Effect of silicon on microstructure of Ti3Al alloys containing niobium

Abstract

The effect of silicon content on the microstructure and phases present in Ti–(20–23)Al–11Nb alloys has been studied in the temperature range 800–1270°C.Four phases, β_o, α₂, O, and a silicide, are formed. The parent β_o is ordered at 1270°C. At 1050°C α₂ is formed which exhibits a higher silicon solubility than the parent β_o. A peritectoid transformation β₀+silicide→α₂ is proposed. Assuming that niobium substitutes for titanium and silicon for aluminium, energy dispersive X-ray spectroscopic data suggest that the α₂ phase, unlike that in binary Ti–Al alloys, is highly stoichiometric and of the form (Ti+Nb)₃(Al+Si). Similarly the silicide corresponds to binary Ti₅Si₃ with the same site substitution as in the α₂ phase. The O phase is orthorhombic and similar in composition to the α₂ which it replaces: its formation is promoted by silicon.

MST/3088     

Evolution of microstructure and phase composition of Ti-3Al-5Mo-4.5V alloy with varied β phase stability

The microstructure evolution and phase composition of an α + β titanium alloy, Ti-3Al-5Mo-4.5V (wt.%), have been investigated. Electron probe micro analysis (EPMA) quantitative results manifest that the stability of β phase decreases with increasing quenching temperature, which is influenced by the significant variation of β-stabilizing elements concentration. Detailed microstructure analysis shows that the β → ω phase transformation does occur when quenching at 750 °C and 800 °C. The ω-reflections change from incommensurate ω-spots (750 °C) to ideal ω-spots (800 °C) as the β stability of the alloy decreases. Further the decrease of β phase stability encourages the formation of athermal α′′ martensite, which has the following orientation relationships: [111]_β//[110]_α′′, [100]_β//[100]_α′′ and [-110]_β//[00-1]_α′′ with respect to the β matrix.     

Calculated ternary diagram of Ti–Al–Si system

Abstract

The phase equilibria between β (body centred cubic, bcc), α (hexagonal closed packed, hcp), Ti₃ Al–α ₂ (hcp), and Ti₅ Si₃ (hcp) in the Ti–Al–Si system have been investigated in the temperature range 700–1200°C. Isothermal sections of the ternary phase diagram have been assessed employing thermodynamic software, which uses the compound energy model to describe the phases mathematically. Available experimental phase equilibria results on the Ti–Al–Si system were used to calculate missing binary and ternary interaction parameters and assess isothermal phase diagrams. Extrapolations in the resulting tie triangles indicate the existence of three eutectoid reactions in the Ti rich corner of the ternary diagram: β → α + Ti₅ Si₃ , α → α ₂ + Ti₅ Si₃ , and β → α ₂ + Ti₅ Si₃ . Additionally, extrapolations in the β + α₂ + α tie triangle observed at 1100°C indicate that two possibilities arise to represent a peritectoid reaction involving α, β, and α ₂phases: β + α₂ → α and β + α → α₂ , depending on the alloy composition and the effect of temperature on the solubitlity of Si in the α phase.     

Cyclic deformation and lattice strain distribution of high Nb containing TiAl alloy

Low cycle fatigue of lamellar TiAl with 8.5 at.-%Nb was studied with a total strain amplitude of 0.28% at three temperatures: room temperature, 750°C and 900°C. At room temperature, the material exhibited cyclic hardening and the fracture mode was mainly interlamellar. At 750°C and 900°C, the material showed cyclic softening and the fracture mode was translamellar. The lattice strain in γ phase was almost tensile and larger tensile lattice strain in γ phase seems detrimental. Besides, the opposite direction of {201}_γ and {100}_α2 lead to crack propagation along α₂/γ interfaces. B2/β_o phase always suffered compressive lattice strain in the tests. The destruction of lamellar microstructure was the reason for colony refinement at 750°C and 900°C.     

