Microstructural Evolution And Properties Of Nb-si Refractory Alloys Fabricated By Powder Metallurgy

Nb-Si refractory alloys usually consist of Nbss, Nb₅Si₃ and/or Nb₃Si, and they have been developed as the new class high temperature structural material because of their high melting point, low density and excellent high temperature mechanical properties. Arc melting is the general method to fabricate Nb-Si alloys, while the casting flaws of component segregation and coarse microstructure damage the properties. Moreover, the intrinsic brittleness of silicides also hinders their practical applications. In the present research, Nb-16Si binary alloys and multi-element ones with the addition of Ti and/or Fe were prepared by mechanical alloying and uniaxial hot pressing and a model of trust chamber were fabricated by powder forging. Meanwhile, the evolution of mechanical alloyed powders, and the effects of mechanical alloying, sintering parameters and alloying elements of Ti and Fe on the microstructures and mechanical properties have been investigated. Moreover, the reinforcing and toughening mechanisms, high temperature compressive deformation behaviors and the defromed microstructures have been studied.The research results of mechanical alloying of Nb-16Si and Nb-16Si-18Ti powder systems showed that the amount of absolute alcohol, particle size of initial Nb powders and addition of Ti affected the ball milling process. During milling, Nb powders were refined into flocculent morphology due to work-hardening resulting from plastic deformation, and a supersaturated solid solution was formed by Si atoms dissolving into Nb lattices. Excess absolute alcohol reduced the refining rate of powders, and the long milling time induced the contaminate to form another solid solution rich in Fe. In addition, under the same milling parameters, the milling progress of ternary powers was slower than that of binary ones due to the lower density of Ti.Nb-16Si binary alloys were prepared by vacuum hot pressing sintering, and the effects of sintering temperature, sintering time and mechanical alloying on the microstructures and mechanical properties were analyzed. The density was mainly determined by sintering temperature, and over 99% relative density were obtained when hot pressed at 1500°C for 60min. Sintered materials contained three phases of Nbss,Nb₃Si and Nb₅Si₃, and pores resulted in the poor properties at lower temperatures. With milling time increasing, the microstructure refined and changed into equiaxed one with average size of about 2μm, and the volume fraction of Nbss decreased. Furthermore, fracture toughness reduced and flexural strength increased because the large size and high content of ductile phase improved room ductility. A model of trust chamber was successfully fabricated by one-time powder forging, and the part contained nice performance and the similar microstructure to sintered material. Moreover, material utilization was improved and the cost was reduced.Influence of the addition of Ti and/or Fe on the microstructures of hot-pressed Nb-Si alloys, and effect of microstructural control on the mechanical properties were studied. The reinforcing and toughening mechanisms of materials with alloying elements were discussed and high temperature compressive deformations and the defromed microstructures were investigated. With the additon of Fe, the strength was enhanced due to the synthesizationof new phase Nb₄FeSi, and nevertheless the microstructure did not change. It is worth noting that the anisotropic microstructures and properties were observed in the materials hot pressed from flaky powders containing Ti. Nbss phases were respectively in shapes of parallel streamlines and mussy sheets on the cross sections parallel and perpendicular to the pressed direction. When propagation directions of the cracks were perpendicular to long strips of Nbss, the fracture toughness was severely improved and the highest value exceeded 16 MPa·m1/2 as a result of plastic deformation, crack deflection and crack bridging. The fracture morphology showed ductile fracture characters of dimples and tear ridges. In addition, flexural strength was enhanced due to solution strengthening when Ti dissolving into Nb, and Nb-Si alloys with Ti consequently possessed superior comprehensive room temperature mechanical properties. In the elevated compressive deformation, when the loading direction was perpendicular to streamlines, strips of Nbss became narrow after being compressed, and the compression ductility was relative to the size of trips. On the other hand, materials deformed through particle rotation and interfacial slip which drove strips of Nbss deflect toward the direction perpendicular to compressive axis.