High-temperature Phase Transformation Research Articles

Spark plasma sintering of graphene platelet reinforced zirconia composites with improved mechanical performance

In this study graphene platelet (GPL)-reinforced yittria stabilised zirconia (YSZ) composites were fabricated using a spark plasma sintering furnace to evaluate a new type of structural material for engineering applications. The effects of the content of GPLs on the microstructures and mechanical performances of the GPL/YSZ ceramic composites were investigated. It is found that GPLs are well dispersed in the YSZ matrix and GPL/YSZ composites are nearly fully consolidated. The addition of GPLs significantly refines the ceramic matrix and a higher percentage of GPLs lead to a finer microstructure. GPLs retain their structural integrity during the high temperature processing and phase transformation of YSZ from the tetragonal to monoclinic occurs with the presence of GPLs. Both hardness and toughness of the YSZ matrix are improved by adding GPLs. A maximum improvement of around 7% and 60% in hardness and toughness are achieved respectively.

TiAl-Nb 합금의 고온상변태와 일방향응고에 관한 연구

TiAl-Nb 합금의 고온상변태와 일방향응고에 관한 연구

On the importance of thermodynamic investigations for the re-assessment of selected ternary Fe-base systems

Reliable thermodynamic data are essential for the design and the production of new alloying systems. Particularly, the knowledge of the high-temperature phase transformations (TLiquid, TSolid, TPerit, Tγ→δ) are important for the solidification and the further processing. Investigations of selected commercial Dual-Phase, TRIP and high-Mn TWIP steels by DTA/DSC measurements show that the experimental results differ significantly from the calculation results of thermodynamic databases with respect to the phase transformation temperature and sequence. Based on these findings, it is very important to identify the defective subsystems of complex alloys in order to optimise the thermodynamic databases. In order to verify a quaternary system, e.g. the Fe-C-Si-Mn system, it is important to check the corresponding ternary subsystems. This was performed by DSC measurements of selected model alloys. By doing so, it was found that in Si- and Mn-alloyed Dual-Phase steels the thermodynamic description of the Fe-Si-Mn system is currently inadequate. This is a very important result, since all new designed steel grades for the automotive industry are based on a Fe-C-Si-Mn matrix.

Embrittlement of a FeCrAlMo alloy exposed to high temperature

Abstract A metallurgical evaluation of a FeCrAlMo alloy was carried out after observing low temperature fracture in several purge lines made of this material. The unexpected material embrittlement was attributed to high temperature phase transformations occurred during a previous exposure of the components at 800 °C. The microstructural analysis showed extensive Mo-rich precipitates and Cr-rich carbides at grain boundaries associated with the failed components. The study also showed by means of compression tests carried out at room temperature evident loss of ductility in the samples exposed at high temperatures as a consequence of the material aging.

An in situ correlative STEM-EDS and HRTEM based nanoscale chemical characterization of solid-liquid interfaces in an aluminium alloy.

The chemistry and the structure of solid-liquid interface in an Al-Si based alloy during high temperature phase transformation were characterized at nanoscale using scanning Transmission Electron Microscopy-EDS and HRTEM. Such studies were until recently limited by large sample drift associated with conventional heating holders. This study was made possible thanks to the modern low-drift MEMS-chip based localized heating technology. The results reveal that (i) the structural interface between solid (111) oriented Si phase and the liquid phase (i.e. decay of crystalline order) coexisting at 600°C is 3.2 nm wide (ii) the STEM-EDS chemical maps show inhomogeneous distribution of the elements with the solid phase being rich in Si and the liquid phase rich in Al (iii) the HRTEM and the HAADF images display respectively dark and bright intensity bands along the interface which could be due to apparent enrichment of Cu at the interface region resulting in enhanced amplitude-contrast (darker band in HRTEM) and Z-contrast (bright band in HAADF) and (iv) intriguingly, the concentration profiles within (i.e. compositional width) and across the solid-liquid interface display element-specific complex and asymmetric variation in the chemical widths.

