定向凝固Nb-Ti-Si基超高温合金的组织与室温条件断裂韧性

采用真空自耗电弧熔炼法制备了Nb-Ti-Si基超高温合金的母合金锭,在2050℃的熔体温度下实现了合金的有坩埚整体定向凝固.测定了电弧熔炼态与定向凝固试样的室温条件断裂韧性,采用SEM,EDS等方法分析了凝固速率V分别为10,20和50 μm/s的整体定向凝固组织、单边切口梁弯曲试样的断口形貌及裂纹扩展路径,并讨论了其断裂机理.结果表明:合金的整体定向凝固组织主要由沿着试棒轴向挺直排列的横截面为多边形的初生(Nb,X)5Si3 (X代表Ti,Hf和Cr元素)棒与耦合生长的层片状Nbss/(Nb,X)5Si3共晶团(Nbss表示铌基固溶体)组成.整体定向凝固显著提高合金的室温条件断裂韧性KQ,且V=50μm/s时的最高,达16.1 MPa·m1/2,较电弧熔炼态试样的KQ提高了50.5％.定向凝固试样中Nbss与(Nb,X)5Si3沿垂直于受力方向的定向排列以及粗糙的Nbss产生的裂纹桥接和偏转,增大了裂纹扩展阻力,从而提高了合金的室温条件断裂韧性.

Microstructure and Room Temperature Fracture Toughness of Directionally Solidified Nb-Ti-Si Based Ultrahigh Temperature Alloy

The master alloy ingot with the nominal composition of Nb-20Ti-16Si-6Cr-5Hf-4Al-2B-0.06Y (at%) was prepared using vacuum consumable arc-melting.The integrally directional solidification of the alloy was conducted in a self-made high vacuum and ultrahigh temperature directional solidification furnace with the use of ceramic crucibles at the melt temperature of 2050℃.Room temperature fracture toughness of both arc-melted and integrally directionally solidified specimens have been measured by single-edge notched bending test.The directionally solidified microstructure,fractographs and crack propagation path of single-edge notched bending specimens of the alloy at different solidifying rate V (10,20 and 50 μm/s) have been investigated by SEM and EDS analyses.The fracture mechanism of the single-edge notched bending test has been discussed.The results show that the integrally directionally solidified microstructure of the alloy is mainly composed of hexagonally cross-sectioned primary (Nb,X)5Si3 columns and coupled lamellar Nbss/(Nb,X)5Si3 eutectic colonies both aligned straightly and uprightly along the growth direction (here X represents Ti,Hf and Cr elements,Nbss denotes Nb solid solution).The directional solidification effects are remarkable.Both the average diameter of eutectic cells and lamellar spacing in them decrease with the increase in solidifying rate.The directional alignment of both Nbss and (Nb,X)5Si3 normal to the notch provides more resistance to the crack initiation and propagation,and therefore the room temperature fracture toughness KQ is improved significantly by the integrally directional solidification.The maximum KQ value occurs for the directionally solidified specimens with V=50 μm/s and is about 16.1 MPa·m1/2.