Experimental investigation of phase equilibria in the Ru–Fe–Al system at 1473 K

The low-Al part of the ternary Ru–Fe–Al phase diagram at 1473 K is established in this work. Due to the very promising properties of B2 ruthenium aluminide, the investigation of the B2 region of this system is of special interest. The experimental work includes diffusion methods, as well as quenching of annealed single-phase and two-phase alloys. The results of the different methods are in good agreement. Optical and scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction are used to investigate the samples. It is shown in this work that a three-component B2 phase exists over a wide composition range.     

The role carbon plays in the martensitic phase transformation of an Fe–Mn–Al alloy

We observed direct evidence that 18R martensite is induced by carbon atoms in the BCC grains of an Fe–27.0wt.%Mn–5.3wt.%Al–0.1wt.%C alloy via high-temperature quenching. A single BCC phase structure formed 18R martensite in the present study. The lowest carbon content found for the formation of 18R martensite is 0.035 wt.% in Fe–Mn–Al alloys.     

In situ neutron diffraction study of texture evolution and variant selection during the α → β → α phase transformation in Ti–6Al–4V

In the present study, in situ phase transformation experiments have been carried out using neutron diffraction to monitor the texture evolution during the α → β → α phase transformation in Ti–6Al–4V with and without 0.4% yttrium additions. The aim of adding yttrium was to control β grain growth above the β transus by Zener pinning. First, both alloys were thermomechanically processed to generate a similar starting α texture and grain morphology. Subsequently, both materials were heat treated above the β transus up to 1250 °C followed by furnace cooling to 210 °C to promote diffusional phase transformation starting from β grain boundaries. In situ texture measurements were taken during α → β → α phase transformation starting at room temperature, 800 °C, 950 °C, above β transus (1050 and 1250 °C), and back to near room temperature. The degree of variant selection was determined by comparing the predicted transformation texture during heating and cooling based on the Burgers relationship and the assumption of no variant selection with the measured textures. It was found that during heating β grows from the pre-existing β and that the β texture evolved even before the β transus was exceeded. The β texture strengthened noticeably above the β transus in the case of conventional Ti–6Al–4V but not Ti–6Al–4V–0.4Y, which was related to β grain coarsening. The level of variant selection was clearly affected by grain coarsening and the formation of β texture components that contribute to the 〈1 1 1〉//normal direction (ND) γ fibre texture rotated about 10° away from ND.     

Phase separation kinetics in a Fe–Cr–Al alloy

The α–α′ phase separation kinetics in a commercial Fe–20 wt.% Cr–6 wt.% Al oxide dispersion-strengthened PM 2000? steel have been characterized with the complementary techniques atom probe tomography and thermoelectric power measurements during isothermal aging at 673, 708, and 748 K for times up to 3600 h. A progressive decrease in the Al content of the Cr-rich α′ phase was observed at 708 and 748 K with increasing time, but no partitioning was observed at 673 K. The variation in the volume fraction of the α′ phase well inside the coarsening regime, along with the Avrami exponent 1.2 and activation energy 264 kJ mol^?1, obtained after fitting the experimental results to an Austin–Rickett type equation, indicates that phase separation in PM 2000? is a transient coarsening process with overlapping nucleation, growth, and coarsening stages.     

Stress-induced phase transformation during superplastic deformation in two-phase Ti–Al–Fe alloy

Ti–5.5Al–1Fe alloys consisting of the h.c.p.-α phase and the b.c.c.-β phase were investigated for microstructural changes during superplastic deformation at temperatures from 1050 to 1200 K. Observed changes occurred in two steps: (1) agglomeration of the β phase to grain boundaries perpendicular to the tensile axis and (2) subsequent increase of the β volume fraction. The β volume fraction after failure was found to increase with increasing deformation temperature. The first step was considered to be induced by the gradient of traction force acting upon grain boundaries. The second step was considered to be induced by stress concentration at grain boundaries of the α phase where the β phase was depleted by agglomeration to the perpendicular boundaries. The phase equilibrium under stressed condition was calculated by increasing the Gibbs energy of the α phase by 500 J/mol relative to that of the β phase. An excellent quantitative agreement was found between calculated results and experimental results of the β volume fraction and the Fe composition in each phase. The present work indicates that the phase transformation accompanied by diffusion can be induced by application of stress of the order of 100 MPa. This new type of stress-induced phase transformation can decrease the β transus temperature by more than 100 K.     

