Effect of thermal fatigue on the structure and properties of Ni3Al-based alloy single crystals

The effect of thermal fatigue during tests of 〈001〉 and 〈111〉 single crystals according to the schedules 100 ai 850°C, 100 ai 1050°C, 100 ai 1100°C at a peak-to-peak stress Δσ_tc = 700–1000 MPa (sum of the maximum tensile and compressive stresses in a thermal cycle) on the structure, the fracture, and the fatigue life of an Ni₃Al-based VKNA-1V alloy is studied. It is found that, at 10³ thermal cycles, the 〈111〉 single crystals have the maximum thermal fatigue resistance at the maximum cycle temperature of 850 and 1050°C, and the properties of the 〈001〉 and 〈111〉 samples are almost the same at the maximum thermal cycle temperature of 1100°C. After thermal cycling at the maximum temperature of 850°C, the γ layers in the two-phase γ′ + γ region in dendrites remain a single-phase structure, as in the as-cast material, and the layer thickness is 100–150 nm. When the maximum thermal cycle temperature increases to 1050 or 1100°C, the discontinuous γ-phase layers in the γ′(Ni₃Al) matrix change their morphology and become shorter and wider (their thickness is 300–700 nm). The nickel-based supersaturated solid solution in these layers decomposes with the formation of secondary γ′(Ni₃Al)-phase (γ′_sec) precipitates in the form of cuboids 50 and 100 nm in size at the maximum cycle temperature of 1050 and 1100°C, respectively. The alternating stresses that appear during thermal cycling cause plastic deformation. As in nickel superalloys, this deformation at the first stage proceeds via the slip of screw dislocations along octahedral {111} planes. Networks of 60° dislocation segments form at γ′/γ interfaces in this case. Fracture begins at the lines of intersection of the slip planes of the {111} octahedron with the sample surface. During fractional, a crack passes from one octahedral plane to another and forms terraces and steps (crystallographic fracture); as a result, the fracture surface bends and becomes curved. In all cases, the fracture surfaces have a mixed brittle-ductile character with a combination of crystallographic and ductile (dimple) fracture elements.     

Effect of directional solidification on the structure and properties of Ni3Al-based alloy single crystals alloyed with W,Mo, Cr,and REM

The effect of the solidification gradient (G = 60 and 150°C/cm) at a solidification rate R = 10 mm/min on the structural parameters and the short- and long-term strength characteristics of blade-type single-crystal workpieces made of a heterophase γ′ + γ VKNA-1V-type γ′(Ni₃Al)-based alloy with low contents of refractory metals is studied. The single crystals have a cellular-dendritic structure: dendrites are heterophase and consist of thin discontinuous nickel-based γ solid solution layers between γ′(Ni₃Al)-matrix regions. Primary γ′-phase precipitates are located in the interdendritic space. An increase in solidification gradient G from 60 to 150°C/cm (by a factor of 2.5) at a solidification rate R = 10 mm/min leads to a decrease in the dendrite arm spacing by ～1.5 times, the size of primary γ′-phase precipitates by 2.5–3 times, and the refinement of γ′ regions between γ layers in dendrite arms and at the periphery of dendrites by 2–3 times. The strength characteristics of the single crystals grown at G = 150°C/cm are higher than those of the single crystals grown at G = 60°C/cm by 10%. An increase in gradient G weakly affects the long-term strength of the single crystals. During long-term high-temperature tests under loading, secondary disperse γ _sec ^′ particles precipitate in the discontinuous γ solid solution layers forming inclusions in two-phase γ′ + γ dendrites, and the morphology of the γ layers changes (they become thicker and shorter). The 〈111〉 VKNA-1V alloy single crystals grown at G = 150°C/cm and R = 10 mm/min have a set of the required properties, namely, a high high-temperature strength over the entire temperature range, moderate high-temperature plasticity, and the absence of the plasticity drop at 800°C (which is characteristic of single crystals with other crystallographic orientations). These properties make 〈111〉 VKNA-1V alloy single crystals promising for working and nozzle gas turbine engine blades, including the blades in “blisk” assembly units.     

