C54 Phase Research Articles

Thermodynamic modelling of the Cr-Nb-Sn system

The Cr-Nb-Sn system has been studied experimentally, by first principles calculation and finally modeled with the Calphad method. The experimental study has been carried out on the Cr-Sn binary system to determine the solubility of Sn in the A2 (Cr) solid solution, but also in the Cr-Nb-Sn ternary system in order to determine phase equilibria of the isothermal section at 800°C and 1100°C. Besides, the formation enthalpies of all the ordered configurations of the C15 and A15 phases and the stoichiometric Nb6Sn5 and NbSn2 phases have been calculated using the Density Functional Theory (DFT). The mixing enthalpies of the A2 binary solid solutions have been estimated using the Special Quasirandom Structures (SQS). All these new experimental and calculated data have been taken into account for a new thermodynamic assessment of the three binary and the ternary systems.

Accurate Estimation of Band Offsets in Group IV Polytype Junctions: A First-Principles Study

Recent advances in the fabrication of group IV homojunctions made by cubic-diamond (3C) and hexagonal-diamond (2H) phases call for a detailed understanding of the physics underlying their electronic structure. In this work, we employ hybrid density functional theory (hybrid-DFT) calculations to study the structural and electronic properties of Si and Ge polytype junctions. Starting from high-resolution transmission electron microscopy (HRTEM) micrographs of 2H/3C Si nanowires, we build a computational model that takes into account both crystal-phase effects and the length of the junction. We obtain accurate estimations of the magnitudes of the band offsets: In particular, we show that 2H/3C Si interfaces induce a type-II band alignment, whereas the Ge structures exhibit a type-I band offset. Furthermore, both Si and Ge homojunctions have direct gaps that are smaller than those of the bulk 3C phase. Finally, we demonstrate that, in all of the considered systems, varying the thickness of the hexagonal phase...

Structural and optical properties of ZnS:Mn micro-powders, synthesized from the charge with a different Zn/S ratio

The influence of Zn/S ratio in the charge on structural and optical properties of ZnS:Mn powders produced by high-temperature self-propagated synthesis was investigated. The samples was shown to consist of mixed-polytypes ZnS crystallites with hexagonal (2H) and cubic (3C) phases, the contribution of the latter increases with the sulfur content in the charge. The most homogeneous size distribution were found at stoichiometric Zn/S ratio. The Zn/S relation affects the Mn incorporation into ZnS lattice. The highest quantity of incorporated Mn is observed at stoichiometric Zn/S relation while lowest one is realized at Zn excess. Besides, the distribution of manganese ions in the blocks, which compose the crystallites, was found to be inhomogeneous, their concentration decreases from crystallites surface to the depth. Mn ions are nearer to the surface in ZnS:Mn synthesized with Zn excess. At Mn concentration in the charge of 1 wt% the shift of ZnS band edge to low energy side is observed, that is ascribed to formation of solid solution ZnS–MnS with lower band gap value.

Effect of rolling deformation on microstructure and texture of spray-deposited magnesium alloy containing Mg-Nd-Zn typed LPSO

Billets of Mg-9Al-3Zn-1Mn-6Ca-2Nd alloy were produced by spray-deposition (the Osprey process). Effect of rolling deformation (T=350°C, ε=5%, 10%, and 15%, respectively) on microstructure and texture evolution of the Mg-9Al-3Zn-1Mn-6Ca-2Nd alloy was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Results show that at pass reduction of ε=5%, 10% and 15% at 350°C respectively, Mg-Nd-Zn typed 24R-LPSO structure was formed in (Ca, Nd)Al2 phase (C15 Laves phase). With the increase in pass reduction (i.e. 5%, 10% and 15%), the texture pole density level of basal texture (0002) changed little and pyramidal texture (101¯3) were increased. In contrast, those of prismatic texture {101¯0} <112¯0> were increased initially and followed by a reduction, indicating texture randomization in the grain-refined Mg alloy. The combined contribution of LPSO phase and C15 phase was key to randomize the texture of the grain-refined Mg alloy. It was noted that the microcosmic plastic deformation of LPSO phase and nanometer-sized dispersed C15 phase impeded dislocation movement, led to dislocation tangles, and facilitated recrystallization.

The formation and stacking faults of Fe and Cr containing Laves phase in Zircaloy-4 alloy

The crystallographic characterization and defect structure of precipitates plays a key role in the properties of Zr alloys. The precipitates in Zircaloy-4 alloy are studied in this work. The Zr-Fe-Cr particles are identified as the dominant precipitates at room temperature with C14 or C15 type Laves structure. The Fe/Cr ratio of C15 type Zr-Fe-Cr phase is slightly lower than that of C14 type phase. The {0001} stacking faults in C14 phase is observed and analyzed for the first time. Some new orientation relationships of C14 and C15 phases with α-Zr are identified.

