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Recrystallization of Single Crystal Nickel-Based Superalloy
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ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2009, 16(6): 75-79
Recrystallization of Single Crystal Nickel-Based Superalloy ZHANG Bing' ,
TAO Chun-hu' , LU Xin" , LIU Chang-kui' , HU Chun-yan' , BAI Ming-yuan'
(1. Failure Analysis Center of AVIC, Beijing Institute of Aeronautical Materials, Beijing 100095, China; Materials Science and Engineering. University of Science and Technology Beijing, Beijing 100083, China)
2. School of
Abstract: A series of experiments of investigating the recrystallization of single crystal DD3 superalloy were carried out. The threshold temperature for recrystallization and the effect of annealing temperature on recrystallization were studied. The results show that the threshold temperature for recrystallization of the shot-peened DD3 samples is between 1 000 'C and 1 050 'C under the condition of annealing for 2 h , and the recrystallization depth increases with the rise of the annealing temperature. Below 1 150 'c, the recrystallization depth increases slowly with the temperature climbing, while above 1 150 ·C, the recrystallization depth increases quickly with the rise of the temperature. The solution of the y' phase is a critical factor of the recrystallization behavior of DD3 superalloy. In addition. the kinetics and microstructural evolution of recrystallization at 1 200 'C were also studied. It is found that the recrystallization progresses rapidly at 1 200 'C through the growth of fully developed recrystallized grains, and the recrystallization process on the shot-peened surface is similar to that of wrought materials. including nucleation of recrystallization, growth of new grains into the matrix, and growth of new grains by swallowing up each other. Key words: single crystal superalloy; recrystallization; shot peening
Because of the excellent high-temperature mechanical properties, single crystal blades and vanes have been introduced into most of the advanced military and civil aircraft engines. Single crystal superalloys were developed to overcome the limited mechanical performance of polycrystalline materials at high temperature. Their superior mechanical properties enable an increased service temperature and thereby an improved overall efficiency of turbines. The superior high-temperature mechanical properties of single crystal superalloys mainly result from the elimination of the grain boundaries perpendicular to the main stress axis[I]. However, recrystallization caused by residual strain is a well-known problem in the production and service of single crystal superalloy parts. There are several possible sources for the necessary plastic deformation during manufacturing and processing of the single crystal parts as well as during service and reconditioning. Since the new grains may introduce disadvantageous orientations and high-angle grain boundaries, they will dra-
matically reduce the creep rupture strength and fatigue life of the single crystal superalloy parts[2.3]. Recrystallization phenomena have been studied extensively in the past several decades. Among these studies, most attentions have been paid to the wrought and powder metallurgy superalloys[4-S] whose microstructures can be optimized by recrystallization during processing. Detailed studies on the recrystallization of single crystal superalloys have seldom been carried out[9.10]. In the present work, the effects of temperature and time on the recrystallization of 003 superalloy have been studied, and the microstructural evolution during recrystallization has also been investigated.
1
Experimental
The material tested in this work is a first-generation single crystal superalloy, 003. The composition of 003 superalloy (mass percent, %) is shown in Table 1. The screw crystal selector and HRS (high rate
Foundation Item: Item Sponsored by Open Foundation of Nondestructive Test Key Laboratory of Chinese Ministry of Education (ZD200729011) Biography:ZHANG Bing(] 978-) • Male. Doctor Candidate; E-mail: [email protected]; Revised Date: December 23, 2008
• 76 •
Journal of Iron and Steel Research. International
Vo!. 16
---------
Table I
%
Chemical composition of DD3 superalloy
C
Cr
Co
W
Mo
AI
Ti
'\Ii
0.009
9. 50
5. 20
0.36
3.63
5.60
2.24
Balance
~50
250
solidification) techniques were employed to produce superalloy 003 in an ISP2/3-0S vacuum induction furnace. Then as-cast 003 superalloy was solutiontreated at 1 250 e for 4 h and aged at 870e for 32 h. The orientation along the axis of the 003 single crystal bars was less than 15° from <001>. Specimens with thickness of 10 mm were cut down along the axis of the bars. The (001) faces of the specimens were ground and then shot peened by steel balls for 3 min under the pressure of o. 3 MPa. In order to avoid oxidation. the samples were tubed in silica glass tubes filled with argon gas. Some samples were annealed at different temperatures ranging from 900 to 1 250 e for 2 h to determine the initial temperature for recrystallization and to investigate the effect of annealing temperature on the recrystallization of D03. To investigate the kinetics and microstructural evolution of recrystallization, some samples were annealed at 1 200 C for different times from 2 min to 4 h. All the heat treatments were followed by air cooling. Metallographic observations were carried out with an optical microscope and a scanning electron microscope (Cambridge S-360)' Metallographic specimens were prepared by metallographic polishing. The polished specimens were etched in the reagent consisting of 50 mL Hel and 50 mL Hj O,;
