Effect Of B On The Microstructure Of A Single Crystal Ni-Based Superalloy

Nickel-base superalloys with excellent high temperature performance, good oxidation resistance and corrosion resistance, are widely used in the field of aviation industry and industrial gas turbines. The development of nickel-base superalloys determines the development of the aviation turbine engines, also decide to the development of the aviation industry. There have many investigations about the influence of the alloy composition to the single crystal superalloy, which mainly focus on the refractory elements rhenium, ruthenium and minor elements carbon and hafnium. However, studies about effects of the minor element boron on the single crystal superalloy are lacking. In this paper, experimental nickel-base superalloys containing different contents of boron are directional solidified under high thermal gradient. The influence of minor element boron on as-cast organization and after heating treatment microstructures are investigated. The main conclusions can be drawn as follows:1. The tested samples with different boron additions are all the dendritic structure. The?/??eutectic and carbides distribute in the interdendritic region. The volume fractions of eutectic increase with the content of boron increasing. The ?? phase in the outward eutectic become more bulky. Under the condition of boron increasing, the volume fraction of carbides also increase, and its morphology has a tendency from the layer to the block and its size become more smaller.2. The primary dendrite arm spacing and ?? phase have not great influence by the boron additions. The rule of element segregation on the dendritic structure with the boron additions can be included as follows: The value of K' can reach to the maximum when the content of boron is 0.010 wt.%, indicating that Re and W seriously segregate on dendrite dry. The K' of Mo and Cr is gradually close to 1, which means the uniform distribution of Mo and Cr in theorganization. Other elements Ni and Co have no significant change.3. The rule of solute partition coefficients on the Solid/Liquid surface: the solute coefficients of Re and W increase significantly. The coefficients of Mo and Ta can reach to the maximum when the content of boron is 0.005%. The solute partition coefficients of Cr and Co have no significant change, which is close to 1.4.Both the liquidus temperature and the range of solidification temperature decreased slowly with the content of boron increasing, but the precipitation temperature of ?? phase increaseS. The ?? phase size changes from the disorder distribution to the uniform distribution after different heat treatment processes, and the morphology of ?? phase become from the irregular shape of square shape to the rules.5.The morphology of ?? phase changes from the regular square to the round with the increase of the B addition, that is to say, B has a power that make the square corner of ?? phase become more inactivating. The boron addition has no obvious effect on the size of ?? phase after the heat treatment proceses.