Preparation Of Yttria Mould Shells And Mechanical Properties Of Directionally Solidified Nb-Si Based Alloy

Nb-Si based ultrahigh temperature alloy has become an alternative material for the new generation of turbine blades due to its low density,high melting point,and considerable mechanical properties at room and elevated temperature.Although investment casting is an ideal forming method for this kind of alloy,the selection range of mould shell materials is greatly limited due to both very high melting point and the existence of a considerable amount of active elements Hf,Ti,etc.,in the alloy.On the other hand,in order to improve their comprehensive mechanical properties,turbine blades in aero-engines have been generally prepared by directional solidification method,which puts forward further requirements on the refractoriness and thermal shock resistance of the mould shells.However,there are few reports on the preparation of its turbine blade components,especially the directionally solidified components,though the research history of this kind of alloy has been up to nearly 30 years.In view of this,the present dissertation will mainly focus on the preparation of the mould shells suitable for the investment casting of Nb-Si based ultrahigh temperature alloy and the mechanical properties of the directionally solidified specimens.The main research contents and results are as follows:The multi-elemental Nb-Si based ultrahigh temperature alloy melt was simplified into 5 kinds of Nb based ternary alloys melt,and then a displacement reaction model between these ternary alloy melts and oxides was established.The Gibbs free energy difference of reaction between the alloy elements and alternative oxides at 1700～2100℃under the influence of activity was calculated,and the interactions between the elements and oxides were revealed.The mould shell material with the highest inertness to each element of the alloy was selected.The calculation results show that elements Nb,Si,Cr do not react with Y₂O₃,La₂O₃,CaO,ZrO₂,CeO₂ and MgO oxides.Element Ti can react with MgO and ZrO₂.Element Hf can react with ZrO₂,CeO₂,MgO and CaO.Element Al can react with MgO.Among all the alternative oxides,the inertness of Y₂O₃ is the highest.Yttria based round tube mould shells were prepared with the selected yttria as the matrix material and La₂O₃,CaO,ZrO₂,CeO₂ and MgO as the modified agents.The densification behavior of the face coat of the mould shells under different process conditions was studied,and the friability of the face coat and the bending strength of the mould shells were also tested and analyzed.The densification results show that La₂O₃+ZrO₂ doping has the greatest effect in improving the densification of yttria based mould shell face coat at the sintering temperature of 1700℃,while CeO₂+ZrO₂ doping will retard the densification of yttria based mould shell face coat.At the sintering temperature of 2000℃in a vacuum furnace with tungsten heating elements,CaO+ZrO₂and MgO+ZrO₂ doping can exert greater acceleration of the yttria grain growth than La₂O₃+ZrO₂ and CeO₂+ZrO₂ doping,but all yttria based mould shell face coats are not completely densified at this sintering temperature.In contrast,the face coat of pure yttria shell is completely densified at the same sintering temperature in the vacuum furnace with graphite heating elements.The performance test results show that the bending strength of yttria based mould shell doped with La₂O₃+ZrO₂ is the highest among all oxides doped yttria based mould shells after sintering at 1700℃.The friability of the mould shell face coat doped with CaO+ZrO₂ is the highest among all yttria based mould shells.The interaction between Nb-Si based ultrahigh temperature alloy and yttria based mould shell in the furnaces with different heating elements were studied based on re-melting experiments.The results show that the vacuum furnace with graphite heating elements is not suitable for re-melting and investment casting of Nb-Si based ultrahigh temperature alloy in the mould shell.When being re-melted in the vacuum furnace with graphite heating elements,a carbides layer composed of C,NbC,TiC and HfC formed on the surface of the master alloy rod,so that it cannot been completely melted and collapsed.There is no interface reaction layer between the alloy and the mould shells,but infiltration zones appear on the side of the mould shells.In contrast,when the alloy is re-melted in the vacuum furnace with tungsten heating elements,all the mould shells are completely filled.Various interface reaction layers emerge between the re-melted alloy and the different mould shells.The interface reaction layer between the re-melted alloy and pure yttria shell is mainly composed of HfO₂+Y₂O₃,while it is single HfO₂between the re-melted alloy and ZrO₂ face coat mould shell.There are two types of interface reaction layers between the re-melted alloy and the yttria based mould shells doped with CaO+ZrO₂,MgO+ZrO₂ and La₂O₃+ZrO₂,which are composed of HfO₂+Y₂O₃ and TiO+HfO₂ respectively.There are also two types of interface reaction layers between the re-melted alloy and the yttria based mould shell doped with CeO₂+ZrO₂,which are composed of HfO₂+Y₂O₃ and single HfO₂ respectively.Nb-Si based ultrahigh temperature alloy was directionally solidified at a melt temperature of 2050℃and withdrawal rate of 100μm/s,and some of the directionally solidified alloy rods were then heat treated at 1350℃for 24h and then 1450℃for 50h.The room and elevated temperature mechanical properties of both directionally solidified and directionally solidified and then heat treated alloys were tested and analyzed.The experimental results show that the room temperature fracture toughness,room temperature tensile property and high temperature tensile stress-rupture property of directionally solidified alloy are better than those of directionally solidified and then heat treated alloy.The average room temperature fracture toughness K_Q of the directionally solidified alloy is 18.3MPa·m^1/2,the average tensile strength is 468MPa,the maximal creep life under 1200℃/70MPa is 108.8h,and the creep rate is 0.03%/h.Finally,a mould shell for investment casting of Nb-Si based ultrahigh temperature alloy turbine blade was prepared,and a directionally solidified blade prototype was successfully fabricated for Nb-Si based ultrahigh temperature alloy at the melt temperature of 2000℃and withdrawal rate of 20μm/s.The directionally solidified microstructure in the blade back consists of lath-shaped primaryγ-（Nb,X）₅Si₃ and columnar Nbss/γ-（Nb,X）₅Si₃ eutectic cells arranged along the axial direction of the blade.Thereinto,"X"stands for the elements Ti,Hf and Cr.