Effect Of Phase Transformation Research Articles

Phase transformation effect on residual stress development in fusion welding of dissimilar stainless steels with different thickness

Phase transformation effect on residual stress development in fusion welding of dissimilar stainless steels with different thickness

Enhancing wear resistance of laser-clad AlCoCrFeNi high-entropy alloy via ultrasonic surface rolling extrusion

The AlCoCrFeNi high-entropy alloy (HEA) coating was synthesized by laser cladding and subsequently processed by ultrasonic surface rolling extrusion (USRE) with varying static loads of 100 N, 200 N, and 300 N. This study aimed to explore the effects of static load on the microstructure, mechanical properties, and tribological characteristics of the coatings. The findings indicate that increased static load results in the precipitation of martensitic phases, grain refinement, enhanced microhardness, and an elevated dislocation density. Specifically, after USRE with 300 N static load, the coating's microhardness reaches 753 HV, 1.4 times greater than the un-USRE coating. This enhancement is attributed to the synergistic effect of martensite phase transformation, grain refinement, and dislocation strengthening. The volume wear rate of the coating decreases to 1.31 × 10−4 mm3·N−1·m−1 following USRE at 300 N, demonstrating superior resistance to abrasive wear due to the high microhardness. Besides, the high-density dislocations facilitate rapid migration and diffusion of oxygen elements, thereby enhancing the oxide layer's protective properties and improving the coating's resistance to oxidation wear.

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.

FCC-Zr phase transformation-induced defect substructure in Zr2Si precipitates equilibrated in Si-modified Zircaloy-4

This work investigates the atomic scale substructure of interfaces in Zr2Si secondary phase precipitates formed by the stresses associated with the α-Zr (HCP) to γ-Zr (FCC) phase transformation and thermal effects in an experimental Si-modified Zircaloy-4 by means of transmission electron microscopy and CSL/DSCL (coincident site lattice/displacement shift complete lattice) models. Analysis of a high-angle twin interface reveals their characteristic principal primary Δg vectors, a typical feature of the singular interfaces, and the sessile twinning disconnections. The step height and length of the interfacial dislocation line, with the Burgers vector parallel to the habit plane, are analysed using the DSCL model. In the second case, a low-angle interface of mixed tilt-twist character is studied and facets of constant length along the densest-packed planes and ledges of inconstant height, a common feature of vicinal interfaces, are identified.

Effects of structural strain and phase transformations on the mechanical properties of alumina-zirconia-graphene composites

Yttria-stabilized-zirconia (YSZ) plus graphene-based-materials composites may present phase transitions. Consequently, it may affect the composite properties. Thus, this work discusses the causality between lattice strain, phase transition, fracture toughness (Kc) and hardness (H) values in alumina+YSZ matrix reinforced with 0 wt.% up to 1.25 wt.% multilayered-graphene (MLG). The conventional sintering was made in a reducing atmosphere. H and Kc were obtained by Vickers indentation. The structural characterization shows that the MLG addition promoted an increase in YSZ lattice parameters and a phase transition, from tetragonal to cubic. As a result, the optimum reinforcement peak in the H and in the Kc happens in different MLG concentrations. The Kc optimum is in lower MLG amount, explained by microstructural features. On the other hand, the optimum H displaced to higher MLG concentration may be effect of increasing lattice parameters and the cubic-YSZ phase proportion.

Shear banding mechanisms in Cu/Ru nano-structured multilayers: effect of phase transformation

Shear banding mechanisms in Cu/Ru nano-structured multilayers: effect of phase transformation

Effect of phase transformation on the densification of LaYO3 pellet

Fabrication process of LaYO3 pellets was studied to develop a reaction-preventing material for casting metallic nuclear fuel. LaYO3 pellets were fabricated by sintering La2O3 and Y2O3 powders. Phase transformation and densification behavior of the LaYO3 were investigated at various temperatures to find optimal conditions of LaYO3 sintering process. Two different LaYO3 structures were formed depending on sintering temperature; an orthorhombic perovskite LaYO3 structure was formed at the sintering temperatures of 1633, 1703, 1743, 1763, 1773, and 1793 K, while a monoclinic LaYO3 structure was formed at a sintering temperature of 1843 K. The Bulk density of sintered LaYO3 pellets increased up to 1763 K, and decreased at the temperatures ranging from 1763 to 1843 K. This study demonstrated that the formation of the LaYO3 phases and the density of the LaYO3 sintered pellets were regulated by varying the sintering temperature. Thus, the fabrication temperature of the high-density LaYO3 sintered pellets was optimized, and the mechanism of the change in density was elucidated.

