and at a larger range of undercooling, grain refinement attributes to solidification shrinkage stress and lattice distortion energy originating from the rapid solidification process. A ‘lamellar eutectic anomalous eutectic’ transition was observed when undercooling exceeds a critical value of ～250 K. When undercooling is small, owing to niobium enrichment in interdendrite, the remaining liquid solidifies as eutectic (γ+Laves phase); whereas, if the undercooling achieves 250 K, the interdendrite transforms from eutectic (γ+Laves phase) to Laves phase, which results from the formation of divorced eutectic arising from the huge variance of the growth velocities of γ and Laves phases.     

Microstructure characterisation and CTE study of Fe—42Ni—Nb invar alloys

Invar alloy has low coefficient of thermal expansion; however, the week strength always limits its application. To circumvent this problem, the authors have designed Fe—42%Ni—Nb invar alloy using midfrequency vacuum induction melting technique. The microstructures of this alloy were characterised by means of XRD, SEM and TEM. The multicomponent composition (NiFeCMnNb) is chosen such that upon chill casting of the alloy the liquid undergoes a metastable reaction, forming a strengthening NbC precipitated phase on γ-(Fe, Ni) solid solution. As a result, the alloy achieves the low coefficient of thermal expansion α_30–100=6·14 × 10^?6 K^?1 ranged from room temperature to 100°C. The results show: in NiFe invar alloys, the primary phases are γ-(NiFe), FeNi₃) Ni and Fe₂C; and with the addition of carbon, manganese and rare earth niobium, NiFeCMnNb invar alloy achieves such primary phases as γ-(NiFe) and Nb_yC_x; and TEM structure consists of superfine structure and its matrix is γ-(NiFe).     

Kinetics of triple-junctions in eutectic solidification: a sharp interface model

The kinetics of triple-junctions (TJs) in eutectic solidification is modeled by the thermodynamic extremum principle (TEP). It consists of two parts. First, TJs as the interaction of interfaces follow the interface kinetics according to which the temperature and concentration at the TJs are determined. This interface part of TJ kinetics is closely related to the eutectic point in the kinetic phase diagram. Second, TJs have their specific kinetics according to which their morphology (e.g., the contact angles in two dimensions) is determined. Using a new solution of solute diffusion in liquid, the TJ kinetics is incorporated into the current lamellar eutectic growth model. The model is applicable to the concentrated alloy systems and can be extended to any kind of eutectics. Simulation results of the rapid solidification of a lamellar Ni₅Si₂–Ni₂Si (γ–δ) eutectic show that both parts of TJ kinetics can play important roles in eutectic solidification and need to be considered to improve the current eutectic theory.     

Effect of Rapid Solidification on the Microstructure and Magnetic-Field-Induced Strain of Co1.36Ni1.21AlFe0.12

The effect of rapid solidification of a suction cast specimen on the microstructure, magnetocrystalline anisotropy, and magnetic-field-induced strain of the Co_1.36Ni_1.21AlFe_0.12 alloy was investigated. The results show that a dual-phase structure consists of L1₀-type twin martensite with a small amount of coexisting A1-type divorced eutectic γ phase in the specimen. After rapid solidification, the network divorced eutectic γ phase was smashed to pieces by strong suction force and the grain grew in a preferred orientation. At the same time, the specimen after rapid solidification has high magnetocrystalline anisotropy and excellent shape recovery. The trend of twin martensitic rearrangements was evaluated by the O'Handley model, and a relatively large magnetic-field-induced strain was obtained in the specimen with rapid solidification.     

Equilibria involving P- and (σ-phases in Ni–Cr–Al–Mo system

Abstract

An investigation is reported of phase equilibria in alloys of the Ni–Cr–AI–Mo system containing 60 and 50 at. –%Ni annealed at 1523 K. The experimental methods used mainly were electron microscopy, electron probe microanalysis, and X–ray diffraction. Four quaternary alloys were used, with compositions (at.–%) lying on a line between Ni₆₀Al₄₀ and Ni₆₀Cr₂₀Mo₂₀,and one alloy containing 50Ni–28Cr–10AI-12Mo.The composition range was chosen to include the P–and σ –phases based on the Ni–Cr–Mo system. The phases involved in equilibria at 1523 K in the 60 at. –%Ni alloy series were γ (Ni–base solid solution), β (based on NiAI), Mo–base solid solution, and P; the 50 at. –%Ni alloy contained γ, β and σ. Partial isothermal sections of the quaternary system at 60 and 50 at. –%Ni have been determined. Eutectics containing either γ + P or γ + β + P were present

