Ss Phase Research Articles

Microstructure and mechanical properties of Nb- and Mo-modified NiTi–Al-based intermetallics processed by isothermal forging

A compositionally modified Ni–45Ti–5Al–2Nb–1Mo (at%) alloy was developed by simultaneously adding Nb and Mo in NiTi–Al-based alloy and processed by forging isothermally. The as-forged Ni–45Ti–5Al–2Nb–1Mo alloy consists of NiTi matrix, Ti2Ni and dispersive (Nb,Ti)SS phases. Mo addition reduces the solubility of Nb in the NiTi matrix and gives rise to the precipitation of numerous nanoscale (Nb,Ti)SS phases (bcc structure) within the grains. The (Nb,Ti)SS phase is semi-coherent to the NiTi matrix with a lattice misfit of 6.7%. The addition of Mo and Nb dramatically enhances the NiTi–Al alloy at high temperatures, which is mainly attributed to the solid solution strengthening of Nb and Mo elements as well as the precipitation strengthening of (Nb,Ti)SS phases. The Ni–45Ti–5Al–2Nb–1Mo alloy is expected to become a high-temperature structural material using at intermediate temperatures with strengths of 650MPa at 923K and 350MPa at 1073K.

The magnetic field induced phase separation in a model of a superconductor with local electron pairing

We have studied the extended Hubbard model with pair hopping in the atomic limit for arbitrary electron density and chemical potential and focus on paramagnetic effects of the external magnetic field. The Hamiltonian considered consists of (i) the effective on-site interaction U and (ii) the intersite charge exchange interactions I, determining the hopping of electron pairs between nearest-neighbour sites. The phase diagrams and thermodynamic properties of this model have been determined within the variational approach (VA), which treats the on-site interaction term exactly and the intersite interactions within the mean-field approximation. Our investigation of the general case shows that the system can exhibit not only the homogeneous phases—superconducting (SS) and non-ordered (NO)—but also the phase separated states (PS: SS–NO). Depending on the values of interaction parameters, the PS state can occur in higher fields than the SS phase (field induced PS). Some ground state results beyond the VA are also presented.

Effects of alloying elements in GH99 superalloy on microstructure evolution of reactive brazing TiAl/GH99 joints

TiAl-based intermetallics and GH99 superalloy were joined by reactive brazing with Ti filler metal. The interfacial microstructure and the effects of main alloying elements Cr, Mo, W and Co originating in the GH99 superalloy on interfacial microstructure evolution of the joint were investigated. The results showed that the alloying elements were mainly concentrated in (Ni, Cr)ss (γ) phase in the zone adjacent to the GH99 superalloy and Ti3Al (α2) + α-Ti + β-Ti phase in the middle zone of the joint. Compared with Ni/Ti/TiAl joint, the alloying elements in GH99 superalloy delayed the formation of liquid phase in GH99/Ti/TiAl joint. Furthermore, these alloying elements dissolving into GH99/Ti/TiAl joint were β-Ti phase stabilizing elements, which promoted the formation of β-Ti + α-Ti + α2 phase. In addition, these alloying elements had positive effects on the ductility and high temperature properties of GH99/Ti/TiAl joint.

The microstructure optimizing of the Nb–14Si–22Ti–4Cr–2Al–2Hf alloy processed by directional solidification

In this work, the microstructure evolution of an Nb–14Si–22Ti–4Cr–2Al–2Hf alloy processed by liquid-metal-cooled directional solidification (LMC) and heat treatment (HT) was investigated. The microstructure in the quasi-steady-state growth region of the LMC ingot consisted of primary (Nb,Ti)SS dendrites and eutectic (Nb,Ti)SS/(Nb,Ti)5Si3. The unstable (Nb,Ti)3Si phase was eliminated and the eutectic with coupled (Nb,Ti)SS and (Nb,Ti)5Si3 occurred. After heat treatment of selection, the fiber reinforcing mash structure was maintained: the (Nb,Ti)SS phase connected to be a continuous matrix and the fine discontinuous fibrous (Nb,Ti)5Si3 was distributed uniformly and parallel to the growth direction on longitudinal section.

