Single-phase Field Research Articles

Isothermal transformations in an Fe-9 pct Ni alloy

Isothermal transformation from austenite in an Fe-9.14 pct Ni alloy has been studied by optical metallography and examination by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the temperature range 565 °C and 545 °C, massive ferrite (α q ) forms first at prior austenite grain boundaries, followed by Widmanstatten ferrite (α W ) growing from this grain boundary ferrite. Between 495 °C and 535 °C, Widmanstatten ferrite is thought to grow directly from the austenite grain boundaries. Both these transformations do not go to completion and reasons for this are discussed. These composition invariant transformations occur below T 0 in the two-phase field (α+γ). Previous work on the same alloy showed that transformation occurred to α q > and α W on furnace cooling, while analytical TEM showed an increase of Ni at the massive ferrite grain boundaries, indicating local partitioning of Ni at the transformation interface. An Fe-3.47 pct Ni alloy transformed to equiaxed ferrite at 707 °C ±5 °C inside the single-phase field on air cooling. This is in agreement with data from other sources, although equiaxed ferrite in Fe-C alloys forms in the two-phase region. The application of theories of growth of two types of massive transformation by Hillert and his colleagues are discussed.

Simulation of ferrite growth in continuously cooled low-carbon iron alloys

The growth of a planar ferrite (α): austenite (y) boundary in low-carbon iron and Fe-Mn alloys continuously cooled from austenite through the (a + y) two-phase field and the a single-phase field was simulated by incorporating carbon diffusion in austenite, intrinsic boundary mobility, and the drag of an alloying element. At a very high cooling rate (≥ 10 3 °C/s), the width of the carbon diffusion spike in austenite approaches the limit at which spikes are viable, so that the growth of ferrite in which carbon is not partitioned can occur even above the a solvus. In this context, the upper limiting temperature of partitionless growth of ferrite is the T 0 temperature. In the presence of drag of an alloying element, e.g., Mn, both carbon-partitioned and partitionless growth of ferrite begins to occur at finite undercoolings from the Ae 3 , T 0 , or α-solvus temperature, at which the driving force for transformation exceeds the drag force. The intrinsic mobility of the a: γ boundary may play a significant role at an extremely high cooling rate (≥10 5 °C/s).

Nucleation of embryos of a second phase by individual impurity atoms

Perturbed angular correlation of gamma rays (PAC) was used to monitor the atomic environments of indium atoms in Ni–Al alloys at temperatures and compositions in the two-phase field between Ni 2Al 3 and NiAl. In earlier PAC measurements at room temperature, the fraction of signal that is characteristic of the Ni 2Al 3 phase greatly exceeded the fraction predicted by the lever-rule, and it was suggested that indium could nucleate small embryos of the Ni 2Al 3 phase in the NiAl single-phase field. Measurements at elevated temperatures are now reported. PAC signals from the Ni 2Al 3 phase are observed to extend into the NiAl single-phase field from 0.5 to 1.0 at.% beyond published phase boundaries that were established by experiments on samples prepared similarly to those in this study. Thus, the new observations support the hypothesis that individual solute atoms can nucleate small crystals of a second phase within a single-phase field. In addition, phase boundaries were calculated from the PAC data, assuming that the lever rule was obeyed, and compared with boundaries from the literature.

Phase composition and microstructure of polycrystalline and epitaxial TaN x layers grown on oxidized Si(001) and MgO(001) by reactive magnetron sputter deposition

