Bcc A2 Research Articles

Understanding the Formation of Complex Phases: The Case of FeSi2

One of the fundamental goals of materials science is to understand and predict the formation of complex phases. In this study, FeSi2 is considered as an illustration of complex phase formation. Although Fe and Si both crystallize with a simple structure, namely, body-centered cubic (bcc A2) and diamond (A4) structures, respectively, it is rather intriguing to note the existence of two complex structures in the Si-rich part of the phase diagram around FeSi2: α-FeSi2 at high temperatures (HT) with a slight iron-deficient structure and β-FeSi2 (also referred to as Fe3Si7) at low temperatures (LT). We re-analyze the geometry of these two phases and rely on approximant phases that make the relationship between these two phases simple. To complete the analysis, we also introduce a surrogate of the C16 phase that is observed in FeGe2. We clearly identify the relationship that exists between these three approximant phases, corroborated by a ground-state analysis of the Ising model for describing ordering that takes place between the transition metal element and the “vacancies”. This work is further supported by ab initio electronic structure calculations based on density functional theory in order to investigate properties and transformation paths. Finally, extension to other alloys, including an entire class of alloys, is discussed.

Increasing ductility via Cu addition in AlxCrFeMnNi: Towards a scrap-based high entropy alloy

Alloys comprising common alloying elements are more amenable to accepting high fractions of scrap. The present study examines the effect of Cu and Al addition on common element containing multiple principal element alloy (CE-MPEA): AlxCuyCrFeMnNi (x = 0, 0.15, 0.3, 0.6, 0.9 and y = 0, 0.07, 0 = 0.14). Alloys were fabricated by arc casting. As expected, after quenching, the microstructures displayed FCC A1, Cr-rich BCC A2 and B2 precipitate phases in varying amounts. Adding small amounts of Cu to low and medium Al alloy groups increased the FCC phase fraction and refined the FCC grains. It is proposed that these microstructural changes account for the increased ductility seen to accompany Cu addition. A hybrid model was successfully employed to predict the hardness of the FCC phase. In contrast to high Cu-containing HEAs from prior studies, Cu additions in the studied alloys did not detrimentally affect the corrosion resistance in idealised polarization corrosion tests in salt solution. This can be ascribed to the absence of a Cu-rich FCC phase at the low Cu levels employed in the present work. The current alloys show promise as a base composition for further development into a tough fine-grained stainless alloy able to be produced using high fractions of scrap input.

Stress contribution of B2 phase in Al0.7CoCrFeNi eutectic high entropy alloy

In this study, the deformation behavior and strengthening contribution of the FCC phase and B2 phase in Al 0.7 CoCrFeNi eutectic high entropy alloys (HEAs) were investigated using in-situ neutron diffraction. An FCC matrix phase exhibited a slip-dominant deformation scheme, while twinning hardly contributed to deformation. Applied macroscopic stresses were rarely redistributed to the BCC A2 phase dispersed within the B2 phase, whereas the stress contribution of the B2 phase, which was initially lower than that of the FCC phase, increased significantly with an increase in plastic strain; hence, its contribution to tensile deformation became predominant. This study allows us to postulate a target value of each phase by microstructural tunning to achieve the desired properties of multicomponent phase HEAs. • Al 0.7 CoCrFeNi high entropy alloy was investigated via in-situ neutron diffraction. • FCC matrix exhibited the slip dominant deformation scheme. • BCC A2 phase rarely accommodated the applied stress during deformation. • Stress contribution of FCC phase was higher at the early stage of deformation. • Stress contribution of B2 phase became predominant beyond 1% strain.

Manufacturing of Ti-Nb-Cr-V-Ni-Al Refractory High-Entropy Alloys Using Direct Energy Deposition.

