Solid-state Phase Transformations Research Articles

Solid-State Phase Transformation Kinetics and Mechanisms in Additively Manufactured Ti–6Al–4V Studied Using In-Situ High-Energy Synchrotron X-ray Diffraction

Solid-State Phase Transformation Kinetics and Mechanisms in Additively Manufactured Ti–6Al–4V Studied Using In-Situ High-Energy Synchrotron X-ray Diffraction

A microscale cellular automaton method for solid-state phase transformation of directed energy deposited Ti6Al4V

Directed energy deposition is a promising additive manufacturing technology that fabricates complex geometries by fusing feed material layer-by-layer. However, the formation mechanism of the directed energy deposited Ti6Al4V solid-state phase transformation process, which is crucial for understanding the process-structure–property relationship, has remained unclear. In this study, a microscale cellular automaton method is proposed to simulate the microstructure evolution process for Ti6Al4V, specifically the β→α/α′ phase transformation process within a few β grains. The method is further integrated with the mesoscale cellular automaton method, which predicts the prior β grain structure, the solid-state phase transformation kinetics model for the prediction of the phase volume fractions, and the finite volume method, which is used for the thermal-fluid flow modelling, providing a temperature field to the former. The integrated numerical framework not only links the thermal history with the phase volume fractions but also provides the columnar β grain structures and acicular α/α′ grain structures in satisfying agreement with the available experimental observation. Moreover, the predictions shed some light on the formation mechanism of the hierarchical α/α′ structure and their particular clusters. The influence of the cooling rate on the α/α′ grain formation is illustrated via the three-layer case simulation. The findings on the formation mechanism of α/α′ are beneficial in tailoring the microstructure of Ti6Al4V for excellent mechanical properties in directed energy deposited Ti6Al4V.

Correction: Phase Selection During Solidification and Solid-State Phase Transformations in an Al-10Ce-8Mn (wt pct) Alloy

Correction: Phase Selection During Solidification and Solid-State Phase Transformations in an Al-10Ce-8Mn (wt pct) Alloy

Predicting metastable oxide-to-hydroxide phase transformations by bulk and interface thermodynamics: Application to the phase stability of aluminium oxides and hydroxides in water

Fundamental understanding of the phase stability of oxides in aqueous environments is of eminent importance for a broad range of disciplines, such as corrosion, tribology, catalysis, medical implants, biosensing and environmental sciences. Most oxide-to-hydroxide phase transformation sequences proceed by consecutive formation of different metastable hydroxide phases towards the most stable bulk hydroxide phase, in accord with Ostwald's Rule of Stages and thus in contradiction with bulk thermodynamics. In this work, a novel unified thermodynamic model is presented for predicting such metastable oxide-to-hydroxide phase transformation sequences by accounting for the energy barrier(s) associated with the creation of new interface(s) between the competing parent and product phase(s). To this end, semi-empirical expressions for the estimation of the interface energies between metals, oxides and/or hydroxides were derived based on the macroscopic atom approach. Application of the model to the phase stability of aluminium in water (at neutral pH) for 298 ≤ T ≤ 500 K predicts a solid-state phase transformation sequence from Al → am-Al2O3 → (pseudo)boehmite, in accord with experimental observations. Subsequent competing formation of the trihydroxide phases bayerite and gibbsite by a dissolution-precipitation on the (pseudo)boehmite surface is equally favoured and critically depends on the hydrolysis of dissolved Al3+ species in solution, as governed by e.g. pH, temperature and the presence of foreign ions in solution.

Phase Selection During Solidification and Solid-State Phase Transformations in an Al-10Ce-8Mn (wt pct) Alloy

Phase Selection During Solidification and Solid-State Phase Transformations in an Al-10Ce-8Mn (wt pct) Alloy

Strong and stable ultrafine-grained Al-Mg alloys via polymer-derived in situ nanoparticles

