8
Fine Grains Forming Process, Mechanism of Fine Grain Formation and Properties of Superalloy 718 Hwa-Teng Lee 1,+1 and Wen-Hsin Hou 2,+2 1 Department of Mechanical Engineering, National Cheng Kung University, No. 1, University Road, Tainan 701, Taiwan, R. O. China 2 Research & Development Center, Gloria Material Technology Corp., Taiwan, R. O. China The mechanical properties of Inconel 718 superalloy are determined primarily by its microstructure and grain size. The grain structure of Inconel 718 is traditionally rened by aging treatment, and a high volume fraction of acicular ¤ phase precipitates before the structure forms. During the following static or dynamic recrystallization process, the existing ¤ phase inhibits recrystallized grain growth and acquires a ne grain structure. In the proposed approach, the Inconel 718 specimens are re-solution heat treated at a temperature higher than the ¤ solvus temperature to ensure thorough dissolution of the precipitated ¤ phase into the austenite matrix and produce a niobium oversaturated matrix. The specimens are then cold compressed to produce a dislocation saturated matrix and are nally recrystallized at 950°C to induce the precipitation of ne ¤ phase. The ¤ phase precipitates exert a strong grain-boundary pinning effect, and thus a ne grain structure is obtained despite the high recrystallization temperature. The average grain size in the rened microstructure is found to be 2-3 μm, which is around half that of the grain size in the specimens prepared using the conventional process. Hardness testing and tensile testing at 25 and 650°C revealed its superior mechanical properties. [doi:10.2320/matertrans.M2011337] (Received October 31, 2011; Accepted December 26, 2011; Published February 22, 2012) Keywords: superalloy, ¤ phase precipitate, recrystallization, cold forming, ne grains 1. Introduction Superalloy 718 possesses an advantageous combination of favorable mechanical properties and good corrosion resist- ance at high temperatures. Thus, it is extensively used throughout the aerospace, petrochemical, and oil and gas industries. The properties of Inconel 718 are determined primarily by its microstructure, particularly the grain size. Studies have shown that the ne grain size of Superalloy 718 results in both a signicant increase in strength and an enhanced low cycle fatigue capability. 1,2) Furthermore, given the presence of ne and stable equi-axed grains with a size of less than 10 μm, Inconel 718 demonstrates a super- plastic property at elevated temperatures and intermediate strains. 2-4) Superalloy 718 comprises an austentic matrix. Thus, the microstructure of forged Inconel 718 components cannot be rened using the same heat treatment processes as those used to rene the grain size in conventional metal forgings. 5) Camus et al. 6) showed that un-recrystallized grains in the microstructure of Inconel 718 hot-worked components may not fully recrystallize when subjected to further thermal deformation. Therefore, dynamic recrystallization has at- tracted signicant attention as a means of producing a ne and evenly-distributed grain structure in Superalloy 718 specimens following hot forming processes. 7-12) The grain renement of Inconel 718 is commonly achieved using the ¤ phase grain boundary pinning effect to inhibit the grain growth. 13-15) The initial ingot is reduced to an intermediate billet diameter, and the billet is then aged at a temperature of around 890-920°C in order to precipitate a high volume fraction of acicular ¤ phase. Hot working, cold forging, or cold rolling is then performed. In cold forming, during the following static recrystallization treatment, the existing ¤ phase exerts a strong grain-boundary pinning effect and prohibits the growth of recrystallized grains, and a ne grain structure is obtained. In hot working, thermomechanical processing is then conducted at a temperature slightly lower than the delta solvus, 1) causing the ¤ morphology to become spherical and to inhibit recrystallized grain growth via a grain-boundary pinning effect. These grain renement methods are known as the Delta Process. The proposed method also uses ¤ phase precipitation to rene grain size. However, the main difference is that the aging treatment is used in place of solution heat treatment at a temperature higher than the ¤ solvus temperature to ensure thorough dissolution of the precipitated ¤ phase into the austenite matrix and produce a niobium oversaturated matrix before cold forming. Therefore, the following static recrys- tallization process prompts a large number of spherical ne ¤ phases to precipitate and inhibit recrystallized grain growth via a grain-boundary pinning effect. Thus, grains that are more uniform and ne are obtained. The conventional process of hot forming has been successfully used to produce Inconel 718 alloy samples with a grain size of as little as ASTM 10 (11.2 μm) without the need for solution annealing. 11) However, maintaining the ¤ phase during the forging process while minimizing its content in the nal component is a major challenge. 16) To prevent the dissolution of the ¤ phase, and to ensure the occurrence of dynamic recrystallization, the deformation process must be performed under enough strain and low strain rate conditions at a temperature between the dissolution temperature of the ¤ phase and the dynamic recrystallization temperature of Inconel 718. 12) The initial grain size of Inconel 718 components is largely dependent upon the nish forging temperature. Although a lower deformation temperature is benecial in reducing the grain size, it also prompts a reduction in the volume fraction of the recrystallized +1 Corresponding author, E-mail: htlee@mail.ncku.edu.tw +2 Graduate Student, National Cheng Kung University Materials Transactions, Vol. 53, No. 4 (2012) pp. 716 to 723 © 2012 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

