effect of the δ phase on the hot deformation behavior of inconel 718

Materials Science and Engineering A 408 (2005) 281–289

Effect of the� phase on the hot deformation behaviorof Inconel 718

H. Yuana, W.C. Liua,b,∗a Key Laboratory of Metastable Materials Science and Technology, College of Materials Science and Engineering, Yanshan University,

Qinhuangdao 066004, PR Chinab Department of Chemical and Materials Engineering, University of Kentucky, 177 Anderson Hall, Lexington, KY 40506, USA

Received in revised form 3 August 2005; accepted 10 August 2005

Abstract

Hot compression tests at different temperatures and strain rates were performed on Inconel 718 solution treated as well as solution treated andthen aged at 900◦C for 1–24 h in order to investigate the effect of the� phase on hot deformation behavior. A hyperbolic-sine Arrhenius-typeequation was used to characterize the dependence of the peak stress on deformation temperature and strain rate. In the case of Inconel 718 with alarge amount of the� phase, the apparent activation energy was determined to be 458 kJ/mol, which was slightly higher than the activation energy

, but dided the flow

d onper-

ior

sentasutiono-size

taged

ingsultsat

fs

of 443 kJ/mol for solution treated Inconel 718. The�′′/�′ phases present in Inconel 718 prior to hot deformation enhanced the peak stressnot affect the peak strain. The� phase present in Inconel 718 not only decreased the peak strain and the peak stress, but also promotsoftening after the peak stress.© 2005 Elsevier B.V. All rights reserved.

Keywords: Inconel 718; Hot deformation; Dynamic recrystallization;� Phase

1. Introduction

Inconel 718 is a nickel-based superalloy extensively usedin the fabrication of critical components for turbine enginesbecause of its excellent mechanical properties at elevated tem-peratures and good corrosion resistance. Since these componentswithstand high alternating stresses and creep loads, the uniformmicrostructure with fine grains must be obtained by controllingforging processes. In the turbine disc application, the standardprocessing, the high strength processing and the direct age pro-cessing have been applied in order to reach the required prop-erties[1,2]. Of these methods, the direct age processing resultsin the highest strength and the finest grains[2,3]. The� phasepresent in Inconel 718 plays an important role in the control ofgrain size during hot working[3]. Although a number of stud-ies have focused on the hot deformation behavior of cast andsolution treated Inconel 718[4–17], a few reports dealt with theeffect of the� phase on hot deformation behavior. In the presentstudy, Inconel 718 was solution treated and then aged at 900◦C

∗ Corresponding author. Tel.: +1 859 2574433; fax: +1 859 3231929.E-mail address: [email protected] (W.C. Liu).

for different times. Hot compression tests were conductesolution treated and aged Inconel 718 at deformation tematures of 900–1180◦C and strain rates from 10−3 to 10−1 s−1.The effect of the�′′/�′ and� phases on hot deformation behavwas determined.

2. Experimental

2.1. Material and sample preparation

The chemical composition of Inconel 718 used in the preinvestigation is shown inTable 1. The as-received material whot rolled bars of 30 mm diameter. These bars were soltreated at 1100◦C for 1 h, followed by water quenching to prduce a single-phase microstructure with an average grainof 96.8�m. In order to study the effect of the� phase on hodeformation behavior, these solution treated samples wereat 900◦C for 1–24 h. The microstructure formed during agwas determined by optical microscopy and SEM. The reshowed that the�′′, �′ and� phases precipitated during aging900◦C. The� precipitation was found to be preceded by the�′′precipitation. After aging at 900◦C for 1 h, a large amount othe �′′ phase precipitated in the matrix, while the� phase wa

0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.msea.2005.08.126

282 H. Yuan, W.C. Liu / Materials Science and Engineering A 408 (2005) 281–289

Table 1Chemical composition of Inconel 718 (wt.%)

C 0.04Ni 52.70Cr 18.53Nb 5.29Mo 3.19Ti 1.01Al 0.63Mn <0.35Si 0.15P <0.015S 0.005Fe Bal.

initiated to precipitate at grain and twin boundaries. After 4 h,the � precipitates grew from the grain boundaries towards thecenter and appeared as long platelets. At the same time, the�precipitation also occurred within the grains. After 24 h, a largeamount of needle-shaped� phase was observed, while the�′′phase almost disappeared. The weight percentages of the�′′, �′and� phases formed during aging at 900◦C were determined bya quantitative phase analysis method of X-ray diffraction[18].The weight percentages of the�′′ and� phases as a functionof aging time are shown inFig. 1. It is seen that as the agingtime increased from 4 to 24 h, the weight percentage of the�phase increased, whereas the weight percentage of the�′′ phasedecreased.

