Skull Melting Research Articles

Origin of the enhanced creep properties of high Nb containing TiAl alloys with the addition of C and Y2O3

In this paper, Ti–43Al–6Nb–1Mo–1Cr, Ti–43Al–6Nb–1Mo–1Cr-0.5C and Ti–43Al–6Nb–1Mo–1Cr-0.5C-0.05Y2O3 alloys were prepared by induction skull melting under argon atmosphere. Creep tests were carried out on three alloys to reveal the effects of C and Y2O3 on the creep properties of alloys and to clarify the precipitation behavior of Ti2AlC and B2 phases. The results show that adding C and Y2O3 can significantly reduce the steady-state creep rate and improve the creep properties of the alloy. After adding C and Y2O3, the parallel decomposition of α2 lamellae occurs, and the fine γ lamellae precipitates from coarse α2 lamellae. The lamellar degradation occurs obviously during creep, and the reaction is α2→γ+B2. The B2 phase preferentially grows into α2 lamellae after nucleation at the lamellar interface. Furthermore, the precipitated B2 phase can induce the formation of stacking faults in α2 lamellae. The strengthening mechanism of C is mainly solid solution strengthening and the precipitation hardening. The dynamically precipitated Ti2AlC during creep can hinder the movement of dislocations and improve the bonding strength of the lamellar interface. Y2O3 mainly plays a role of second phase strengthening, which can share the stress of the matrix and prevent the crack propagation. Finally, the voids nucleation and crack propagation process of the alloy are analyzed and summarized.

Optimizing energy efficiency in induction skull melting process: investigating the crucial impact of melting system structure

Induction skull melting (ISM) technology could melt metals with avoiding contamination from crucible. A long-standing problem of ISM is that the low charge energy utilization and inhomogeneous fields have obstructed its application in many critical metal materials and manufacturing processes. The present work investigated the problem through the structure optimization strategy and established a numerical electromagnetic-field model to evaluate components’ eddy current loss. Based on the model, the effect of crucible and inductor structure on charge energy utilization, etc. was studied. Furtherly, the charge energy utilization was increased from 27.1 to 45.89% by adjusting the system structure. Moreover, structure modifications are proposed for enhancing electromagnetic intensity and uniformity, charge soft contact and uniform heating. The work constructed a basis for framing new solutions to the problem through ISM device structure optimization.

Plasma-arc skull melting and casting of austenitic steel with super equilibrium nitrogen content

Plasma-arc skull melting and casting of austenitic steel with super equilibrium nitrogen content

Short-time creep behavior of (TiB + TiC + Y2O3) reinforced titanium matrix composite in the range of 600 °C to 700 °C

A titanium matrix composite reinforced by TiB, TiC and Y2O3 was prepared by induction skull melting. The creep behavior of the composite was tested at the temperature range from 600 °C to 700 °C in short time to investigate the deformation mechanism and microstructure involution. The as-cast composite shows a typical basket-weave microstructure. At the stress of 150 MPa, the stress exponents of the composite are 3.48 at 600 °C and 5.33 at 700 °C, corresponding to the deformation mechanism mainly controlled by solute-dragging and dislocation climbing respectively. Reinforcements added hinder dislocations movement effectively, which improves the creep resistance of the composite. As the creep temperature rises, the solubility of β-Ti increases, accompanied by the precipitation of silicides with different morphologies. Silicides play a role in pinning dislocations and hindering interface from migration. The dislocation movement and rearrangement during creep lead to the formation of sub-grains.

Heat transfer in β-Ga2O3 crystal grown through a skull melting method

Heat and mass transfer in β-gallium oxide crystals grown via a skull melting method were numerically studied. Specifically, we calculated the temperature and velocity distributions in a furnace for the skull melting method. The distributions covered the entire system: the growing crystal, melt, raw material, basket, and coils generating heat through the radio frequency electromagnetic field. We determined the dependences of temperature and melt flow velocity distributions on the coil position.The results indicate that the melt flow direction is opposite to that in the Czochralski method and that a descending flow exists near the periphery of the melt. The results also show that the maximum temperature of the melt, which affects the overheating conditions of the melt, depends on the position of the coil.

