Effect of boron addition on the microstructure and stress-rupture properties of directionally solidified superalloys

Abstract

This study is focused on the effect of boron addition, in the range of 0.0007wt% to 0.03wt%, on the microstructure and stress-rupture properties of a directionally solidified superalloy. With increasing boron content in the as-cast alloys, there is an increase in the fraction of the γ′/γ eutectic and block borides precipitate around the γ′/γ eutectic. At a high boron content of 0.03wt%, there is precipitation of lamellar borides. Upon heat treatment, fine block borides tend to precipitate at grain boundaries with increasing boron content. Overall, the rupture life of the directionally solidified superalloy is significantly improved with the addition of nominal content of boron. However, the rupture life decreases when the boron content exceeds 0.03wt%.

The Minor Precipitation at the Final Stage of U720Li Solidification

“…c 0 ), which has also been presented in Ref. [19]. In addition, B is rejected into the residual liquids during the whole solidification process because it is a typical positive segregation element.…”

The role of boron in modifying the solidification and microstructure of nickel-base alloy U720Li

“…So it is almost impossible for boron to get into the octahedral sites of the face-centered cubic g dendrite during solidification. As the solidification proceeds, boron atoms are continually expelled into the liquid, then a boron rich layer is formed at the front of the solid/liquid interface [27], and the higher the boron content the thicker the layer. The boron rich layer naturally acts as an obstacle for the transition of constituent elements from the liquid to the solid and also the backward reaction.…”

“…In this case, it is important to know how boron influencing the eutectic precipitation in U720Li alloy. However, the mechanisms by which boron affects eutectic (g þ g 0 ) formation are not well understood [27].…”

Effect of Titanium on Hot Deformation Behaviors of Boron Microalloyed Steel

“…Generally speaking, hardenability occurs because boron atoms easily segregate to prior austenite grain boundaries, lowering the grain boundary energy while retarding the transformation of austenite to ferrite. [9][10][11] Advantageously, grain boundary segregation of boron in steel occurs via two mechanisms: (1) equilibrium and (2) non-equilibrium segregation. Qualitatively, non-equilibrium segregation proves to be the dominant contributing factor in conventional heat treatment conditions of steel.…”