High-entropy alloys (HEAs) are composed of 5–35 at% of five or more elements, have high configurational entropy, do not form intermetallic compounds, and have a single-phase face-centered cubic structure or body-centered cubic structure. In particular, refractory HEAs (RHEAs), based on refractory materials with excellent mechanical properties at high temperatures, have high strength and hardness at room temperature and excellent mechanical properties at low and high temperatures. In this study, the Ti-Nb-Cr-V-Ni-Al RHEAs were deposited using direct energy deposition (DED). In the microstructure of Ti-Nb-Cr-V-Ni-Al, the sigma, BCC A2, and Ti2Ni phases appeared to be different from the BCC A2, BCC B2, and Laves phases predicted in the phase diagram. This microstructure was similar to that of the casted Ti-Nb-Cr-V-Ni-Al and had a constructed fine grain size. It was found that the growth of these microstructures was due to the DED process, which has a fast solidification rate. The fine grain size caused high hardness, and the microhardness of the Ti-Nb-Cr-V-Ni-Al was measured to be about 900 HV. In addition, in order to analyze the thermal properties of Ti-Nb-Cr-V-Ni-Al composed of the refractory material, the heat-affected zone (HAZ) was analyzed through a preheating test. The HAZ was decreased, owing to the high thermal diffusivity of Ti-Nb-Cr-V-Ni-Al.

Evolution of the phase structure, magnetic domain structure, and magnetic properties of annealed Fe72Ga28 thin films

In contrast to the disordered body-centered cubic (bcc) A2 phase, the ordered face-centered cubic (fcc) L12 phase exhibits negative magnetostriction (λ) and has a high elastic modulus (E). Therefore, an in-depth understanding of the microstructural evolution of Fe–Ga thin films from bcc A2 phase to fcc L12 phase is essential to obtain Fe–Ga thin films with desirable properties. The transformation of the phase structure, magnetic domain structure, and magnetic properties of Fe72Ga28 thin films is analyzed in this study. The phase transformation of the Fe72Ga28 thin film from A2 to A2 L12 occurred after heat treatment, as determined by high-resolution transmission electron microscopy (HRTEM) and glancing-incidence X-ray diffraction (GI-XRD). Non-continuous lattice distortion at the phase boundaries of A2 and L12 was also observed, as shown by geometric phase analysis (GPA) and electron backscattered scattering detection (EBSD). Numerous edge dislocations at the phase boundaries were revealed by HRTEM. A shift in the magnetic domain structure from the maze-like domain to maze-like domain and honeycomb-like domain was detected by magnetic force microscopy (MFM). The variation in magnetic properties was determined to be related to the structural transformation and ununiform lattice distortion between the phase boundaries of the A2 and L12.

Theoretical Study on Structural Stability and Elastic Properties of Fe25Cr25Ni25TixAl(25-x) Multi-Principal Element Alloys.

Material genetic engineering studies the relationship between the composition, microstructure, and properties of materials. By adjusting the atomic composition, structure, or configuration of the material and combining different processes, new materials with target properties obtained. In this paper, the design, and properties of the ordered phases in Fe25Cr25Ni25TixAl(25-x) (subscript represents the atomic percentage) multi-principal element alloys are studied. By adjusting the percentages of Ti and Al atoms, the effect of the atomic percentage content on ordered phases’ structural stability in multi-principal element alloys are studied. Thermodynamic analysis predicted the composition phase and percentage of the alloy. Formation heat, binding energy, and elastic constants confirmed the structural stability and provide a theoretical basis for designing alloys with target properties. The results showed that the disordered BCC A2 phase and the ordered BCC B2 phase are the ductile phases, while the Laves phase is brittle. The research method in this paper is used to design multi-principal element alloys or other various complex materials that meet the target performance.

Temperature-Dependence of Static Magnetic Properties of FeGa Thin Film Fabricated by Pulsed Laser Deposition

FeGa thin films have been deposited onto (100)-oriented GaAs and (001)-oriented Si substrates at different temperatures by pulsed laser deposition. It has been shown that at different substrate temperatures, magnetic properties of FeGa films greatly change, which is mainly dependent on different crystal textures. The presence of the structure consisting of D03-ordered grain phase and bcc α-Fe crystal structure (A2) has been revealed using X-ray diffraction analysis. The diffraction peak of FeGa (001) direction, which ‎corresponds to the D03-ordered granular phase, was observed when the temperatures of the GaAs substrate were 400 and 600°C. The diffraction peak that corresponds to (110) direction of the bcc A2 structure has appeared at 800°C. However, the FeGa film on the Si substrate did not demonstrate any obvious structural change. The changes in the magnetic properties can be mainly attributed to changes in the roughness and the lattice mismatch between FeGa and Si.‎

