Effects of temperature and ferromagnetism on the γ-Ni/γ′-Ni3Al interfacial free energy from first principles calculations

The temperature dependencies of the γ(f.c.c.)-Ni/γ′-Ni₃Al(L1₂) interfacial free energy for the {100}, {110}, and {111} interfaces are calculated using first-principles calculations, including both coherency strain energy and phonon vibrational entropy. Calculations performed including ferromagnetic effects predict that the {100}-type interface has the smallest free energy at different elevated temperatures, while alternatively the {111}-type interface has the smallest free energy when ferromagnetism is absent; the latter result is inconsistent with experimental observations of γ′-Ni₃Al-precipitates in Ni–Al alloys faceted strongly on {100}-type planes. The γ(f.c.c.)-Ni/γ′-Ni₃Al interfacial free energies for the {100}, {110}, and {111} interfaces decrease with increasing temperature due to vibrational entropy. The predicted morphology of γ′-Ni₃Al(L1₂) precipitates, based on a Wulff construction, is a Great Rhombicuboctahedron (or Truncated Cuboctahedron), which is one of the 13 Archimedean solids, with 6-{100}, 12-{110}, and 8-{111} facets. The first-principles calculated morphology of a γ′-Ni₃Al(L1₂) precipitate is in agreement with experimental three-dimensional atom-probe tomographic observations of cuboidal L1₂ precipitates with large {100}-type facets in a Ni-13.0 at.% Al alloy aged at 823 K for 4096 h. At 823 K this alloy has a lattice parameter mismatch of 0.004 ± 0.001 between the γ(f.c.c.)-Ni-matrix and the γ′-Ni₃Al-precipitates.     

Phase evolution and crystallography of precipitates during decomposition of new “tungsten-free” Co(Ni)–Mo–Al–Nb γ–γ′ superalloys at elevated temperatures

This article reports the microstructural stability and consequent phase decomposition including the appearance of topologically close-packed (TCP) phases at high temperature of recently discovered tungsten-free γ–γ′ alloys of base composition Co–10Al–5Mo–2Nb with or without the addition of Ni and Ti. On prolonged aging at 800 °C of the Co–10Al–5Mo–2Nb alloy, needle-shaped DO₁₉-ordered precipitates with stoichiometry of Co₃(Mo, Nb) start appearing in the microstructure. In addition, growth of cellular domains from the grain boundaries featuring a three-phase composite lamellar structure could be observed. These phases are fcc γ-Co with composition different from the original matrix, CoAl with B2 ordering and Co₃(Mo, Nb) with DO₁₉ ordering. All the phases exhibit well-defined crystallographic orientation relationships. The decomposition of the alloys depends on the solvus temperature of the γ′ phase. The Ni-containing alloy exhibits no phase decomposition until 100 h of aging at 800 °C without any significant effect on γ′ volume fraction (76 %). However, at 950 °C, the alloy decomposes leading to the appearance of four different phases including TCP phases: a Cr₃Si-type cubic phase, a hexagonal Laves phase, rhombohedral μ phase, and solid solution of Co phase. The γ–γ′ microstructure in the Co–10Al–5Mo–2Nb and Co–30Ni–10Al–5Mo–2Ta alloys is not stable at 800 and 950 °C, respectively, on long-term aging. This shows that the measured solvus temperatures (i.e., 866 and 990 °C) are metastable solvus temperatures. We also report that the Ti-containing alloy exhibits superior stability with no evidence of either TCP phase formation or any other decomposition of γ′ precipitates, even after aging at 950 °C for 100 h.     