The Cr-Nb-Si system: Improved thermodynamic modelling and its use in simulation of Laves phase in steel

Thermodynamic modelling of the Cr-Nb-Si ternary system is revised considering new findings in the binary Cr-Nb and extension to multicomponent systems. Thermodynamic model parameters of intermetallic phases are re-optimized based on density functional theory (DFT) calculations and experimental data. Particular attention was given to the transformations of Laves phase polytypes (i.e. C14 and C15). The calculated phase stabilities of technologically relevant Laves phase polytypes C14 and C15 are discussed. Their extension to multicomponent systems is tested for typical steel grades used in high-temperature applications. C14 is the dominant phase in high Cr ferritic steel.

Microstructure evolution of diffusion welded 304L/Zircaloy4 with copper interlayer

Diffusion-welding of 304L stainless steel with Zircaloy4 was performed with a copper interlayer. All the samples are composed of a reaction layer and a diffusion-affected zone in the Zircaloy4. Two reaction regimes were identified: (i) involving a liquid phase on the Zircaloy4 side leading to a fast dissolution and (ii) involving solid state reaction on the 304L stainless steel which is consequently less affected. The reaction layers contains numerous phases that were identified as the C36- and C14-Fe2Zr Laves phases and the cubic and tetragonal polymorphs of the FeZr2 intermetallic. The multiplication of the phases with Cu addition results in the interaction with chromium since the Cr-rich C36 phase and the t-FeZr2 phases have limited solubility in Cu and Cr, respectively, leading to the stabilization of phases where this solubility is increased, e.g. the C14 Laves phase and the c-FeZr2. While in the processing conditions tested the copper interlayer was totally dissolved for all the temperature >1223K, leading to the occurrence of embrittling Laves and intermetallic phases, the presence of copper lead to a significant softening of the weld. The best properties were obtained with the microstructures formed at 1223K.

Exploring the coordination change of vanadium and structure transformation of metavanadate MgV2O6 under high pressure.

Raman spectroscopy, synchrotron angle-dispersive X-ray diffraction (ADXRD), first-principles calculations, and electrical resistivity measurements were carried out under high pressure to investigate the structural stability and electrical transport properties of metavanadate MgV2O6. The results have revealed the coordination change of vanadium ions (from 5+1 to 6) at around 4 GPa. In addition, a pressure-induced structure transformation from the C2/m phase to the C2 phase in MgV2O6 was detected above 20 GPa, and both phases coexisted up to the highest pressure. This structural phase transition was induced by the enhanced distortions of MgO6 octahedra and VO6 octahedra under high pressure. Furthermore, the electrical resistivity decreased with pressure but exhibited different slope for these two phases, indicating that the pressure-induced structural phase transitions of MgV2O6 was also accompanied by the obvious changes in its electrical transport behavior.

High-Temperature Deformation Behavior of (Mo&lt;sub&gt;0.85&lt;/sub&gt;Nb&lt;sub&gt;0.15&lt;/sub&gt;)Si&lt;sub&gt;2&lt;/sub&gt; Crystals with C40/C11&lt;sub&gt;b&lt;/sub&gt; Lamellar Microstructure

The effect of alloying element (such as Cr, Zr, and Ir) addition on the high-temperature creep deformation behavior of C40/C11b lamellar-structured (Mo0.85Nb0.15)Si2 silicide crystals was examined. The results indicated that these additions all lead to a decrease in the steady-state creep strain rate (SSCR) when the applied stress is parallel to the lamellar interface. To clarify the origin of this, the dependence of the creep deformation behavior on the microstructure was determined in detail. As a result, it was found that the C40 phase acts as a strengthening phase during the deformation of the C40/C11b duplex-phase crystals. The variant-1-type C11b phase grains, whose loading orientation is parallel to [001], also acts as an effective strengthening component. The decrease in SSCR by Cr or Zr addition is attributed to the increase in volume fraction of those C40 phase and C11b-V1 grains. The refinement of microstructure by Ir addition was also found to result in a modest decrease in the SSCR.