Studies on Synthesis and Characterization of Mo Based In Situ Composite by Silicothermy Co-reduction Process

The results of an in situ synthesis of refractory metal–intermetallic composite (RMIC), Mo-16Cr-4Si (wt pct) multiphase alloy and its characterization, are presented in this study. The alloy was prepared from the oxides of molybdenum and chromium by their co-reduction with Si metal powder as a reductant. The exothermic nature of these reactions resulted in the formation of consolidated composite as a product in a single step. The thermodynamic aspects of exothermic reactions were studied by thermogravimetry/differential thermal analyzer. As-reduced alloys were remelted by arc melting and heat treated to obtain a homogenous microstructure. The evolution of phases and microstructures qA studied by X-ray diffraction, scanning electron microscopy, and energy-dispersive spectrum analysis. The multiphase alloy consisted of Mo3Si and discontinuous (Mo, Cr) (ss) phase with a volume percentage of 28 pct. The synthesized alloys were characterized with respect to composition, phases, microstructure, hardness, and oxidation behavior.

Dielectric properties of anodic oxide film on Nb solid solution/Nb 2N two phase alloys

The structure and dielectric properties of an anodic oxide on Nb solid solution (Nb ss)/Nb 2N two phase alloys were investigated. The anodized specimens were covered with two types of anodic oxides on the Nb ss and Nb 2N phases, respectively. The relative permittivity and leakage current density of the anodized specimens can be explained by the combination of the two oxides; the relative permittivity of the specimens increased with the volume fraction of the oxide on the Nb ss phase, because the relative permittivity of the oxide on the Nb ss phase is higher than that on the Nb 2N phase. The leakage current was suppressed irrespective of the constitute phases as compared with the anodized pure Nb, because it was reduced in both the oxides on the Nb ss and Nb 2N phases.

(Ca,Sr)CO 3 aqueous–solid solution systems: From atomistic simulations to thermodynamic modelling

The uptake of strontium in calcium carbonates is a topic of sustained interest in (radio)geochemistry. The available data on (Ca,Sr)CO 3 aqueous–solid solution (Aq–SS) systems with the calcite ( R 3 ¯ c ) and aragonite ( Pmcn) structures are reconciled using a stepwise approach from atomistic to thermodynamic modelling consisting of: (1) Quantum–mechanical and force-field calculations aimed at the prediction of standard thermodynamic properties of a hypothetical pure SrCO 3( R 3 ¯ c ) compound; (2) Force-field calculations on supercell structures of end members with defects of incoherent atoms in the cluster expansion framework, followed by Monte Carlo simulations of the excess SS mixing properties in both R 3 ¯ c and Pmcn structures; (3) Thermodynamic modelling of Aq–SS systems with SS phases of both structures using the Gibbs Energy Minimization (GEM) code in two scenarios: (i) aqueous solution in equilibrium with one SS phase of either Pmcn or R 3 ¯ c structure; (ii) aqueous solution equilibrated with two SS phases having different structures. The (ii) case was investigated using two approaches: (A) The two SS introduced as separate phases, with the excess mixing in each phase described by an own Guggenheim polynomial and (B) End members of different structures combined into a single calcite–strontianite phase using the Darken’s Quadratic Formulation (DQF) mixing model. For the aragonite–strontianite SS system, a nearly symmetric solvus is predicted in good agreement with the available calorimetric and electrochemical data. The solubility data at low Sr concentrations are qualitatively reproduced with the DQF model. For the (Ca,Sr)CO 3 SS with the calcite structure, a slightly asymmetric, moderately positive excess Gibbs free energy of mixing is predicted and shown to agree well with known equilibrium distribution coefficients for trace Sr in calcite and with the predicted solubility product of the hypothetical SrCO 3( R 3 ¯ c ) end member. Although each SS system with the calcite and aragonite structures shows a nearly symmetric moderate deviation from the ideal mixing, the GEM calculations of Aq–SS equilibria including two SS phases result in a broad, strongly asymmetric “miscibility gap” with maximum x = 0.003 mole fraction of Sr in calcite. These results are instructive because many polyvalent cations showing a strong uptake in calcite (e.g. actinides) are expected to form end member compounds with structures different from the host mineral. The combination of atomistic simulations with GEM thermodynamic modelling provides a new suite of computer-aided tools for modelling such geochemically complex systems. The accuracy of thermodynamic values predicted from atomistic simulations is shown to be comparable to that of the best experimental data.

