Gamma prime and TCP phases and mechanical properties of thermally exposed nickel-base superalloy

The effect of long-term thermal exposure and casting superheat on microstructure, topologically close-packed (TCP) phases, γ?′ precipitation and mechanical properties of an experimental Ni-base superalloy were studied. The investigated alloys were produced by investment casting process under two levels of superheat. After solution heat treatment, at 1180°C for 2?h followed by air cooling; the two alloys under investigation were isothermally exposed at 845°C for 24, 200, 1000, and 1500?h. The long-term thermal exposure conditions have a significant impact on the precipitation and morphology of TCP and γ?′ phases. The σ phase precipitated as needle and platelet shapes, whereas the μ phase formed in plate and agglomerated shapes. The μ phase has high concentration of Cr, Mo, W, and Co, while the σ phase has high percentages of Ni and Ti. The μ phase was precipitated after thermal exposure of 200?h in the case of high superheat specimen and after 1000?h in low superheat specimen. The η phase found was also a thick plate-like shape in the microstructure of both alloys in the interdendritic zones. The optimum size and volume fraction of γ?′ precipitates were obtained after being thermally exposed for 200 and 1000?h for high and low superheat alloys, respectively. Consequently, the highest hardness level was achieved at the optimum conditions of γ?′ precipitates in high and low superheat alloys.     

Early stages of precipitation and microstructure control in Mg–rare earth alloys

Hardening precipitation frequently occurs in Mg–rare earth (RE) alloys after heat treatment in the 150–200°C range. Early stages of precipitation have been studied in detail by transmission electron microscopy in two Mg–RE alloys (Mg–Y–Gd and Mg–Y–Nd). Two types of structures may be involved in the precipitation sequence: a DO₁₉ phase and the so-called orthorhombic β′ phase. The structural relationship between DO₁₉ and β′ phases has been established in underaged and overaged states from the observations at peak ageing. We show that the earliest precipitates play a key role in the selection of phases developing in overaged states. Depending on the habit plane of the precipitates present in the early states, either the DO₁₉ or the β′ phase will grow in further ageing. The Mg–Y–Gd and Mg–Y–Nd alloys illustrate the different microstructures resulting from such selection. Due to the selective growth of the β′ phase, the Mg–Y–Gd alloys are characterized by a fine scale microstructure which provides improved mechanical properties.     

The chemical composition of a metal/ceramic interface on an atomic scale: The Cu/MgO {111} interface

The chemical composition profile across a Cu/MgO {111}-type heterophase interface, produced by the internal oxidation of a Cu(Mg) single-phase alloy at 1173 K, is measured via atom-probe field-ion microscopy with a spatial resolution of 0.121 nm; this resolution is equal to the interplanar spacing of the {222} MgO planes. In particular, we demonstrate directly that the bonding across a Cu/MgO {111}-type heterophase interface, along a <111> direction common to both the Cu matrix and an MgO precipitate, has the sequence Cu|O|Mg... and not Cu|Mg|O...; this result is achieved without any deconvolution of the experimental data. Before determining this chemical sequence, it was established, via high-resolution electron microscopy, that the morphology of an MgO precipitate in a Cu matrix is an octahedron faceted on {111} planes with a cube-on-cube relationship between a precipitate and the matrix; that is, {111}_Cu//{222}_MgO and <110>_Cu // <110>_MgO.     

Precipitate characterisation in metallic systems by small-angle X-ray or neutron scattering

Small-angle scattering enables the extraction of precise, quantitative information about nano-scale precipitate microstructures. It can be used with X-rays (SAXS) or neutrons (SANS). This paper presents simple methods for extracting information on the precipitate size and volume fraction from SAS spectra. The various possibilities for obtaining precipitate size are reviewed, and the meaning of their differences is discussed. Examples of applications for complex precipitate microstructure measurements are given in the following areas: kinetic in-situ measurements in Fe–Cu and Fe–Nb–C alloys, non-stoichiometric precipitation in an Al–Zn–Mg–Cu alloy studied by anomalous SAXS (ASAXS), and precipitation mapping in weld cross-sections.     

Microstructural evolution and phase transformation in the liquid-solid Al/Ni diffusion couple

The liquid-solid Al/Ni diffusion couple was successfully fabricated by annealing at 1373?K for 48?h followed by water-quenching. Cross-sectional scanning and transmission electron microscopic analyses show that the multilayered diffusion zones comprise the following sequence of layers: γ-Ni(Al) | γ′-Ni₃Al | β′-NiAl | Ni-rich β-NiAl | β-NiAl. The Ni-rich β-NiAl upon quenching undergoes a martensitic transformation from β (B2) to β′ (L1₀). The β′ martensite is found to be internally twinned on the {111}<112>system. The volume changes and strains due to martensitic phase transformation, the precipitation of γ′-Ni₃Al from γ-Ni(Al) and lattice mismatch between Ni-rich β-NiAl and β-NiAl in the Al/Ni diffusion couple are quantitatively determined. The cuboidal γ′ phase coherently precipitates cube-on-cube in γ-Ni(Al). Composition fluctuations existing in the supersaturated solid solution γ-Ni(Al), provide sufficient driving force for the precipitation and facilitate nucleation and growth of the γ′ phase under isothermal annealing.     

