Phase transformation and magnetic properties of annealed Fe52.2Co46V1.8 alloy

In this work, FeCoV alloys were annealed in vacuum at various temperatures for different time, with and without applying an external magnetic field, respectively. The effect of the annealing on their magnetic properties has been investigated in regard with microstructural characterizations. X-ray diffraction analysis shows that the alloy is characterized of α-Fe bcc structure. It has been found that an order-disorder phase transformation took place locally in the surface around 993 K. In addition, α-γ phase transformation occurs at the same temperature at which the Curie point is observed in the TG curve. Coercivity is decreased with increasing annealing temperatures, and changed from about 3.2 kA/m to 78 A/m when the annealing temperatures are increased to 1173 K. Microstructural observations show that the decrease of coercivity after annealing is contributed mainly by grain growth. The coercivity of the alloy is further decreased by 32 A/m after magnetic field annealing at 1033 K. Hysteresis loops of the alloys after field annealing at 1033 K featured more rectangular shape with smaller coercivity, compared with those after vacuum annealing.     

Production of superconducting Cu-Nb-Sn alloy by hot extrusion of rapidly solidified powder

A Cu_92.5Nb₅Sn_2.5 alloy has been rapidly quenched into a powder by ultrasonic gas atomization. The structure of the powder consists of finely dispersed Nb particles in a Cu-Sn matrix. The powder has been consolidated by hot extrusion at 650°C. The extrusion does not change the phases present in the material appreciably but annealing for 100 h at 650°C causes a transformation of the Nb particles to Nb₃Sn. The extruded material shows superconductivity. The critical temperatureT _c, of the asquenched alloys is 8.0 K but increases to 15.6 K after annealing for 100 h at 923 K. The upper critical magnetic field, \(H_{C_2 } \) , and critical current density,J _c, were 3.4 kOe at 4.2 K and 7.4×10⁵ Am^?2 at zero applied field and 4.2 K, respectively. The appearance of superconductivity is attributed to the proximity effect of the Nb₃Sn phase particles which are sufficiently well distributed to satisfy the leak distance and the critical particle size.     

Metastable order-disorder transition and sigma phase formation in Fe-V binary alloys

The influence of binary and ternary alloying compositions and heating rate on theσ formation reaction are studied. Several ways are developed to supress the stableα phase formation and to understand the metastable CsCl-type order-disorder transition in Fe-V alloys. The Fe-V ordered phase has a metastable critical transition temperature of between 850 and 880° C at equiatomic composition, and the so called “650° C anomaly” corresponds to the 550° C anomaly in Fe-Co alloys. Theσ formation reaction does not commence most easily at equiatomic composition but at the iron-rich side of the 40 at. % V alloy. The Influence of ternary alloying additions on theσ formation in Fe-V alloys depends on the relative stabilities of the binaryσ phase with iron or vanadium.     

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.     

Mechanical properties of ductile Fe-Ni-Zr and Fe-Ni-Zr (Nb or Ta) amorphous alloys containing fine crystalline particles

Melt-quenched Fe_60–80Ni_10–30Zr₁₀ and Fe₇₀Ni₂₀Zr_10–x (Nb or Ta) _x (x2 at %) alloy ribbons with the duplex structure consisting of amorphous and bcc phases were found to exhibit hardness and tensile strengths higher than those of the totally amorphous alloys. The volume fraction of the bcc phase was intentionally allowed to alter in the range 0% to 60% by changing the composition and sample thickness. The bcc phase has an average particle size of 75 nm for the Fe-Ni-Zr alloys and 50 nm for the Fe-Ni-Zr-Nb alloys, and the lattice parameter is much larger than that of pure -Fe because of the dissolution of large amounts of zirconium, niobium and/or tantalum. The hardness and tensile strength of the duplex alloys increase with amount of bcc phase and reach about 880 DPN and 2580 MPa, which are higher by about 20% to 30% than those of the amorphous single state, at an appropriate volume fraction of bcc phase. As the volume fraction of the bcc phase increases further, the duplex alloys become brittle and the tensile strength decreases significantly. The enhancement of strength was considered to be due to the suppression of shear slip caused by fine bcc particles dispersed uniformly in the amorphous matrix. It was thus demonstrated that an optimum control of melt-quenched structure results in the formation of ductile Fe-based amorphous alloys containing fine crystalline particles.     

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).     

Structural transition and atomic ordering of Ni49.8Mn28.5Ga21.7 ferromagnetic shape memory alloy powders prepared by ball milling

The Ni_49.8Mn_28.5Ga_21.7 powders of micro-scale irregular equiaxial particles are prepared by ball milling method, and characterized by XRD, DSC and SEM techniques. The powders are found to contain disordered fct structure. Upon heating to high temperatures, the crystal structure of the as-milled powder is found to evolve from disordered fct to disordered bcc and then to a Heusler-type structure sequentially. The critical temperature for the transition from the bcc phase to the Heusler phase is 360 °C. This phase transition process is also a disorder–order transition. An atomic ordering model similar to the grain nucleation and growth is established to explain the annealing temperature dependence of the phase transformation temperature.     

