Phase Evolution Mechanism Research Articles

Phase stability and microstructural evolution of Ti2AlNb alloys-a review

Ti2AlNb alloys are potential next-generation aerospace materials due to their lightweight and high-temperature strength and oxidation resistance properties. Heat treatment of this alloy results in five major microstructures, including fully B2 and α2 + B2, lamellar, bimodal lamellar, equiaxed, and duplex O-phase microstructure. Recently, widmanstätten and plate-like O phase have also been reported. Modern manufacturing methods have made fabrication of high-temperature alloys easy and economical. However, they need post-heat treatments to improve the microstructure and mechanical properties. In this review article, a sequence of various developments in the study of features and thermo-mechanical control on the evolution of various microstructures of the Ti2AlNb intermetallic alloys will be presented. In addition, other aspects such as orientation relationships between different phases, factors affecting grain growth and, phase evolution mechanisms will be discussed. The effect of each heat treatment on every feature of the microstructure will be summarized in each section.

Microstructure and Phase Evolution Mechanism in Hot-pressed and Hot-deformed Nd-Fe-B Magnets with Nd85Cu15 Addition

Chemical composition and structure characteristics of the intergranular phases in Nd-Fe-B magnets play a key role in regulating the magnetic properties. However, it has still been a challenge to understand the evolution mechanism of grain boundary phases in hot-pressed and hot-deformed Nd-Fe-B magnets due to the complex phase composition and structure. In this work, grain boundary modification using Nd85Cu15 alloy has been applied to hot-pressed and hot-deformed Nd-Fe-B magnets. The structure evolution mechanism of the grain boundary phase during hot-pressing was studied by TEM analysis, which shows that an overall reaction, Nd + Nd26Ga7Co10 + Nd2Fe14B → Nd3Co + Nd5Fe2B6 + Nd6(Fe,Co)13Ga, occurs at 600°C in hot-pressed magnet with Nd85Cu15 addition. When the hot-deformation was conducted at 675°C abnormal grain growth was observed in hot-deformed magnets with or without Nd85Cu15 addition. In contrast, as the hot deformation temperature decreased to 630 °C, the uniform and ultra-fine Nd2Fe14B grains formed in the Nd85Cu15 doped magnet, which shows a significantly improved coercivity of 2.03 T and a squareness factor of 0.93.

Low temperature and pressureless synthesis of MgAlON: qualitative analysis and formation evolution

Abstract By calculating the Gibbs free energy of a magnesium aluminium oxynitride (MgAlON) system, pure MgAlON was obtained by pressureless sintering at 1 570 °C in a nitrogen atmosphere using Al, Al2O3, and MgO as raw materials. The formation mechanism of MgAlON and phase evolution of the Al-Al2O3-MgO-N2 system were investigated. The aluminium was melted at approximately 660 °C and formed AlN in the range 740- 1 250 °C. The reaction of AlN, Al2O3, and MgO started to form a primary crystalline MgAlON phase at 950 °C, and as the temperature increased, the minerals continuously dissolved in the primary crystalline phase.

Low temperature and pressureless synthesis of MgAlON: qualitative analysis and formation evolution

Abstract By calculating the Gibbs free energy of a magnesium aluminium oxynitride (MgAlON) system, pure MgAlON was obtained by pressureless sintering at 1 570 °C in a nitrogen atmosphere using Al, Al2O3, and MgO as raw materials. The formation mechanism of MgAlON and phase evolution of the Al-Al2O3-MgO-N2 system were investigated. The aluminium was melted at approximately 660 °C and formed AlN in the range 740- 1 250 °C. The reaction of AlN, Al2O3, and MgO started to form a primary crystalline MgAlON phase at 950 °C, and as the temperature increased, the minerals continuously dissolved in the primary crystalline phase.

Analysis of crystallographic orientation and morphology of microstructure during hot working for an alpha/beta titanium alloy

This work focuses on analysis of microstructure morphology and crystallographic orientation for Ti-17 alloy during hot working. The results show that alpha phase and beta phase influence each other and there is a coordinate deformation between them. The non-uniform deformation is observed under small deformation conditions. The observing area can be divided into small deformation zone (area L) and large deformation zone (area H). Both alpha and beta phases remain the initial morphology, and they have better capability of coordinate deformation in area L, while coordinate capability is weak in area H in which alpha phase is globularized. Correspondingly, the Burgers orientation relations are well preserved in area L, but the orientation relations are more or less destroyed in area H. Dynamic recovery is the main mechanism of beta phase evolution when height reduction is lower. By contrast, the continuous dynamic recrystallization (CDRX) of beta phase gradually dominates the deformation pattern as the deformation increases. An uniformly globularized alpha structure is obtained under large deformation condition. The unsynchronized rotation of alpha phase around <11−20> pole occurs during deformation, which leads to the uniform crystal structure inside the same alpha lamellae. This process is an important step of globularization of the lamellar structure.