TaN x is presently used in a variety of hard coating, wear-resistant, and diffusion barrier applications. However, the Ta–N system is inherently complex with more than 11 reported equilibrium and metastable phases and there has been little systematic study of the synthesis of these materials. Here, we report the results of an investigation of the ultrahigh vacuum reactive magnetron sputtering of Ta as a function of the N 2 fraction f N 2 in mixed Ar/N 2 discharges together with the phase composition and microstructure of TaN x layers grown on MgO(001) and oxidized Si(001). Unlike the Ti–N system, for which TiN is the terminal phase, the abundance of N-rich phases in the Ta–N system results in the film deposition rate R and the N/Ta ratio of as-deposited layers varying continuously with f N 2 , even for values > f N 2 *, where f N 2 * is the N 2 fraction corresponding to the maximum rate of N 2 uptake by deposited Ta. Phase composition results are summarized in a phase map plotted as a function of film growth temperature T s (100–800°C) and f N 2 . In pure Ar, the films are tetragonal β-Ta at T s<150°C, bcc α-Ta at T s>400°C, and a mixture of the two phases at intermediate temperatures. α-Ta layers grown on MgO(001) at T s>500°C are epitaxial with a strain-driven 45° in-plane rotation with respect to the substrate: (001) α-Ta∣∣(001) MgO with [110] α-Ta∣∣[100] MgO. A series of lower nitrides — TaN 0.1, Ta 4N, and Ta 2N — are formed over narrow ranges of f N 2 between 0 and 0.10. This is followed by a wide single-phase field at T s ≤ 650°C corresponding to the growth of metastable B1 NaCl-structure δ-TaN x with x ranging from 0.94 ( f N 2 =0.10) to 1.37 ( f N 2 =0.275). The thermodynamically-stable hexagonal ε-TaN phase is formed at higher growth temperatures. δ-TaN x layers grown on MgO(001) at T s=550–650°C are epitaxial, exhibiting a cube-on-cube relationship: (001) δ-TaN∣∣(001) MgO with [100] δ-TaN∣∣[100] MgO. The relaxed lattice constant of δ-TaN x (001) layers decreases linearly from 0.4350 nm with x=0.94 to 0.4324 nm with x=1.37. Finally, layers grown in pure N 2 are a two-phase mixture of δ-TaN x and body-centered tetragonal TaN x .

A Comparative Study of (Cu-III-Se2)x-(FeSe)1?x Alloys (III: Al, Ga, In) (0 ? x ? 1) by X-Ray Diffraction, Differential Thermal Analysis and Scanning Electron Microscopy

Polycrystalline samples of (Cu-III-Se 2 )-(FeSe) 1-x alloys (III: Al, Ga and In) were prepared by the usual melting and annealing method in the entire composition range 1≤ x < 0. X-Ray Diffraction (XRD), Differential Thermal Analysis (DTA) and Scanning Electron Microscopy (SEM) techniques were used to characterize the products. From SEM technique, the stoichiometric relation for each sample was checked to be very close to the nominal values, with deviations less than 10%. XRD measurements showed that, for the three alloy families, there is a wide single-phase field, with a chalcopyrite-like structure, for 1 < x < 0.5. The lattice parameters vary linearly with composition and the variation can be predicted using Jaffe and Zunger's model for chalcopyrites, Pauling's set of covalent radii, and the covalent radius of 1.35 A for Fe. From DTA measurements, thermal transitions in the temperature range 300 K < T < 1400 K were obtained. The melting point decreases from that of extreme compounds (x = 0 and x x = 1) towards an intersection point (local minimum) at around x = 0.3. These results suggest that under adequate thermal treatments, the single-phase field could be increasing up to around x 0.3. In a closer analysis, a pronounced local minimum has been observed at x = 2/3 for Ga and In-based alloys, suggesting that the composition x 2/3 is a new class of compounds, the (Cu-III) 2 FeSe 5 .

Determination of γ′ solution temperature in Re-rich Ni-base superalloy by small-angle neutron scattering

A harmful segregation of heavy elements (e.g.W, Mo, Re) during solidification of Ni-base superalloys can only be eliminated by using a homogenizing heat treatment, which needs to be carried out in the single-phase (γ) field above the γ′ solvus temperature but below the solidus temperature. Small-angle neutron scattering (SANS) was employed forin situobservation of the dissolution of precipitates in an Re-rich superalloy. The temperature dependence of the relative volume fraction and the size distribution of smaller γ′ precipitates, and the specific surface of large inhomogeneities as well as some other parameters were determined from the two-dimensional scattering curves measured for as-cast and heat-treated samples. Overlap of the incipient melting region with the region where a certain amount of precipitates remained undissolved was observed, thus complicating a determination of the temperature at which all γ′ precipitates are already dissolved. Nevertheless, conclusions about the temperature at which the precipitates dissolve and about the temperature at which the incipient melting starts could be formulated. The total scattering probability is suggested as the measure of the overall homogeneity of the distribution of elements in the sample. The temperature dependence of this parameter indicates the optimum solution procedure.

