Phase Transformation Behavior Research Articles

Different phase transformation behaviors of B2-CuZr crystalline phase and their associated mechanical properties by molecular dynamics using different potentials

Metastable CuZr-based alloys have attracted much attention due to their high glass-forming ability and shape memory effect. Many experiments have underscored the pivotal role of the B2-CuZr phase as an effective inclusion to improve both the strength and ductility of metallic glass matrix composites. The improved ductility can be attributed to the dilatation induced by martensitic transformation, while the strengthening effect is owed to the higher strength and hardness of the martensite phase compared to the austenite phase. Well-designed atomistic simulations can predict the mechanical behavior of materials before experiments and offer sufficient information on the atomic scale. The selection of an appropriate interatomic potential is a primary concern in any atomistic simulation and needs to be carefully considered to ensure reliable results. The uniaxial tensile behavior of B2-CuZr crystalline phase is investigated by molecular dynamics simulations using five typical potentials, namely the potentials developed by Mendelev-2007, Mendelev-2009, Mendelev-2016, Mendelev-2019 and Cheng-2009. The phase transformation behavior during tension, Young’s modulus, and yield strength of the simulated samples exhibit variations when different potentials are employed. Notably, only by employing Mendelev-2019 and Cheng-2009 potentials, the strength and Young’s modulus of the transformed phase are higher than the untransformed austenite phase, corresponding well with experimental results. This work emphasizes the impact of different interatomic potentials on phase transformation behaviors within a specific simulation context.

Preparation of Polydopamine Functionalized HNIW Crystals and Application in Solid Propellants.

The application of hexanitrohexaazaisowurtzitane (HNIW) as an oxidizer in solid propellants aligns with the pursuit of high-energy materials. However, the phase transformation behavior and high impact sensitivity of HNIW are its limitations. Due to the strong adhesion and mild synthesis conditions, polydopamine (PDA) has been employed to modify HNIW. However, the method suffers from a slow coating process and a non-ideal coating effect under short reaction time. Herein, oxygen-accelerated dopamine in situ polymerization coating method was developed. It was found that oxygen not only reduced the coating time but also contributed to forming a dense and uniform PDA layer. HNIW@PDA coated in oxygen for 6 h exhibited the most favorable performance, with a delay of 20.8 °C in the phase transition temperature and a reduction of 145.45% in the impact sensitivity. The -OH groups on the surface of PDA enhanced the interaction between HNIW and polymer binders, resulting in a 20.36% reduction in the dewetting percentage. The lower content of PDA in HNIW@PDA (1.17%) resulted in minimal variation in the heat of explosion for HNIW@PDA-based HTPB propellant (6287 kJ/kg) in comparison to HNIW-based HTPB propellant (6297 kJ/kg). Hence, HNIW@PDA-based propellants are expected to offer an alternative with promising safety and mechanical performance compared to existing HNIW-based propellants, thus facilitating the application of HNIW in high-energy propellants. This work presents a low-cost method for efficiently inhibiting the phase transformation of polycrystalline explosives and reducing the impact sensitivity. It also offers a potential approach to enhance the interfacial interaction between nitro-containing explosives and polymer binders.

Analysis of high temperature phase transformation behavior of ultra-low-carbon steel using X-ray topography, diffraction, and synchronized temperature measurement

Despite the technical importance of δ/γ phase transition in steel alloys, conventional thermal analysis techniques like calorimetry lack precise kinetics measurement. This study introduces an integrated approach combining X-ray topographic imaging, diffraction, and precise temperature measurement to overcome these limitations. This methodology allows for the accurate quantification of recalescence due to release of latent heats from solidification and solid-state phase transformation. It also enables tracking of the fractional volume changes of the δ phase during continuous cooling. We demonstrate that in Ultra-Low Carbon steels, the δ to γ transformation occurs rapidly in the single γ phase region far below the solidus temperature in the phase diagram, and this transformation accelerates further in the higher cooling rate. Furthermore, it can be inferred that in phase transformation, its rate becomes a critical factor in the temperature increase resulting from the released latent heat, overshadowing the influence of undercooling during continuous cooling.