2 2. 1
Results and Discussion Effect of temperature on recrystallization
After annealing at different temperatures for 2 h , the depth of the recrystallized layers of the shot-peened samples was measured. The relationship between the recrystallization depth and the annealing temperature is shown in Fig. 1. It can be seen that the threshold temperature for recrystallization of the shot-peened samples is between 1 000 C and 1 050C under the condition of annealing for 2 h. At the temperature below 1 150C. the recrystallization depth increases slowly with the temperature climbing. At the temperature above 1 150C. the recrystallization depth increases rapidly with the rise of temperature. The effect of second phase particles on the growth of recrystallized grains is related to the volume fraction (F v) and particle size (r) of the second phase. It is
E
:f ~
150
CI
50
o ----1 000
Fig. 1
1 050
1 100 1 150 Ternperature/T'
1 200
1 250
Recrystallization depth vs annealing temperature (annealed for 2 h)
generally accepted that, if Fv/r>O. 2 rm-I. recrystallization will be inhibited! II. In the 003 samples after solution and aging heat treatments, the y' volume fraction (Fv) is as high as 63 %, and the average size of the cubical coherent y' particles (r) is about o. 3 rID 12 • F \' / r is about 2 rm -I , far greater than o. 2 rm I, so it can be concluded that the presence of the y' particles will severely retard the growth of the recrystallized grains. At the temperature below 1 150C, only a small fraction of y' particles have been dissolved, so the grain boundaries move slowly inwards owing to the pinning and retardation of the residual y' particles. At the temperature above 1 150C, with the rise of temperature, the amount of the dissolved y' particles increases rapidly, and the reduction of retardation to migrating grain boundaries leads to the rapid increase in the recrystallization depth. The different dissolving characters of the y' phase in different temperature ranges determine the rate of recrystallization. Annealing temperature has a significant effect on the recrystallization mode of 003. At the temperature below 1 200 C, cellular discontinuous recrystallization may be observed (Fig. 2). The strainfree recrystallized cells consist of large columnar y' particles in the y matrix. The columnar y' particles elongate in the growth direction of the cells. The columnar y' phase in the cellular microstructure is different from the cubical y' phase in the matrix in both shape and size, so it can be assumed that original y' was dissolved at the recrystallization interface and then reprecipitated as columnar y' particles in the new grains. Oblak J M et al studied cellular recrystallization in a nickel-based superalloy by transmis-
Recrystallization of Single Crystal Nickel-Based Superalloy
Issue 6
Fig. 2
Cellular recrystallization microstructure after heat treatment at 1 100 't for 2 h
sion electron microscopy'". They found that the orientation of the y' in the cellular microstructure was different from that of the v' in the matrix, and the recrystallizaion front was a high-angle interface. Therefore, dissolution of original t' must have occurred in the vicinity of the high-angle interface. They assumed that the dissolution of original y' in the vicinity of the high-angle interface was a critical factor of recrystallization. Cellular microstructure was also observed by Porter A et a1C4] when they studied the recrystallization of several nickel-based superalloys of different y volume fractions. According to their studies, the dissolution and reprecipitation of the y phase were attributed to the high solubility and diffusivity of the migrat recrystallizationinterface. During the migration of recrystallized grain boundaries, the recrystallization interface takes nearly all the y' that encounters into solution. As a result, the migrating grain boundaries become
(a) Overview;
Fig. 3
• 77 •
supersaturated with y'-forming elements soon. This supersaturation is unstable and is relieved by the production of large columnar y' particles, At 1 200 °C, rapid recrystallization occurs in the surface layer through the growth of recrystallized grains (Fig. 3). The size and shape of y' particles in the recrystallized grains are similar to those of y' particles in the matrix. Annealing twins can be clearly seen in the recrystallized grains. The annealing twins resulted from the redistribution of atomic layers at the advancing boundaries'F". In the DD3 samples after solution and aging heat treatments, the smaller size of y' particles leads to lower y' solvus temperature. 1 200 'C is high enough to dissolve nearly all v' particles into the matrix. This means that there are few residual y' particles to be dissolved during the migration of recrystallized grain boundaries. Low local solute concentration near the migrating boundaries 'cannot provide sufficient supersaturation for the nucleation of large columnar y' particles, before substantial diffusion of solute along the boundaries to other precipitation sites occurs. During cooling down, some large y' particles precipitate at the boundaries because of the local solute concentration.