Epitaxial NaCl structure δ-TaNx(001): Electronic transport properties, elastic modulus, and hardness versus N/Ta ratio

While metastable B1-NaCl-structure δ-TaNx is presently used in a variety of hard coating, wear-resistant, and diffusion barrier applications, it is a complex material exhibiting a wide single-phase field, x≃0.94–1.37, and little is known about its fundamental properties. Here, we report physical properties of epitaxial δ-TaNx layers grown as a function of x on MgO(001) by ultrahigh vacuum reactive magnetron sputter deposition. The room-temperature resistivity (ρ=225 μΩ cm), hardness (H=30.9 GPa), and elastic modulus (E=455 GPa) of δ-TaNx(001) are independent of x over the range 0.94–1.22. However, changes in the electronic structure associated with increasing x&amp;gt;1.22 lead to an increase in ρ with a decrease in H and E. All δ-TaNx(001) layers exhibit negative temperature coefficients of resistivity between 20 and 400 K due to weak carrier localization. δ-TaNx is superconducting with the highest critical temperature, 8.45 K, obtained for layers with the lowest N/Ta ratio, x=0.94. Based upon the above results, combined with the fact that the relaxed lattice constant a0 shows only a very weak dependence on x, we propose that the wide phase field in δ-TaNx is due primarily to antisite substitutions of Ta on N (and N on Ta) sites, rather than to cation and anion vacancies. To first order, antisite substitutions in TaNx are isoelectronic and hence have little effect on charge carrier density. At sufficiently high N/Ta ratios, however, simple electron-counting arguments are no longer valid since large deviations from stoichiometry alter the character of the band structure itself.

Ca4Nb2O9–CaTiO3: Phase Equilibria and Microstructures

CaTiO3–Ca4Nb2O9 is a quasi-binary section, and all the phases participating in equilibrium are solid solutions of the binary end-members. The phases are perovskite-based ABO3-type, with a common stoichiometry Ca[CaxNbyTiz]O3. The differences between the lower-temperature phases is in the type of ordering between Ca/Nb/Ti ions occupying the B-site, as well as in the type of octahedral tilting. The proposed phase diagram has the following single-phase fields: (1) cubic disordered C (Pm3m); (2) a series of disordered/tilted CaTiO3-based polymorphs T, O1, and O2 (Pnma) with different combinations of tilting; (3) disordered/tilted O′2 (Pnma), same as O2 but lower in Ti; (4) ordered/tilted HTP2 (P21/c) with 1:1 ordered (111)c planes; (5) ordered/tilted LT1/6 (P21/c) with 1:2 ordered (111)c planes. The single-phase fields are connected with the following reactions of the phases: (1) high-temperature monotectoid C→C′+O′2; (2) miscibility gap C→C′+C″; (3) eutectoid HTP2→LT1/6+O2; (4) a series of peritectoid reactions between O′2 and CaTiO3 polymorphs; (5) an ordering transition O′2→HTP2. In addition, a metastable transition to the triclinic LT1/4 phase (with 1:3 ordered (111)c planes) occurs for a wide range of compositions (0<x<0.6).

Ridge direction of thermal conductivity contours in ternary Ni 3Ga phase

Thermal conductivity of Ni 3Ga-X ternary alloys was comprehensively surveyed for 16 kinds of ternary elements X. The direction of ridge in thermal conductivity contours in γ′ single phase field agrees with that of solubility lobe of γ′ phase in a ternary phase diagram. The ridge direction is determined by the sublattice occupation of a ternary element.