The effect of heat treatment on the mechanical and structural properties of one-part geopolymer-zeolite composites

This contribution presents the results of structural and compressive strength investigations on cured and high-temperature treated silica-based one-part geopolymer-zeolite composites. The specimens were synthesized from two different silica sources, sodium aluminate and water. The phase content as well as the compressive strength of the cured composites varied depending on the starting mix-design and the silica feedstock. Besides geopolymeric gel, A-type zeolites and hydrosodalites were the major reaction products. One of the silica feedstocks yielded significantly higher compressive strength (19MPa), while the other one appears to cause less variation in phase content. Strength testing indicated an improvement on heating up to 200–400°C (28MPa) followed by a moderate decrease up to 700°C. Above 700°C the systems underwent new phase formation and shrinkage (volume decrease) deformations. After exposure at 1000°C the different mixes consisted of a mix of several stuffed silica phases, almost pure hexagonal nepheline or amorphous phase. Depending on the mix-design, the onset temperature of the high temperature phase transformations varied.

Exploring phase stability, electronic and mechanical properties of Ce–Pb intermetallic compounds using first-principles calculations

The phase stability, electronic and mechanical properties of Ce–Pb intermetallics have been investigated by using first-principles calculations. Five stable and four metastable phases of Ce–Pb intermetallics were verified. Among them, CePb2 has been confirmed as HfGa2-type structure. For Ce5Pb3, the high pressure phase transformation from D8m to D88 with trivalent Ce has been predicted to occur at P=1.2GPa and a high temperature phase transformation has been predicted from D8m to D88 with tetravalent Ce at 531.5K. The calculated lattice constants of the five stable phases are in good agreement with experimental values. The electronic density of states, charge density and electron localization function of Ce3Pb have been calculated, which indicated that the Ce and Pb show ionic behavior. The polycrystalline bulk modulus, shear modulus, Young's modulus, and Poisson's ratio are also estimated from the calculated single crystalline elastic constants. All of the calculated elastic constants satisfy mechanical stability criteria. The microhardness and mechanical anisotropy are predicted. The anisotropic nature of the Ce–Pb intermetallic compounds are demonstrated by the three-dimensional orientation dependent surfaces of Young's moduli and linear compressibility are also demonstrated. The longitudinal, transverse and average sound velocities and the Debye temperatures are also obtained in this work. The Ce3Pb has the largest Debye temperature of 192.6K, which means the Ce3Pb has a highest melting point and high thermal conductivity than other compounds.

Thermodynamic optimization of individual steel database by means of systematic DSC measurements according the CALPHAD approach

Reliable thermodynamic data are essential information required for the design of new steel types and are a prerequisite to effective process optimization and simulation. Moreover, it is important to know the exact temperatures at which the high-temperature phase transformations (TLiquid, TSolid, TPerit, Tγ→δ) occur in order to describe the solidification sequence and to describe further processing parameters. By utilizing DTA/DSC measurements, our earlier experimental studies of selected commercial DP, TRIP and high-Mn TWIP steels, have indicated that currently commercially available databases can often not be utilised to reliably describe the behaviour and microstructural development in such complex alloy systems. Because of these ostensible deficiencies, an experimental study was undertaken in an attempt to determine the pertaining thermodynamic data to analyse the behaviour of the important five- component Fe-C-Si-Mn-Al alloy system. High purity model alloys with systematic alloy variations were prepared and utilized in order to determine the influence of individual alloying elements in this complex, but industrially important alloy system. The present study provides new validated experimental thermodynamic data and analysis of the five-component Fe-C-Si- Mn-Al system, which will allow the construction of new phase diagrams, prediction of solidification sequences and the assessment of micro-segregation.

A modified hot thermocouple apparatus for the study of molten oxide solidification and crystallization

The hot thermocouple technique (HTT) is an experimental method primarily used for studying high temperature phase transformations and interactions. The thermocouple driver is the main component of the apparatus, allowing heating and temperature measurement of the thermocouple that acts as a heating element and temperature sensor simultaneously. Previous setups have employed an ac (alternating current) power feed to the thermocouple, creating inherent limitations to the time interval and frequency of temperature sensing and control. In this article, the development of a dc (direct current)-based thermocouple driver is discussed. The new setup allows higher frequency (480 Hz) of heating pulses applied to the thermocouple. The higher switching frequency improves the thermocouple temperature reading accuracy and decreases the time interval between the measurements by a factor of eight, compared to existing HTT devices. The development of a dc power HTT apparatus, its calibration, and examples of its use in the studies of the crystallization of oxide mixtures are presented in this article.