Microstructure and Phase Transformation Behavior of NiTiCu Shape Memory Alloys Produced Using Twin-Wire Arc Additive Manufacturing

NiTiCu thin walls were produced by twin-wire arc additive manufacturing (T-WAAM) using commercial NiTi and Cu wires as the feedstock materials. This approach aims to solve the problems typically associated with large phase transformation hysteresis in NiTi shape memory alloys. The microstructure, mechanical properties, and phase transformation behavior of the as-deposited NiTiCu alloy were comprehensively examined. The results revealed that the as-deposited NiTiCu alloy was well-formed, with its microstructure showed columnar, equiaxed, and needle-like grains, depending on the location within the deposited walls. The microhardness gradually increased from the first to the third layer. The Cu content was 20.80 at%, and Cu-based precipitates were formed in the as-deposited NiTiCu. The volume fractions and lattice parameters of the matrix and precipitates in the as-deposited NiTiCu material were analyzed using high-energy synchrotron X-ray diffraction. The martensitic phase was identified as a B19 crystal structure, and the as-deposited NiTiCu underwent a one-step B2-B19 phase transformation. The tensile strength and fracture strain were approximately 232 MPa and 3.72%, respectively. In particular, the addition of Cu narrowed the phase transformation hysteresis of the as-deposited NiTiCu alloy from 24.4 to 7.1 ℃ compared with conventional binary NiTi alloys. This study expands the potential of T-WAAM in modifying the phase transformation behavior of NiTi-based ternary alloys.

Phase transformation behavior and mechanical properties of NiTi shape memory alloys fabricated by directed laser deposition

Directed laser deposition shows promise as a method for fabricating complex structure NiTi shape memory alloy parts in engineering, aerospace, automobile, and biomedical fields. The dislocation density has an impact on phase transformation behavior and mechanical properties of NiTi manufactured by directed laser deposition. In this work, NiTi samples were produced by directed laser deposition, and the mechanism of martensitic phase transformation and its interactions with dislocation density were revealed through molecular dynamics. Laser power and scanning speed are the main factors affecting dislocation density, the phase compositions of the samples are B2, B19′, and Ti2Ni phase, and the samples with dislocation density below 2 × 1015 m−2 show single-step B2–B19′ martensitic transformation, stress plateau and improved elongation, the samples with dislocation density above the value exhibit no significant transformation peak and no stress plateau. The movement of (100) twin planes and the growth of advantageous variants are observed in the simulation which can explain stress-induced martensitic reorientation and the stress plateau phenomenon. The bending test suggested that the sample under the optimal parameters has an 85 % recovery ratio within 8 % pre-strain.

Phase transformation behavior and shape memory effect of nickel-titanium shape memory alloy using the magnetic field assisted wire electrical discharge machining