Effect of phase transformation on the electrochemical supercapacitive behavior of thermally exfoliated zirconia

Amorphous and crystalline zirconia (ZrO2) are successfully synthesized through a rapid thermal exfoliation process. The crystal structure and phase purity of exfoliated materials are examined using XRD. Crystalline ZrO2 reveals the formation of a tetragonal phase. Amorphous zirconia possesses a specific surface area of 45 m2/g. As synthesized, amorphous zirconia provides a specific capacity of 1406C/g at 2 A/g (3-electrode measurements). A two-electrode asymmetric supercapacitor of activated carbon, amorphous zirconia (ZrO2//AC), exhibits a better electrochemical performance than its crystalline phase. The amorphous ZrO2-based asymmetric supercapacitor gives a specific capacitance of 160 F/g at 3 A/g. The device delivered an energy density of 47.6 Wh/kg at a power density of 2.2 kW/kg. These high energy and power density values of asymmetric amorphous ZrO2 supercapacitors outperform most of the reported ZrO2-based electrode materials. Amorphous ZrO2 is a promising electrode material in terms of its capacitance retention of ∼75 % over 10,000 cycles. The as-developed device has a rate capability of 35 % at a current density of 25 A/g and a self-discharge current of 2.6 mA.

Effect of phase transformation in cold sintering of aluminum hydroxide

Cold sintering of gibbsite and boehmite, which are difficult to densify by conventional sintering processes that require heating above 1000 °C owing to thermal decomposition, was attempted at various CSP temperatures. When boehmite particles were cold sintered, densification did not progress substantially, with a maximum relative density of 62% achieved at 200 °C. On the other hand, when gibbsite particles were cold sintered, the relative density of the hardened bodies increased significantly with temperature up to 150 °C, with a maximum value of 82%. However, when gibbsite was cold sintered at temperatures above 200 °C, phase transformation from gibbsite to boehmite took place, and the relative density of the hardened body decreased significantly. The precipitated boehmite particles bonded to each other to form a three-dimensional network, and the hardened body fabricated at 250 °C showed a compressive strength of 115 MPa despite a low relative density of 62%.

Micro-Vickers Hardness of Cu and Cu2O Dual Phase Composite Films Electrodeposited from Acidic Aqueous Solutions Containing Polyethylene Glycol

Metallic copper (Cu) and copper oxide (Cu2O) dual phase composite films were fabricated via a cathodic reduction process in an acidic aqueous solution dissolving polyethylene glycol (PEG) to investigate the structure and micro-Vickers hardness. By dissolving PEG in an aqueous electrolyte, the cathode potential was depolarized to the noble region during the electrodeposition, and the average crystallite size of electrodeposited Cu/Cu2O composite films was decreased down to around 40 nm. The metallic copper films electrodeposited from the solution without PEG was preferentially orientated in (220), while that containing PEG was composed of Cu and Cu2O dual phase composite films with random crystal orientation. The micro-Vickers hardness of the Cu/Cu2O composite films that were electrodeposited from the solution containing PEG was achieved up to 2.53 GPa. This improvement in mechanical performance can be explained by the grain refinement effect and the electrochemical phase transformation effect from a copper metallic state to the oxide.

Strength-ductility synergy of an additively manufactured metastable high-entropy alloy achieved by transformation-induced plasticity strengthening