Formation of Surface Corrosion-Resistant Nanocrystalline Structures on Steel

Engineering materials with nanocrystalline structure could be exploited under simultaneous action of mechanical loading and corrosion environments; therefore, their corrosion resistance is important. Surface nanocrystalline structure was generated on middle carbon steels by severe plastic deformation using the method of mechanical pulse friction treatment. This treatment additionally includes high temperature phase transformation and alloying. Using a complex of the corrosive, electrochemical and physical investigations, it was established that nanocrystalline structures can be characterized by lower or increased corrosion resistance in comparison with the reference material. It is caused by the action of two confronting factors: arising energy level and anticorrosive alloying of the surface layer.

Rapid Solidification and Phase Transformations in Additive Manufactured Materials

In recent years, additive manufacturing (AM) has emerged as a promising manufacturing technique to enable the production of complex engineering structures with high efficiency and accuracy. Among the important factors establishing AM as a sustainable manufacturing process is the ability to control the microstructures and properties of AM products. In most AM processes, such as laser sintering (LS), laser melting (LM), and laser metal deposition (LMD), rapid solidification and high-temperature phase transformations play primary roles in determining nano- and microstructures, and consequently the mechanical and other properties of AM products. 1 This topic of JOM is dedicated to summarizing the current research efforts in the area of rapid solidification and phase transformations in additively manufactured materials. A brief summary follows below of 10 journal articles in this topic. AM results in microstructures that typically differ from those produced by more conventional processing. Cheruvathur et al. explore the phases and microstructures of laser powder-bed fusion additively manufactured 17-4 precipitation hardenable (PH) stainless steel, including the role of post-build thermal processing. They determined that homogenization effectively eliminates the as-built solidification structure, resulting in 90% martensite and 10% austenite that exhibits similar microhardness values to conventionally wrought-processed 17-4 PH.

Calorimetric Study of Phase Stability and Phase Transformation in U-xZr (x = 2, 5, 10 wt pct) Alloys

A comprehensive calorimetric study of high-temperature phase equilibria and phase transformation characteristics in U-xZr (x = 2, 5, 10 wt pct) alloys has been undertaken, as a function of heating and cooling rates. It is found that the following sequence of phase transformation takes place upon slow heating in annealed U-2 wt pct Zr alloy: α + α′ + δ-UZr2 → α + γ 2 → β + γ 2 → β + γ 1 → γ. For alloys of 5 and 10 wt pct Zr, the additional presence of a miscibility gap (γ 1 U-rich bcc + γ 2 Zr-rich bcc) in the high-temperature γ(bcc) phase region resulted in the following transformation sequence: α + α′ + δ-UZr2 → α + γ 2 → β + γ 2 → γ 1 + γ 2 → γ. Further, it has been demonstrated that depending on the nature of starting microstructure, namely whether it is α eq + δ-UZr2, or a mix of α′ + α eq + δ-UZr2 phases, the relative extents of two possible co-occurring modes of the first on-heating phase transformation step differ. In case of starting microstructure having mixture of three phases α′ + α eq + δ-UZr2, it is found that α′-martensite relaxation via α′ + α eq + δ-UZr2 → α eq + δ-UZr2 constitutes the first on-heating thermal response. The α′-martensitic relaxation is very closely followed by the dissolution of δ-UZr2. The co-occurrence of these two events gives rise to a composite thermal arrest in a normal dynamic calorimetry profile. However, if the starting microstructure is the one having the equilibrium mix of α eq and δ-UZr2, then only the peritectoidal dissolution of δ-UZr2 is found in the calorimetry profile. Unless, a very slow cooling rate of the order of 0.1 K min−1 is adopted from high-temperature γ(bcc) phase, it is not possible to obtain 100 pct of α eq phase along with equilibrium amount of δ-UZr2. At normal and high cooling rates, it is possible to suppress the diffusional decomposition of γ to varying extents. The direct γ → α′-martensite transformation has been observed at sufficiently higher cooling rates. It has been also noticed that even after γ → α′-martensite transformation the precipitation of δ-UZr2 phase is possible at lower temperature during non-isothermal cooling. Further, the critical cooling rate required for γ → α′ displacive transformation is found to decrease with increasing Zr content. For U-2, 5, and 10 wt pct Zr alloys, it is found to be of the order of, 60, 20, 10 K min−1, respectively. The cooling rate from high-temperature γ(bcc) is found have a strong influence on microstructure evolution as well. The kinetic aspects of α → β diffusional transformation that occurs on heating have been modeled in terms of Kolmogorov–Johnson–Mehl–Avrami formalism, and it is found that the transformation is effectively controlled by the diffusion of Zr in α′-orthorhombic phase. Continuous heating and cooling transformation diagrams have also been obtained for U-2 wt pct Zr alloy.

