Doping Modification Of BaZrO₃ By Y³⁺/Ca²⁺and Its Interfacial Reaction With Titanium Melt

Titanium alloys are widely used in aerospace,chemical industry and biomedical fields due to their high specific strength,good corrosion resistance and biocompatibility.The industrial development of titanium alloys is limited by its long production cycle and high energy consumption.The vacuum induction melting technology is applied to melt titanium alloys by our group,and this technology has low energy consumption and can obtain homogeneous titanium alloys.However,the refractories that do not react with titanium melt for vacuum induction melting are required.BaZrO₃ refractory was used in titanium melting and some achievements were achieved.But BaZrO₃ refractory would dissolve in the titanium melt and then contaminated the high content titanium alloys.The high stability refractories,Y₂O₃and CaO were doped into BaZrO₃ to prepare BaZrO₃-based refractories successively,but the doping amount was relatively single,and the obtained refractories are two phases.In order to further improve the stability of BaZrO₃,this paper optimizes the doping amount and preparation process of powders,to obtain Y₂O₃ and CaO doped BaZrO₃ refractories with good sintering performance and strong corrosion resistance for melting titanium alloys,and the dissolution kinetics and thermodynamic factors of refractories in titanium alloy melt were studied,then the sintering properties,hydration resistance and stability of Y₂O₃ and CaO doped BaZrO₃ refractories were also studied.This paper mainly includes the following three parts:1.The Y₂O₃ doped BaZrO₃ powders and crucible were prepared by the mole ratio BaCO₃:ZrO2:Y₂O₃=0.5:0.485:0.015,and the phase was analyzed,then the relationship between ball milling time and powder size and ceramic density were also studied,and the sintering property of Y³⁺doped BaZrO₃ was improved by adding the sintering aid TiO2.The results showed that Y³⁺doped BaZrO₃ powder and crucible were all the single phase perovskite structure,no secondary phase were found;The powder size of Y³⁺doped BaZrO₃ by ball milling 8h is 2.252μm,and the relative density of Y³⁺doped BaZrO₃ ceramic is the highest,but its only about78.9%,there are many pores on the ceramic surface;When adding 2wt%TiO2sintering additive,the single phase cubic perovskite of Y³⁺doped BaZrO₃ refractory with relative density of 97.4%can be obtained.2.The stability of Y³⁺doped BaZrO₃ refractory was investigated by using Y³⁺doped and non-doped BaZrO₃ crucible to melt Ti₂Ni alloy（66mol%Ti）under the same melting conditions.The relationship between the density of refractories and corrosion resistance of titanium melt was investigated,and the mechanism of corrosion resistance of crucible was analyzed from thermodynamics and kinetics.The results showed that:（1）Y³⁺doped BaZrO₃ crucible exhibits good corrosion resistance to Ti₂Ni melt,and the thickness of the erosion layer is about 1700μm,the grains in the crucible inner wall after melting the alloy are relatively complete,just part of the grain boundaries become blurred by melt erosion and dissolution,while the thickness of the undoped BaZrO₃ erosion layer is 2000μm,and the lamellar structure appears in the crucible inner wall.（2）The relative densities of Y³⁺doped BaZrO₃ crucibles with and without2wt%TiO2 sintering additives are about 97.4%and 78.9%,respectively,and the average grain sizes are 8μm and 2μm,respectively.After melting Ti₂Ni alloy with the two kinds of crucibles,the thickness of the erosion layer is 1700μm and 4000μm,respectively.（3）The thermodynamic calculations show that the Gibbs free energy of formation is-2203.7KJ/mol,which is less than-2198.7KJ/mol for undoped BaZrO₃.The crucibles were eroded and the titanium alloys were contaminated are all due to the dissolution reaction of refractories in Ti₂Ni melt.According to Ginsting equation theory,the crucible with higher density and larger grain size can effectively reduce the dissolution reaction rate of refractories in the melt.3.The refractories of Y³⁺and Ca²⁺doped BaZrO₃ are prepared by different powder size CaO（5.5μm and 300nm）,then optimized CaO doping process,the sintering property and hydration resistance of Y³⁺and Ca²⁺doped BaZrO₃refractories were investigated,and determined the optimum preparation process of crucible.Ti₂Ni alloy was melted in the Y³⁺and Ca²⁺doped BaZrO₃ crucible which was prepared by optimum process,and the feasibility of the crucible to melt high titanium alloy was estimated.The results showed that:（1）3mol% Y³⁺and 10mol%Ca²⁺doped BaZrO₃ powders were prepared,and no secondary phase was detected by XRD.The sintering properties of Y³⁺doped BaZrO₃ refractories can be improved effectively by adding CaO after solid phase synthesis,and the densification and hydration resistance of Y³⁺doped BaZrO₃ with nano CaO（300nm）were the best,with the relative humidity of 56⁶0%,and the weight gain rate was only about 0.05%after 63 days.（2）Ti₂Ni alloy was melted by two crucibles of Y³⁺doped BaZrO₃ with two kinds of CaO（5.5μm and 300nm）.After melting,the thickness of the erosion layer was about 310μm and 70μm respectively,and the powder of erosion layer was not only detected[Ba1-xCax][Zr1-yYy]O₃,but also BaCO₃ phase.（3）Ti₂Ni alloy was melted by Y³⁺/Ca²⁺doped BaZrO₃ crucible and Y³⁺doped BaZrO₃ crucible respectively.After melting,the thickness of erosion layer was about300μm for the former,and the oxygen content in the alloy was about 0.0844wt%;the thickness of erosion layer was about 350μm for the latter,and the oxygen content in the alloy was 0.1026wt%.