Phase Transformation Properties Research Articles

OBTAINING OF ALKALINE-FREE ALUMINOSILICATE GLASS CERAMICS BY SOL-GEL METHOD AND INVESTIGANING OF ITS PROPERTIES

The glass-crystalline ceramic materials in SiO2–Al2O3–RO (RO–MgO, SrO, BaO and their combinations) systems have been synthesized by the sol-gel method. The factors influencing on the sol-gel formation process are revealed. The effect of the composition and temperature-time modes of heat treatment on the crystallization properties of gels, phase transformations, phase composition, granulometry and shape of the resulting particles has been studied. The nature of phase transformations in powders is revealed depending on the temperature-time conditions of heat treatment. It is shown that the dominant crystalline phases, depending on the composition, are anorthites or their solid solutions, their formation temperatures have been established: hexagonal celsian is formed at a temperature of 1350 °C, monoclinic strontium anorthite – at 1400 °C, cordierite – at 1450 °C. The influence of pressing and heat treatment conditions on the physic-mechanical, dielectric and thermal properties of the obtained materials is investigated. The synthesized glass-ceramics with low dielectric constant and dielectric loss tangent, high deformation temperatures (above 1500 °C) is a promising replacement for the spodumene-eucryptite and cordierite sitalls currently used as radio-transparent materials, the disadvantage of which is insufficiently high operating temperatures (not higher 900 – 1100 °C).

Relaxor ferroelectric and dielectric properties of (1–x)Ba(Zr0.1Ti0.9)O3–xBa(Mg1/3Ta2/3)O3 ceramics

The environmentally-friendly (1–x)Ba(Zr0.1Ti0.9)O3–xBa(Mg1/3Ta2/3)O3 (x=0, 0.02, 0.04, 0.06, 0.08) relaxor ferroelectric ceramics were prepared by the conventional solid-state method and sintered in air at 1400 °C for 2 h. SEM and XRD analyses were utilized to study the surface morphologies and the crystalline structures, respectively. The effects of Ba(Mg1/3Ta2/3)O3 on the phase transformation, dielectric and ferroelectric properties of Ba(Zr0.1Ti0.9)O3 ceramics were also investigated. It is found that the average grain size of (1–x)Ba(Zr0.1Ti0.9)O3–xBa(Mg1/3Ta2/3)O3 (BZT–BMT) perovskite single-phase ceramics decreases as the content of Ba(Mg1/3Ta2/3)O3 (BMT) increases. The relaxor ferroelectric behavior with diffuse phase transition and well-defined frequency dispersion of dielectric maximum temperature is found for the ceramic with increasing x values. 0.98BZT–0.02BMT ceramic shows very good dielectric properties with the relative permittivity and the dielectric loss, measured at 100 kHz as 6034 and 0.01399 respectively at room temperature. Both remnant polarization and coercive field decreased with increasing BMT content, indicating a transition from the ferroelectric phase to the paraelectric phase at room temperature.

Heat treatment of hot-isostatic-pressed 60NiTi shape memory alloy: Microstructure, phase transformation and mechanical properties

60NiTi alloy is considered to be a promising material for specialized bearing and gear applications due to its high hardness, strength, and low modulus. However, fabricating 60NiTi through conventional processing methods is challenging due to the brittleness and poor workability. In this study, 60NiTi with high relative density was successfully fabricated directly from pre-alloyed powder through hot isostatic pressing. The effects of solution and aging treatments on microstructure and mechanical properties were systematically studied by advanced characterization techniques. The hot-isostatic-pressed 60NiTi showed low average hardness and elastic strain due to the formation of a soft Ni3Ti phase and B2 NiTi matrix. Solution treatment above 1000 °C dissolved the Ni3Ti phase and promoted the formation of nanoscale Ni4Ti3 precipitates, which significantly improved the hardness, strength, and elastic strain of 60NiTi. The formation of the Ni4Ti3 phase can be mainly attributed to the driving forces induced by the chemical supersaturation and mechanical stress concentration. Finally, the phase transformation mechanisms during heat treatment and compression test were discussed.

