Effects Of Hf,B And Cr Additions On The Microstructure And Properties Of Nb-Si Based Ultrahigh Temperature Alloys

Development of aero-technology requires engines to have higher thrust-weight ratio,i.e.,the turbine blade materials have to experience higher temperatures and possess more exceptional mechanical and environmental properties.Because of their high melting points,low densities,as well as excellent and balanced mechanical properties at both room and elevated temperatures,Nb-Si based ultrahigh temperature alloys have shown to be potential as next generation high temperature structural materials applied in aeroengines.However,poor oxidation-resistant performance is a major barrier to their high temperature application in the field of aerospace.Alloying,as an effective method of ameliorating the integrated properties,especially oxidation resistance of the alloys,has been employed.In recent years,researches on Hf,B and Cr alloying have been extensively reported but mainly focused on simple ternary or quaternary Nb-Si based alloy systems.Moreover,studies regarding synergetic influence among Hf,B and Cr elements as well as compositional optimization etc.have been rarely involved.It is therefore imminent and essential to investigate the effects of Hf,B and Cr additions on the microstructure and properties of the alloys systematically,with the aim of developing novel Nb-Si based ultrahigh temperature alloys and realizing their application as soon as possible.The main researches and results in this paper are as follows:Fifteen multi-component Nb-Si based ultrahigh temperature alloys with nominal compositions of Nb-22Ti-16Si-3Al-xHf-yB-zCr(at.%)were prepared by vacuum non-consumable arc melting and then heat-treated at 1450? for 50 h.The effects of Hf,B and Cr additions on the phase selection,phase stability,phase transformation,non-equilibrium and equilibrium microstructures of the alloys have been investigated.The results show that both B and Cr additions promote the hypereutectic structures but Hf addition has no influence on the eutectic point of the alloys.The addition of Hf or B alone completely suppresses the formation of ?(Nb,X)5Si3("X" represents Ti,Hf and Cr elements)whilst promotes the formation ?(Nb,X)5Si3 and ?(Nb,X)5Si3.Furthermore,the combined additions of Hf and B have an extremely strong influence of promoting ?(Nb,X)5Si3 formation.However,Cr addition has no influence on the crystal structures of 5-3 silicides.The combined additions of Hf and Cr promote the formation of Cr2(Nb,X)containing three-phase eutectic structures.After 1450?/50 h heat-treatment,?(Nb,X)5Si3 has completely transformed into ?(Nb,X)5Si3 in the alloy without Hf or B addition,and a(Nb,X)5Si3 has partly transformed into y(Nb,X)5Si3 in the alloy with Hf addition alone,regardless of the presence or absence of Cr.Nbss and a(Nb,X)5Si3 equilibrium is observed in the alloys without Hf or B;Nbss,a(Nb,X)5Si3 and ?(Nb,X)5Si3 equilibrium in the alloys with Hf or B alone;Nbss and ?(Nb,X)5Si3 equilibrium in the alloys with both Hf and B,respectively.In addition,in the presence of B,relatively lower Ti or(Ti + Hf)concentrations can still stabilize y(Nb,X)5Si3.With increasing the Hf content in the alloy,the amount of primary silicides does not change obviously.The silicides are a(Nb,X)5Si3 and ?(Nb,X)5Si3 in the alloys without Hf and with 2 at.%Hf,and y(Nb,X)5Si3 in the alloys with 4 or 8 at.%Hf,respectively.The formation of both ?(Nb,X)5Si3 and(Nb,X)3Si is promoted by the higher addition of B(5 or 10 at.%)and a eutectoid transformation reaction(Nb,X)3Si?Nbss + ?(Nb,X)5Si3 has occurred in the alloys during heat-treatment.Moreover,the amounts of both primary silicides and Cr2(Nb,X)containing three-phase eutectic exhibit a rising trend with increase in B content in the alloy.The higher addition of Cr(10 at.%)still does not change the crystal structure of 5-3 silicide(only ?(Nb,X)5Si3 in the presence of Hf and B)but significantly increases the amounts of both Cr2(Nb,X)containing three-phase eutectic and primary silicides.After heat-treatment,Cr2(Nb,X)Laves phase disappears in the alloys with lower addition of Cr(less than 5 at.%)while still can be observed in the alloys with higher addition of Cr(10 at.