Synthesis and Mossbauer studies of mesoporous γ-Fe2O3

A method of synthesis of mesoporous γ-Fe₂O₃ by thermal decomposition of iron citrate has been proposed. Investigations of the crystalline and magnetic structure of obtained materials were done. Nanodispersed maghemite (γ-Fe₂O₃) with the sizes of coherent scattering regions of about 4–7 nm consisted of one phase only after gel sintering at 200, 250 and 300 °C. The particles of synthesized materials were both in magnetically ordered, and superparamagnetic states, and they formed a grid-like mesoporous structure. The influence of magnetic dipole interparticle interaction on the parameters of Mossbauer spectra was observed. A phenomenological model of the differences between nanodispersed γ-Fe₂O₃ magnetic microstructures obtained after annealing at different temperatures was presented.     

The effect of Ta on structure and magnetic properties in Fe–N films

Microstructure and magnetic properties of Fe–Ta–N alloy films near the eutatic composition were studied. The four systematic alloy films with different Ta content were prepared by reactive sputtering. The dependence of structures and magnetic properties on Ta and annealing were investigated by VSM and X-ray diffraction. It is found that Ta atoms replace Fe in α-Fe lattice and have strong affinity for nitrogen, which inhibits the formation of γ-Fe₄N phase in Fe–Ta–N films. The TaN phase precipitates in grain boundaries and suppresses the growth of α-Fe(N) crystalline during annealing. Coercivity varies with the change of microstructure.     

Interfacial characteristics of Zn/AZ31/Zn clad sheets fabricated by hot roll-bonding and annealing

The Zn/AZ31/Zn tri-layer clad sheets are prepared by hot roll-bonding technology and post-roll annealing in this study and the interfacial microstructure, mechanical and corrosion properties are investigated. The annealing temperature can influence the interfacial bonding behaviour. Upon annealing at 200°C, a good metallurgical bonding interface composed of Mg₄Zn₇ and MgZn₂ phases is obtained. A higher temperature (300°C) can induce the precipitation of the brittle Mg₂Zn₁₁ phase at the interface. The clad sheets annealed at 200°C can attain the maximum bonding strength among all the clad sheets investigated. Importantly, the corrosion rate can be obviously reduced by cladding pure Zn layer on both sides of AZ31 alloys, which provides another effective method to improve the corrosion resistance of magnesium alloys.     

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.     

Oxidation behaviour of an intermetallic Ti–Al–Cr–Zr bond coat on a γ–TiAl based TNB alloy with 7YSZ thermal barrier coating

Abstract

Intermetallic titanium aluminide alloys are attractive light-weight materials for high temperature applications in automotive and aero engines. The development of γ-TiAl alloys over the past decades has led to their successful commercial application as low pressure turbine blades. The operating temperatures of γ-TiAl based alloys are limited by deterioration in strength and creep resistance at elevated temperatures as well as poor oxidation behaviour above 800 °C. Since improvement in oxidation behaviour of γ-TiAl based alloys without impairing their mechanical properties represents a major challenge, intermetallic protective coatings have aroused increasing interest in the last years.

In this work, a 10 μm thick intermetallic Ti–46Al–36Cr–4Zr (in at.-%) coating was applied on a TNB alloy using magnetron sputtering. This layer provided excellent oxidation protection up to 1000 °C. Microstructural changes in this coating during the high temperature exposure were extensively investigated using scanning and transmission electron microscopy. The coating developed a three-phase microstructure consisting of the hexagonal Laves-phase Ti(Cr,Al)₂, the tetragonal Cr₂Al phase and the cubic τ-TiAl₃ phase. After long-term exposure the three-phase microstructure changed to a two-phase microstructure of the hexagonal α₂-Ti₃Al phase and an orthorhombic body-centred phase, whose crystal structure has not yet been definitely identified. On the coating, a thin protective alumina scale formed. Applying this intermetallic layer as bond coat, thermal barrier coatings (TBCs) of yttria partially stabilized zirconia were deposited on γ-TiAl based TNB samples using electron-beam physical vapour deposition. The results of cyclic oxidation testing (1 h at elevated temperature, 10 min. cooling at ambient temperature) revealed a TBC lifetime of more than 1000 h of cyclic exposure to air at 1000 °C. The ceramic topcoat exhibited an excellent adhesion to the thermally grown alumina scale which contained fine ZrO₂ precipitates.     