in the as–solidified 60 at. –%Ni alloys, while a γ + β + σ eutectic was present in the 50 at–%Ni alloy; some compositional data for eutectic phases were obtained by analytical transmission electron microscopy. High hardness values (from ~ 500 to 670 HV) were obtained in the as–solidified alloys.

MST/265     

Interface temperature measurement of M2C and M6C eutectic carbides in the Fe–Mo–C system

The High speed cast iron, which is used for hot rolling parts, needs high fracture toughness and wear resistance. To improve these properties, the control of eutectic carbides, M₃C, M₇C₃,M₆C and MC is important by adding elements such as Cr, W, V and Mo.

The aim of this study is to estimate which carbide will solidify under certain solidification conditions and compositions. This prediction criterion can be gained by measuring the interface temperature of each carbide in various samples with different solute elements, composition and growth rate.

In this report, the solidified temperature of γ + M₂C and γ + M₆C eutectic carbide in the Fe–Mo–C ternary system in the composition range near to the eutectic monovariant line, was measured during the unidirectional solidiication process. The relationship between solidified interface temperature and growth rate was obtained. In eutectic solidification along the γ + M₆C monovariant line, a coefficient of undercooling, the k value, was obtained.

The authors have already measured the k values of other eutectic carbides, such as γ + M₃C, austenite + M₇C₃, and γ + VC in Fe–Cr–C and Fe–V–C system. The paper also discusses the relationships between these properties of eutectic carbides.     

Interface temperature measurement of M2C and M6C eutectic carbides in the Fe–Mo–C system

The High speed cast iron, which is used for hot rolling parts, needs high fracture toughness and wear resistance. To improve these properties, the control of eutectic carbides, M₃C, M₇C₃, M₆C and MC is important by adding elements such as Cr, W, V and Mo.The aim of this study is to estimate which carbide will solidify under certain solidification conditions and compositions. This prediction criterion can be gained by measuring the interface temperature of each carbide in various samples with different solute elements, composition and growth rate.In this report, the solidified temperature of γ+M₂C and γ+M₆C eutectic carbide in the Fe–Mo–C ternary system in the composition range near to the eutectic monovariant line, was measured during the unidirectional solidification process. The relationship between solidified interface temperature and growth rate was obtained. In eutectic solidification along the γ+M₆C monovariant line, a coefficient of undercooling, the k value, was obtained.The authors have already measured the k values of other eutectic carbides, such as γ+M₃C, austenite+M₇C₃, and γ+VC in Fe–Cr–C and Fe–V–C system. The paper also discusses the relationships between these properties of eutectic carbides.     

Properties of NbC-TiAl and TiB2-TiAI composites produced by plasma transferred arc process

Intermetallic compound based composites (IMCs) consisting of particles of NbC and TiB₂ in Ti-36 wt-%Al (NbC-TiAl and TiB₂-TiAl respectively), with varying volume fraction of reinforcing particles, were produced by a plasma transferred arc process. In NbC-TiAl IMCs, the hardness and 0·2% compressive proof stress increased steadily with increasing volume fraction of NbC whereas the tensile strength was lower than that of the unreinforced TiAl regardless of the volume fraction of NbC. In TiB₂-TiAl IMCs, the hardness and 0·2% compressive proof stress exhibited maximum values at a volume fraction of 5 vol.-%TiB₂. The maximum tensile strength of ～500 MN m^?2, which is almost twice that of the unreinforced TiAl, was obtained at a volume fraction of 5 vol.-%TiB₂. The initial improvement of mechanical properties due to the addition of TiB₂ was considered to be caused by the reinforcing effect of the TiB₂ particles, grain refinement, and the disappearance of γ grains in 3–5 vol.-%TiB₂-TiAl IMCs. The deterioration of the mechanical properties observed for a volume fraction of >5 vol.-%TiB₂ may be attributed to the increase in the amount of y grains with increasing volume fraction of TiB₂ particles from 7 to 15 vol.-% and the increase in coarse TiB₂ particles that can act as crack initiation sites in tensile tests.

