Dynamic Recrystallization Behavior of a Directionally Solidified Superalloy

Abstract

Directionally solidified (DS) nickel-base superalloys are widely used for turbine blades in aircraft engines and industrial gas turbines. The surface recrystallization, detrimental to mechanical properties of DS superalloys, is an influencing factor in the engineering failures. [1][2][3][4] For conventional polycrystalline metals, recrystallization is an effective approach to optimizing microstructure and thus to improving mechanical property, such as grain refining and strength improving. Industrially, the static recrystallization temperature is at which the metal with 70 % cold deformation mostly accomplishs its recrystallization course in one hour. The dynamic recrystallization refers to the recrystallization process during the hot working operation which temperature is higher than the static recrystallization one. [4] However, for DS superalloy, aim of recrystallization researches is to provide effective measures to reduce or even supress surface recrystallization of DS components. When DS components are deformed by the improper operations such as severe grit-blating or polisihing and subsequently heated above the static recrystallization temperature, the static recrystallization occurs. [5][6][7][8] The static recrystallization temperature of DS nickel-base superalloy, dominated by the solution behavior of gamma prime (c′), is close to the dissolution temperature of c′ and about 0.8 ∼ 0.9 times the absolute melting temperature. [9][10] In addition, DS components may be dynamically recrystallized because of the rigorous service conditions even if the operation temperature is slightly lower than that of the static recrystallization.Little information has been available in the open literature on the dynamic recrystallization of DS superalloys, especially on the dynamic recrystallization mechanism. [11][12][13][14] This paper investigates the dynamic recrystallization behavior of a DS nickel-base superalloy. The main purpose is to find out the dynamic recrystallization mechanism and thus to provide measures against the dynamic recrystallization of DS components. Experimental Details MaterialThe alloy investigated was produced by directional solidification and the longitudinal axis was in the [001] direction with not more than 10°deviation. The nominal chemical composition in weight percentage was as follows: C 0.14, Cr 9.5, Co 6.0, W 5.3, Mo 3.8, Al 6.0, Ti 1.8, B 0.02, and balance Ni. The DS nickel-base superalloy was treated by solution (1220°C/4 h, air cool) and subsequent aging (870°C/32 h, air cool). In the heattreated state, the superalloy had a uniform fine cuboidal c′ precipitate, typically 0.1 lm ∼ 0.15 lm in size and approximately 0.6 vol. fraction. The static recrystallization temperature of this superalloy is above 1000°C. [15] Testing ProcedureFull size creep specimens with the gage portion of 52 mm in length and 10 mm in diameter were machined from the DS superalloy. Tensile creep tests were carried out using the single lever type creep machine at three temperatures, i.e. 800°C, 850°C and 900°C und...

Effect of cyclic oxidation on the tensile behavior of directionally solidified CM-247LC Ni-based superalloy at 870°C

“…Oblak and Owczarski [15] reported this phenomenon in as early as 1968 where they observed recrystallization in cold swaged Udimet-700 alloy when it was annealed at 980 • C. Subsequently, many others have also reported the phenomenon in various equiaxed, DS and single crystal superalloys [1,3,7,8,16]. In general, RX in DS superalloys occurs during heat treatment or long-term service when the starting strained ␥-␥ structure of the superalloy transforms into strain-free equiaxed grains of ␥-␥ .…”

“…The oxide growth stresses generated during the high temperature exposure (either cyclic or isothermal) might also have a limited contribution to the driving force for the above phase transformation [1,8]. In most previous studies, large prior strains (up to 50%) have been imposed for inducing RX inside a superalloy [15,16].…”

“…The thickness of this zone for B10 and B100 samples was 40 and 50 m, respectively. The formation of such a zone, which is usually called as recrystallized (RX) zone, has been previously reported in case of both DS and single crystal Ni-base superalloys [1][2][3]7,8]. It is known that the ␥-␥ structure of the starting DS substrate undergoes cellular transformation during oxidation leading to the formation of the above RX zone [1].…”

“…However, the effect of surface damage caused by oxidation exposure on the mechanical properties of DS superalloys has remained a relatively less investigated area. Although limited studies have been carried out in creep and fatigue [7,8], very little information is available on how the oxidation-related surface damage affects the tensile properties of DS superalloys [3].…”

Recrystallization and its influence on microstructures and mechanical properties of laser solid formed nickel base superalloy Inconel 718

“…It was found that the LSFed nickel based superalloy Inconel 718 was mainly composited of γ-phase matrix in the form of columnar dendrite growing almost in the deposition direction, resulted in the obvious anisotropic structures and mechanical properties [7][8][9]. Owing to the repeated rapid heating and rapid cooling along with the moving of high power laser beam, residual thermal stresses were remained in the LSFed materials, which is beneficial for driving the recrystallization process in subsequent heat treatment.…”