A revision of the Fe-Ni phase diagram at low temperatures (<400 °C)

The low-temperature Fe-Ni phase diagram was assessed experimentally by investigating Fe-Ni regions of meteorites using high resolution analytical electron microscopy techniques. The present phase diagram differs from the available experimental phase diagram based on observations of meteorite structure, but it is consistent with the available theoretical diagram in that α/Ni₃Fe equilibrium was found at low temperatures. The a phase containing 3.6 wt.% Ni is in local equilibrium with the γ′ (Ni₃Fe) phase containing 65.5 wt.% Ni, while the γ′' (FeNi) phase is present as a metastable phase. The new phase diagram incorporates a monotectoid reaction (γ₁ → α + γ₂, where (γ₁ is a paramagnetic fcc austenite, a is a bcc ferrite, and γ₂ is a ferromagnetic fcc austenite) at about 400 °C, a eutectoid reaction (γ₂ → α + γ′) at about 345 °C, and a miscibility gap associated with a spinodal region at low temperatures. The miscibility gap is located between 9.0 and 51.5 wt. % Ni at ～200 °C. The new low-temperature Fe-Ni phase diagram is consistent with all the phases observed in the metallic regions of meteorites.     

Determination of the Fe-rich portion of the Fe-Ni-S phase diagram

The low-temperature, Fe-rich portion of the Fe-Ni-S phase diagram was determined from Fe-Ni-S alloys (2.5,5,10,20, and 30 wt.% Ni, 10 wt % S, balance Fe) heat treated at 100 °C intervals from 900 to 300 °C. The microstructure and microchemistry of the phases in the heat treated Fe-Ni-S alloys were studied using a high-resolution field-emission gun (FEG) scanning electron microscope (SEM), electron probe microanalyzer (EPMA), and analytical electron microscope (AEM). Tieline compositions were obtained by determining the average phase composition and by measuring compositional profiles across interphase interfaces with the EPMA and AEM. At 600 °C and below, at least one phase was <1 Μm in size requiring the use of the AEM for analysis. The measured α + FeS, γ+ FeS, and α + γ + FeS boundaries in the Fe-rich corner of the Fe-Ni-S isotherms are consistent with previous studies. However, two new phases were observed for the first time coexisting with γ and FeS phases: FeNiγ′′ (∼52 wt.% Ni) at 600 and 500 °C and Ni ₃ Fe, ordered Ll ₂,γ′ (∼64 wt.% Ni) at 400 °C. New ternary isotherms are given at 600,500, and 400 °C that include the newly determined γ+γ′′ + FeS and the γ + γ′ + FeS three-phase fields. The effects of S on the phase boundaries of the α + γ phase field and the application of the Fe-Ni-S phase diagram to explain the microstructure and microchemistry of the metallic phases of stony meteorites are also discussed.     

Formation of an <Emphasis Type="Italic">L</Emphasis>1<Subscript>0</Subscript> superstructure in austenite upon the α → γ transformation in the invar alloy Fe-32% Ni

Structure of a metastable austenitic invar alloy Fe-32% Ni preliminarily quenched for martensite and subjected to α → γ transformation using slow heating to various temperatures (430–500°C) with the formation of variously oriented nanocrystalline lamellar austenite, which was subjected to an additional annealing at 280°C (below the calculated temperature of ordering of the γ phase), has been studied electron-microscopically. An electron diffraction analysis revealed the presence of an L1₀ superstructure in the disperse nickel-enriched nanocrystalline γ phase both after annealing at 280°C and in the unannealed alloy immediately after α → γ transformation upon slow heating to 430°C.     

Experimental Phase Diagram Investigations in the Ni-Rich Part of Al-Fe-Ni and Comparison with Calculated Phase Equilibria

The phase equilibria in the section Ni₃Fe-Ni₃Al and phase boundaries of γ′(Ni₃Al) at 1000 °C were studied by a combination of powder X-ray diffraction (XRD), differential thermal analysis (DTA), and electron probe microanalysis (EPMA). The existence of a continuous solid solution at 450 °C was confirmed by a linear decrease of the lattice parameter from Ni₃Al to Ni₃Fe. The phase boundaries of γ′ with γ and the B2-type phase were determined at 1000°C by EPMA. A vertical section of the phase diagram from Ni₃Al to Ni₃Fe above 450 °C, including the liquidus temperatures, is proposed based on the DTA investigations. The invariant four-phase equilibrium U: L + γ′ = γ + B2 is found to occur at 1366 ± 1 °C. The experimental data are compared with a calculated phase diagram obtained by extrapolation from the corresponding binary data sets.     