Effect of 2–6 at.% Mo addition on microstructural evolution of Ti-44Al alloy

In order to understand the effect of Mo alloying on the microstructural evolution of TiAl alloy, the as-cast microstructure, heat treated microstructure characteristic, and hot compression microstructure evolution of Ti-44 A1 alloy have been studied in this work. The as-cast microstructure morphology changes from(γ+α_2)lamellar colony and β/β_0+γ mixture structure to β/β_0 phase matrix widmannstatten structure,when Mo content increases from 2 at.% to 6 at.%. Affected by the relationship between β phase and αphase, the angles between the lamellar orientation and the block β/β_0 phase are roughly at 0°, 45° and90°. Comparing with heat treatment microstructure, the hot compression microstructure contains lessβ/β_0 phase, however, the β/β_0 phase containing 2 Mo alloy and 3 Mo alloy hot compressed at 1275 ℃ has the inverse tendency. In addition,(α_2 +γ) colony is decomposed by the discontinuous transformation.     

A Study on Thermal Properties and α(hcp) → β(bcc) Phase Transformation Energetics in Ti–5 mass% Ta–1.8 mass% Nb Alloy Using Inverse Drop Calorimetry

Accurate measurements of enthalpy increment (H _T ? H _298.15) values have been made on a Ti–5 mass% Ta–1.8 mass% Nb alloy using the inverse drop calorimetry technique in the temperature range from 463 K to 1457 K. The measured enthalpy increment values show a steady increase with temperature in both α-hcp and β-bcc solid solution regions. It is found that both the onset as well the completion of the α → β phase change are demonstrated by a marked deviation of the enthalpy increment behavior from the otherwise smooth variation encountered in the respective low-temperature α- and high-temperature β-phase domains. The transformation start (T _s) and finish (T _f) temperatures of the α → β phase change are found to be (1072±10) K and (1156±10) K, respectively. In the actual α → β phase transformation region, the variation of the enthalpy with the progress of transformation is found to follow a sigmoidal shape which is in line with the diffusive nature of the phase transformation. An estimation of the total enthalpy change associated with the α → β phase transformation (Δ°H _tr) has been made by assuming a simple diffusion limited kinetic model for the phase change. The net enthalpy change for the α → β transformation is found to be 76 J · g^?1. The measured temperature variation of the enthalpy increment in both α- and β-phase regimes are fitted to simple analytical functional forms to obtain temperature-dependent estimates of the specific heat, C _P . The total specific heat change associated with the α → β phase transformation ${\Delta^{\circ}{C_{P}^{\alpha}}^{\rightarrow{\beta}}}$ is estimated to be 904 J · kg^?1 · K^?1.     

Synthesis of magnetite nanoparticles by thermal decomposition of ferrous oxalate dihydrate

Two different polymorphs of ferrous oxalate dihydrate were synthesized by precipitation of ferrous ions with oxalic acid: α-Fe(C₂O₄) · 2H₂O with a monoclinic unit cell is obtained after precipitation and ageing at 90 °C, whereas the orthorhombic β-type is formed after precipitation at room temperature. The morphology of the oxalate crystals can be tailored from prismatic crystals of the α-polymorph over star-like aggregates of α/β-mixtures to non-agglomerated crystallites of β-oxalate. Thermal decomposition in air gives hematite at T ≥ 250 °C; if the thermolysis reaction is performed at low oxygen partial pressures (e.g., T = 500 °C and p _O2 = 10^?25 atm) magnetite is obtained. The synthesized magnetite is stoichiometric as signaled by lattice parameters of a ₀ = 8.39 Å. The thermal decomposition of ferrous oxalate is monitored by thermal analysis, XRD, and IR-spectroscopy. The morphology of the oxalate crystals is preserved during thermal decomposition; the oxalates are transformed into spinel particle aggregates of similar size and shape. The crystallite size of the magnetite particles increases with temperature and is 40 or 55 nm, if synthesized from β-oxalate at 500 °C or 700 °C, respectively. The saturation magnetization of the magnetite particles decreases with decreasing particle size. Since the particles are larger than the critical diameter for superparamagnetic behavior they display hysteresis behavior at room temperature.     

Phase transformation and sintering behavior of Ca2P2O7

The phase transformation and sintering behaviors of Ca₂P₂O₇ with different phase composition were investigated by using X-ray powder diffraction (XRD), dilatometery and scan electron microscopy techniques in this paper. It is found that although α-Ca₂P₂O₇ (high-temperature form) could be kept during its cooling process, it is metastable and retransformed into β-Ca₂P₂O₇ (low-temperature form) at about 950 °C during its reheating process. This reversible phase transformation was discussed from the point view of polyhedral distortion. The sample from β-Ca₂P₂O₇ calcined at 1000 °C/2 h densifies much faster than that from α-Ca₂P₂O₇ and α+β mixture, and bulk density as high as 98% TD can be obtained as it is sintered at 1150 °C. A dense α-Ca₂P₂O₇ free of micro-crack could not be obtained whatever from the powder of β-Ca₂P₂O₇ or α-Ca₂P₂O₇ or α+β mixture. Such different sintering behavior was explained in related to the reversible phase transformation between α- and β-Ca₂P₂O₇.     