Phase-field study for the pre-precipitation process of L12-Ni3Al phase in Ni–Al–V alloy

The microscopic phase-field dynamic model is employed to study the pre-precipitation process of Ni–Al–V alloy. Our computer simulation results show that there exists pre-precipitated phase with L1₀ structure before L1₂ phase formed, temperature and elastic strain energy play significant effect on the pre-precipitated phase with L1₀ structure. The elastic strain energy can induce the formation of L1₀ pre-precipitated phase. Under the same temperature, the greater the elastic strain energy is, the easier L1₀ pre-precipitated phase will form; under the same elastic strain energy, with the temperature increasing, the incubation period and existent time of L1₀ pre-precipitate phase are prolonged, the number of L1₀ precipitated phase also increases, and the formation of the L1₂ phase are delayed.     

Phase-field simulations of α → γ precipitations and transition to massive transformation in the Ti–Al alloy

A phase-field model for the solid–solid α → γ transition of Ti–Al binary alloys is presented based on analytical Gibbs free energies and couplings to the thermodynamical database ThermoCalc. The equilibrium values recover the α + γ phase boundaries. Morphological transitions from diffusive to massive (partitionless) growth are observed on increasing the initial mole fraction of aluminum. Temporal evolution of the interface shows a $\sqrt{t}$ behavior for diffusive and a linear behavior for massive growth, which is in accordance with theoretical predictions. An estimate of the interfacial mobility of Ti–Al based on the Burke–Turnbull equation is calculated. The expression of the mobility follows an Arrhenius law. Using the derived interfacial mobility, the calculated interfacial velocities of the massive transformation are in quantitative agreement with those observed in experiments.     

Identification of the structure of a new Al–U–Fe phase by electron microdiffraction technique

The particles of an unknown intermetallic phase with the approximate composition Al₁₀Fe₂U were observed in a ternary Al–Fe–U alloy. The structure of this phase was investigated in a transmission electron microscope using a microdiffraction technique based on analysis of the symmetry and relative positions of reflections in the zero-order and high-order Laue zones. The phase has an orthorhombic C-centered unit cell with lattice parameters a=8.900, b=10.190 and c=8.993 Å; its crystal symmetry can be described by the Cmcm space group.     

The formation mechanism of the O phase in a Ti3Al–Nb alloy

It is reported that there are several different transformation mechanisms of the O phase in different heat treatment conditions in the Ti₃Al based alloys. However, very little work has been carried out on the α₂→O phase transformation in the Ti₃Al–Nb alloys of Nb amounts exceeding 12 at%. In this paper, the formation mechanism of the O phase in the Ti–24Al–14Nb–3V–0.5Mo (at%) alloy has been carried out by means of TEM and HRTEM. The results show that the O phase is directly derived from the primary equiaxed α₂ grains with a fine streak contrast, and exists in multivariant forms owing to its different orientations after the alloy is solution treated at 1000°C for 1 h followed by water quenching (WQ) and aged at 650°C for 24 h. The O plates in the primary equiaxed α₂ grains exist not only in the form of a single variant, but also in the form of fine α₂+O mixtures. The analysis indicates that the formation of the O phase is the result of a phase decomposition, that is the introduction of niobium as the preferred β stabilizer makes the supersaturation of niobium in the primary α₂ grains, and the α₂ phase containing Niobium separates into Niobium lean and Niobium rich regions through the Niobium diffusion: α₂→α₂(Nb-lean)+O(Nb-rich). Niobium rich regions transform to the ordered orthorhombic phase (O phase) with a lattice distortion and only a very small composition change. It appears, therefore, that the transformation involves nucleation, growth and coarsening of the O phase by a diffusion mechanism.     

