Transformation Of Metastable Phase Research Articles

Metastable phase transformations and mechanical properties of an as-extruded TiAl-based alloy during two-step heat treatment

Metastable phase transformations of TiAl-based alloys provide a pathway for grain refinement and performance improvement. In this paper, the microstructure evolution of an as-extruded Ti-47Al-2Cr-0.2Mo alloy during quenching and tempering was investigated by scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM), and the mechanical properties of the alloy were further evaluated by tensile tests. The results indicated that the analogous Σ3 coincidence site lattice (CSL) and γ/γ true twin interface appeared in both the Widmanstätten structure (γw) and feathery structure (γf), implying that sympathetic nucleation mechanism was responsible for the nucleation of the metastable phases. A 6 H intermediate phase with the long period stacking ordered structure (LPSO) was discovered after short-term water quenching, which provided an evidence supplement of the sympathetic nucleation mechanism. The transformation path of the metastable γw phase was proposed to follow the sequence of α/α2→6 H→γ. Apparent refinements of both grains and α2/γ lamellar colonies ((α2+γ)L) were exhibited after metastable phase transformation. The ultimate tensile strengths of the heat-treated alloy at whether room temperature (RT) or 800 °C were obviously improved, and the elongation at RT also kept at a considerable level.

Effect of metastable precipitate phase transformation on reheat cracking in SS347H welding

Reheat cracking occurred in the welded heat affected zone (HAZ) of the SS347H tube during high temperature applications of approximately 600 °C. The reheat cracks occur when the inside of the grain is strengthened, causing grain boundary embrittlement. In this study, the intra-granular strengthening phase transformation of the HAZ causing the reheat crack was analyzed using TEM.The hardness of SS 347H HAZ with cracks is measured to be 221HV on average, which is about 25HV higher than 196HV of HAZ without cracks. This intra-granular strengthening is due to the formation of intra-granular precipitate phases. The precipitated phase is ultimately NbC, but undergoes an intermediate phase transformation due to diffusion of Nb and C components. The phase transformation consists of the following steps: “(i) SA (solution annealing), (ii) Zone A, (iii) Zone B, and (iv) NbC precipitation.”This is a process in which Nb/C components diffuse and segregate at the reheating temperature from a solid solution state of Nb/C components, and NbC with a size of several tens of nanometers is precipitated. As metastable phases, Zones A and B exist in the intermediate process of precipitation, which is the strengthening stage where intra-granular hardness is maximized. Zone A and B in the pre-precipitation stage means that a new lattice plane is formed as the segregation of component elements forms coherency with the matrix. The relationship between the base and new lattice planes is confirmed by Double Diffraction in the TEM diffraction diagram.The root cause of reheat cracking in SS 347H HAZ is believed to be the intra-granular strengthening phase caused by zone A and zone B as a metastable phase transformation in an intermediate stage before NbC precipitation. In order to suppress reheat cracking, it is desirable to precipitate stable NbC from the metastable phases of Zones A and Zone B through stabilization heat treatment.

Precipitation evolution of Al-Zn-Mg-Cu-(Ag) alloys with a low Zn/Mg ratio

The influence of minor Ag on the precipitation evolution of the Al-4.2Zn-2.8Mg-1.0Cu (wt.%) alloy from early stages to over-aged stages at 150 °C was investigated. Surprisingly, co-precipitation of strengthening phases T′ and η′ are found in both Ag-free and Ag-added alloys. With Ag addition, precipitation of both T′ and η′ is refined and increased, such that the age-hardening capabilities and peak-aged tensile strength are improved. In addition, the quantitative proportion of η′ precipitates increases with the increase of Ag content due to the increase in the (Zn+Cu)/Mg ratio of nucleating particles. The narrowed precipitate-free zones (PFZs) are considered responsible for the undiminished fracture elongation in Ag-added alloys. Essentially, these effects of Ag are closely related to the strong Ag-vacancy and Ag-solute interactions. In over-aged stages, the Ag-added alloys still possess higher hardness values compared to the Ag-free alloy, which is related to precipitate coarsening mechanisms. The Ag-free alloy follows classical coarsening behavior by solid solution mediated diffusion, while the Ag-added alloy follows two possible coarsening mechanisms, coalescence of aggregates and diffusion of atoms. The smaller average size and higher residual number density of precipitates benefited from the slow diffusion-controlled coarsening behavior depending on the precipitate composition characteristics of the two-stage differentiation and the precipitate distribution characteristics of high-density dispersion in early-aged stages could explain why the hardness of Ag-added alloy keeps at a higher level than that of Ag-free alloy even after 1000 h ageing. Meanwhile, the transformation of metastable phases to stable phases is inhibited due to the addition of Ag, such that GP zones, T′, η′, η and T phases coexist even after 14 d of ageing. In terms of phase composition, the addition of Ag decreases the ratio of Mg/(Al+Zn) in T-type phase. For the Ag-added alloy, the sum concentration of Zn+Mg in η′ phase is about 10 at.% higher compared to T′ phase, and η phase continues to have a high sum concentration of Zn+Mg, besides, the Zn/Mg ratio and Cu concentration exhibit obvious differences from T phase.