Concurrent design for NiAl-based (β/γ′) two-phase alloys by controlling microstructure and texture

Control of the crystallography of Ni₃Al(γ′) precipitates along grain boundaries of NiAl(β) was systematically studied using β bicrystals with controlled orientations. γ′ phase preferentially precipitated along β grain boundaries showing a film-like shape. The variants of γ′ precipitates were uniquely selected, which satisfies the Kurdjumov–Sachs (K–S) relation with a neighboring β grain and deviates from the relation with another adjacent β grain. In the course of tensile deformation, fracture occurred preferentially at the (β/γ′-film) interface deviating from the K–S relation and the fracture stress decreased with increasing deviation angle from the K–S relation. For improvement of the coherency at the irrational (β/γ′-film) boundaries, the control of microstructure and crystal orientation distribution in (β/γ′) two-phase polycrystals was next attempted by thermomechanical processing. After hot-compression in β phase region and subsequently annealing in (β/γ′) two-phase region, γ′ phase transformed from β phase with 〈111〉_β fiber texture satisfying the K–S relation, resulting in the formation of 〈110〉_γ′ fiber texture. In particular, a large number of (β/γ′-film) boundaries became partially coherent. This thermomechanical processing was effective in controlling the crystallography of γ′-film along β grain boundaries and leads to the harmonic design of strength and ductility for (β/γ′) two-phase alloys.     

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     

Effect of cooling rate on the morphology of γ′ precipitates in a nickel–base superalloy under directional solidification

The morphological evolution of γ′ precipitates in a nickel-based superalloy K5 was studied by zone melting directional solidification under vacuum conditions. The results show that at the lower cooling rate of 12.42 K s^–1, γ′ precipitates remand big cuboids, γ′ particles become smaller at the cooling rate ranges from 12.42 to 38.80 K s^–1. For a rather fast cooling rate of 50.16 K s^–1, γ′ particles retain a spherical shape. The experiments show that big cuboids will become unstable and split into several small one at the lower cooling rate of 1.1 K s^–1. The mechanism of the evolution of the γ′ morphologies is also analyzed by introducing a new parameter-shape factor which classifie the total energy into several energy levels. Based on this, the effect of the cooling rate on the γ′ morphology is discussed.     

Concurrent design for NiAl-based (β/γ′) two-phase alloys by controlling microstructure and texture

Control of the crystallography of Ni₃Al (γ′) precipitates along grain boundaries of NiAl(β) was systematically studied using β bicrystals with controlled orientations. γ′ phase preferentially precipitated along β grain boundaries showing a film-like shape. The variants of γ′ precipitates were uniquely selected, which satisfies the Kurdjumov–Sachs K–S) relation with a neighboring β grain and deviates from the relation with another adjacent β grain. In the course of tensile deformation, fracture occurred preferentially at the (β/γ′-film) interface deviating from the K–S relation and the fracture stress decreased with increasing deviation angle from the K–S relation. For improvement of the coherency at the irrational (β/γ′-film) boundaries, the control of microstructure and crystal orientation distribution in (β/γ′) two-phase polycrystals was next attempted by thermomechanical processing. After hot-compression in β phase region and subsequently annealing in (β/γ′) two-phase region, γ′ phase transformed from β phase with 〈111〉_β fiber texture satisfying the K–S relation,resulting in the formation of 〈110〉γ′ fiber texture. In particular, a large number of ′/γ′-film) boundaries became partially coherent. This thermomechanical processing was effective in controlling the crystallography of γ′-film along β grain boundaries and leads to the harmonic design of strength and ductility for (β/γ′) two-phase alloys.     

Influence of minor additions on characteristics of Cu–Al–Ni alloy

Abstract

The aluminium and nickel contents of Cu–Al–Ni alloy are varied to relate the parent phase chemistry to its shape memory behaviour. Rare earth and grain refining elements (titanium, zirconium, boron, etc.) are added in minor quantities to assess their effects on the grain refinement of the alloy and also on its shape recovery behaviour. It is observed that increasing the aluminium and nickel contents decreases the shape recovery temperature whereas minor additions are found to increase it. The alloys have been aged in the parent as well as the martensitic phase to investigate the influence of minor additions on their aging response. It is observed that precipitation of γ₂ phase occurs during the initial stage of aging of the ternary alloy. The aging behaviour is monitored via changes in resistivity and hardness of the alloys during aging. Minor additions are found to retard the precipitation of γ₂ phase during aging. Titanium and rare earths particularly reduce the tendency for grain coarsening in the alloy. It is further observed that two types of martensite, β′₁ and γ′₁, are produced in the alloys under investigation. The transformation temperatures of these martensites are also related to the aluminium content of the alloy.

