Synthesis And Superplasticity Of Ultra Fine-Grained Nb-Si-Fe Refractory Alloys

Niobium has superior characteristic, i.e., high melting temperature (2468℃), good ductility and fracture toughness at room temperature. Moreover, Nb-Si system refractory alloys have a wide Nb₅Si₃ and Nb solid solution two-phase region; therefore, eutectic and eutectoid reactions offer a variation of microstructures to prepare Nb-Si refractory alloys. So the refractory alloy with Nbss and niobium silicides has the most potential as the next generation ultra-high temperature structural materials.Arc melting is a general method to fabricate the Nb-Si systems refractory alloys. As cast structure consists of original niobium and nonequilibrium Nb+Nb3Si. Because of unstable microsture of Nb3Si phase at high temperature, as cast materials should be heat treated at 1800℃for 100 h. Moreover, in order to remove defect of as cast materials, materials must be hot extruded after heat-treatment.Due to the reduced diffusion path in the refined microstructures and the increased free energy by mechanical alloyed (MA), it is easy to produced ultra-fine refractory alloys. Therefore, in this research the MA process was introduced as a key process to develop Nb-Si refractory alloys with ultra-fine and homogeneously distributed phase. There is hardly any report about Nb-Si refractory alloys prepared by MA + hot pressing sintering up to now. So in this paper the Nb-xSi-2Fe (x=3, 6, 10, 16) refractory alloys were prepared by MA and hot pressing sintering; mechanical properties at room temperature and high temperature were also researched. It is interesting to note that Nb-16Si-2Fe refractory alloy has excellent superplasticity. Superplasticity of Nb-Si system refractory alloys has not been reported up to now.Nb-Si-Fe powders with excellent sintering property were synthesized by MA. Effects of milling time, milling speed and addition of alcohol on powder sizes and morphlogy were studied. Sub-micro Nb-16Si-2Fe powders with particle size of 0.3μm and some particle size less than 100nm can be obtained by milling for 30h at 250rpm, which mainly consist of Nb, Fe, Nb₅Si₃ phase and metastable Nb3Si phase.Nb-Si-Fe system refractory alloys were prepared by hot pressing sintering. The refractory alloys sintered at 1500℃for 1h has high relative theoretic density (99.6%). The microstructures of all Nb-Si-Fe refractory alloys consist of the Nbss and uniformly dispersed three kinds of intermetallic Nb3Si and Nb₅Si₃, and Nb₄Fe₃Si₅ phase. The average size of grains of these refractory alloys is about 3μm and the grain shape is essentially equiaxed. The grain boundaries are clear and sharply facetted. Intermetallic particles mainly located at multi-grain junction of Nb grains. But there are sub-micro intermetallic particles within the Nb grians.The as sintered at 1500℃Nb-Si-Fe system refractory alloys have excellent mechanical properties at room temperature. The hardness and elastic modulus of Nb-16Si-2Fe are 11.2GPa and 330GPa repectively. The fracture toughness of Nb-3Si-2Fe reaches 14.6 MPa·m1/2. The Nb₄Fe₃Si₅ phase has improved fracture toughness of Nb-Si-Fe refractory alloys. The tensile elongation of Nb-Si-Fe refractory alloys at 1300℃is between 54% and 95% depening on Si content. The tensile strength of Nb-Si-Fe system refractory alloys at 1300℃is between 112MPa and 237MPa.The tensile elongation of the Nb-16Si-2Fe refractory alloy reaches 185%, 338%, 320% respectively, at 1350℃,1400℃and 1500℃with initial strain rate of 2.31×10-4 s-1. The maximum elongation of the Nb-16Si-2Fe refractory alloy was obtained at a temperature which is 91℃higher than the solidus temperature of 1359℃. A significant amount of liquid phase Nb₄Fe₃Si₅ occurs during superplastic deforming; the volume of Nb₄Fe₃Si₅ liquid phase was about 24%. The deformation strain mainly results from the sliding/rotation of solid Nbss, Nb3Si and Nb₅Si₃ grians accompanied by strectching of the liquid phase Nb₄Fe₃Si₅. Grain growth is restrained by four-phase structure of Nb-16Si-2Fe refractory alloy.The extrusion deformation of a big extrusion ratio of 30 was undertaken at 1600℃. There is texture structure along extrusion directon, which is attributed to fracture toughness. The fracture toughness increases from 11.2 MPa.m1/2 to 20.1 MPa.m1/2 after extrusion. Model of engine combustion chamber was successfully extruded one-off at 1600℃. That material can be processed to parts with good quality by means of simple extrusion technique. Moreover, the utilization rate of the material was high and cost has been greatly cut down.