Microstructure And Properties Of Nb-Ti-Si Based Ultrahigh Temperature Alloys Prepared By Powder Metallurgy Route

Nb-Ti-Si based ultrahigh temperature alloys,as kinds of promising ultrahigh temperature structural materials due to their high melting points,moderate densities and excellent room-and high temperature mechanical properties,could be expected to have a wide and important application prospect in aerospace and astronautic fields.However,it's rather difficult to prepare these alloys by traditional smelting method due to their ultrahigh melting points.At such high smelting temperatures,it's very easy to yield composition change and contamination.To obtain a relatively uniform and pure ingot,remelting these alloys under ultra-high vacuum by several times is required,which,however,leads to the problems of high preparation cost,long production flow and so on,seriously hindering the practical application of these alloys.Nb-Ti-Si based ultrahigh temperature alloys with uniform microstructure and excellent comprehensive properties are expected to be obtained using powder metallurgy route.Preparation of high-quality powders is the prerequisite of the following powder consolidation.However,up to now,there have been few systematic studies on the effect of mechanical alloying process on the internal structure,composition change and impurity content of Nb-Ti-Si based ultrahigh temperature alloy powders.Moreover,the effect of hot press sintering process on the microstructure and phase formation of Nb-Ti-Si based ultrahigh temperature alloys,as well as their mechanical and high temperature oxidation resistance,has not been revealed up to now.In view of these,the mechanical alloying behavior of the elemental powders of Nb-Ti-Si based ultrahigh temperature alloys?the nominal composition is Nb-?20,22?Ti-15Si-5Cr-3Hf-3Al?at.%??has been revealed.Then,the bulk specimens of the alloys were prepared via hot press sintering.The influence of mechanical alloying process and hot press sintering process on the microstructure and properties of alloy has been elucidated.The following conclusions have been reached:At different ball mill speeds?300,400 and 500 rpm?,with increase in ball milling time,ductile particles such as Nb and Ti first plastically deform into flakes and brittle particles such as Si break into finer particles;then these particles are cold welded and agglomerated into large particles.As ball milling continues,these agglomerates are refined and become uniform,while their morphology gradually becomes spherical.Almost the size of all the powder particles has undergone a process of first increasing and then decreasing.The higher is the rotation speed of the ball mill,the faster the refining speed of the powder particles.At 300 rpm even after 70 h ball milling,the particles are coarse and their size distribution is still uneven,while after 20 h ball milling at 400 rpm,the particle size is fine,uniform and spheroidal;when the rotation speed is increased to 500 rpm,the particle size of the powders is smaller after ball milling of the same time.For all these three rotation speeds,lamellar microstructure is formed inside the powder particles at the very early stage.However,lamellar microstructure exists inside the particles all the time when ball milling at 300 rpm.When the rotation speed is increased to 400 rpm,the lamellar microstructure disappears after ball milling for40 h,and the inside of the particles is composed of a single uniform microstructure.With further increasing the rotation speed to 500 rpm,the lamellar microstructure disappears completely after ball milling for 20 h.At all these three rotation speeds,alloying elements Ti,Cr,Si,etc.dissolve into Nb crystal lattice to form Nb solid solution?Nbss?,resulting in decrease in Nbss lattice parameters during ball milling.The grain size of Nbss decreases and the microstrain in it increases during ball milling.When ball milling at 300 rpm,elemental particles such as Ti,Cr,Si etc.always exist.After ball milling at 400 rpm for 40 h,alloying elements such as Ti,Cr and Si have completely dissolved into Nb lattice to form supersaturated Nbss;when ball milling is performed at 500 rpm,the solution process is completed after ball milling for 20 h,and then the alloy is completely amorphized after ball milling for 40 h.Considering the fact that the impurity content of the powder particles is significantly increased at 500 rpm,the optimal ball milling rotation speed should be 400 rpm.For the ball milling process of the elemental powders of Nb-Ti-Si based ultrahigh temperature alloys,process control agent must be added due to the considerable amount of plastic powders.When methanol and ethanol are added for ball milling,HfC is easily produced as impurities.The lubrication effect of normal hexane is insufficient due to its extremely high volatility,resulting in severe cold welding during ball milling.Stearic acid,as a solid process control agent,will lead to excessive lubrication when the amount of stearic acid is excessive,resulting in very slow powder mechanical alloying.However,when the amount of stearic acid is too low,the lubrication effect is insufficient,thus the particle cold welding is serious,resulting in coarse particle size and the formation of HfC impurity.The optimal process control agent for ball milling is stearic acid,with an appropriate additive amount of 1.25 wt.%.The Nb-Ti-Si based ultrahigh temperature alloys powders have been prepared using the optimal ball milling process parameters,and then consolidated by hot press sintering at 40 MPa pressure stress and different temperatures?1250,1350,1400,1450 and 1500??respectively.The bulk Nb-Ti-Si based ultrahigh temperature alloys prepared at different temperatures are all composed of Nbss,?Tiss?Ti based solid solution?and??Nb,X?₅Si₃.The microstructure of the alloy is fine and equiaxed during hot press sintering at 1450?;when the hot press sintering temperature is lower than1450?,supersaturated Nbss with insufficient diffusion reaction exists in the alloy microstructure;when the sintering temperature higher than 1450?,the microstructure coarsens again.With increase in hot press sintering temperature,the density of Nb-Ti-Si based ultrahigh temperature alloys increases continuously and reaches its maximum at 1450?,meanwhile the contents of?Tiss and??Nb,X?₅Si₃ in the alloy also gradually increase.The Nb-Ti-Si based ultrahigh temperature alloys prepared by hot press sintering at 1450?has considerably higher fracture toughness and Vickers hardness.When the Nb-Ti-Si based ultrahigh temperature alloys prepared by hot press sintering at different temperatures are oxidized at 1250?,the oxide films all fall off.The oxide film has obvious layered structure?i.e.outer layer and inner layer?and the alloy substrate has an obvious internal oxidation zone.After oxidation at 1250?for the same time,the alloy prepared by hot press sintering at 1450?has the lowest oxidation weight gain.TiO₂ forms by the preferential reaction of?Tiss and O in the alloy;after that,the growth of the oxide film is mainly progressed through the inward diffusion of O and outward diffusion of Nb to form Nb₂O₅,and then TiNb₂O₇ is formed by the reaction of TiO₂ and Nb₂O₅.The oxide film is mainly composed of Nb₂O₅,TiNb₂O₇,TiO₂ and a small amount of amorphous silicate.Comprehensively considering the microstructure,mechanical properties and oxidation resistance,1450?is a suitable hot press sintering temperature to prepare Nb-Ti-Si based ultrahigh temperature alloys.