Microstructural characterization of Mo–12Si–8.5B alloy subjected to diffusion annealing treatment

Abstract Microstructural evolutions in the solidification and subsequent diffusion annealing treatment of Mo–12Si–8.5B alloys have been studied by using XRD, OM, SEM and TEM. Because the annealing temperature (1900 °C) is in the vicinity of four-phase equilibrium eutectic reaction (1950 °C) point of L = Mo 3 Si + Mo 5 SiB 2 (T 2 )+Mo ss (Mo solid solution), the dendrites and lamellar phase have almost been dissolved. Dislocations, Mo ss precipitates and subgrain boundary have been observed in the T 2 phase while the other phases are not found. In addition, dislocations are from excess vacancy during the annealing treatment and play important roles for the Mo ss precipitate. The dislocations can serve as the heterogeneous nucleation sites and compose of sub-boundary which refines the grain size to some extent. The results observed in this study can be used as a reference for future work on the relationships between microstructure and mechanical property of the Mo–Si–B alloys.

Influence of Cr on the microstructure and mechanical properties of Ti–Si Eutectic Alloys

Abstract As promising candidates for new high-temperature structural materials, Ti – Si eutectic alloys are required for the next generation of advanced aircraft engines. In this paper, microstructures of the Ti – Si eutectic alloys with various Cr contents were investigated by means of X-ray diffraction, scanning electron microscopy and light optical microscopy. The results show that with the increase of Cr content up to 9 wt.%, β-Ti (Si, Cr)ss matrix and Ti5Si3 form in the as cast alloy, however, the α-Ti (Si, Cr)ss phase disappears. Cr element solute in β-Ti and Ti5Si3 can significantly change the morphology and distribution of Ti5Si3 intermetallic phase. This greatly affects the fracture behavior and mechanical properties of as-cast Ti – Si eutectic alloys.

Microstructural evolution of Cu-1 at% Ti alloy aged in a hydrogen atmosphere and its relation with the electrical conductivity

Copper alloys with titanium additions between 1 and 6 at% Ti emerge currently as attractive conductive materials for electrical and electronic commercial products, since they exhibit superior mechanical and electrical properties. However, their electrical conductivity is reduced owing to the residual amount of Ti solutes in the Cu solid solution (Cu ss) phase. Since Cu shows only poor reactivity with hydrogen (H), while Ti exhibits high affinity to it, we were inspired by the idea that hydrogenation of Cu–Ti alloys would influence their microstructure, resulting in a significant change of their properties. In this contribution, the influence of aging under a deuterium (D 2) atmosphere of Cu-1 at% Ti alloys on their microstructure is investigated to explore the effects on the electrical conductivity. The specimens were investigated by means of transmission electron microscopy (TEM), field ion microscopy (FIM), computer-aided field ion image tomography (cFIIT), and atom probe tomography (APT). At an early aging stage at 623 K in a D 2 atmosphere of 0.08 MPa, ellipsoidal α-Cu 4Ti precipitates are formed in the alloy, and during subsequent aging, δ-TiD 2 is competitively nucleated instead of growth of α-Cu 4Ti particles. The co-precipitation of α-Cu 4Ti and δ-TiD 2 efficiently reduces the Ti concentration of Cu ss matrix, particularly in the later aging stages in comparison to the aging in vacuum conditions. The electrical conductivity of the alloy aged in the D 2 atmosphere increases steeply up to 48% International Annealed Copper Standard (IACS) after 1030 h, while it saturates to approximately 20% IACS in the alloy aged in vacuum. The outstanding increase of electrical conductivity during aging in D 2 atmosphere can be basically explained by the reduction of Ti solute concentration in Cu ss matrix.