Microstructure evolution, phase transformation and mechanical properties of IN738 superalloy fabricated by selective laser melting under different heat treatments

Selective laser melting (SLM) is a new method for manufacturing IN738 superalloy hot-end components. This study evaluated the microstructure evolution, phase transformation, and mechanical properties of IN738, fabricated using SLM before and after different heat treatments. The results showed that the microstructure of the as-SLMed sample was mainly columnar, with cell grains aligned in the building direction (BD) due to the high cooling rate. Many Laves phases and nanoscale Ti-, Nb-, and Ta-rich MC carbides precipitated in the sub-grain boundaries. The grains exhibited full recrystallization and serious coarsening at a solution temperature of 1260 °C. The residual Laves phases completely dissolved when the solution temperature rose above 1140 °C. After solution heat treatment (SHT), the primary MC carbides obviously grew up and were distributed as grains on the grain boundaries, and a large number of fine and homogeneous γ′ phases precipitated from the γ matrix. After aging heat treatment (AHT), hardness, ultimate tensile strength (UTS), and elongation (EL) decreased due to an increase in γ′ size and a reduction in uniformity. With increasing aging temperatures, this situation worsened. However, the yield strength (YS) increased under AHT at 750 °C, which was attributable to the formation of chain-like Cr-rich M23C6 and MC carbides on the grain boundaries. Finally, the overall performance of SLMed IN738 samples improved under solution treatment at 1200 °C for 2 h, followed by aging at 750 °C for 24 h, with increases of UTS and YS from 1205 MPa to 1,351, and from 850 MPa to 1319 MPa, respectively.

Visualized-experimental investigation on a mini-diameter loop thermosyphon with a wide range of filling ratios

The mini-diameter loop thermosyphon, which has an excellent heat transfer performance with simple, compact structure and pump-free advantages, can effectively solve the problem of heat transfer under high heat load in a confined space. The mini-diameter loop thermosyphon exhibits different gas-liquid transport and heat transfer patterns than the conventional-diameter one due to various gas-liquid phase transformation properties and a strong friction force in the mini-diameter channel. Furthermore, data on the visual analysis of gas-liquid transport and heat transfer properties of the mini-diameter loop thermosyphon under various filling ratios is still lacking. In this paper, a visible experimental research on a mini-diameter loop thermosyphon (3 mm) in a wide filling ratio range(44% -97%) is undertaken. The maximal heat transfer capacity increases initially and subsequently falls with the filling ratio, according to the testing data. At a low filling ratio, the flow pattern changes as slug flow - churn flow - annular flow with the heat load increase, leading to a low loop thermal resistance, and dry out occurs at 185 W heat load under 50% filling ratio. The gas-liquid flow shifts to slug flow-geyser boiling-bubble flow at high filling ratios (73% and 82%), and the bubble departure diameter steadily decreases with increasing heat load. Consequently, the heat transfer performance (thermal resistance) has been enhanced, and the maximum heat transfer capacity has been increased to 280 W. Single-phase flow is plainly seen in the loop when the filling ratio reaches 91% or even 97%, thus reduced heat transfer capacity. Therefore, the high-filling-ratio mini-diameter loop thermosyphon is better for high-load heat dissipation. However, a low thermal resistance shows at the low filling ratio, but the heat load is also low. As a result, a loop thermosyphon with a low filling ratio should be selected for the low heat load situation.

Effect of Oxygen Diffusion During the Post-Processing of Ti6Al4V Lattice Structures Fabricated by the Selective Laser Melting Process

Abstract Post-processing of Ti6Al4V lattice structures fabricated using selective laser melting (SLM) was performed using hot isostatic pressing (HIPing) and heat treatment (HT) to mitigate the undesired effect of rapid cooling during SLM. Oxygen diffusion during post-processing had a significant influence on the microstructure and subsequently the mechanical properties of the lattices. Oxygen content analysis was conducted to confirm the oxygen diffusion through the strurts’ peripheries. The effect of oxygen diffusion during the HIPing and sub-transus HT (600–800 °C) regimes on the phase transformation, failure mechanisms, and mechanical properties of the lattices was investigated. Results revealed that the transformation of the originally formed α′ martensite was dependent on the post-processing temperature. This transformation resulted in a decrease in yield strength. The decrease in failure strain (ductility) for all treated conditions was related to oxygen diffusion, forming near-surface α-case.

