Investigations Of Microstructures Of Nb-Coated C_f/SiC And Platinum Based Alloy

The nozzle of rocket engines is an extremely critical component which is designed for orbital corrections or carrying the satelite from the launch rocket up to the altitude of its service obrit.The properties of the nozzle material is directly related to that of engine.With the advance of aerospace engineering,to further improve the operating temperature of the engine is highly desired.It is urgent to develop high temperature structural materials with a much higher melting point to serve in aggressive environment.Cf/SiC composite and platinum-based alloys are two important candidate materials for the nozzle of rocket engines.On one hand,Cf/SiC composite is one of the candidate materials due to its excellent properties such as low density,high temperature resistance and high specific strength.However,due to its intrinsic brittleness and the difficulties in the manufacturation of complex large-size structural parts,it has important practical significance to join the CdSiC composite with niobium transition ring.On the other hand,platinum-based alloys with high melting point and high temperature oxidation resistance are one of the important development directions of ultra-high temperature oxidation resistance structural materials.However,studies on CVD Nb transition joining and the addition of rare earth elements into Pt-Rh alloys have been rarely reported.Since the microstructure is a key factor to determine the properties of materials,it is necessary to study the fine microstructural characteristics of the above-mentioned two materials,such as the interface structure of Cf/SiC-Nb and the distribution of rare earth in Pt-Rh alloy.In this paper,microstructures of Cf/SiC-Nb and Pt-Rh-0.2La-0.2Ce materials were investigated by means of optical microscopy(OM),scanning electron microscopy(SEM),X-ray diffraction(XRD),transmission electron microscopy(TEM)and secondary ion mass spectrometer(SIMS).The main conclusions are summarized as follows:1.Nb coatings on the Cf/SiC composites were prepared by chemical vapor deposition(CVD)with the use of NbCl5 and H2 as the precursors at 1100℃.The maximum room temperature shear strength of the joint is 37.75 MPa.The layered structures were formed by elemental diffusion and chemical reactions.From the interface to the top of Nb,the sequence of layers is the interfacial reaction zone,fine equiaxed grained Nb zone and coarse columnar grained Nb zone.A sandwich-like interfacial reaction zone(an intermittent Nb5Si3 layer between two NbC layers,denoted as NbC|γ-Nb5Si3|NbC)was formed between Nb coating and Cf/SiC composite.Nanoscale NbC particles were formed near the interface of the Cf/SiC composite,while needle-shaped γ-Nb2C was formed in the Nb coating.Their formation is due to the interdiffusion of Nb and C elements.An orientation relationship was identified betweenγ-Nb2C and Nb matrix,which is[1120]γ//[001]M,(0002)γ//(110)M.Hydrogen was found to exist in the NbC phase and the micro-cracks at the interface by ToF-SIMS analysis,which has a negative effect on the interfacial bonding properties.Therefore,argon should be used as protective atmosphere during the heating and cooling cycles to reduce the hydrogen absorption.The formation mechanisms of the interfacial structures between Nb coating and Cf/SiC composite were discussed based on thermodynamics,reaction kinetics and elemental diffusion.2.After changing the H2 flow rate and heat treatment,the Nb coating still retains the layered structure.With the increase of H2 flow rate,the columnar characteristics of the coarse columnar grained Nb zone become more obvious,and the grains become more elongated along the growth direction.The small grains in the fine equiaxed Nb zone near the interface changed from regular and compact arrangement to irregular and loose arrangement.The total thickness of the interfacial reaction layer decreases with the increase of H2 flow rate.After heat treatment,the columnar grains grew laterally and the equiaxed grain size at the interface increased.With the same holding time,the thickness of the B layer(γ-Nb5Si3)and the total thickness of diffusion reaction layer increase with the temperature of heat treatment,but the total thickness increases slightly.At the samet temperature of heat treatmen,the holding time has a minor impact on the total thickness of the interfacial reaction layer.After heat treatment,massive carbides β-Nb2C are also precipitated in the Nb layer,and there is a specific OR between the β-Nb2C and Nb matrix,which can be identified as[10 1 0]β//[001]M,(0001)β//(110)M,.These findings are valuable for figuring out strategies to optimize the deposition process of Nb coating by CVD method and improve the interfacial bonding properties of Nb-coated Cf/SiC composites.3.Anew Pt-based alloy,Pt-25Rh-0.2La-0.2Ce(wt.%)was successfully developed by non-consumable arc melting technology.The alloy was identified to be consisted of two balanced phases,a(Pt,Rh)solid solution phase with a FCC structure(a=0.4048 nm)and a Pt5RE intermetallic phase with a hexagonal structure(as a strengthening phase).The grain size of the matrix ranges from 50 to 100 μm.Its composition is 76.18Pt-23.82Rh(wt.%),which is close to the designed composition Pt-25Rh(wt.%).The Pt5RE intermetallic phase has three types of forms distributed in the alloy:precipitates in the matrix with a grain size of 0.6～1.2 μm;strip-like intergranular phase along the grain boundary with a size of 2～3 μm;and eutectic mixture coexisted with(Pt,Rh)ss.There is OR between the Pt5RE and(Pt,Rh)ss phases in the eutectic structure,which can be identified as[1010]P//[01 1]M,(0001)P//(111)M.After forging and drawing,the eutectic mixture decomposed,which cause the formation of densely distributed Pt5RE and a small amount of rare earth oxides(La,Ce)O2 appeared.The rare earth oxides and substrate have a clear orientation relationship[111]O//[111]M,(022)O//(022)M.Twins appear in the matrix due to large deformation.4.The Pt-25Rh-0.2La-0.2Ce(wt.%)alloy was tested at 1500℃ under a load of 5 MPa.After that,a large number of dislocations appear in the matrix,they are typical dislocations with Burgers vector a/2[101].The dislocations piled up and tangled in ahead of the Pt5RE phases.Through the alloying of a small amount of RE(La and Ce),the Pt5RE intermetallics were in-situ formed in the alloy,which can effectively improve the high-temperature strength and creep resistance.The Pt5RE intermetallics are distributed on the grain boundaries have a pinning effect,and those in the matrix can strongly impede the movement of dislocations.By adding RE,the high temperature comprehensive performance of the alloy is significantly better than that of Pt-30Rh,which is not only a breakthrough in the research of precious metal materials,but also reduces the amount of Rh to reducing markedly the cost of the alloy.