Subsurface defect detection using phase evolution of line laser-generated Rayleigh waves

An unreported phenomenon of phase evolution of Rayleigh ultrasonic waves with subsurface defects is observed and systematically explored for detection of subsurface defects using non-contact line laser ultrasonic technique and numerical simulation. The mechanism of phase evolution of Rayleigh wave signals is explained by the interference of the reflected and direct Rayleigh wave, explored by finite element analysis. Both experiments and simulation show distinct peak evolution of the Rayleigh wave signals with the width and depth of subsurface defect. A dimensionless parameter (|Neg|/Pos), defined by the ratio of absolute negative peak to positive peak of Rayleigh wave, is proposed to evaluate the phase evolution of Rayleigh wave with defect width and depth, which is further used to quantify the subsurface defects. The phase evolution of Rayleigh waves can act as a robust and sensitive feature to detect subsurface defects using laser-generated ultrasound, which has promising applications in life prediction and health monitoring of various engineering structures.

Research on preparation process for the in situ nanosized Zr(Y)O2 particles dispersion-strengthened tungsten alloy through synthesizing doped hexagonal (NH4)0.33·WO3

In this article, the in situ nanosized Zr(Y)O2 particles uniformly distributed in the tungsten alloy was successfully prepared through synthesizing doped α-HATB powder and spark plasma sintering process. The processing route involves a molecular-level liquid–liquid doping technique that causes a large number of nanosized particles within tungsten grains. The synthesis mechanism of α-HATB and phase evolution were detailedly investigated. Over 75% of W–Zr(Y)O2 powders particles are less than 3 μm. The determined Sw values (5.7), coefficient of uniformity Cu (2.3) of the W-0.5% Zr(Y)O2 doped tungsten are 5.7 and 2.3, respectively, both indicating the narrower size distribution. The average size of Zr(Y)O2 particles in prepared W alloy are about 250 nm under SEM observation. Through milling of these powders, the particles in W-0.5%Zr(Y)O2 alloy and 92.25W-4.9Ni-2.1Fe-0.75ZrO2 can further decrease to less than 100 nm in size, which are 8–10 times smaller than those in the state-of-the-art review.

Fabrication of Bi-2223 High Temperature Superconducting Tapes With Groove Rolling Process

Bi 1.76 Pb 0.34 Sr 1.93 Ca 2.02 Cu 3.06 O 10+δ (Bi-2223) high temperature superconducting tapes were fabricated with Powder in tube (PIT) process. The traditional drawing process was partially replaced by groove rolling. The influence of cold working method on the filament density, the phase evolution mechanism as well as superconducting properties has been systematically studied. The groove rolling process could obviously enhance the filament density of Bi-2223 green wires, which thus led to the change of Bi-2223 formation rate. Meanwhile, although the heat treatment process has not been thoroughly optimized, the residual secondary phase content was still high, the critical current has been improved for more than 20% with groove rolling process, which should be attributed to the improved intergrain connections. On the other hand, the groove rolling process was also effective on the fabrication of Bi-2223 tapes with larger filament number, such as 85 or 121 to maintain the uniformity of filament deformation.

The formation of intermetallics in Ti/steel dissimilar joints welded by Cu-Nb composite filler

Cu–Nb flux-cored wires with different Nb contents (5 at. %, 20 at. % and 30 at. %) were designed to butt join explosion-welded Ti/steel bimetallic plate. The lower Nb content in Cu5Nb filler results in the formation of extensive Cu–Ti compounds. Brittle Fe2Nb intermetallics form when the Nb content increases to ∼30 at. %. The joints welded by Cu20Nb filler are crack free. Simple models are developed to explain the possible phase evolution mechanisms in the welded joint with Cu20Nb filler. The brittle intermetallics (Fe2Ti, Fe2Nb and FeTi) are dispersed and are surrounded by soft Cu phase, which significantly improve the cracking resistance of the weld metals. Cracks emerge and propagate along the regions rich in α-Fe + Fe2Nb, CuTi2 phases during the three-point bending tests. The tensile strength of the butt joints is ∼334 MPa. Fractography of the tensile samples is corresponding well to the microstructures.