Phase field simulation of morphology evolution and coarsening of γ′ intermetallic phase in Ni–Al alloy

Abstract

The morphology evolution and coarsening behaviour of γ′ (Ni₃Al) phase in Ni–Al alloys were investigated by using the Cahn–Hilliard and Ginzberg–Laudan phase field equations, and the composition, order parameter and average particles size evolution coupling with the internal elastic strain were studied. Cubic γ′ phase was presented firstly and then the rectangle particles were formed by means of coarsening; the mechanisms of γ′ phase coarsening are Ostwald ripening or coalescence of neighbouring particles. The average particle size of γ′ phase increases rapidly at the initial stage, and it slows down at the coarsening dominated stage. The average particle size follows a cubic law of ‘Lifshitz–Slyozov’ encounter modified theory in the coarsening regime.     

Experimental study and thermodynamic modeling of the Al–Co–Cr–Ni system

Abstract

A thermodynamic database for the Al–Co–Cr–Ni system is built via the Calphad method by extrapolating re-assessed ternary subsystems. A minimum number of quaternary parameters are included, which are optimized using experimental phase equilibrium data obtained by electron probe micro-analysis and x-ray diffraction analysis of NiCoCrAlY alloys spanning a wide compositional range, after annealing at 900 °C, 1100 °C and 1200 °C, and water quenching. These temperatures are relevant to oxidation and corrosion resistant MCrAlY coatings, where M corresponds to some combination of nickel and cobalt. Comparisons of calculated and measured phase compositions show excellent agreement for the β–γ equilibrium, and good agreement for three-phase β–γ–σ and β–γ–α equilibria. An extensive comparison with existing Ni-base databases (TCNI6, TTNI8, NIST) is presented in terms of phase compositions.     

Recrystallization,texture evolution,and magnetostriction behavior of rolled (Fe81Ga19)98B2 sheets during low-to-high temperature heat treatments

In order to study the texture evolution and magnetostriction behavior in the rolled Fe–Ga–B sheets during the heat treatments from low to high temperatures, (Fe₈₁Ga₁₉)₉₈B₂ sheets were prepared and investigated. The phase structure, recrystallization, grain size, texture evolution, and magnetostriction behavior during the annealing from 525 to 1200 °C for 1–5 h were investigated using X-ray diffraction, electron backscattering diffraction, and standard strain-gauge measurements. Results indicated that the primary recrystallization temperature for 1-h annealing was found as 525–575 °C in (Fe₈₁Ga₁₉)₉₈B₂ sheets. Annealing the sample below 575 °C for 1 h, the release of rolling stress and increase of 〈100〉 η-fiber texture during the primary recrystallization jointly resulted in a rapid improvement in magnetostriction. After annealed between 575 and 1100 °C for 1 h, the grains of the sheets underwent a normal growth, and the three (α-, γ- and η-fiber) types of textures kept an approximate balance, leading to a plateau of magnetostriction around 75 ppm. When the abnormal grain growth proceeded above 1100 °C for 1 h, the proportion of η-fiber texture markedly increased, and the magnetostriction was subsequently increased to 97 ppm. For longer annealing durations, the strong ideal cube texture (η-fiber) was firstly formed and then changed to undesired texture (γ-fiber), producing a corresponding magnetostriction peak of 136 ppm at 2 h for the annealing at 1200 °C. The clear correlation among heat treatments, recrystallization, texture, and magnetostriction provides an essential understanding for Fe–Ga–B alloy sheets.     

Superconductivity in Cu-V-Si alloys

The superconducting properties of copper ternary alloys containing small amounts of vanadium and silicon have been investigated. The wire samples of three alloys—Cu₉₀V_7.5Si_2.5, Cu₈₈V₉Si₃, and Cu₈₄V₁₂Si₄—show a resistive superconducting transition around 17 K, the characteristic transition temperature of the A15 compound V₃Si, followed by a plateau and a second transition between 5 and 10 K. The resistivity, however, does not drop to zero down to 2.5 K. The maximum drop in resistivity is observed for the alloy containing minimum amounts of vanadium and silicon. Complete super-conductivity is absent in these alloys for two reasons. First the effective superconducting volume fraction in these alloys is well below the threshold value obtained by either the effective medium theory or the site percolation theory for solid conduction. Second, the superconducting phase does not precipitate in a fine structure in the matrix and a strong proximity effect does not operate. Experimental evidence confirms the formation of brittle phase V₃Si in the cast alloys which does not get elongated to the same extent as the matrix when rolled or drawn. This reduces the effective volume fraction of V₃Si in the drawn wires. Annealing of these wires causes the superconducting particles to coalesce and grow in size, increasing the interparticle distances to the detriment of superconductivity in these alloys.     