Oxidation of Nb–Si–Cr–Al in situ composites with Mo, Ti and Hf additions

The effects of Mo, Ti and/or Hf additions on the isothermal oxidation behaviour of Nb–Si–Cr–Al-based in situ composites in static air at 800 °C and 1200 °C were studied for the as-cast and heat-treated materials. After heat treatment at 1500 °C, the microstructures of all the alloys consisted only of the niobium solid solution (Nb ss) and Nb 5Si 3 phases. The addition of Ti and the decrease of the Mo concentration to 2 at.% improved the oxidation resistance of the alloys at 800 °C dramatically. The Hf addition had no significant effect on the oxidation behaviour of Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf (at.%) at 800 °C. The oxidation resistance of the alloys was found to be sensitive to the volume fraction of Nb ss. The coarsening and the increase of the volume fraction of the Nb ss phase in the heat-treated alloys were mainly responsible for the degradation of their oxidation resistance. Preferential attack of the Nb ss phase was observed in all the alloys. Pesting oxidation behaviour was exhibited at 800 °C by the Nb–18Si–5Al–5Cr–5Mo (as-cast), Nb–24Ti–18Si–5Al–5Cr–5Mo (as-cast), Nb–24Ti–18Si–5Al–5Cr–2Mo (heat-treated) and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf (heat-treated) alloys. Pesting of the alloys occurred due to a combination of the presence of elements that oxidised rapidly and the restricted deformation capability of the scales. Spallation of oxide scales during cooling was observed for the as-cast and heat-treated Nb–24Ti–18Si–5Al–5Cr–2Mo and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf alloys at 1200 °C. In the diffusion zones formed in the alloys without Hf addition, the 5–3 silicide was not oxidised. The TiNbO 4, TiNb 2O 7, Ti 2Nb 10O 29 and silicon oxide were present in the scales formed on the Nb–24Ti–18Si–5Al–5Cr–2Mo and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf alloys at 1200 °C and Hf oxide was also present in the latter alloy. At 1200 °C, in the Ti containing alloys, large Ti oxide particles formed at the Nb ss and 5–3 silicide interface and fine Ti oxide particles formed inside the Nb ss. Needle-like Hf oxide formed inside the 5–3 silicide in the heat-treated Hf containing alloy, which resulted in the degradation of its oxidation resistance. No protective oxide scale was formed on any of the alloys studied in this work.

Phase stability and alloying behavior in the Mo-Si-B system

The effect of transition metal (TM) substitution for Mo has been examined in terms of the phase stability and multiphase microstructures in the Mo-Si-B ternary system. The metal-rich portion of the ternary Mo-Si-B system at equilibrium is comprised of thermally stable bcc Mo(ss) phase, a ternary-based Mo5SiB2 (T2) phase, and a binary-based metal-rich silicide (Mo3Si [A15]). The structures that are developed by following systematic alloying with a wide range of TMs, which are substitutional in both Mo(ss) and T2 phases (group IVB, VB, and VIB metals), have been analyzed to elucidate the roles of the substitution on the stability of the three phase fields of Mo(ss) + T2 + Mo3Si. In particular, the borosilicide ternary-based T2 phase shows an extended solid solution with a wide range of TMs. The extended solubility in the T2 phase essentially mimics the alloying behavior of the TM-based bcc phase. The critical factor for the phase stability appears to be the existence of a unique feature of bcc-like TM clusters within the T2 lattice structure. The combined criteria of atomic size factor and the valence electron concentration per atom (e/a) have been used to elucidate the observed alloying behavior.

