Influence of heat treatment on microstructure and tensile behavior of a hot isostatically pressed nickel-based superalloy

Abstract

A nickel-based superalloy powder RR1000 has been hot isotatically pressed (HIPed) and heat treated to produce different microstructures. Microstructures were investigated using a scanning electron microscope (SEM). Tensile testing was performed at room temperature and 700ºC and the deformed samples were examined using SEM and transmission electron microscope (TEM). It was found that in the as-HIPed condition the microstructure consisted of coarse and irregularshaped primary and secondary γ′ together with a low volume fraction of fine γ′ (<50nm in diameter). Solution treatment below the γ′ solvus followed by air cooling resulted in the formation of finer cuboidal secondary γ′ (350nm-750nm) and medium-sized spherical tertiary γ′ (100nm-200nm). This led to an improvement of both the 0.2% yield strength and ultimate tensile strength. Ageing of the solution-treated or of the as-HIPed samples at 760°C resulted in the precipitation of a high population of fine γ′ (around 50nm) which further increased the strength. Within the resolution limit of the current TEM analysis, deformation at room temperature seemed to occur mainly by dislocations cutting through secondary γ′ and very fine γ′, accompanied by the formation of stacking faults within these precipitates; most of the mediumsized tertiary γ′ precipitates in solution-treated and aged samples were not cut through but were surrounded by dislocations. Deformation at 700℃ happened by dislocations cutting through γ′ precipitates and γ matrix, leading to the formation of extended stacking faults across both γ and γ′. It is suggested that the optimum treatment of the powder superalloy is to solution-treat and age after HIPing; this results in properties which are comparable with those of thermomechanically processed samples.

Influence of Heat Treatment on Microstructures and Mechanical Properties of NiCuCrMoTiAlNb Nickel-Based Alloy

“…In general, the morphology of γ phase is complicated and diverse, and it is closely associated with the heat-treatment regime. Qiu et al stated that in a hot isostatical pressed nickel-based alloy, solution treatment below the solvus of γ phase leads to the formation of finer secondary γ phase with a cuboidal shape as well as a medium-size tertiary with a spherical shape [19]. Qiu et al found that during aging heat treatment of the DZ483 nickel-based superalloy, most of the γ phases exhibit a typical cuboidal shape in the dendrite core, whereas some of the γ phases show a butter-fly shape in the interdendritic zone [26].…”

“…The interaction between dislocations and γ precipitates has an important influence on the plastic deformation mechanisms of nickel-based alloy. The plastic deformation mechanisms of Ni-based alloys are substantially influenced by deformation temperatures, where dislocation slip or dislocation climb plays a key role [18,19,35]. In addition, several novel deformation mechanisms are also found in a few nickel-based alloys containing γ precipitates, which result from heat treatment.…”

“…Heat treatment significantly influences the microstructures of nickel-based superalloys, which further affect the corresponding mechanical properties [14][15][16][17]. In the recent literature, some researchers have reported that appropriate heat treatment modes can be used for enhancing strength [18,19], increasing hardness [20], and improving creep strength [21]. It can be generally accepted that the ability of appropriate heat treatment to enhance the mechanical properties of Ni-based alloys results from precipitation of intermetallic phases.…”

Review and Analysis of Powder Prior Boundary (PPB) Formation in Powder Metallurgy Processes for Nickel-based Super Alloys

“…However, for most AA powder, three intrinsic problems can arise when it is HIPed. They are: prior particle boundary (PPB) precipitate networks [19][20][21][22], thermal induced porosity (TIP) and ceramic inclusion [23]. During HIPing, trace amounts of gas entrapped in powder particles could be compressed and dissolved into the powder particle surface under high temperature and high pressure; As a consequence, during subsequent high temperature heat treatment, TIP may be found in the HIPed super alloys.…”

Hot Ductility of Incoloy 901 Produced by Vacuum Arc Remelting

“…Concern of low hot ductility is why the hot deformation behavior of superalloys has drawn much attention in the recent decades. [1][2][3][4][5] The existence and segregation of detrimental elements such as Sulfur and phosphorous to the grain boundaries has been known very detrimental to the intrinsic ductility of steels and superalloys. 6) For this reason, the advanced techniques such as electro-slag remelting (ESR) and vacuum arc remelting (VAR) are often used for clean ingot casting of special steels and superalloys.…”