Studies On The Microstructure Of SiC Fiber Reinforced TiAl Composites

SiCf/TiAl matrix composites are regarded as high temperature structurematerials with great prospects of application due to their low density, excellent hightemperature strength, high temperature oxidation resistance and high temperaturecreep resistance. At present, for SiCf/TiAl matrix composites, the research is mainlyfocused on interfacial reaction and the matrix of SiCf/TiAl composites is seldominvestigated. Due to the adoption of different TiAl alloy and different SiC fiber, theinterfacial reaction products and corresponding reaction mechanisms are different too.Therefore, it is necessary to study the matrix of the composites and furtherinvestigate the interfacial reaction mechanism. Besides, numerical calculation andcomputer simulation are very useful tools by which we can deal with someexperimental difficulties or verify some experimental phenomena. In theexperimental parts of this research, by means of scanning electron microscope, X-raydiffractometer and transmission electron microscope, we focus on SiCf/TiAlcomposites in the following aspects:(1) the microstructural evolution of SiCf/TiAlcomposite matrix during the fabrication process of fiber-foil-fiber method,(2) themicrostructural evolution of SiCf/TiAl composite matrix during the fabricationprocess of matrix coated fiber method,(3) the interfacial reaction of SiCf/C-coating/TiAl composite,(4) the interfacial reaction of SiCf/B4C-coating/TiAlcomposite. And in the chapter of numerical calculation and computer simulation, westudy (5) the variation of the chemical component of coating and target surface, and(6) the matrix evolution of SiCf/TiAl composite during the fabrication process ofmatrix coated fiber method.In the composite fabricated by foil-fiber-foil method, the microstructure of thematrix alloy from the position close to SiC fiber to the position far away from SiCfiber is: a layer of single γ-phase close to SiC fiber; α2/γ lamellae microstructureadjacent to single γ-phase layer; a mixed microstructure composed of α2/γ lamellae, γgrains and B2particles along grains far away from SiC. The distribution of matrixmicrostructure is closely related to the diffusion of C and Al elements, and is affectedby the matrix plastic deformation and the accumulation energy of plastic deformation.During the fabrication process of composite by matrix coated fiber method, atthe initial stage of sputtering, due to low temperature and difficulty in nucleation, amorphous TiAl forms on the surface of SiC fiber firstly; at the following stage oftemperature increasing-temperature holding, part of amorphous TiAl transforms tometastable α-Ti(Al), and small part of amorphous TiAl transforms to TiAl3due toaggregation of Al element; at the stage of hot isostatic processing (HIP) at highertemperature, amorphous TiAl, α-Ti(Al) and TiAl3transform to stable γ-TiAl and α2-Ti3Al.The study of interfacial reaction in SiCf/C-coating/TiAl composites shows thatthree kinds of reaction products, TiC, Ti2AlC and (Ti,V)5(Si,Al)3, exist between C-coating and TiAl matrix. TiC is close to C-coating, and (Ti,V)5(Si,Al)3is close toTiAl matrix. Ti2AlC layer exists as columnar grains between TiC and (Ti,V)5(Si,Al)3layers. The TiC layer can be divided into fne-grained layer and coarse-grained layer.The fne-grained TiC layer contains amorphous carbon, and the coarse-grained TiClayer is separated from fne-grained TiC by an intermittent carbon layer. In addition,small amount of TiC particles are also formed between carbon-coating and SiC.The study of interfacial reaction in SiCf/B4C-coating/TiAl composites shows thatfive kinds of reaction products, C, TiB2, TiC, TiB and Ti2AlC, exist between B4C-coating and TiAl matrix. These five reaction products form four reaction layers:amorphous C layer on the surface of B4C-coating|a mixed layer composed of TiB2+amorphous C|TiC layer|a mixed layer composed of TiB+Ti2AlC close to TiAlmatrix.A simple sputtering model is established in this paper, and the variation ofchemical component of both coating and target surface is calculated by numericalcalculation. The calculated results conform to the common recognition of the law ofsputtering, and the calculated result of coating component is consistent with theexperimental result.Molecular dynamics method is used to simulate the evolution process of TiAlmatrix alloy during matrix coated fiber method, and the results show that this methodcan satisfyingly represent the transformation process of matrix alloy, i.e. amorphousTiAlâ†'amorphous TiAl+α-Ti(Al)â†'γ-TiAl+α2-Ti3Al.