Effects of C and Cr content on high-temperature microstructures of Fe–9Al–30Mn–xC–yCr alloys

This investigation elucidated the effects of C and Cr content on the high-temperature microstructures of Fe–9Al–30Mn–xC–yCr alloys by means of optical microscopy and transmission electron microscopy. With increasing Cr content, the phase transition sequence within the α phase was found to be α + B2 → α + B2 + DO₃ → α + DO₃. And with increasing C content, a γ → (γ + κ) phase transition was observed within the γ phase. The κ phase carbides ((Fe,Mn)₃AlC_x) had an ordered L′1₂-type structure with lattice parameter a = 0.368 nm and were formed by a spinodal decomposition during quenching. The amounts of Cr₇C₃ increased with the C and Cr content. Moreover, the Al and Mn content played important roles in expanding the (α + γ) region. These features have not been previously reported in the Fe–Al–Mn–C–Cr alloy system.     

Microstructure and oxidation behaviors of near-α Ti-6.5Al-4Sn-4Zr-0.5Mo-based alloys with Ir addition

The microstructure and oxidation behaviors of near α-Ti-based alloys with small amount of iridium (Ir) additions were investigated. The microstructure of both Ir-free and Ir-containing alloys was observed to consist of α + β Widmanstätten colonies. The β lamellae gradually became continuous with increasing Ir additions since Ir acted as a β-stabilizer in the alloys. Isothermal oxidation test indicated that Ir addition reduced the oxidation resistance at 650 °C; while at 750 °C, the adherence of thermally grown oxides was enhanced, and a thin Al₂O₃-enriched layer on the oxide scale was promoted in the Ir-containing alloy, which suggests that Ir addition was effective in improving oxidation resistance of near-α-based alloys at 750 °C.     

Microstructure and Texture Formation During Near Conventional Forging of an Intermetallic Ti–45Al–5Nb Alloy

Texture formation was studied in an intermetallic Ti‐45at%Al‐5at%Nb alloy after uniaxial compression and near conventional forging. Depending on the deformation conditions the texture of the γ‐TiAl phase is formed by pure deformation components, components related to dynamic recrystallization, or transformation components. This changing corresponds with microstructural observations. The α₂‐Ti₃Al and the α‐Ti(Al) phase show a similar texture as it is known for Ti and Ti‐base alloys after compressive deformation at elevated temperatures. In contrast to the γ texture, no significant change of the α/α₂ texture was observed in the temperature range between 800 °C and just below the α‐transus temperature (T_α = 1295 °C).     

Thermal decomposition of β-FeOOH

β-FeOOH particles were prepared by a forced hydrolysis of the 0.1 M FeCl₃ + 5·10⁻³ M HCl solution, whereas sulfated β-FeOOH particles were prepared by forced hydrolysis of the 0.1 M FeCl₃ solution containing 5·10⁻³ M quinine hydrogen sulfate (QHS). β-FeOOH particles, as well as sulfated β-FeOOH particles, were thermally treated up to 600 °C. The samples were characterized using DTA, XRD, FT-IR and TEM. β-FeOOH particles showed a cigar-type morphology, whereas bundles of β-FeOOH needles were obtained in the presence of QHS. Heating of β-FeOOH particles at 300 °C and above yielded α-Fe₂O₃ particles. Specific adsorption of sulfate groups showed a strong effect on the thermal decomposition of β-FeOOH particles. Upon heating of sulfated particles between 300 and 500 °C the formation of an amorphous phase and a small fraction of α-Fe₂O₃ were observed. Needle-like morphology of amorphous particles in these samples was preserved. At 600 °C, α-Fe₂O₃ particles were obtained; however, they were much smaller than those obtained by heating a pure β-FeOOH.     

