Evaluation of Ca-doped BaZrO3 as the crucible refractory for melting TiAl alloys

In this study, a new Ca-doped BaZrO₃ refractory was designed by using thermodynamics approaches and tested for its applicability for vacuum induction melting (VIM) of TiAl alloys. The influence of CaO on the BaZrO₃ phase constitution and microstructure, as well as the key features of the TiAl melt interaction with the Ca-doped BaZrO₃ crucibles were investigated by X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM). Results revealed that the Ca-doped BaZrO₃ refractory consisted of Ba_1-xCa_xZrO₃ and CaO phases. An obvious interaction occurred during the melting of the TiAl alloy in the Ca-doped BaZrO₃ crucible along with the generation of BaAl₂O₄ as a reaction product, with formation of a reaction layer up to 5?µm thick. Dissolution of Ca-doped BaZrO₃ refractory in the TiAl melt was the main reason for the alloy-crucible reaction. Moreover, the Ca-doped BaZrO₃ crucible was found to substantially reduce the contamination of the TiAl alloy, with lower oxygen concentration as compared with other conventional oxide crucibles. Overall results confirmed that vacuum induction melting using the Ca-doped BaZrO₃ refractory can be considered as an appropriate method for the fabrication of TiAl alloys.     

Phase and microstructural evolution of BaZrO3-CaZrO3 refractory and its interaction with titanium alloy melt

In this study, the effect of CaZrO₃ additives on the phase and microstructure of BaZrO₃ refractory were investigated as well as the interaction with titanium alloy melts. Results revealed that no second phase was observed in the BaZrO₃ refractory with 10 and 20 mol% CaZrO₃ additives, respectively, and the incorporation CaZrO₃ additives promoted the densification and the growth of grains due to the generation of BaZrO₃-CaZrO₃ solid solutions. Whereas, the appearance of CaZrO₃ phase was in the BaZrO₃ refractory with 30 mol% additive, indicating that the additive content excessed the solid solubility limit in the BaZrO₃ refractory. Interaction analysis indicated that CaZrO₃ additive could improve the performance of BaZrO₃ refractory for resisting the penetration of Ti₂Ni alloy melt. Meanwhile, an increasing extent of melt contamination by the BaZrO₃ refractory was also detected with the increasing content of CaZrO₃ additive. According to the thermodynamic calculation, an obvious increase in Gibbs free energy of formation for the BaZrO₃ refractory was confirmed with the CaZrO₃ additive, resulting in the increasing extent of the dissolution-corrosion between the refractory and the titanium alloy melts.     

Role of Ca additive on the effect of phase composition and microstructure of BaZrO3 refractory and its interaction with titanium alloys

In this study, the effect of different Ca additives (Ca(OH)₂, CaO, and nano-CaCO₃) on the composition and microstructure of the fused BaZrO₃ crucible were investigated, as well as their interaction with Ti₂Ni alloy during vacuum induction melting. Results showed that the crucibles had the same phase compositions when the doping amounts of Ca additives were the same. Three kinds of Ca additives provided three different grain sizes of CaO precursors to participate in the solid solution reaction. When the doping amount of the Ca additive was 5.3 wt.%, the ZrO₂ phase in the fused BaZrO₃ was disappeared, and the CaZrO₃ phase was founded. The composition of the crucibles doped with 7.7 wt.% Ca additives only consisted of Ba_1-xCa_xZrO₃ and CaZrO₃ phase. The relative densities and erosion resistance of the crucibles were improved effectively with the increase of Ca additive content. Moreover, the crucible doped with 7.7 wt.% nano-CaCO₃ additive exhibited the highest density and the thinnest erosion layer. The Ba_1-xCa_xZrO₃ and CaZrO₃ phases were dissolved by the alloy melt, according to the interaction analysis. In addition, the crucible doped with nano-CaCO₃ additive had a higher content of refractory element concentration in the alloy melts in comparision with the other two kinds of crucibles due to their higher number of pores.     