In-situ probing phase evolution and electrochemical mechanism of ZnMn2O4 nanoparticles anchored on porous carbon polyhedrons in high-performance aqueous Zn-ion batteries

Inspired by the successful application of spinel LiMn2O4 cathode in Li-ion batteries, analogous spinel ZnMn2O4 (ZMO) is regarded as a promising cathode for rechargeable aqueous Zn-ion batteries (ZIBs). Nevertheless, clear Zn2+ storage mechanism and phase transition of spinel ZMO is still scarce. Herein, in the first time, we report an in-situ Raman study in dynamically probing the phase and structure evolution of a spinel ZMO-based cathode during charging-discharging process, in which spinel ZMO nanoparticles are anchored on porous carbon polyhedrons (PCPs). By in-situ investigation, it is demonstrated that the electrochemical mechanism can be attributed to the highly reversible phase transformation between spinel ZMO and λ-type MnO2 upon Zn2+ insertion/extraction, which is driven by stepwise oxidation and reduction reactions of Mn3+/Mn4+ along with efficient charge carriers. Furthermore, the resultant ZMO@PCPs composite as cathode delivers a large reversible capacity of 125.6 mAh g−1 at a high current density of 1 A g−1 after 2000 cycles, representing superior long-term cyclic stability (capacity retention of 90.3%) and remarkable rate capability in aqueous ZIBs. As a proof of concept, high-performance flexible aqueous ZIBs are fabricated and represent stable electrochemical performance under various deformation states, indicating their potential applications in portable and wearable electronics.

Study of the Phase-Evolution Mechanism of an Fe-Se System at the Nanoscale: Optimization of Synthesis Conditions for the Isolation of Pure Phases and Their Controlled Growth.

The iron selenide (Fe-Se) family of nanoparticles (FexSey-where x/y ranges from 1:2 to 1:1) has been fabricated by a thermal decomposition method. The control over solution chemistry has been developed by intensively investigating the effect of reaction parameters by means of wide-angle X-ray scattering, leading to the rich insights into the phase-evolution mechanism of the Fe-Se system. The phase transformation followed the FeSe2 → Fe3Se4 → Fe7Se8 → FeSe sequence in the temperature range of 110-300 °C. The deep mechanistic insight helped in the identification of optimized conditions needed to crystallize the individual phase of the Fe-Se system as well as control of the morphology, crystalline phase purity, and thermal stability of the obtained Fe-Se nanoparticles.

Effects of contact body temperature and holding time on the microstructure and mechanical properties of 7075 aluminum alloy in contact solid solution treatment

Contact solid solution treatment (CST) of the 7075 aluminum alloy can significantly shorten the solution time, reduce energy consumption and improve production efficiency compared with traditional heating furnace solid solution treatment (FST). CST experiments with different process parameters were carried out with a self-developed experimental device. The effects of contact body temperature and holding time on the microstructure and mechanical properties of the 7075 aluminum alloy in CST were studied by mechanical property tests and microstructure observations. The secondary phase evolution mechanism was explored, and the strengthening mechanisms were also quantitatively evaluated. The results revealed that the solubility of the secondary phase increased as the contact body temperature and holding time were increased, but after a long period at high temperature, the solid solution portion of the secondary phase (mainly Mg2Si) coarsened, resulting in a decrease of the mechanical properties. The secondary phase dissolved most when the sample was pressed at 495 °C for 30 s, and the tensile strength reached 565 MPa after aging, which was better than the T6 strength of 560 MPa. In addition, Orowan precipitation strengthening played a dominant role rather than grain boundary strengthening, and its strength increase was about 442 MPa.

Tunable polymorph and morphology synthesis of iron oxalate nanoparticles as anode materials for lithium ion batteries

Iron oxalate (FeC2O4), a promising anode material for lithium ion batteries (LIBs), is typically crystallized in orthorhombic-type phase. When the synthesis temperature rises, β-FeC2O4·H2O can transform into high-crystalline α-phase. At present, the detail evolution mechanism of crystal structure for two kinds of iron oxalate polymorphs is seldom studied. Through in-depth experiments, the tunable polymorph and morphology of iron oxalate can be obtained by controlling certain conditions. The oxalic acid complex ([Fe(C2O4) x]−2(x−1)), as unstable intermediate state, can change the reaction thermodynamics and kinetics, thus promoting the nucleation and growth of crystals. Higher temperature and longer reaction time would availably reinstitute stable hydrogen bonds and enhance the disordered state between successive stack layers, then resulting in various morphologies and phase-transition process. Due to short diffusion path and stable effective channels for Li+ ions, the mixed state of iron oxalate exhibits higher reversible capacity and more excellent cycling stability. Understanding the phase evolution mechanism of iron oxalate is critical to controllable synthesis, and ultimately, enhancing the performance of future LIBs.