MST/1744     

Constitution of Ni3Al–Ni3Mo–Ni3W section of Ni–Al–Mo–W system

Abstract

The isothermal section of the Ni–Al–Mo–W system has been studied at 75 at.-%Ni at temperatures of 1523 and 1273 K. Constitutional data have been determined using electron probe microanalysis, X-ray diffraction, and microscopical examination. The alloys studied lay in the range 12·5–15 at.-%Al, 2·5–7·5 at.-%Mo, and 2·5–7·5 at.-% W. The phases present at 1523 K were γ, γ′, and α (based on the Mo–W continuous series of solid solutions); at 1273 K, NiMo(δ′) was also encountered. The γ/γ′ mismatch values lay in the range ?0·03 to ?0·75%. In the as-solidified state, the alloys consisted predominantly of γ-phase containing γ′-precipitates formed in the solid state.

MST/462     

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     

Ternary alloy films of Ni-Cr-Al for thin film resistors

Multicomponent thin alloy films based on Ni-Cr with additions of aluminium have been used in the G.D.R. for the production of discrete thin film resistors for more than 10 years. Additions of aluminium stabilize the layer and reduce the temperature coefficient of resistance (TCR) to values of about zero. Three regions have been found up to now in the system Ni-Cr-Al which result in highly stable resistive films with a very low TCR. The properties of these films are determined by the content and the distribution of the phases which are present in the amorphous film. The following phases have been found in structural investigations of vacuum-annealed films: α phase (chromium), β phase (NiAl), γ phase (Ni₃Cr), γ′ phase (Ni₃Al) and γ-γ′ phase (Ni₃(Cr, Al)). Of the three regions, one with less than 3 at.% Al, one with 28–32 at.% Al and one with 45–60 at.% Al, the first two regions have been used for the production of thin film resistors up to now. Films of the third region, which has only been found during the last few years, can be obtained by sputtering.     

Microstructure evolution and mechanical properties of GH984G alloy with different Ti/Al ratios during long-term thermal exposure

GH984G alloy is a low cost Ni–Fe based wrought superalloy designed for 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants. In this paper, the microstructure evolution and tensile properties of GH984G alloy with different Ti/Al ratios during thermal exposure at different high temperatures are investigated. Detailed microstructure analysis reveals that the Microstructure of alloys with different Ti/Al ratios are similar after standard heat treatment, and the primary precipitates are γ′, MC, M₂₃C₆ and M₂B. However, η phase precipitates at grain boundary in the alloy with high Ti/Al ratio after thermal exposure at 750 °C for 570 h. By contrast, the microstructure stability of the alloy with lower Ti/Al ratio is excellent. There is no detrimental phase even if after thermal exposure at 750 °C for 5000 h in the alloy with lower Ti/Al ratio. γ′ coarsening plays a great role on the tensile strength, and the critical size range of γ′ could be defined as approximately 27–40 nm. The influence of η phase on tensile strength has close relationship with its volume fraction, the high volume fraction results in the decrease of tensile strength. The tensile strength of the alloy with lower Ti/Al ratio is obviously higher than the alloy with higher Ti/Al ratio and the yield strength has no obvious decrease during long-term thermal exposure at 700 °C. It is demonstrated that the thermal stability of microstructure and mechanical properties of GH984G alloy can be improved by moderately decreasing Ti/Al ratio to satisfy the requirement of A-USC plants.     

Precipitation hardening owing to δ′ Al3Li in low activation Al–3 Li–12Si–3Mg alloy

Abstract

The precipitation hardening behaviour owing to ordered δ′ Al₃Li in an Al–3Li–12Si–3Mg (wt-%) based alloy was investigated during aging between room temperature and 523 K after quenching from between 723 and 823 K. The induced radioactivity level of this alloy is very low, suitable for fusion reactor structures. Effects of aging temperature, solutionising temperature, and pretreatments on precipitation hardening were examined in detail. Tensile properties were also examined in the as quenched condition as a function of testing temperature between room temperature and 673 K. Yield stress exhibited a positive temperature dependence between room temperature and 523 K and decreased abruptly above 523k with increasing test temperature. The precipitation behaviour of δ′ and the strengthening mechanisms owing to δ′ were discussed.

