High-temperature Synchrotron X-ray Diffraction Research Articles

Melting Curve of Black Phosphorus: Evidence for a Solid-Liquid-Liquid Triple Point.

Black phosphorus (bP) is a crystalline material that can be seen as an ordered stacking of two-dimensional layers, which results in outstanding anisotropic physical properties. The knowledge of its pressure (P)-temperature (T) phase diagram, and in particular, of its melting curve is fundamental for a better understanding of the synthesis and stability conditions of this element. Despite the numerous studies devoted to this subject, significant uncertainties remain regarding the determination of the position and slope of its melting curve. Here we measured the melting curve of bP in an extended P, T region from 0.10(3) to 5.05(40) GPa and from 914(25) to 1788(70) K, using in situ high-pressure and high-temperature synchrotron X-ray diffraction. We employed an original metrology based on the anisotropic thermoelastic properties of bP to accurately determine P and T. We observed a monotonic increase of the melting temperature with pressure and the existence of two distinct linear regimes below and above 1.35(15) GPa, with respective slopes of 348 ± 21 and of 105 ± 12 K·GPa-1. These correspond to the melting of bP toward the low-density liquid and the high-density liquid, respectively. The triple point at which solid bP and the two liquids meet is located at 1.35(15) GPa and 1350(25) K. In addition, we have characterized the solid phases after crystallization of the two liquids and found that, while the high-density liquid transforms back to solid bP, the low-density liquid crystallizes into a more complex, partly crystalline and partly amorphous solid. The X-ray diffraction pattern of the crystalline component could be indexed as a mixture of red and violet P.

Activation energy and crystal growth coefficient determination of t-ZrO2 using high-temperature synchrotron X-ray diffraction

Abstract Activation energy and crystal growth coefficient of t-ZrO2 (tetragonal zirconia) were determined through an in-situ synchrotron radiation X-ray diffraction (SR-XRD) experiment. The t-ZrO2 precursor was synthesized through (a) purification of zircon sand from Kereng Pangi, Kalimantan to zircon powder, (b) alkali fusion, and (c) co-precipitation. High-temperature SR-XRD experiments were carried out with varying holding times of 1, 2, and 3 hours at 900, 950, and 1000 °C, respectively, where the formation and crystal growth of t-ZrO2 were observed. The diffraction patterns show that t-ZrO2 has formed at 900°C for 1 hour and it grows accordingly. The effect of the heating temperature and holding time on the crystal growth shows that the size of hot crystals increases with increasing temperature and holding time. It is found that the activation energy of t-ZrO2, in general, decreases with holding time, i.e. from 75.3 kJ/mol (3 h) to 57.3 (1h). Meanwhile, Beck’s crystal growth coefficient value is between 0.3 at 900 °C and 0.2 at 1000 °C.

Anisotropic thermo-mechanical response of layered hexagonal boron nitride and black phosphorus: application as a simultaneous pressure and temperature sensor.

Hexagonal boron nitride (hBN) and black phosphorus (bP) are crystalline materials that can be seen as ordered stackings of two-dimensional layers, which lead to outstanding anisotropic physical properties. Knowledge of the thermal equations of state of hBN and bP is of great interest in the field of 2D materials for a better understanding of their anisotropic thermo-mechanical properties and exfoliation mechanism towards the preparation of important single-layer materials like hexagonal boron nitride nanosheets and phosphorene. Despite several theoretical and experimental studies, important uncertainties remain in the determination of the thermoelastic parameters of hBN and bP. Here, we report accurate thermal expansion and compressibility measurements along the individual crystallographic axes, using in situ high-temperature and high-pressure high-resolution synchrotron X-ray diffraction. In particular, we have quantitatively determined the subtle variations of the in-plane and volumetric thermal expansion coefficients and compressibility parameters by subjecting these materials to hydrostatic conditions and by collecting a large number of data points in small pressure and temperature increments. In addition, based on the anisotropic behavior of bP, we propose the use of this material as a sensor for the simultaneous determination of pressure and temperature in the range of 0-5 GPa and 298-1700 K, respectively.