Dissolution of BaZrO3 refractory in titanium melt

Although numerous investigations have studied BaZrO₃ as a crucible refractory for melting titanium alloys, the interaction mechanism between them has not been clarified. In this study, a set of three designed alloys with different Ti composition (TiNi, Ti_1.5Ni, and Ti₂Ni) were melted in the BaZrO₃ crucibles. By using the X‐ray diffraction, optical, and scanning electron microscopy analysis, the interactions between the BaZrO₃ crucibles and the titanium melts were investigated. It was found that dissolution of the BaZrO₃ refractory into the titanium melts resulted in the crucible erosion and the melt contamination, the degree of which were both increased with the increasing of Ti content in the melts. The dissolution reaction could be determined as follows: . The oxygen content dissolved in TiNi, Ti_1.5Ni, and Ti₂Ni melts was thermodynamically calculated as 0.0055, 0.1922, and 0.2263 wt%, respectively, which were in agreement with the experimental results.     

A novel potential ceramic material for melting Ti6Al4V alloy: A solid solution of BaZrO3 and CaZrO3

Vacuum induction melting technology is a promising low-cost method for producing high-quality titanium alloy. The key challenge lies in the development of ceramic crucibles with excellent chemical stability for titanium alloy corrosion. In this work, (Ba_1−x,Ca_x)ZrO₃ ceramic was designed and synthesised via pressureless sintering of a mixture of BaZrO₃ and CaZrO₃ powders. X-ray diffraction and scanning electron microscopy analyses showed that a new phase, Ba_0.8Ca_0.2ZrO₃, was formed after heat treatment at 1700 °C. Vacuum induction melting experiments of the Ti6Al4V alloy were carried out using a Ba_0.8Ca_0.2ZrO₃ crucible. Compared with the original BaZrO₃ crucible and CaZrO₃ crucible, the erosion layer of the Ba_0.8Ca_0.2ZrO₃ crucible was significantly reduced by approximately 85∼92.5%. The interface between the crucible and the alloy was clearly visible, and there was no obvious element diffusion between the alloy and the material. This shows that Ba_0.8Ca_0.2ZrO₃ is highly promising as a crucible material for melting Ti6Al4V alloys.     

Failure mechanism of the Y2O3 doped BaZrO3/Al2O3 composite ceramic mould during directional solidification of TiAl-Based alloys

Directional solidification of Ti–46Al–8Nb (at.%) intermetallic in the Y₂O₃ doped BaZrO₃/Al₂O₃ composite ceramic mould was carried out using Bridgman apparatus. To increase the success rate of Ti–46Al–8Nb single crystals preparation and improve the quality of the ingots, the failure mechanism of the mould in directional solidification experiments was evaluated. Nucleation and propagation of cracks in the moulds were investigated by tracing each key process of the experiment, the effect of pores in the mould on the target alloy was revealed by studying the mould/metal interface. The results show that the macrocracks in the facecoat of the mould would lead to the leakage of the alloy melt. Furthermore, the alloy melt would infiltrate into the mould through large-size pores, which would increase the oxygen content of the target alloy, and also form inclusions containing O, Zr, Si and Y elements in the alloy ingot.     

Sintering behavior of Y-doped BaZrO3 refractory with TiO2 additive and effects of its dissolution on titanium melts

In this study, the sintering behavior of Y-doped BaZrO₃ with TiO₂ additive and effects of its dissolution on titanium melts were investigated. In order to overcome the difficulty of Y-doped BaZrO₃ sintering performance, the 0-5 wt% TiO₂ was added into Y-doped BaZrO₃ and its densification was investigated by density analyzer, scanning electron microscope (SEM) and X-ray diffraction (XRD). Thereafter, the interface reaction between two crucibles (without and with 2 wt% TiO₂) and titanium alloys, the thermodynamics and kinetics of dissolution reaction were also investigated. The results showed that Y-doped BaZrO₃ was rarely dense without TiO₂ additive and its relative density was just 88%, while after doping 2 wt% TiO₂, the relative density was more than 97%. However, with the excessive TiO₂(>2 wt%) doping, the secondary phase was observed by SEM and XRD. After melting titanium-rich alloys (Ti₂Ni, 66 mol% Ti) by Y-doped BaZrO₃ crucibles without and with 2% TiO₂ additive, the erosion layer of the two crucibles was approximately 4000 and 1700 μm, respectively. It was also found that the dissolved reaction rate was related to the density and grain size of Y-doped BaZrO₃ ceramic; the higher density and larger grain size ceramics can effectively prevent the crucible from being eroded by Ti₂Ni melt.     