First-principles studies of the structural, elastic, and lattice dynamical properties of ZrMo2 and HfMo2

ABSTRACTA comprehensive investigation of the structural, elastic, and lattice dynamical properties for ZrMo2 and HfMo2 with C14, C15, C36, and CeCu2 phases are conducted using density functional total energy calculations. The results have showed that C15 phase for both materials is energetically more stable than C14, C36 and CeCu2 phases. We have also estimated the mechanical behaviours of these compounds, including mechanical stability, bulk modulus, Young's modulus, shear modulus, Poisson's ratio, ductility, and anisotropy. Additionally, the lattice dynamical properties are analyzed and discussed exhaustively for these phases. The calculated properties agree well with available experimental and theoretical data.

Possible quantum liquid crystal phases of helium monolayers

The second-layer phase diagrams of $^4$He and $^3$He adsorbed on graphite are investigated. Intrinsically rounded specific-heat anomalies are observed at 1.4 and 0.9 K, respectively, over extended density regions in between the liquid and incommensurate solid phases. They are identified to anomalies associated with the Kosterlitz-Thouless-Halperin-Nelson-Young type two-dimensional melting. The prospected low temperature phase (C2 phase) is a commensurate phase or a $\textit{quantum hexatic}$ phase with quasi-bond-orientational order, both containing $\textit{zero}$-$\textit{point}$ defectons. In either case, this would be the first atomic realization of the $\textit{quantum liquid crystal}$, a new state of matter. From the large enhancement of the melting temperature over $^3$He, we propose to assign the observed anomaly of $^4$He-C2 phase at 1.4 K to the hypothetical supersolid or superhexatic transition.

Preparation of Mo(Si,Al)2 feedstock used for air plasma spraying

In order to prepare high quality Mo(Si,Al)2 feedstock characterized with C40 phase, higher Al doping amount and excellent flowability, Mo(Si1–x,Alx)2 with different Al contents (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) were synthesized by self-propagating high-temperature synthesis first and Mo(Si0.6,Al0.4)2 was confirmed as the suitable material through X-ray diffraction analysis. A series of tests with different parameters of induction plasma spheroidization were applied to improving the flowability of feedstock. Mo(Si,Al)2 feedstock with excellent flowability (26.2 s/50 g) was prepared through adding hydrogen into sheath gas and decreasing the powder feeding rate. The composition segregation occurred in the spheroidized powder after Al consumption and oxidation. The inhomogeneous structure of the same particle was caused by the asymmetric heating and cooling when particle passed through the plasma jet.

Tetrahedral Order in Liquid Crystals

We review the impact of tetrahedral order on the macroscopic dynamics of bent-core liquid crystals. We discuss tetrahedral order comparing with other types of orientational order, like nematic, polar nematic, polar smectic, and active polar order. In particular, we present hydrodynamic equations for phases, where only tetrahedral order exists or tetrahedral order is combined with nematic order. Among the latter we discriminate between three cases, where the nematic director (a) orients along a 4-fold, or (b) along a 3-fold symmetry axis of the tetrahedral structure. For the optically isotropic T_d phase, which only has tetrahedral order, we focus on the coupling of flow with e.g. temperature gradients and on the specific orientation behavior in external electric fields. For the transition to the nematic phase, electric fields lead to a temperature shift that is linear in the field strength. Electric fields induce nematic order, again linear in the field strength. If strong enough, electric fields can change the tetrahedral structure and symmetry leading to a polar phase. We briefly deal with the T phase that arises when tetrahedral order occurs in a system of chiral molecules. To case (a) belong (i) the non-polar, achiral, optically uniaxial D2d phase with ambidextrous helicity and with orientational frustration in external fields, (ii) the non-polar tetragonal S4 phase, (iii) the non-polar, orthorhombic D2 phase that is structurally chiral featuring ambidextrous chirality, (iv) the polar orthorhombic C2v phase, and (v) the polar monoclinic C2 phase. Case (b) results in a trigonal C3v phase that behaves like a biaxial polar nematic phase. Finally we discuss some experiments that show typical effects related to the existence of tetrahedral order.