Nuclear resonance investigation of hydrogen mobility in chromous acid (α-HCrO2)

Nuclear relaxation times (T₁ and T₂) for protons in α-HCrO₂ were found to be temperature independent between 24–400°C. The absence of motional narrowing requires H⁺ diffusion times to satisfy τ jump ? T₂ = 30 μsec at 400°C. Thus α-HCrO₂ does not appear promising as a hydrogen ion solid electrolyte.     

Homogeneous and heterogeneous precipitation mechanisms in a binary Mg–Nd alloy

Parallel modes of precipitation mechanisms in an Mg–Nd alloy were revealed by examining an isothermally annealed high pressure die cast alloy at 177 °C for up to 100 h. Broadly, precipitate evolution was observed to occur concurrently on dislocations and within the surrounding α-Mg matrix. However, it was observed that the presence of dislocation accelerated the precipitate formation kinetics significantly. In contrast to the accepted precipitation pathway in the Mg–Nd system, i.e., SSSS → GP zones → β″ → β′ → β₁ → β → β_e, dislocations were found to preferentially facilitate the formation of β′ and β₁ precipitates even at the very early stages (5 h) of annealing. Within the same time frame, a homogeneous distribution of Nd-rich pockets was observed throughout the α-Mg matrix, along with the β′ and β₁ precipitates decorating dislocation lines. Results further indicate that these Nd-rich regions initiated precipitation within the parent α-Mg matrix. The formation of these Nd-rich pockets was explained on the basis of a miscibility gap in the α-Mg phase at 177 °C. Our results demonstrate that the presence of dislocations influence strongly the phase-transformation pathways in Mg-rare earth alloys by facilitating the formation of selective precipitate phases.     

Microstructural evolution of grain refined in situ Sip/ZA27 composite during partial remelting

Abstract

The microstructural evolution of Si particle reinforced ZA27 (Si_p/ZA27) composite refined by Zr and rare earth was investigated during partial remelting. The phase transformations and the effect of the Si particles were also discussed. The results indicated that a semisolid microstructure with small and spheroidal primary particles could be obtained after being partially remelted. The microstructural evolution was divided into four steps, the initial rapid coarsening, structure separation, spheroidisation and final coarsening, which resulted from the phase transformations of α+η+?→β, η+β+?+Si→L and β→α′+L, Si+α′→L, and Si+α′→L and L→Si+α′ respectively. The variation of the primary particle size with heating time obeyed the Lifshitz, Slyozov and Wagner law during the final coarsening stage. The Si particles had no effect on the general microstructural evolution steps of the ZA27 matrix. But due to the effect of Si element on the as cast microstructure, the final coarsening step could be further divided into two stages. Furthermore, the evolution progress of the four steps was slowered due to the existence of the Si particles. The size of the Si particles first decreased due to their partial remelting and then coarsened because of Ostwald ripening, and their morphologies first became blunt and then spheroidal.     

Aspects of phase equilibria in Fe/Co/2.5 to 3.0% V alloys

Remendur alloys, one of which is used as the base material in remanent reed sealed contacts for the communications industry, are ternary alloys containing approximately equal iron and cobalt with 2 to 4 wt % vanadium. The available equilibrium diagram for this system does not provide precise positioning of the phase boundaries in the regions of commercial interest and, consequently, does not permit accurate determination of the amounts of the phases present or their compositions. This paper reports on the precise determination of five tie lines in theα ₁+γ two-phase field in the region of interest, by metallographic and microprobe techniques. In the 900 to 950° C range, this field was found to be narrower than expected from published data. Submicron fcc (γ) particles form during annealing at 600° C by decomposition of a non-equilibrium bcc (α₂) phase into a secondary ordered bcc phase (α′₁) and stabilizedγ. Deformation, by drawing and by stamping, enhances coercivity in these alloys by promoting a more uniform, more finely divided dispersion ofγ particles. Annealed microstructures are especially sensitive to vanadium content, annealing temperature, and annealing time.     