Thermal behavior of borax, Na2B4O5(OH)4·8H2O

Abstract Globally, borax is one of the most important borate minerals, both industrially and from a technological viewpoint. Despite its importance, there have been only a few reports on the structural changes of borax upon heating. In this study, we have investigated the thermal behavior and structural characteristics of borax using thermal analysis, in situ high-temperature synchrotron X-ray diffraction, in situ variable-temperature single-crystal X-ray diffraction, and quantum chemical calculations. Differential scanning calorimetry (DSC) curve showed a large endothermic peak at 349 K corresponding to the dehydration reaction of borax. X-ray diffraction (XRD) pattern remained almost unchanged up to 353 K. Above 363 K, the XRD peaks gradually became less intense until they disappeared at 403 K. The unit-cell volume continuously increased with increasing temperature and became constant just before its phase transition to tincalconite. The volumetric thermal expansion coefficient between 113 and 323 K was 7.84 × 10–5 K–1. The a and c lattice parameters exhibited a slight increase trend with increasing temperature, whereas the b lattice parameter increased significantly. Therefore, the thermal expansibilities followed the order b/b0 >> c/c0 > a/a0. The increasing b lattice parameter was closely related to the elongation of the O4···H7B hydrogen-bond interaction along the b-axis. Na-O bond lengths were isotropically expanded with increasing temperature, whereas the B-O bond lengths and angles remained unchanged even after the phase transition to tincalconite. Molecular orbital calculations revealed that an electron cloud shared by two borate tetrahedra led to the formation of a large electron cloud distribution over the B4O5(OH)4 cluster. The intramolecular interactions with substantial covalent character made the cluster quite rigid. The existence of borate minerals containing the B4O5(OH)4 clusters in their structure evidenced the presence of moderately acidic or alkaline water, wherein the borate minerals grew via the incorporation of B4O5(OH)4 clusters. Our results indicate that the connection geometry of the fundamental building block consisting of Bφ3 triangles and Bφ4 tetrahedra (φ: O2–, OH–) can potentially be used as a palaeoenvironmental indicator.

In situ study of phase transformations in electron beam additive manufactured Ti-6Al-4 V titanium alloy by high temperature synchrotron X-ray diffraction and TEM

The microstructure and phase composition of the EBAM Ti-6Al-4 V alloy samples were studied by the methods of in situ high temperature synchrotron X-ray diffraction and transmission electron microscopy (TEM) during heating from room temperature to a temperature of 1373 K. In the initial state, the main phase of the EBAM Ti-6Al-4 V alloy the samples was the α phase, the second phases were β + α″. The volume fraction of the β phase does not exceed 5%, α″ - 3%. It was found that the new α″ phase was formed at the temperature of 1073 K due to the phase transition α→α″. The α″ phase volume fraction increased as the heating temperature increased to the temperature of 1223 K and then decreased with a further increase of the temperature due to the α″→β phase transition. The dependences of the lattice parameters of the α, β and α″ phases on the heating temperature were revealed. It was established that the crystal lattice of the α phase was distorted during heating. The α phase passed into the unstable state. An increase of the c/a parameters ratio was accompanied by the formation of the α″ and β phases.

Formation and evolution of hierarchical microstructures in a Ni-based superalloy investigated by in situ high-temperature synchrotron X-ray diffraction

Hierarchical microstructures are created when additional γ particles form in γ’ precipitates and they are linked to improved strength and creep properties in high-temperature alloys. Here, we follow the formation and evolution of a hierarchical microstructure in Ni86.1Al8.5Ti5.4 by in situ synchrotron X-ray diffraction at 1023 K up to 48 h to derive the lattice parameters of the γ matrix, γ’ precipitates and γ particles and misfits between phases. Finite element method-based computer simulations of hierarchical microstructures allow obtaining each phase's lattice parameter, thereby aiding peak identification in the in situ X-ray diffraction data. The simulations further give insight into the heterogeneous strain distribution between γ’ precipitates and γ particles, which gives rise to an anisotropic diffusion potential that drives the directional growth of γ particles. We rationalize a schematic model for the growth of γ particles, based on the Gibbs-Thomson effect of capillary and strain-induced anisotropic diffusion potentials. Our results highlight the importance of elastic properties, elastic anisotropy, lattice parameters, and diffusion potentials in controlling the behavior and stability of hierarchical microstructures.

