Microstructures and high-temperature mechanical properties of a directionally solidified Ni-based superalloy: Influence of boron content

Highlights

• High boron promotes the formation of multi-phase eutectic-like constituent.

• Incipiently melted region appears around multi-phase eutectic-like constituent.

• Incipiently melted region consumes secondary γ′ forming elements.

• Incipiently melted region destroys high-temperature tensile and stress rupture properties.

Abstract

The effects of boron content on the microstructure and mechanical properties of a directionally solidified Ni-based superalloy were investigated. The increase of boron content significantly increases the amount of γ/γ′ eutectic, impels the formation of multi-phase eutectic-like constituent, and promotes the morphological transformation of MC carbides from rod-like to blocky. After standard heat treatment, the incipient melting occurs in the high boron alloy, which can lower the dissolving temperature of γ/γ′ eutectic. With increasing boron content, the volume fraction of secondary γ′ phase decreases significantly and the homogeneity and alignment of secondary γ′ show a decrease, and the tensile strengths at 900 °C decrease slightly mainly due to the strengthening effect of much more tertiary γ′ phases. The wide γ matrix channel facilitates the movement of dislocations, which is somewhat beneficial to the elongation, while the incipiently melted region (IMR), the decreased volume fraction of secondary γ′ phase and the less homogeneity of secondary γ′ phase are rather harmful to the elongation. Consequently, the elongation is drastically reduced. The existence of a large amount of IMRs is principally responsible for the reduction of the stress rupture property. Tertiary γ′ phases re-dissolving in γ matrix channel facilitates the movement of dislocations between large secondary γ′ phases, which is beneficial to the elongation of the alloys. In addition, the IMRs provide masses of crack sources, and consume the precipitation strengthening elements (Ti, Ta) and the solid solution strengthening elements (Cr, W and Mo), which plays the major role in the degradation of the high-temperature tensile or stress rupture properties.

Introduction

Boron element generally tends to segregate at grain boundaries (GBs) during the solidification of superalloys, but boron segregation and precipitation of boride were also reported to coexist in the interdendritic regions of EP962 alloy, and the formations of borides took place around the γ/γ′ eutectics during solidification [1]. According to the studies by Yan et al. [2], the addition of boron promoted the formations of γ/γ′ eutectics and low melting phases, and affected the transverse properties of a directionally solidified superalloy. Decker and Freeman [3] thought that the addition of minor elements such as boron and zirconium could enhance the stability and strength of grain boundaries (GBs) in a complex heat-resistant superalloy. It was demonstrated by Zhao et al. [4] that boron played a role for the creep rupture life of a second-generation Ni-based single crystal superalloy DD11. They attributed this phenomenon to the decreases in the γ/γ′ misfit and the solid solution strengthening effect, and the formation of script-like M₃B₂ phase. Bocchini et al. [5] found that boron segregated strongly at GBs without the formation of boride, thus improving the creep strength of the baseline ternary alloy. Zhao et al. [6] investigated the effect of boron and zirconium additions on the hot tearing of Ni-Based Superalloy IN792. The results revealed that zirconium and boron additions enhanced the hot tearing susceptibility due to the formation of continuous liquid films at GBs during the final solidification stage. Garosshen et al. [7] reported that the addition of boron with an amount being more than its solubility limit resulted in the precipitation of M₃B₂ boride, which causes no further improvement in creep rupture properties of a P/M Ni-based Superalloy. Hosseini et al. [8] indicated that boron could increase the mismatch and elastic strain between the γ and γ′ phases, and thus improve the tensile properties of the wrought Ni-based superalloy ATI 718Plus. Despite such large amount of work reporting the role of boron in superalloys [[1], [2], [3], [4], [5], [6], [7], [8]], the underlying mechanism about its effect on mechanical properties is still under debate. Particularly, the excessive B contents would result in mass of multi-phase eutectic-like constituent in DZ444 alloy [9]. The multi-phase eutectic-like constituent was also often observed in many other Ni-based alloys, such as Rene80, IN939 and IN738 [[10], [11], [12]]. However, the influence of this phase on the microstructure and mechanical properties is also not well understood.

In the present work, the directionally solidified Ni-based DZ444 superalloys with two different (low and high) B contents were selected as the target materials, to examine the effect of B content (or the existence of multi-phase eutectic-like constituent) on the heat treated microstructure and high-temperature mechanical properties of DZ444 alloy, and the relevant mechanisms are also discussed.