Crystal-plasticity modeling of phase transformation–viscoplasticity coupling in high-temperature shape memory alloys

A coupling between phase transformation and viscoplasticity is observed in HTSMAs during actuation. The dislocations generated and retained phases accumulated are responsible for the coupling. It results in functional fatigue, which is reflected as an alteration in the functional properties of the alloy, and an increase in the irrecoverable deformations. The objective of the present study was to develop a theoretical framework to account for the interactions between viscoplasticity and phase transformation to simulate the alteration in functional properties and generation of irrecoverable deformations. A crystal-plasticity based multi-scale approach was followed to develop the framework. This approach accounts for the mechanisms of: phase transformation, transformation induced plasticity, accumulation of retained martensite, plasticity (in a rate-independent manner), and viscoplasticity (in a rate-dependent manner). The coupling was accounted by a direct effect of the dislocation densities produced by the irrecoverable mechanisms onto the transformation resistance and retained martensite. This resulted in an indirect effect on the functional properties of phase transformation. On simulating the response of single crystals and polycrystals, anisotropic responses are captured at the grain scale (from single crystals), and a nearly isotropic response is captured at the macro scale (from polycrystals). The results show consistency between the functional property trends (such as TT and hysteresis) and those observed experimentally at the macroscale. The contributions of this study are presenting an effect of: (i) texture, (ii) thermal cycling rate and (iii) functional fatigue, through the response of several randomly oriented single crystals and a polycrystal of Ni–Ti–Hf HTSMA.

Effect of laser scanning speed on the microstructure, phase transformation and mechanical property of NiTi alloys fabricated by LPBF

This study investigated the effects of laser scanning speed on the microstructure, phase transformation and properties of NiTi alloys fabricated by laser powder bed fusion (LPBF). In this study, the contributions of metallurgical factors under different scanning speeds, such as Ni evaporation, Ni4Ti3 precipitation, dislocations and internal stress, to the transformation temperature and transformation latent heat were clarified through specially designed experiments. Ni evaporation is found to have the most profound effect, followed by precipitation. Increasing scanning speed is found to reduce Ni loss, thus cause less increase in the transformation temperature and transformation heat of the LPBF-NiTi alloys. Increasing scanning speed also increases the microstructure non-uniformity and thus widens the transformation temperature interval. The orientations of residual stress exhibit strong crystallographic stiffness dependence. The LPBF-NiTi alloys with different scanning speeds all exhibited high strains (>13.4%) and excellent shape memory effect. A LPBF-NiTi honeycomb structure exhibited 96% shape recovery rate after a 60% pre-compressive deformation. Besides, there is an optimum scanning speed for minimum porosity and smallest average pore size. However, the pore structure is found to have weak influence on the tensile behaviour of the LPBF-NiTi alloys, possibly due to the high defect tolerance of the martensitic transformation.

Effect of heat treatment on microstructure evolution, phase transformation and mechanical properties of dual phase Cu-Zn alloy

Effects of heat treatment conditions on the microstructure evolution, phase transformation and mechanical properties of dual phase Cu-Zn alloy were systematically investigated. Meanwhile, the relationship between the nano-hardness and the macroscopic hardness was also discussed in the study. The results show that the β phase rich in Zn, has the phase transformation temperature from 456.5 °C to 827.1 °C. With the increasing of heat treatment temperature, significant (200) (220) α phase to (200) (211) β phase transformation occur and the β phase fraction increases from as-received 13.1–41.3%. The grain becomes coarser and grows rapidly from 32.6 µm to 49.7 µm from temperature 550 °C to 800 °C, the increase of β phase results in the strength increases and the fracture elongation decreases. The β phase fraction has not varied significantly at temperature 750 °C and 800 °C, the strength has a further increase which is mainly attributed to a part of fine needles of Widmanstätten phase begin to form. At the same displacement, the β phase exhibits a much larger load than that of the α phase, the material pile-ups around indent edges of the α phase becomes more evident indicating that the α phase is softer and has a better ductile. The big difference in the predicted hardness and the macroscopic hardness when heat treatment at 750 °C and 800 °C is mainly attributed to the formation of the fine needles of Widmanstätten phase.