This study investigated the microstructures and mechanical properties of a metastable high-entropy alloy (HEA) Fe34Co34Cr20Mn6Ni6 produced by laser powder bed fusion (LPBF) and compared them with those of an as-cast one. The LPBF-processed HEA exhibited a face-centered cubic (FCC) structure due to the high cooling rate of the laser-induced melt pools. In contrast, the as-cast HEA featured a mass of hexagonal close-packed (HCP) phase within the FCC matrix due to the elemental segregation resulting from low cooling rates. The LPBF-processed HEA exhibited superior strength-ductility synergy when compared to its as-cast counterpart. The yield strength, ultimate strength, and plasticity of the LPBF-processed HEA were 305 MPa, 808 MPa, and 18.9 %, respectively, which were much higher than those of the as-cast HEA (171 MPa, 463 MPa, and 7.3 %). This strength-ductility synergy was attributed to the in-situ formation of a fine HCP phase through the stress-induced phase transformation (TRIP) effect. The fine HCP phase provided abundant FCC/HCP interfaces, thus enhancing strong back stress hardening and facilitating the deformation uniformity. In contrast, the as-cast HEA displayed coarse and straight FCC/HCP interfaces that hindered back stress hardening. Moreover, the interaction between the pre-existing HCP phase and the stress-induced HCP phase in the as-cast HEA tended to cause stress concentration and subsequent crack initiation, leading to reduced ductility in the as-cast HEA. These findings are expected to provide valuable insights for a better understanding of additively manufactured TRIP-assisted HEAs.

Study of the effect of multiple phase transformations and relaxation annealing on the microstructure of a martensitic TiNi alloy in different structural states

The work was devoted to studying the effect of multiple phase transformations in the temperature range of phase transformations of a TiNi alloy with a martensitic structure in various initial states - coarse-grained, ultrafine-grained, nanostructured. The conducted studies have shown that in the process of thermal cycling, the accumulation of defects in the crystalline structure occurs - dislocations, which contribute to a decrease in the size of structural elements. Subsequent relaxation annealing, slightly reducing the dislocation density, made it possible to obtain a stable and more equilibrium microstructure, including the nanostructural state.

Simulation of SMA-based engineering applications considering large displacement and rotation, thermomechanical coupling and partial phase transformation

Shape memory alloys (SMAs) undergo large recoverable deformation due to their inherent diffusionless martensitic phase transformation. Two factors need to be taken into account to incisively emulate the behaviour of SMA based structures: (i) consideration of large displacement and rotation (LDR) effect, and (ii) capturing the consequence of transformation induced material level coupling. In this study, both these effects are apprehended by extending the infinitesimal strain-based constitutive model of SMA as proposed by Lagoudas et al. (2012), assuming small elastic strain but with finite inelastic strain. To preclude any spurious stress generation out of large rotation, the Jaumann stress rate is used, and incremental objectivity for finite rotation between two successive time instants is conserved by rotating the strain increment and spin tensor to the rotation-independent mid-point configuration following Hughes–Winget (Hughes and Winget, 1980) algorithm. In addition, the contribution of thermoelastic and latent heat evolved during transformation is taken into consideration in the thermal equilibrium equation. Moreover, the effect of partial phase transformation yielding minor hysteresis loop has also been captured. The equilibrium equation is expressed in the current configuration following the Updated Lagrangian formulation. The mechanical and thermal equilibrium equations are solved concurrently in block matrix form using the Newton–Raphson (NR) iterative technique, considering the coupling terms resulting from the phase transformation. The efficacy and robustness of the developed finite element model are corroborated through various practical applications of SMA-based members, e.g., SMA ring, SMA-actuated beam, morphing of corrugated airfoil, orthodontic palatal expander, compliant gripper, undergoing LDR while subjected to different thermomechanical loading conditions. The combined effects of LDR along with thermomechanical coupling yield a stiffening behaviour during loading and sluggish response at the time of thermal recovery.

Phase transformation and incidental effects of metastable crystalline TiAlN on the material removal mechanism

Spinodal decomposition of Ti1−xAlxN crystal structure significantly impacts their physical properties, particularly at the atomic scale. In this study, we employed systematic three-dimensional molecular dynamics simulations to understand the process of material removal in TiAlN. Orthogonal simulations were conducted with cutting depth and Al content as variables to analyze their impact on the phase transformation mechanism and the resulting surface finish. The simulations reveal that the aluminum content and depth of cut significantly influence the phase transformation process of supersaturated TiAlN crystals through spinodal decomposition. It is observed that the aluminum content has a maximum stable solubility of approximately 0.6 in TiAlN crystals, above which the system undergoes significant spinodal decomposition during cutting to produce hexagonal phases with lower hardness. Increasing the aluminum content leads to wider phase change and deeper sub-surface damage in the system. On the other hand, the cutting depth will promote phase transformation by affecting the stress distribution, thereby increasing the root mean square roughness of the height profile.