Application of CALPHAD Method for Predicting of Concentration Range of Amorphization of Transition Metals Melts

Early, the efficiency of the CALPHAD (Calculation of Phase Diagrams) method to a targeted search for compositions of amorphous alloys has been shown. The method for predicting the ranges of amorphization is based on the calculation of diagrams of metastable phase transformations between supercooled melts and boundary solid solutions on the base of pure elements. In this work, the model parameters for thermodynamic properties of liquid alloys and boundary solid solutions were summarized in a self-consistent database for the multicomponent Cu–Fe–Ni–Ti–Zr–Hf system. Such database for the multicomponent system is based on a common set of model parameters for boundary binary and ternary systems. This database was used to predict the concentration ranges of amorphization for the quinary Cu–Fe–Ni–Ti–Zr, Cu–Fe–Ni–Ti–Hf and boundary ternary and quaternary systems. The results of calculations are presented along sections in quaternary and quinary systems. The ternary and quaternary equiatomic alloys along with high entropy CuFeNiTiZr and CuFeNiTiHf alloys are trapped into prognosed composition ranges of amorphization. Predicted composition space of amorphization for melts of the Fe–Ni–Ti–Zr system is shown on the concentration tetrahedron. Based on the obtained results, a new criterion for predicting the concentration regions of amorphization of multicomponent melts is proposed, according to which the presence of a sufficient content of metals that are electron acceptors and donors is a chemical factor that affects the thermodynamic stability of melts and determines their glass-forming ability. For multicomponent melts of the Cu–Fe–Ni–Ti–Zr–Hf system the concentration ranges of amorphization correspond to the simultaneous fulfillment of the conditions xFe + xNi + xCu > 0.25 and xTi + xZr + xHf > 0.15, where Fe, Ni, and Cu are electron acceptors and Ti, Zr, and Hf are electron donors.

Cu-based metastable microstructural template and its role in the heat flux application

The present paper focuses on developing an optimised strength-conductive alloy in Cu–Fe–Si system. We present the effect of a cost-effective rapid-casting route on modifying the microstructure. The non-equilibrium phase transformation events are tracked and the evolution sequences of metastable phases in the microstructure are elucidated. The metastable phase transformation of the precipitates from DO19- to L12-ordered structure and its role in enhancing thermal conductivity are presented. A density functional theory (DFT)-based calculation also complements the experimental evidence of the relative stability of the evolved metastable phases. One of the key findings is the evolution of L12-ordered precipitates via face centred cubic (FCC)-based interfacial matrix template.

Structures and energies of computed silicon (001) small angle mixed grain boundaries as a function of three macroscopic characters

Small angle grain boundaries comprised of dislocation structures have unique impacts on the performances of crystalline materials. In this atomistic study, we report structure-property correlations of Si (001) small angle mixed grain boundaries (SAMGBs) across three macroscopic GB characters (tilt character, twist character, and an implicit rotation character between them). Firstly, for the computed SAMGB energies spanning the three-dimensional GB character space, a revised Read-Shockley relationship is proposed and fitted, which hints that the energy trends follow the Read-Shockley formalism from the insight of GB decomposition. GB structural transitions from dislocation to amorphous structures are also given as a function of dislocation core radii, allowing the estimation of small angle GB proportion. Then, the proportion, topology and structural signatures of different SAMGB types defined from the ratio between the tilt and twist angles are presented. By extracting the transformation of metastable SAMGB phases, the formation process of these (001) SAMGB structures is characterized as the energetically favorable dislocation glide and reaction, from which the dislocation density function is derived. The relevant results about SAMGB energies and structures are validated and supported by theoretical calculations and experimental observations, respectively.