Ultrafine-grained Al-Mg alloys provide immense promise as lightweight metallic materials for load-bearing applications, contributing significantly to energy efficiency and CO2 emission reduction. However, for prevalent bulk ultrafine-grained Al-Mg alloys fabricated by a combined mechanical alloying and thermomechanical consolidation, the presence of remaining polymers, which are used as a process control agent during milling, jeopardizes the mechanical performance of the consolidated materials and thus significantly limit their broad applications. An ideal solution to overcome this critical issue is to turn these polymers into useful nanoparticles, enabling simultaneous improvements in strength and grain size stability. In this study, we present an approach to currently achieve strength enhancement and grain size control by incorporating polymer-derived in situ nanoparticles. Through a combined process involving polymer-assisted mechanical alloying, extrusion at the eutectic temperature, and annealing-driven solid-state phase transformation, we achieved a strong and stable ultrafine-grained Al-Mg alloy. After post-heat treatment at 440 °C (i.e. 0.76 Tm; Tm is the absolute melting temperature of pure Al) for 24 h, the compressive yield strength exhibited minimal reduction from ∼1.3 GPa for the as-extruded alloy to ∼1 GPa for the as-annealed alloy. Similarly, the average grain size only experienced a slight increase, from 106 to 145 nm after post-annealing. These findings offer practical insights into the design of mechanically robust and thermodynamically stable ultrafine-grained metallic materials via a new strategy of polymer-derived in situ nanoparticles.

In-situ scattering and calorimetric studies of crystallization pathway and kinetics in a Cu-Zr-Al bulk metallic glass

The crystallization route and kinetics of a Cu-Zr-Al bulk metallic glass (BMG), based on Cu50Zr50, were investigated using in-situ synchrotron small- and wide-angle X-ray scattering (SAXS/WAXS), in-situ high-energy X-ray diffraction (HEXRD), and differential scanning calorimetry (DSC). The acquisition of kinetic diagrams of SAXS/WAXS enables the direct construction of the overall phase transformation route from room temperature to the onset temperature of solid state phase transformation upon heating. It is revealed that the studied alloy undergoes multiple steps (at least five steps) before reaching the high-temperature thermodynamic equilibrium. This is distinct from the commonly believed simple path of glass → Cu10Zr7 + CuZr2 → B2-CuZr predicted by the Cu-Zr equilibrium phase diagram. In addition to thermodynamics, we emphasize that kinetic factors play a prominent role in the phase transformation process of Cu-Zr-Al BMGs. Furthermore, isothermal crystallization kinetics analyses conducted by in-situ HEXRD and DSC indicate that the formation of the initial crystalline phase, specifically the Cu10Zr7, follows an interface-controlled quasi-polymorphic process with an increasing nucleation rate. The findings of this study may provide novel insights into the crystallization mechanism of glass-forming supercooled liquids from a thermophysical perspective.

Preparation of bainite phase microstructure by laser-electromagnetic induction hybrid solid-state phase transformation

Increasing the proportion of bainite and martensite phases is one of the effective ways to enhance the toughness of medium-carbon low-alloy steel(42CrMo). In this paper, the surface heat treatment of 42CrMo steel is carried out by using a laser heat source to achieve complete austenitization, then with electromagnetic induction heating for several times to implement insulation, and eventually to promote bainite phase transformation. In conjunction with the temperature history curves, OM, SEM, and EBSD are used to characterize the bainite morphology and percentage of bainite via different parameters of the hybrid process. The results demonstrate that the hybrid process combined with twice electromagnetic induction heating can yield a considerable proportion of bainite phase in 42CrMo steel. The resulting duplex structure, consisting of bainite and a minor fraction of martensite, exhibits a significant increase (81.1%) in tensile strength, while maintaining similar impact toughness. The elongation after fracture only has a slight reduction of 5.5% compared to that of the substrate. This innovative process provides a practical alternative to traditional bainite isothermal treatment method which must perform in a heat treatment furnace. Therefore, it is presented as a promising solution for enhancing the mechanical properties of medium-carbon low-alloy high strength steels.

