Microstructure And Property Of Eutectic Superalloy Directionally Solidified Under High Temperature Gradient

In this paper, two high temperature gradient directional solidification apparatus with liquid metal cooling (LMC) and electron-beam floating zone melting (EBFZM) were used. Longitudinal and transverse metallographic specimens for microscopic observation were prepared using conventional techniques. The morphology evolution was observed with NEOPHOT- 1 metaloscope and AMRAY 1 OOB SEM. The volume fraction, the transverse area and the spacing of the TaC fibers were measured with Leica Quantimet 500 instrument. JEM-2000CX TEM was used to observe the y phase. In order to optimize the parameter of preparation for the composite, role of the composition and solidification rate and temperature gradient on primary phase precipitation behavior were researched. On these basis, the effect of solidification rate on the solidified structure, cooperation growth zone and microstructures such as TaC rods volume fraction, TaC rods average spacing, TaC average transverse area and y'phase in Ni-TaC system eutectic were systematically investigated with LMC. The relationship between solidification processing controlling and solidification rnicrostructure was established. The influence of the microstructure on mechanical property, and rupture mode of the composite at the room and elevated temperature were discussed. It was found that primary phases precipitating could be bated in some extent by positive temperature gradient and low solidification rate. Lamellar eutectic composite with compositions up to 2.9 wt.% away from the eutectic could be grown with solidification rate varying from 1 .05~tm/s to 7.1 4km/s. The spacing is linear with reciprocal value of growth rate square root. The lamellar spacing obeys the relationship with X2V= 83.14.tm/s in this eutectic system. During directional solidification of Ni-Nb near eutectic alloy by EBFZM, the composite structures could be prepared from off-eutectic alloy at a velocity of I .67~.tm/s. When the rate is higher than I .7j.tm/s, the primary phase precipitates and has stochastic orientation, and the plate-like thickness is uneven. The eutectic changes from regular orientation to unstable orientation, finally to orientation disorder with increasing traversing velocity. The inter-rod spacing and transverse area of TaC, as a function of solidification rate, are given as X=8.414v4985 and S=.7485v+5.594 in Ni-TaC composites. The inter-rod spacing of TaC obeys relationship with X=12.935v~498, and the transverse area of TaC versus solidification rate is S= .657v+5.1235 in Ni-TaCI6 composite. The volume fraction of TaC can be changed with increasing solidification rate. In the composite of Ni-TaC and Ni-TaC 16, the TaC-matrix interface structure is similar to regular broken line. In the meanwhile, irregular broken line and wave mode and fishbone like interface morphologies were observed by TEM. The fibers of eutectic directionally grow by a model of a pyramid. The varieties of TaC transverse section are mainly influenced by edge-instability of TaC and growth-rate anisotropy of crystal planes. The tensile strength and ductility of the Ni-TaC 16 composites at room temperature is obviously improved with increasing the volume fraction of TaC and decreasing the inter-rod spacing and the diameter. The rupture mode of the composite at room and elevated temperature depends upon the work-hardening characteristics of the nickel alloy matrix.