Tempering of an additively manufactured microsegregated hot-work tool steel: A high-temperature synchrotron X-ray diffraction study

Rapidly solidified microstructures, produced by concentrated heat sources, have gained increasing attention due to the widespread expansion of additive manufacturing techniques. In laser powder bed fusion of tool steels, the as-built microstructure consists of a microsegregated network of cell-like walls decorated with retained austenite (RA). This inhomogeneous elemental distribution leads to phase transformation sequences dictated by local – instead of global – equilibrium. This work uses synchrotron X-ray diffraction to study the kinetics of retained austenite decomposition during tempering of a microsegregated as-built AISI H13 tool steel. The RA decomposition products are also characterized by hardness tests, scanning electron microscopy, and atom probe tomography. Present results evidence the formation of high temperature para-equilibrium (T < 650 °C) or equilibrium (T ≥ 650 °C) microstructures. The para-equilibrium microstructural path evidences isothermally stable RA, which transforms into fresh martensite upon cooling due to carbon depletion. However, the cell-like microsegregation arrangement is retained due to insufficient homogenization of substitutional elements. The equilibrium microstructural path results in isothermal transformation of RA into ferrite + carbides, as well as dissolution of the cell-like microsegregation structure. These results provide insights towards the understanding and development of optimized heat treatment cycles appropriate for such increasingly common inhomogeneous microstructures. The results are relevant not only to laser powder bed fusion, but also to other localized melting/remelting processes with associated high cooling rates.

Structure and properties of two superionic ice phases

In the phase diagram of water, superionic ices with highly mobile protons within the stable oxygen sublattice have been predicted at high pressures. However, the existence of superionic ices and the location of the melting line have been challenging to determine from both theory and experiments, yielding contradictory results depending on the employed techniques and the interpretation of the data. Here we report high-pressure and high-temperature synchrotron X-ray diffraction and optical spectroscopy measurements of water in a laser-heated diamond anvil cell and reveal first-order phase transitions to ices with body-centred and face-centred cubic oxygen lattices. Based on the distinct density, increased optical conductivity and the greatly decreased fusion enthalpies, we assign these observed structures to the theoretically predicted superionic ice phases. Our measurements determine the pressure–temperature stability fields of superionic ice phases and the melting line, suggesting the presence of face-centred cubic superionic ice in water-rich giant planets, such as Neptune and Uranus. The melting line determined here is at higher temperatures than previously determined in static compression experiments, but it is in agreement with theoretical calculations and data from shock-wave experiments.

Diagram of constituent crystalline phases in a Nd–Fe–B–Cu sintered magnet by in-situ high-temperature synchrotron X-ray diffraction and its thermodynamic interpretation

A crystalline phase diagram of a Nd–Fe–B–Cu sintered magnet up to 1100 °C was determined through in-situ synchrotron X-ray diffraction using a new sample mounting method that prevents high-temperature contact reactions between the sample and the quartz capillary tube container. Using this newly developed method, we successfully observed almost identical phase diagrams during heating and cooling, indicating thermodynamic equilibrium. In addition, we obtained changes in the constituent phase fraction after quenching. Temperature-reversible changes in the dhcp-Nd fraction were observed from 475 °C to 650 °C, corresponding to a eutectic reaction in the Nd–Cu phase. The fcc-NdOx fraction decreased with the increase in the Nd2O3 fraction above 1000 °C, which behavior was attributed to a phase change from fcc-NdOx to high-temperature liquid and hcp-Nd2O3 phases. The measurements verify the thermodynamic database recently constructed for CALPHAD calculations of the Nd–Fe–B–Cu–O system by assuming the local thermodynamic equilibrium of the Nd oxides within the microstructure of the magnet.

The Role of Water and Hydroxyl Groups in the Structures of Stetindite and Coffinite, MSiO4 (M = Ce, U).