%).The isothermal oxidation behavior of the fifteen Nb-Si based ultrahigh temperatures alloys was evaluated in static air at 1250? for 1 or 50 h.The effects of Hf,B and Cr additions on oxidation resistance of the alloys have been investigated.The results show that all the scales formed on the alloys upon oxidation at 1250? for 1 or 50 h spall out after cooling.A double-layer structure of the scale(i.e.outer and inner layers)and an internal oxidation zone are observed for the oxidized alloys.The outer layer of the scale is mainly composed of TiNb2O7,(Ti,X)O2(solid solution oxide)and amorphous silicate particles,and its corresponding inner layer Ti2Nb10O29,TiO2,Nb2O5,HfO2,amorphous silicate and unoxidized silicides.The oxides in the internal oxidation zone are mainly TiO2 and HfO2.The oxidation resistance of the alloy is the worst without Hf,B or Cr addition,and slightly ameliorated with Hf addition alone(based on the dynamic-segregation theory for reactive element).The addition of B or Cr alone obviously ameliorates the oxidation resistance of the alloy by increasing the amount of silicide and compactness of the scale,decreasing the thickness of the inner layer and its proportion over the whole scale,and promoting the selective oxidation of Ti in the internal oxidation zone.The addition of Cr can promote the formation of(Ti,X)O2 which is beneficial to the oxidation resistance of the alloy.The alloy has better oxidation resistance with combined additions of B and Cr than the addition of B or Cr alone.Moreover,the oxidation resistance of the alloy is further ameliorated by increasing its B or Cr content.In the presence of B or Cr,the addition of Hf does not ameliorate but obviously degrades the oxidation resistance of the alloy by decreasing the compactness of the scale,increasing the thickness of the inner layer and its proportion over the whole scale,suppressing the selective oxidation of Ti and promoting the formation of HfO2 in the internal oxidation zone.In addition,in the presence of both B and Cr,the oxidation resistance of the alloy is firstly ameliorated slightly by the lower addition of Hf(2 at.%),and then degraded by the higher addition of Hf(especially 8 at.%)due to the more severe internal oxidation.Single edge notched three-point-bending experiments and hardness measurements of the specimens of the fifteen Nb-Si based ultrahigh temperatures alloys were preformed.The effects of Hf,B and Cr additions on the room-temperature fracture toughness(KQ),microhardness and macrohardness of the alloys have been investigated.High temperature compressive measurements of the specimens of the alloys with different Hf contents were also conducted at 1250? with a strain rate of 5×10-3 s-1.The effects of Hf content on high temperature compreSSive properties of the alloys have been investigated.The results show that Cr is detrimental to the KQ of the alloy.The KQ of the alloy is degraded by the addition of Hf or B alone but hardly influenced by their combined additions.The Kq of the alloy is further degraded by increasing its B(5 and 10 at.%)or Cr(10 at.%)content due to formation of more primary silicides in the alloys.In the presence of B and Cr,the KQ of the alloy exhibits a rising trend with increase in Hf content in the alloy(especially obvious at 8 at.%).The hardness of y(Nb,X)5Si3,a(Nb,X)5Si3 and ?(Nb,X)5Si3 decreases successively.Because of possessing significantly higher B concentration,(Nb,X)3Si has a higher hardness than(Nb,X)5Si3.After heat-treatment,the hardness of(Nb,X)5Si3 slightly decreases while that of(Nb,X)3Si increases to some extent.The hardness of eutectic structure and the macrohardness for Cr-containing alloys are higher than those for Cr-free alloys.Moreover,the macrohardness of the alloy further increases with higher B(5 and 10 at.%)or Cr(10 at.%)additions.The hardness of primary silicides and eutectic structures in the alloys,as well as their macrohardness all exhibit an increasing trend with Hf addition(especially obvious at 8 at.%).After heat-treatment,the macrohardness of Hf-containing alloys obviously decreases,particularly in the absence of Cr.In addition,the high temperature compressive strength of the alloy is firstly increased by the lower addition of Hf(2 and 4 at.%)based on its solid solution strengthening,and then degraded by the higher addition of Hf(8 at.%)due to the embrittlement of the alloy,especially y(Nb,X)5Si3.