Structural, vibrational and thermal characterization of phase transformation in l-histidinium bromide monohydrate single crystals

l-Histidinium bromide monohydrate (LHBr) single crystal is a nonlinear optical material. In this work the high temperature phase transformation and the thermal stability of single crystals of LHBr was investigated by X-ray diffraction, thermogravimetric analysis, differential thermal analysis, differential scanning calorimetry and Raman spectroscopy. The results showed the LHBr phase transformation of orthorhombic (P212121) to monoclinic system (P 1 2 1) at 120 °C, with the lattice parameters a = 12.162(1) Å, b = 16.821(2) Å, c = 19.477(2) Å and β = 108.56(2)°. These techniques are complementary and confirm the structural phase transformation due to loss water of crystallization.

Effect of aluminum on microstructure, mechanical properties and pitting corrosion resistance of ultra-pure 429 ferritic stainless steels

Effect of aluminum on microstructure, mechanical properties and pitting corrosion resistance of ultra-pure 429 ferritic stainless steels has been investigated. Aluminum can significantly increase the ratio of equiaxed crystal grains, but the promotion effect has great relation with aluminum content. Aluminum can stabilize ferrite phase and significantly reduce recrystallization temperature. Increased aluminum content can also lead to the precipitate of AlN and Al2O3 at higher temperature. The increased amount of AlN may partly contribute to the reduced nitrogen element to form austenite at high temperature, hence the high temperature phase transformation of α+γ→α occurs. The fine and large number of Al2O3 particles can refine grain size and then promote recrystallization. The highest intensity of γ-fiber texture {111}〈112〉 is observed in the steel with 0.19wt.% aluminum, which can improve the formability of steels. With the increase of aluminum content, the tensile strength increases linearly but the elongation and plastic strain ratio first increase then decrease, the working hardening index varies slightly among the steels. Appearance of Al2O3 inclusions with small size and decreased content of MnS benefit pitting corrosion resistance. However, the large dimension Al2O3 inclusions have significantly negative influence on pitting corrosion resistance.

First-principles calculations of the high-temperature phase transformation in yttrium tantalate

The high-temperature phase transition between the tetragonal (scheelite) and monoclinic (fergusonite) forms of yttrium tantalite (${\mathrm{YTaO}}_{4}$ ) has been studied using a combination of first-principles calculations and a Landau free-energy expansion. Calculations of the Gibbs free energies show that the monoclinic phase is stable at room temperature and transforms to the tetragonal phase at 1430 \ifmmode^\circ\else\textdegree\fi{}C, close to the experimental value of $1426\ifmmode\pm\else\textpm\fi{}7$ \ifmmode^\circ\else\textdegree\fi{}C. Analysis of the phonon modes as a function of temperature indicate that the transformation is driven by softening of transverse acoustic modes with symmetry ${E}_{u}$ in the Brillouin zone center rather than the Raman-active ${B}_{g}$ mode. Landau free-energy expansions demonstrate that the transition is second order and, based on the fitting to experimental and calculated lattice parameters, it is found that the transition is a proper rather than a pseudoproper type. Together these findings are consistent with the transition being ferroelastic.

Broadband inelastic light scattering study on relaxor ferroelectric Pb(In1/2Nb1/2)-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals

The broadband inelastic light scattering spectra of ternary Pb(In1/2Nb1/2)-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals were investigated as a function of temperature and crystal orientation by combining Raman and Brillouin spectroscopies. The angular dependence of the strong Raman peak located at ∼50 cm−1 was investigated at 300 °C. The intensity variation of this mode with rotation angle was compatible with the F2g mode of Fm3¯m symmetry, suggesting that this mode arises from the 1:1 chemical order at the B-site in this perovskite structure. The temperature evolution of the polar nanoregions was associated with the growth of two central peaks and the change in the intensity of some Raman peaks, which were known to be sensitive to the rhombohedral symmetry. Both relaxation processes exhibited partial slowing-down behaviors with a common critical temperature of ∼160 °C. Poling the crystal along the [001] direction induced abrupt changes in some of the Raman bands at the rhombohedral–tetragonal phase transition. On the other hand, the diffuse tetragonal–cubic phase transition was not affected by the poling process. This high-temperature phase transformation seems to be smeared out by the inherent disorder and strong random fields enhanced by the addition of Pb(In1/2Nb1/2) into Pb(Mg1/3Nb2/3)O3-PbTiO3.