MST/3434     

Current opinion in medium manganese steel

Abstract

Medium Mn steels have been actively investigated due to their excellent balance between material cost and mechanical properties. The steels possess a single α′ martensite phase in hot and cold rolled states and multiphases after intercritical annealing. Many studies have focused on investigating the influences of chemical composition and annealing conditions on the microstructure, particularly the grain size and retained γ (γ_R), and on the tensile properties. The steels exhibit high strength and good ductility due to transformation induced plasticity occurring in γ_R, whose volume fraction is approximately 0·2–0·4. The present review summarises the important results of previous studies about the effects of both intercritical annealing conditions and alloying elements on the microstructure and tensile properties of medium Mn steels.     

Microstructure evolution of directionally solidified Nb–Si alloy

Abstract

By taking the method of liquid–metal cooled directional solidification, alloys with a nominal composition of Nb–14Si–24Ti–10Cr–2Al–2Hf (at-%) were prepared under different conditions. Alloys were initially directional solidified with different withdrawal rates (R?=?1·2, 6, 18 mm min^?1) at 1750°C and subsequently heat treated at 1450°C for 10 h. These processes aimed to investigate the microstructure of directionally solidified (DS) and heat treated (HT) alloys by XRD, SEM, and EDS. The microstructure of DS alloy was composed of (Nb,Ti)_SS, (Nb,Ti)₅Si₃, and Laves phase Cr₂Nb, and the former two components formed (Nb,Ti)_SS+(Nb,Ti)₅Si₃ eutectics. In addition, (Nb,Ti)₅Si₃ laths only presented in DS1·2 alloy. With the increasing withdrawal rates, the microstructure of alloy altered from hypereutectic into pseudo-eutectic, accompanied with the eutectic morphology transformation from petaloid into coupled. Also, the dimension of constituent phases reduced. However, after heat treatment, the constituent phases did not change. The petaloid morphology of eutectics in DS specimens disappeared and coupled eutectic transferred into network. The block or needle-like Cr₂Nb gathered along the boundary between (Nb,Ti)₅Si₃ and (Nb,Ti)_SS, and the overall alloy composition became homogenisation.     

Ni3AI–Ni3Cr–Ni3Ta section of Ni–Cr–AI– Ta system

Abstract

The constitution of the 75 at.%Ni section of the Ni–Cr–Al– Ta system has been determined at 1523 and 1273 K. Alloys annealed at these temperatures have been studied using electron probe microanalysis and X–ray diffraction, and their microstructures and associated hardness values have also been examined. The isothermal sections at 1523 and 1273 K contain the following phases: γ+γ′+Ni₃Ta, and Ni₆TaAI, with the following three–phase equilibria between them: γ+γ′+Ni₆TaAI and γ+Ni₃Ta+Ni₆TaAl. The γ′–phase contains up to ~9 at.–%Ta. Some observations on as–cast structures have also been made.

MST/208     

Preparation of eutectics by directional solidification of quaternary melts

This paper presents a theoretical analysis of quasi-equilibrium directional solidification of a quaternary melt. We consider the variation in the composition of phases in each portion of the sample and changes in phase composition for various types of phase reactions. The results indicate that the melt trajectory during directional solidification may belong only to those phase diagram elements corresponding to the crystallization of binary, ternary, or quaternary eutectics or single-phase crystallization regions. Using the directional solidification of a melt with the composition (at %) Cu 29.37, Ni 17.72, Fe 5.91, and S 47.00, we obtained a sample consisting of zones with different phase compositions: [Ni _z (Fe,Cu)_{1 − z} ]S_{1 ± δ} single-phase zone, [Ni _z (Fe,Cu)_{1 − z} ]S_{1 ± δ} + Cu_{5 ± x} Fe_{1 ± x} S₄ binary eutectic mixture, and [Ni _z (Fe,Cu)_{1 − z} ]S_{1 ± δ} + Cu_{5 ± x} Fe_{1 ± x} S₄ + (Ni _z Fe_{1 − z} )S₂ ternary eutectic mixture. In going from one zone to another, new phases appear and the average composition of the sample changes sharply, whereas the compositions of the melt and solid solution present in neighboring zones vary continuously. These results are consistent with theoretical concepts.     

