The microstructure of a single crystal superalloy after different aging heat treatments
来源期刊：Rare Metals2018年第3期
论文作者：Xiao-Dai Yue Jia-Rong Li Xiao-Guang Wang
文章页码：210 - 216
摘 要：To study the influence of aging heat treatments on the microstructure of single crystal superalloys with high content of refractory elements and optimal the aging heat treatment regimes, a single crystal superalloy containing 22 wt% refractory elements was investigated.Results show that for the experimental alloy, even the homogenization-solution heat treatment for 25 h cannot homogenize the alloying elements completely. During primary aging heat treatment, γ’ phase grows larger and turns to regular cubes. Higher aging temperature induces larger γ’ cubes. For specimens with primary aging heat treated at 1120 ℃,γ’ morphology does not change apparently during secondary aging heat treatment. For specimens with primary aging heat treatment at 1150 ℃,γ’phase in interdendrite grows obviously comparing with that in dendrites. By analyzing the precipitating kinetics of γ’phase, it is found that owning to the dendrite segregation and different aging heat treatment temperatures, γ’ phase at different regions grows under the control of different factors at different aging heat treatment stages. The two controlling factors that are driving forces of phase transformation and element diffusion rate induce obviously different growth rates of γ’ phase. As a result, the γ’-precipitating behaviors are variable based on different solute concentrations and aging temperatures. For advanced single crystal superalloys that are supposed to be used at relatively high temperatures, the final γ’ size after aging heat treatment is suggested to be close to the crossing point of diffusion controlling curve and driving force controlling curve corresponding to the serving temperature. And then,high-temperature properties can be improved.
稀有金属(英文版) 2018,37(03),210-216
Xiao-Dai Yue Jia-Rong Li Xiao-Guang Wang
Science and Technology on Advanced High Temperature Structural Materials Laboratory,Beijing Institute of Aeronautical Materials
收稿日期：8 October 2014
基金：financially supported by the Foundation of Beijing Institute of Aeronautical Materials (No.KJSJ150109);
Xiao-Dai Yue Jia-Rong Li Xiao-Guang Wang
Science and Technology on Advanced High Temperature Structural Materials Laboratory,Beijing Institute of Aeronautical Materials
Abstract：
To study the influence of aging heat treatments on the microstructure of single crystal superalloys with high content of refractory elements and optimal the aging heat treatment regimes, a single crystal superalloy containing 22 wt% refractory elements was investigated.Results show that for the experimental alloy, even the homogenization-solution heat treatment for 25 h cannot homogenize the alloying elements completely. During primary aging heat treatment, γ' phase grows larger and turns to regular cubes. Higher aging temperature induces larger γ' cubes. For specimens with primary aging heat treated at 1120 ℃,γ' morphology does not change apparently during secondary aging heat treatment. For specimens with primary aging heat treatment at 1150 ℃,γ'phase in interdendrite grows obviously comparing with that in dendrites. By analyzing the precipitating kinetics of γ'phase, it is found that owning to the dendrite segregation and different aging heat treatment temperatures, γ' phase at different regions grows under the control of different factors at different aging heat treatment stages. The two controlling factors that are driving forces of phase transformation and element diffusion rate induce obviously different growth rates of γ' phase. As a result, the γ'-precipitating behaviors are variable based on different solute concentrations and aging temperatures. For advanced single crystal superalloys that are supposed to be used at relatively high temperatures, the final γ' size after aging heat treatment is suggested to be close to the crossing point of diffusion controlling curve and driving force controlling curve corresponding to the serving temperature. And then,high-temperature properties can be improved.