Monotropic type transformations of diamond below the Berman-Simon line

We construct the phase diagram of a monotropic type for carbon under the Berman-Simon line. It describes metastable phase transformations occurring in diamond when the graphitization is delayed. However, thus far, monotropic phase transition of metastable diamond could not be experimentally observed. To produced delay conditions, we introduced at diamond surface a droplet of graphite melt with a temperature of about 4800K. We demonstrate that thermal shock provided mechanical stress and competition between phase transformations occurring at two high temperature and high pressure branches. At the so called “fast graphitization line” (reported by F.P. Bundy et al., Carbon 34 (1996) 141) the transformations take place tending to the spontaneous diamond graphitization, whereas at the line of monotropic phase diagram, they tend toward the melting of diamond. The transition can start as the graphitization, whereas change-over to the second branch can occur as a result of rapid unloading of mechanical stresses. The melting wins this is competition due to its faster kinetics. We have also found out the characteristic features underlying the initiation of processes near the “fast graphitization line” as are dealing with the spontaneous formation in diamond of a network of compressed graphite, and with the melting of such graphite in the course of its expansion taking place owing to the loss of mechanical stability of diamond caused by the network. This mechanism, apparently reveals a liquid-like nature of the transformations at the “fast graphitization line”.

Electron Microscopy Study of Structural Defects Formed in Additively Manufactured AlSi10Mg Alloy Processed by Equal Channel Angular Pressing

This study focused on electron microscopy studies of microstructural defects formed in an additively manufactured (AM) AlSi10Mg alloy as a result of post-deformation by equal channel angular pressing (ECAP), with the aim of elucidating the fundamental deformation mechanisms that govern the plasticity of both the aluminium matrix and the silicon phase. This article focused on the process of grain refinement, metastable phase transformations, and microstructural defects such as stacking faults or amorphous areas that severely disrupt the face-centred cubic (FCC) crystal lattice symmetry. The findings presented in this study imply that deformation twinning, phase transformation, and amorphization are not mutually exclusive modes of Si phase deformation. Both can occur at an ECAP temperature of 150 °C. At a deformation temperature of 100 °C, amorphization is the dominant deformation mode of the Si phase. It was also discovered that dislocation slip was the predominant deformation mode of Al matrix at 150 °C, while at 100 °C, additionally stacking faults were formed within the Al matrix. The present findings provide not only a fundamental understanding of the deformation micro-mechanism of the SLMed AlSi10Mg alloy but also open a new horizon for the development of the next generation of structural materials.

Laser Powder Bed Fusion Processing of Soft Magnetic Fe–Ni–Si Alloys: Effect of Hot Isostatic Pressing Treatment

Laser powder bed fusion (L-PBF)-processed high-silicon steel has great advantages in freely designed electric engines, and various studies have been conducted in this field. However, the analysis of both the mechanical and magnetic properties, focusing on the multiscale microstructure under as-fabricated and heat-treated conditions, which is indispensable for industrial applications, has not been performed. In this study, an Fe–Ni–Si sample was fabricated using the L-PBF process. Subsequently, the following hot isotropic pressing (HIPing) process was employed as a post heat treatment step for the Fe–Ni–Si alloys. The effects of HIPing on the microstructure were investigated, focusing on the metastable stable phase transformation in the Fe–Ni–Si system. X-ray diffraction results showed single-phase fcc γ (Fe, Ni) in the L-PBF-processed samples before and after HIPing. Moreover, the acicular Ni/Si-rich structure (formed in the as-fabricated L-PBF sample because of its high cooling rates) transformed to the equilibrium austenite, Ni3Si, and FeNi3 phases during HIPing. After HIP, the compressive modulus and strength increased from 11 GPa and 650 MPa to approximately 18 GPa and 900 MPa, respectively. The magnetic properties were evaluated via a hysteresis loop, and the coercivity increased from 1.8 kA/m and to 2.9 kA/m after the HIPing process.

Study of Ti–Si in situ composite processing by multi-stage eutectic solidification

Abstract A titanium composite alloy (Ti –Al –Si – Zr) manufacturing process using eutectic solidification was studied by simultaneous thermal analysis up to 1480 °C. Phase transformations predicted by sections of the phase stability diagram (ThermoCalc optimisation) have been observed. The structure of the alloy was also studied by scanning electron microscopy (SEM), light optical microscopy (LOM) and X-ray microanalysis. Several patterns, unexplained on the basis of the stable phase diagrams of the system, are revealed to be associated with metastable phase transformations. The results can be used for a determination of composite applications and the optimisation of its casting technology.

