Si对高Nb-TiAl合金组织及室温拉伸性能的影响

EFFECT OF Si ADDITION ON THE MICROSTRUCTURE AND ROOM TEMPERATURE TENSILE PROPERTIES OF HIGH Nb-TiAl ALLOY

研究硅化物（Nb5Si3相）析出对高Nb-TiAl合金组织及室温拉伸性能的影响.实验结果表明,硅化物脱溶析出温度在1000~1200℃之间,析出物位于片层团晶界处、b（B2）相偏析处以及片层之间.添加Si元素后,合金室温拉伸性能有所增加.因为Nb5Si3相的形成使得b（B2）相稳定元素Nb含量下降,导致脆性相b（B2）相体积减少.但是,含Si高Nb-TiAl合金经过热处理后,室温拉伸性能随热处理温度提高而逐步降低.因为沿片层析出的硅化物会导致裂纹沿片层产生与增殖,而且应力会导致硅化物进一步析出,加速裂纹扩展.而且,Si的加入会导致γ相区扩大,在1280~1300℃之间形成γ单相区.硅化物析出在片层边界处,会导致块状γ＋b（B2）相组织,脆化晶界;而硅化物析出在片层内部会导致二次γ板条形成,割裂了初始片层组织.

High Nb-TiAl alloys, which being regarded as a new generation TiAl alloy, had attracted more and more attention for their higher operating temperature and better oxidation resistance than conventional TiAl alloys.It was found that silicide particles in high Nb-TiAl alloys were Nb5Si3 rather than Ti5Si3 precipitated in TiAl alloys. In this work, the effect of Nb5Si3 phase on the microstructure and room- temperature tensile properties of high Nb- TiAl alloy was studied. The experimental results showed that the precipitation temperature of silicide was between 1000~1200 ℃. Precipitates located in the colony boundary, b（B2） segregation and between γ/a2 lamella. The tensile properties of as-cast alloy with Si addition increased. Because the formation of Nb5Si3 precipitates resulted in the reduction of Nb content, which was one of b（B2） phase stable elements. Therefore, the volume fraction of b（B2） phase obviously decreased due to Si addition. However, after heat treatments, the tensile properties of Si containing high Nb-TiAl alloy gradually reduced with the increasing of heat treatment temperature.Silicide particles which precipitated along lamella leaded to generation and propagation of cracks. Moreover, silicide particles further precipitated due to tensile stress which increased the rate of crack propagation. Si addition leaded to g phase area expanded. γ single-phase region formed between 1280~1300 ℃. Silicide precipitated in colony boundary resulted in bulk γ＋b（B2） phases, which weaken the grain boundaries. While silicide precipitated in lamella leaded to formation of secondary γ lath which split the initial lamella microstructure.