Keyword：
Single crystal superalloy; Aging heat treatment; γ' precipitation; Kinetics;
Author： Xiao-Dai Yue e-mail:yuexiaodai0126@126.com;
Received： 8 October 2014
1 Introduction
For aero-engines,increasing turbine entry temperature is the most effective approach to improve thermodynamic efficiency.Accordingly,excellent high-temperature properties are necessary for materials in the hot section of aeroengines,in particular the turbine blades
Nowadays,nickel-based single crystal superalloys are regarded as the main material for turbine blades of aeroengines as this class of material offers a good balance of required properties under severe high-temperature conditions.To cater for the increasing service temperature of turbine blades,more and more refractory elements are added to newer generations of single crystal superalloys
These refractory elements that include Mo,Re,Ta,W,and Nb may give solid-solution strengthening to the matrix phase and precipitate phase,and increase the alloy mechanical properties at high temperatures.At the same time,the segregation tendency is also increased because of the sluggish solute diffusivity of these refractory elements
The heat treatment of single crystal superalloys involves a homogenization-solution heat treatment and several aging heat treatments.The homogenization-solution heat treatment is to dissolveγ'and homogenize alloying elements.The aging heat treatment is to adjustγ'morphology.Most of the aging heat treatments of single crystal superalloys consist of a primary aging heat treatment and a secondary aging heat treatment.Sometimes,a third aging heat treatment is also included.After the aging heat treatment process,fine and regularγ'cubes will distribute in theγmatrix.
Currently,most heat treatment studies of single crystal superalloys focus on homogenization-solution heat based on the fact that it is increasingly difficult for newer generations of single crystal superalloys to homogenize alloying elements
In this study,a single crystal superalloy containing22.0 wt%refractory elements was investigated.After an identical homogenization-solution heat treatment for 25 h,different primary aging heat treatments were done at 1120and 1150℃,respectively.Then,all specimens underwent the same secondary aging heat treatment.After observing the microstructures at different stages,theγ'-precipitating kinetics was studied.Based on all experimental and analyzing results,suggestions about designing the aging heat treatment for newer generations of single crystal superalloys were put forward.
2 Experimental
To cater for the component characteristic of high-generation single crystal super alloys,the refractory elements(W+Mo+Ta+Re+Nb) were added in the experimental single crystal superalloy (hereafter called SX4 for short) sum up to 22.0 wt%.In addition,a certain amount of Ru was also added.
The single crystal bars (15 mm in diameter and 170 mm in length) were cast with crystal selection method in a directionally solidified furnace.Experimental specimens with15 mm in diameter and 10 mm in length were cut from above bars.Based on abundant thermal tests,two heat treatment regimes,as listed in Table 1,were chosen.In Table 1,the solution temperature 1340℃was about 5℃lower than the incipient melting temperature.The homogenization and solution time were decided after considering the homogenization efficiency,solution effect,and practical feasibility for industrial manufacture comprehensively.After homogenization-solution heat treatment,specimens were separated into two groups and underwent different primary aging heat treatments whose temperatures were 1120 and 1150℃,respectively.The former one was the general primary aging heat treatment temperature of most single crystal superalloys,and the later one was designed to cater for the temperature capability of the fifth-generation single crystal superalloys.Finally,an identical secondary aging heat treatment process was conducted,which was also a general secondary aging heat treatment regime for most single crystal superalloys.
After etching the specimens with a solution of CuSO4+HCl+H2SO4+H2O,the microstructures were examined by optical microscope (OM,Leica DM-4000 M)and field emission scanning electron microscopy (FESEM,JEOL S-4800).Then,the experimental results were analyzed with the assistant of kinetics analysis.Based on above analysis,suggestions on designing aging heat treatment regimes for newer generations of single crystal superalloys were put forward.
3 Results and discussion
3.1 As-cast microstructure
The as-cast microstructures of SX4 alloy are shown in Fig.1.Figure 1 demonstrates obvious dendrite segregationin the as-cast specimen.Regular primary and secondary dendrite arms are shown in Fig.la.No third dendrites arm can be found.Theγ'phase in interdendrites is much larger than that in dendrites because mostγl-forming elements segregate in interdendrites during solidification.
Table 1 Heat treatment regimes conducted in the experiment
Fig.1 Microstructures of as-cast SX4 alloy:a OM image of dendrites structure,b FESEM image ofγ/γ'in dendrite,c FESEMimage ofγ/γ'in interdendrite,and d FESEM image of eutectic structure
Three specimens were taken for each stage,and five fields of each specimen were chosen to calculate theγ'sizes in dendrites and interdendrites.The meanγ'sizes for respective stages and regions are calculated by Eq.(1),as given in Table 2.