716-Fine Grains Forming Process, Mechanism of Fine Grain Formation and Properties of Superalloy 718

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  • Fine Grains Forming Process, Mechanism of Fine Grain Formationand Properties of Superalloy 718

    Hwa-Teng Lee1,+1 and Wen-Hsin Hou2,+2

    1Department of Mechanical Engineering, National Cheng Kung University,No. 1, University Road, Tainan 701, Taiwan, R. O. China2Research & Development Center, Gloria Material Technology Corp., Taiwan, R. O. China

    The mechanical properties of Inconel 718 superalloy are determined primarily by its microstructure and grain size. The grain structure ofInconel 718 is traditionally rened by aging treatment, and a high volume fraction of acicular phase precipitates before the structure forms.During the following static or dynamic recrystallization process, the existing phase inhibits recrystallized grain growth and acquires a ne grainstructure. In the proposed approach, the Inconel 718 specimens are re-solution heat treated at a temperature higher than the solvus temperatureto ensure thorough dissolution of the precipitated phase into the austenite matrix and produce a niobium oversaturated matrix. The specimensare then cold compressed to produce a dislocation saturated matrix and are nally recrystallized at 950C to induce the precipitation of ne phase. The phase precipitates exert a strong grain-boundary pinning effect, and thus a ne grain structure is obtained despite the highrecrystallization temperature. The average grain size in the rened microstructure is found to be 23 m, which is around half that of the grainsize in the specimens prepared using the conventional process. Hardness testing and tensile testing at 25 and 650C revealed its superiormechanical properties. [doi:10.2320/matertrans.M2011337]

    (Received October 31, 2011; Accepted December 26, 2011; Published February 22, 2012)

    Keywords: superalloy, phase precipitate, recrystallization, cold forming, ne grains

    1. Introduction

    Superalloy 718 possesses an advantageous combination offavorable mechanical properties and good corrosion resist-ance at high temperatures. Thus, it is extensively usedthroughout the aerospace, petrochemical, and oil and gasindustries. The properties of Inconel 718 are determinedprimarily by its microstructure, particularly the grain size.Studies have shown that the ne grain size of Superalloy718 results in both a signicant increase in strength andan enhanced low cycle fatigue capability.1,2) Furthermore,given the presence of ne and stable equi-axed grains with asize of less than 10 m, Inconel 718 demonstrates a super-plastic property at elevated temperatures and intermediatestrains.24)

    Superalloy 718 comprises an austentic matrix. Thus, themicrostructure of forged Inconel 718 components cannot berened using the same heat treatment processes as those usedto rene the grain size in conventional metal forgings.5)

    Camus et al.6) showed that un-recrystallized grains in themicrostructure of Inconel 718 hot-worked components maynot fully recrystallize when subjected to further thermaldeformation. Therefore, dynamic recrystallization has at-tracted signicant attention as a means of producing a neand evenly-distributed grain structure in Superalloy 718specimens following hot forming processes.712)

    The grain renement of Inconel 718 is commonly achievedusing the phase grain boundary pinning effect to inhibit thegrain growth.1315) The initial ingot is reduced to anintermediate billet diameter, and the billet is then aged at atemperature of around 890920C in order to precipitate ahigh volume fraction of acicular phase. Hot working, coldforging, or cold rolling is then performed. In cold forming,

    during the following static recrystallization treatment, theexisting phase exerts a strong grain-boundary pinning effectand prohibits the growth of recrystallized grains, and a negrain structure is obtained. In hot working, thermomechanicalprocessing is then conducted at a temperature slightly lowerthan the delta solvus,1) causing the morphology to becomespherical and to inhibit recrystallized grain growth via agrain-boundary pinning effect. These grain renementmethods are known as the Delta Process.The proposed method also uses phase precipitation to