Orthosilicates adopt the zircon structure types (I41/amd), consisting of isolated SiO4 tetrahedra joined by A-site metal cations, such as Ce and U. They are of significant interest in the fields of geochemistry, mineralogy, nuclear waste form development, and material science. Stetindite (CeSiO4) and coffinite (USiO4) can be formed under hydrothermal conditions despite both being thermodynamically metastable. Water has been hypothesized to play a significant role in stabilizing and forming these orthosilicate phases, though little experimental evidence exists. To understand the effects of hydration or hydroxylation on these orthosilicates, in situ high-temperature synchrotron and laboratory-based X-ray diffraction was conducted from 25 to ∼850 °C. Stetindite maintains its I41/amd symmetry with increasing temperature but exhibits a discontinuous expansion along the a-axis during heating, presumably due to the removal of water confined in the [001] channels, which shrink against thermal expansion along the a-axis. Additional in situ high-temperature Raman and Fourier transform infrared spectroscopy also confirmed the presence of the confined water. Coffinite was also found to expand nonlinearly up to 600 °C and then thermally decompose into a mixture of UO2 and SiO2. A combination of dehydration and dehydroxylation is proposed for explaining the thermal behavior of coffinite synthesized hydrothermally. Additionally, we investigated high-temperature structures of two coffinite-thorite solid solutions, uranothorite (UxTh1-xSiO4), which displayed complex variations in composition during heating that was attributed to the negative enthalpy of mixing. Lastly, for the first time, the coefficients of thermal expansion of CeSiO4, USiO4, U0.46Th0.54SiO4, and U0.9Th0.1SiO4 were determined to be αV = 14.49 × 10-6, 14.29 × 10-6, 17.21 × 10-6, and 17.23 × 10-6 °C-1, respectively.

High-Temperature Thermodynamics of Cerium Silicates, A-Ce2Si2O7, and Ce4.67(SiO4)3O

Lanthanide disilicates and oxyapatites have potential roles in high-temperature applications as thermal (TBC) and environmental barrier coatings (EBC) or possible alteration phases in geological nuclear waste repositories. However, those Ce3+-bearing silicates have only been limitedly studied. In this work, we performed detailed structural and thermodynamic investigations on A-Ce2Si2O7 (tetragonal, P41) and Ce4.67(SiO4)3O (hexagonal, P63/m). The high-temperature structural behaviors and coefficients of thermal expansion were determined by in situ high-temperature synchrotron X-ray diffraction (HT-XRD) implemented with Rietveld analysis and thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC). A-Ce2Si2O7 was found to be stable in N2 and air up to ∼1483 K with an isotropic thermal expansion along the a- and c-axes (αa = 12.3 × 10–6 K–1 and αc= 12.4 × 10–6 K–1). Ce4.67(SiO4)3O had a slow partial oxidation between 533 and 873 K to a new nonstoichiometric phase Ce3+1.67-xCe4+xCe3+3(SiO4)3O1+0.5x, followed by a thermal decomposition to CeO2 and SiO2 at ∼1000 K in air. By using high temperature oxide melt solution calorimetry at 973 K with lead borate as the solvent, the standard enthalpy of formation was determined for A-Ce2Si2O7 (−3825.1 ± 6.0 kJ/mol) and Ce4.67(SiO4)3O (−7391.3 ± 9.5 kJ/mol). These thermodynamic parameters were compared with those of CeO2, CeSiO4, and other silicate oxyapatites for examining their chemical stability in high-temperature environments relevant for aeronautical applications, mineral formation, and nuclear fuel cycle