Flexible electrospun iron compounds/carbon fibers: Phase transformation and electrochemical properties

Flexible electrodes are currently widely studied for applications in energy storage, due to their elasticity and lightweight. Here we describe the fabrication of flexible electrodes obtained by embedding different iron-compound nanoparticles in carbon fibers. The composites are prepared by electrospinning followed by subsequent thermal annealing. We elucidate the phase evolution process of iron compounds in the carbon fibers, i.e., Fe3O4, Fe3O4/Fe3C and Fe3C being produced in the carbon fibers at 450 °C, 620 °C and 800 °C, respectively. In addition, we investigate the electrochemical performance of Fe3O4/carbon fibers (Fe3O4/CF), Fe3O4/Fe3C/carbon fibers (Fe3O4/Fe3C/CF) and Fe3C/carbon fibers (Fe3C/CF). When used as anode materials for lithium-ion batteries (LIBs), Fe3O4/CF, Fe3O4/Fe3C/CF and Fe3C/CF electrodes exhibit discharge capacities of 255.3, 478.1 and 169.2 mAh g−1 after 650 cycles at 1 A g−1, corresponding to 50.2%, 99.3% and 39.3% of the capacities after the second cycle, respectively. Compared with Fe3O4/CF and Fe3C/CF electrodes, the Fe3O4/Fe3C/CF electrode delivers higher capacity and better cycling stability. The excellent electrochemical performance of Fe3O4/Fe3C/CF is attributed to the high theoretical specific capacity of Fe3O4 and the excellent catalytic activity of Fe3C. Amorphous carbon fibers can buffer large volume changes during the lithium-ion insertion/extraction process. Finally, we evaluate the lithium storage properties of Fe3O4/CF and Fe3C/CF. Our work provides insights on the phase evolution and lithium storage mechanisms for iron-compound nanoparticles embedded in carbon fibers.

Effect of Crystallographic Anisotropy on Phase Transformation and Tribological Properties of Ni-Rich Niti Shape Memory Alloy Fabricated by Lpbf

Effect of Crystallographic Anisotropy on Phase Transformation and Tribological Properties of Ni-Rich Niti Shape Memory Alloy Fabricated by Lpbf

Study on microstructure, phase transformation behavior of T15 high-speed steel and the effect of adding REEs

Rare earth elements (REEs) are a vital modifier in tool & die steel. Their effects on the phase transformation behavior, microstructure characteristics and property of as-cast T15 high-speed steel (HSS) were investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffractometry (XRD), differential scanning calorimetry (DSC) and dilatometry. The experimental results show that the as-cast microstructure of samples is refined and ameliorated after adding REEs, and the total quantity of eutectic carbides with hard and brittle properties is significantly reduced, which contribute to lower Rockwell hardness levels and higher bending strength. The morphology of grain boundaries carbides turns from lamellar to blocky polygonal. During the reheating process, REEs can reduce the thermal expansion coefficient of studied steel before austenitizing and promote the dissolution of alloying elements into austenite matrix. During cooling, REEs delays the occurrence of phase transformation, but it is favorable to the increase of the expansion amount of martensite transformation. Additionally, the microhardness value also increases with the increase of cooling rate. When the cooling rate is 20 °C/s, the full martensite transformation will occur and the microhardness of HSS1 reaches the maximum value of HV738, greater than that of RE-free HSS0 (HV701).

Effects of Hydrothermal Treatment on the Phase Transformation, Surface Roughness, and Mechanical Properties of Monolithic Translucent Zirconia.