Measurement of γ′ precipitates in a nickel-based superalloy using energy-filtered transmission electron microscopy coupled with automated segmenting techniques

Precipitates of the ordered L1₂ γ′ phase (dispersed in the face-centered cubic or FCC γ matrix) were imaged in Rene 88 DT, a commercial multicomponent Ni-based superalloy, using energy-filtered transmission electron microscopy (EFTEM). Imaging was performed using the Cr, Co, Ni, Ti and Al elemental L-absorption edges in the energy loss spectrum. Manual and automated segmentation procedures were utilized for identification of precipitate boundaries and measurement of precipitate sizes. The automated region growing technique for precipitate identification in images was determined to measure accurately precipitate diameters. In addition, the region growing technique provided a repeatable method for optimizing segmentation techniques for varying EFTEM conditions.     

Stress-assisted nucleation and growth of γ″ and γ′ precipitates in a Cu–1.2 wt%Be–0.1 wt%Co alloy aged at 320°C

An investigation by high-resolution transmission electron microscopy of the precipitation process during ageing a Cu–1.2?wt%Be–0.1?wt%Co alloy at 320°C has revealed that the transition phases follow a γ″→ γ″?+?γ′?→?γ sequence. The γ′ phase heterogeneously precipitates on the γ″ phase. The effects of an external stress on the nucleation and growth of disc-shaped γ″ and plate-shaped γ′ precipitates have been examined for the alloy aged at 320°C. A compressive stress applied in the [001] direction during ageing preferentially accelerates the nucleation and growth of the γ″ variant normal to the [001] axis among three crystallographically equivalent variants and the specific four γ′ variants formed on the γ″ variant normal to the [001] axis. A tensile stress does not significantly affect those of γ″ and γ′ precipitates. The critical diameter of the disc-shaped γ″ nucleus is estimated as about 1?nm from evaluation of the interaction energy between the applied stress and the misfit strains of γ″ precipitates. It is proposed that applied external stress does not affect the diffusion rate but the interphase boundary velocity.     

Damage mechanism of nickel-based creep-resistant alloys strengthened by the Laves phase at the grain boundary

ABSTRACT

This study proposes a design guideline for polycrystal Ni-based model alloys with high ductility and 100-MPa creep rupture strength beyond 800°C and 10^5?h. These alloys are strengthened by both the precipitation of fine γ′ particles inside the grain and the Laves phase at the grain boundary. For investigating the damage mechanism, transformation from the non-equilibrium Laves phase to the σ phase at the grain boundary and formation of the equilibrium needle-like Laves phase inside the grain are promoted by increasing the Fe concentration. The rupture time of Fe-free alloys significantly increases because of the equilibrium Laves phase at the grain boundary owing to a suitable Mo equivalent. In particular, W addition can help achieve high-temperature creep strength. The precipitate-free zone (PFZ) is predominantly formed by prior migration at the grain boundary without precipitation. Creep rupture occurs at the precipitation/matrix interface in the PFZ. Therefore, transformation control from the Laves to the σ phase at the grain boundary suppresses creep degradation. Consequently, a Ni-based alloy with strength >100?MPa and rupture elongation >20% at 800°C and 10^5?h is fabricated using Larson–Miller parameter conversion, and the alloy design guideline’s validity is confirmed.     

Shearing of γ′ precipitates in Ni-base superalloys: a phase field study incorporating the effective γ-surface

An extended phase field model of dislocations in Ni-base superalloys is presented. It incorporates the recently developed effective γ-surfaces for both matrix and precipitate phases, obtained from atomistic simulations. These novel γ-surfaces feature extrinsic stacking faults as additional local minima. Thus, they offer an increased number of available dislocation dissociation pathways within the phase field system. The new model has been used to simulate a variety of mechanisms for γ′ precipitate shearing proposed in literature. A critical assessment is made based on the modelling observations.     