Robust spin-ice freezing in magnetically frustrated Ho2GexTi2−xO7 pyrochlore

Structural analysis of spin frustrated Ho2GexTi2−xO7 (x = 0, 0.1, 0.15 & 0.25) pyrochlore oxides has been performed using high resolution x-ray diffraction pattern and low temperature synchrotron x-ray diffraction pattern. The effect of positive chemical pressure on the spin dynamics of Ho2GexTi2−xO7 has been analysed through the study of static (M–T and M–H; magnetisation against temperature & magnetisation against magnetic field) and dynamical (ac susceptibility) magnetic measurements. In lower temperature regime (∼2 K), such systems are predominantly governed by competing exchange (Jnn) and dipolar (Dnn) magnetic interactions. Magnetic measurements indicate that the application of increased chemical pressure in Ho2Ti2O7 matrix propels the system towards diminished ferromagnetic interaction. Dipolar coupling constant remains almost unchanged but Curie–Weiss temperature (θcw) reduces to −0.04 K from 0.33 K (for an applied magnetic field; H = 100 Oe) with increasing x in Ho2GexTi2−xO7. Positive chemical pressure establishes the dominance of Ho–Ho antiferromagnetic interaction Jnn over dipolar interaction Dnn. Spin relaxation feature corresponding to thermally activated single ion freezing (Ts∼15 K) is shifted towards lower temperature. This chemical pressure-driven Ts shift is ascribed to the alteration in crystal field effect, which reduces the activation energy for singe ion spin freezing. The reduction in the activation energy indicates crystal field-phonon coupling in Ho2GexTi2−xO7 system. The robustness in spin ice freezing (second spin relaxation feature in ac susceptibility curve) remains unaffected with increasingly chemical pressure. This spin freezing (‘2 in-2 out’ spin arrangement in tetrahedra) is related to quantum tunnelling phenomenon, at Tice ∼ 2 K. It indicates that majority of spins still follows the ‘ice rule’ in Ho2GexTi2−xO7 even after the application of chemical pressure.

Thermal Analysis of High-Entropy Rare Earth Oxides.

Phase transformations in multicomponent rare earth sesquioxides were studied by splat quenching from the melt, high temperature differential thermal analysis and synchrotron X-ray diffraction on laser-heated samples. Three compositions were prepared by the solution combustion method: (La,Sm,Dy,Er,RE)2O3, where all oxides are in equimolar ratios and RE is Nd or Gd or Y. After annealing at 800 °C, all powders contained mainly a phase of C-type bixbyite structure. After laser melting, all samples were quenched in a single-phase monoclinic B-type structure. Thermal analysis indicated three reversible phase transitions in the range 1900–2400 °C, assigned as transformations into A, H, and X rare earth sesquioxides structure types. Unit cell volumes and volume changes on C-B, B-A, and H-X transformations were measured by X-ray diffraction and consistent with the trend in pure rare earth sesquioxides. The formation of single-phase solid solutions was predicted by Calphad calculations. The melting point was determined for the (La,Sm,Dy,Er,Nd)2O3 sample as 2456 ± 12 °C, which is higher than for any of constituent oxides. An increase in melting temperature is probably related to nonideal mixing in the solid and/or the melt and prompts future investigation of the liquidus surface in Sm2O3-Dy2O3, Sm2O3-Er2O3, and Dy2O3-Er2O3 systems.

Influence of spinodal decomposition and fcc→w phase transformation on global and local mechanical properties of nanolamellar CVD fcc-Ti1-xAlxN coatings

Recently, it was shown that annealing of nanolamellar CVD fcc-Ti1-xAlxN at temperatures of 1000-1200 °C results in the formation of complex phase fields consisting of still intact nanolamellar face centered cubic (fcc) zones, side by side with non-lamellar fully decomposed and transformed fcc and wurtzite (w) zones. It can be assumed that the observed phase fields and their microstructure strongly correlate with their mechanical properties. Consequently, this work focuses on the investigation of the effects of spinodal decomposition and fcc→w phase transformation of a nanolamellar CVD fcc-Ti0.2Al0.8N coating on the corresponding global and local mechanical properties. The sequence of spinodal decomposition and fcc→w phase transformation of a compact coating sample was investigated by in situ high temperature synchrotron X-ray diffraction up to a maximum temperature of ~1250 °C. Conventional nanoindentation experiments on the surfaces of samples annealed between 900 to 1300 °C in vacuum were performed to illustrate the age hardening and overaging behavior. Finally, the influence of the observed phase fields on the local mechanical properties was investigated by correlative SEM/EBSD and nanomechanical mapping experiments on a cross-section of a coating annealed at 1050 °C. Maps of the lateral microstructure, phase composition, Young´s modulus and hardness of the coating were successfully obtained with a resolution of ≤100 nm. The lateral phase fields could be clearly identified and correlated with the observed mechanical properties. The results indicate that age hardening of nanolamellar CVD fcc-Ti0.2Al0.8N coatings occurs homogeneously, while overaging is associated to the fcc→w transformation and thus, locally confined.