Microstructure Investigation on Metal Hydride Alloys by Electron Backscatter Diffraction Technique

The microstructures of two metal hydride (MH) alloys, a Zr7Ni10 based Ti15Zr26Ni59 and a C14 Laves phase based Ti12Zr21.5V10Ni36.2Cr4.5Mn13.6Sn0.3Co2.0Al0.4, were studied using the electron backscatter diffraction (EBSD) technique. The first alloy was found to be composed of completely aligned Zr7Ni10 grains with a ZrO2 secondary phase randomly scattered throughout and a C15 secondary phase precipitated along the grain boundary. Two sets of orientation alignments were found between the Zr7Ni10 grains and the C15 phase: (001)Zr7Ni10A//(110)C15 and [100]Zr7Ni10A//[0 1 ¯ 1]C15, and (01 1 ¯ )Zr7Ni10B//( 1 ¯ 00)C15 and [100]Zr7Ni10B//[313]C15. The grain growth direction is close to [313]Zr7Ni10B//[ 1 ¯ 11]C15. The second alloy is predominated by a C14 phase, as observed from X-ray diffraction analysis. Both the matrix and dendrite seen through a scanning electron microscope arise from the same C14 structure with a similar chemical composition, but different orientations, as the matrix with the secondary phases in the form of intervening Zr7Ni10/Zr9Ni11/(Zr,Ni)Ti needle-like phase coated with a thin layer of C15 phase. The crystallographic orientation of the C15 phase is in alignment with the neighboring C14 phase, with the following relationships: (111)C15//(0001)C14 and [1 1 ¯ 0]C15//[11 2 ¯ 0]C14. The alignments in crystallographic orientations among the phases in these two multi-phase MH alloys confirm the cleanliness of the interface (free of amorphous region), which is necessary for the hydrogen-storage synergetic effects in both gaseous phase reaction and electrochemistry.

The isothermal section of the Zr[sbnd]Cr[sbnd]Cu ternary system at 580 °C

The 580 °C isothermal section of the ZrCrCu ternary system was determined by means of X-ray diffraction, scanning electron microscopy and electron probe microanalysis. This isothermal section contained 10 single-phase regions, 18 two-phase regions and 9 three-phase regions. No ternary compound was found at 580 °C in the system. The solubility of Cu in the ZrCr2 cubic Laves C15 phase and the solubility of Cr and Cu in the terminal α-Zr solid solution phase were determined. The site occupation of element Cu in the ZrCr2 cubic Laves C15 phase was determined by Rietveld refinement. The Cu prefers to occupy the position of Cr. The CuZr phase, which is not stable at 580 °C in the binary CuZr system, was confirmed to exist at this temperature in the ZrCrCu ternary system. This is probably due to that the dissolving of Cr in the CuZr phase increases its thermodynamic stability.

Point defects and Zn-doping in defective Laves phase C15 MgCu2: A first-principles study

Employing first-principles calculations, the point defect in C15 Laves phase MgCu2 is studied. The formation enthalpies of point defects and defective C15 MgCu2 compound show that the dominating defect structure is MgCu and CuMg antisite defect on Mg-rich and Cu-rich side of off-stoichiometry, respectively. The concentration of various point defects as a function of composition of MgCu2 is analyzed from grand canonical statistics. The calculated local geometrical variation around defect demonstrates an apparent size effect, and the electronic densities of states and charge density difference distributions indicate the gradually weaker bonding from MgCu to VCu and from CuMg to VMg systems. Furthermore, doping of Zn atom in defective MgCu2 has been investigated. It is found that Zn has a stronger preference for occupying the Cu sublattice than for Mg sublattice because of the relative lower formation enthalpy. The electronic structure further reveals that the stability of Zn-doped system is associated with the stronger covalent bonds between Cu–Cu and Zn–Cu atoms.

Studies on the Synergetic Effects in Multi-Phase Metal Hydride Alloys

The electrochemical reactions of multi-phase metal hydride (MH) alloys were studied using a series of Laves phase-related body-centered-cubic (BCC) Ti15.6Zr2.1V43Cr11.2Mn6.9Co1.4Ni18.5Al0.3X (X = V, B, Mg, Y, Zr, Nb, Mo, La, and Nd) alloys. These alloys are composed of BCC (major), TiNi (major), C14 (minor), and Ti2Ni (minor) phases. The BCC phase was found to be responsible for the visible equilibrium pressure plateau between 0.1 MPa and 1 MPa. The plateaus belonging to the other phases occurred below 0.005 MPa. Due to the synergetic effects of other non-BCC phases, the body-centered-tetragonal (BCT) intermediate step is skipped and the face-centered-cubic (FCC) hydride phase is formed directly. During hydrogenation in both gaseous phase and electrochemistry, the non-BCC phases were first charged to completion, followed by charging of the BCC phase. In the multi-phase system, the side with a higher work function along the grain boundary is believed to be the first region that becomes hydrogenated and will not be fully dehydrided after 8 h in vacuum at 300 °C. While there is a large step at approximately 50% of the maximum hydrogen storage for the equilibrium pressure measured in gaseous phase, the charge/discharge curves measured electrochemically are very smooth, indicating a synergetic effect between BCC and non-BCC phases in the presence of voltage and charge non-neutrality. Compared to the non-BCC phases, the C14 phase benefits while the TiNi phase deteriorates the high-rate dischargeability (HRD) of the alloys. These synergetic effects are explained by the preoccupied hydrogen sites on the side of the hydrogen storage phase near the grain boundary.