Investigation on the fatigue crack growth behavior of welded joints in EH690 high-strength marine steel

EH690 high-strength steel (HSS) is extensively utilized in marine engineering due to its exceptional strength. The fatigue crack growth (FCG) behaviors of EH690 HSS welded joints in different zones (base metal (BM), heat-affected zone (HAZ), and weld metal (WM)) were investigated by conducting FCG tests with various stress ratios (R). The results demonstrate that the WM and HAZ materials exhibit enhanced resistance to FCG compared to the BM material. The higher stress ratios result in increased fatigue crack growth rates (FCGRs). The welded residual stress (WRS) distribution in the welded joints was predicted considering solid-state phase transformation (SSPT). Subsequently, a comprehensive analysis of the WRS influence on FCG behavior shows that the residual compressive stress enhances the material’s resistance to FCG. Additionally, the effect of R on the FCG behavior of the material was investigated by employing the Walker model, and the applicability of the Walker model was discussed as well. Finally, the FCG mechanisms of the different zones in the welded joint were investigated from a microscopic perspective, further exploring the influence of R on the FCG behavior.

Thermo-kinetic insights into deformation mechanism in the phase-transforming nanostructured Fe alloy

Solid-state phase transformations (SSPTs) being the most widely versatile routes to tailor microstructures are less utilized in the design of nanostructured (NS) alloys, resulting in a lack of understanding of the significance of SSPTs in tuning mechanical properties. Here, we combine nanostructuring with reverse austenite transformation to make a phase-transforming NS Fe alloy consisting of ultrafine ferrite and austenite grains. By comparison with the coarse-grained (CG) counterpart, the NS sample exhibits high strengths (i.e., the yield and the ultimate tensile strengths are 816 and 1170 MPa, respectively), good ductility with a uniform elongation of 53 %, and superior strain hardening capacity, under multiple strengthening mechanisms and the activation of transformation induced plasticity. Regarding the mechanism in forming the NS sample, grain refinement is revealed to markedly affect the austenite formation kinetics by generating sluggish growth behavior accompanied by sufficient solute partitioning. Then, we clarify the difference in mechanical responses of the phase-transforming NS and CG samples using a physically based microstructures-properties model. Further based on the thermo-kinetic theory of generalized stability, we show that the simultaneous increase of strength and ductility in the NS sample relative to the CG sample is originated from the deformation physics with higher driving force and higher generalized stability. From the thermo-kinetic perspective, our work demonstrates that the SSPTs hold substantial promise in optimizing the microstructures and the mechanical properties of NS Fe alloys and are expected to find wide application in the development of other NS materials.

Evolution of phase ratio and its effect on residual stress for 2205 duplex stainless steel multipass welded joints by thermo-metallurgical-mechanical model

The control of phase ratio and residual stress is a crucial subject for the structural integrity of duplex stainless steel welded components. Therefore, this paper aims to clarify the evolution of phase ratio and its effect on residual stress for duplex stainless steel multipass welded joints by a thermo-metallurgical-mechanical coupled welding model, alongside the crucial experiments which contribute to the development and validation of the coupled model. The developed model takes both the precipitation and dissolution behavior of austenite under repeated thermal cycles into consideration, thereby demonstrating a remarkable ability to predict the distribution of phase ratio and residual stress. The influencing mechanism of solid-state phase transformation on residual stress is discussed extensively. The results reveal that, for the duplex stainless steel welded joints by gas tungsten arc welding, excessive austenite is often formed within the welding zone particularly for the middle passes, while it is the occurrence of ferritization at the heat affected zone. The effect of phase transformation on residual stress is mainly presented by the change of mechanical properties and phase volume. The overtransformation from ferrite to austenite tends to induce a higher tensile residual stress, vice versa, compressive stress. In addition, it is challenging to achieve phase balance through heat input alone. However, a smaller heat input is recommended to minimize residual stress while ensuring welding penetration. This paper serves as a theoretical foundation for controlling the phase ratio and mitigating residual stress during the welding of duplex stainless steel, thereby contributing to the enhancement of weld quality and structural reliability.

Neutron diffraction analysis of microstructural evolution and mechanical behavior in an additively manufactured multiphase alloy

Abstract Additive manufacturing (AM) involves an unprecedented thermal history during solidification and post-melt high-temperature exposure, leading to unique microstructural evolution. In this study, we employed neutron-diffraction-based microstructural analysis to better understand the microstructural evolution and mechanical behavior of AM alloys, with a particular focus on multiphase alloys. Samples of Ti−6Al−4V alloy used as a model material were prepared using electron beam powder bed fusion (EB-PBF) under varying building conditions. Time-of-flight neutron diffraction (TOF-ND) measurements were performed using an iMATERIA (BL20), J-PARC, Japan. Using Rietveld texture analysis (RTA), we revealed the textural evolution during hierarchical microstructural development from solidification to solid-state phase transformations in the EB-PBF process. The effects of building conditions on the textures in the as-built states and their evolution during subsequent tensile loading were analyzed.