Microstructure evolution and mechanical properties of Nb–Si based alloy processed by electromagnetic cold crucible directional solidification

In this paper, the samples with the composition of Nb–22Ti–16Si–3Cr–3Al–2Hf (at.%) have been successfully processed by electromagnetic cold crucible directional solidification, and the microstructure evolution and mechanical properties of the samples were investigated as a function of withdrawal rate. The microstructures of all the samples were composed of primary Nbss, Nbss + α-(Nb, Ti)₅Si₃ eutectic and Nbss + Ti-rich (Nb, Ti)₅Si₃ eutectic. With the increase of withdrawal rate, the microstructure became finer, which could be characterized by the decreased mean diameter of cellular eutectic, mean diameter of eutectic Nbss particles and mean interphase spacing. Meanwhile, the room temperature fracture toughness decreased first, and then increased; the high temperature tensile strength increased gradually. The volume fraction, size and aspect ratio of Nbss and the number of Nbss/(Nb, Ti)₅Si₃ interfaces codetermined the fracture toughness. The more number of Nbss/(Nb, Ti)₅Si₃ interfaces was conducive to impeding the movement of dislocation, which caused the improvement of tensile strength.     

Solidification pathway and phase transformation behavior in a beta-solidified gamma-TiAl based alloy

The phase transformation behavior of an as-cast Ti-42Al-5 Mn(at.%) alloy after subsequent quenching from 1380 ℃ to 1000 ℃ was investigated based on the differential thermal analysis(DTA),electron probe micro analyzer-backscattered electrons(EPMA-BSE),transmission electron microscope(TEM) and X-ray diffraction(XRD).The results show that,the solidification path can be summarized as follows:Liquid→Liquid+β→β→β→α→β+α+γ→β_o+α_2+γ→β_o+γ+α_2/γ→β_o+γ+α_2/γ+β_(o,sec),with the phase transformation α→β temperature(T_β)=1311℃,phase transformation γ→β temperature of(T_(γsolv))=1231℃,phase transformation α_2→α or β_o→β temperature(T_(α2→α)/T_(β_o→β))=1168 C,eutectoid temperature(T_(eut))=1132 ℃ and T_(α_2/γ→βo,sec)≈1 120℃.In comparison with Ti-42 Al alloy,the T_(eut) and T_(γsolv)are slightly increased while both the Tp is decreased obviously by 5% Mn addition.When quenched from the temperature of 1380-1260 ℃,the martensitic transformation β→α' could occur to form the needlelike martensite structure in β area.This kind of martensitic structure is much obvious with the increase of temperature from 1260℃ to 1380 ℃.When the temperature is below T_(γsolv)(1231℃),the γ grains would nucleate directly from the β phase.For the temperature slightly lower than T_(eut)(1132℃),the dotted β_(o,sec) phases could nucleate in the lamellar colonies besides the γ lamellae precipitated withinα_2 phase.Finally,at room-temperature(RT),the alloy exhibits(p_o+α_2+γ) triple phase with microstructure of β_o+lamellae+γ,of which the lamellar structure consists of α_2,γ and β_(o,sec) phases.The phase transformation mechanisms in this alloy,involving β→α',β→γ,α_2→α_2/γ and α_2→β_(o,sec) were discussed.     

Effects of boron and silicon on microstructure and isothermal solidification during TLP bonding of a duplex stainless steel using two Ni–Si–B insert alloys

Abstract

The influence of B and Si on microstructure and isothermal solidification during transient liquid phase (TLP) bonding of a duplex stainless steel using MBF-30 (Ni–4·5Si–3·2B, wt-%) and MBF-35 (Ni–7·3Si–2·2B, wt-%), was investigated. Based on experimental studies, the formation of Ni₃B within the joint centreline is dependent on B content; the morphology of Ni₃Si is dominated by Si concentration. There was a deviation between the times for complete isothermal solidification obtained by the experiment and the conventional TLP bonding model. Isothermal solidification of the liquid dependent on different solidification regimes is suggested to be a controlling factor contributing to the change in the rate of isothermal solidification observed using the two filler metals.