Abnormal grain growth in annealing cast Cu–Al–Mn–V alloys and their superelasticity

Metals are usually difficult to fabricate bulk materials with a large grain size over cm-scale only through heat treatment with neither directional solidification nor macroscopic deformation. This paper reports a novel method to obtain large grains and single crystals over cm-scale only through simply annealing polycrystalline cast Cu–Al–Mn–V shape memory alloys. The results show that the abnormal grain growth (AGG) directly occurs when cast alloys are annealed at 1173 K, and ultra-large grains and single crystals are obtained after quenching. The AGG results from unique microstructure of cast Cu–Al–Mn–V alloys. All cast alloys consist of L21-Cu2AlMn parent and completely coherent bcc A2(V) nanoparticles due to bcc phase separation. The results further show that the dissolution of A2(V) nanoparticles into the matrix during annealing results in a continuous misorientation gradient within the matrix grains. It may provide the driving force for the rapid migration of grain boundaries, leading to the AGG. Furthermore, the single crystals close to exhibit good superelasticity. Cu-13.7Al-8.6Mn-1.4 V single crystal shows the largest full superelasticity of 7.0%, and Cu-13.2Al-9.8Mn-0.7 V single crystal exhibits the largest superelastic strain of 5.4%.

Excellent superelasticity and fatigue resistance of Cu-Al-Mn-W shape memory single crystal obtained only through annealing polycrystalline cast alloy

In this study, the cast Cu-13.96Al-9.84Mn-0.51W shape memory alloy consisted of L21-Cu2AlMn parent with completely coherent bcc A2(W) nanoparticles due to bcc phase separation. The abnormal grain growth happened only through annealing cast polycrystalline alloy. The results showed that a continuous misorientation gradient within the matrix grains with a deviation of below 2° formed after the dissolution of A2(W) nanoparticles during annealing. This continuous misorientation gradient may result in the abnormal grain growth. The single crystal specimens were directly obtained from one of large grains. The single crystal close to [100] orientation exhibited very excellent superelasticity and fatigue resistance. Here full superelasticity up to 12% and the largest superelastic strain up to 8.5% were displayed, being comparable to commercial Ni-Ti shape memory alloys. Furthermore, the single crystal almost remained full superelasticity during 5 × 103 cycles. From the points of raw material cost, preparation technology and functional properties, the Cu-Al-Mn-W single crystal shows good potential as new superelastic material.

Low-cost Cu-based shape memory single crystals obtained by abnormal grain growth showing excellent superelasticity

Metal single crystals are generally obtained by directional solidification with sophisticated equipment and process. This method is not only very expensive but also very time-consuming. In this study, we report a new approach to produce Cu-Al-Mn-Mo shape memory alloys with single crystals through annealing cast polycrystalline alloys. The microstructure of the cast alloys has a common characteristic that the L21-Cu2AlMn parent is completely coherent with bcc A2(Mo) precipitates due to bcc phase separation. The dissolution of bcc A2(Mo) nanoparticles during annealing process results in abnormal grain growth because of the formation of a continuous misorientation gradient within the grains. The single crystals obtained from large grains exhibit excellent superelasticity. When the orientation of single crystals is close to [−210], the largest full superelasticity is up to 10%, and the largest superelastic strain is up to 6.7%. It is expected that such microstructural design can also be applied to the production of large grains and even single crystals of other alloy systems with phase separation.

Microstructural Evolution and Phase Formation in 2nd-Generation Refractory-Based High Entropy Alloys.

Refractory-based high entropy alloys (HEAs) of the 2nd-generation type are new intensively-studied materials with a high potential for structural high-temperature applications. This paper presents investigation results on microstructural evolution and phase formation in as-cast and subsequently heat-treated HEAs at various temperature-time regimes. Microstructural examination was performed by means of scanning electron microscopy (SEM) combined with the energy dispersive spectroscopy (EDS) mode of electron probe microanalysis (EPMA) and qualitative X-ray diffraction (XRD). The primary evolutionary trend observed was the tendency of Zr to gradually segregate as the temperature rises, while all the other elements eventually dissolve in the BCC solid solution phase once the onset of Laves phase complex decomposition is reached. The performed thermodynamic modelling was based on the Calculation of Phase Diagrams method (CALPHAD). The BCC A2 solid solution phase is predicted by the model to contain increasing amounts of Cr as the temperature rises, which is in perfect agreement with the actual results obtained by SEM. However, the model was not able to predict the existence of the Zr-rich phase or the tendency of Zr to segregate and form its own solid solution—most likely as a result of the Zr segregation trend not being an equilibrium phenomenon.