Enhanced corrosion resistance of Ti[sbnd]Al[sbnd]Mo alloy through solid state transformation driven by rapid solidification

Rapid solidification of undercooled liquid Ti50Al44Mo6 alloy was realized by electromagnetic levitation. Primary (βTi) dendrite grew rapidly from melt with decreasing temperature. As undercooling increased, nucleation rate and growth driving force of primary (βTi) dendrites increased. Growth velocity of primary (βTi) dendrite increased significantly, reaching 13.5 m·s-1 at the maximum undercooling (233 K). After solidification, primary (βTi) dendrite decomposed simultaneously through β→α→α2 transformation and martensite transformation β→γ. Homogeneity of solute distribution in primary (βTi) phase affects the solid-state phase transformation mode. Solid-state phase transition was mainly dominated by diffusion-controlled β→α→α2 transformation at small undercooling. Solid-state phase transition gradually was dominated by displacive martensite transformation at deep undercoolings, and corresponding microstructure was mainly characterized by more refined martensite needles. The refined microstructure and martensite transformation domination contributed to the formation of passivation films with improved corrosion resistance. Moreover, this weakens micro-galvanic effect, significantly reduces size of pits, maintains corrosion scales over pits to effectively alleviate the corrosion process, and consequently enhances corrosion resistance.

Development and experimental validation of a thermo-metallurgical-mechanical model of the laser metal deposition (LMD) process

The current paper presents the development and experimental validation of a thermo-metallurgical-mechanical model for a multi-pass Laser Metal Deposition (LMD) process, with the objective of accurately predicting the resultant constituent phases and residual stresses. The thermal model is calibrated using temperature readings and is shown to accurately capture the transient temperature field and extent of the fusion zone associated with the multi-pass LMD process. The metallurgical model incorporates the kinetics of ongoing solid-state phase transformations (SSPTs) and tempering reactions. The predictions are validated via hardness measurements, demonstrating a very good agreement with the predictions. The developed mechanical model predicts the residual stress field, which is validated using X-ray diffraction measurements, and the comparison also shows a very good agreement between the predictions and measurements. The validated numerical model is then used to explore alternative LMD strategies showing that the choice of deposition strategy can significantly impact resultant constituent phases and residual stresses.

High-fidelity level-set modeling of diffusive solid-state phase transformations for polycrystalline materials

The formation of microstructures in metallic alloys during hot metal forming involves simultaneous metallurgical complex phenomena. Traditional high-fidelity numerical frameworks used on the polycrystalline scale tend to focus on single-phase microstructures or isolate phase transformations from grain boundary migration mechanisms. The level-set method is highlighted as effective in proposing a global framework for modeling multiphase polycrystalline materials and diffusive solid-state phase transformations. This framework includes novel techniques for efficient large-scale microstructural representation, strong coupling with ThermoCalc software for real-time thermodynamic data, application for ternary alloys and beyond by taking solute drag aspects, and the use of advanced nucleation models. Numerous applications are then illustrated.

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.

Effect of Solid-State Phase Transformation and Transverse Restraint on Residual Stress Distribution in Laser-Arc Hybrid Welding Joint of Q345 Steel.

A Q345 steel butt-welded joint was manufactured using laser-arc hybrid welding (LAHW) technology, and its microstructure, microhardness, and residual stress (RS) distribution were measured. Using ABAQUS software, a sequentially coupled thermo-metallurgical-mechanical finite element method was employed to model the welding RS distribution in the LAHW joint made of Q345 steel. The effects of solid-state phase transformation (SSPT) and transverse restraint on the welding RS distribution were explored. The results show that a large number of martensite phase transformations occurred in the fusion zone and heat-affected zone of the LAHW joint. Furthermore, the SSPT had a significant effect on the magnitude and distribution of RS in the LAHW joint made of Q345 steel, which must be taken into account in numerical simulations. Transverse restraints markedly increased the transverse RS on the upper surface, with a comparatively minor impact on the longitudinal RS distribution. After the transverse restraint was released, both the longitudinal and transverse RS distributions in the LAHW joint reverted to a level akin to that of the welded joint under free conditions.