2. 2
Kinetics and microstructural evolution The recrystallization kinetics was studied by measuring the depth of recrystallized layers formed at 1 200 'C for different annealing times from 2 min to 4 h. The results are shown in Fig. 4. Recrystallization of DD3 occurs very rapidly at 1 200 ·C. After annealing for 2 min, recrystallized grains have' occurred. The grain boundaries of the new grains move
(b) Magnification
Microstructures of the recrystallized grains after heat treatmentat 1 200 't for 2 h
• 78 •
Journal of Iron and Steel Research. International
Vol. 16
--------------------
•
200
•
180 160
~ 140
5
~ 120
Col
100 80 60
0
50
100
150
200
250
Time/min
Fig. 5 Fig. 4
Recrystallization depth vs time (annealed at 1 200 "C )
rapidly inwards from the surface, and after 1 h , the growth of the new grains inwards is almost complete. Fig. 5 shows the microstructural evolution after annealing at 1 200 'C for 4 min. It can be seen that recrystallization begins on the shot-peened surface, and then the new grains grow inwards into the matrix. Grain nucleation merely starts from the surface for the following two reasons. One reason is that nucleation along a free surface will reduce new interface, which means that less interface energy will be generated. The other is that there is a strain gradient in the deformed layer from the surface. This gradient might cause nucleation of new grains on the surface and growth of new grains inwards. The depth of the recrystallized grains differs, depending on the precise location. Recrystallization progresses much faster at the dendritic core regions than at the interdendritic regions. On the shot-peened surface, recrystallization first occurs in the dendritic core regions where y' dissolves faster [Fig. 6 (a)]. Then the recrystallized grains grow into the interdendritic regions where y' dissolves much more slowly. After annealing for almost 14 min, the primary recrystallization on the
Fig. 6
Microstructure of the recrystallized grains after heat treatment at 1 200 'c for 4 min
surface is finished [Fig. 6 (b)]. Then the new grains on the surface continue to grow up by swallowing up each other with the prolongation of annealing time [Fig. 6 (c)]. The recrystallization process on the shot-peened surface is similar to that of wrought materials, including nucleation of recrystallization, growth of new grains into the matrix, and growth of new grains by swallowing up each other. Fig. 7 schematically displays the recrystallization process on the shot-peened surface with the prolongation of annealing time.
3
Concl usions
(1) Under the condition of annealing for 2 h , the threshold temperature for recrystallization of the shot-peened samples is between 1 000 'C and 1 050e. At the temperature below 1 150C, the recrystallization depth increases slowly with the rise of temperature. At the temperature above 1 150 'c, the recrystallization depth increases rapidly with the rise of temperature. ( 2) Annealing temperature has a significant effect on the recrystallization mode of DD3. At the temperature below 1 200e, cellular discontinuous
Recrystallization process on the shot-peened surface
Recrystallization of Single Crystal Nickel-Based Superalloy
Issue 6
Fig. 7
Schematic representation of recrystallizaion process on shot-peened surface
recrystallization may be observed. At the temperature above 1 ZOO ·c, rapid recrystallization occurs in the surface layer through the growth of recrystallized grains. ( 3) Recrystallization of DD3 occurs very rapidly at 1 ZOO ·C. After annealing for Z min, recrystallized grains have occurred. After 1 h , the growth of the new grains inwards from the surface is almost complete. (4) On the shot-peened surface, recrystallization first occurs in the dendritic core regions where t' dissolves faster. Then the recrystallized grains grow into the interdendritic regions where "'{' dissolves much more slowly. After the primary recrystallization is finished, the new grains on the surface continue to grow up by swallowing up each other with the prolongation of annealing time. The recrystallization on the shot-peened surface is similar to that of wrought materials, including nucleation of recrystallization, growth of new grains into the matrix, and growth of new grains by swallowing up each other.