Spectrum analysis of voltage noise of moving vortex lattice and dynamic phase transition

Using Langevin molecular dynamics method, we have studied the voltage noise spectrum generated by current-driven vortices as a function of the magnetic field an d current. It is found that the broad band noise (BBN) at low frequencies decrea ses with increasing magnetic field, it decreases at the magnetic field near th e vortex dynamic transition field FP， and the Lorentze narrow ba nd noise (NBN) at high frequencies appears. As the magnetic field increases near the melt ing field Fm, the NBN peak goes up and shifts towards higher freq uencies. In the elastic motion region of vortices with strong driving currents, NBN exhib its the washboard-like shape. This behavior arises from the periodic modulation of the translational velocity of vortices, suggesting that there is a Bragg gla ss phase with the translation order in the vortex lattices of the layered superc onductors.

On the formation mechanism of BaTiO 3–BaZrO 3 solid solution through solid-oxide reaction

The formation of solid solution composition BaTi 0.6Zr 0.4O 3, from BaCO 3, TiO 2 and ZrO 2 powders has been studied through solid-oxide reaction using TGA/DSC, XRD. BaCO 3 decomposes at much lower temperature in the mixture due to the presence of TiO 2. BaTiO 3 (BT) and BaZrO 3 (BZ) start forming from the temperature range 700–800 °C without intermediate formation of BaO or other titanate/zirconate phases. Rate of BaTiO 3 formation is higher than BaZrO 3 formation due to the lower activation energy (34.3 kcal/mol) required for titanate formation than zirconate (48.4 kcal/mol). Solid solution then forms by diffusion of BaTiO 3 into BaZrO 3 from 1300 °C onwards. Lattice parameter of initial SS phase is higher, indicating the phase has a coherent interface with zirconate and higher in Zr-concentration than the final product. Activation energy for the SS formation (133 kcal/mol) indicates that Ba and/or O diffusion through SS layer may limit the reaction.

Preparation and characterization of SrCo 0.8Fe 0.2O 3− δ–SrSnO 3 oxygen-permeable composite membrane

Sn-containing SrCo 0.8Fe 0.2O 3− δ (SCF) oxygen-permeable dense ceramic membranes were prepared and characterized. The solid solubility of Sn in SCF was found not higher than 2.5 mol%; beyond that value, the membrane existed as a composite comprising an oxygen-permeable SCF phase and oxygen-imperious SrSnO 3 (SS) phase. The inclusion of 15 mol% of SS phase in SCF gave rise to an increase of sintering temperature from 1100 to 1280 °C. By exposing a 1.2-mm-thick composite to air at one side and flowing helium at the other side, oxygen permeated through the membrane at rates of 1.3×10 −6 and 5×10 −8 mol cm −2 s −1 at 1000 and 560 °C, respectively. The Arrhenius plot of oxygen permeation flux showed a bending at 737 °C for the composite, about 80 °C lower than that for the single-phase SCF. Moreover, the oxygen permeability of the composite decreased less pronouncedly with decreasing temperature than that for SCF, especially in the temperature range below the bending point. The composite also showed a reduced thermal expansion compared with SCF, which would improve the mechanical compatibility of the membrane with other components in a membrane assembly.

