The effect of nitrogen on secondary grain nucleation in nickel base single crystal superalloys

Abstract

The commercial casting process for single crystal turbine components produces a significant number of aerofoils which are rejected due to the growth of secondary grains that form high angle boundaries with the principal crystal. Many of these secondary grains occur as a consequence of preferred solidification from geometric features on the component, or by nucleation from freckle chains which occur due to constitutional segregation effects. However, manufacturing experience shows that there are secondary grains in single crystal castings which occur from no obvious source. This paper summarises the work carried out to investigate the possible origins of such grains. The work described in this paper covers those solidification events which are influenced by nucleation and growth under directional solidification conditions. From an evaluation of potential heterogeneous nuclei, the research identified nitrogen gas as the most powerful agent in promoting secondary grains. The probable source of nitrogen was identified as originating from the outgassing of the investment mould when cast under vacuum. The desorption of nitrogen is followed by the formation of titanium nitride compounds which act as secondary grain nuclei by either forming large inclusions or films which interrupt dendrite growth, or which act as substrates for atomic attachment from the liquid metal and subsequent crystal growth.