Structure and Magnetic Properties of Magnetostrictive FeGa Film on Single-Crystal (100) GaAs and (001) Si Substrate Fabricated by Pulsed Laser Deposition

FeGa thin film has been deposited on (100)-oriented GaAs and (001)-oriented Si substrates with different film thicknesses and laser energy densities at room temperature by pulsed laser deposition system. Materials structure and static magnetic of FeGa film have great changes depending on the substrate and energy density of pulsed laser. X-ray diffraction reveals the presence of first-order order–disorder structure of D03-ordered grain phase and disordered bcc A2 structure on GaAs substrate. The coercivity and remanence of FeGa film on GaAs substrate ratio show a regular dependence on the thickness and energy densities. However, film on Si substrate did not exhibit structure change, which can be attributed to a large lattice mismatch between FeGa and Si.

A new beta titanium alloy system reinforced with superlattice intermetallic precipitates

Titanium alloys traditionally lack a nm-scale intermetallic precipitate that can be exploited for age-hardening from solid solution. Here such a strengthening concept is developed in the Ti-Fe-Mo system, with it being found that a high temperature β (bcc A2) single-phase field for homogenisation can be obtained, which following ageing (750°C/80h) precipitated B2 TiFe <100nm in size. The orientation relationship was found to be ⟨100⟩A2//⟨100⟩B2, {100}A2//{100}B2, with a misfit of −6.1%. The alloy was found to be very hard (HV0.5=6.4GPa) and strong (σy, 0.2=1.9GPa) with a density of 6.68gcm−3. TEM observation and micropillar deformation showed that the precipitates resist dislocation cutting.

Superconductivity in U-T alloys (T = Mo, Pt, Pd, Nb, Zr) stabilized in the cubic γ-U structure by splat-cooling technique

Abstract We succeed to retain the high-temperature (cubic) γ-U phase down to low temperatures in U-T alloys with less required T alloying concentration (T = Mo, Pt, Pd, Nb, Zr) by means of splat-cooling technique with a cooling rate better than 10 6 K/s. All splat-cooled U-T alloys become superconducting with the critical temperature T c in the range of 0.61 K–2.11 K. U-15 at.% Mo splat consisting of the γ-U phase with an ideal bcc A2 structure is a BCS superconductor having the highest critical temperature (2.11 K).

Structure, Electrical Resistivity and Superconductivity of Low-alloyed γ-U Phase Retained to Low Temperatures by Means of Rapid Cooling

The γ-U phase alloys can be retained down to low temperatures with less required alloying concentration by using the splat-cooling technique with a cooling rate better than 106 K/s. Doping with 15 at.% Mo, Pt, Pd, Nb leads to a stabilization of the cubic γ-U phase, while it requires much higher Zr concentrations (≥30 at.% Zr). All U–T splats become superconducting with T c in the range of 0.61–2.11 K. A good agreement of the experimentally observed specific-heat jump at T c with that from BCS theory prediction was obtained for U-15 at.% Mo consisting of the γ-U phase with an ideal bcc A2 structure.

The structural and magnetic properties of rapidly solidified Fe100−xGax alloys, for 12.8≤x≤27.5

This work presents the findings from structural, magnetic, and thermal investigations of melt-spun ribbons of Fe100−xGax alloys where 12.8≤x≤27.5at%. X-ray diffraction analysis from the as-spun samples indicates that all of the compositions form the disordered bcc A2 phase. Thermo-magnetic observations of the Fe-rich compositions where x=12.8, 17.5 and 19.5 show temperature dependent magnetisation curves where the A2 bcc phase is retained through the heating and cooling cycles. However, the compositions of x=22.5 and 27.5 both show anomalies, where a marked increase in magnetisation is detected at high temperatures indicating a substantial structural reconfiguration. Repetition of these measurements confirm that both structural and magnetic hysteresis occurs in both these compositions; however, in the x=22.5 composition these changes are eventually removed but not so for the x=27.5 sample. Repetitive thermal measurements using differential scanning calorimetry of the x=27.5 sample coupled with direct comparison to the thermo-magnetic measurements ascertain excellent evidence for the structural and magnetic transitions that are observed. In agreement with phase diagrams, X-ray diffraction of both annealed and quenched samples confirms the proposed structural transformations involving D03, L12 and D019 phases.

