Research On Shock Synthesis And Reaction Mechanism Of Nb/Si And Nb/Al Intermetallic Compound

Shock loading can quickly generate extremely high temperature and pressure within substance and therefore offers a new way for the synthesis of materials,especially for those materials with demanding synthetic conditions.Binary niobium intermetallies have found potential applications in high performance components due to their low density and excellent high temperature strength as well as physical properties.However,it is very hard to synthesize such materials through conventional methods due to their high melting point,complex crystal structure.In addition,the formation reaction of such intermetallic compounds is an exothermic process.Theoretically,the reaction can be self-sustaining once activated,but few conventional methods can be effectively applied to make use this advantage because the activation barrier energy is very high.In such case,shock loading is advantageous for the activation of this reaction by utilizaing its extreme high temperature and high pressure produced,compared to other approaches.However,the flyer-impact reaction synthesis,especially for such unique intermetallic compounds,still faces challenges in terms of technical issues and basic understanding of the reaction mechanism.In particular,no successful recovery experiments have been reported so far on the shock synthesis of niobium silicides with higher shock intensity?flyer velocity>2.0 km/s?.Based on this,the dissertation aimed to explore the impact synthesis of nibium-silicon/nibium-aluminum intermetallic compounds as well as the reaction mechanisms underlying.With the help of two-stage light gas gun and flyer impact technology,the impact recovery experiment of strong shock loading for nibium-silicon and nibium-aluminum powder mixtures respectively was performed.The external loading conditions?flyer velocity?,the initial state of the powder mixture reactants?porosity and ball milled pretreatment?,and the influence of the elememetal component system?nibium-silicon and nibium-aluminum?were systematically studied.From the perspective of shock-induced chemistry,the recovery samples of binary niobium intermetallies were characterized and analyzed comprehensively and the reaction mechanisms were discussed and fathomed.The main contents of the research and fingdings are as following:1)The bulk niobium silicides?Nb₅Si₃?was successfully obtained by impact recovery of the nibium-silicon powder mixture with the initial ratio of Nb₅Si₃ stoichiometric ratio at high impact strength.Characterization and analysis of the recovered samples synthesized by impact recovery experiment showed that the shock intensity was decisive for the impact chemical reactions and characteristics of products between the nibium-silicon powders.At the lower shock intensity?the flyer velocity is 1.98 km/s?,the nibium-silicon mixture hardly undergoes a chemical reaction,and only a very small amount of NbSi₂ compound is obtained in some localized regions.When the higher shock intersity?the flyer velocity reaches 2.23km/s?,partial"solid-liquid"chemical reaction activation occurs at the interface of niobium-silicon,and NbSi₂ phase compound is formed,indicating that the occurrence of chemical reaction is a thermodynamic and kinetic factor common to the microscopic region.With the further improvement of impact strength?the flyer velocity is 2.63 km/s?,the niobium silicide?Nb₅Si₃?with a stoichiometric ratio was obtained,which was characterized by a"liquid phase"crystallization reaction.At the highest shock intensity?the flyer velocity of 2.82km/s?,the bulk Nb₅Si₃ intermetallic compound with high temperature?-Nb₅Si₃ phase was obtained,and the structure was fine and homogeneous.2)By studying the influence of the shock-induced chemical reaction of nibium-silicon intermetallic compounds in the initial state?porosity and ball milled pretreatment?of different starting powder mixed reactants,it was found that the sample with low porosity?10%?hardly reacted.When the porosity is 20%,the nibium-silicon powder undergoes a partial chemical reaction to form a NbSi₂ compound.As the porosity increases to 35%,a complete reaction occurs to obtain a Nb₅Si₃ intermetallic compound under the same impact conditions.It was confirmed that at high porosity,the high temperature generated by the pore collapse of the reactants is the main cause of complete reaction in the powder reactant.The impact reaction experiments on the ball-milled niobium-silicon powder mixture pretreated at different times showed that the NbSi₂ compound was synthesized by partial reaction of ball milled pretreatment for 1 hour at the same shock intensity?flyer velocity?.As the ball milled time increased to 5 hours,the product phase not only had NbSi₂compounds,but also Nb₅Si₃ intermetallic compounds.When the ball milled pretreatment time was further increased to 10 hours,niobium silicides?Nb₅Si₃?was obtained under the same impact conditions.It indicates that the ball milled pretreatment can effectively reduce the threshold of the impact-induced reaction of the niobium-silicon binary powder,and as the ball milling time increases,the impact reaction of the niobium-silicon powder particles proceeds more completely under the same impact loading conditions.3)The shock-induced chemical reaction behavior of the niobium-aluminum powder mixture with the initial ratio of Nb₃Al stoichiometric ratio was studied by changing the reactant composition system.Through the characterization analysis of the impact recovery product of the niobium-aluminum powder mixture,it was found that the impact compression can induced the reaction between niobium-aluminum to synthesize a single-phase Nb₃Al compound.and when the shock intensity is low?the flyer velocity of1.97 km/s?,only the partial reaction of the powder mixture occurs,and the impact reaction main product obtained is the aluminum-rich compound NbAl₃.As the shock strength increases?the flyer velocity is 2.27km/s?,the Nb₂Al compound in the product is the main component phase.At the highest impact strength?the flyer velocity of 2.61km/s?,the chemical reaction of high-melting Nb₃Al compound was activated in the highly deformed niobium and aluminum particles,and the intermetalli compound Nb₃Al was synthesized.The reaction was complete and the reaction product is relatively dense and uniform.4)The underlying mechanisms associated with the impact synthesis of nibium-silicon and nibium-aluminum intermetallic compounds are discussed.It is proposed that the impact reaction is a complex cascade process from physical structrue,mechano-chemical to thermo-chemical interactions.The extrinsic factors such as external impact loading,initial state of the reactants as well as intrincis properites of the reactant components per se play decisive roles in determining the type/stage of the impact chemical reaction,the extent to which the reaction occurs and therefore the microstructure of the reaction products.