Unlocking the Origins of Highly Reversible Lithium Storage and Stable Cycling in a Spinel High‐Entropy Oxide Anode for Lithium‐Ion Batteries

AbstractDeveloping high‐capacity conversion‐type anodes with superior durability substituting conventional graphite anodes is urgently desired to improve the energy density of lithium‐ion batteries (LIBs). However, fatal capacity decay during cycling of the conversion‐type anodes, which is primarily due to their inevitable structural degradation and continuous solid‐electrolyte interphase reformation induced by drastic volume change, has highly restricted their commercialization. And, the interrelated effects of phase transformation, structural evolution, and electrochemical characteristics of the conversion‐type anodes during cycling remain poorly understood. Herein, the findings on the fabrication and understanding of a previously unexplored entropy‐stabilized spinel oxide, (Co0.2Mn0.2V0.2Fe0.2Zn0.2)3O4 as a promising conversion anode for LIBs, exhibiting not only moderate volume change character but also highly reversible capacities of ≈900 mAh g−1 for 500 cycles at 0.2 A g−1 and ≈500 mAh g−1 for 2000 cycles at 3 A g−1, respectively, are reported. Evidenced by in situ transmission electron microscopy coupled with theoretical calculations, its underlying mechanism underpinning highly reversible Li storage is explicitly revealed, which originates from reversible phase transformation and domain reconstruction during cycling. Moreover, the origin of small volume change is also clearly clarified. This work provides renewed mechanistic insights into designing high‐capacity and durable conversion‐type electrode materials for high‐performance LIBs.

Effect of phase transformation on the ductile to brittle transition behavior of Ti–V–Nb–Zrx body-centered cubic high-entropy alloys at elevated temperatures

Single-phase body-centered cubic (BCC) lightweight Ti–V–Nb–Zrx solid-solution alloys were fabricated by reducing the Zr concentration within the range of 0–5 at% to improve the mechanical properties of equimolar TiVNbZr alloy at elevated temperatures. The fabricated Zrx alloys indeed exhibited greater strength above 800 °C than TiVNbZr, but some special mechanical responses occurred at 800 °C: (ⅰ) superficial cracks with different lengths and amounts along the high-angle grain boundaries (HAGBs) on the compressed sample surface, and (ⅱ) near-brittle fracture of the tensile stress-strain curves. Based on the analysis of the deformed and undeformed surface, local stress concentration and phase transformation were responsible for the cracks and difference in compression behavior, and atmospheric oxygen, deformation localization and phase transformation were mainly responsible for the tensile fracture difference at 800 °C. Phase transformation promote stress dispersion, combined with better oxidation resistance, which make the Zr-containing alloys fractured via the transgranular mode in tensile test. In particular, the Zr3 alloy exhibited work hardening and tensile ductility. Based on the tensile and compression properties of the Zrx alloys, it was found that at RT and 600 °C, atomic size difference (δ) governed alloy strength; from 800 to 1000 °C, δ and melting temperature (Tm) co-operatively governed alloy strength; at 1200 °C, alloy strength was independent of δ, but dependent on Tm. The fabricated Ti–V–Nb–Zrx alloys have medium mechanical properties but a small strength loss at high temperatures, and are suitable for various RT deformation processing methods.

Revealing the effect of crystallinity and oxygen vacancies of Fe-Co phosphate on oxygen evolution for high-current water splitting

Strategically tuning the composition and structure of transition metal phosphates (TMPs) holds immense promise in the development of efficient oxygen evolution reaction (OER) electrocatalysts. However, the effect of crystalline phase transformation for TMPs on the catalytic OER activity remains relatively uncharted. In this study, we have deftly orchestrated the reaction process of anion-etched precursor to induce the amorphization process of FeCo-POx from crystalline to amorphous states. The as-obtained amorphous FeCo-POx (A-FeCo-POx) exhibited an optimized OER performance with a low overpotential of 270 mV at a current density of 10 mA cm−2, which could be attributed to the flexibility of its amorphous structure and the synergistic effect of oxygen vacancies. Moreover, when incorporated into an overall water splitting (OWS) device configured as A-FeCo-POx(+)||Pt/C(–), it displayed long-term solid stability, sustaining operation for 300 h at a current density of 200 mA cm−2. This work not only provides valuable insights into understanding the transformation from crystalline to amorphous states, but also establishes the groundwork for the practical utilization of amorphous nanomaterials in the field of water splitting.