    rene grain size. However, the main difference is that theaging treatment is used in place of solution heat treatment at atemperature higher than the solvus temperature to ensurethorough dissolution of the precipitated phase into theaustenite matrix and produce a niobium oversaturated matrixbefore cold forming. Therefore, the following static recrys-tallization process prompts a large number of spherical ne phases to precipitate and inhibit recrystallized grain growthvia a grain-boundary pinning effect. Thus, grains that aremore uniform and ne are obtained.The conventional process of hot forming has been

    successfully used to produce Inconel 718 alloy samples witha grain size of as little as ASTM 10 (11.2 m) without theneed for solution annealing.11) However, maintaining the phase during the forging process while minimizing its contentin the nal component is a major challenge.16) To prevent thedissolution of the phase, and to ensure the occurrence ofdynamic recrystallization, the deformation process must beperformed under enough strain and low strain rate conditionsat a temperature between the dissolution temperature ofthe phase and the dynamic recrystallization temperatureof Inconel 718.12) The initial grain size of Inconel 718components is largely dependent upon the nish forgingtemperature. Although a lower deformation temperature isbenecial in reducing the grain size, it also prompts areduction in the volume fraction of the recrystallized

    +1Corresponding author, E-mail: [email protected]+2Graduate Student, National Cheng Kung University

    Materials Transactions, Vol. 53, No. 4 (2012) pp. 716 to 7232012 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

  • grains.17) Therefore, in many 718 forgings, dynamicrecrystallization may be incomplete.Lu14,15) produced an Inconel 718 sheet with a ne grain

    size of ASTM 12 (5.6 m) via integration of a great volumeof existing phase, cold rolling, and recrystallization.However, a large volume fraction of phase precipitateswas precipitated, leading to a reduction in the tensile strengthas a result of the consumption of the strengthening elementniobium (Nb). Moreover, in Refs. 14,15), the specimencondition before the ne grain treatment, the grain renementmechanism, and the tensile strength of the nished productwere not discussed.Accordingly, the present study proposes an alternative

    method for rening the grain structure of Inconel 718:solution heat treating the initial specimens (i.e., hot rollingand then solution heat treatment at 968C) at high temper-ature to ensure a thorough dissolution of the precipitated phase and then performing a cold forming process to producea dislocation saturated matrix with high strain energy. Finally,a recrystallization process is performed to prompt theprecipitation of ne phase, thereby restricting graingrowth by means of the grain-boundary pinning effect.The mechanism by which the grain structure is rened isexamined. In addition, the effect of phase precipitation onthe tensile strength of the nished component is evaluated byperforming tensile tests at temperatures of 25 and 650C.

    2. Experimental Section

    The composition of the Superalloy 718 used in the presentexperiments is shown in Table 1. In the experimentalprocedure, Inconel 718 ingots with a diameter of 400mmwere fabricated using a Vacuum Induction Melting (VIM)process followed by Vacuum Arc Remelting (VAR). Anopen-die pre-forging process was performed to reduce thediameter of the ingots to 350mm. A nal forging processwas then performed using a four-hammer precision forgingmachine to reduce the circular ingots to square billets withdimensions of 133mm 133mm. Each billet was hot rolledto a diameter of 22mm, solution heat treated at 968C for 1 h,and then peeled to a diameter of 19.84mm.The materials were then compressed at room temperature

    using a 50T universal tensile testing machine. The recrystal-lized structure of the compressed sample was evaluated by

    performing metallographic tests. Finally, standard metallo-graphic techniques were used to prepare specimens forobservation via an optical microscope (OM), an imageanalyzer, a Transmission Electron Microscope (JEOL JEM-2100F CS STEM), and a Scanning Electron Microscope(SEM). Mechanical properties were determined by micro-hardness and a 10T universal tensile testing machineequipped with a high-temperature furnace.