[3J
[4J [5J
[6J
[7J
[8J
[9J
[10J
[11J
References : [lJ
[2J
• 79 •
TAO Chun-hu, ZHANG Wei-fang. LI Yun-ju, et al, Recrystallization of Directionally Solidified and Single Crystal Superalloy [J]. Failure Analysis and Prevention, 2006. 1(4): 1 (in Chinese). Goldschmidt D. Paul U, Sahm P R. Porosity Clusters and Recrystallization in Single-Crystal Components [A]. Antolovich S D, Stusrud R W, MacKay R A, et al , eds, Superalloys 1992
[12J
[13J
[CJ. Warrendale: TMS, 1992. 155. Khan T, Caron P, Nakagawa Y G. Mechanical Behavior and Processing of DS and Single Crystal Superalloys [JJ. Journal of Metals, 1986,38: 16. Porter A, Ralph B. The Recrystallization of Nickel-Base Superalloys [JJ. Journal of Materials Science, 1981, 16: 707. Porter A J, Brian Ralph. Recrystallization of a Nickel-Base Superalloy , Kinetics and Microstrutrural Development [JJ. Materials Science and Engineering. 1983.59: 69. Oblak J M. Owczarski W A. Cellular Recrystallization in a Nickel-Base Superalloy [I]. Transactions of the Metallurgical Society of AIME. 1968. 242: 1563. Winberg L, Dahlen M. Recrystallization in a Powder Metallurgy Nickel-Base Superalloy [I]. Journal of Materials Science, 1978, 13(11): 2365. HU Ben-Iu , CHEN Huan-rning , JIN Kai-sheng , et al. Static Recrystallization Mechanism of FGH95 Superalloy [n. The Chinese Journal of Nonferrous Metals. 2004. 14(6): 901 (in Chinese). Bond S D, Martin J W. Surface Recrystallization in a Single Crystal Nickel-Based Superalloy [I]. Journal of Materials Science, 1984. 19: 3867. Burgel R, Portella P D. Preuhs J. Recrystallization in Single Crystals of Nickel Base Superalloys [AJ. Pollock T M. Kissinger R D. Bowman R R. et al , eds, Superalloys 2000 [CJ. Warrendale: TMS. 2000. 229. CHEN De-hou, WEI Shu-yue , WU Zhong-tang , et al. The Creep Behaviour of a Nickel-Base Single Cystal Superalloy [JJ. Journal of Materials Science and Technology, 1986, 2: 378. Lindsley B. Pierron X. Sub-Solvus Recrystallization Mechanisms in UDlMET~ Alloy 720LI [AJ. Pollock T M. Kissinger R D, Bowman R R. el al , eds, Superalloys 2000 [CJ. Warrendale: TMS, 2000. 59. Cullity B D. Elements of X-Ray Diffraction [MJ. 2nd ed, Reading: Addison-Wesley, 1978.
ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2009, 16(6): 75-79
Recrystallization of Single Crystal Nickel-Based Superalloy ZHANG Bing' ,
TAO Chun-hu' , LU Xin" , LIU Chang-kui' , HU Chun-yan' , BAI Ming-yuan'
(1. Failure Analysis Center of AVIC, Beijing Institute of Aeronautical Materials, Beijing 100095, China; Materials Science and Engineering. University of Science and Technology Beijing, Beijing 100083, China)
2. School of
Abstract: A series of experiments of investigating the recrystallization of single crystal DD3 superalloy were carried out. The threshold temperature for recrystallization and the effect of annealing temperature on recrystallization were studied. The results show that the threshold temperature for recrystallization of the shot-peened DD3 samples is between 1 000 'C and 1 050 'C under the condition of annealing for 2 h , and the recrystallization depth increases with the rise of the annealing temperature. Below 1 150 'c, the recrystallization depth increases slowly with the temperature climbing, while above 1 150 ·C, the recrystallization depth increases quickly with the rise of the temperature. The solution of the y' phase is a critical factor of the recrystallization behavior of DD3 superalloy. In addition. the kinetics and microstructural evolution of recrystallization at 1 200 'C were also studied. It is found that the recrystallization progresses rapidly at 1 200 'C through the growth of fully developed recrystallized grains, and the recrystallization process on the shot-peened surface is similar to that of wrought materials. including nucleation of recrystallization, growth of new grains into the matrix, and growth of new grains by swallowing up each other. Key words: single crystal superalloy; recrystallization; shot peening
Because of the excellent high-temperature mechanical properties, single crystal blades and vanes have been introduced into most of the advanced military and civil aircraft engines. Single crystal superalloys were developed to overcome the limited mechanical performance of polycrystalline materials at high temperature. Their superior mechanical properties enable an increased service temperature and thereby an improved overall efficiency of turbines. The superior high-temperature mechanical properties of single crystal superalloys mainly result from the elimination of the grain boundaries perpendicular to the main stress axis[I]. However, recrystallization caused by residual strain is a well-known problem in the production and service of single crystal superalloy parts. There are several possible sources for the necessary plastic deformation during manufacturing and processing of the single crystal parts as well as during service and reconditioning. Since the new grains may introduce disadvantageous orientations and high-angle grain boundaries, they will dra-
matically reduce the creep rupture strength and fatigue life of the single crystal superalloy parts[2.3]. Recrystallization phenomena have been studied extensively in the past several decades. Among these studies, most attentions have been paid to the wrought and powder metallurgy superalloys[4-S] whose microstructures can be optimized by recrystallization during processing. Detailed studies on the recrystallization of single crystal superalloys have seldom been carried out[9.10]. In the present work, the effects of temperature and time on the recrystallization of 003 superalloy have been studied, and the microstructural evolution during recrystallization has also been investigated.
1
Experimental
The material tested in this work is a first-generation single crystal superalloy, 003. The composition of 003 superalloy (mass percent, %) is shown in Table 1. The screw crystal selector and HRS (high rate
Foundation Item: Item Sponsored by Open Foundation of Nondestructive Test Key Laboratory of Chinese Ministry of Education (ZD200729011) Biography:ZHANG Bing(] 978-) • Male. Doctor Candidate; E-mail: [email protected]; Revised Date: December 23, 2008
• 76 •
Journal of Iron and Steel Research. International
Vo!. 16
---------
Table I
%
Chemical composition of DD3 superalloy
C
Cr
Co
W
Mo
AI
Ti
'\Ii
0.009
9. 50
5. 20
0.36
3.63
5.60
2.24
Balance
~50
250
solidification) techniques were employed to produce superalloy 003 in an ISP2/3-0S vacuum induction furnace. Then as-cast 003 superalloy was solutiontreated at 1 250 e for 4 h and aged at 870e for 32 h. The orientation along the axis of the 003 single crystal bars was less than 15° from <001>. Specimens with thickness of 10 mm were cut down along the axis of the bars. The (001) faces of the specimens were ground and then shot peened by steel balls for 3 min under the pressure of o. 3 MPa. In order to avoid oxidation. the samples were tubed in silica glass tubes filled with argon gas. Some samples were annealed at different temperatures ranging from 900 to 1 250 e for 2 h to determine the initial temperature for recrystallization and to investigate the effect of annealing temperature on the recrystallization of D03. To investigate the kinetics and microstructural evolution of recrystallization, some samples were annealed at 1 200 C for different times from 2 min to 4 h. All the heat treatments were followed by air cooling. Metallographic observations were carried out with an optical microscope and a scanning electron microscope (Cambridge S-360)' Metallographic specimens were prepared by metallographic polishing. The polished specimens were etched in the reagent consisting of 50 mL Hel and 50 mL Hj O,;