Table 2 Meanγ'size of SX4 at different stages (HS:homogeniza tion-solution)
where r isγ'size (assumingγ'phase distributes as regular cubes in the y matrix,and r represents the length of cube side),A is the actual area of the field view,and
3.2 Microstructure after homogenization-solution heat treatment
The microstructures of SX4 alloy after homogenizationsolution heat treatment are shown in Fig.2.After homogenization-solution heat treatment,eutectic is eliminated completely.No incipient melting exists.Fine and irregularγ'cubes exhibit in both dendrites and interdendrites.That is becauseγ'phase cannot precipitate completely and turns to regular cubes during rapid air cooling process.Moreover,γ'cubes in interdendrites are obviously larger than those in dendrites,which means that dendrite segregation still exists after a homogenization-solution heat treatment as long as 25 h,including solution heat treatment for 13 h.
3.3 Microstructures after aging heat treatment
Micros true tures after primary aging heat treatments are shown in Figs.3 and 4.It can be seen that allγ'cubes grow larger and turn to regular cubes.Theγ'cubes in Group 2are larger than those in Group 1,and this difference is much more obvious in interdendrites than in dendrites.Ideal negative mismatch can be identified from the high cubical level.The size gaps between dendrites and interdendrites still exist.The mean sizes ofγ'cubes in dendrites and interdendrites were also calculated by Eq.(1),and the results are given in Table 2.
Fig.2 Microstructures of SX4 alloy after homogenization-solution heat treatment:a OM image of dendrite,b FESEM image ofγ/γ'in dendrite,and c FESEM image ofγ/γ'in interdendrite
Fig.3 FESEM images of SX4 alloy with primary aging heattreatment temperature of 1120℃:aγ/γ'in dendrites after primary aging heat treatment,bγ/γ'in interdendrites after primary aging heat treatment,cγ/γ'in dendrites after secondary aging heat treatment,and dγ/γ'in interdendrites after secondary aging heat treatment
Fig.4 FESEM images of SX4 alloy with primary aging heattreatment temperature of 1150℃:aγ/γ'in dendrites after primary aging heat treatment,bγ/γ'in interdendrites after primary aging heat treatment,cγ/γ'in dendrites after secondary aging heat treatment,and dγ/γ'in interdendrites after secondary aging heat treatment
Here,it is interesting to notice that after secondary heat treatment,γ'cubes do not change obviously in both dendrites and interdendrites for Group 1,while for Group 2 theγ'cubes in interdendrites grow larger apparently compared with those in dendrites.
The microstructures of specimens after secondary aging heat treatment with greater enlargement in Fig.5 show that no second and thirdγ'phases are observed in y channel.That is becauseγ'phase is fully precipitated after a secondary aging heat treatment as long as 32 h at 870℃.
Fig.5 FESEM images of SX4 alloy of specimens after secondary aging heat treatments:a primary aging heat treatment at 1120℃and b primary aging heat treatment at 1150℃
4 Discussion
According to above experimental result,both primary aging heat treatments makeγ'phase grow larger and turn to regular cubes.After that,γ'phase becomes more cubical during secondary aging heat treatment.
For all specimens,γ'size gaps exist all the time.But theγ'growth regulations are different for the two groups.For Group 1,γ'sizes hardly change after secondary aging heat treatment.As a result,γ'size gap does not change.For Group2,γ'cubes in interdendrites grow larger during secondary aging heat treatment,while those in dendrites hardly change obviously,which leads to a larger size gap ofγ'cubes.
In spite ofγ'size,theγ'-precipitating behaviors are also different for SX4 alloy or aging heat treatments.According to Han et al.
4.1 γ'-precipitating kinetics during aging heat treatment
Precipitation strengthening and solution strengthening are two major strengthening modes of single crystal superalloys.The precipitation strengthening mainly depends on the distribution and morphology ofγ'phase.After solidification,theγ'precipitates are irregular and coarse.To get a better precipitation strengthening effect,it is always desirable to have fine,uniform,and coherent cubicalγ.As a result,a homogenization-solution heat treatment has to be conducted.During homogenization-solution heat treatment,γ'phase dissolves intoγphase.Then,fine and irregularγ'phase precipitates during following rapid cooling process.After the homogenization-solution heat treatment,a primary aging heat treatment and a secondary aging heat treatment are necessary to obtain desiredγ'morphology.
