Skull Melting Technique Research Articles

Enhanced high temperature tensile and creep properties of a β-solidified γ-TiAl alloy with the hybrid addition of C and Y2O3

β-solidified γ-TiAl alloys are popular lightweight structural materials well suited for high temperature applications, however, whose service temperature limit is greatly restricted due to the insufficient high temperature strength and creep resistance. In present work, a hybrid reinforced β-solidified γ-TiAl alloy was successfully developed with the hybrid addition of C and Y2O3 by induction skull melting (ISM) technique, accordingly, the microstructure, high temperature tensile and creep properties of the alloy are detailly discussed. The results indicate that the hybrid addition of C and Y2O3 can improve the high temperature tensile strength of this nearly lamellar TiAl alloy with the increase of about 13.7%. More importantly, creep resistance of this alloy is significantly enhanced, which is attributed to multiple strengthening mechanisms: (ⅰ) the second phase strengthening from multi-scale Y2O3 particles and dynamically precipitated H–Ti2AlC carbides; (ⅱ) mechanical twins and twin intersections can act as special obstacles to dislocation movement, which is of great significance in decreasing the steady-state creep rate. Particularly, this alloy exhibits excellent microstructure stability during creep. On the one hand, nano-scale Y2O3 and dynamically precipitated H–Ti2AlC particles could pin the B2/γ interface to strengthen lamellar colony boundary. On the other hand, dispersed B2 phase particles could precipitate along α2 lamellae and grow up with the consumption of the α2 phase, which will inhibit the degradation of α2/γ lamellae. This study offers a new composition design approach to optimize high temperature performance of β-solidified γ-TiAl alloys.

Effect of heat treatment on the structure and mechanical properties of partially gadolinia-stabilized zirconia crystals

ABSTRACT (ZrO2)1-x (Gd2O3) x (x= 0.028–0.04) is heat-treated at 1600°C in air or in vacuum. Partially gadolinia-stabilized zirconia crystals are grown by the skull melting technique. The effect of annealing on the phase compositions, structures, and mechanical properties of the crystals is studied using X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and indentation. A comparison with previously obtained data on the structure and properties of crystals after growth shows that annealing the crystals leads to the redistribution of gadolinia between the tetragonal phases: the t phase is depleted of gadolinia while the t’ phase becomes enriched. The gadolinia-enriched t’ phase undergoes multistage twinning. Thus, the sizes of the twins in the t’ phase are far smaller than those in the t phase. Annealing the crystals increases their fracture toughness, and this increase is larger for all crystals after air annealing. The increase in the fracture toughness of the crystals is caused by a decrease in the gadolinia concentration in the transformable t phase, which enhances the effect of the transformation hardening mechanism. The fracture toughness of the crystals is further increased by the ferroelastic hardening mechanism, the effect of which is enhanced with an increase in the quantity of the t’ phase in the crystals.

Effect of nano Y2O3 addition on microstructure and room temperature tensile properties of Ti–48Al–2Cr–2Nb alloy

Ti–48Al–2Cr–2Nb alloy reinforced with ceramic particle Y2O3 was fabricated by induction skull melting (ISM) technique in a vacuum environment. The thermodynamic feasibility of the reactions between nano Y2O3 and molten alloy has been considered. The as-cast microstructure of Ti–48Al–2Cr–2Nb adding nanoY2O3 during melting was analyzed by OM, SEM, EDS and TEM. It is concluded that multiple reinforcements are synthesized and they show different shapes. Y2O3 grows in regular geometry shapes when the content of Y2O3 is about 0.1 at. % and grows in dendritic shapes when the content of Y2O3 is below 0.05 at.%. The work shows that adding nano Y2O3 could refine microstructure of TiAl and improve strength and elongation of TiAl alloy effectively within a certain range.

