Research On Room And High Temperature Properties Of Boride Ceramic Particles Reinforced Niobium-Titanium Matrix Composites

With the rapid development of energy and electric power industry,higher standard are required for structural materials with high temperature resistance,good oxidation resistance and excellent strength and toughness.The operating temperature of traditional superalloy is only 1000?-1200?.Even the silicon carbide with fantastic high temperature performance can be only used at 1650?.Therefore,it's important to develop a new kind of ultra-high temperature materials for power industry.In this study,Nb-xTi(x=10,20,30,40wt.%)alloys were firstly fabricated by hot pressing sintering and the influence of Ti content on the strength and toughness were investigated.Experimental results show that Nb-20wt.%Ti alloy is the best choice for the matrix of metal matrix ceramic composites.Then,xZrB₂-NbTi(x=0,15,30,45,60wt.%)composites with density?97%were prepared by hot pressing sintering(1600?/30MPa)with matrix of Nb-20wt.%Ti alloy and reinforcement phase of ZrB₂ ceramic particles.Scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffractometer(XRD)and universal testing machine were used to investigate the microstructure,phase composition,mechanical properties at room and high temperature,and oxidation properties at high temperature.The effect of microstructure on mechanical and oxidation properties was analyzed.The toughening mechanism,strengthening mechanism and oxidation resistance properties of the composites were obtained.In Nb-xTi alloys with different Ti contents,the fracture toughness increases with increasing Ti content to the highest value and then decreases,The fracture toughness of Nb-30wt.%Ti at room temperature is the highest,which is 24.5MPa·m1/2.The yield strength at room temperature increases linearly with Ti content.The highest yield strength is 1548.9MPa when the Ti content is 40wt.%.The yield strength at 800? decreases linearly with the content of Ti,and the yield strength of Nb-40wt.%Ti at 800? is the lowest,which is 414.8MPa.As for xZrB₂-NbTi composites,the main phases are(Ti,Nb,Zr)ss solid solution matrix,needle-shaped Ti-rich(Nb,Ti)B and nearly spherical Nb-rich(Ti,Nb)B.The content of borides increases with increasing ZrB₂ particles.When the ZrB₂ content exceeds 45wt.%,residual ZrB₂ particles and(Ti,Nb)B cluster can be observed.Both the quantity and morphology play an important role on the strength and toughness of xZrB₂-NbTi composites.Experimental results of room temperature mechanical properties show that,the highest toughness value of xZrB₂-NbTi composites is 12.0MPa·m1/2,which is about 3 times higher than previous study.Crack bridging,crack defection and pull-out mechanisms caused by(Ti,Nb)B phase are the main reasons for high toughness.The strength of xZrB₂-NbTi composites depend on the combining effect of dislocation strengthening mechanism caused by(Nb,Ti)B phase and the weakening effect caused by(Ti,Nb)B phase and residual ZrB₂ particles due to stress concentration.When there are more residual ZrB₂ particles in the material,the composites fracture in a brittle manner along the shear zone under external force and then strength decreases obviously.The maximum strength at room temperature is 1785.6MPa.Experimental results of high temperature 800?-1300? mechanical properties show that,when the content of ZrB₂ is fixed,the yield strength and fracture toughness of xZrB₂-NbTi decrease linearly with the increase of experimental temperature from 800? to 1300?.When the test temperature is a constant,the yield strength reaches highest value at 45 at.%ZrB₂-NbTi and the fracture toughness reaches highest value at 30 at.%ZrB₂-NbTi.Particle strengthening,fine grain strengthening and residual stress field strengthening are the main strengthening mechanisms for high temperature strength.The retention of toughness of the matrix and the toughening mechanisms caused by(Ti,Nb)B phase are the main reasons for high temperature toughness.When temperature is higher than 1200?,the toughness of 60 wt.%ZrB₂-NbTi material increases significantly duo to the martensitic transformation toughening mechanism of zirconia.Experimental results of high temperature 800?-1300? oxidation resistance show that,the oxidation products of xZrB₂-NbTi composites includes TiNb2O7,Ti2Nb10O29,Nb2O5,Nb2Zr6O17,TiO2 and ZrO2.Nb2Zr6O17 and TiO2 are beneficial to the integrity of the oxide layer which can improve the oxidation resistance.The order of the destruction ability to the oxide layer of lamellar Nb2O5,loose Ti2Nb10O29 and TiNb2O7 is:Nb2O5>Ti2Nb10O29>TiNb2O7.At 800?-1000?,the film-like Nb2Zr6017 increases with increasing ZrB₂ content while the lath-shaped Nb2Os decreases.Film-like Nb2Zr6O17 can heal the cracking of oxide layer caused by growth stress of Nb2O5.As a result,the higher ZrB₂ content,the better oxidation resistance.At 1200?,the volatilization of B₂O3 leads to a broken Nb2Zr6O17 film.The volume effect of ZrO2 martensitic transformation and the growth of Nb2O5 result in excessive stress in the oxide layer.Thus,oxidation resistance becomes worse at higher ZrB₂ content.