On the Modification of Hydration Resistance of CaO with ZrO2 Additive

Various ZrO₂/CaO samples were fabricated by cold isostatic pressing and sintered at 1750°C for 4 h. It was observed that the sample with 12% ZrO₂ additive possessed the good hydration resistance and had the lowest apparent porosity of about 0.75%; its weight additive stored after 56 days was less than 0.6 wt%, and it contributed to the occurrence of CaZrO₃ on the surface of CaO. The CaO crucible with 12 mol% ZrO₂ additive did not react with titanium melt during melting TiNi alloy. This provides a support for searching a new refractory with the good hydration resistance for induction melting titanium alloys.     

Corrosion resistance of calcium zirconate crucible to titanium-copper melts

The corrosion resistance of refractory materials to the titanium alloy melts is vital for the production of titanium alloys by vacuum induction melting. In this study, the corrosion behavior of calcia-stabilized zirconia, solid state synthesized calcium zirconate, and fused calcium zirconate refractory suffering Ti-5 wt% Cu melts were investigated at 1680 ℃ for 15 min of soaking time by the cup test method. It was found that the three crucibles directly dissolved into the titanium melt, then generated Ti (Zr, O) and CaZrO₃ in the infiltration layer, and eventually developed a porous Ti₃O layer in the lining. Besides, the contamination of Ti-5 wt% Cu alloy (oxygen: 5.3 wt%; zirconium: 6.01 wt%; calcium: 0.42 wt%) by fused calcium zirconate crucible was significantly less than the solid state synthesized one (oxygen: 5.83 wt%; zirconium: 6.14 wt%; calcium: 0.43 wt%), implying that the production method of calcium zirconate notably affected the impurity of titanium alloys.     

The modification of surface,size and shape of barium zirconate powder via salt flux

The “top-down” process via direct conversion of the micro (μm)-to-submicroscale (sub-μm) particle was applied in this work by using eutectic chloride salts to prepare BaZrO₃. The particle size at optimum condition could be decreased by more than 10 times from 2.1 ± 0.9 μm to 168 ± 23 nm without destroying the 1:1 of Ba:Zr stoichiometry. The uniform sub-μm-BaZrO₃ powder was sintered in order to obtain ~98% dense ceramic at 1400°C/10 h, which is significantly lower than the 1650°C in normal cases. The microwave dielectric constant, tan δ, and quality factor were also determined. Furthermore, this method also was applied to lead-free piezoelectric material in the 0.87BaTiO₃–0.13BaZrO₃–CaTiO₃ (0.87BT–0.13BZ–CT) system. The particle size of 0.87BT–0.13BZ–CT was reduced greatly from >10 µm to 2.8 ± 0.4 µm. It can be proved that salt flux dissolution method enables high-purity with uniform sub-micro/nanometer powder production in one step by using simple laboratory equipment and low-cost raw materials.     

CaZrO_3耐火材料的制备及其与钛合金熔体的界面反应

合成了一种CaO-ZrO_2系复合耐火材料,用模压法成型,分别在1 650℃和1 680℃烧结制成坩埚.使用此坩埚在感应炉中熔化钛合金(TiNi),在真空条件下, 1 500℃保温5min.对在1 680℃烧结成的坩埚进行了抗水化性能测试.用X射线衍射仪对CaZrO_3材料进行物相分析.用扫描电子显微镜观察钛合金与此材料的界面反应层的微观形貌,结合能谱仪进行微区成分分析.结果显示:1 680℃烧成的CaZrO_3材料具有良好的抗水化性;用此坩埚熔化TiNi合金后,液态钛合金和耐火材料的界面反应层厚度约为30μm,界面反应层中,Ca,Zr,Ti,Ni等元素无明显扩散.     

Interaction between CA6-MA crucible and molten wrought Ni-based superalloys

The interfacial behavior between CaAl₁₂O₁₉-MgAl₂O₄ spinel (CA₆-MA) crucible and molten wrought Ni-based superalloys during the vacuum induction melting process have been comprehensively investigated, with Al₂O₃ crucible included for comparison. Experimental and theoretical results suggest that CA₆-MA crucible exhibits higher thermodynamic stability and poor wettability to the superalloy melt. Benefited from these merits, the superalloy ingot with cleaner surfaces and more metallic luster is obtained in the CA₆-MA crucible. Furthermore, the total oxygen content in the superalloy ingots obtained from CA₆-MA crucible is 17.9 ppm, which is considerably lower than that from Al₂O₃ crucible (29.7 ppm). These results indicate that CA₆-MA crucible can be a promising candidate for melting wrought Ni-based superalloys.     