Structure and transport properties of melt grown Sc2O3 and CeO2 doped ZrO2 crystals

In this work we report a study of (ZrO2)1−x−y(Sc2O3)x(СeO2)y solid solution crystals (x = 0.08–0.10; y = 0.005–0.015) grown by skull melting technique. The crystal structure of the material was studied using X-ray diffraction, Raman spectroscopy and impedance spectroscopy. The study showed that optically homogeneous and transparent crystals cannot be grown from the melt in the experimental composition range. For overall stabilizing oxide concentrations of above 10 mol% the crystals contained a cubic phase and a rhombohedral one, whereas at concentrations of lower than 10 mol% a cubic and a tetragonal phases coexisted. Ceria introduction into the (ZrO2)1−x(Sc2O3)x system increases its high-temperature ionic conductivity. The highest ionic conductivity was observed in 0.5 mol% ceria containing crystals.

ANISOTROPIC MECHANICAL PROPERTIES AND HARDENING MECHANISMS IN ZRO2–Y2O3 SOLID SOLUTION CRYSTALS

Abstract. The anisotropy of the mechanical properties of single crystal ZrO2 — 2.8 mol.% Y2O3 solid solutions has been studied. The crystals have been grown by skull melting technique. The microhardness and fracture toughness have been tested for different crystallographic planes by indentation with different indenter diagonal orientations. The study shows that the microhardness of the material depends on the crystallographic orientation but slightly whereas the fracture toughness varies for different planes. The maximum fracture toughness has been observed in the crystal specimen cut laterally to the orientation. We have studied the anisotropy of the microhardness in the material for different indenter diagonal orientations. The maximum fracture toughness has been obtained for the {100} plane and the indenter diagonal orientation. The phase composition inside and outside the indents on the {100}, {110} and {111} surfaces for 20, 3 and 1 N loads has been studied in local areas using Raman spectroscopy. The degree of the tetragonal−monoclinic transition has been evaluated for different crystallographic planes and different indenter diagonal orientations. The tetragonal−monoclinic transition proves to be anisotropic, and this affects the transformation hardening mechanism. The maximum amount of the monoclinic phase is present in the vicinity of the indent in the {100} plane for the indenter diagonal orientation. The highest fraction toughness has also been observed in the {100} plane for the indenter diagonal orientation. Probably, the abovementioned indenter diagonal orientation provides for the maximum stress concentration along the coherent conjugation planes between the tetragonal and the monoclinic phases during the tetragonal−monoclinic transition, i.e. (100)t||(100)m and [001]t||[010]m.

Anisotropy of mechanical properties and hardening mechanism in ZrO2–Y2O3 solid solution crystals

The anisotropy of the mechanical properties of crystalline ZrO2 – 2.8mol.% Y2O3 solid solutions has been studied. The crystals have been grown by skull melting technique. The microhardness and fracture toughness for different crystallographic planes have been tested by indentation with different indenter diagonal orientations. We show that the microhardness of the material depends on the crystallographic orientation but slightly whereas the fracture toughness varies for different planes. The maximum fracture toughness has been observed in the specimen cut out from the crystal laterally to the <100> orientation. We have studied the microhardness anisotropy for different indenter diagonal orientations. The maximum fracture toughness has been obtained for the {100} plane and the <100> indenter diagonal orientation.The phase composition inside and outside the indents on the {100}, {110} and {111} surfaces for 20, 3 and 1N loads has been studied using local Raman spectroscopy. The degree of the tetragonal-monoclinic transition has been assessed for different crystallographic planes and different indenter diagonal orientations. We show that the tetragonal-monoclinic transition is anisotropic, this affecting the transformation hardening mechanism. The maximum amount of the monoclinic phase has been detected in the vicinity of the indent in the {100} plane for the <100> indenter diagonal orientation. The highest fraction toughness has also been observed in {100} plane for the <100> indenter diagonal orientation. Probably, this indenter diagonal orientation provides for the maximum stress orientation along the coherent conjugation planes between the tetragonal and the monoclinic phases during the tetragonal-monoclinic transition, i.e. (100)t || (100)m and [001]t || [010]m.