High-Temperature Thermal Cycling Effect on the Irreversible Responses of Lattice Structure, Magnetic Properties, and Electrical Conductivity in Co2.75Fe0.25O4+δ Spinel Oxide

We report high-temperature synchrotron X-ray diffraction (SXRD), magnetization, and current-voltage (I-V) characteristics for the samples of Co2.75Fe0.25O4 ferrite. The material was prepared by chemical reaction of the Fe and Co nitrate solutions at pH ∼ 11 and subsequent thermal annealing. Physical properties of the samples were measured by cycling the temperature from 300 K to high temperature (warming mode) and returning back to 300 K (cooling mode). The lattice structure showed sensitivity to high measurement temperatures. Magnetization curves showed a defect-induced ferromagnetic phase at higher temperatures and superparamagnetic blocking of the ferrimagnetic particles near to 300 K or below. Electrical conductivity exhibited a thermal hysteresis loop at higher measurement temperatures. The samples exhibited new form (not studied so far) of surface magnetism in Co rich spinel oxides and irreversibility phenomena in the lattice structure, magnetization, and conductivity on cycling the measurement temperatures.

Thermal expansion of α-boron and some boron-rich pnictides

Thermal expansion of α-rhombohedral boron (α-B12) and two isostructural boron-rich pnictides (B12P2 and B12As2) has been studied between 298 and 1280 K by high-temperature synchrotron X-ray diffraction. For all studied phases no temperature-induced phase transitions have been observed. The observed temperature dependencies of the lattice parameters and unit cell volumes were found to be quasi-linear. Variation of the thermal expansion coefficients in the group of boron-rich pnictides (B13N2 – B12P2 – B12As2) was analyzed.

Selective laser melting enabled additive manufacturing of Ti–22Al–25Nb intermetallic: Excellent combination of strength and ductility, and unique microstructural features associated

To realize near net-shaping of hard-to-process intermetallics is an often challenging but critical issue to their wider industrial applications. In this work, we report that an intermetallic Ti–22Al–25Nb has been successfully fabricated by selective laser melting (SLM). The as-printed samples show a high room-temperature ultimate tensile strength ∼1090 MPa and excellent ductility ∼22.7%; both values are higher than most conventionally fabricated Ti–22Al–25Nb intermetallic. We clarify the mechanical performance achieved by detailed microstructure analysis, including dislocation and phase constitution. It is proposed that high-density dislocation networks significantly contribute to the strength and ductility, which are further enhanced by the favorable phase constitution, including the nano-scale O phase precipitates within the disordered β phase and disappearance of the brittle α2 phase in the microstructure. Phase evolution during solidification, particularly regarding the O phase's formation, has also been clarified using in-situ laser heating, high-temperature synchrotron X-ray diffraction and Scheil simulation. It is demonstrated that the O phase formation involves both displacive transition (B2 → B19) and chemical ordering (B19 → O). The metastable B19 phase (as an intermediate stage) may be formed by shearing cubic B2 phase along (110)[1−11]direction into an orthorhombic structure under high residual stress. Furthermore, a demonstrative part of turbine blade has been fabricated to highlight the importance of SLM in fabricating critical structural part like the hard-to-process intermetallics.

Synthesis and Characterization of Double Solid Solution (Zr,Ti)2(Al,Sn)C MAX Phase Ceramics.