Cylindrical compression specimens of 12 mm in height and8 mm in diameter were machined from the solution treated andaged materials. Grooves of 0.2 mm deep were machined into thends of the specimens to retain the graphite lubricant during thcompression tests.

2.2. Hot compression test

A computer-controlled, servo-hydraulic Thermecmaster-Zmachine was used for the compression tests. The specimewere induction-heated, and the temperature was controlled by

FW

thermocouple welded at the mid-span of the specimen. In orderto reduce the friction, a graphite lubricant was used betweenthe specimen and the cross-head. Hot compression tests wereconducted in an argon atmosphere. The specimens were heatedto the deformation temperature at a rate of 10◦C/s and heldfor 2 min in order to obtain a stable temperature prior to defor-mation. The specimens were deformed to a true strain of 0.76at the given temperatures and strain rates. All specimens wereimmediately quenched to room temperature in less than 1 s afterdeformation. For different aging treatments, the used deforma-tion temperatures and strain rates are given inTable 2.

The deformed specimens were sectioned parallel to compres-sion axis at one-half of radius, ground and mechanically pol-ished. Specimens were examined optically, and the volume frac-tion of recrystallized grains was determined by point counting.

3. Results and discussion

3.1. Hot deformation behavior of solution treated Inconel718

3.1.1. Stress–strain curvesFig. 2 shows the stress–strain curves for solution treated

Inconel 718 deformed at temperatures of 900–1180◦C and strainrates from 10−3 to 10−1 s−1. These curves are in agreement withthe usually observed deformation characteristics of nickel-baseds luew urtheri witha orlyd eninga iates amicr imu-l iticals

3stress

o eaks tively.T n tem-p reasedw strainr ation

TD ferenta

A

N 180

A

A
ig. 1. Relationship between the weight percentage of the� and�′′ phases (W�,
�′′ ) and aging time during aging at 900◦C.

ee

nsa

uperalloys[8,11–13]. The flow stress increased to a peak vaith increasing strain and then decreased as the strain f

ncreased. The initial rapid rise in stress was associatedn increase in dislocation density and the formation of poeveloped subgrain boundaries, as a result of work hardnd dynamic recovery. In the alloys with low or intermedtacking fault energy, e.g. nickel or stainless steel, the dynecovery proceeded slowly. The high dislocation density stated the occurrence of dynamic recrystallization once a crtrain was exceeded.

.1.2. Development of constitutive relationshipThe peak stress can be selected as the representative

f each curve.Figs. 3 and 4show the dependence of the ptress on deformation temperature and strain rate, respeche classic interdependence of the peak stress, deformatioerature and strain rate can be seen, i.e. the peak stress incith decreasing deformation temperature and increasing

ate. The peak strain also increased with decreasing deform

able 2eformation temperature and strain rate applied in the present work for difging treatments

ging treatment Deformation temperature (◦C) and strain rate (s−1)

o aging treatment T: 900, 940, 980, 1020, 1060, 1100, 1140 and 1ε̇: 100, 10−1, 10−2 and 10−3

t 900◦C for 1, 2,4, 8 and 12 h

T: 900, 940, 980 and 1020ε̇: 100 and 10−2

t 900◦C for 24 h T: 900, 940, 980 and 1020ε̇: 100, 10−1, 10−2 and 10−3

H. Yuan, W.C. Liu / Materials Science and Engineering A 408 (2005) 281–289 283

Fig. 2. Stress–strain curves for solution treated Inconel 718 deformed at strain rates of: (a) 100 s−1; (b) 10−1 s−1; (c) 10−2 s−1; (d) 10−3 s−1.

temperature. However, the peak strain did not increase withincreasing strain rate. The behavior is similar to the resultsobserved by Guimaraes and Jonas[7], Garcia et al.[11] andSrinivasan and Prasad[19].

For the range of deformation conditions employed, the peakstress as a function of deformation temperature and strain rate

Fig. 3. Relationship between the peak stress and deformation temperature forsolution treated Inconel 718.

was analyzed through a hyperbolic-sine Arrhenius-type equa-tion [20–23]:

ε̇ = A[sinh(ασp)]n exp

(−Q

RT

)(1)

Fig. 4. Relationship between the peak stress and strain rate for solution treatedInconel 718.


whereA (s−1) andα (MPa−1) are materials constants,n a con-stant closely related to the strain rate,Q the activation energy ofdeformation (J/mol),R the universal gas constant,ε̇ the strainrate (s−1), T the deformation temperature in Kelvin andσp isthe peak stress (MPa). From equation(1), we can write

ln sinh(ασp) = −1

nln A + 1

nln ε̇ + 1

n

Q

RT(2)

