Effects of secondary precipitation on recrystallization in Co-base superalloy DZ40M
ZHAO Yang(赵 阳), WANG Lei(王 磊), YU Teng(于 腾), SONG Xiu(宋 秀)
Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education,
Northeastern University, Shenyang 110004, China
Received 28 July 2006; accepted 15 September 2006
Abstract: A series of experimental studies were conducted on the recrystallization of directionally solidified cobalt-base superalloy DZ40M. It is found that the secondary M23C6 precipitation influences the size and shape of the recrystal grains. When the annealing temperature is below 1 473 K, a large amount of the fine secondary M23C6 precipitations are distributed around the primary carbides, and such carbides impede the movement of grain boundary because the effect, the size and shape of recrystal grains become irregularly. When the temperature exceeds 1 473 K, the recrystal grains grow rapidly due to the dissolved secondary M23C6 precipitation.
Key words: DZ40M alloy; Co-base superalloy; secondary precipitation; recrystallization; pinning force
1 Introduction
Because of less grain boundaries perpendicular to the orientation of stress directionally solidified alloys have excellent properties such as the high heat-resistance, fatigue strength, creep strength and ductility. However, there are several possible sources for plastic deformation during manufacturing and application, such as mechanically polish and grit-blasting and local stress concentration[1-2]. It is quite unsafe that the recrystallization phenomenon takes place in the blades with such plastic deformation during the heat treatment or working process. Recrystallization may greatly reduce the fatigue life and the stress rupture strength of the directionally solidified blades[3-4]. Therefore, recrystallization is seriously pernicious for the mechanical properties and must be avoided in directionally solidified superalloy.
DZ40M alloy is a new directionally solidified Co-base superalloy developed by Institute of Metal Research of Chinese Academy of Sciences in recent years[5]. Many investigations have been done on the microstructure and mechanical properties of DZ40M alloy[5-7]. However, little information is available in the open literature concerning the recrystallization of DZ40M alloy. The purpose of the present research is to investigate the effect of secondary precipitation on recrystallization in directionally solidified cobalt-base superalloy DZ40M.
2 Experimental
The master alloy was prepared in a conventional vacuum furnace with a mold withdrawal device. The nominal composition (mass fraction, %) of the alloy is: Ni 11, Cr 25, W 7.5, Mo 0.2, Ti 0.15, Ta 0.25, Al 0.8, Zr 0.15, C 0.45, B 0.05, balance Co. The board (220 mm×7 mm×12 mm) of the alloy was produced at a withdrawal speed of 7 mm/min and with a thermal gradient of about 50 K/cm at the solid/liquid interface. All the samples for experiments were cut by EDM.
The samples were indented with different loads at room temperature using Brinell testing machine HBD-3000 with the spherical indenter of diameter 5 mm. The sample shape and load direction were shown in Fig.1.
Following indentation, the samples were placed in a preheat furnace and annealed at 1 323-1 573 K. In order to maintain the microstructure of the alloy at different annealing temperatures, all the samples were water quenched after annealing. The microstructures were examined with OLYMPUS GX71 optical microscope. The etched solution is CuCl2(5 g)+HCl(100 mL) +C2H5- OH(100 mL). Energy dispersive analysis of X-ray (EDAX) was conduced on SHIMADZU SSX-550 scanning electron microscope (SEM).
Fig.1 Sample shape and load direction
3 Results and discussion
3.1 Microstructure under different conditions
The microstructure of as-cast DZ40M alloy is composed of cobalt-base solid solution and the primary carbides M7C3 and MC. The solid solution matrix is columnar grains with〈001〉direction parallel to the growth axis[6-7]. Fig.2 shows the microstructures of the as-cast and deformed DZ40M alloys. It can be seen that the alloy matrix is well-developed columnar grains with 〈001〉direction parallel to the sample axis. After deformation, the first dendrite axis at the bottom of indentation is distorted. Deformation is obtained by indentation to accelerate the recrystallization behavior.
After annealing, the microstructure is changed obviously (Fig.3). The primary carbides are thinned out considerably and a large number of fine profusion particles are distributed, agglomerating around the primary carbides. The recrystallization microstructure can be seen at the bottom of the indentation. The size and shape of the recrystal grains are irregular. And some twin crystals can be seen in the recrystal grains.