Creep deformation of single crystals of new Co–Al–W-based alloys with fcc/L12 two-phase microstructures

The creep deformation behaviour of single crystals of Co–Al–W-based alloys with γ?+?γ′ two-phase microstructures has been investigated in tension under a constant stress of 137?MPa in air at 1000°C as a function of the γ′ solvus temperature and the volume fraction of the γ′ phase. When described by the creep strain rate versus time curve, the creep deformation of Co–Al–W-based alloys consists of transition and accelerating regions without a steady-state region, as observed in many modern nickel-based alloys. However, the creep strength of the present Co–Al–W-based alloys is comparable with nickel-based superalloys of the first generation but is much weaker than those of the second and higher generations. Unlike in nickel-based superalloys, the so-called p (parallel)-type raft structure, in which the γ′ phase is elongated along the tensile axis direction, is formed during creep in Co–Al–W-based alloys, being consistent with what is expected from the positive values of lattice misfit between the γ and γ′ phases. As a result, of the alloys investigated, the best creep properties are obtained with the alloy possessing the highest volume fraction (85%) of the γ′ phase, which is far larger than usual for nickel-based superalloys (55–60%).     

Filling the voids in silicon single crystals by precipitation of Cu3Si

This article presents a method to decorate all open volume defects in a silicon single crystal by Cu₃Si precipitation. Si single crystals are being used for the determination of Avogadro's number with sufficient accuracy (1 part in 10⁸) to allow the establishment of a physical standard for the kilogram. The method described here can be used to certify that the open volume in such crystals is sufficiently small. The voids in this work were formed artificially by annealing He-implanted silicon wafers. Annealing after application of a copper nitrate solution to the surface followed by slow cooling produced Cu₃Si precipitates in the η′ phase in the voids. To fill all the voids completely it proved necessary to coat the surfaces, after application of the nitrate, with a pure Cu layer as well. Slow cooling keeps the supersaturation of Cu small but sufficient to precipitate Cu into the voids. Formation of dislocation loops and stacking faults, often seen during Cu silicide precipitation in silicon after fast cooling, can be minimized. Other transition metals, such as Au, Ni, and Fe, were found to be less suitable than Cu for filling voids.     

HRTEM study of the effect of deformation on the early precipitation behaviour in an AA6060 Al–Mg–Si alloy

The effect of 10% pre-ageing deformation on the early precipitation behaviour in an AA6060 Al–Mg–Si alloy aged 10?min at 190°C was investigated by high-resolution transmission electron microscopy (HRTEM) in ?100?Al projections. The precipitate nucleation was heterogeneous since all precipitates were found to grow on dislocation lines. The pre-ageing deformation suppresses growth of Gunier–Preston zones and β″ phase. The resulting precipitates are still largely coherent with the aluminium matrix. They appear with two main morphologies; one consists of independent, small cross-sections arising from needles with disordered β′ and B′ structures. The other morphology is a much more continuous decoration where precipitates have elongated and conjoined cross-sections and where a particular precipitate phase could not be determined. All precipitates in this work were found to contain a common near-hexagonal sub-cell (SC) with projected bases a?=?b?≈?0.4?nm. This strongly indicates that they are built over the same Si network, which recently has been demonstrated to exist in all precipitates in the Al–Mg–Si(–Cu) system. For the discrete morphology type the network has one hexagonal base vector parallel to or very near a ?510?Al direction. For the continuous type, one base vector falls along a ?100?Al direction. This orientation of the network is different from previous studies of ternary Al–Mg–Si alloys and must be a direct consequence of the deformation.     

The effect of Cu on precipitation in Al–Mg–Si alloys

TEM investigations of two alloys isothermally heat treated at 175°C and 260°C show how Cu additions to the Al–Mg–Si system affect precipitation. Both alloys had a solute content Mg?+?Si?=?1.3 at.%, 0.127 at.% Cu, but with Mg/Si 0.8 and 1.25. Cu-containing Guinier-Preston (GP) zones and three types of Q′ precursors are identified as most common phases at peak-hardness conditions, whereas β″ accounts for maximum 30% of the total number of precipitates. The precursors have needle (L and S precipitates) or plate (C precipitate) morphologies. They consist of different arrangements of Al, Mg and Cu atoms on a grid defined by triangularly arranged Si planes parallel with and having the same period as {100} Al planes. The Si grid is composed of nearly hexagonal sub-cells of a?=?b?=?4.05?Å, c?=?4.05?Å. The Cu arrangement on the grid is often disordered in the needle precursors. The plate precursor is ordered, with a monoclinic unit cell of a?=?10.32?Å, b?=?8.1?Å, c?=?4.05?Å, γ?=?101°.     

