ting of many individual particles,have been observed somewhere in the VIM ingots.The morphology of a typical inclusion cluster in the VIM ingot of GH169 is shown in Fig.1,b.These small particles contained carbon and niobium in a certain degree beside nitrogen and titanium (see Fig.2,b,and Table 2). Fig.2 Wave spectrun photos of pure TiN powder (a)and jnclusions in the VIM Ingot cf GH169 (In-718)(b) The results obtained demonstrated that the pure TiN powder added either adhered to the Al2O3 crucible wall,or decomposed in the liquid metal.From the experimental conditions and results,(ie.1060 ppm TiN powder,having a size of about 20um,completely decomposed in two minites),it was supposed that the decomposition rate of TiN powder be very high. Work 2 indicated that the TiN solubility corresponds to contents between 25 and 40 ppm N for the commonly used forging alloys.From Table 1,it can be seen that the nitrogen content in the original forging bar is higher than 40 ppm,but about 1000 ppm TiN (corresponds to about 240 ppm N)has decomposed in the liquid metals. To explain this result,the possibility of TiN powder decomposition in the liquid metal is discussed. During the VIM process,the reactions CN3-N2() (1) TiN(e)=〔Ti)+〔N) (2) simultaneously occur in the various regions.The denitrification reaction (1) occurs at the vaccum atmosphere/liquid metal interface.The nitrogen content of liquid metal in equilibrium with the partial pressure of nitrogen in vacuum atmosphere is very low,as the low nitrogen alloys (e.g.the In-718 containing 15 ppm N)can be melted in VIM furnace.The decomposition reaction (2)occurs within the melt.The nitrogen content of liquid metal in equilibrium with the 504， · 宜 ， 一 · ， ， 。 刀 卜 一 ， 一 ， ， 召 ， ， 犷 · ， ， 。 ， 。 ， ， 、 、 、 “ 、 玄一 ‘、 “ ‘ ’ 。 〔 〕 〔 〕 。 ， 一 一 一 尸