The Effect of Reaction Conditions and Presence of Magnesium on the Crystallization of Nickel Sulfate

Recycling of valuable metals such as nickel is instrumental to meet the need from the dramatic increase in electric vehicle battery production and to improve its sustainability. Nickel required in the battery manufacture can be recovered from the hydrometallurgical industrial process streams by crystallization of nickel sulfate. Here, crystallization of nickel sulfate is studied from an industrial point of view, investigating the effects of temperature, seeding and presence of magnesium on the formation of various solid phases for the evaluation of their potential influence on the process design. Results showed that the precipitating phase was dictated both by seed amount and reaction temperature. Transformation of metastable phases both in suspension and in a dry state was observed over time. Presence of magnesium was shown to promote formation of NiSO4·7H2O in solution and increased its stability in a dry form. In their dry state, nickel sulfate that was formed in the absence of magnesium transformed towards α-NiSO4·6H2O, whereas those precipitated in the presence of high magnesium concentrations transformed towards β-NiSO4·6H2O, indicating that magnesium inhibited the phase transformation towards α-NiSO4·6H2O. Knowledge about various solid phases of varying crystal morphology and stability can be used as input to decisions for the best suited solid product type and how this relates to the initial conditions of the sidestreams.

Transformation of the Metastable Al6Fe Intermetallic Phase during Homogenization of a Binary Al-Fe Alloy

Within the scope of this research the transformation of the Al6Fe metastable phase was analyzed via Differential Scanning Calorimetry (DSC), optical and Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). A binary Al-Fe1.1 low-impurity alloy was produced with refined raw materials in a controlled environment. With a cooling rate of 35 K/s, solidification of the Al6Fe metastable phase was achieved. The samples were homogenized at 600 °C for 2–24 h. Results of a qualitative analysis of metallographic samples show that the transformation began on grain boundaries, forming an Fe-phase free region, but after 2 h began to take place within the eutectic region. The transformation is mostly complete after 12 h, but after 24 h of homogenization it is fully complete as all samples, except the 24 h homogenized one, contain both the metastable Al6Fe and the stable Al13Fe4 phase.

Metal cyamelurates: structural diversity caused by kinetic and thermodynamic controls

The kinetic control of chemical reactions makes it possible both to isolate metastable substances with properties different from those of thermodynamically stable phases and to obtain information useful for studying the crystal nucleation and further transformations of metastable phases into stable ones. Metal cyamelurates are suitable subjects for kinetic control in synthesis due to their easy crystallization and short reaction times. In this work, cobalt, manganese, magnesium, copper, aluminum, chromium, calcium, and lead cyamelurates were obtained and their crystal structures were determined. Most syntheses at room temperature take place under kinetic control, and this often leads to a mixture of phases. KM(C6N7O3)·5H2O (M = Co(II), Mn(II)) powders consisted of one crystalline compound. An increase in temperature leads to thermodynamic control, which is proved by registration of Mn(C6N7O3H)·5H2O and KMg(C6N7O3)·5H2O crystalline phases. It was possible to isolate Ca(C6N7O3H)·6H2O and Pb(C6N7O3H)·3H2O from the mixture by washing samples with a weak acid solution. As a result of the work, five different structural types were obtained, including KCu(C6N7O3)∙2H2O and KM(C6N7O3H)2·6H2O (M = Al, Cr(III)). Despite the diversity of salts obtained as a result of thermodynamic or kinetic control, practically the same building blocks form the crystal structure of cyamelurates. The hypothesis of nucleation in a colloidal micelle with electric double layer can be applied to the compounds listed above.

Bulk Crystalline 4H-Silicon through a Metastable Allotropic Transition.

We report the synthesis of bulk, highly oriented, crystalline 4H hexagonal silicon (4H-Si), through a metastable phase transformation upon heating the single-crystalline Si_{24} allotrope. Remarkably, the resulting 4H-Si crystallites exhibit an orientation relationship with the Si_{24} crystals, indicating a structural relationship between the two phases. Optical absorption measurements reveal that 4H-Si exhibits an indirect band gap near 1.2eV, in agreement with first principles calculations. The metastable crystalline transition pathway provides a novel route to access bulk crystalline 4H-Si in contrast to previous transformation paths that yield only nanocrystalline-disordered materials.