For SX4 alloy,the refractory element content sums up to as much as 22.0 wt%.It is quite difficult to homogenize solute completely.As a result,even after a homogenization-solution heat treatment as long as 25 h,the interdendrites are still segregated withγ'-forming elements of Ti,Al,Ta,and Nb,while dendrites are segregated withγ-forming elements of Co,Cr,W,Mo,and Re.That is why theγ'cubes in interdendrites are larger than those in dendrites all the time.
During the cooling process after homogenization-solution heat treatment,a mass of fineγ'phases nucleate in the matrix.And then,γ'phase starts to grow larger during aging heat treatment.According to the phase transformation kinetics,the growth rate ofγ'is decided by either driving force of phase transformation or element diffusion rate betweenγandγ'
For theγ'growth controlled by the driving force of phase transformation,theγ'growth rate can be calculated by Eq.(2)
where u is the growth rate ofγ'phase,r is the size ofγ',t is aging time,δis single atom thick,Q is the activation energy of element diffusing fromγtoγ',whose diffusion frequency is expressed by v,and T is Kelvin temperature.
For a given aging heat treatment temperature,the only variable parameter in Eq.(2) is t.After doing the integral over t,Eq.(3) can be obtained.
For theγ'growth controlled by element diffusion rate,theγ'growth rate can be calculated by Eq.(4)
where D is the diffusion coefficient overall the alloy and C0,
After aging heat treated for some time,the solute concentrations become comparably constant.As a result,t can be assumed to be the only variable parameter in Eq.(4).After doing the integral over t,γ'size can be calculated by Eq.(5)
The whole growth process ofγ'phase is analyzed by Fig.6 which is drawn according to Eqs.(3) and (5).Based on the fact that the precipitation is controlled by the slower factor,theγ'precipitation process can be separated into two parts.When t<t0 (where t0 is the aging time corresponding to the intersection of the curves in Fig.6),γmatrix is highly supersaturated withγ'-forming elements,and the growth ofγ'phase is controlled by the driving force of phase transformation,which means a linear increase inγ'size.With the increase in aging time,condenser depression decreases.When the time gets to the t0 point,γ'growth turns to be controlled by solute diffusion,which leads to a much lower growth rate ofγ'phase.
Fig.6 Variation ofγ'size (r) over aging time (t)
4.2γ'precipitating kinetics in different regions
For the SX4 alloy,there is still obvious dendrite segregation after a homogenization-solution heat treatment up to 25 h,and the solute concentrations are different between dendrites and interdendrites.Figure 7 is built for this situation.
According to Fig.7,the aging heat treatment of single crystal superalloys with dendrite segregation can be separated into three stages.In Stage 1,namely t<t1 (where t1and t2 are the aging time corresponding to the intersection of the curves in Fig.7),the precipitation in both dendrites and interdendrites is controlled by the driving force of phase transformation,and r increases linearly.In Stage 2,namely t1<t<t2,solute diffusion starts to control the precipitation in dendrites,which slows down the growth rate ofγ'phase.At the same time,the precipitation in interdendrites is still controlled by the driving force of phase transformation.As a result,the size gap ofγ'in different regions appears and increases rapidly.In Stage 3,namely t>t2,γ'phases in both dendrites and interdendrites are controlled by solute diffusion rate,and the growth rate ofγ'phase becomes sluggish since t2.Consequently,if the time of aging heat treatment is long enough,γ'phase will end up with certain size gap between dendrites and interdendrites.In addition,theγ'size gaps will be obviously different if the time of aging heat treatment is at different sides of t2.
Based on above analysis,an appropriate homogenization-solution heat treatment is necessary to reduce theγ'size gaps between dendrites and interdendrites.If the service temperature is close to the temperature of aging heat treatment,the aging time should be longer than to get stable micro structure.However,if the service temperature is much lower than the aging heat treatment temperature,the time of aging heat treatment can be cut down,without inducing big difference ofγ'sizes.
Fig.7 Variation ofγ'size (r) over aging time (t) in single crystal superalloys with dendrite segregation
Moreover,if microstructure stability is the top priority,which is one of the key points of creep property,gettingγ'sizes of larger than the value that comes from t2 corresponding to service temperature after aging heat treatment is necessary.