Quasi phase-pure (>98 wt %) MAX phase solid solution ceramics with the (Zr,Ti)2(Al0.5,Sn0.5)C stoichiometry and variable Zr/Ti ratios were synthesized by both reactive hot pressing and pressureless sintering of ZrH2, TiH2, Al, Sn, and C powder mixtures. The influence of the different processing parameters, such as applied pressure and sintering atmosphere, on phase purity and microstructure of the produced ceramics was investigated. The addition of Sn to the (Zr,Ti)2AlC system was the key to achieve phase purity. Its effect on the crystal structure of a 211-type MAX phase was assessed by calculating the distortions of the octahedral M6C and trigonal M6A prisms due to steric effects. The M6A prismatic distortion values were found to be smaller in Sn-containing double solid solutions than in the (Zr,Ti)2AlC MAX phases. The coefficients of thermal expansion along the ⟨ a⟩ and ⟨ c⟩ directions were measured by means of Rietveld refinement of high-temperature synchrotron X-ray diffraction data of (Zr1- x,Ti x)2(Al0.5,Sn0.5)C MAX phase solid solutions with x = 0, 0.3, 0.7, and 1. The thermal expansion coefficient data of the Ti2(Al0.5,Sn0.5)C solid solution were compared with those of the Ti2AlC and Ti2SnC ternary compounds. The thermal expansion anisotropy increased in the (Zr,Ti)2(Al0.5,Sn0.5)C double solid solution MAX phases as compared to the Zr2(Al0.5,Sn0.5)C and Ti2(Al0.5,Sn0.5)C end-members.

Selective Laser Melting Enabled Additive Manufacturing of Ti-22Al-25Nb Intermetallic: Excellent Combination of Strength and Ductility, and Unique Microstructural Features Associated

To realize near net-shaping of hard-to-process intermetallics is an often challenging but critical issue to their wider industrial applications. In this work, we report that an intermetallic Ti-22Al-25Nb has been successfully fabricated by selective laser melting (SLM). The as-printed samples show a high room-temperature ultimate tensile strength ~1090 MPa and excellent ductility ~22.7%; both values are higher than most conventionally fabricated Ti-22Al-25Nb intermetallic. We clarify the mechanical performance achieved by detailed microstructure analysis, including dislocation analysis and phase constitution analysis. High-density dislocation networks significantly contribute to the strength and ductility, which are further enhanced by the favorable phase constitution, including the nano-scale O phase precipitates within the disordered β phase and disappearance of the brittle α2 phase in the microstructure. Phase evolution during SLM has also been clarified using in situ heating, high-temperature synchrotron X-ray diffraction and Scheil simulation, particularly regarding the O phase's formation. It is demonstrated that O phase is formed by shearing primary cubic B2 phase along (110)[111] direction into an orthorhombic structure under high residual stress. Furthermore, a demonstrative part of turbine blade has been fabricated to highlight the importance of SLM in fabricating critical structural part like the hard-to-process intermetallics.

Combined computational and experimental investigation of high temperature thermodynamics and structure of cubic ZrO2 and HfO2

Structure and thermodynamics of pure cubic ZrO2 and HfO2 were studied computationally and experimentally from their tetragonal to cubic transition temperatures (2311 and 2530 °C) to their melting points (2710 and 2800 °C). Computations were performed using automated ab initio molecular dynamics techniques. High temperature synchrotron X-ray diffraction on laser heated aerodynamically levitated samples provided experimental data on volume change during tetragonal-to-cubic phase transformation (0.55 ± 0.09% for ZrO2 and 0.87 ± 0.08% for HfO2), density and thermal expansion. Fusion enthalpies were measured using drop and catch calorimetry on laser heated levitated samples as 55 ± 7 kJ/mol for ZrO2 and 61 ± 10 kJ/mol for HfO2, compared with 54 ± 2 and 52 ± 2 kJ/mol from computation. Volumetric thermal expansion for cubic ZrO2 and HfO2 are similar and reach (4 ± 1)·10−5/K from experiment and (5 ± 1)·10−5/K from computation. An agreement with experiment renders confidence in values obtained exclusively from computation: namely heat capacity of cubic HfO2 and ZrO2, volume change on melting, and thermal expansion of the liquid to 3127 °C. Computed oxygen diffusion coefficients indicate that above 2400 °C pure ZrO2 is an excellent oxygen conductor, perhaps even better than YSZ.