Fully coupled numerical analysis for induction skull melting technology applied for metallic glass component alloy

High-purity melting is critical for casting metallic glass components, which would be achieved by developing the induction skull melting (ISM) technology used in casting process. The melting process of metallic glass components during ISM is urgently desired to be understood, which is rare so far. Based on this, a full-scale coupling simulation model for ISM technology with finite element method (FEM) was proposed, which contains all physical fields engaged in the melting processes. Furthermore, the metallic glass melting process was experimentally and numerically investigated based on the full-scale coupling model. The distribution of the multi-physics field within the charge was elaborated based on the numerical simulation results, especially electromagnetic field contribution, heat transfer, meniscus shape and melt reflux zone. Furthermore, due to its high viscosity, a long holding time to improve melt homogeneity is necessary for metallic glass melt. The chief aim of this work is to understand of induction melting process for melting amorphous master alloys using induction skull melting technology and to provide a foundation for the casting method to form bulk metallic glasses.

Compressive creep behavior of high Nb containing TiAl alloy: Dynamic recrystallization and phase transformation

In this work, compressive creep behavior of Ti-43Al-6Nb-1Cr-1.5V alloy fabricated by vacuum induction skull melting technology was investigated at 800-950 °C. The results show that compressive creep behavior of this alloy strongly depends on temperature. At 800 °C and 850 °C, this alloy exhibits conventional three-stage creep characteristics with a well-defined steady-state creep stage, and creep strain is small. However, at 900 °C and 950 °C, this alloy exhibits unconventional creep characteristics, and creep strain increases sharply. Microstructure observation proves that this is mainly due to the occurrence of dynamic recrystallization (DRX). During creep, continuous dynamic recrystallization and discontinuous dynamic recrystallization appear simultaneously, but the latter is more significant. At 800 °C and 850 °C, creep deformation mainly depends on dislocation motion, while at 900 °C and 950 °C, DRX also greatly contributes to creep deformation. During creep, recrystallized grains preferentially form at β-segregation and α-segregation, and rising temperature from 850 °C to 900 °C facilitates the sharp increase of DRX fraction. The occurrence of DRX reduces dislocation density, which greatly softens the alloy. Besides, more α2 grains generate at γ grain boundary and grain boundary/twin intersection during creep, and maintain Blackburn orientation relationship with γ grains. This is the result of stress-induced γ→α2 phase transformation.

Creep behavior and microstructure evolution of titanium matrix composites reinforced with TiB, TiC and Y2O3

Titanium matrix composites reinforced with (TiB + TiC) and (TiB + TiC + Y2O3) were prepared by induction skull melting. The creep test was carried out at 650 °C and 120−160 MPa, and the microstructure evolution was characterized in detail by XRD, SEM and TEM. The results show that the microstructure of the two composites is basket-weave structure. Under the same creep condition, the composite with the addition of Y2O3 has lower steady-state creep rate. After creep, silicides precipitated at the α/β interface, around dissolved β phases and reinforcements, which have a strong pinning effect on the dislocation movement. Due to the stacking fault structure through TiB whisker, the size of silicides around TiB is significantly larger than that around TiC and Y2O3. Solute-drag creep is the main creep mechanism, and the dislocation movement is also affected by α/β interface, reinforcements and silicides.

Break the superheat temperature limitation of induction skull melting technology

Overcoming the limitation of low superheat temperature problem is key for the applications of the induction skull melting technology in many important high melt point metals used in fields such as aerospace. The present paper presents an advanced numerical study to explain the mechanism of melt superheat temperature. A coupling model between electromagnetic, thermal, phase and fluid flow fields was established, which accurately predict the temperature and meniscus shape in the research domain. The simulation results show that the forced convective heat transfer caused by electromagnetic force in the melt and the current passing through phenomenon between molten melt and crucible were the key to influence the superheat temperature and electrical efficiency. Furthermore, it was discovered that convection suppression in the melt by employing an external DC coil and increasing the thermal resistance between the skull and the crucible could increase the melt superheat temperature effectively. Increasing the thermal resistance between the skull and the crucible also leads to an increase in the superheat temperature, which could be achieved by enhancing the interface roughness. The present work would shed lights on the design of new generation induction crucible to obtain high superheat temperature melt.