4.3 γ'precipitation at different aging temperatures
To analyze theγ'growth behaviors at different temperatures,the growth rate curves ofγ'at different temperatures are shown in Fig.8 (assuming the alloy is completely homogeneous).According to Fig.8,the curve can also be pided into three parts.Before t5 (where t4 and t5 are the aging time corresponding to the intersection of the curves in Fig.8),γ'phases precipitated at lower temperature are always smaller than those at higher temperature.But the size gap for different temperatures becomes smaller after t4.After t5,different temperatures will not lead to differentγ'sizes.
Therefore,if the aging temperature is much lower than service temperature,γ'phase is likely to get a linear growth during service.At the same time,applied stress will accelerate the rafting and de-rafting process,which decreases the rupture life.If a single crystal super alloy is aging heat treated at T5 (corresponding to t5) and serviced at T4 (corresponding tot4),the aging time should be close to t5 to avoid above stress aging during service.
4.4 Effect of aging heat treatment on mechanical properties
Above analysis shows that only whenγ'size is larger than a certain value,can a single crystal superalloy get relatively stable micros true ture during service.Theγ'growth curves of SX4 at 1150,1120,and 870℃with a material simulating software JMatPro are shown in Fig.9.Figure 9shows that with temperature increasing,γ'growth rate increases.It is reported that 300-500 nm is the optimalγ'size range of single crystal superalloys
Fig.8 Variation ofγ'size (r) over aging time (t) at different aging temperatures (T4>T5)(r4 and r5 beingγ'size corresponding to t4 and t5,respectively)
However,by considering the alloy strength,the size ofγ'phase is also needed to be limited for effective dislocation blocking.Equation (6) shows the influence of precipitation strengthening on the increase in yield strength
whereΔτis yield strength increment,G is shear modulus of the alloy,b is Burgers vector,andλis the space length betweenγ'.
For single crystal superalloys,λcan be calculated using Eq.(7)
Fig.9 Calculated results ofγ'growth curve of SX4 alloy at 1150,1120,and 870℃
whereλis the space length betweenγ'andf is the content ofγ'phase (vol%).
For an invariable aging heat treatment temperature,the shear modulus,Burgers vector,and content ofγ'phase can be regarded as constant.Hence,Eqs.(6) and (7) could be combined to Eq.(8),where the only variable parameter is r.
According to Eq.(8),the increase in yield strength caused by precipitatedγ'phase is inversely proportional toγ'size,which means that largerγ'size will decrease the strength of single crystal superalloy.
The above discussion concludes that ifγ'phase is not large enough,it will not keep stable when servicing at higher temperatures.Meanwhile,ifγ'phase is not fine enough,it will affect the precipitation strength.Comprehensively,for new single crystal superalloys that are supposed to be utilized at relatively high temperatures,a series of aging heat treatment experiments must be done to find out the optimum point,which is suggested to be close to the crossing point of the solute diffusion controlling curve and driving force controlling curve corresponding to the serving temperature.Then,high-temperature properties can be improved.
5 Conclusion
In this research,the effect of aging heat treatment on the microstructure of a single crystal superalloy was studied.Results show that for the SX4 alloy,dendrite segregation cannot be completely eliminated by a homogenizationsolution heat treatment as long as 25 h.Consequently,theγ'size in dendrites is always larger than that in interdendrites.For specimens with primary heat treatment at1120℃,theγ'growing stage controlled by the driving force of phase transformation ends during primary heat treatment.As a result,γ'size does not change much during secondary heat treatment.For specimens with primary heat treatment at 1150℃,theγ'growing stage controlled by the driving force of phase transformation only ends in dendrites during primary heat treatment.As a result,theγ'phase in interdendrites grows obviously during the secondary heat treatment,while theγ'phase in dendrites hardly changes.To get better high-temperature properties,theγ'size has to be large enough to avoid stress aging during service and fine enough to get higher strength.Consequently,to optimize the aging heat treatment regime,an appropriate homogenization-solution heat treatment is needed to prevent too large gaps betweenγ'phase in dendrites and interdendrites;the general service temperature has to be considered to calculate the necessaryγ'growth curves;the finalγ'size after aging heat treatment is needed to be close to the crossing point of solute diffusion controlling curve and driving force controlling curve corresponding to the serving temperature.
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