In situ correlation between metastable phase-transformation mechanism and kinetics in a metallic glass

A combination of complementary high-energy X-ray diffraction, containerless solidification during electromagnetic levitation and transmission electron microscopy is used to map in situ the phase evolution in a prototype Cu-Zr-Al glass during flash-annealing imposed at a rate ranging from 102 to 103 K s−1 and during cooling from the liquid state. Such a combination of experimental techniques provides hitherto inaccessible insight into the phase-transformation mechanism and its kinetics with high temporal resolution over the entire temperature range of the existence of the supercooled liquid. On flash-annealing, most of the formed phases represent transient (metastable) states – they crystallographically conform to their equilibrium phases but the compositions, revealed by atom probe tomography, are different. It is only the B2 CuZr phase which is represented by its equilibrium composition, and its growth is facilitated by a kinetic mechanism of Al partitioning; Al-rich precipitates of less than 10 nm in a diameter are revealed. In this work, the kinetic and chemical conditions of the high propensity of the glass for the B2 phase formation are formulated, and the multi-technique approach can be applied to map phase transformations in other metallic-glass-forming systems.

Superheated metastable water atomization: fine atomized water plumes diagnostics and disperse characteristics; summary of results, applications in power generation and new techniques.

Complex experimental investigation of superheated flow and dispersion characteristics of sprays in atomizing nozzles of various types is made. The comparative analysis of experimental and calculating investigations of thermo physic and hydro- gas-dynamic flow and disperse characteristics was carried out for various types of spray nozzles. It has shown that the main process, which determine peculiarities of superheated metastable water flow and phase transformation in spray nozzles, is quick (explosive) boiling up. And the optimal variant for superheated water atomization and achieving the most fine spray plumes, which contain mainly submicron droplet fraction, at initial temperature 240°C (and higher) is atomization by means of contracting-expanding nozzles. In such nozzles the explosive boiling is accomplished by quick transformation of foamed liquid into vapor-droplet flow. Diagnostic peculiarities are considered for these optically dense spray plumes; the auxiliary measurement methods were worked out for estimation superheated water high atomization level. The experimental and calculating data dimensionless treatment was made for achieving extremely fine (mainly submicron) superheated water spray. The dimensionless parameter, which determines necessary water (or some other liquids, for example – fuels) superheating level was derived. The possible technical applications of superheated water sprays are described, and up-to-date state of corresponding projects is given.

Synergistic Erosion-Corrosion Behavior of AISI 2205 Duplex Stainless Steel Elbows in Potash Brine-Sand Slurry and the Associated Microstructural Changes

Slurry erosion-corrosion damage to materials used in mineral processing industries occurs due to the combined effects of electrochemical and mechanical processes. In this study, AISI 2205 duplex stainless steel elbows were tested in a flow loop using slurries consisting 10, 20, 30, 40 and 50 wt.% sand particles and saturated potash brine. Erosion-corrosion tests were conducted at flow velocities of 2.5 and 4.0 m/s. The results of the gravimetric analysis revealed that erosion-corrosion rates increased with increasing particle concentration at both velocities. The elbows exhibited negative synergy for all operating conditions, which is attributed to the presence of passive oxide film on the surface. Microstructural examination of the eroded surfaces showed that materials removal occurred by cutting and plastic deformation. Electron backscattered diffraction analysis suggests that plastic deformation, which occurred approximately 4 µm below the eroded surface, led to the transformation of metastable austenite phase to martensite.

Ion-induced Transformation of Metastable Phases

Ion-induced Transformation of Metastable Phases

On the S-phase precipitates in 2024 aluminum alloy: An atomic-scale investigation using high-angle annular dark-field scanning transmission electron microscopy

Abstract

Influence of the Ti/Al/Nb ratio on the structure and properties on intermetallic layers obtained on titanium by non-vacuum electron beam cladding

Ti-Al-Nb based intermetallic layers of sufficient quality and thickness were obtained by non-vacuum electron beam cladding on the surfaces of Ti workpieces. Optical microscopy and X-ray tomography did not reveal any dramatical defects in the structure of cladded layers. X-ray diffraction as well as scanning and transmission electron microscopy were applied to thoroughly investigate the structure and phase composition of coatings. It was found that non-equilibrium cooling conditions of coatings provided by fast removal of heat to untreated Ti substrate after the electron beam cladding was terminated induced the proceeding of metastable phase transformations. For example, γ-phase formation was suppressed in these coatings. In coatings with 8 and 20 at.% Nb (46 and 43% Al respectively) along with ordered with α2, formation of disordered solution of the alloying elements in α-Ti took place. In high-Nb alloys β(B2) phase has undergone the diffusionless transformation to ω’, which is the intermediate phase in β → ω and the coating with the maximum Nb content characterized by appearance of γ1 as a main phase. ω-phase had negative influence to hardness and wear resistance of coatings, however, generally this paremeter increased in 1.3–1.75 times compared to cp-Ti. The high temperature creep and oxidation properties decreased proportionally with increasing Nb and decreasing Al content in the cladded layers.