Directional Solidification Techniques And The Microstructure Control Of An Advanced Nb-Nb Silicide Based In-Situ Composites

Directionally solidified refractory metal matrix eutectic in-suit composites possess excellent mechanical and physical properties such as low density, high enduring temperatures, good room-temperature fracture-toughness and high elevated-temperature strength, therefore are expected to be used as a new type of ultra-high temperature structural materials.In this paper, an ingot of niobium-niobium silicide based in-situ composites (RMICs) was prepared by arc melting process, and Nb-Nb3Si/ Nb5Si3 in-situ composites with a uniformly orientated microstructure were produced in a high temperature gradient directional solidification apparatus named Electron Beam Floating Zone Melting (EBFZM). The relationships between the processing parameters and the characteristics of the solidified microstructure have been investigated. The influence of the microstructure on the mechanical properties has been revealed and the rupture mechanism at room temperature has been discussed.It has been found that the temperature gradient ahead of the solid/liquid interface is almost linearly increased with the increasing of withdrawing rate. The microstructure of as-cast RMICs is composed of primary Nb-based solid solution dendrites and two-phase eutectic colonies of Nb3Si or Nb5Si3 and Nb-based solid solution. The strengthening phases (Nb3Si or Nb5Si3) has been refined by the directional solidification process. The amount of Nb3Si/Nb5Si3 increases, their morphology tends to be more regular, their distribution becomes more homogeneously with the increasing of withdrawing rate. The microstructure is better oriented along the longitudinal axis of the samples when the withdrawing rate is between0.6 and 6.0mm/min. At the same withdrawing rate, the microstructure changes from the beginning part to the ending part of the samples. According to the analysis of the XRD and the TEM, it has been confirmed that the main microstructure of RMICs is composed of Nb-based solid solution, Nb3Si and Nb5Si3 phases, and the orientation relationship between Nb-based solid solution and Nb3Si phase is [141]Nb3si// [111]Nb.The tensile ruptures of both the as-cast and the directionally solidified samples are of brittle fracture mode at room temperature. Both the tensile strength and elongation of RMICs are increased by the directionally solidification process. The maximum of ob is 778.4MPa and that of KIC is 46.3MPa m1/2. The appearance of the fracture surfaces is brittle cleavage. The cracks extend along the matrix/strengthening-phases interfaces and form the cleavage steps and tear ridges. There appear some very small dimples on the fracture surfaces of the directionally solidified samples, which come from the Nb-based solid solution particles in the big plates of the strengthening phases, but no dimples on the fracture surfaces of the as-cast sample.It can be concluded from above results that niobium-niobium silicide based in-situ composites (RMICs) with a uniformly orientated microstructure can be produced by the EBFZM with high temperature gradient, and their mechanical properties can be increased.