Improvement of room temperature ductility of stoichiometric Ni3Al by unidirectional solidification

Stoichiometric polycrystalline Ni₃Al was grown undirectionally by a floating zone method. The solidification structure was strongly dependent on the growth rate. A columnar grained structure with a strong 〈111〉 texture was obtained with the growth rate over 13 mm/h. The alloy exhibited a large tensile elongation of 17% at room temperature without the addition of boron. The combined mode of transgranular fracture and intergranular fracture was observed on the fracture surfaces, with traces of crystallographic slip. It is considered that the large room temperature ductility in this alloy is attributed to the substantial suppression of intergranular crack initiation and its catastrophic propagation.     

X-ray diffraction study of the structural evolution in a grade 1469 (Al-Cu-Li) alloy during the production of hot-rolled sheets

X-ray diffraction is used to study the structural changes in a grade 1469 (Al-Cu-Li) alloy with a high lithium content that occur during the production of hot-rolled sheets according to the ingot → pressed strip → hot-rolled sheet schedule. In the pressed strip, a multicomponent Bs {110}〈112〉, Cu {211}〈111〉, and S {123}〈111〉 texture forms, which is typical of articles pressed from such alloys, and an unusual intense single-component texture of the Ex₁ {011}〈111〉 type forms in the hot-rolled sheets. Its formation is stimulated by cross rolling of the sheets. The low strength characteristics of the hot-rolled sheets after heat treatment are related to an elevated heating temperature used for quenching and to the oriented precipitation of the lamellar particles of the hardening T ₁ phase.     

Rare-earth metals in nickel aluminide-based alloys: III. Structure and properties of multicomponent Ni<Subscript>3</Subscript>Al-based alloys

The possibility of increasing the life of heterophase cast light Ni₃Al-based superalloys at temperatures higher than 0.8T _m of Ni₃Al is studied when their directional structure is additionally stabilized by nanoprecipitates, which form upon additional alloying of these alloys by refractory and active metals, and using special methods for preparing and melting of an alloy charge. The effect of the method of introducing the main components and refractory reaction-active and surface-active alloying elements into Ni₃Al-based cast superalloys, which are thermally stable natural composite materials of the eutectic type, on the structure-phase state and the life of these alloys is studied. When these alloys are melted, it is necessary to perform a set of measures to form particles of refractory oxide cores covered with the β-NiAl phase and, then, γ′_prim-Ni₃Al phase precipitates during solidification. The latter phase forms the outer shell of grain nuclei, which provides high thermal stability and hot strength of an intermetallic compound-based alloy. As a result, a modified structure that is stabilized by the nanoprecipitates of nickel and aluminum lanthanides and the nanoprecipitates of phases containing refractory metals is formed. This structure enhances the life of the alloy at 1000 °C by a factor of 1.8–2.5.     

Influence of the initial concentration of hydrogen and modes of vacuum annealing on the structure, phase composition, and mechanical properties of sheet billet of the VT6 alloy: Part 1. Influence of the initial hydrogen concentration and modes of vacuum annealing on the phase composition and the average diameter of grain of the VT6 alloy

The possibilities of using reversible hydrogen doping in combination with warm rolling for milling the structural components and the formation of a heterophase structure are shown by the example of the VT6 alloy. Alloy doping with 0.3?C0.7 wt % hydrogen and the formation of a ?? + ?? + ??₂ structure is accompanied by the enhancement of the local nonuniformity of deformation and an increase in the number of recrystallization nuclei during warm sheet rolling. The relation between the initial hydrogen concentration and the temperature of vacuum annealing (VA) with the phase composition and the size of structural components is established.     

Structure–phase characteristics and the mechanical properties of single-crystal nickel-based rhenium-containing superalloys with carbide–intermetallic hardening

Single crystals of rhenium-containing ZhS32-VI, ZhS32U nickel superalloys with the 〈001〉, 〈011〉, and 〈111〉 crystallographic orientations have been produced by directional solidification. The alloying element segregations and the thermal stability of the microstructure consisting of a γ solid solution and hardened by precipitates of the γ′ phase and MC carbides are studied. The crystal lattice parameters of the γ′ and γ phases; the γ/γ′ misfit; and the liquidus, solidus, and γ′-solvus temperatures of the alloys have been found. The temperature dependence of the γ′-phase solubility has beeisn determined. The temperature–orientation dependences of the tensile strength characteristics in the range 20–1150°C and the low-cycle fatigue at 850°C of the alloy single crystals with the 〈001〉, 〈011〉, and 〈111〉 orientations are presented.     