Characterization of Cubic Zirconia as a Lens Material Suitable for Autonomous Driving

As the development of autonomous driving technology is now in full swing, the demand for miniaturized optical modules mounted on various sensors has increased. Particularly, the optical lens used for such autonomous driving must demonstrate stable performance and durability despite rapid changes in the external environment. In this regard, cubic zirconia (CZ) can be used as an optical lens due to its high refractive index, which is above 2.1 in visible and near-infrared wavelengths, along with its chemical and mechanical durability. Thus, in this paper, we investigated the temperature-dependent physical properties of CZ fabricated by the skull melting method. The temperature coefficient of the refractive index (dn/dT) of the fabricated CZ plate in the temperature range of 25–100 °C decreased from 9.76 × 10−5/K to 7.00 × 10−5/K as the wavelength increased from 447.0 nm to 785.0 nm. The estimated Abbe number decreased from 33.98 at 25 °C to 33.12 at 100 °C, while the measured coefficient of thermal expansion (CTE) was 9.91 × 10−6/K, which revealed that the dn/dT value of the CZ plate was more affected by the dispersion than by the CTE. In addition, the CZ samples with a high refractive index, coated with a dielectric multilayer showed a high average transmittance of 98.2% at the investigated wavelengths, making it suitable for miniaturization or wide-angle optical lens modules. To secure the durability required for automobile lenses, the variation in the surface profile of the CZ before and after the external impact was also analyzed, revealing much better performance than TAF glass. Therefore, the observed results demonstrate that the CZ material is suitable for use as an optical lens for autonomous vehicles.

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 TiB, TiC and Y2O3 on tensile properties and creep behavior at 650 ℃ of titanium matrix composites

Two kinds of (TiB + TiC + Y2O3)/α-Ti composites with different TiB and TiC contents and the corresponding matrix alloy were prepared by induction skull melting. The results showed that the matrix alloy possessed a typical widmanstatten structure while the composites displayed a basket-weave structure characterized by α lamellae with different orientations. Obvious TiB/TiC structures were observed with the increase of reinforcements, which were very stable in the tensile and creep processes at high temperature. The ultimate tensile strength at 650 ℃ increased from 589 MPa to 730 MPa and 812 MPa with the increase of reinforcements. The enhancement in strength of the composites was elucidated on the load transfer strengthening of the reinforcements. The creep properties of the composites at 650 ℃ have been significantly improved due to the impediment of reinforcements and silicides to dislocation movement. The strengthening effect increased with the increase of reinforcements and stress. At 650 ℃/290 MPa, compared with the matrix alloy, the creep life of composite with highest reinforcements content was increased by 931% and the steady-state creep rate was reduced by 93%. According to the different stress levels, the composites exhibited different creep behaviors. At 230 MPa, the sub-grains were obvious and the number of actuated dislocations is small. At 290 MPa, the dislocation pile-up was serious and the obviously coarsened silicides lost their pinning effect.

Effect of stress and temperature on creep behavior of a (TiB + TiC + Y2O3)/α-Ti composite

A (TiB + TiC + Y2O3)/α-Ti composite was prepared by induction skull melting to investigate the microstructure evolution, high temperature tensile properties and the creep behavior. The results show that the original microstructure of the composite is a basket-weave structure with a little equiaxed α phase. The UTS of the composite is 710 MPa at 650 °C and 581 MPa at 700 °C. Stress exponents of the composite are found to 4.5 at 650 °C and 3.9 at 700 °C, respectively, showing the dislocation climbing is the dominant mechanism of the creep process at both temperatures. Reinforcements including TiB, TiC and Y2O3 can effectively impede the movement of dislocations, which contributes to the improvement of creep properties. The formation of equiaxed α phase is promoted by the addition of TiB and can decrease the creep resistance to some extent. Microstructure instabilities occur during creep process, especially the precipitation of silicides, which is strongly associated with the temperature and the stress. At 650 °C, the composite has S2-type silicide with ellipsoid morphology and (Ti, Zr)x(Sn, Si)y type silicide with equiaxed morphology. At 700 °C, the composite has S1-type silicide with rod morphology. The increase of the stress will promote the growth and rearrangement of the silicides. The precipitation of silicides can be observed in the crystal defects region near the TiB and Y2O3 reinforcements. These silicides have strong pinning effect on dislocations and improve the creep resistance.