    3. Results and Discussion

    3.1 Observations of annealed microstructureIn the proposed grain rene method, before cold forming,

    the specimen was re-solution treated at a temperature higherthan solvus, and the grain structure transformed to coarsegrains. Normally, coarse grains will decrease the performanceof grain renement. To investigate the effects of coarsegrains and the timing of phase precipitation on the grainrenement results, the initial annealed samples were furthersolution heat treated at temperatures of 1030 and 1060C,respectively, for 1 h and then quenched in water. Figure 1(a)presents an SEM micrograph of the microstructure of theinitial Inconel 718 sample solution heat treated at 968C for1 h. As shown, the sample has a typical austenitic micro-structure, with twin boundaries located within the individualgrains. The grain size was found to be around 18 m. Inaddition, the micrograph shows the presence of rod-like phase precipitates at the grain boundaries. Figures 1(b) and1(c), illustrating the microstructures of the samples solutionheat treated at 1030 and 1060C, respectively, show thatgrain growth occurred in both specimens. The grain size ofthe sample solution heat treated at 1030C was around60 m, while that of the sample heat treated at 1060C wasaround 68 m. In addition, in both samples, the phaseprecipitates observed at the grain boundaries in the initialannealed sample were fully dissolved within the matrix.

    3.2 Characteristics of phase precipitation3.2.1 Analysis of phase precipitation using Gleeble

    thermomechanical simulatorDilatometric measurement has been widely used in the

    study of phase transformation.1820) The characteristics ofthe phase precipitated during the thermal process wereinvestigated via dilatometry combined with a Gleeble 3500.

    Table 1 Chemical composition of Superalloy 718 (unit: mass%, Fe: balance).

    Item C Si Mn P S Ni Cr Mo Cu

    AMS5662

    Min 50.00 17.00 2.80

    Max. 0.08 0.35 0.35 0.015 0.015 55.00 21.00 3.30 0.30

    Value 0.05 0.08 0.03 0.006 0.001 52.29 18.34 3.12 0.02

    Item Al Co Ti B Nb Ta Pb Bi Se

    AMS5662

    Min 0.20 0.65 4.75

    Max. 0.80 1.00 1.15 0.006 5.50 0.05 0.0005 0.00003 0.0003

    Value 0.54 0.13 0.97 0.004 5.15 0.03 0.0000(1) 0.0000(1) 0.0000(4)

    Fine Grains Forming Process, Mechanism of Fine Grain Formation and Properties of Superalloy 718 717

  • Specimens with free compressed reduction were heattreated at a temperature of 1030C for 1 h and were thenimmediately cooled in water. A cylindrical sample with alength of 15mm and a diameter of 10mm was machinedfrom the heat treated bar. The specimen was mounted in aGleeble 3500 thermomechanical simulator.The specimens were heated to a temperature of 950C at a

    rate of 1Cs1, held at this temperature for 15min, and thencooled to 400C at a rate of 0.5Cs1. The change indiameter of the specimens over the duration of the constanttemperature stage of the heating/cooling cycle was measuredusing a dilatometer.The corresponding results are presented in Fig. 2. The

    negative slope of the blue line indicates that the diameter ofthe specimen shrank during the thermal aging process. Thissuggests that some phase with a density higher than that ofthe austenite matrix was immediately precipitated as soon asthe sample temperature reached 950C.

    Figure 3 presents the general physical properties of theprecipitate density curve simulated by JMatPro for Inconel718. The results suggest that the precipitates are most likely or BB phase. Since the density of the precipitates is higherthan that of the austenite matrix, volume shrinkage couldhave resulted. Therefore, the specimen was further inves-tigated by TEM to conrm the structure of the precipitates.The TEM micrograph of the tested dilatometric specimen

    in Fig. 4(a) illustrates that the matrix microstructure con-tained some dislocations, which were arranged in planararrays. The matrix diffraction pattern presented in Fig. 4(b)shows the single phase diffraction pattern and indicates thatno other phase, such as BB phase, precipitated in the matrix.The TEM image presented in Fig. 5(a) shows that the samplecontained needle-like precipitates at the austenite grainboundaries. The selected area diffraction (SAD) patternspresented in Figs. 5(b)5(d) show that the precipitates were phase. This result seems inconsistent with the TTT and PTTcurves presented in Refs. 21) and 22), respectively, whichsuggest that phase precipitation occurs only after 10min ata temperature of 950C. The apparent difference between thetwo sets of results can be attributed to the greater sensitivityof the Gleeble dilatometric method used in this study

    (a)

    (b)

    (c)

    Fig. 1 SEM micrographs of various solution heat treated samples: (a) heattreated at 968C; (b) heat treated at 1030C; and (c) heat treated at1060C.

    Fig. 2 The variation of the specimen diameter over the constant temper-ature stage of the heating/cooling cycle.

    Fig. 3 Density of various precipitate phases.

