| The working blades of aviation turbine engine are mainly made from Ni based alloys presently, and the maximum operating temperature is1100℃, however, the inlet temperature of the new generation turbine engine will reach1200-1400℃, which is beyond the working temperature of Ni based alloy. Compared with Ni based alloys, Nb-Si based alloys possess higher melting points, lower densities (6.6-7.2g/cm3) and excellent high temperature strength. However, the mismatching between the room and high temperature mechanical properties of Nb-Si based alloy has limited their applications, the alloying and directional solidification (DS) process are the effective methods to conquer the problem. Up to now, Nb-Si based alloys own two alloying systems: one is the high strength system, which is alloyed by W, Mo and Mn; the other is the low density system, which is alloyed by Ti, Cr and Al. The previous studies indicate that the room temperature fracture toughness of the former was lower and the density was higher, but the room temperature fracture toughness of the latter was higher and the density was lower. Therefore, the low density system was chosen as the investigated subject in my paper. The mechanical property of Nb-Si based alloy is related with the microstructure, including the volume fraction, size and aspect ratio of Nbss, the species and morphology of silicide and the number of Nbss/Nb5Si3interfaces. However, the microstructure can be controlled by directional solidification processing parameters. The electromagnetic cold crucible directional solidification was used to prepare Nb-Si based alloys in this paper, and the processing parameters included heating power, withdrawal rate and holding time. A series of Nb-Si based alloy samples were prepared successfully by orthogonal test, and the relationship between microstructure and processing parameters, mechanical properties and the relationship between mechanical property and processing parameters had been investigated in this paper.Castep software was used to calculate the effect of Ti, Cr, Al and Hf on the impurity formation energy, total state density, valence charge density and elastic constants of Nb and α-Nb5Si3, and the calculation results were the foundation of composition design. The nominal composition of the investigated Nb-Si based alloy was Nb-22Ti-16Si-3Cr-3Al-2Hf (at.%) in this thesis. The calculation results indicate that Ti, Cr and Al could be solute in Nb; Ti and Hf could be solute in α-Nb5Si3. Ti doped Nb unit cell resulted in the increase of total density state of Fermi surface, Cr and Al doped Nb unit cell caused the decrease of total density state of Fermi surface; Ti and Hf doped in α-Nb5Si3unit cell led to the decrease of total density state of Fermi surface. Ti doped in Nb unit cell posed the valence charge density of the system increase, Cr and Al doped in Nb unit cell caused the valence charge density of the system decrease; Ti and Hf doped in α-Nb5Si3unit cell caused the local degree of valence charge weaken and the valence charge density of the system decrease. Ti doped in Nb unit cell caused the bulk modulus, shear modulus and elastic modulus decrease; Cr doped in Nb unit cell caused the bulk modulus, shear modulus and elastic modulus increase; Al doped in Nb unit cell caused the bulk modulus decrease, shear modulus and elastic modulus increase; Ti and Hf doped in α-Nb5Si3unit cell caused the bulk modulus, shear modulus and elastic modulus decrease.The initial growth zone and skull are the essential parts of the electromagnetic cold crucible directionally solidified samples, and the parts pose material waste, but the length of the initial growth zone and the thickness of the skull can be as small as possible by controlling the processing parameters. The comprehensive assessment result of initial growth zone length and skull thickness indicates that the effect factor order was v>t>P; the optimization processing parameters were P=50kW, v=0.8mm/min, t=3min. The factor order of the effect of processing parameters on the S/L interface instability degree was: t>v>P. With the increase of holding time, the S/L interface instability degree decreased gradually; with the increase of withdrawal rate, the S/L interface instability degree increased gradually; with the increase of heating power, the S/L interface instability degree increased gradually. Those indicate that the increase of holding time, the decrease of withdrawal rate and the increase of heating power were conducive to making the S/L interface planar. The factor order of the effect of processing parameters on the primary dendrite arm spacing and secondary dendrite arm spacing of primary Nbss was: v>P>t. With the increase of withdrawal rate, the primary dendrite arm spacing and secondary dendrite arm spacing of primary Nbss decreased gradually; with the increase of heating power, the primary dendrite arm spacing and secondary dendrite arm spacing of primary Nbss increased gradually; with the increase of holding time, the primary dendrite arm spacing and secondary dendrite arm spacing of primary Nbss increased first, and then decreased.The steady-state growth region microstructures of the DS samples were composed of Nbss, α-Nb5Si3and a little of γ-Nb5Si3. The temperature gradient had an influence on the coupled growth of Nbss/Nb5Si3, the increase of heating power and decrease of withdrawal rate were conducive to the coupled growth of Nbss/Nb5Si3. The effect of withdrawal rate, the mean diameter of eutectic Nbss particles (MD), the average interphase spacing (λ) and the mean diameter of cellular eutectic (DE) decreased gradually. Under different heating power, the relationships between MD, λ, DE, and withdrawal rate (v) were: MD=-4.497v+8.741(45kW), MD=-3.515v+8.013(50kW), MD=-3.886v+7.295(55kW); λ=19.091v-0.485(50kW), λ=32.150v-0.248(55kW); DE=85.823v-0.346(50kW)，DE=66.816v-0.475(55kW). The processing parameters had an effect on the contents of alloying elements in each phase and lattice parameters of the constituent phases.With the increase of withdrawal rate, the room temperature fracture toughness (KQ) decreased first, and then increased; with the increase of holding time, KQ decreased first, and then increased; with the increase of heating power, KQ decreased gradually. During the room temperature fracture process, the fracture of Nbss exhibited the river pattern and tearing ridge, which indicates that both the cleavage fracture and plastic stretching fracture took place in Nbss. Nb5Si3showed the cleavage fracture. The secondary crack and the proper weakening of the Nbss/Nb5Si3interface bond strength were conducive to the improvement of KQ. With the increase of heating power, withdrawal rate and holding time, the high temperature tensile strength increased first, and then decreased. The processing parameters affected the microstructure of Nb-Si based alloy, including grain boundary and phase boundary, and the microstructure determined the mechanical properties of the alloy. The comprehensive assessments of the room temperature fracture toughness and high temperature tensile strength suggest that the optimization processing parameter was: P=50kW, v=0.4mm/min, t=6min. |
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