Microstructural Characteristics Of Nb-Si Based Ultrahigh Temperature Alloys

Nb-Si based ultrahigh temperature alloys have attracted much attentions recently for their potential applications as high temperature structural materials due to their high melting points,relatively low densities and excellent high temperature strengths.However,their main drawbacks,such as low fracture toughness at room temperature and poor oxidation resistance at high temperatures still need to be overcome.Alloying with moderate amounts of appropriate elements is the most effective,economical and widely applicable method to inhibit those disadvantages.Alloying elements of Ti,Hf,Cr,Al have been added in Nb-Si alloys and Nb-Ti-Si-Cr-Al and Nb-Ti-Si-Cr-Al-Hf systems have been developed,which can get good balance between mechanical properties and oxidation resistance.Influences of alloying elements on the microstructure and then the performance of Nb-Si based alloys have been investigated in the last decades.Moreover,directional solidification could enhance the comprehensive mechanical performances of Nb-Si based alloys significantly.The integrally directional solidification technique could produce fine and regularly aligned microstructure with high stability and better performance than other directional solidification methods for Nb-Si based alloys.However,the detailed microstructure characteristics such as phase transformation of constituent phases,formation characteristics of precipitates and preferred growth orientation of Nbss/Nb₅Si₃eutectics have not been systematically investigated,which are important to the service performance of the Nb-Si based alloys.In the present dissertation,the microstructures of as-cast and heat-treated Nb-Ti-Si-Cr-Al alloys have been studied by transmission electron microscopy?TEM?,electron backscattered diffraction?EBSD?as well as in-situ TEM observation.?Nb₅Si₃ transforms into?Nb₅Si₃ and Nbss during the homogeneous heat-treatment,with formation of?Nb₅Si₃ variants with different orientation.Formation of these orientation variants is accompanied by motion of dislocations,suggesting a diffusion-controlled phase transformation.Crystallographic orientation relationships?ORs?,interfacial structures and formation characteristics of the particle like and?Nb₅Si₃ have been analyzed.Ti precipitates with fcc structure have formed in the as-cast Nb-Ti-Si-Cr-Al alloys.The precipitates inside Nbss would increase significantly upon heating and dissolve back into Nbss at 1450 ^oC,while bulk fcc-Ti precipitates partially transform into hcp-Ti lamellae through a martensitic transformation during heat-treatment.The precipitation mechanisms of fcc-Ti precipitates in Nbss and hcp-Ti lamellae in fcc-Ti matrix have been analyzed.Furthermore,hcp-Ti lamellae experience a series of phase transformation and finally transform into fcc-Ti during heating in TEM.Two periodic structures nucleate and act as modulated structures in the process of hcp-Ti to fcc-Ti phase transformation.The transformation proceeds through motion of partial dislocations and formation of growth ledges.Origins of the two periodic structures and ledge model for the phase transformation have been discussed.Our findings provide a comprehensive insight of structural phase transformation of Ti precipitates in Nb-Ti-Si based alloys.Microstructure of Nb-Ti-Si-Cr-Al-Hf alloys is composed of primary Nb₅Si₃ and euetctics of Nbss/Nb₅Si₃.The structure of Nb₅Si₃ varies from?Nb₅Si₃ to?Nb₅Si₃ with Hf or Ti additions increased.Upon heat-treatment,three types of intermetallics precipitates would form inside Nbss,namely,?Nb₅Si₃,?Nb₁₁Si₄ and Cr₂Nb.The morphology,interface types and ORs with Nbss matrix of these three kinds of precipitates have been characterized.Hexagonal?Nb₅Si₃plates precipitate from Nbss with ORs of[111]_Nb//[0001]_?and?101!?_Nb//?11!00?_?and with coherent interfaces.The interfacial structure has been interpreted with the Coincidence site lattice/Displacement shift complete lattice?CSL/DSC?model,forming an array of phase boundary dislocations?PBDs?on the interface.PBDs with Burgers vectors of b"?=₃¹<1!21!>_Nbon?111?_Nb plane might promote the precipitation of?Nb₅Si₃.Furthermore,?Nb₅Si₃ precipitates adopt OR of[010]_Nb//[0001]_??OR-I?and[010]_Nb//[112!3]_??OR-II?with Nbss during in-situ heating TEM observation.These two types of?Nb₅Si₃ precipitates all experience three stages of expansion,adjustment and dissolution during heating.However,the dissolution temperature of?Nb₅Si₃ precipitates with OR-II is 125 ^oC higher than that of?Nb₅Si₃ precipitates with OR-I.The results suggest that crystallographic characteristics of precipitates have great influence on their stability.Stability of these two orientational?Nb₅Si₃ precipitates have been discussed using Bramfitt planar disregistry theory.Silicide precipitates of?Nb₁₁Si₄ usually exhibit homogeneous distribution with acicular and tetrapod morphologies,forming different orientation relationships?ORs?with Nbss.Formation of these morphologies might be caused by the decreased symmetry during the phase transformation of Nbss??Nb₁₁Si₄+Nbss₁.In some cases,the?Nb₁₁Si₄ precipitates also adopt heterogeneous distribution with plate morphology and identical ORs with Nbss.The Laves phase precipitate,Cr₂Nb,possesses C15structure and contains high densities of stacking faults?SFs?.It always precipitates along defects inside Nbss or Nbss/?Nb₅Si₃ interfaces,forming incoherent interfaces with them.The coupling precipitation behaviors and precipitation reactions of?Nb₁₁Si₄,Cr₂Nb and?Nb₅Si₃ in the Nb-Ti-Si based alloys have been discussed.Crystallographic characteristics have great effects on the performance of directionally solidified Nb-Ti-Si based in-situ composite,which is mainly composed of parallel Nbss/?Nb₅Si₃ eutectic cells along the longitudinal axis of the specimens.Nbss and?Nb₅Si₃grow up in a coupled manner and exhibit preferred growth orientation along[111]_Nb and[0001!]_?zone axes,respectively.Under these orientations,two types of coherent interfaces have been formed between the two phases,corresponding to the different morphologies of?Nb₅Si₃.The preferred growth orientations and coherent interface structures might promote the coupled growth and enhance the stability of the eutectic cells of this in-situ composite.During heat-treatment,numerous hcp-Ti precipitates formed inside Nbss.These hcp-Ti precipitates form an OR of[111]_Nb//[112!0]_hcp-Ti,?11!0?_Nb//?0001?_hcp-Ti and coherent interfaces with Nbss.Moreover,Ti precipitates inside Nbss form the same ORs as Nbss precipitates inside Tiss.Their interfacial types match well with“Edge-to-Edge”model,forming interfaces with low interfacial energy.