Non-isothermal crystallization kinetics of a Fe–Cr–Mo–B–C amorphous powder

Fe–Cr–Mo–B–C amorphous powders are usually used in thermal spraying, spark plasma sintering or 3D printing to prepare coatings or large-sized bulk amorphous alloys. However, their non-isothermal crystallization kinetics is far from being investigated in detail because of their relatively complicated crystallization behavior. In this work, the phase evolution, crystallization kinetics and crystallization mechanism of Fe–Cr–Mo–B–C (Cr: 25–27 wt%, Mo: 16–18 wt%, B: 2–2.2 wt%, C: 2–2.5 wt%) amorphous powder during non-isothermal crystallization are analyzed by X-ray diffraction, scanning electron microscope and differential scanning calorimetry together with Ozawa method and local Avrami exponent. The peak temperature of the first precipitated phase is less sensitive to the heating rate. In the non-isothermal crystallization process upon constant-rate heating to elevated temperatures the phase sequence is: α-Fe, M23(C, B)6, M7C3 and FeMo2B2 (M = Fe, Cr, Mo). The apparent activation energy of crystallization of the amorphous powders obtained using Kissinger’s method is between 385 and 557 kJ/mol, which is higher than that of most iron-based amorphous alloys reported so far, indicating a relatively high stability against crystallization. a-Fe and FeMo2B2 have a similar transformation mechanism: the early phase transition is completed by diffusion controlled growth with an increasing nucleation rate; as the crystallized volume fraction increases, the nucleation rate decreases, and nucleation does not occur even in the later stage of crystallization. The crystallization mechanism of M23(C, B)6 and M7C3 is similar: when the crystallized volume fraction α is higher than 0.1, only crystal growth occurs. This might be due to the fact that the large number of interfaces formed between the early precipitated phase and the amorphous matrix promote nucleation, rendering nucleation complete at the stage when the crystallized volume fraction α is less than 0.1. Therefore, the first and fourth crystallization events are diffusion controlled, the second and third crystallization events are primarily governed by grain growth.

A Mechanistic Analysis of Phase Evolution and Hydrogen Storage Behavior in Nanocrystalline Mg(BH4)2 within Reduced Graphene Oxide.

Magnesium borohydride (Mg(BH4)2, abbreviated here MBH) has received tremendous attention as a promising onboard hydrogen storage medium due to its excellent gravimetric and volumetric hydrogen storage capacities. While the polymorphs of MBH-alpha (α), beta (β), and gamma (γ)-have distinct properties, their synthetic homogeneity can be difficult to control, mainly due to their structural complexity and similar thermodynamic properties. Here, we describe an effective approach for obtaining pure polymorphic phases of MBH nanomaterials within a reduced graphene oxide support (abbreviated MBHg) under mild conditions (60-190 °C under mild vacuum, 2 Torr), starting from two distinct samples initially dried under Ar and vacuum. Specifically, we selectively synthesize the thermodynamically stable α phase and metastable β phase from the γ-phase within the temperature range of 150-180 °C. The relevant underlying phase evolution mechanism is elucidated by theoretical thermodynamics and kinetic nucleation modeling. The resulting MBHg composites exhibit structural stability, resistance to oxidation, and partially reversible formation of diverse [BH4]- species during de- and rehydrogenation processes, rendering them intriguing candidates for further optimization toward hydrogen storage applications.

High‐temperature flexural strength of SiC ceramics prepared by additive manufacturing

AbstractSilicon carbide (SiC) ceramics, as a kind of candidate material for aero‐engine, its high‐temperature performance is a critical factor to determine its applicability. This investigation focuses on studying the high‐temperature properties of SiC ceramics fabricated by using additive manufacturing technology. In this paper, SiC ceramics were prepared by combining selective laser sintering (SLS) with precursor infiltration and pyrolysis (PIP) technique. The microstructure, phase evolution, and failure mechanism after high‐temperature tests were explored. SiC ceramic samples tested at room temperature (RT), 800°C, 1200°C, 1400°C, and 1600°C demonstrated bending strengths of 220.0, 226.1, 234.9, 215.5, and 203.7 MPa, respectively. The RT strength of this material can be maintained at 1400°C, but it decreased at 1600°C. The strength retention at 1400°C and 1600°C were 98% and 92%, respectively. The results indicate that the mechanical properties of SiC ceramics prepared using this method have excellent stability. As the temperature increases, the bending strength of the specimens increased slightly and reached the peak value at 1200°C, and dropped to 203.7 MPa at 1600°C. Such an evolution could be mainly due to the crack healing, and the softening of the glassy phase.