Linear statistical regression methods cannot be used directly todetermine the values ofα, A, n andQ because there are fourconstants ofα, A, n andQ in equation(2). In order to calculatethe values ofA, n andQ, we first gave a value ofα, and then cal-culated the values ofA, n, Q and the residual sum of squares byfitting the experimental data. The residual sum of squares shouldbe as a function ofα. The value ofα was estimated from the min-imum residual sum of squares to be 0.012 MPa−1. The value ofαrepresents the stress reciprocal upon which the material changesfrom power to exponential stress depend. After the optimumvalue ofα was determined, the values ofA,n andQ were found tobe:A = 4.8× 1014 s−1, n = 2.36 andQ = 443 kJ/mol. Thus, equa-tion (1) can be expressed as

ε̇ = 4.8 × 1014[sinh(ασp)]2.36 exp

(−443000

RT

)(3)

The activation energy of deformation for solution treatedI tione lb to stressc (ε thep withi

3rved

b eso t

F nt

Fig. 6. Microstructures of Inconel 718 deformed at a strain rate of 100 s−1 andat: (a) 1020◦C, (b) 1060◦C and (c) 1180◦C.

deformation temperatures. The volume fraction of recrystal-lized grains was determined by point counting and depicted inFig. 7as a function of deformation temperature at different strainrates. The volume fraction of recrystallized grains was found to

nconel 718 was 443 kJ/mol, which compared with activanergies of 400 kJ/mol by Medeiros et al.[17], 423 kJ/moy Weis [8] and 485 kJ/mol by Garcia et al.[5]. The effecf deformation temperature and strain rate on the peakan be expressed by the Zener-Hollomon parameterZ =

˙ exp(Q/RT ), s−1). Fig. 5 shows the relationship betweeneak stress and theZ parameter. The peak stress increased

ncreasing theZ parameter.

.1.3. Distributive map of dynamic microstructureThe microstructure of hot deformed materials was obse

y optical microscopy.Fig. 6shows the typical microstructurf the samples deformed at a strain rate of 100 s−1 and differen

ig. 5. Relationship between the peak stress (σp) and theZ parameter for solutioreated Inconel 718 (Q = 443 kJ mol−1, T = 1173–1453 K,̇ε = 10−3 to 100 s−1).


Fig. 7. Relationship between the percentage of dynamic recrystallized grains(Xv) and deformation temperature at different strain rates for solution treatedInconel 718.

increase with deformation temperature at a given strain rate.The curves shifted to high temperature with increasing strainrate, i.e. the temperature of 50% recrystallization increased withincreasing strain rate. IfXv = 10 and 90% are defined as thestart and finish of dynamic recrystallization, the distributivemap of dynamic microstructure can be presented, as shown inFig. 8. At low temperatures and high strain rates, only dynamicrecovery took place during hot deformation. Fully recrystallizedmicrostructures were obtained at elevated temperatures and lowstrain rates. The dynamically recrystallized grain size increasedwith increasing deformation temperature. Between two defor-mation conditions, the deformed austenite was partiallyrecrystallized.

It is noted from Fig. 2 that the stress–strain curves atlow temperatures and high strain rates exhibited a declinebeyond the peak. However, the observation of microstructurerevealed that dynamic recrystallization did not take place in thecases. This behavior can be explained by deformation heating[23].

Fig. 8. Distributive map of dynamic microstructure at a strain of 0.76 for solutiontreated Inconel 718.

3.2. Hot deformation behavior of Inconel 718 with a largeamount of needle δ phase

3.2.1. Stress–strain curvesFig. 9shows the typical stress–strain curves at temperatures

of 900–1020◦C and strain rates from 10−3 to 100 s−1 for Inconel718 solution treated and then aged at 900◦C for 24 h. Thestress–strain curves exhibited a similar pattern to those of solu-tion treated materials. The difference in the stress–strain curvesbetween solution treated and aged samples was that the flowstress of the aged samples dropped much faster and much lowerafter the peak stress than that of the solution treated samples.