2 2. 1
Results and Discussion Effect of temperature on recrystallization
After annealing at different temperatures for 2 h , the depth of the recrystallized layers of the shot-peened samples was measured. The relationship between the recrystallization depth and the annealing temperature is shown in Fig. 1. It can be seen that the threshold temperature for recrystallization of the shot-peened samples is between 1 000 C and 1 050C under the condition of annealing for 2 h. At the temperature below 1 150C. the recrystallization depth increases slowly with the temperature climbing. At the temperature above 1 150C. the recrystallization depth increases rapidly with the rise of temperature. The effect of second phase particles on the growth of recrystallized grains is related to the volume fraction (F v) and particle size (r) of the second phase. It is
E
:f ~
150
CI
50
o ----1 000
Fig. 1
1 050
1 100 1 150 Ternperature/T'
1 200
1 250
Recrystallization depth vs annealing temperature (annealed for 2 h)
generally accepted that, if Fv/r>O. 2 rm-I. recrystallization will be inhibited! II. In the 003 samples after solution and aging heat treatments, the y' volume fraction (Fv) is as high as 63 %, and the average size of the cubical coherent y' particles (r) is about o. 3 rID 12 • F \' / r is about 2 rm -I , far greater than o. 2 rm I, so it can be concluded that the presence of the y' particles will severely retard the growth of the recrystallized grains. At the temperature below 1 150C, only a small fraction of y' particles have been dissolved, so the grain boundaries move slowly inwards owing to the pinning and retardation of the residual y' particles. At the temperature above 1 150C, with the rise of temperature, the amount of the dissolved y' particles increases rapidly, and the reduction of retardation to migrating grain boundaries leads to the rapid increase in the recrystallization depth. The different dissolving characters of the y' phase in different temperature ranges determine the rate of recrystallization. Annealing temperature has a significant effect on the recrystallization mode of 003. At the temperature below 1 200 C, cellular discontinuous recrystallization may be observed (Fig. 2). The strainfree recrystallized cells consist of large columnar y' particles in the y matrix. The columnar y' particles elongate in the growth direction of the cells. The columnar y' phase in the cellular microstructure is different from the cubical y' phase in the matrix in both shape and size, so it can be assumed that original y' was dissolved at the recrystallization interface and then reprecipitated as columnar y' particles in the new grains. Oblak J M et al studied cellular recrystallization in a nickel-based superalloy by transmis-
Recrystallization of Single Crystal Nickel-Based Superalloy
Issue 6
Fig. 2
Cellular recrystallization microstructure after heat treatment at 1 100 't for 2 h
sion electron microscopy'". They found that the orientation of the y' in the cellular microstructure was different from that of the v' in the matrix, and the recrystallizaion front was a high-angle interface. Therefore, dissolution of original t' must have occurred in the vicinity of the high-angle interface. They assumed that the dissolution of original y' in the vicinity of the high-angle interface was a critical factor of recrystallization. Cellular microstructure was also observed by Porter A et a1C4] when they studied the recrystallization of several nickel-based superalloys of different y volume fractions. According to their studies, the dissolution and reprecipitation of the y phase were attributed to the high solubility and diffusivity of the migrat recrystallizationinterface. During the migration of recrystallized grain boundaries, the recrystallization interface takes nearly all the y' that encounters into solution. As a result, the migrating grain boundaries become
(a) Overview;
Fig. 3
• 77 •
supersaturated with y'-forming elements soon. This supersaturation is unstable and is relieved by the production of large columnar y' particles, At 1 200 °C, rapid recrystallization occurs in the surface layer through the growth of recrystallized grains (Fig. 3). The size and shape of y' particles in the recrystallized grains are similar to those of y' particles in the matrix. Annealing twins can be clearly seen in the recrystallized grains. The annealing twins resulted from the redistribution of atomic layers at the advancing boundaries'F". In the DD3 samples after solution and aging heat treatments, the smaller size of y' particles leads to lower y' solvus temperature. 1 200 'C is high enough to dissolve nearly all v' particles into the matrix. This means that there are few residual y' particles to be dissolved during the migration of recrystallized grain boundaries. Low local solute concentration near the migrating boundaries 'cannot provide sufficient supersaturation for the nucleation of large columnar y' particles, before substantial diffusion of solute along the boundaries to other precipitation sites occurs. During cooling down, some large y' particles precipitate at the boundaries because of the local solute concentration.