Nickel-titanium shape memory alloy (NiTi-SMA) can restore its original shape after deformation, making it show great application in the fields of biomedicine, aerospace, etc. The wire electrical discharge machining (WEDM) has the advantage of machining micro and complex structures, which can greatly promote the application of NiTi-SMA. However, there are issues such as heat affected layer (HAL) on the surface of WEDM, which can affect the functionality of NiTi-SMA. Therefore, when using WEDM to NiTi-SMA, more attention should be paid to the functional changes after machining. This article primarily examine is the impact of peak current and pulse on time on the phase transformation behavior, shape memory effect, and mechanical properties of NiTi-SMA during WEDM, and introduces into the magnetic field assisted (MFA) process to improve the machining performance of NiTi-SMA. The results indicate that the HAL formed on the surface of NiTi-SMA is primarily composed of the irreversible phase transformation of B2 austenite. This layer induces a hardening effect, resulting in an increase in the phase transition temperature and a decrease in the phase transition energy (∆HM-A), as a result, the martensitic phase transformation behavior is suppressed. When the peak current and pulse on time increase, the surface quality of the NiTi-SMA processed decreases. The thickness of the HAL and microhardness increase, causing an increase in the phase transition temperature of the NiTi-SMA processed samples. Additionally, the transformation energy ∆HM-A, shape recovery rate, and mechanical properties decrease, and after recovery, more surface cracks appear and show larger extensions. MFA-WEDM can improve the surface quality of NiTi-SMA processing, reduce the thickness of the HAL, and effectively enhance the phase transformation temperature and energy ∆HM-A of the NiTi-SMA processed samples. With a magnetic field strength of 0.45 T, the tensile shape recovery rates of 98%, 94.4%, and 91.0% were achieved at 2%, 5%, and 8% strain, respectively. This represents an improvement of 8.9% compared to the maximum shape recovery rate with conventional WEDM. At the same time, it also inhibits the generation and propagation of surface cracks after deformation recovery. Additionally, MFA-WEDM improves the mechanical properties of NiTi-SMA, and reduces the thickness of the brittle fracture region by 49.7% compared to conventional WEDM. Therefore, MFA-WEDM can effectively enhance the shape memory effect of NiTi-SMA. However, it is important to avoid excessive peak currents and pulse on time to achieve optimal shape memory functionality.

Accelerated improvement in tensile superelasticity of electron beam directed energy deposition manufactured NiTi alloys by artificial thermal cycling combined with low temperature aging treatment

ABSTRACT The low-temperature aging treatment at 250°C can significantly improve the tensile superelasticity of NiTi alloys fabricated by electron beam directed energy deposition (EB-DED). However, it requires a very long aging duration (up to 200 h) to achieve excellent tensile superelasticity due to inherent coarse grain size. To accelerate the aging process, the high-density dislocations are introduced by artificial thermal cycling treatment prior to the aging treatment (the original dislocation content in EB-DED processed NiTi alloys is very low), which will promote the subsequent uniform precipitation of nanoscale Ni4Ti3 particles during low-temperature aging treatment. The phase transformation behaviour always maintains a stable two-stage martensitic phase transformation. Under a cyclic tensile test at 6% strain, 24 h aged sample with thermal cycling maintains a recovery rate exceeding 90% even after 10 cycles, comparable to the performance of the sample aged for 200 h without thermal cycling, indicating a substantial improvement in aging efficiency.

Post-dynamic transformation from α to β phase and reverse transformation in Ti-10V-2Fe-3Al alloy during interrupted hot deformation

Interrupted hot deformation tests were carried out to study the phase transformation behavior of Ti-10V-2Fe-3Al alloy during isothermal holding after deformation. The results show that post-dynamic transformation (post-DT) from α to β phase occurred first during isothermal holding and the meta-β phase retransformed to α phase after a long holding time. This phenomenon is quite different from α+β titanium alloys in which only reverse transformation (RT, βα) took place during isothermal holding. Microstructure evolution of β-matrix was also investigated and it was found that static recovery was the main softening mechanism. Furthermore, phase transformation has a great influence on the static softening and mechanical response of interrupted hot deformation. Due to the occurrence of post-DT, the yield strength of reloading pass reduced and became lower than the initial pass when the holding time exceeds 900 s. And significant static softening was achieved. The driving and opposing force for post-DT and RT were derived and compared based on thermodynamic analysis. Since the deformation and holding temperature is close to β-transus temperature, post-DT has a lower energy barrier which can be easily overcome by stored energy. This may explain why the phase transformation direction in Ti-10 V-2Fe-3Al alloy is opposite to that in α+β titanium alloys during isothermal holding.