Structure and conductivity of yttria and scandia‐doped zirconia crystals grown by skull melting

In this paper a detailed study of the (ZrO2)1‐x(Y2O3)x (x=0.025–0.15), (ZrO2)1‐x(Sc2O3)x (x = 0.06 – 0.11) and (ZrO2)1‐x‐y(Sc2O3)x(Y2O3)y (x=0.07 – 0.11; y=0.01 – 0.04) solid solution crystals grown by skull melting technique is presented. The structure, phase composition, and ion conductivity of the obtained crystals were investigated by X‐ray diffraction, transmission electron microscopy, Raman scattering spectroscopy, and impedance spectroscopy. Maximum conductivity as (ZrO2)1‐x(Y2O3)x and (ZrO2)1‐x(Sc2O3)x solid solution crystals is observed for the compositions containing 10 mol% stabilizing oxide, and the conductivity of 10ScSZ is ~3 times higher than for 10YSZ. Experiments on crystal growth (ZrO2)1‐x‐y(Sc2O3)x(Y2O3)y solid solutions showed that uniform, transparent crystals 7Sc3YSZ, 7Sc4YSZ, 8Sc2YSZ, 8Sc3YSZ, 9Sc2YSZ, 9Sc3YSZ, 10Sc1YSZ, and 10Sc2YSZ are single phase crystal containing t″ phase. It is established that a necessary condition of melt growth of (ZrO2)1‐x‐y(Sc2O3)x(Y2O3)y single‐phase crystals is the total concentration of the stabilizing oxides from 10 to 12 mol%. The addition of Y2O3 affects the (ZrO2)1‐x‐y(Sc2O3)x(Y2O3)y solid solution conductivity different ways and depends on the Sc2O3 content in the starting composition. The effects of structure, phase composition, concentration, and type of stabilizing oxides on the electrical characteristics of obtained crystals are discussed.

Microstructure evolution of Ti–46Al–6Nb–(Si,B) alloys during heat treatment with W addition

The cast ingots of Ti–46Al–6Nb–0.25Si–0.2B and Ti–46Al–6Nb–0.5W–0.25Si–0.2B (at%) were made by induction skull melting (ISM) technique. A series of heat treatments (HTs) were conducted to research the microstructure evolution of both alloys. Microstructure and tensile property were examined by scanning electron microscope (SEM), X-ray diffraction (XRD), transmission electron microscope (TEM), and tensile testing machine. The results show that microsegregation (liquid segregation and solid segregation) is exacerbated by the addition of 0.5 at% W; the addition of Nb, W in TiAl alloy makes the phase transition difficultly take place; then, the microstructures and tensile properties of both alloys are improved after certain HT processes; finally, the thicknesses of the γ/α2 lamellae after a certain HT process are significantly affected by the number of residual γ phases before the furnace-cooling moment.

Growth and characterization of low yttria-doped fully cubic stabilized zirconia-based single crystals

For the first time, fully cubic stabilized zirconia-based single crystals with large nitrogen contents and optimized anion vacancy concentrations, interesting for new electrochemical devices such as nitrogen sensors, were prepared by a two-step approach. This two-step approach combines growth of nitrogen-free zirconia single crystals via the skull melting method and their subsequent direct nitridation to form cubic nitrogen doped zirconia-based single crystals. In the initial step, zirconia crystals with various yttria contents (2, 3, 4 and 6mol%) were prepared by the skull-melting technique. As determined by X-ray powder diffraction, tetragonal stabilized crystals were obtained, containing monoclinic segregations, when the yttria content of the grinded crystals was below 4mol%. Subsequently, the crystals were nitrided with nitrogen gas at 1500°C using different reaction times. Nitrogen content and crystallographic identity of the nitrided samples were investigated, revealing successful nitrogen incorporation and, hence, full cubic stabilization of the initial nitrogen-free samples also for the lowest yttria content. The degree of deviation from the cubic fluorite-type structure is discussed with respect to crystallographic parameters (‘pseudo-cubic factor’ etc.). The stabilization of cubic or tetragonal phases to ambient temperature is related to the anion vacancy concentration with respect to different impacts of the anion vacancies generated either by yttria or nitrogen incorporation (effective vacancy concentration [Veff]).