The dependence of the peak stress on deformation temper-ature and strain rate is shown inFigs. 10 and 11, respectively.The peak stress increased with decreasing deformation tempera-ture and increasing strain rate. In comparison to solution treatedInconel 718, it is noted that the hot deformation behavior ofInconel 718 was affected by the� phase. The� phase decreasedthe peak stress, especially at high strain rates and at low tem-peratures (Fig. 11). Therefore, it is to say that the� phase had asoftening effect on the hot deformation of Inconel 718. The peak

F f the� phaa

ig. 9. Typical stress–strain curves for Inconel 718 with a large amount ond (b) 10−2 s−1.

se at deformation temperatures of 900–1020◦C and strain rates of: (a) 100 s−1


Fig. 10. Effect of deformation temperature on the peak stress for Inconel 718with a large amount of the� phase.

stress as a function of deformation temperature and strain ratewas analyzed by equation(1). The value ofα was determined tobe 0.012 MPa−1. The hot deformation equation for Inconel 718with a large amount of the� phase is given by

ε̇ = 4.5 × 1015[sinh(ασp)]2.19 exp

(−458000

RT

)(4)

For Inconel 718 with a large amount of needle� phase, the acti-vation energy of deformation was calculated to be 458 kJ/mol,which was slightly higher than that for solution treated Inconel718. The relationship between the peak stress and theZ parame-ter is shown inFig. 12. The peak stress increased with increasingtheZ parameter.

3.2.2. Dependence of the peak strain on deformationtemperature and strain rate

Figs. 13 and 14show the dependence of the peak strain ondeformation temperature and strain rate, respectively. The peakstrain depended strongly on deformation temperature and strain

F largea

Fig. 12. Relationship between the peak stress (σp) and the Z parameterfor Inconel 718 with a large amount of the� phase (Q = 458 KJ mol−1,T = 1173–1293 K,̇ε = 10−3 to 100 s−1).

Fig. 13. Effect of deformation temperature on the peak stress for Inconel 718with a large amount of the� phase.

Fig. 14. Effect of strain rate on the peak strain for Inconel 718 with a largeamount of the� phase.

ig. 11. Effect of strain rate on the peak stress for Inconel 718 with amount of the� phase.


Fig. 15. Relationship between the peak strain (εp) and the Z parameterfor Inconel 718 with a large amount of the� phase (Q = 458 kJ mol−1,T = 1173–1293 K,̇ε = 10−3 to 100 s−1).

rate. The expression normally established to interpret this depen-dence has the following form:

εp = A1Zn1 (5)

whereA1 andn1 are material constants. The values ofA1 andn1were determined by regression analyses of experimental data.The result is given by the following equation:

εp = 1.07× 10−5Z0.21 (6)

Fig. 15shows the relationship between the peak strain and theZ parameter. The peak strain increased with increasing theZparameter.

3.3. Effect of the γ ′′/γ ′ and δ phases on hot deformationbehavior

In order to study the effect of aging time on hot deforma-tion behavior, hot compression tests were performed on Inconel718 solution treated and then aged at 900◦C for 0, 1, 2, 4, 8,12 and 24 h at deformation temperatures of 900–1020◦C andstrain rates of 10−2 and 100 s−1. Typical stress–strain curvesare shown inFig. 16. According to the variation in stress–straincurves, it is possible to determine the influence of the�′′/�′and� phases formed during aging at 900◦C on hot deformationbehavior.

F(

ig. 16. Typical stress–strain curves at deformation temperatures of 900–102◦C anc) 12 h.

0d a true strain rate of 1 s−1 for Inconel 718 aged at 900◦C for: (a) 2 h; (b) 4 h;


3.3.1. Effect of the γ ′′/γ ′ and δ phases on the peak stressFig. 17shows the effect of aging time at 900◦C prior to hot

deformation on the peak stress. It is noted that the effect of agingtime on the peak stress depended on deformation temperature. Inthe case of deformation at 900◦C, the peak stress first increasedwith aging time and then decreased after about 2 h. In the case ofdeformation at high temperatures above 940◦C, the peak stressdecreased slowly with increasing aging time.

The effect of aging time on the peak stress can be attributedto the�′′/�′ and� phases prior to deformation. During aging at900◦C, the�′′/�′ phases first precipitated, and the amount ofthe �′′/�′ phases increased with increasing aging time. There-fore, the peak stress also increased increasing aging time due tothe Zener pinning effect of the�′′/�′ phases. The�′′/�′ phasespresent in Inconel 718 enhanced the peak stress. After 4 h, theamount of the�′′/�′ phases decreased with increasing aging time,whereas the amount of the� phase increased (Fig. 1). The changein the�′′/�′ and� phases resulted in a decrease in the peak stresswith aging time. At high deformation temperatures, the�′′/�′phases were dissolved during annealing prior to deformationsince the deformation temperatures were higher than the�′′/�′solvus. Therefore, the peak stress changed slightly with increas-ing aging time.