This study aimed to investigate the effects of hydrothermal treatment on four types of monolithic, translucent, yttria-stabilized, tetragonal zirconia polycrystals (Y-TZPs). Two commercially available Y-TZP brands-SuperfectZir High Translucency (Aidite Technology Co, China) and Katana HT (Kuraray Noritake Dental, Japan) were assessed. For each brand of Y-TZP, materials of four coloring types, including noncolored (NC), colored by staining (CS), precolored (PC), and multilayered (ML) specimens were investigated after hydrothermal aging in an autoclave at 134°C/0.2 MPa for 0 (control group), 5, 10, and 20 hours. The tetragonal-to-monoclinic phase transformation, surface roughness, flexural strength, and structural reliability (Weibull analysis) were measured and statistically analyzed (α=0.05). The subsurface microstructure was analyzed with scanning electron microscopy. The group ML exhibited the lowest flexural strength and Weibull characteristic strength among the four coloring types (p<0.05). Slight increases in the monoclinic phase volume, flexural strength, and Weibull characteristic strength were observed after hydrothermal aging (pall<0.05). Regardless of coloring type, no significant effects of aging on the Weibull modulus or surface roughness were found for the tested materials. Compared with the Katana HT cross-sections, the SuperfectZir High Translucency cross-sections exhibited a similar but thicker transformation zone. The coloring procedure and material type were found to affect the mechanical properties and aging resistance of translucent monolithic Y-TZP ceramics. Regardless of the aging time, the surface roughness of the tested Y-TZP ceramics remained unchanged.

Microstructure Evolution, Phase Transformation and Mechanical Properties of In738 Superalloy Fabricated by Selective Laser Melting Under Different Heat Treatments

Microstructure Evolution, Phase Transformation and Mechanical Properties of In738 Superalloy Fabricated by Selective Laser Melting Under Different Heat Treatments

Synthesis and characterization of Ge-Ag-Sb-S-Se-Te high-entropy thermoelectric alloys

Multielement alloying is an appealing approach for suppressing thermal conductivity of thermoelectric materials. In this study, we synthesized GeTe-based high-entropy alloys with notable (S, Se) substitution at Te sites and (Ag, Sb) at Ge sites. The Ge0.82Ag0.08Sb0.1S0.5Se0.1Te0.4 exhibits an extremely low thermal conductivity of ∼ 0.66 W/m⋅K and a high Seebeck coefficient (>250 μV/K) over a temperature range of 150 – 400 °C. The influence of lattice distortion on phase transformation and transport properties of Ge0.9-2xAg2xSb0.1S0.5Se0.1Te0.4 (x = 0 – 0.06) was investigated.

In-situ synthesis of high-quality pseudoelastic NiTi alloys with intrinsic Ni4Ti3 phase precipitation using laser DED

Laser directed energy deposition (DED) has been used for fabrication of Nickel-Titanium (NiTi) alloys. The raw material can be pre-alloyed NiTi powders or premixed Ni and Ti powders. Using premixed Ni and Ti powders to in-situ synthesize NiTi alloys shows more potential with advantages of cost-effectiveness and flexibility in tailoring the mechanical and phase transformation properties. However, a common problem for in-situ synthesis of NiTi lies in the difficulty in obtaining sufficiently dense parts due to the alloying effects, liquid capillary effects, and the difference in diffusivity between Ni and Ti. The porosity could be greatly affected by the powder morphology. For the first time, this paper reports the in-situ synthesis of high-quality pseudoelastic NiTi alloys using spherical powders in the laser DED process. The comparisons of the NiTi alloys in-situ synthesized using powders with different morphologies are made. The manufacturing defect formation mechanisms with different powder morphologies (spherical and irregular) are firstly discussed. It is found that the NiTi parts in-situ synthesized using spherical powders have comparable density with those fabricated using pre-alloyed powders. Moreover, Ni4Ti3 precipitates are only observed in the NiTi parts in-situ synthesized using spherical powders. The mechanical properties (pseudoelasticity, Young's modulus, and microhardness) are also affected by the powder morphology. The experimental results indicate that high-quality NiTi alloys can be in-situ synthesized with reduced manufacturing defects, the formation of Ni4Ti3 precipitates, and improved pseudoelasticity and microhardness.

