| At present the maximum working temperature for Ni-based superalloy has been approaching 1150℃, which exceeds eighty percent of the melting point of Ni-based superalloys, therefore Ni-based superalloys can’t meet the requirement of next generation aeroengine turbine engine, that they should work at the temperature between 1200℃ and 1400℃. In contrast, Nb-Si based alloys has high melting point(above 1900℃), low density(6.6~7.2g/cm3), high rigidity, good high temperature strength and excellent high temperature creep resistantce, which make it one of high temperature structural materials with potential. However, the toughness of Nb-Si alloys at room temperature is low, and its high temperature strength is not enough. Therefore, it has importantly theoretical and practical significance to prepare alloys with excellently comprehensive mechanical properties at room and high temperature.The alloy this paper chosen was Nb-Ti-Si based alloy, which was successfully prepared to Nb-22Ti-16Si-3Cr-3Al-2Hf(at.%) square ingots with different processing parameters by electromagnetic cold crucible directionally solidification technology with the addition of a certain of Titanium element. The effect of directionally solidified processing parameters, such as heat power and withdrawal rate, on the structure and mechanical properties was analyzed. The directional structure, with coupled growth(Nb,Ti)ss/(Nb,Ti)5Si3 along the solidification direction, was acquired and exhibits good comprehensive mechanical properties.Macrostructure of directionally solidified samples was composed of initial growth zone, directional growth zone which accounted for more than 60% of the entire length of samples, final solidification zone, skull and solid-liquid interface. The initial growth zone, which contained big(Nb,Ti)5Si3 blocks with sharp corners, had little difference with indirectional cast structure. Fine α-(Nb,Ti)5Si3 evenly distributed in(Nb,Ti)ss matrix in the skull zone. The solid-liquid interface presented ―ω‖ type at the macro, which was related to the motion staggered fluid swinging in the hump, and dendritic growth or cell dendritic growth at the micro related to the relative growth speed of(Nb,Ti)ss phase and(Nb,Ti)5Si3 phase. The final solidification zone was composed of fine equiaxed dendrite(Nb,Ti)ss with(Nb,Ti)5Si3 distributing among them.The(Nb,Ti)ss phase and(Nb,Ti)5Si3 phase were coupled growth paralleling to the withdrawal direction in the directional growth zone in samples, with eutectic structures between them. And eutectic structures mainly presented three types of morphologies, namely mesh, dendritic cells and lamellar. The transversal section was composed of primary α-(Nb,Ti)5Si3 blocks, eutectic cells and eutectic intercellular after the samples were cut cross. With the increasing of withdrawal rate, the content of primary α-(Nb,Ti)5Si3 phase decreased or even disappeared. Most structure in the eutectic cell were(Nb,Ti)ss+α-(Nb,Ti)5Si3, but(Nb,Ti)ss+γ-(Nb,Ti)5Si3 in the eutectic intercellular. The Titanium element having relatively low melting point was easy to be rich in eutectic intercellular and developed to be Ti-rich(Nb,Ti)5Si3 phase. That was γ-(Nb,Ti)5Si3 which most distributed in eutectic intercellular zone.The average diameter of eutectic cells decreased with the increase of withdrawal rate, but increased first and then decreased with the increase of heat power. The processing parameters had little effect on the content of ductile(Nb,Ti)ss phase, and it was hard to enhance the content of(Nb,Ti)ss phase significantly. With the increase of heat power, as well as withdrawal rate, the interphase spacing showed a tendancy of decreasing.The Vickers hardness of directional growth zone in the whole samples, which approximately presented linear correlation with the the content of ductile(Nb,Ti)ss phase V(Nb,Ti)ss, was between 609.59 HV and 703.64 HV and satisfied the relationship HV=1143.63-9.91V(Nb,Ti)ss. And the hardness of γ-(Nb,Ti)5Si3 phase was higher than that ofα-(Nb,Ti)5Si3 phase. The fracture toughness of cast alloys was 8.93 MPa·m1/2, but after directionally solidification it can be enhanced to 13.21 MPa·m1/2 at the most. This was related to tortuous crack propagation path and toughening mechanism in directional structure, including crack deflection, bridging, branching and so on. The room temperature fracture toughness of directional solidified alloys was obviously higher than that of cast alloys. In addition, it increased with the increasing withdrawal rate, and increased first then slightly decreased with the increasing heat power. The cast compression specimen presented shear fracture with 45°, but the directionally solidified specimen fractured paralleling to the axial direction with tearing ridges and cleavage facet in the fractography.The tensile strength level at 1250℃ of directionally solidified alloys was preceded that of cast alloys and the maximum rise was 53%. The fractography plane was all perpendicular to the loading direction.(Nb,Ti)5Si3 was obvious brittle facture morphology with large cleavage facet and(Nb,Ti)ss was ductile fracture morphology with tearing ridges. The arrangement that silicide directionally distributed in(Nb,Ti)ss matrix, which followed the strengthen mechanics of fiber reinforced composite materials, was able to effectively improve the high-temperature tensile properties. |
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