3.2 Relationship between precipitation and recrysta- llization
As shown in Fig.3, the primary carbides dissolve after annealing and a large number of fine secondary precipitates appear. The microstructure of the secondary precipitations was examined by scanning electron microscopy (SEM) with an energy dispersive analysis of X-ray (EDAX) an shown in Fig.4. EDAX indicates that the precipitates are Cr-rich carbides. In Fig.4, relatively high cobalt peak is resulted from the alloy matrix due to narrowness of the fine precipitates and a limited resolution of EDAX. At a higher temperature, the as-cast alloy is thermodynamically unstable. The primary carbides, both M7C3 and MC carbides dissolve sluggishly and the secondary M23C6 carbide precipitates quickly. They concentratively distribut around the primary carbides and exiguously distribut in the center of grains[7].
Fig.2 Microstructures of as-cast (a) and deformed (b) DZ40M alloy
Fig.3 Recrystallization microstructure after annealed at 7.5 kN, 1 393 K for 60 min: (a) Lower magnification; (b) Higher magnification
In DZ40M alloy, the carbides contribute significantly to strengthening and the finer secondary carbides pin up dislocation easily and leads to the precipitation hardening effect[7]. Meanwhile, the existence of these precipitates may inevitably exert great influence on the recrystallization process[8-9]. Fig.5 shows the microstructures of recrystal grain and the secondary precipitations. Once nucleation, the recrystallized grains will grow rapidly into the surrounding deformed region where a large amount of second precipitations exist (Figs.5(a) and (b)). As recrystallization proceeds, i.e. grain-boundary migration continues, it will inevitably be overcome by pinning force of the precipitations[9-10]. As shown in Figs.5(c) and (d), the secondary carbides at the grain boundary impede the movement of dislocation and suspend the recrystallization process. Because of the pinning force of the precipitations, the size and shape of the recrystal grains become irregular. It is indicated that the secondary precipitation has a great effect on the size and shape of the recrystal grains.
Fig.4 SEM image(a) and EDAX spectrum(b) of secondary precipitation at 1 223 K for 60 min
Fig.5 Secondary precipitations near recrystal grain boundaries: (a) 10 kN, 1 423 K, 30 min; (b) 15 kN, 1 423 K, 30 min; (c) 15 kN,
1 323 K, 120 min; (d) 10 kN, 1 423 K, 60 min
Fig.6 shows the recrystallization microstructures of DZ40M alloy at different annealing temperatures. Both the fraction of secondary precipitations and recrystal grain size are changed obviously. When the temperature is higher, the recrystal grains become larger and the fraction of secondary precipitations decreases. At 1 373 K, a higher fraction of precipitation appears in the matrix. When the annealing temperature exceeds 1 473 K, the secondary precipitations can be rarely observed.
The effect of the pinning force of the small particles on the grain boundary migration is determined by using the well-known equation: FZ=3f γb/D, where, f is the volume fraction of the grains, γb, boundary energy, and D, the diameter of the precipitates[11]. In this calculation, γb is about 1/3γH to be 0.5-0.6 J/m2. At 1 373 K, f ≈23.31% and D≈1.84 μm, giving FZ≈20.90 MPa. However, at 1 473 K, f ≈7.14% and D≈1.90 μm, giving FZ≈6.20 MPa. The Zener force largely decreases with the decrease of the precipitations in DZ40M alloy.
To illustrate the change of recrystallization microstructure with the decrease of the pinning force the size of the recrystal grains and the depth of the recrystallization which is the depth of the recrystallization area beneath indentation were measured. It can be seen from Fig.7 that both the recrystal grains and the depth of the recrystallization have a larger increasing velocity at 1 473 K corresponding to the bally low fraction of precipitation. The secondary carbides could be mostly dissolved at the temperature above 1 423 K in Co-base superalloy[12]. So, the secondary precipitation volume fraction is very low for the dissolving behavior and the limited pinning of the grain boundaries is expected at 1 473 K. As a result, the recrystal grain size and the depth of recrystallization increase rapidly with the decrease of the fraction of secondary precipitation when the temperature is higher than 1 473 K.
Fig.6 Recrystallization microstructures annealed at different temperatures(15 kN, 60 min, WQ): (a) 1 373 K; (b) 1 423 K; (c) 1 473 K; (d) 1 523 K
Fig.7 Grain size (a) and average depth (b) of recrystallization at different annealing temperatures for 60 min
4 Conclusions
1) The secondary precipitation strongly influences the size and shape of the recrystal grains by retarding to the recrystal boundary migration.
2) The secondary M23C6 carbide precipitates around the primary carbides in DZ40M alloy and the primary carbides are thinned out greatly after annealing.
3) Since the pinning force of the secondary precipitation decreases rapidly at the temperature over 1 473 K, the recrystal grain size and the depth of the recrystallization increase faster.
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(Edited by CHEN Wei-ping)
Corresponding author: WANG Lei; Tel: +86-24-83687725; E-mail: wanglei@mail.neu.edu.cn