Crystallographic laws governing the formation of the structure of pseudo-single crystals of some 3<Emphasis Type="Italic">d</Emphasis> metals and related alloys

Methods of metallography, X-ray diffraction, transmission electron microscopy, and dilatometry have been used to study the formation of the structure of pseudo-single crystals of nitrogen-bearing steel Kh18AG20 upon the δ → γ (bcc → fcc) transformation, in pseudo-single crystals of pure cobalt and binary alloy Co-29.7% Ni upon the β → α (fcc → hcp) transformation, and in pseudo-single crystals of zirconium upon the β → α (bcc → hcp) transformation. It has been established that the precipitation of austenite from ferrite during the δ → γ transformation in a single crystal of the Kh18AG20 steel occurs via a crystallographically ordered mechanism with the fulfillment of orientation relationships close to the Kurdjumov-Sachs orientation relationships. In the volume of the pseudo-single crystal there were realized six orientations of austenite with the retention at room temperature of a significant fraction of residual δ (α) ferrite. It has been shown that in the process of cooling of the β single crystals of cobalt, Co-29.7% Ni alloy, and zirconium to below the temperature of the β → α transition there formed several crystallographic orientations of the α phase that are grouped into packets. In each packet, there exist crystals of the α phase of one orientation. In accordance with the Wassermann orientation relationships, in the pseudo-single crystals of cobalt and Co-29.7% Ni alloy there are realized packets of four variants. In the pseudo-single crystal of zirconium, six variants of packets based on the Burgers orientation relationships are realized.     

The phase equilibria of N3Al evaluated by directional solidification and diffusion couple experiments

Three solid phases are involved in the phase equilibria of the important intermetallic compound N1₃Al near its melting point, β, γ′(Ni₃AI) and γ. The generally-accepted phase diagram involves a eutectic reaction between γ′ and γ, but some recent studies agree with an older diagram due to Schramm, 5 which has a eutectic reaction between the β and γ′ phases. This work uses diffusion couple experiments to evaluate the equilibria at temperatures just below the liquidus, and it examines the liquidus region using quenched, directional solidification experiments that preserve the microstructures formed at the solidification front. At the growth rate studied, 61 μm/s, the eutectic forms between the γ and γ′ phases, as in the Schramm diagram.     

The phase equilibria of N3Al evaluated by directional solidification and diffusion couple experiments

Three solid phases are involved in the phase equilibria of the important intermetallic compound N1₃Al near its melting point, β, γ′(Ni₃AI) and γ. The generally-accepted phase diagram involves a eutectic reaction between γ′ and γ, but some recent studies agree with an older diagram due to Schramm, 5 which has a eutectic reaction between the β and γ′ phases. This work uses diffusion couple experiments to evaluate the equilibria at temperatures just below the liquidus, and it examines the liquidus region using quenched, directional solidification experiments that preserve the microstructures formed at the solidification front. At the growth rate studied, 61 μm/s, the eutectic forms between the γ and γ′ phases, as in the Schramm diagram.     

Partial phase diagram of the Ti-Al binary system

The equilibrium temperature-composition coordinates of the β/(β + α), (β + α)/α,α/(α + γ), and (α γ @#@)/γ phase boundaries were determined for the binary Ti-Al phase diagram through the temperature range 1150 to 1400 °C by means of diffusion couples with subsequent EPMA examination. This was supplemented with microstructural examination of a two-phase alloy. No peritectoidal decomposition reaction of the type α→ β + γ reported by Murray [88Mur] at 1280 °C was found. Results indicate that the a phase persists to temperatures well above 1400 °C. The phase boundaries from the present work are in agreement with the Mc Cullough [88Mcc] binary phase diagram.     

Thermodynamic optimization of the Co-Zn system

Literature information and authors’ experimental data have been used for the evaluation of optimized polynomial coefficients serving to calculate the cobalt (Co)-zinc (Zn) phase diagram. The programs BINGSS and THERMO-CALC have been used for the optimization. The binary liquid phase, the solid Co-based face-centered-cubic (fcc) and hexagonal close-packed solutions, as well as the intermediate β-, β₁-, and γ-compounds have been treated as disordered substitutional phases. The phases with narrow homogeneity ranges (δ, γ₁, and γ₂) have been modeled as stoichimetric Co₂Zn₁₅, CoZn₇, and CoZn₁₅, respectively. The calculated phase diagram and thermodynamic quantities are in agreement with the experimental data. For the first time, a eutectoid decomposition (at around 658 K) of the fcc solutions has been predicted. Moreover, the calculations have shown the possibility for a magnetically induced miscibility gap involving both forms (paramagnetic and ferromagnetic) of the fcc solutions.     

Detection of the ɛ phase and the Headley-Brooks orientation relationships upon α → γ transformation in the Fe-32% Ni alloy

The structure of an Fe-32% Ni alloy preliminarily quenched for martensite and subjected to α→ γ transformation upon a slow heating to different temperatures (430–500°C) has been studied by the electron-microscopic method. There has been observed an intermediate ɛ phase with an hcp lattice and rarely encountered Headley-Brooks bcc/fcc orientation relationships, which differ from the Kurdjumov-Sachs relationships. The networks of reflections of the ɛ phase have been observed in electron-diffraction patterns of the Fe-32% Ni alloy after both a slow heating to 430°C without annealing and a slow heating to 500°C with a subsequent annealing at 280°C; the Headley-Brooks relationships between the α matrix and the γ phase, which are typical for increased temperatures of phase transformations, have been observed in the samples after a slow heating to 500°C with annealing.