Textures and plastic anisotropy in γ-TiAl

A polycrystalline alloy of composition Ti-36 wt % Al consisting mainly (about 95 vol. %) of γ-TiAl has been deformed in compression at 450°C as well as in rolling at 1040°C. The textures of the deformed specimens were measured and analyzed in terms of orientation distribution functions (ODFs). The textures after hot rolling show a cube-like component (100) [010] with an alignment of the c-axis with the transverse direction. A comparison of measured compression textures with those simulated on the basis of the Taylor theory of polycrystal deformation leads to the following conclusions. Both the “easy” {111} 〈110〉 and “hard” {111} 〈101〉-slip modes of deformation occur in γ-TiAl at 450°C. The critical resolved shear stresses (CRSSs) for these two slip modes differ by a factor of less than 2, the CRSS for {111}〈110〉-slip being higher than that for {111}〈101〉-slip. The rolled specimens show a pronounced plastic anisotropy, which can only be explained on the basis of microstructural considerations.     

Phase and structural transformations in corrosion-resistant steels upon high-pressure torsion and heating

High-pressure torsion (HPT) at a pressure of 6 GPa and room temperature is found to form a nanocrystalline structure in corrosion-resistant austenitic 05Kh15N9D2TAMF and 08Kh18N10T steels and a submicrocrystalline structure in corrosion-resistant ferritic 08Kh18T1 steel and armco iron. X-ray diffraction analysis of both austenitic steels reveals the γ → α and γ→ ?→ α martensitic transformations during HPT at room temperature. After HPT, the strain hardening in the austenitic and ferritic steels is approximately the same and mainly determined by nano- and submicrocrystalline structures, and the role of alloying and phase composition weakens. The thermal stability of the hardening in the austenitic and ferritic steels is almost the same, ～400°C. As a result of HPT, the austenitic 08Kh18N10T and ferritic 08Kh18T1 steels acquire an axial texture with the predominant 〈211〉_γ direction in austenite and the 〈110〉_α and 〈311〉_α directions in martensite and ferrite, respectively. The axial texture is retained in both steels up to a heating temperature of 750°C.     

Effect of the method of producing Ni3Al-based alloy single crystals on the macro- and microhomogeneity of component distribution,structure, and properties

The mechanisms of hardening heterophase Ni₃Al-based cast alloys, which are thermally stable natural eutectic composites, are studied in the operating temperature range. The distribution of basic and alloying elements and impurities in macrovolumes along the height of a charge billet prepared in a vacuum induction furnace is analyzed. The effect of the deviation of the axis of intermetallic alloy single crystals from the 〈111〉 orientation on their mechanical properties is considered. It is shown that the deviation from this orientation within 2.5°–5.4° does not affect the short-term strength characteristics and substantially affects the ductility characteristics of the single crystals. The effect of the method of introducing basic components and refractory reaction- and surface-active alloying elements in the alloys on the structure-phase state of Ni₃Al-based alloys and their service life is investigated.     

Effect of Ca and Rare Earth Additions on the Texture,Microhardness, Microstructure and Structural Properties of As-Cast Mg–4Al–2Sn Alloys

In this work, alloys with nominal composition of Mg–4Al–2Sn–xRE–yCa (where RE = rare earth, x = 0, 1, 3 and y = 0, 1) have been prepared using tilt casting method. Prepared as-cast samples have been investigated by X-ray diffractometry, optical microscopy and scanning electron microscopy techniques as well as hardness measurement. The texture, microstructure and structural parameters of samples were also refined from X-ray diffraction patterns utilizing the Rietveld method and generalized spherical-harmonic model. It was found that with addition of rare earth and calcium elements, intermetallic phases of γ-Mg₁₇Al₁₂ and β-Mg₂Al₃ disappeared in cast alloys while small amount of Al₁₁RE₃ and CaMgSn intermetallics phases are formed. The texture factor of α-Mg as a main phase of samples was decreased with addition of rare earth up to 1 % and increased with more addition of rare earth elements. According to the results, with addition of rare earth elements, texture of Mg phase changes from 〈112〉 direction to 〈100〉 and 〈002〉 directions while Ca addition causes the texture in 〈002〉 direction. The microhardness of Mg–4Al–2Sn alloy was enhanced with addition of rare earth and calcium elements which is in agreement with the expected trend based on computed phase fraction of the samples. Addition of 1 wt% of calcium causes a dramatic change in the morphology and chemical composition of intermetallic phases, from acicular shape with composition of Al₁₁RE₃ into fine feather of CaMgSn intermetallic phase which accumulated in cluster morphologies in interdendritic regions.     