High temperature transformations of the Au7Cu5Al4 shape-memory alloy

The β-phase of Au7Cu5Al4 undergoes a reversible shape-memory phase transformation, however there has been some uncertainty regarding the crystal structure or structures of the parent phase. Here we show that, under equilibrium conditions, the parent phase possesses the L21 structure between its Ap (about 79°C) and ∼630°C, and the B2 primitive cubic structure between ∼630°C and its melting point. It melts directly from B2 into the liquid state and hence never achieves the random bcc A2 structure that has been previously mooted. Splat-cast samples of the alloy are martensitic, proving that development of equilibrium order and defect concentration are not pre-requisites for the A→M transformation to occur.

Magnetic force microscopy study of heat-treated Fe 81Ga 19 with different cooling rates

Phase constitution and domain structures of heat-treated Fe 81Ga 19 cooled from 800 °C at different rates have been studied by X-ray diffraction (XRD) and magnetic force microscopy (MFM). Heat-treated samples were annealed at 800 °C for 3 h, followed by water quenching, air cooling (∼160 °C/min), furnace cooling (∼5 °C/min) and slow cooling (0.1 °C/min). Both the as-quenched and air-cooled Fe 81Ga 19 had a single disordered bcc A2 phase, but exhibited obvious difference in alignment of stripe domains. Dendrite domains were observed in the furnace-cooled sample with the formation of the ordered D0 3 phase in the A2 matrix. For the slow-cooled Fe 81Ga 19 sample, L1 2 phase was found to be precipitated and showed a plate domain structure with a relatively weaker contrast. The great thermal history dependence of the initial domain structure of Fe 81Ga 19 samples is considered to be responsible for the change in their magnetostrictive behaviors.

The magnetostriction of Fe–(18− x) at% Ga– x at% Al (3≤ x≤13.5) alloys

The ingot of Fe–(18− x) at% Ga– x at% Al (3≤ x≤13.5) alloys was prepared from high purity elements using a high vacuum arc melting system. The X-ray diffraction patterns indicated that the alloys were disordered bcc A2 structure. The magnetostriction of the alloys was measured and the effect of partial substitution of Ga with Al on the magnetostriction of the alloys was investigated. Fe–9 at% Ga–9 at% Al alloy, the optimizing magnetostrictive alloy was found in Fe–(18− x) at% Ga– x at% Al (3≤ x≤13.5) alloys. The saturated magnetostriction of the directional solidification Fe–9 at% Ga–9 at% Al rod is up to 135 ppm for 0 MPa and 221 ppm for 53 MPa. It was found that the alloy has the high linearity of the magnetostriction curve, the low hysteresis and saturated magnetic field, which suggests the directional solidification Fe–9 at% Ga–9 at% Al alloy is a potential candidate for magnetostrictive actuator and transducer applications.

Neutron diffraction studies of magnetostrictive Fe–Ga alloy ribbons

Melt-spun Fe–Ga ribbons were prepared and some ribbons were annealed at 1000°C for 1h then slowly cooled to room temperature. X-ray diffraction patterns revealed no evidence of texture and only bcc phase in the as-quenched ribbons. However, high-resolution neutron diffraction patterns gave more information on the structure of these ribbons. Only diffractions from the disordered bcc A2 phase were found in as-quenched ribbons with 15, 17.5, and 19.5at.% Ga content, without any trace of satellite peaks or splitting peaks from the proposed Ga–Ga pairing superlattice structure. The broadening of the base of the (110) peaks for all samples except the as-quenched 15at.% Ga ribbon might indicate the existence of some kind of short range ordering. Ribbons developed L12 phase after annealing especially in the Fe 19.5at.% Ga ribbon where the formation of L12 phase reduced the Ga content in the remaining A2 phase and decreased its lattice parameter dramatically. D03 phase formed in the as-quenched 22.5at.% Ga ribbon and the following annealing treatment transformed more A2 phase into D03 phase.