Study On The Role Of Trace Element Boron In Nickel Base Single Crystal Superalloy

Nickel-based single crystal superalloy has overcome the grain boundary defects and effectively improved the mechanical properties of the alloy.It has become the first choice material for advanced aero-engine and gas turbine blade.Existing researche shows that the microstructure of Ni-base single crystal superalloy has a great influence on the improvement of mechanical properties,and the influence of trance element boron on the microstructure of Ni-base single crystal superalloy is also very significant.Therefore,the third generation nickel base single crystal superalloy DD90 was used as the parent alloy,and four nickel-based single crystal superalloy samples with different boron content were prepared by directional solidification technology,and the prepared samples were treated by different heat treatment processes.This paper mainly study on the effect of trace element boron on microstructure of the casting and after heat treatment,including the dendrite structure,?/?'eutectic structure,the morphology of carbides,?'phase and segregation of elements.At the same time,the carbides and boride phases precipitated in the alloy were analyzed.The main conclusions are as follows:1.The effects of boron addition on the as-cast dendritic structure,?/?'eutectic structure,apparent microporosity and ?' phase were studied.The addition of boron has no obvious effect on the primary dendritic spacing.The number of ?/?'eutectic structure in the alloy increases significantly with the increase of boron content.There are two kinds of eutectic morphologies in the as-cast alloy,namely radial and petal shape.The addition of boron significantly reduced the number of microshrinkage in the alloy.Under the casting state,the size of ?' phase of dendrite core is obviously smaller than that of interdendritic,and the size and volume fraction of ?' phase increase with the increase of boron content.2.The influence of the content of trace element boron on the solidification characteristic temperature of nickle-based single crystal superalloy was studied.The addition of boron obviously reduces the solidus temperature and liquidus temperature of the alloy,and the solidus temperature and the liquidus temperature decreased more obviously with the increase of boron content.The effect of boron addition on the dissolution temperature of ?' phase is not obvious.The crystallization temperature interval also increases with the increase of boron content.3.The effects of boron on dendritic structure,y/y'eutectic structure and ?' phase after heat treatment were studied.The effect of boron on dendrite and y/y'eutectic were basically consistent under three heat treatment process.The number of residual dendrite and eutectic decreases with the increase of boron content.The effect of boron on the size and volume fraction of ?' phase were consistent.The size and volume fraction of y'phase increased with the increase of boron content.But with the change of boron content,the size of y'phase did not increase obviously.The y matrix channel narrowed with the increase of boron content.Compared with the cast state,the size of the ?' phase after heat treatment is obviously smaller than that of the interdendritic,and the volume fraction of the ?' phase is much larger than that of the cast state.In comparison with two step solution treatment and one step solution treatment,it is obvious that the two step solution precipitation ?' phase distribution is uniform,the size is larger,the cubic degree is higher,and the volume fraction is larger.4.The effects of boron on carbides and boride between cast state and heat treatment were studied.In the cast state,skeleton and block carbides were precipitated in four kinds of alloys.And the skeleton carbides and block carbides precipitated in the cast state are all MC-type carbides and rich in Ta.After heat treatment,block carbides were precipitated in all four alloys,and no skeleton carbides was found.After heat treatment,the type of carbide was the same as that of the cast state,and all of them were MC-type carbide rich in Ta.The change of boron content had no obvious effect on the morphology and precipitation type of carbide.The boride phase is precipitated in the alloy with boron addition,the boride precipitates around the carbide and its morphology is smaller.The boride is short rod like in the cast state,the main component of the short rod like boride is Cr,Mo,W,Re,and it is a Cr-rich boride phase.After heat treatment,the morphology of boride is rod and granular,both of which contain Cr,Mo,W and Re.The composition of boride is similar to that of as-cast boride,and it is a Cr-W rich boride phase.After heat treatment,the boride is mostly in the shape of rod.5.The effect of boron on solidification segregation behavior was studied.With the increase of boron content,the segregation degree of W?Re?A1 and Ta in the alloy increased,but the effect of boron on the segregation of Cr?Co and Mo element was not significant.With the increase of boron content,the segregation degree of Re and Ta is more significant than that of W and Al.