Anomalous temperature dependence of flow stress in a Fe 3Al alloy

Iron aluminides have attracted much interest since 1930s when the excellent corrosion resistance was noted in alloys with the composition of more than about 18 at.% Al. These alloys have relatively low material cost, due to the reduced usage of strategic elements like Cr, Mo and Ni, and a lower density than stainless steels. Their tensile strength is also comparable to those of ferritic and austenitic steels. These advantages have led the iron aluminide alloys being considered for many applications in industries needing sulfidation and oxidation resistance (1). However, the poor ductility at ambient temperatures and an abrupt drop in strength above 600 C have limited these alloys for structural applications. In the past years, extensive efforts have been devoted to understanding and improving the metallurgical properties of iron aluminides with the aim of producing more strong, ductile, and corrosion-resistant materials for structural applications. These studies have resulted in significant contributions to the understanding of the fabrication and mechanical properties of iron aluminides. Deformation behavior in iron aluminides is now known to depend on composition, temperature, and the presence or absence of ordered structures. Recent studies have demonstrated that improved engineering ductility of 10--15% can be achieved in wrought Fe{submore » 3}Al-based iron aluminide alloys, through the control of composition and microstructure. The effect of strain rate on the deformation behavior of Fe{sub 3}Al alloys, especially on the anomalous temperature dependence of strength is of interest recently and more systematic investigation is now necessitated. Load relaxation test has been generally regarded as a very effective technique to measure the strain rate sensitivity over a wider range of strain rates with very little microstructural changes and has been applied to the plasticity of various rate-sensitive materials. In the present study, the iron aluminide alloys with 27.6 at.% Al having the various single-phase field, i.e. the disordered solid solution ({alpha}), an Fe{sub 3}Al with an imperfectly ordered B2 structure, and an ordered Fe{sub 3}Al with the D0{sub 3} structure were first prepared. A series of load relaxation tests was then carried out at the various temperatures ranging from 300 to 800 C to investigate the effect of the tensile strain rate on the anomalous flow behavior.« less

Effects of ternary additions on thermal conductivity of NiAl

Ternary addition generally degrades an excellent conduction properties of NiAl. The contour map of thermal conductivity in NiAl–X β single phase field is drawn for the ternary elements X=Ti, Cu and Ge. It is revealed that each element has a characteristic ridge direction in the contour map. This feature is discussed based on the preference substitutional site of a ternary element, and eighteen ternary elements with a sufficient solubility into NiAl are classified into three groups according to the ridge direction. Thermal conductivity of NiAl-based alloys increases with increasing temperature, however the ridge direction in the contour map remains unchanged.

Potential of rapid heat treatment of titanium alloys and steels

Rapid heat treatment (RHT) of titanium alloys and steels, which includes rapid heating into the single-phase field, β and γ of titanium alloys and steels, respectively, is reviewed. Heating rate is an important parameter that affects the mechanism and kinetics of phase and/or structural transformation. Refinement of grain structure, formation of micro-chemical inhomogeneity and substructure in the high temperature phase following RHT are addressed. Thermo–kinetic effects during rapid heating of material with an initial metastable (quenched or deformed) microstructure are discussed. The response of titanium alloys and steels to RHT is compared. The improvement in mechanical properties of both material system following RHT is also presented.

Discontinuous coarsening of primary α2/γ lamellae at colony boundaries in γ-TiAl-based alloys

Abstract The discontinuous coarsening of primary α 2 /γ lamellae at lamellar colony boundaries was studied by TEM and optical microscopy for the Ti–40Al, Ti–42Al and Ti–45Al (in at%) alloys which were isothermally aged at 1000°C for various time after being solution-treated in the α single-phase field, and then water quenched to room temperature. Discontinuous coarsening nuclei form at lamellar colony boundaries and grow into the neighboring colony. With increasing Al concentrations in the alloys, the degree of the discontinuous coarsening becomes greater. The size of discontinuous coarsening cells increases linearly with time in the first hour for the Ti–42Al alloy and the growth velocity is about 1.17×10 −8 m s −1 . However, at larger times, the microstructure stabilizes and further discontinuous coarsening occurs at much slower rates.

