Thermodynamic Analysis And Mechanical Properties Of Nb-Si Based Alloy Alloyed With C Element

With the rapid development of aerospace technology,the engine,which is the "heart" of aerospace vehicles,is the core of the development of the aerospace industry.Materials are the foundation of the engine's performance.Nb-Si alloy has a high melting point(>1750?)and a low density(7.2g/cm3),making it a highly potential superstructure material.However,the room temperature fracture toughness of Nb-Si alloy is poor,which greatly limits its wide application in industry and engineering.The room temperature fracture toughness and other mechanical properties of Nb-Si alloy depend on its microstructure,especially the morphology and distribution of Nb5Si3,Nb3 Si and other intermetallic compounds.For the multi-component Nb-Si alloy,due to the addition of alloying elements,under the combined action of multiple elements,intricate reactions will occur during the smelting process,which is bound to affect the microstructure of the Nb-Si alloy,that is,the phase composition and the phase Shape and distribution have an impact.Therefore,in this paper,the thermodynamic model of mediema and Toop is used to establish the thermodynamic calculation model of Nb-Si-Ti-C alloy,to predict the Gibbs free energy of the intermetallic compounds in the alloy system within a certain temperature range,and to carry out the possible precipitation phases.Forecast.The casting process is used to select the Nb-16Si-24 Ti alloy with better comprehensive properties as the original alloy,and Ti element is used as the carrier to alloy the C element.The effect of element C on the microstructure,phase composition and distribution,and mechanical properties of Nb-Si alloy was analyzed.And studied the correlation between microstructure and mechanical properties.First,based on the miedema model,the excess Gibbs free energy of Ti-C,Nb-Si,Si-C,Ti-Si and other binary alloy systems and the activity ai of each component in the binary system are calculated.According to the Toop model,the excess Gibbs free energy of Nb-Ti-C and Nb-Ti-Si ternary alloy systems and the activity of each component in the ternary alloy system are obtained.Substituting the obtained activity expression of each binary system component into the standard Gibbs free energy change formula of the formation reaction of each intermetallic compound,the Gibbs free energy change of the formation reaction of each intermetallic compound in the binary alloy system can be obtained The relationship between temperature T.Combining the component points on the phase diagrams of each binary system,the data processing software is used to perform linear fitting to obtain a linear relationship with T,and the obtained linear relationship is substituted into the ternary alloy system for the formation reaction of intermetallic compounds.In the Booth free energy expression,thecorresponding parameters and empirical parameters of each element are substituted,and finally the Gibbs free energy change of the intermetallic compound formation reaction can be calculated,and the metals can be inferred based on the obtained Gibbs free energy change value the size of the thermodynamic driving force for the precipitation of inter-compounds.After the above calculations,three phases of Ti C,Ti5Si3 and Nb5Si3 may be generated.At the same time,as the content of C element increases,the Gibbs free energy of the formation reaction of each phase will also change,that is,the C element will weaken the thermodynamic driving force of the formation reaction of the two phases of Ti C and Ti5Si3 and suppress the two phases.Generation;at the same time,the thermodynamic driving force of the formation reaction of the Nb5Si3 phase is improved,and the formation of the Nb5Si3 phase is promoted.Nb-16Si-24Ti-x C(x=0,1,2,3,4)alloy was prepared by vacuum non-consumable arc melting equipment,and the prepared alloy was analyzed and room temperature Compression performance and room temperature fracture toughness test.The results show that the addition of C element changes the solidification path of Nb-Si alloy from hypoeutectic to hypereutectic.Element C will promote the formation of Nbss phase,but as the content of element C increases,the content of Nbss phase tends to be stable.With the increase of C content,the primary phase of Nb3 Si gradually increases the content,volume and aspect ratio.When the atomic ratio of element C is 1,a large amount of Nb5Si3 phase appears in the structure,and when the atomic ratio is 2,it almost disappears.At the same time,a large number of clusters with the primary phase of Nb3 Si as the core and surrounded by a dense eutectic structure? Shaped structure.With the further increase of the content of C element,the Nb5Si3 phase gradually precipitates from Nb3 Si,the Nb3 Si phase increases,and the fine eutectic structure decreases.The addition of C element can effectively improve the room temperature fracture toughness and room temperature compressibility of Nb-Si alloy.The addition of C element will affect the morphology,content and distribution of each phase,and then affect the mechanical properties.The room temperature fracture toughness of the alloy with the atomic ratio of 2 is the highest,which is 132% higher than that of the original alloy.With the increase of the C element,the room temperature fracture toughness of the alloy shows a trend of first increasing,then decreasing and increasing.The alloy with an atomic ratio of 1 has the highest compressive strength,an increase of 25%compared to the original alloy,and the best compression performance.C element will also promote the alloy from brittle fracture to plastic deformation.The Nbss phase and fine eutectic structure in the structure can improve the room temperature fracture toughness,and Nb5Si3 is relatively beneficial to increase the compressive strength.The coarse primary Nb3 Si phase has an adverse effect on the room temperature fracturetoughness and compressive strength.