Microstructure of Stress‐Induced Martensite in Micron‐Sized Zirconia Single Crystal Spheres

Martensitic phase transformation of zirconia (ZrO2), when realized from the superelastic effect, has immense potential in energy applications, such as micro‐actuation and high‐energy dissipation/damping. In this study, martensitic transformation in ZrO2 is studied using a newly developed microscale phase field model, which is scale‐independent and contains an averaged gradient term. The model is implemented using the FEniCS package in Python. With a thorough explanation of the model construction, the mechanics of phase transformation are detailed, and the material behaviors under different stress conditions are discussed. Mesh sensitivity, multivariant evolution, and the effect of particle size are analyzed to understand the martensite evolution in superelastic ZrO2. This work provides new insight into the martensitic phase transformation behaviors of micron‐sized spherical ZrO2 particles.

Stable and wide temperature range superelasticity in an Mo-doped nanocrystalline Ni–Ti–Mo shape memory alloy

The phase transformation and superelastic behavior of nanocrystalline Ni48Ti50Mo2 alloy wires were investigated. Nanocrystalline (NC) alloys were obtained via severe plastic deformation of cold-drawing followed by annealing at low temperatures. The results indicate that the NC Ni48Ti50Mo2 undergoes a single-step B2→R phase transformation and a two-step B2→R→B19′ phase transformation in coarse crystals (CG). The NC alloy wire exhibits 8 % recoverable strain under a tensile stress of 1.1 Ga and superelasticity exists in a wide temperature range from −60 °C to 100 °C. After 50 cycles of tension fatigue testing, the superelastic upper plateau stress reduction rate is 7 % with almost no residual strain accumulation, which has excellent superelastic stability compared with fine crystals and coarse crystals. Transmission electron microscope (TEM) reveals an average grain size of 20 nm. The stable superelasticity is attributed to the nanocrystalline reinforced matrix, which hinders dislocation generation and slip.

Effect of heat treatment on phase transformation behavior and properties of Ni-Co-Mn-Al-Y shape memory alloy by laser direct energy deposition

Laser direct energy deposition (LDED) of Ni-Co-Mn-Al-Y magnetic shape memory alloys (MSMAs) with high elastocaloric properties has important application prospects in solid-state refrigeration. However, because the microstructure of laser non-equilibrium metallurgy does not have reversible martensitic transformation, further regulating the microstructure of deposited samples through scientific post-heat treatment to obtain better properties is still a top priority. In this work, Ni-Co-Mn-Al-Y alloy samples were prepared by an optimized LDED process, and then the deposited samples were heat-treated with different temperatures. The results showed that the sample subjected to heat treatment with 550 °C × 3.5 h + 1100 °C × 20 h had maximum entropy change of the phase transformation, compressive properties, and elastocaloric effect. The compressive fracture strength was up to 1327 MPa, the maximum compressive strain was up to 26.2%, and an adiabatic temperature change was up to −4.3 K under the condition of rapid unloading. Characterization of the microstructure changes of the alloy under 11 cycles of compression revealed that the deformation mechanism was mainly planar sliding of dislocations in the martensite and dislocation cutting the γ(Y-rich) phase. The alloy prepared by the optimizing LDED and heat treatment process had better cycle stability and preliminary refrigeration capacity.

Effect of geometry on the coefficient of performance of Ni–Ti elastocaloric parts manufactured via powder bed fusion

This study investigates the greater coefficient of Performance (COP) exhibited by the Ni–Ti lattice geometry (LS) over the solid geometry (SS) for solid-state heating and cooling. The LS's advantageous COP can be attributed to factors including variations in phase transformation behavior, a larger surface area facilitating enhanced heat transfer, and a greater volume of water heated per cycle owing to the cavities. Moreover, the LS demands lower compression forces due to its lower mass and geometry, which contribute to its heightened energy efficiency. Remarkably, the LS achieved a maximum temperature span of 11.2 °C at 40 kN compression force, whereas the SS required 80 kN to attain a similar 12.9 °C span. Both the SS and LS showcased a non-linear correlation with compression force which was attributed to Lauder's transformation and pseudo-elastic behaviors. Additionally, the temperature span of the SS exhibited symmetrical behavior during heating-cooling cycles. Conversely, the LS displayed asymmetry, with a more substantial temperature change during the cooling phase than the heating phase.