The influence of rolling temperature on texture evolution and variant selection during α → β → α phase transformation in Ti–6Al–4V

The role of starting texture in variant selection has been studied during α → β → α transformation in Ti–6Al–4V. By hot rolling at different temperatures followed by recrystallization, material with either a strong basal texture or a strong transverse texture was generated. Subsequently, both conditions were heat-treated above the β transus followed by slow cooling. The degree of variant selection was assessed by comparing the strength of the measured and predicted α texture from high temperature β texture, assuming equal occurrence of all possible variants during β → α transformation. It was found that, even though the material rolled originally at 800 °C displayed a stronger α texture after β heat treatment, it was the material rolled originally at 950 °C that showed greater variant selection. The variant selection mechanism is discussed in terms of the generated β texture and common 〈1 1 0〉 poles in neighbouring β grains selecting a similar α variant on both sides of the prior β grain boundary. Predictions of possible 〈1 1 0〉 pole misorientation distributions for the two investigated β textures showed that the combination of texture components generated during rolling Ti–6Al–4V at 950 °C increases the likelihood of having β grain pairs with closely aligned (1 1 0) planes compared to rolling at 800 °C. Therefore, it can be proposed that avoiding the generation of certain combinations of β texture components during thermomechanical processing has the potential for reducing variant selection during subsequent β heat treatment.     

Quasicrystalline phase formation in the mechanically alloyed Al–Cu–Fe system

Al–Cu–Fe samples were prepared by ball milling powders of elemental Al, Cu, Fe (first route) and of elemental Al mixed with previously mechanically alloyed Cu–Fe solid solution (second route). Phase and structure transformations by annealing the as-milled powders were investigated by differential scanning calorimetry, X-ray diffraction and Mössbauer spectroscopy. The influence of the thermodynamic driving forces, namely the heat of mixing, positive for the Cu–Fe system and negative for the Al–Fe and Al–Cu systems, was discussed and correlated to the sequence of phase transformations during heating.     

Isothermal section of the Er–Fe–Al ternary system at 800 °C

Physico-chemical analysis techniques, including X-ray diffraction and Scanning Electron Microscope–Energy Dispersive X-ray Spectroscopy, were employed to construct the isothermal section of the Er–Fe–Al system at 800 °C. At this temperature, the phase diagram is characterized by the formation of five intermediate phases, ErFe_12?xAl_x with 5 ≤ x ≤ 8 (ThMn₁₂-type), ErFe_1+xAl_1?x with ?0.2 ≤ x ≤ 0.75 (MgZn₂-type), ErFe_3?xAl_x with 0.5 < x ≤ 1 (DyFe₂Al-type), Er₂Fe_17?xAl_x with 4.74 ≤ x ≤ 5.7 (TbCu₇-type) and Er₂Fe_17?xAl_x with 5.7 < x ≤ 9.5 (Th₂Zn₁₇-type), seven extensions of binaries into the ternary system; ErFe_xAl_3?x with x < 0.5 (Au₃Cu-type), ErFe_xAl_2?x with x ≤ 0.68 (MgCu₂-type), Er₂Fe_xAl_1?x with x ≤ 0.25 (Co₂Si-type), ErFe_2?xAl_x with x ≤ 0.5 (MgCu₂-type), ErFe_3?xAl_x with x ≤ 0.5 (Be₃Nb-type), Er₆Fe_23?xAl_x with x ≤ 8 (Th₆Mn₂₃-type), and Er₂Fe_17?xAl_x with x ≤ 4.75 (Th₂Ni₁₇-type) and one intermetallic compound; the ErFe₂Al₁₀ (YbFe₂Al₁₀-type).     