MST/1100     

The effects of Co and Ti additions on microstructures and compressive strength of Udimet710

The effects of Co and Ti additions on the microstructures and compressive strength of the Ni-based superalloy Udiemt710 (U710) were investigated. The preliminary results showed that the Ni₃Ti-type (η) phase was observed in the alloys with low Co and Ti content; while a (Ni_xCo_1?x)₃Ti phase with a hexagonal structure was detected in the alloys with high Co and Ti content. The γ′ → η → (Ni_xCo_1?x)₃Ti phase transformation was discussed in terms of Co and Ti contents. The γ′ morphology changed from spherical to cuboidal with increasing Co and Ti content. Compressive tests showed that the alloys with Co and Ti addition possessed higher yield strength than the base alloy, U710. The strength increase could be ascribed to solid–solution strengthening of the γ and γ′ phases as a result of the Co and Ti additions, and the higher volume fraction of γ′ in the new alloys.     

Effect of heat treatment on microstructure and mechanical properties of Cu–6.9Ni–2.97Al–0.99Fe–1.06Mn alloy

ABSTRACT

The effect of heat treatment on the mechanical properties and microstructures of Cu–6.9Ni–2.97Al–0.99Fe–1.06Mn alloys was investigated. The results show that the microstructure of the as-cast alloy mainly consists of an alpha-copper matrix and γ-phase Ni₃Al particles. The microstructure of the alloy after solution treatment at 950°C for 2?h is a single-phase alpha-copper supersaturated solid solution and the second-phase strengthening disappears. After ageing treatment at 550°C for 6?h, the γ-phase particles are fully precipitated, and the mechanical properties of the alloy are significantly improved. The tensile strength is increased from 305 to 588?MPa. Quasi-cleavage fracture with shallow dimples appeared in the Cu–6.9Ni–2.97Al–0.99Fe–1.06Mn alloy aged at 550°C for 6?h.     

Stacking faults of metastable γ″ phase precipitated in an Ni-15Cr-8Fe-6Nb alloy

Abstract

The stacking faults of a metastable γ″ phase precipitated in a nickel base superalloy, a modified JIS NCF 3 type alloy (X–750M), are investigated by TEM and an X-ray diffraction method. The γ″ precipitates are circular shaped plates at the early stage of aging and they become elliptic or irregular shaped plates at the latter stage of aging at temperatures up to 1033 K. Contrast which suggests the existence of stacking faults on {112}_γ″ planes can be seen in many of the large γ″ precipitates extracted from the specimens aged at 1033 K for 36 ks or more. It is clear that the values of γ″ ?γ lattice mismatch increase with increasing aging time from the measurement of lattice constants of the γ and the γ″ phases. The formation of stacking faults on {112}_γ″ planes in the large γ″ precipitates is due to the movement of an a/6 <disp-formula><graphic href="splitsection1-m1.tif"/></disp-formula> partial dislocation introduced by γ″ ?γ coherency strain. Since a part of the stacking sequence has a similar crystal structure to that of stable δ phase precipitates in a γ″ phase, the formation of stacking faults in the γ″ precipitates is considered to be favourable for their stabilisation.     

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.     