Strain-Assisted Phase Transformation in Two-Dimensional Transition-Metal Dichalcogenides.

Two-dimensional polymorphic transition-metal dichalcogenides have drawn attention for their diverse applications. This work explores the complex interplay between strain-induced phase transformation and crack growth behavior in annealed nanocrystalline MoS2. Employing molecular dynamics (MD) simulations, this research focuses on the effect of grain size, misorientation, and annealing on phase evolution and their effects on the mechanical behavior of MoS2. First, examining phase transformation in monocrystalline MoS2 under various stress states reveals distinct behaviors depending on the initial phase (1T or 2H) and crystallographic orientation with respect to loading directions. Notably, transformation from a layered hexagonal to a body-centered tetragonal structure is more noticeable when strain in a zigzag direction is applied to the 1T sample. As such, single crystalline MoS2 with a 1T phase exhibits a 16% lower fracture stress in the armchair direction compared to that with a 2H phase. On the other hand, the 1T phase shows a 5% higher phonon lifetime compared to the 2H phase with similar phonon group velocities. Next, the influence of thermal energy and mechanical stress on the phase transformation of nanocrystalline MoS2 is investigated through annealing and quenching cycles, uncovering 60 and 44% irreversibility of phase transformation for an average grain size of 3 and 11 nm, respectively. Besides, the evolution of nanocrystalline samples with different initial phases and grain sizes is studied under uniaxial and biaxial stress. This study shows an inverse pseudo-Hall-Petch effect with exponents of 0.11 and 0.09 for 2H and 1T, respectively. The study reveals that phase transformation can occur concurrently with crack initiation and propagation with the 1T phase exhibiting a 19% lower grain size sensitivity of fracture stress compared to the 2H phase.

Wire-based electron beam additive manufacturing of NiTiNb shape memory alloy: Microstructure, phase-transformation behavior and mechanical properties

NiTiNb shape memory alloy (SMA) is considered to have great potential in aerospace due to its wide phase-transformation hysteresis. Here, we innovatively used electron-beam additive manufacturing (EBAM) to fabricate a NiTiNb SMA thin-walled structure. The phase-transformation temperature, microstructure evolution, and mechanical properties were investigated in detail. The as-built NiTiNb SMA comprised a NiTi matrix and different morphologies of β-Nb phase at room temperature. The microstructure homogeneity and mechanical properties of NiTiNb prepared by EBAM significantly improved compared with those of NiTiNb powder-based AM technology. Our work provided a promising method for the preparation of defect-free and microstructure homogeneous NiTiNb SMA by AM. This method further stimulated the potential of additively manufactured SMA components for industrial applications.

Sulfuric acid leaching of a complex zinc sulfide concentrate with high copper and lead concentrations under oxygen pressure

ABSTRACT This paper presents the use of an oxygen pressure-based acid leaching process for the selective extraction of basic metals from a complex zinc ore containing high concentrations of lead (6.00%) and copper (2.47%). The effects of leaching temperature, ore particle size distribution, leaching time, liquid-to-solid ratio, and oxygen partial pressure on the leaching efficiencies of zinc and copper were investigated, and the phase transformation behaviour of lead compounds was studied. Under the optimal conditions that is, a particle size of −48/+40 μm, initial sulfuric acid concentration of 150 g/L, liquid-to-solid ratio of 6:1, leaching time of 150 min, leaching temperature of 150°C, and oxygen partial pressure of 0.8 MPa – the leaching efficiencies of zinc, copper, and iron were 98.85%, 82.69%, and 33%, respectively. The leaching residue showed that almost all of the zinc sulfide was dissolved as ZnSO4, and most of the negatively charged bivalent sulfur was converted to zero-valent element sulfur. Through the pressure leaching reaction, galena in the concentrate was converted to lead jarosite (PbFe6(SO4)4(OH)12) and elemental sulfur. The formation of lead jarosite facilitated the removal of a significant portion of iron during leaching, reducing the subsequent purification burden.