Thallium cuprate 1223 type.: I. Its elaboration and dependence of chemical and thermal stability and of superconducting transition temperature on Tl/Pb and Sr/Ba substitutions

The formation of samples with nominal composition (Tl x Pb 1− x )(Sr 1− y Ba y ) 2Ca 2Cu 3O z was followed by X-ray diffraction study. With the chosen elaboration conditions, the 1223 phases were formed in two steps with a 1212 type phase observed as an intermediate compound. From the results, the limits of the 1223 single phase field were determined for temperature below 930°C and under 1 bar of oxygen. Samples with 91% purity were obtained and for compositions located outside the limits of the monophased field, a Tl/Pb–Sr/Ba-1212 and a substituted Tl/Pb and Sr/Ba perovskite were found as the main impurities.

Effect of interfacial kinetic barriers on interface motion in binary diffusion couples

A finite difference scheme is used to examine the effect of misfit strains and interfacial kinetic barriers to the establishment of local thermodynamic equilibrium on the evolution of a coherent interface in a binary diffusion couple. Impediments to the transformation of one phase into the other and to attaining chemical equilibrium at the interface were considered using a matrix of interfacial kinetic coefficients which coupled thermodynamic driving forces with interfacial velocity and fluxes. Interfacial kinetic barriers result in a decrease in the interfacial velocity, a change in temporal power laws, and large shifts in the time-dependent interfacial compositions, sometimes up to 20 at. pct from the time-independent equilibrium values. The interfacial compositions can shift into either the two-phase field or single-phase field depending upon a number of materials parameters, including the initial compositions of the phases comprising the diffusion couple.

Phase equilibria of the Si-Ge-Ti system relevant to the reactions between SiGe alloys and Ti

The semiconductor-rich region of the Si-Ge-Ti ternary isotherm at 900°C was determined by metallography, x-ray diffraction, and electron microprobe analysis. The sample alloys were prepared by arc-melting. These alloys were brought to equilibrium by annealing at 900°C for 400 h. It was confirmed that at 900°C, TiSi2 and TiGe2 form a continuous solid solution Ti(Si1−yGey)2 with the C54 crystal structure. It was also shown that, other than Ti(Si1−yGey)2 and Si1−xGex, there is not any binary or ternary phase within the Si-Ge-TiGe2-TiSi2 trapezoid region. Between the Ti(Si1−yGey)2 and Si1−xGex single-phase fields is the Ti(Si1−yGey)2-Si1−xGex two-phase region. The tie-lines for this two-phase region were determined. The tie-lines tilt slightly toward the TiSi2 and Ge corners. In other words, at equilibrium, the silicon to germanium atomic ratio is larger in Ti(Si1−yGey)2 than in Si1−xGex (x>y). This tendency for tie-lines to tilt toward the TiSi2 and Ge corners had been proposed in the literature as the reason for the interesting microstructure evolution during the reactions between SiGe alloys and Ti. In addition, the possible diffusion paths for the reactions between SiGe alloys and Ti were discussed based on the obtained isotherm. Recognizing Si and Ge have higher mobilities in Ti(Si1−yGey)2, it is predicted that for SiGe the extent of concentration change is large but occurs over a shorter distance, and for TiSi2 the extent of concentration change is small but occurs over a longer distance.

Analysis of high temperature flow stress of titanium alloys IMI 550 and Ti-10V-2Fe-3AI during isothermal forging

High temperature flow stress data obtained from laboratory scale isothermal forging tests on two titanium alloys, IMI550 (Ti-4Al-4Mo-2Sn-0.5Si) and Ti-10V-2Fe-3Al, have been correlated using a generalised hot working equation. The (α + β) titanium alloy IMI 550 was studied at temperatures in the range 900–950°C, high in the (α + β) phase field, and over a range of strain rates from 4.2 × 10−4 S−1 to 4.2 × 10−2 S−l. The metastable β titanium alloy Ti-10V-2Fe-3Al was studied at temperatures in the range 740–780°C, high in the (α + β) phasefield, and also at 820–950°C in the β field, over the same range of strain rates. A good correlation between the experimental data and the hot working equation was observed for both alloys. Values of the activation energy for hot working have been considered, in conjunction with microstructural evidence, in relation to the dynamic restoration processes taking place. The activation energy for hot working of Ti-10V-2Fe-3Al in the β field has been found to be 185 kJ mol−1, which supports the view that dynamic recovery is the only restoration mechanism in the single phase field. In the(α + β) field, ‘effective’ activation energy values have been determined for both Ti-10V-2Fe-3Al and IMI 550, and these values were significantly higher than those for β forging, especially in the case of IMI 550. However, for both alloys, the dominant restoration mechanism at temperatures high in the (α + β) field is considered to be dynamic recovery of the β phase.