Synergistic effects of Gd and Co on the phase evolution mechanism and electrochemical performances of Ce2Ni7-type La-Mg-Ni-based alloys.

The influences of Gd and Co co-substitution for La and Ni on phase structures, and electrochemical properties of La0.83-xGdxMg0.17Ni3.35-2xCo2xAl0.15 (x = 0-0.83) alloys were investigated. All the alloys contained A2B7-type (Ce2Ni7- and Gd2Co7-type) phase, Pr5Co19-type phase, PuNi3-type phase and CaCu5-type phase. The partial replacement of Gd and Co for La and Ni increased the phase abundance of the Ce2Ni7-type superstructure and decreased cell volumes, which contributed to a better hydrogen absorption capacity, cyclic stability and HRD. Compared to those of single Gd- or Co-substitutions, the synergistic effects of Gd and Co on the overall electrochemical properties of alloys were significant. Such a superior overall electrochemical performance may result from appropriate cell volumes and anti-pulverization abilities.

The Effect of Sintering Temperature on Phase Evolution and Sintering Mechanism of Ceramic Proppants with CaCO3 Addition

Low-cost ceramic proppants were successfully prepared from natural bauxite and solid waste coal gangue via CaCO3 additive. 40 wt% of bauxite in raw materials was replaced by coal gangue, which significantly reduced the manufacturing costs. The apparent density, bulk density, acid solubility and breakage ratio of the proppant sintered at different temperatures were systematically investigated. The phase composition and morphological structure were determined using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the amount of liquid phase affected the solid phase reaction velocity by changing sintering mechanism. When the sintering temperature was 1350 °C, the optimum size of the mullite crystal particles and the optimum amount of the liquid phase were observed and the samples exhibited the best performance.

Micro/Nanoengineered α-Fe2 O3 Nanoaggregate Conformably Enclosed by Ultrathin N-Doped Carbon Shell for Ultrastable Lithium Storage and Insight into Phase Evolution Mechanism.

The Fe-based transition metal oxides are promising anode candidates for lithium storage considering their high specific capacity, low cost, and environmental compatibility. However, the poor electron/ion conductivity and significant volume stress limit their cycle and rate performances. Furthermore, the phenomena of capacity rise and sudden decay for α-Fe2 O3 have appeared in most reports. Here, a uniform micro/nano α-Fe2 O3 nanoaggregate conformably enclosed in an ultrathin N-doped carbon network (denoted as M/N-α-Fe2 O3 @NC) is designed. The M/N porous balls combine the merits of secondary nanoparticles to shorten the Li+ transportation pathways as well as alleviating volume expansion, and primary microballs to stabilize the electrode/electrolyte interface. Furthermore, the ultrathin carbon shell favors fast electron transfer and protects the electrode from electrolyte corrosion. Therefore, the M/N-α-Fe2 O3 @NC electrode delivers an excellent reversible capacity of 901 mA h g-1 with capacity retention up to 94.0 % after 200 cycles at 0.2 A g-1 . Notably, the capacity rise does not happen during cycling. Moreover, the lithium storage mechanism is elucidated by ex situ XRD and HRTEM experiments. It is verified that the reversible phase transformation of α↔γ occurs during the first cycle, whereas only the α-Fe2 O3 phase is reversibly transformed during subsequent cycles. This study offers a simple and scalable strategy for the practical application of high-performance Fe2 O3 electrodes.

Atomic atmosphere: a way to understand phase evolution during vanadium slag roasting at the atomic level.

To increase vanadium recovery, it is critical to understand the intrinsic phase evolution during vanadium slag roasting. This work proposes the novel concept of `atomic atmosphere' to effectively evaluate and recognize the microscopic state of the vanadium atom in vanadium slag before and after roasting. Atomic atmosphere includes the chemical state, chemical environment and spatial environment of the atom, and provides a universal way to explore the microscopic phase evolution mechanism during the metal-extraction process from minerals.