Stability of B2 ordered phase in the Ti-rich portion of Ti–Al–Cr and Ti–Al–Fe ternary systems

The stability region of the B2 phase at 1000°C in the Ti-rich part of the Ti–Al–Cr and Ti–Al–Fe ternary systems are investigated by energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) using two-phase alloys and diffusion couples. It is established that the critical boundaries of the A2/B2 continuous ordering transition are functions of both the Al and Fe or Cr contents, and the phase equilibria between the α₂ and the β and between the β and FeTi (B2) phases are strongly affected by the A2/B2 order–disorder transition. By extrapolating these ternary data to the Ti–Al binary and using the Bragg–Williams–Gorsky approximation a metastable A2/B2 ordering boundary is postulated to exist at 1000°C in the vicinity of 23.5 at%Al in the Ti–Al binary system.     

Effect of titanium on the kinetics of the σ-phase formation in a coarse-grained Fe–Cr alloy

The influence of titanium on the kinetics of the σ-phase formation promoted by an isothermal annealing at T=973 K was studied in coarse-grained quasi-equiatomic Fe–Cr alloys by using the ⁵⁷Fe Mössbauer Spectroscopy. It was found that the kinetics could be well-described in terms of the Johnson–Avrami–Mehl equation. The addition of titanium (up to 3 at%) was revealed to effect the transformation kinetics in the following way: for x_Ti⩽1.5 at%, its presence accelerates the process with the highest transformation rate for x_Ti=0.3 at%, for x_Ti⩾1.5 at%, titanium retards the formation of the σ-phase. Quantitatively, the effect of titanium on the kinetics was described in terms of a change of the effective activation energy.     

The formation of ε-Fe3–2N phase in a nanocrystalline Fe

The formation kinetics of ε-Fe_3–2N phase in a nanocrystalline α-Fe, which was processed by surface mechanical attrition treatment, is found to be obviously increased with respect to that in the coarse-grained form. This can be attributed to the much enhanced heterogeneous nucleation rate at numerous grain boundaries in the nanocrystalline α-Fe.     

Oxidation of Fe–Si,Fe–Al and Fe–Si–Al alloys in CO2–H2O gas at 800 °C

Binary Fe–(1, 2, 3)Si and Fe–(2, 4, 6)Al, and ternary Fe–(2, 3)Si–(4, 6)Al alloys (all in wt%) were oxidised in Ar–20% CO₂, with and without H₂O, at 800 °C. All binary alloys except Fe–6Al, in all gases, formed a thin outer layer of Fe₃O₄, an intermediate Fe₃O₄ + FeO layer, an inner FeO + Fe₂SiO₄ (or FeAl₂O₄) layer and internally precipitated SiO₂ (or FeAl₂O₄). Ternary alloys and Fe–6Al developed a protective Al₂O₃ layer beneath Fe₂O₃ in Ar–20% CO₂. Water vapour affected ternary alloy oxidation only slightly, but Fe–6Al oxidized internally in high H₂O-content gas, and its scale was non-protective.     

On the formation of intermetallics in Fe–Al system – An in situ XRD study

Self-propagating high-temperature synthesis (SHS) was previously proposed as alternative preparation route for Fe–Al intermetallics. However, this process was not optimized and the mechanism and kinetics of the phases' formation was not fully clarified up to current days. In this work, in situ high energy X-ray diffraction analysis was carried out during the SHS process and the mechanism of the intermetallic' formation in FeAl25 powder mixture was described during rapid heating and isothermal annealing at 800 °C as well as during a slower continuous heating to 900 °C. During slower heating, the formation of Fe₂Al₅ and FeAl₂ intermetallics starts below the melting point of aluminium. When the heating rate is high, intermetallics are created after melting of aluminium. During long-term annealing, all of the phases can be transformed to FeAl phase when fine powders were applied. Detailed mechanism is proposed in this paper and kinetics of the intermetallics' formation is described.