In-situ synchrotron X-ray diffraction study of the effects of grain orientation on the martensitic phase transformations during tensile loading at different strain rates in metastable austenitic stainless steel

In this work, in-situ high-energy X-ray diffraction was used to analyze the effects of strain rate and austenite (γ) grain orientation on the strain-induced martensitic transformation in metastable austenitic stainless steel 301LN. The diffraction measurements were carried out at strain rates ranging from 10−3 s−1 to 1 s−1 continuously without interrupting the experiment and thus creating nearly adiabatic conditions at the highest studied strain rate. The results indicate that <100>γ fiber-oriented grains preferentially transform at the strain rate of 10−3 s−1 when the true strain is above 0.10, whereas the <111>γ fiber-oriented grains transform only at later stages of plastic deformation. The phase transformation rate of the <111>γ and <100>γ fiber-oriented grains decreases with increase in strain rate. A theoretical model based on stacking fault width as a function of external stress and temperature (stacking fault energy) was used to predict lower-bound estimates for the critical tensile stress needed to start ε-martensite and α’-martensite phase transformations. The model can predict the experimentally observed phase transformation behavior of the <111>γ fiber orientations at all strain rates but is unable to predict the decrease of phase transformation rate of <100> fiber-oriented γ grains with increase in strain rate, which could be related to change in dislocation structure.

Effect of solution treatment on the microstructure, phase transformation behavior and functional properties of NiTiNb ternary shape memory alloys fabricated via laser powder bed fusion in-situ alloying

Post-heat treatment is commonly employed to improve the microstructural homogeneity and enhance the mechanical performances of the additively manufactured metallic materials. In this work, a ternary (NiTi)91Nb9 (at.%) shape memory alloy was produced by laser powder bed fusion (L-PBF) using pre-alloyed NiTi and elemental Nb powders. The effect of solution treatment on the microstructure, phase transformation behavior and mechanical/functional performances was investigated. The in-situ alloyed (NiTi)91Nb9 alloy exhibits a submicron cellular-dendritic structure surrounding the supersaturated B2-NiTi matrix. Upon high-temperature (1273 K) solution treatment, Nb-rich precipitates were precipitated from the supersaturated matrix. The fragmentation and spheroidization of the NiTi/Nb eutectics occurred during solution treatment, leading to a morphological transition from mesh-like into rod-like and sphere-like. Coarsening of the β-Nb phases occurred with increasing holding time. The martensite transformation temperature increases after solution treatment, mainly attributed to: (i) reduced lattice distortion due to the Nb expulsion from the supersaturated B2-NiTi, and (ii) the Ti expulsion from the β-Nb phases that lowers the ratio Ni/Ti in the B2-NiTi matrix, which resulted from the microstructure changes from non-equilibrium to equilibrium state. The thermal hysteresis of the solutionized alloys is around 145 K after 20% pre-deformation, which is comparable to the conventional NiTiNb alloys. A short-term solution treatment (i.e. at 1 273 K for 30 min) enhances the ductility and strength of the as-printed specimen, with the increase of fracture stress from (613 ± 19) MPa to (781 ± 20) MPa and the increase of fracture strain from (7.6 ± 0.1)% to (9.5 ± 0.4)%. Both the as-printed and solutionized samples exhibit good tensile shape memory effects with recovery rates >90%. This work suggests that post-process heat treatment is essential to optimize the microstructure and improve the mechanical performances of the L-PBF in-situ alloyed parts.