The atomic structure of A1-B2 interfaces in a NiBe alloy

The atomic structure of the interfaces between a Ni-based solid solution with f.c.c.-structure (α-phase) and β-precipitates (NiBe with B2-structure) in a Ni-12 at.% Bi alloy has been investigated by field ion microscopy (FIM) and computer simulation of the images. The precipitates were found to be plate-shaped with an aspect ratio of about 10. Most of the plates were formed roughly but not exactly on (001)-planes of the matrix. The phase boundary is extremely narrow and does not exceed 2–3 atomic layers in thickness. The orientation relationship between α- and β-phases was found to be the Baker-Nutting one, where (001)_α//(001)_β with 〈110〉_α//〈110〉. The observed morphology of the precipitates and the structure of the interfaces were analysed on the basis of an anisotropy of the interfacial energy, the effect of the elastic strains set up during the phase separation and the growth mechanism of the precipitates.     

Features of the structure of porous titanium nickelide during reaction sintering with an aluminum additive

Porous cylindrical samples of a titanium nickelide-based alloy containing 0.5, 1.5, and 2.0 at % Al of 2.5 × 30 mm in size were obtained by two-stage sintering. The first stage was solid-phase sintering at t ≥ 900°C, and the second stage was liquid-phase sintering at t > 1000°C. Starting from existing notions of the reaction diffusion in the Ti-Ni-Al system, the structure of the obtained alloy is analyzed. It is established by scanning electron microscopy and electron probe microanalysis that a multiphase alloy is formed during the first sintering. This alloy contains isolated regions of the TiNi phase, the isolation of which prevents the propagation of the front of the martensite phase transformation in the ready sample. The heterophase structure of the Ti₅₀Ni_{50 − x} Al _x porous alloy, which is formed after the first sintering, becomes more uniform after carrying out the second one at a higher temperature.     

Structural Changes of High-Temperature Nickel Alloy Containing Rhenium and Lanthanum on Heat Treatment

The properties of high-temperature nickel alloys for manufacturing depend on the thermal stability of the structure, the particle size, the shape, the quantity of strengthening γ' phase, and the strength of the γ solid solution. Such alloys are strengthened by the addition of rhenium and lanthanum. In the present work, the structure and phase composition of high-temperature nickel alloy with added rhenium (0.4 at %) and lanthanum (0.006 at %) are qualitatively and quantitatively investigated. The methods employed are transmission diffraction electron microscopy and scanning electron microscopy. The alloy structure is considered in three states: after directed crystallization (the initial state, sample 1); after directed crystallization, annealing at 1150°C for 1 h, and annealing at 1100°C for 480 h (sample 2); and after directed crystallization, annealing at 1150°C for 1 h, and annealing at 1100°C for 1430 h (sample 3). Primary and secondary phases are observed in the superalloy. The primary phases are γ' and γ. They form the structure of the alloy and are present in the form of γ' quasi-cuboids separated by γ layers. The secondary phases due to the presence of rhenium and lanthanum are β NiAl, AlRe, NiAl₂Re, σ, χ, and Ni₃La₂. The secondary phases seriously disrupt the structure of the γ + γ' quasi-cuboids. The rhenium and lanthanum do not uniformly fill the whole alloy volume, but only appear in local sections. Therefore, in all three states of the alloy, only some volume of γ + γ' quasicuboids is disrupted. Analysis of the secondary phases’ morphology shows that the σ particles are thin needles, whereas the Ni₃La₂ particles have internal structure with characteristic contrast and are relatively thick. Interestingly, the σ phase and Ni₃La₂ are deposited at the same locations. The introduction of rhenium and lanthanum changes the phase composition of the alloy, suppressing the formation of γ phase. The particles of secondary phase are localized in individual sections of the alloy with specific periodicity. The secondary phases are refractory: the melting point is about 1600°C for β phase, 2600°C for σ phase; and 2800° for χ phase. Thanks to the formation of refractory secondary phases and their periodic distribution in the structure, the strength of the superalloy with added rhenium and lanthanum is increased.     