Creep deformation and rupture behavior of a high Nb containing TiAl alloy reinforced with Y2O3 particles

This study presents the creep deformation and rupture behavior of Ti–48Al–6Nb–0.05Y 2 O 3 alloy prepared by induction skull melting (ISM). Creep studies were carried out at 800 °C and 300 MPa for various time periods. The fine defect structures evolve throughout the creep tests and govern the deformation of Y 2 O 3 -bearing alloy. The deformation is mainly supported by the dislocation climb in γ phase, which takes place by the formation of jog-pairs. With the increase in creep time, the increasing climb component in α 2 phase is beneficial for the co-deformation of lamellar structure. The propensity of mechanical twinning also increases during creep. The formation of deformation twins in lamellar γ phase more readily occurs than that in massive γ phase owing to the lower nucleation energy barrier. Besides, stress concentration around the Y 2 O 3 particles can promote deformation twin nucleation. Several cracks were formed along the colony boundaries perpendicular to the loading direction in the middle of secondary stage. The propagation and coalescence of intergranular cracks and cavities in the tertiary stage are the main causes for the failure of specimens. • The creep deformation of the Y 2 O 3 -bearing TiAl alloy is controlled by dislocation climb and mechanical twinning. • Y 2 O 3 particles are effective obstacles to dislocation motion. • Y 2 O 3 particles promote the formation of deformation twins. • The creep failure mechanism of the alloy is discussed.

Direct Rolling of TiCp/Ti–6Al–4V Composite for Improved Microstructure, Mechanical Properties, and High‐Temperature Oxidation Resistance

Herein, the 5 vol% TiCp/Ti–6Al–4V composite is prepared by induction skull melting (ISM) and direct rolling in near‐β phase region. The microstructure, mechanical properties, and high‐temperature oxidation behavior of the as‐cast and as‐rolled composites are systematically investigated. The investigations of microstructure reveal that the as‐cast coarse α/β lamellar matrix microstructure is translated to refined bimodal microstructure after direct rolling. And the coarse and segregated TiC particles tend to be refined and uniformly distributed in the matrix. Tensile test results exhibit that a well‐matched strength and elongation is obtained after direct rolling. The tensile strength at room temperature (RT) and high temperature (650 °C) increases to 1291.9 and 472.5 MPa, respectively, while the as‐rolled composite maintains a good elongation of 6.7% and 26.8%, respectively. The strengthening mechanism can be attributed to grain boundary strengthening and the load‐bearing capability of TiC. Isothermal oxidation test indicates that as‐rolled composite exhibits better oxidation resistance than as‐cast composite both at 550 and 650 °C. The improved oxidation resistance is mainly a result of the significant refinement of oxides and consequently retarding the diffusion process.

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.

Phase transformation and microstructure evolution of a beta-solidified gamma-TiAl alloy