Investigation on the equivalent strength of Portland cement and PC-CSA blends: Experimental findings and thermodynamic modelling

Mixing calcium sulfoaluminate (CSA) cement and Portland cement (PC) can provide several benefits for certain applications, especially reducing the carbon footprint of PC. This study investigated the heat of hydration, hydration products, mechanical properties, autogenous shrinkage and microstructure of blended cement, and phase composition calculations were also made with thermodynamic modelling. The results show that the autogenous shrinkage of the cement paste is effectively mitigated due to the swelling produced by the rapid hydration of ye'elimite before 1 d. However, when the CSA admixture is 15 wt%, many large holes and obvious cracks can be found by three-dimensional rendition of X-ray computed tomography (X-CT), this may be attributed to the adverse effects of phase transformation, specifically the conversion of ettringite into monosulfate. Combined with the thermodynamic modelling results, the hydration of PC and CSA in the blended cement is a competitive relationship, and the presence of AH3 is not found in the PC-rich blended systems. In addition, the compressive strength of the blended mortars with addition of 5 wt% CSA at 28 d reached 61.5 MPa which is equivalent to that of plain PC. However, the strength of 15 wt% CSA addition is about 30 MPa lower than that of 5 wt% CSA. This is significantly correlated with AFt/AFm, and once AFt/AFm exceeds a certain limit, the strength of PC-CSA blends is significantly lower than that of plain PC.

Effect of grain boundary engineering on grain boundary character distribution and deformation behavior of a TRIP-assisted high-entropy alloy

This study carried out two-step thermo-mechanical processing (TMP) in grain boundary engineering (GBE) to optimize the grain boundary character distribution (GBCD) and tensile properties of a transformation-induced plasticity-assisted high-entropy alloy (TRIP-HEA). The evolution and mechanism of GBCD in the HEA were characterized and discussed using electron backscatter diffraction (EBSD). The tensile properties of the HEA after undergoing different TMPs were evaluated through room-temperature uniaxial tensile tests, and the influence of GBE on the deformation mechanisms was investigated using techniques such as EBSD and transmission electron microscopy (TEM). The results indicated that the fraction of low Σ coincident site lattice (CSL, Σ ≤ 29) boundaries exhibited a decreasing trend followed by an increasing trend with an increase of second-step cold rolling strain during TMPs, and the overall fraction of Ʃ9 + Ʃ27 boundaries showed an increasing trend. When the reduction ratio was 10%, the low-ƩCSL boundaries fraction reached ~66.35%. In terms of tensile properties, all GBE specimens showed improvement in strength and ductility. When the reduction ratio of second-step cold rolling was 20%, the yield strength was 506.7 MPa, which was ~59% higher than base material (BM) sample. The ultimate tensile strength was 885.6 MPa, ~26% greater than BM sample. In addition, when the reduction ratio of second-step cold rolling was 3%, the total elongation was 52.6%. The main deformation mechanisms of the HEA were dislocation slip, stacking fault (SF), nano-twining, and phase transformation. The optimization of GBCD had multiple effects on the deformation mechanisms. It manifests in several ways, such as the enhancement of strain uniformity through ΣCSL grain boundaries, improving the ability of dislocation multiplication and stress-induced cracking resistance, as well as the strengthening due to the combined effects of phase transformation and grain refinement.

Removing the Two-Phase Region in Spinel LiMn2O4 Towards High-Rate, High-Energy Density Partially Disordered Spinels

Resource constraints have become critical for the Li-ion industry. Spinel LiMn2O4 presents a cheaper, more sustainable alternative to traditional layered Li-ion cathodes, but its capacity is constrained by a two-phase transition at ~3V associated with a large, inhomogeneous volume change that leads to particle cracking and poor cycling stability. In contrast, the partially disordered spinels (PDS) have more sloping, solid-solution-like behavior, resulting in better rate capability and more homogeneous volume change. In this talk, we demonstrate that PDS achieves this by introducing cation disorder, which can shrink, and even fully remove, the two-phase region. By analyzing samples from cluster expansion-based Monte Carlo simulations, we identify the mechanisms that increase Li solubility in the LiMn2O4 phase and lead to this shortened ~3V plateau. These results provide guidance on the optimal level of disorder in spinels to achieve both solid-solution behavior and good Li mobility and also highlight more generally how disorder can be utilized to reduce the effects of problematic phase transformations in ordered frameworks.