Fracture characteristics of notched investment cast TiAl alloy through in situ SEM observation

TiAl alloys were produced by investment casting method combined with induction skull melting (ISM) technique. In situ scanning electron microscopy (SEM) was utilized to study the fracture characteristics and crack propagation of a notched investment cast TiAl specimens in tension under incremental loading conditions. The whole process of crack initiation, propagation and failure during tensile deformation was observed and characterized. The results show that the fracture mechanism was sensitive to not only the microcracks near the notched area but also lamellar orientation to loading axis. The high tensile stress leads to the new microcracks nucleate along lamellar interfaces of grains with favorable orientation when local stress intensity reaches the toughness threshold of the material. Thus, both plasticity and high tensile stress are required to cause notched TiAl failure.

Microstructure evolution, hot deformation behavior and mechanical properties of Ti-43Al-6Nb-1B alloy

A novel high Nb, low Al contained TiAl alloy Ti-43Al-6Nb-1B (at %) was fabricated by induction skull melting (ISM) technique. The as-cast alloy exhibits fully lamellar (FL) microstructure with mean colony size of 90 μm. By means of thermophysical simulation, the hot deformation mechanism at 1250 °C is concluded as dislocation slip and twinning in γ phase; bending, rotating and elongation of lamellar colonies are the primary deformation modes. A large-size TiAl pancake was successfully produced by triple-step canned forging at 1250 °C. The forged alloy displays duplex (DP) microstructure consisting of elongated lamellar colonies and γ grains. Typical DP and FL microstructures were further obtained by different heat treatments. The tensile strength increases rapidly from 670 MPa in as-cast state to 975 MPa in as-forged condition at room temperature, with elongation increasing from 0.4% to 1.5%. The as-forged alloy maintains strength higher than 950 MPa until 750 °C; also the DP and FL materials hold excellent strength more than 830 MPa and 770 MPa respectively. The superior tensile properties after forging are mainly ascribed to the microstructure refinement and homogenization, the lamellar spacing reducing, as well as the low Al and relatively low Nb additions in this novel high Nb containing TiAl alloy.

Effect of Forging on Microstruture and Mechanical Properties of &lt;i&gt;In Situ&lt;/i&gt; TiB&lt;sub&gt;w&lt;/sub&gt;/Ti Composite

Ti-6Al-2.5Sn-4Zr-0.7Mo-0.3Si-0.3Y alloy matrix composite reinforced with 7%vol TiB whiskers was fabricated by Induction Skull Melting (ISM) technique and one-direction forged technology utilizing the reaction between titanium and TiB2. The microstructure, room temperature and high temperature mechanical properties have been presented and discussed. Microstructural analysis of the composites revealed that the microstructure was significantly refined and TiB whiskers were made to align the longitudinal direction after forging. It shows that the tensile strength and ductility of the composites has a significant improvement, especially at room temperature.

Luminescence properties of Sm3+-doped polycrystalline ZrO2

We obtained samarium-doped zirconia using two different routes. In one, atomic layer deposited thin crystalline films were doped by using ion implantation; this sample was mainly monoclinic. The other method, the skull-melting technique, yielded polycrystalline bulk zirconia containing both monoclinic and tetragonal phases of ZrO2. Thorough photoluminescence (PL) measurements of Sm emission in these materials were performed using pulsed laser excitation at 405, 320 and 230nm, respectively corresponding to direct, defect-related and host-sensitized excitation. Both samples exhibited well-resolved emission series of Sm3+. In general, the recorded spectra may be considered as superpositions of two different sets of lines attributable to Sm3+ centers in different crystalline phases of ZrO2. These results have been confirmed by time-resolved measurements, which also suggest that all emission lines originate from a common initial state (4G5/2) with a lifetime of about 1ms. As expected, the host-mediated excitation leads to a prolonged decay profile attributed to the retarded energy transfer from host to guest.