In this study, TiAl alloy with nominal composition of Ti-43.5Al-6V-1Cr was fabricated by inductive skull melting technology. Microstructure evolution and microstructure formation mechanism were invested by quenching experiments. The results show that Ti-43.5Al-6V-1Cr alloy consists of lamellar colony (α2/γ), isolated γ phase and B2 phase. Phase transition equations and reaction temperatures during heating from RT to 1350 ℃ were identified: α2→γ, 835 ℃; α2→B2 + γ, 990 ℃; α2 + B2 + γ→B2 + γ, 1132 ℃; B2 + γ→α + γ, 1144 ℃; γ→α, 1183 ℃; α + γ→α + β + γ, 1278 ℃; α + β + γ→α + β, 1290 ℃; α + β→β, 1313 ℃. It’s found that adding of β-stabilizing elements makes β phase field of Ti43.5Al6V1Cr alloy expand to Al rich side, decreases temperature of β→α transition by 127 ℃ and causes β + α + γ phase field appearing in the phase diagram. Orientation relation (111)B2//(110)γ was found between B2 and isolated γ phase, besides (110)B2//(111)γ(K-S relation), the match of the two phases declares that β phase is parent phase of isolated γ phase. Above all, the phase transformation process during heating and cooling was deduced. During heating it’s: α2/γ + B2 + γ→B2 + γ→α + γ→β + α + γ→α + β→β, while during cooling it’s L→L + β→β→α + β→α + β + γ →α + B2 + γ→α2/γ + B2 + γ. Besides, the microstructure evolution mechanism was explained and depicted in schematic.

Heat Transfer in Induction Melting of Aluminum Oxide in Cold Crucible

Abstract This article aims to explain the mechanism of heat transfer between melt and cold crucible (CC) in processes referred to as induction skull melting (ISM). Several experiments were performed for this purpose. In these experiments, all relevant electrical and mechanical quantities that affect the heat transfer between the melt and the cold crucible were monitored. Firstly, this article deals with the description of the equipment used in the experiments. Aluminum oxide was considered as experimental material (EM). Therefore, the article also deals with the design of a suitable cold crucible for its melting. Subsequently, the article deals with the implementation of the experiment itself and its phases. Finally, based on the data obtained during melting processes, this paper deals with the calculation of the real heat transfer from the melt to the cold crucible. The main contribution of this article is a comprehensive view of the values of mechanical and electrical quantities that accompany the melting of the considered experimental material (EM) in the designed cold crucible. In conclusion, the values of thermal quantities obtained in the performed experiments are compared with numerical simulation.

High temperature creep behavior of an as-cast near-α Ti–6Al–4Sn–4Zr-0.8Mo–1Nb–1W-0.25Si alloy

The high temperature creep behavior of a near-α Ti–6Al–4Sn–4Zr-0.8Mo–1Nb–1W-0.25Si alloy prepared by induction skull melting has been studied under the conditions of 625 °C-675 °C and 150 MPa–250 MPa using uniaxial creep tests in this work. Results indicated that the incoherent secondary phase particle, silicide, mainly precipitated from the α/β lath interface, which has a positive effect on hindering the movement of dislocations and pinning α/β lamellae boundaries during creep. The stress exponent and apparent activation energy was 7.2 at 650 °C and 417.6 kJ/mol at 200 MPa, respectively. The high stress exponent and apparent activation energy indicated that the creep process was controlled by dislocation climb and slip impeded by silicides. The main failure mechanism of the as-cast alloy during creep was the initiation, growth, and coalescence of cracks at the grain boundary, which was proven from the intergranular fracture feature.

Chemical composition analysis on industrial scale ingots and castings of TiAl alloys

The chemical composition variation of the TiAl-4722 alloys was examined in a batch of the industrial scale master ingots, and in the corresponding castings prepared by conventional vacuum arc remelting (VAR) combined with induction skull melting (ISM) and investment casting processes. The content changes of major elements and interstitial elements were evaluated based on the chemical analysis at the top and bottom of the ingots and castings. Results show that the contents of C, N, H, Fe and Si have almost no change in the ingots and castings, suggesting that the chemical analysis on these elements can be based on the batch analysis. The O content keeps almost the same in different ingots, but exhibits relatively large differences in castings, which was probably influenced by the reaction between the shell mold and the molten alloy, and the spalling of face coat of the shell mold during casting. For the major elements of Al, Nb and Cr, the composition difference between the top and the bottom of the ingots is less than that of the castings. But for the O element, the trend is different, especially for the castings, suggesting that the investment casting is a homogenization process for Cr and Nb, but a differentiation process for O. The contents of major elements in castings fluctuate mainly in the same range as that in the ingots, indicating that the contents of the major elements are controllable during investment casting.