Structural characterization of iron-based bulk metallic glass alloys produced by centrifugal casting

In this study bulk metallic glass alloys in the Fe-Zr-B-M (M=Al, Co, Cu, Mo, Ni, W) systemwere produced by using a centrifugal casting method. The thickness ofthe samples that could be produced with an almost completely amorphousstructure was found to be as high as ～2 mm for alloy Fe60Co8Zr10Mo5W2B15. Alloying element additions or replacements have resulted in a considerable effect on glass-forming ability. Among others, Al and Cu were found to have the most deteriorative effects. The ternary alloy Fe70Zr10B20, from which bulk metallic glass alloys can be formed by cleverly implementing several alloying elements, was seen to have a different phase transformation behavior when compared to alloy Fe60Co8Zr10Mo5W2B15. For the latter alloy, the phase transformation process and microstructure evolution were found to be extremely sensitive to variation of the cooling rate.     

Study on the stability of different refractories during the melting process of a K4169 Ni-based superalloy

The composition of the refractory strongly affects the cleanliness of the alloy. K4169 Ni-based superalloys were melted in different types of refractories in this study. The cleanliness of the Ni-based superalloy and phase transformation of the refractory were observed by X-ray fluorescence (XRF), X-ray diffraction (XRD), and scanning electron microscopy energy dispersive spectroscopy (SEM‒EDS). The high-temperature stabilities of a Y₂O₃-based refractory, MgO-based refractory, and Al₂O₃-based refractory during melting with a Ni-based alloy were compared. The oxygen content was also lowest, and no Y₂O₃-containing inclusions were observed in the Ni-based alloy melted with the Y₂O₃-based refractory at 1823 K. Inclusions with 21%–29% MgO and a phase composed of Al, Mg and O with an area of approximately 1300 μm² were observed in the alloy. This indicates that the dissolution and erosion of the Y₂O₃-based refractory were weak, and obvious physical erosion and chemical dissolution of the MgO-based refractory occurred during the melting process of the Ni-based alloy. The width of the refractory phase adhered to the boundary of the Ni-based alloy increased in the order Y₂O₃-based refractory (15 μm- 23 μm)< Al₂O₃-based refractory (93 μm- 285 μm)< MgO-based refractory (3.5 mm–3.6 mm), indicating that the adhesive strength of the MgO-based refractory with the Ni-based alloy was strongest. The interaction between the refractory material, Ni-based alloy and inclusions was analyzed based on thermodynamic calculations by Factsage software. The effects of dissolution of the three refractory types on the formation and transformation of the new phases and inclusions were estimated. The thermodynamic results were in good agreement with the experimental results.     

Some properties of fine-grained mixtures of ZrSiO4-ZrO2

Conclusions The effect of additions of ZrO₂ in monoclinic and cubic modifications and also of Zr(OH)₄ on the swelling, thermal-shock resistance, and glass-resistance of high-density, zircon refractories has been studied.It is shown that the most effective addition for lowering the additional growth of the refractory almost fivefold was a mixture consisting of 9.5% monoclinic ZrO₂ and 0.5% Zr(OH)₄. This additive also increases significantly the thermal-shock resistance of the zircon refractories and the glass resistance as a result of the lower decomposition of ZrSiO₄, and the higher concentration of ZrO₂ in the contact zone. The density and electrical resistivity of the refractory is maintained at a high level.Translated from Ogneupory, No. 8, pp. 48–53, August, 1982.     

Performance of BaZrO3/Y2O3 dual-phase refractory applied to TiAl alloy melting

Vacuum induction melting is a potential process for the preparation of TiAl alloys with good homogeneity and low cost. But the crucial problem is a selection of high stability refractory. In this study, a BaZrO₃/Y₂O₃ dual-phase refractory was prepared and its performance for melting TiAl alloys was studied and compared with that of a Y₂O₃ refractory. The results showed the dual-phase refractory consisted of BaZr_1-xY_xO_3-δ and Y₂O₃(ZrO₂), exhibited a thinner interaction layer (30 μm) than the Y₂O₃ refractory (90 μm) after melting the TiAl alloy. Although the TiAl alloys melted in the dual-phase and Y₂O₃ refractory exhibited similar oxygen contamination (<0.1 wt%), the alloy melted in the dual-phase refractory had smaller Y₂O₃ inclusion content and size than that in the Y₂O₃ refractory, indicating that the dual-phase refractory exhibited a better melting performance than the Y₂O₃ refractory. This study provides insights into the process of designing highly stable refractory for melting TiAl alloys.     