Microstructure and tensile properties of a cost-affordable and ultrahigh-strength metastable β titanium alloy with a composition of Ti-6Al-1Mo-1Fe-6.9Cr

A cost-affordable and ultrahigh-strength metastable β titanium alloy with a composition of Ti-6Al-1Mo-1Fe-6.9Cr has been designed by high-throughput diffusion couple technique. The alloy was heat treated along different regimes to investigate the effects of heat treatments on phase transformation, microstructure evolution, and tensile properties. The results showed that the aging strength of the alloy after solution treatment at 840 °C was higher than that at 800 °C due to the more αs precipitation during aging, while solution treatment at 840 °C and aging caused slight decrease of tensile ductility because of the lower αp-phase fraction during solution treatment. Moreover, the formation TiCr2 Laves phase at aging temperatures above 540 °C resulted in remarkable reduction of tensile ductility. Owing to the multiple-hierarchy α precipitations dispersed into the β matrix, two excellent strength-ductility combinations were successfully obtained for the alloys after solution treatment at 800 °C/840 °C and aging at 470 °C with the ultimate tensile strength and tensile elongation values of 1395 MPa/16.0% and 1614 MPa/9.7%, respectively.

Water distribution and moisture-absorption in egg-white derived peptides: Effects on their physicochemical, conformational, thermostable, and self-assembled properties

Three egg-white derived peptides (DHTKE, MPDAHL, and FFGFN) were characterized with hydrophilia and water distributions. The effect of moisture exposure on their properties at 75% relative humidity for 30 h were further investigated. LF-NMR tests revealed that strong bound-water (relaxation time < 10 ms) accounted for more than 80% of total water in peptides after moisture-absorption. The absorbed water led to the pH of three peptides increase, antioxidant activities in vitro decrease, and diverse changes in their functional group vibrations, molecular hydrophobicity, and phase transformation properties. Compared to dried samples, the hydrated-DHTKE was pyrolyzed and hydrated-MPDAHL was oxidized over 160 °C, while the glass transition, melting, and crosslink temperatures of FFGFN all decreased after moisture-absorption. Moreover, the results indicated that moisture-absorption in FFGFN powder enhanced the surface-hydrophobicity of FFGFN-hydrogel and accelerated its self-organizations. This study provides a comprehensive understanding of moisture-absorption effects on peptides, with these changes potentially impacting storage recommendations and scientific interpretations.

Theoretical studies of pressure induced structural phase transformation and elastic properties in nickel silicide

In this study, we have explored pressure dependent structural phase transformation and related volume collapse in the vicinity of transition in Nickel Silicide (NiSi) with the help of an effective inter-ionic potential approach. It was observed that by employing an effective interionic interaction approach, which is basically based on phenomenological models of lattice dynamics, includes the long-range Coulomb, van der Waals (vdW) interaction, and the short-range repulsive interaction in the framework of the Hafemeister and Flygare approach. Our results indicate that the material parameters evaluated by such an approach provide a successful evaluation of the equation of state for change in volume with respect to the applied pressure. The estimated value of phase transition pressure (Pt) is 25 GPa for NiSi, which is in line with the information that we have in the literature. This transition corresponds to the transformation of a B1 (NaCl) type structure to a B2 (CsCl) type structure and is a first-order phase transition. This first order phase transformation showed a volume collapse of about 11.44% at 25 GPa for NiSi. Later on, the relationship between pressure and the variation of second-order elastic constants is examined in greater detail.

Effect of Ti on the Structure and Mechanical Properties of TixZr2.5-xTa Alloys.

To determine the effects of Ti and mixing entropy (ΔSmix) on the structure and mechanical proper-ties of Zr-Ta alloys and then find a new potential energetic structural material with good me-chanical properties and more reactive elements, TixZr2.5−xTa (x = 0, 0.5, 1.0, 1.5, 2.0) alloys were investigated. The XRD experimental results showed that the phase transformation of TixZr2.5−xTa nonequal-ratio ternary alloys depended not on the value of ΔSmix, but on the amount of Ti atoms. With the addition of Ti, the content of the HCP phase decreased gradually. SEM analyses revealed that dendrite morphology and component segregation increasingly developed and then weakened gradually. When x increases to 2.0, TixZr2.5−xTa with the best mechanical properties can be ob-tained. The yield strength, compressive strength and fracture strain of Ti2.0Zr0.5Ta reached 883 MPa, 1568 MPa and 34.58%, respectively. The dependence of the phase transformation and me-chanical properties confirms that improving the properties of Zr-Ta alloys by doping Ti is feasible.