Mechanical properties and structure of rapidly solidified bulk Fe89 − xHf4Ta1Cu1Gd1SixB4 (x = 0–15) and Fe74Hf4Ta1Cu1Gd1LaySi15 − yB4 (y = 7) alloys

Authors investigated mechanical properties of microcrystalline Fe89−xHf4Ta1Cu1Gd1SixB4 (x=0–15) and Fe74Hf4Ta1Cu1Gd1LaySi15−yB4 (y=7) alloys obtained by copper mold injection casting to a form of rods, 3mm in diameter. Structure of the samples was analyzed by Cu K-α powder X-ray diffractometry and the unit cell parameters of identified h-Fe2Hf0.8Ta0.2 (C14 Laves), fcc-Fe16Hf6Si7 (D8a) and bcc-Fe phases were calculated. The study revealed that ascending Si content leads to reduction in values of both unit cell parameters of C14 phase in the alloys studied while causes the expansion of D8a lattice; decrease in secondary dendrite arm spacing in the middle of the rods; stepwise improvement of yield and compressive strength at the expense of plastic deformation region, what was attributed to the precipitation of C14 Laves/D8a in the interdendritic regions of bcc-Fe; decrease in flexural strength with no effect on deflection-at-yield and exponential increase in hardness of the studied alloys. There is also clear correlation between unit cell volume of fcc-Fe16Hf6Si7 phase, yield and compressive strength on Si content. For the alloys with x=0–13, relation between yield strength and Vickers hardness was described with logarithmic function. It was found, that the lattice expansion of bcc-Fe and, in lesser extent, C14 Laves phase is correlated with the value of bending modulus in the whole composition range.

Effect of TiO2 addition on the properties of Ti3Si(Al)C2 based ceramics fabricated by reactive melt infiltration

In this paper, Ti3Si(Al)C2 based ceramics were fabricated by reactive melt infiltration (RMI) of TiC/TiO2 preforms with liquid silicon. The microstructure, phase composition, and mechanical properties of the Ti3Si(Al)C2 based ceramics have been investigated to understand the effect of phase composition of the preforms on the formation mechanisms of Ti3Si(Al)C2. The preforms with different content of TiO2 infiltrated at 1500°C with liquid silicon for 1h were composed of Ti3Si(Al)C2, Al2O3, TiC, TiSixAly and residual Al. The prior generated Al2O3 phases inhibited the dispersion of Ti3Si(Al)C2 phases, resulting in the drastically grain growth of Ti3Si(Al)C2. Subsequently, the microstructure with gradually increasing Ti3Si(Al)C2 grain size resulted in the decrease of the bending strength and fracture toughness of samples. When the content of TiO2 reached 20wt%, the bending strength reached the maximum, 326.6MPa. The fracture toughness attained the maximum, 4.3MPam1/2, when the content of TiO2 was 10wt%.

EFFECTS OF ROLLING DEFORMATION ON TEXTURE AND LPSO PHASE OF SPRAY-DEPOSITED MAGNESIUM ALLOYS CONTAINING Nd

Mg alloys have been applied widely as structural materials over the past decades, with low density, high specific strength, stiffness, specific elastic modulus, and high recycling rate. However, their features of poor ductility and formability at room temperature have limited their application due to hexagonal close-packed crystal structure with less independent slip systems. Grain refinement and texture randomization are two means to activate other slip systems. In this work, the billets of Mg-9Al-3Zn-1Mn-6Ca-2Nd alloy produced by spraying deposition method (the Osprey process) were studied in order to analyze the effect of rolling deformation at 350 °C and pass reduction e =20%, 25% and 30% on texture and microstructure evolution of an extruded size-asymmetry Mg alloy by SEM, TEM and XRD. The results show that under the condition of reduction of e =20% at 350 °C, a long-period stacking ordered phase with 24R structure was formed in (Ca, Nd)Al2 phase (C15 Laves phase ). All of basal texture (0002), prismatic texture {1010} , and pyramidal texture {1012} were activated, with pole density level weakened while pass reduction increased (e =20% , 25% and 30% ), namely, texture randomization achieved in Mg alloy, with main causes of nanometer-sized dispersed C15 phase impeding dislocation movement 第938-944页 pp.938-944