Analysis of high temperature flow stress of titanium alloys IMI 550 and Ti-10V-2Fe-3AI during isothermal forging

High temperature flow stress data obtained from laboratory scale isothermal forging tests on two titanium alloys, IMI550 (Ti-4Al-4Mo-2Sn-0.5Si) and Ti-10V-2Fe-3Al, have been correlated using a generalised hot working equation. The (α + β) titanium alloy IMI 550 was studied at temperatures in the range 900–950°C, high in the (α + β) phase field, and over a range of strain rates from 4.2 × 10−4 S−1 to 4.2 × 10−2 S−l. The metastable β titanium alloy Ti-10V-2Fe-3Al was studied at temperatures in the range 740–780°C, high in the (α + β) phasefield, and also at 820–950°C in the β field, over the same range of strain rates. A good correlation between the experimental data and the hot working equation was observed for both alloys. Values of the activation energy for hot working have been considered, in conjunction with microstructural evidence, in relation to the dynamic restoration processes taking place. The activation energy for hot working of Ti-10V-2Fe-3Al in the β field has been found to be 185 kJ mol−1, which supports the view that dynamic recovery is the only restoration mechanism in the single phase field. In the(α + β) field, ‘effective’ activation energy values have been determined for both Ti-10V-2Fe-3Al and IMI 550, and these values were significantly higher than those for β forging, especially in the case of IMI 550. However, for both alloys, the dominant restoration mechanism at temperatures high in the (α + β) field is considered to be dynamic recovery of the β phase.

Segregation of yttrium ions to domain boundaries of tetragonal zirconia

The domain structures in tetragonal zirconia (t-ZrO ) formed by the 2 diffusionless cubic-to-tetragonal transformation have been examined in ZrO2 3 mol%Y O-12 mol% TiO, which has a wide single t-ZrO field. During 2 3 2 2 annealing in the single-phase field, the domain boundaries with antiphaseboundary-like contrast tend to be facetted with a habit plane close to one of the {111} planes. Transmission electron microscopy-energy-dispersive spectroscopy analysis shows that yttrium ions segregate to the domain boundaries. Since the domain boundaries have a cubic structure, they must be stabilized by the segregation of yttrium ions, which are known to stabilize cubic zirconia.

Tl/Pb and Sr/Ba cuprates of type 1212: compositional effect on the purity and on the superconducting properties

The effect of Sr/Ba and Tl/Pb substitutions has been investigated for Tl 1− x Pb x (Sr 1− y Ba y ) 2CaCu 2O 7− δ (0≤ x and/or y≤1) compounds produced by conventional solid state reaction. The observed phases were characterized by X-ray powder diffraction, scanning electron microscopy and their nominal composition has been controlled by plasma emission spectroscopy. A tentative single phase field is proposed at T≤960°C. For x=0.5, the highest weight fraction of the (Tl 0.5Pb 0.5)(Sr/Ba)-1212 phase was found to be 96.4% and 94% when Ba increases from 0 to 0.2. It drastically decreases when Ba content is over 0.3. For Ba over 0.3, (Tl/Pb)(Sr/Ba)-1223 and perovskite (Tl/Pb)(Sr/Ba)O 3 are observed as main impurities and their content increases with Ba stoichiometry. The unit cell parameters of (Tl 0.5Pb 0.5)(Sr/Ba)-1212 phase increase when Sr atoms are progressively replaced by Ba atoms. Structural refinement leads to conclude that 10 at% of Tl would be substituted on Ca sites and less than 2 at% of Ca could be substituted on Sr sites. The distance between the Cu atom and the apical O atom of the square pyramid increases only slightly when the Ba content raises. The critical temperature remains between 80 and 90 K and seems to be affected by the substitution Sr/Ba.