Effects of microalloying and magnetic annealing on microstructures and properties of magnetic immiscible copper matrix alloys

The magnetic immiscible copper matrix alloys are extensively industrial applications due to their comprehensive properties such as high strength, high conductivity, high thermal conductivity and excellent magnetic permeability. Here, we investigated the effects of the addition of Co and Al elements on the microstructural evolution of magnetic Cu–Fe alloys (CFAs) and comprehensive properties after annealing under the static magnetic field (SMF). The results show that the microalloying was effective in enhancing the mechanical and magnetic properties but negative for the electric conductivity. The addition of Co element significantly suppressed the grains recrystallization of CFAs during the annealing process and improved the thermal stability of CFAs. Al elements homogeneously dispersed in the sample. SMF facilitated the nucleation and precipitation processes of the secondary precipitated particles in the Cu-rich matrix through the introducing the extra magnetic Gibbs free energy and also promoted the structural transformation of the secondary precipitated particles from FCC to BCC. This work indicates that SMF can be effectively used to modify the magnetic materials with exceptional comprehensive properties during the solid state phase transformation process. It also provides valuable guidance for the development of novel materials with superior comprehensive performance.

Formation and long-time exposure aging of oxides on Ni-Cr and Ni-Cr-X (X = Mo, W) alloys in acidic chloride solutions: Ramifications towards corrosion resistance

The passivation of Ni-22Cr, Ni-22Cr-6Mo, and Ni-22Cr-6Mo-3W (wt%) was characterized from 100 s to 10 days in 0.1 M NaCl in pH 4 at various potentials in the passive range. The influence of aqueous solution exposure time in NaCl (exposure aging), passivation potential, as well as Ni-Cr-X alloy composition (Mo or W) on passive film growth, composition, oxidation state, molecular identity, and inner versus outer film layering were investigated. Initial passive film formation was dominated by Ni(II). Cr(III) oxides and hydroxides tended to enrich upon formation and exposure aging. Corundum and rock salt are speculated to undergo a solid-state phase transformation to form NiCr2O4 at long times under certain conditions. The concept of the time-potential-transformation diagrams to describe the evolution of passive film phases and protectiveness is introduced. Factors limiting Cr(III) enrichment were discussed. Aliovalent Mo and W ions were detected in passive films, particularly after aging. The roles of Cr enrichment, Ni depletion, and Mo and/or W aliovalent cations after oxide aging towards protectiveness to passive and localized corrosion in chloride (Cl-) solutions were discussed.

Short study on mechanism of morphology change of γ′ precipitates in IN718 Plus

γ′ Ni3Al phase (L12 structure) strengthened superalloy has become an important strengthening phase in Ni based superalloys because of its excellent high-temperature service performance, and has been widely used. The process of γ′ phase precipitation has also received a lot of attention and research. However, most studies focused on its growth rate. There were few studies on the morphology transformation of γ′ phase, and the morphology transformation process is complex. The mechanism of γ′ phase transformation process was systematically analyzed in this paper. Firstly, through molecular dynamics calculation, the most stable morphology of the γ′ phase in the nickel matrix was obtained under ideal conditions. The stability of spherical precipitates is higher than that of cubic precipitates. When the temperature is greater than 800 K, the stability is more obvious.Then through the micro EDS analysis of the γ′ phase in the morphology transformation process, it was shown that the soft impingement phenomenon exists in the nickel base alloy with high γ′ phase volume fraction. Further, by the derivation of the formula of solid-state phase transformation, it was found that the growth rate of precipitate is directly proportional to the concentration gradient of Al in the matrix at the interface, while increases with the rising of the Al concentration in the matrix at the interface. Finally, the way and mechanism of soft impingement affecting the morphology transformation were analyzed. The precipitates formed by soft impingement are nearly cubic precipitates with rounded edges and corners.