2. 2
Kinetics and microstructural evolution The recrystallization kinetics was studied by measuring the depth of recrystallized layers formed at 1 200 'C for different annealing times from 2 min to 4 h. The results are shown in Fig. 4. Recrystallization of DD3 occurs very rapidly at 1 200 ·C. After annealing for 2 min, recrystallized grains have' occurred. The grain boundaries of the new grains move
(b) Magnification
Microstructures of the recrystallized grains after heat treatmentat 1 200 't for 2 h
• 78 •
Journal of Iron and Steel Research. International
Vol. 16
--------------------
•
200
•
180 160
~ 140
5
~ 120
Col
100 80 60
0
50
100
150
200
250
Time/min
Fig. 5 Fig. 4
Recrystallization depth vs time (annealed at 1 200 "C )
rapidly inwards from the surface, and after 1 h , the growth of the new grains inwards is almost complete. Fig. 5 shows the microstructural evolution after annealing at 1 200 'C for 4 min. It can be seen that recrystallization begins on the shot-peened surface, and then the new grains grow inwards into the matrix. Grain nucleation merely starts from the surface for the following two reasons. One reason is that nucleation along a free surface will reduce new interface, which means that less interface energy will be generated. The other is that there is a strain gradient in the deformed layer from the surface. This gradient might cause nucleation of new grains on the surface and growth of new grains inwards. The depth of the recrystallized grains differs, depending on the precise location. Recrystallization progresses much faster at the dendritic core regions than at the interdendritic regions. On the shot-peened surface, recrystallization first occurs in the dendritic core regions where y' dissolves faster [Fig. 6 (a)]. Then the recrystallized grains grow into the interdendritic regions where y' dissolves much more slowly. After annealing for almost 14 min, the primary recrystallization on the
Fig. 6
Microstructure of the recrystallized grains after heat treatment at 1 200 'c for 4 min
surface is finished [Fig. 6 (b)]. Then the new grains on the surface continue to grow up by swallowing up each other with the prolongation of annealing time [Fig. 6 (c)]. The recrystallization process on the shot-peened surface is similar to that of wrought materials, including nucleation of recrystallization, growth of new grains into the matrix, and growth of new grains by swallowing up each other. Fig. 7 schematically displays the recrystallization process on the shot-peened surface with the prolongation of annealing time.
3
Concl usions
(1) Under the condition of annealing for 2 h , the threshold temperature for recrystallization of the shot-peened samples is between 1 000 'C and 1 050e. At the temperature below 1 150C, the recrystallization depth increases slowly with the rise of temperature. At the temperature above 1 150 'c, the recrystallization depth increases rapidly with the rise of temperature. ( 2) Annealing temperature has a significant effect on the recrystallization mode of DD3. At the temperature below 1 200e, cellular discontinuous
Recrystallization process on the shot-peened surface
Recrystallization of Single Crystal Nickel-Based Superalloy
Issue 6
Fig. 7
Schematic representation of recrystallizaion process on shot-peened surface
recrystallization may be observed. At the temperature above 1 ZOO ·c, rapid recrystallization occurs in the surface layer through the growth of recrystallized grains. ( 3) Recrystallization of DD3 occurs very rapidly at 1 ZOO ·C. After annealing for Z min, recrystallized grains have occurred. After 1 h , the growth of the new grains inwards from the surface is almost complete. (4) On the shot-peened surface, recrystallization first occurs in the dendritic core regions where t' dissolves faster. Then the recrystallized grains grow into the interdendritic regions where "'{' dissolves much more slowly. After the primary recrystallization is finished, the new grains on the surface continue to grow up by swallowing up each other with the prolongation of annealing time. The recrystallization on the shot-peened surface is similar to that of wrought materials, including nucleation of recrystallization, growth of new grains into the matrix, and growth of new grains by swallowing up each other.
[3J
[4J [5J
[6J
[7J
[8J
[9J
[10J
[11J
References : [lJ
[2J
• 79 •
TAO Chun-hu, ZHANG Wei-fang. LI Yun-ju, et al, Recrystallization of Directionally Solidified and Single Crystal Superalloy [J]. Failure Analysis and Prevention, 2006. 1(4): 1 (in Chinese). Goldschmidt D. Paul U, Sahm P R. Porosity Clusters and Recrystallization in Single-Crystal Components [A]. Antolovich S D, Stusrud R W, MacKay R A, et al , eds, Superalloys 1992
[12J
[13J
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