Formation of heterogeneous interfaces in the TiAl/Ti2AlNb layered composite and its effect on the fracture toughness

TiAl/Ti2AlNb intermetallic-intermetallic laminated (IIL) composites featuring brittle/ductile heterogeneous interfaces were fabricated through vacuum hot-pack rolling. The microstructures and the phase transformation behaviors of the interfaces of the IIL composites before and after annealing at 900 °C/6 h were investigated. The heterogeneous interfaces are composed of four distinct regions, individually I (βo+γ+α2), II (βo/B2+ω) (brittle part), III (O lath), and IV (equiaxed O) (ductile part) regions from TiAl to Ti2AlNb side. Notably, after annealing, an equiaxed O band approximately 50 μm wide was observed in region IV of the interface. In addition, a significant microhardness variation was observed between regions II and IV of the interface, where region II exhibited higher hardness compared to the TiAl alloy, and region IV displayed lower hardness than the Ti2AlNb alloy. The enhanced fracture toughness of the IIL composites, three times that of the TiAl base alloy, is attributed to the formation of the brittle/ductile heterogeneous interfaces and the layered design incorporating the Ti2AlNb alloy. The corresponding toughening mechanism was further discussed. The brittle II region plays a role in increasing crack branching, while the ductile IV region inhibits the propagation of microcracks and prevents the formation of main cracks. This work highlights the crucial role of the brittle/ductile heterogeneous interface in the toughening of laminated composites. Furthermore, the discovery of the O band provides novel insights into the design of TiAl/Ti2AlNb heterostructures.

Effects of Calcination Conditions on the Structural and Electrochemical Behaviors of High‐Nickel, Cobalt‐Free LiNi0.9Mn0.1O2 Cathode

AbstractEliminating cobalt from high‐nickel layered oxide cathodes lowers the cost of lithium‐ion batteries for electric vehicles. However, cobalt‐free cathodes with high Mn4+ and Ni2+ contents are prone to Li/Ni mixing after synthesis, potentially compromising battery energy density, rate capability, and cycling stability. Without cobalt facilitating cation ordering in the layered structure, the degree of Li/Ni mixing in cobalt‐free cathodes depends heavily on the calcination conditions. In this study, a systematic exploration of calcination temperatures and LiOH ratio for LiNi0.9Mn0.1O2 (NM‐90) provides detailed insights into the optimal synthesis conditions for high‐capacity cobalt‐free cathodes with extended cycle life. Surprisingly, high Li/Ni mixing does not necessarily lead to poor cycling stability whereas low Li/Ni mixing does not guarantee a long cycle life. More importantly, although excessive calcination temperature can further decrease Li/Ni mixing, it does not necessarily enhance capacity. Instead, the pernicious effects from the H2 → H3 phase transition are amplified due to a pronounced two‐phase reaction. An extensive suite of chemical and structural characterization methods uncovers a correlation between elevated calcination temperature, phase transformation, cation ordering, and capacity fading behavior: “overcooking” high‐nickel, cobalt‐free cathodes induce structural arrangement toward that of LiNiO2, with exacerbated lattice distortion and surface instability accelerating capacity fade.

Interaction between phase transformation and static recrystallization during annealing of rolled TC18 titanium alloy

TC18 titanium alloy was subjected to hot rolling at 840 °C, followed by annealing experiments at 830, 860, and 890 °C for different time lengths. The effects of annealing temperature and time on phase transformation and static recrystallization (SRX) behavior, as well as the interaction between phase transformation and SRX, were studied. The results showed that at the low annealing temperature of 830 °C, the microstructure underwent β → α phase transformation and α grain growth with the increase of annealing time, and the SRX of β phase proceeded through subgrain nucleation mechanism induced by α particles. At the middle annealing temperature (860 °C), the α → β phase transformation occurred before the SRX of β phase, and the SRX of β phase was dominated by the grain boundary (GB) bulge mechanism. Meanwhile, the reduction of α phase content accelerated the SRX rate and grain growth. As the annealing temperature (890 °C) exceeded the β transus temperature, the α phase completely transformed to the β phase, and the SRX of the β phase proceeded with a very fast grain growth rate. In addition, the interaction between phase transformation and SRX affected the distribution of α particles. The large-sized α particles (830 °C) were located at the triple GB junction and hindered the SRX β grain growth, while smaller α particles (860 °C) were bypassed by β GB and dispersed within the SRX β grains.