Diffusional constrained crystal nucleation during peritectic phase transitions

The casting behaviour, microstructure formation and resulting mechanical properties of materials that undergo a peritectic phase transition during solidification are largely determined by the kinetics of this high-temperature phase transformation during the production process. However, current nucleation models do not accurately predict the nucleation behaviour and phase transition kinetics in many polycrystalline materials. Utilizing a newly developed experimental technique, we have performed in situ observations to study the nucleation behaviour of a newly forming intermediate phase by using the peritectic phase transition in Fe–C and Fe–Ni alloys as examples. In our experiments as well as by using thermodynamic and kinetic arguments we demonstrate that nucleation of a new intermediate (peritectic) phase can be constrained in the presence of solute diffusion fields that form during primary solidification due to an increase in the Gibbs free energy barrier for nucleation. We found a strong correlation between the magnitude of these diffusion fields and the resulting nucleation undercooling required for the formation of a new phase. Our study casts new light on, and clarifies for the first time, the much-debated underpinning reason for the occurrence of massive phase transformations occurring during solidification processing at large nucleation undercoolings. These new insights contribute to the improvement of nucleation theory and allow more accurate predictions on nucleation events and, in turn, physical properties of materials that undergo phase transitions in the course of materials production processes.

High-temperature phase transformation and low friction behaviour in highly disordered turbostratic graphite

Microstructure of turbostratic graphite consists of a large amount of pores and voids which eventually convert to plates and flakes constituting layered microfolding upon high-temperature heat treatment. This treatment results in apparent phase transformation from 3D to 2D structured graphite where stacking order between adjacent layers is absent. Interestingly, low friction coefficients 0.08 and 0.06 were measured in 2D graphite in ambient atmosphere. This is explained in terms of passivation of carbon dangling bonds by the formation of C–O and C–OH where both physisorbed and chemisorbed oxygen species are present. In contrast, these values were as high as 0.17 and 0.19 in 3D graphite. At significantly less humid atmosphere, progressive increase of friction coefficient (up to the value of 0.8) with sliding distance is observed in both 3D and 2D graphites. This consistent increase in friction coefficient is ascribed to gradual loss of residual passivating chemical species such as oxygen and H2O molecules due to tribochemical reaction. Interestingly, the structure of graphite remains similar while Raman spectra obtained from the locations of wear track, where ultra-low and high friction coefficient is measured.

High Temperature Phase Transformation in a Cold-Rolled Fe-Mn-Si Alloy

The internal friction of a cold-rolled Fe-Mn-Si alloy has been investigated using a multifunctional internal friction apparatus though forced vibration method from room temperature to 950 °C. It has been shown that an internal friction peak is found on the IF-T curves during first heating at around 640 °C for the cold-rolled Fe-Mn-Si alloy. The internal friction peak is confirmed to be crystallizing peak of amorphous. The amorphous is resulted from the cold-rolling of the Fe-Mn-Si alloy.

Nucleation and Crystallization Kinetics of a Multicomponent Lithium Disilicate Glass by in Situ and Real-Time Synchrotron X-ray Diffraction

In this study, the high-temperature phase transformation of a multicomponent lithium disilicate glass was investigated by in situ and real-time synchrotron X-ray diffraction in the SiO2–Li2O–P2O5–Al2O3–ZrO2 glass system. Quantitative phase analysis via the Rietveld method was performed on the high-resolution data aiming to reveal the crystallization sequence, crystallization kinetics, and the role of P2O5 on nucleation. It is found that the nucleation of lithium metasilicate (LS) and lithium disilicate (LS2) in this complex glass is triggered by the steep compositional gradients associated with the disordered lithium phosphate (LP) precursors in the glass matrix. The LS2 crystals grow at the expense of the LS, cristobalite, and quartz phases in the glass during the isothermal crystallization process at 770 °C. The nucleation kinetics is temperature dependent, and the induction period of nucleation is longer at a lower temperature.