The effects of Ta on the stress rupture properties and microstructural stability of a novel Ni-base superalloy for land-based high temperature applications

A novel polycrystalline Ni-base superalloy was developed for land-based high temperature applications, such as isothermal forging dies and industrial gas turbines. The alloy possessed surprisingly high stress rupture life of 52 h at 1100 °C/118 MPa which is comparable to the first generation single crystal (SC) superalloy and exhibited good microstructural stability. The effects of Ta addition on the phase change, stress rupture properties and microstructural stability of the alloy were investigated. The results indicated that Ta is a γ′-former and promotes the formation of eutectic γ′. The alloys with ∼7 vol.% eutectic γ′ possess higher stress rupture life at 1100 °C/118 MPa than the alloys with higher ∼20 vol.% eutectic. However, ∼20 vol.% excessive eutectic phases will enhance the stress rupture life at intermediate temperature of 760 °C for 686 MPa but weaken high temperature stress rupture properties. The (Al + Ta) content over 14.4 at.% led to the formation of large amounts of eutectic γ′ and exceeded the solubility of W and Mo in the residue liquid pool, which then promoted the precipitation of primary α-(W,Mo) and M₆C phases. Tantalum was also found as a strong MC carbides forming element. The order of ability to form monocarbide decreased from NbC to TaC to TiC. 6Al–0Ta (wt.%) alloys possessed good microstructural stability with no harmful topologically close-packed (TCP) phases being observed after thermal exposure at 850 °C/3000 h, 900 °C/1000 h. Only trace amounts of secondary plate-like M₆C carbides appeared in Ta-free and 5Al–4Ta (wt.%) alloys at 1100 °C/100–500 h. However, excessive Ta addition will destabilize the alloy and large amounts of secondary plate-like M₆C carbides precipitated after thermal exposure at 1100 °C. The transmission electron microscopy (TEM) and selected area electron diffraction (SAED) results showed the existence of the plate-like M₆C carbides.     

Influence of composition on monomodal versus multimodal γ′ precipitation in Ni–Al–Cr alloys

This study investigates the influence of alloy composition on γ′ precipitation in Ni–8Al–8Cr and Ni–10Al–10Cr at.% during continuous cooling from a supersolvus temperature. When subjected to the same cooling rate, Ni–8Al–8Cr develops a monomodal population, whereas Ni–10Al–10Cr develops a multimodal (primarily bimodal) population of γ′ precipitates. The bimodal γ′ precipitate size distribution in Ni–10Al–10Cr alloy can be attributed to two successive nucleation bursts during continuous cooling while the monomodal γ′ size distribution in Ni–8Al–8Cr results from a single nucleation burst followed by a longer time—wider temperature window for nucleation resulting in a larger number density of precipitates. Three-dimensional atom probe investigations reveal that while local equilibrium is achieved across the γ/γ′ interface, far-field γ compositions still retain an excess of Al and a depletion of Cr with respect to equilibrium, thus giving direct experimental evidence of the driving force for subsequent nucleation bursts during continuous cooling in case of Ni–10Al–10Cr. Contrastingly, in case of the Ni–8Al–8Cr alloy, while the γ phase retains an excess of Al and a depletion of Cr with respect to equilibrium after continuous cooling to room temperature, the additional driving force is consumed during subsequent isothermal annealing by growth of the large number of γ′ precipitates without any new nucleation.     

Vapor pressure determination for solid and liquid La<Subscript>2</Subscript>S<Subscript>3</Subscript> using boiling points

A high-temperature technique was developed for vapor pressure determination of solid and liquid γ-La₂S₃ (we called it the boiling point technique). Melting temperatures and total vapor pressures were measured for incongruently vaporizing γ-La₂S₃ at 1853–2210 K and 0.3–3.0 atm pressures. Having compared the slopes of the log p(S₂) versus 1/T plots measured by various techniques, we recommend the equation log p(S₂) [atm] = (6.31 ± 0.15) ? (12720±310)T ^?1 for T = 1021–2013 K as the most reliable for practical use.     

γ′ precipitation in some model nickel based superalloys

Abstract

An investigation is reported of the precipitation of γ′ from supersaturated γ in model Ni based superalloys containing (at.-%) 75%Ni, 15 or 12·5%Al, 10 or 7·5%Mo, 2·5%Ta or Won aging in the range 1073–1373 K for periods up to 100 h. For short aging periods the γ/γ′ mismatch was positive, but on prolonged aging the y lattice parameter increased and that of γ′ decreased giving negative mismatch; this change is attributed substantially to the partitioning of Mo to γ.

MST/1047