Effects of Ag or Si on precipitation in the alloy Al-2.5 mass% Cu-1.5 mass% Mg

Calorimetric measurements and electron microscopy observations were performed on Al-2.5?mass% Cu-1.5?mass% Mg alloys containing also 0.4, 1 or 2% Ag or 0.5% Si, in order to improve understanding of the relationships between precipitation processes and age hardening. The analogous behaviour of calorimetric and hardness data confirms that the first hardening stage is initiated in all alloys by GPB zone formation which occurs via a nucleation and growth controlled mechanism. The vacancy-trapping effect of Mg is increased by Ag and Si additions and leads to slower precipitation kinetics. Consequently, refined GPB zones sizes are obtained leading to an increase in hardness with respect to the ternary alloy. During the second hardening stage, the formation of the more stable S′ phase increases the total amount of strengthening precipitates in the ternary alloy. Phases typical for binary Al–Cu alloys form additionally in the Si-containing alloy. In the Ag-bearing alloys, precipitation of the hardening X′ phase occurs the earlier the higher the Ag content; it is followed by S′ precipitation. During heating of the ternary alloy, the S′ phase forms after substantial dissolution of GPB zones and of the S′′ phase identified by high resolution electron microscopy; this contradicts the concept of a continuous precipitation sequence.     

Ferromagnetic order in silicon-manganese alloys with phase separation

A phenomenological model of high-temperature ferromagnetism in silicon-manganese alloys has been proposed taking into account phase separation in these alloys, where manganese-rich particles of the secondary phase (precipitate MnSi_{2 − z} with z ≈ 0.25–0.30) are formed inside a manganese-depleted matrix of almost pure silicon. Precipitate MnSi_{2 − z} is considered as the silicide MnSi_1.7 containing a certain number of magnetic defects whose origin is due to the presence of weakly hybridized 3d orbitals of manganese. The silicide MnSi_1.7 is a weak band ferromagnet in which strong fluctuations of the spin density (paramagnons) are present at a temperature much higher than its Curie temperature. It has been shown that the ferromagnetic exchange interactions between the magnetic moments of defects in precipitate exists due to thermal excitations of the spin density and the ferromagnetic order can appear at a temperature much higher than the Curie temperature of the silicide. The spatial structures and characteristics of this order have been described in the framework of the proposed approach for both homogeneous bulk precipitate and precipitate particles of various shapes and sizes. The short-range magnetic order near the bulk phase transition has been analyzed taking into account inhomogeneities in the distribution of magnetic defects in precipitate. The experimental data on the magnetic properties of silicon-manganese alloys have been interpreted in terms of the theoretical results obtained in this work.     

相场法研究Fe-Cu-Mn-Al合金富Cu相析出机制

基于Ginzburg-Landau理论采用连续相场法模拟了Fe-15%Cu-3%Mn-x Al(质量分数x=1%, 3%, 5%)合金在873 K等温时效时纳米富Cu析出相沉淀机制及Al含量对富Cu相析出的阻碍效应.通过计算成分场变量和结构序参数,研究了富Cu析出相的形貌、颗粒密度、平均颗粒半径、生长和粗化动力学.研究结果表明:在时效早期阶段,纳米富Cu相通过失稳分解机制析出,由于原子扩散速率存在差异,从而形成以富Cu相为核心的核壳结构.随着时效时间延长,富Cu相析出物结构由体心立方转变为面心立方.其中Al和Mn原子在富Cu核外偏析形成Al/Mn簇,可以将其视为阻碍富Cu析出相形成的缓冲层;在沉淀过程中,随着Al含量的增大, Al/Mn金属间相促进了缓冲层的生长,阻碍富Cu析出相的生长和粗化.     

Mechanisms of high-temperature creep of nickel-based superalloys under low applied stresses

[001] single-crystal specimens of the superalloys CMSX-4 and CMSX-10 were tested for creep at 1100°C under tensile stresses between 105 and 135?MPa, where they show pronounced steady creep. The deformed superalloys were analysed by density measurements, scanning electron microscopy and transmission electron microscopy which supplied information about porosity growth, evolution of the γ–γ′ microstructure, dislocation mobility and reactions during creep deformation. It is shown that, under the testing conditions used, steady creep strain mostly results from transverse glide–climb of (a/2) ?011? interfacial dislocations. A by-product of the interfacial glide–climb are vacancies which diffuse along the interfaces to growing pores or to a ?100? edge dislocations climbing in the γ′ phase. Climb of a ?100? dislocations in the γ′ phase is a recovery mechanism which reduces the constraining of the γ phase by the γ′ phase, thus enabling further glide of (a/2) ?011? dislocations in the matrix. Moreover the γ′ dislocations act as vacancy sinks facilitating interfacial glide–climb. The creep rate increases when the γ–γ′ microstructure becomes topologically inverted; connection of the γ′ rafts results in extensive transverse climb and an increase of the number of a?100? dislocation segments in the γ′ phase.