摩擦对IF钢铁素体区热轧和退火织构的影响

采用X射线衍射仪分析IF钢铁素体区热轧织构以及退火织构的演化,在实验室热轧机上进行了IF钢的铁素体区热轧,研究了摩擦对IF钢铁素体区热轧、退火织构的影响。结果表明：无润滑轧制时,钢板表层形成强高斯织构组分{110}〈001〉,弱γ纤维织构,导致再结晶织构中高斯组分强度高,γ纤维织构强度低;润滑轧制时,钢板表层高斯织构组分强度降低,{100}〈011〉、γ纤维织构强度提高,退火后γ纤维织构强度提高。钢板中心受摩擦作用影响较小,轧制过程中发展为较强的α和γ纤维织构,退火后γ纤维织构成为主要织构组分。     

Fracture toughness of 304 stainless steel in an 8 tesla field

Metastable austenitic steels are often used as structural materials in superconducting magnets, in which they are subject to high magnetic fields at 4.2 K. The most common structural alloy is 304 stainless steel. Recent work on the fracture toughness of 304 indicates that the toughness may change significantly at 4.2 K in a high magnetic field. In this study, an increase in the fracture roughness of 304 specimens tested at 4.2 K in an 8T magnetic field is observed relative to the fracture toughness of specimens tested in 0 T. Part of this increase is ascribed to martensite (α′) transforming ahead of the crack tip during the J_Ic test.     

Mobile dislocations at the α2/γ phase boundaries in intermetallic TiAl/Ti3Al-alloys

Ti₃Al/TiAl interfaces of four titaniumaluminium alloys with and without chromium additions were examined in detail by tilting experiments using conventional transmission electron microscopy (TEM). Careful adjustment of weak beam conditions showed that the TiAl- as well as the Ti₃Al- phase contain interfacial dislocations which accommodate the lattice misfit between both phases. In the most ductile TiAl47Cr1Si0.2 alloy only one set of 〈10〉 interfacial dislocations in the TiAl phase was found. Most of them possess screw character and will leave the interface during plastic deformation. Consequently, this alloy exhibits remarkable ductility at room temperature.     

Structural heat-resistant β-NiAl + γ′-Ni<Subscript>3</Subscript>Al alloys of the Ni–Al–Co system: I. Solidification and structure

When analyzing the ternary Ni–Al–M phase diagrams, where M is a group VI–VIII transition metal, we chose the Ni–Al–Co system, where the γ′ and γ phases are in equilibrium with the β phase, as a base for designing alloys with the following physicochemical properties: a moderate density (≤7.2 g/cm³) and satisfactory heat resistance at temperatures up to 1300°C. The structure formation in heterophase β + γ′ alloys during directional solidification is studied. It is found that, in contrast to cobalt-free β + γ′ alloys (where the γ′-Ni₃Al aluminide forms according to the peritectic reaction L + β ? γ′), the alloys with 8–10 at % Co studied in this work during directional solidification at 1370°C contain the degenerate eutectic L ? β + γ. The transition from the β + γ field to the β + γ′ + γ field occurs in the temperature range 1323–1334°C, and the γ′ phase then forms according to the reaction β + γ ? γ′.     

Comparison of the Effect of Individual and Combined Zr and Mn Additions on the Fracture Behavior of Al-Cu-Li Alloy AA2198 Rolled Sheet

The effect of individual and combined addition of dispersoid-forming alloying elements Zr and Mn on the fracture behavior of the Al-Cu-Li alloy 2198 has been investigated by the Kahn tear test. Overall, the standard baseline 2198 alloy containing only Zr exhibited the best performance, while the alloy with the combined presence of Zr and Mn was slightly inferior. The lowest properties were seen for a Zr-free 2198-0.4Mn alloy variant. In the T351 temper fracture initiated at coarse constituent particles that formed large cavities and microvoid sheets linked the initial sites of void growth. In the Mn-containing alloys microvoids clearly nucleated at the coarser Al₂₀Cu₂Mn₃ dispersoids within the microstructure, while this was not identifiable for the finer coherent Al₃Zr dispersoids. However, this difference in the mechanism of cavity linkage had little effect on the overall toughness of the materials, which was more closely related to the effect of Mn and Zr on the level of recrystallization. Extended artificial aging promoted grain boundary decohesion due to the precipitation of high densities of T₁ particles on GBs and favored a cleavage fracture mode. Particle decohesive fracture was also promoted by T₁ precipitation on the Mn dispersoids.