Nitrogen-added Al rare-earth alloys for thin film transistors

The effects of nitrogen addition to aluminum-rare-earth alloys were investigated. Yttrium and gadolinium were employed as respective alloy components. The electrical properties and hillock densities of alloy films were investigated, and their nanostructures were studied by cross-sectional transmission electron microscopy. Nitrogen effectively decreases the grain size, and causes the columnar structure, that is generally present in aluminum-based alloys, to disappear. Nitric aluminum-rare-earth alloys have a strong resistance to hillock formation, and formed no micro-voids even after annealing at 450°C. An N₂ flow rate of 1.3–10% in the sputtering gas gives a low hillock density of 2.0E+1–7.7E−1 pcs/mm² after annealing at 350°C in both nitric Al-rare-earth alloys. In the case of patterned nitric aluminum-yttrium alloys, an N₂ flow rate of less than 1.3% causes large side-hillocks after annealing at over 350°C. As an optimum value, an N₂ flow rate of 2.5% results in a low hillock density of 1.9E+1 pcs/mm² and a low resistivity of 8.6 μΩ cm after annealing at 350°C.     

Precipitation in a nickel-beryllium alloy

Precipitation was studied in a nickel 1.89 wt% beryllium alloy over the temperature range 400 to 750° C. Below 650° C, the precipitation sequence is analogous to that in the widely studied Cu-Be system: $$\alpha (supersaturated solid solution) \to \alpha + GP zones \to \alpha + \gamma '_{NiBe} \to \alpha + \gamma _{NiBe} $$ where the equilibrium γ _NiBe phase is formed by a discontinuous reaction consuming the intermediate γ～' _NiBe. The morphology of the discontinuous γ _NiBe was too irregular to assign a habit plane. Numerous orientation relations were observed between the α _Ni and the discontinuous γ _NiBe. The kinetics of the reaction were found to follow a linear growth with time. The activation energy of 58±3 kcal mol^?1 is less than bulk diffusion of nickel but greater than grain boundary self diffusion. Above 650° C, the discontinuous reaction front is arrested before completion because the equilibrium γ _NiBe forms by continuous coarsening of the γ′ _NiBe phase. The continuous γ _NiBe has a habit plane of {113}_α. Three closely related orientations were observed at constant ageing parameters for the equilibrium continuous phase. Each has been separately proposed in the literature as a unique relation for the copper-beryllium system. For nickel-beryllium either all three are uniquely present or the accuracy of selected area diffraction technique is such that the small differences in orientation cannot be distinguished.     

Effect of annealing on microstructure and mechanical properties of bulk nanocrystalline Fe3Al alloy with 5 wt.% Cu prepared by aluminothermic reaction

Microstructure and mechanical properties of bulk nanocrystalline Fe₃Al based alloy with 5 wt.% Cu prepared by aluminothermic reaction before and after annealed at 873, 1073 and 1273 K for 8 h were investigated. Microstructures of the alloy before and after the annealing consisted of a Fe-Al-Cu matrix, a little Al₂O₃ sphere and Fe₃AlC_x fiber phases. The matrix of the alloy before the annealing was composed a nanocrystalline phase with disordered bcc crystal structure and a little amorphous phase. The amorphous phase disappeared after the annealing and Fe₃Al phase with ordered DO₃ structure appeared in the alloy after annealed at 1073 and 1273 K in the matrix of the alloy. Size of the Fe₃AlC_x fiber phase increased with the annealing temperature. The alloy after the annealing had better plasticity, higher yield strength than that of the alloy before the annealing, and the alloy after annealed at 1273 K had the highest yield strength.     

Effect of annealing conditions and concentration of oxygen vacancies on vanadium doped SrBi2Ta2O9

This paper studied the annealing effects on the dielectric characteristics of vanadium doped SrBi₂Ta₂O₉ (SBT). SBT was synthesized at 1000 °C and vanadium doped SBT at 900 °C by solid-state reaction. Crystallization structure and phase purity of the prepared ceramic samples was observed by X-ray diffraction analysis. XRD analysis indicated a single layered perovskite structure without any secondary phases up to 15% of vanadium doping in SBT ceramics. Detailed dielectric study on vanadium doped SBT ceramics indicated that post-sinter annealing enhances the peak dielectric permittivity, which is attributed to the increased homogeneity in the system at atomic scale upon annealing. Annealing for larger time interval suppresses the permittivity growth beyond transition temperature which gives a direct evidence for the existence of lower valance state of vanadium (V⁺⁴) in as-sintered SBTV ceramics and also the permittivity growth is related to the oxygen ions or oxygen vacancies created during sintering. UV–Vis spectroscopy was also performed to confirm the lower valance state of the vanadium ions in the ceramics.