Partially stabilized zirconia single crystals: growth from the melt and investigation of the properties

The PSZ crystals of ZrO 2–(2–5 mol%)Y 2O 3 compositions have been grown by the skull-melting technique. The effects of composition and crystallization conditions on the microstructure and properties have been studied. The crystallization conditions have been shown to affect the crystal size and compositional homogeneity. Decreasing crystallization rate has resulted in increasing of crystal size and decreasing of the concentration of impurities as compared to initial oxides. The morphology of as-grown facets was considered a key to the material microstructure. The material formation depends on the initial composition, the crystallization conditions and subsequent annealing. The distribution of the ZrO 2–Y 2O 3 solid solution components was found to be uniform for (2–5 mol%)Y 2O 3 concentration range at 10–40 mm/h growth rate range. The K eff values determined are close to 1.

BaZrO 3: Skull-melting, characterisation and interactions with YBCO

BaZrO 3 is supposed to be an inert crucible or substrate material for YBa 2Cu 3O 7−x (YBCO) superconductors. Therefore we grow single crystals of BaZrO 3 using the skull-melting technique and examine their interaction with YBCO-flux. A very small reaction layer of (Ba,Zr,Cu)O x is formed; Zr can not be detected by EDX analysis in the flux volume.

The microhardness of σ-Al2O3grown by the skull-melting technique

The microhardness of σ-Al₂O₃grown by the skull-melting technique

Electron Microscope investigation of crack path selection and crack interface interactions in a W-ZrO2(Y2O3) directionally solidified eutectic (DSE)

Metal-ceramic composites play a vital materials role in many technologically important industries due to their unique electrical, thermal and structural properties. The mechanical properties of metal-oxide materials are largely controlled by the properties of the metal-ceramic interface, specifically its structure and chemistry which govern the bonding between the constituents. For a brittle matrix composite, the fracture properties of the interface can determine the failure response of the composite as a whole.The W-ZrO2(Y2O3) directionally solidified eutectic was grown by a skull melting technique described elsewhere. Composites were grown with 10 mol % yttria and ~ 6 w% tungsten. Figure 1 is a scanning electron micrograph showing that the tungsten grows as parallel fibers approximately lujn in diameter in a zirconia matrix. Selected area diffraction of the composite (Figure 2) reveals the growth direction to be: &lt;110&gt;zrO2 // &lt;11l&gt;w.Electron microscopy of microindentation cracks, of pre-existing microcracks due to thermal expansion anisotropy and of short cracks inadvertently introduced during specimen preparation shows four different crack-interface interactions:

The distribution coefficients of rare earth ions in cubic zirconium dioxide

Cubic zirconium dioxide crystals are grown with the skull melting technique. The effective distribution coefficients for Nd 3+, Sm 3+ and Er 3+ as dopants are determined experimentally as a function of the crystal growth velocity. With the Burton-Prim-Slichter theory, the equilibrium distribution coefficients can be calculated. The distribution coefficients of all other trivalent rare earth ions can be estimated by applying the correlation towards the ionic radii.

Determination of the distribution coefficients of the rare earth ions Er3+ and Nd3+ in yttria-stabilized c-ZrO2 single crystals

Abstract We describe investigations of growth of cubic zirconium dioxide single crystals doped with rare-earth ions. The cubic phase is stabilized down to room temperature by adding 12 mole% Y2O3. The crystals are grown using the skull-melting technique. For colouring they are doped with Nd2O3, and Er2O3. We investigate the effective distribution coefficient of these dopants for crystals grown with growth velocities in the range of 4 to 18 mm/h. From optical absorption data the dopant concentration is calculated. The use of the Burton-Prim-Slichter (BPS) equation allows to estimate the equilibrium distribution coefficient of Nd3+(k 0 = 0.43) and Er3+(k 0 = 1.11) in cubic zirconium dioxide.

Absorption and emission spectra of magnesium oxide single crystals

MgO single crystals have been prepared by the skull melting technique. They are characterized by their absorption and transmission. We compare the spectra of not intentionally doped samples prepared by the above mentioned technique with those grown by the more conventional arc melting method. Furthermore we demonstrate, that the skull melting technique allows also to produce homogeneously doped samples and show some selected data of MgO:Ni, MgO:Co and MgO:Cr.