Hydrogenation-induced fragmentation in Ta–Ni alloy

Hydrogen-induced fragmentation behavior of Ta–Ni binary alloys is investigated to understand the mechanism of hydrogen pulverization of refractory metals and alloys and to develop the powder fabrication process by hydrogenation. Single-phase alloy of Ta solid solution (Ta ss) exhibits no change in sample shape by hydrogenation at room temperature, while two-phase alloys of Ta ss and Ta 2Ni Laves phase are broken into fragments. Crack propagation occurs preferentially in the brittle Ta 2Ni phase rather than in the ductile Ta ss phase. When the volume fraction of brittle Ta 2Ni increases with increasing Ni content, hydrogen-induced fragmentation is enhanced. The lattice parameter after hydrogenation increases in Ta ss, but not in Ta 2Ni. Nanosized clusters with Moiré patterns are observed in a HRTEM image of hydrogenated Ta ss, and Debye rings corresponding to tantalum hydride β-TaH appear in the associated diffraction pattern. It is suggested that the hydrogen fragmentation is attributed to the absorption of a large amount of hydrogen, the interfacial strain introduction by lattice expansion and hydride formation, and crack formation at brittle constituent phase(s) and hydride.

Early stages of interface reactions between AlN and Ti thin films

The early stages of interface reactions between AlN and Ti thin films were investigated using x-ray diffractions, Auger electron spectroscopy, cross section transmission electron microscopy (XTEM), and high resolution XTEM. The AlN/Ti bilayers were deposited on a molybdenum substrate using reactive and nonreactive magnetron sputtering techniques. After deposition, the bilayers were heat treated for 1–10 h at 600 °C in a nitrogen atmosphere. Decomposition of the AlN layer took place at the AlN/Ti interface and its products, Al and N, reacted with Ti to produce a AlN/Al3Ti/Ti2N/Ti3Al/α-(Ti, Al)ss phase sequence. This phase sequence is not consistent with the Ti–Al–N phase diagram and is believed to be the outcome of the particular conditions that prevail in the thin film and correspond to a particular set of kinetic parameters. A model that explains the development of the phase sequence and predicts its evolution after prolonged heat treatments is put forward. The applicability of such a solid-state reaction technique for producing functionally graded coating for wear and corrosion resistance applications is discussed.

Transition Metal Alloying and Phase Stability in the Mo-Si-B System

ABSTRACTThe effect of transition metal substitution for Mo on the phase stability and multi-phase microstructures in the Mo-Si-B ternary system has been examined. The metal-rich portion of the ternary Mo-Si-B system at equilibrium is comprised of thermally stable BCC Mo(ss) phase, a ternary-based Mo5SiB2 (T2 phase), binary-based metal-rich silicides (Mo3Si [the A15 phase] and Mo5Si3 [the T1 phase]) and borides (Mo2B and MoB phases). Systematic alloying with selected transition metals which are substitutional in both Mo(ss) and T2 phases such as Cr, V, Nb, W, Ti and Hf, has been performed to elucidate the roles of the substitution on the stability of the three phase fields of Mo(ss) + T2 + A15 and T2+ T1 + A15. The potential of the alloying effects on the microstructure design and control of the solidification pathways is further detailed.

Microstructural mechanical properties and yield point effects in Mo alloys

The mechanical properties of two phase Mo–Si–B alloys were analyzed with nanoindentation techniques and atomic force microscopy. A nanoindenting atomic force microscope allows local determinations of hardness and modulus of the T2 phase and Mo(ss) phase. The T2 phase and Mo(ss) phase in these alloys show a hardness of 21 and 8.5 GPa, respectively. The modulus of the T2 phase is nearly 10% higher than that for the Mo(ss) phase. Large pop-ins are found during indenting in the T2 phase but not in the Mo(ss) phase which has a higher surface roughness.

Seismic Observations of Splitting of the Mid-Transition Zone Discontinuity in Earth's Mantle

The transition zone of Earth's mantle is delineated by globally observed discontinuities in seismic properties at depths of about 410 and 660 kilometers. Here, we investigate the detailed structure between 410 and 660 kilometers depth, by making use of regional stacks of precursors to the SS phase. The previously observed discontinuity at about 520 kilometers depth is confirmed in many regions, but is found to be absent in others. There are a number of regions in which we find two discontinuities at about 500 and 560 kilometers depth, an effect which can be interpreted as a "splitting" of the 520 kilometer discontinuity. These observations provide seismic constraints on the sharpness and observability of mineralogical phase transitions in the mantle transition zone.