SHS quenching as a route to bulk glassy Fe<Subscript>34</Subscript>Co<Subscript>34</Subscript>B<Subscript>10</Subscript>Si<Subscript>14</Subscript>Nb<Subscript>8</Subscript> alloys

The method of SHS quenching combined with dynamic annealing was used to prepare bulk amorphous metallic glass alloys Fe₃₄Co₃₄B₁₀Si₁₄Nb₈ in a two-stage process. The structural properties and microstructure of thus prepared materials were found to be close to those produced by other methods, such as melting-injection-molding and melt spinning.      

Effect of Zirconium on the Structure and Congruent Crystallization of a Supercooled Calcium Aluminosilicate Melt

The influence of zirconium as a nucleating agent on the congruent crystallization and relevant physical properties of a supercooled calcium aluminosilicate melt of a composition close to CaAl₂SiO₆ has been investigated up to 6 mol% ZrO₂. Zirconium marginally affects rheological and structural properties, decreasing the viscosity of the Zr‐free melt by no more than 0.25 log unit and, as observed by Raman spectroscopy, not changing significantly the polymerization state of the material. Whereas the Zr‐free melt crystallizes congruently and heterogeneously from the sample surface to yield yoshiokaite, a stuffed derivative of the nepheline structure, addition of zirconia promotes instead bulk crystallization of tetragonal ZrO₂ and then of yoshiokaite. The latter process takes place in two stages: dissolved Zr first promotes homogeneous precipitation of zirconia before yoshiokaite crystallizes congruently from a Zr‐depleted volume of melt around zirconia precipitates. This process makes zirconium, and probably other poorly soluble oxides, valuable to control congruent crystallization in silicate glass‐ceramics. From the recorded thermograms, an enthalpy of crystallization of 40 and 46 kJ/mol has been determined at 1060 and 1140 K, respectively, for CaAl₂SiO₆ yoshiokaite, a very low value that is likely due to the extensive atomic disorder of crystals precipitating at high degrees of supercooling.     

Processing of vacuum heat-treated MgO-densified silicon nitride crucible for molten cast iron handling

Silicon nitride with 3% MgO powder mix is uniaxially and cold isostatically pressed to form a green Si₃N₄ crucible. Liquid phase sintering was applied to the green Si₃N₄ crucible at 1600 °C for 30 min under the nitrogen atmosphere. Intergranular Mg–Si–O–N glass remained between the silicon nitride grains which reacted with the molten metal during melting. This grain boundary glass was removed by vacuum heat treatment at 1575 °C for 5 h. The vacuum heat treated crucible was used to melt cast iron to examine reactions between the molten metal and Si₃N₄ ceramic crucible. EDX spectra across the Si₃N₄–cast iron interface and XRD for silicon nitride sample after cast-iron melting side surface analysis were carried out. Optical microscopy and SEM image analysis were made to examine the interaction between Si₃N₄ crucible and cast iron melt. Surprisingly, no reaction was observed between the vacuum heat treated crucible and melted cast iron.     

Stimulation of densification during the reduction of U3O8 to UO2 by atmosphere control

A reduction process in the head-end for pyroprocessing has been adopted to avoid oxidation attack on the molybdenum crucible during sintering. The reduction process is employed to reduce U₃O₈ pellets to UO₂ prior to sintering. This allows elimination of the oxygen source, which causes oxidation attack during sintering, thereby permitting the use of a metallic crucible. However, little densification occurs due to the low reduction temperature limited by the INCONEL crucible. Consequently, the amount of material scraps from the pellets increases, thus creating an additional processing burden due to its high radioactivity. To reduce the amount of scraps, densification should be enhanced. This study suggests a simple atmospheric control strategy and clarifies its effects. With the atmospheric control, a higher bulk density and better attrition resistance were obtained in comparison to without this strategy. This can be explained in terms of O/U ratio dependent diffusion kinetics during the reduction of U₃O₈ to UO₂.