Sputtered austenitic manganese steel

Manganese steel of the typical Hadfield composition (1%C, 13%Mn) is well known for its remarkable properties of abrasion and wear resistance under loads of sufficient severity to cause work hardening. Hadfield steel coatings about 75 to 100 μm thick were deposited on tantalum and metallurgically polished copper substrates using a hollow cathode sputtering apparatus. Substrate temperatures ranged from 1000°C to those provided by liquid nitrogen cooling. X-ray diffraction measurements indicated that the occurrence of austenite (γ) and ferrite (α) phases in the as-deposited coatings was consistent with the equilibrium phase diagram: γ phase at 1000°C, γ + α from 600 to 400°C and α below 400°C. Metallographic cross sections revealed (1) that the structure varied from fibrous to columnar with increasing substrate temperature and (2) that the γ and the γ + α deposits included a dispersed iron carbide phase. Coatings deposited at temperatures above 400°C exhibited a moderate hardness (KHN ≈ 350 kg mm^?2). However, those deposited at lower temperatures (< 300°C) exhibited a distorted b.c.c. (martensite-like) structure and yielded hardnesses equivalent to the highest values reported for work-hardened Mn steel (KHN = 750 kgmm^?2).     

Ordering of interstitial nitrogen and oxygen in vanadium near to V8N and V8O

The ordered structure of the V₈N subnitride was studied by X-rays, electron diffraction and electron microscopy. V₈N exists in two different modifications (α′ and α″). The vanadium sublattice of both phases is pseudo-tetragonal, in reality triclinic, with lattice parameters: $\text{α}{}_{\mathrm{<mtext>o</mtext>}}\text{=}\text{b}{}_{\mathrm{o}}\text{= 3.114}\text{A}\text{?}$, $\text{c}{}_{\mathrm{o}}\text{= 2.994}\text{A}\text{?}$, α_o = β_o = 90.5^o ± 0.1^o, γ_o = 90^o. The proposed unit cell of α′-V₈N has dimensions: a = 2√2 a_o, c = 2c_o and corresponds to V₃₂N₄. Doublets of nitrogen atoms occupy two sets of octahedral cavities whose shortest axes are aligned along the x and y directions of the sublattice in an ordered fashion. The α″-V₈N phase is a periodically twinned modification of the α′-V₈N, the twin plane being of the (001) type. The V₈O suboxide has the same structure as the α″-V₈N, the sublattice parameters being: $\text{a}{}_{\mathrm{o}}\text{=}\text{b}{}_{\mathrm{o}}\text{= 3.11}\text{A}\text{?}$, $\text{c}{}_{\mathrm{o}}\text{= 2.994}\text{A}\text{?}$, α_o = β_o = 90.3^o ± 0.1^o, γ_o = 90^o.     

Cooling rate effects on the microstructure evolution in the βo zones of cast Ti–45Al–8.5Nb–(W,B, Y) alloy

The cooling rate effects on the β_o → ω_o phase transformation in Ti–45Al–8.5Nb–(W, B, Y) (at.%) alloy were investigated by annealing the alloy at 950 °C followed by different cooling methods. The morphology and the distribution of the ω-related phases were analyzed by TEM. The amount and morphology of the ω-related phases are very sensitive to the cooling rate. The ω-related phases could not be resolved in the water-quenched sample whereas it grew into nano-particles in the air-cooled sample. In the furnace-cooled sample, the ω_o phase with B8₂ structure grew into micron-sized particles and occupied the whole β_o area. The nucleation of the ordered ω embryos can be explained by the well accepted displacive mechanism because of the instability of the β_o(B2) structure, accompanied by a short-range diffusion process between neighboring {111}_βo planes. However, the growth of the ω-related phases is controlled by a long-range diffusion process. Due to ready nucleation and growth, the ordered ω formation is bound to occur in as-cast and heat-treated high Nb–TiAl alloys.     

Refinement of the structure of K2Cr2O7

The parameters for the structure of K₂Cr₂O₇ have been refined from 7511 observed reflections; $\text{a}{}_{\mathrm{o}}\text{= 7.4200(6)}\text{A}\text{?}$, $\text{b}{}_{\mathrm{o}}\text{= 13.399(3)}\text{A}\text{?}$, $\text{c}{}_{\mathrm{o}}\text{= 7.3845(9)}\text{A}\text{?}$, cosα = ?0.1396(2) (α = 98°2'), cosβ = ?0.0154(2) (β = 90°53'), cosγ = ?0.1078(2) (γ = 96°11'). The discrepancy factor R(F_o²) = 0.0702 with a type 2 extinction correction. The average CrO distances are 1.609Å (unshared) and 1.783Å (shared).