3.3.2. Effect of the γ ′′/γ ′ and δ phases on the peak strainFig. 18shows the effect of aging time on the peak strain. It is

s e wasl withi endedo uresa peaks

inedba hem n didn Thisi ain.T effect

F aturea

Fig. 18. Effect of aging time on the peak strain at given deformation temperaturesand strain rates.

of the� phase. The effect of the� phase on the peak strain alsodepended on deformation temperature and strain rate. In the caseof deformation at a strain rate of 1 s−1 and temperatures of 900and 940◦C, a small amount of the� phase at grain boundariesdecreased the peak strain. After aging at 900◦C for 4 h, the�phase started to precipitate within the grains. Maybe becausethe effect of the� phase within the grains was less than that atgrain boundaries, the peak strain decreased slowly as the agingtime increased from 4 to 12 h. After aging for 12 h, the amountof � phase increased significantly with increasing aging time,and the� phase appeared as a cross-net, which resulted in afurther decrease in the peak strain. In the case of deformationat high temperatures and low strain rates, the dynamic recov-ery and recrystallization took place easily. Thus, the effect ofthe� phase on the peak strain decreased with decreasing theZparameter.

3.3.3. Effect of the δ phase on the extent of flow softeningComparison of the stress–strain curves inFigs. 2 and 9

revealed that the flow stress of Inconel 718 with a large amountof the � phase dropped much faster and much lower after the

F tem-p

een that the peak strain was unchanged when the aging timess than 1 h. After aging for 1 h, the peak strain decreasedncreasing aging time. The decrease in the peak strain depn the deformation condition. Deformation at low temperatnd high strain rates resulted in a significant decrease in thetrain.

The effect of aging time on the peak strain can be explay the�′′/�′ and� phases formed during aging at 900◦C. Afterging at 900◦C for 1 h, the�′′/�′ phases precipitated in tatrix. During subsequent hot deformation, the peak straiot alter in comparison to solution treated Inconel 718.

ndicates that the�′′/�′ phases do not affect the peak strhe decrease in the peak strain can be attributed to the

ig. 17. Effect of aging time on the peak stress at given deformation tempernd strain rates.

sig. 19. Effect of aging time on the extent of the flow softening at giveneratures and strain rates.


peak stress than that of solution treated Inconel 718. This indi-cates that the� phase promotes the flow softening. In order toillustrate the degree of decrease in the flow stress after the peakstress, the ratio of the flow stress at a strain of 0.6 to that at thepeak stress is defined as the extent of flow softening.Fig. 19shows the relationship between the extent of flow softening andaging time. It is seen that theσ0.6/σp value hardly varied withthe aging time when the aging time was less than 1 h. After1 h, theσ0.6/σp value decreased with increasing aging time. Thedecrease in theσ0.6/σp value can also be attributed to the effectof the � phase on the hot deformation of Inconel 718. The�phase present in Inconel 718 not only decreased the peak strainand the peak stress, but also promoted the flow softening afterthe peak stress.

4. Conclusions

(1) In the case of solution treated Inconel 718, the apparentactivation energy was determined to be 443 kJ/mol. The rela-tionship among the peak stress, deformation temperatureand strain rate can be described by the following equation:

ε̇ = 4.8 × 1014[sinh(ασp)]2.36 exp

(−443000

RT

)

(2) The relationship between the fraction of recrystallizedrateribu-ated

( eto bsesshipstra

( ,r.r

( vior18o no

affect the peak strain. The� phase present in Inconel 718not only decreases the peak strain and the peak stress, butalso promotes the flow softening after the peak stress.

Acknowledgments

The authors would like to acknowledge the financial sup-port of the National Natural Science Foundation of China underContract Number 59971039 and Shenyan Liming Engine Man-ufacturing Company for providing materials.

References

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grains and deformation temperature at different strainwas determined, and the dynamic microstructure disttion map at a strain of 0.76 was pictured for solution treInconel 718.

3) In the case of Inconel 718 with a large amount of th�phase, the apparent activation energy was calculated458 kJ/mol. The� phase present in Inconel 718 increaslightly the apparent activation energy. The relationamong the peak stress, deformation temperature andrate can be described by the following equation:

ε̇ = 4.5 × 1015[sinh(ασp)]2.19 exp

(−458000

RT

)

4) In the case of Inconel 718 with a large amount of the� phasethe peak strain increases with increasing theZ parameteThe relationship between the peak strain and theZ parametecan be expressed as

εp = 1.07× 10−5Z0.21

5) The�′′/�′ and� phases affect the hot deformation behaof Inconel 718. The�′′/�′ phases present in Inconel 7prior to hot deformation enhance the peak stress, but d

s

e

in

t

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effect of the δ phase on the hot deformation behavior of inconel 718

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