    H.-T. Lee and W.-H. Hou718

  • compared to the standard metallographic techniques used inRefs. 21,22).It was observed that the precipitate had two different

    orientations. Thus, it can be inferred that the precipitatesrotated to nd another suitable habit plane variant parallelto the grain boundary.23) Since the grains in a single-phasepolycrystalline specimen generally have many differentorientations, many different types of grain boundary arepossible.24)

    3.2.2 Effect of compression strain on phase precip-itation

    Cylindrical specimens with a diameter of 10mm and alength of 15mm were cold compressed to reductions of 0, 25,and 50%, respectively. The various samples were then heattreated at a temperature of 950C for 5, 10, and 15min,respectively, and then quenched immediately in water. Thevolume fraction of phase in each sample was then

    determined using an image analyzer at a magnication of1500. The results presented in Fig. 6 are the same as inother studies,25,26) showing that the volume fraction of the phase increases with both increasing strain (i.e., an increasingreduction ratio) and increasing aging time.Cold forming creates dislocations. Moreover, the disloca-

    tion density increases as the reduction ratio increases.27) Thedislocations serve as nucleation sites for phase andaccelerate the diffusion of the alloy elements. Thus, theapparent activation energy of phase precipitation decreasesas the cold compression reduction ratio increases. However,the most important result is that as strain increased to 50%, itevidently accelerated the precipitation of phase, and thiscriterion was applied to the design of the grain renementprocess. In other words, cold forming promotes phaseprecipitation, particularly as strain exceeds 50%, and istherefore an effective means of retarding grain growth,thereby achieving a more rened grain structure.

    3.3 Effect of starting grain size on grain renementThis section investigates the relationship between the

    starting grain size, the applied compression strain, and therecrystallized grain size. In performing the investigation, asmall number of initial samples annealed at 968C were re-solution heat treated at temperatures of 1030 and 1060C for1 h and were then quenched in water to acquire the grain sizesof 18, 60 and 68 m, respectively. The specimens were thenmachined into cylindrical samples with a diameter and lengthof 10 and 15mm, respectively. The various specimens werethen compressed at a strain rate of 15mm/min to nalreductions of 0, 24, 37, 50, and 66%, respectively. Thecompressed specimens were recrystallized via heat treatmentat 950C for 15min and were then quenched immediately inwater.Figures 7(a)7(c) illustrate the grain size distribution and

    the ne grain recrystallization ratio in the initial specimenunder various compression ratios and recrystallization treat-ment. Note that the range between the blue and red linesin each gure represents the grain size distribution at thecorresponding reduction ratio. In other words, a narrowerrange indicates that the specimen has a more uniform grainstructure. In every case, it can be seen that the grain size wasrened, and the ratio of ne grains increased, as the reductionratio increased. Under a reduction ratio of 50% [see

    Fig. 4 TEM bright eld image and diffraction pattern (a) the matrix containsome dislocations which were arranged in planar arrays in Inconel 718sample heated at 950C for 2.5min. (b) diffraction pattern correspondingto matrix.

    Fig. 5 TEM bright eld images and diffraction patterns: (a) needle-shaped phase precipitates at the grain boundary in Inconel 718 sample heated at950C for 2.5min. (b) SAD pattern corresponding to location a; (c) SADpattern corresponding to location b; and (d) SAD pattern corresponding tolocation c.

    Fig. 6 Variation of volume fraction of phase precipitation with aging timeat 950C as a function of compression strain.

    Fine Grains Forming Process, Mechanism of Fine Grain Formation and Properties of Superalloy 718 719

  • Fig. 7(a)], the initial sample, with 18 m, obtained ne grainswith a dimension of less than 5 m and had the most uniformgrain size distribution following compression, since the initialmicrostructure was more rened. For initial coarse grain sizesof 60 and 68 m, ne grains with a dimension of less than5 m were obtained when a reduction ratio of 66% wasapplied [see Figs. 7(b) and 7(c)]. Especially, the ratioof ne grains within the microstructure was superior to theinitial grain size of 18 m.In general, the results presented in Fig. 7 show that for all

    three samples, the grain size in the compressed micro-structure is signicantly dependent on the initial grain size for

    cold compression ratios of less than 50%. In the samplesprepared using the grain renement method proposed in thisstudy, re-solution heat treatment at a temperature higher thanthe solvus temperature prior to compression resulted incoarse grain. However, the nest grain size and the largestnumber of ne grains were obtained by the proposed methodwhen applying a compression ratio of 66%.

    3.4 The effect of pre-treatment on grain renementTo compare the grain renement effect of the proposed

    method with that of the conventional process method, theinitial specimen (i.e., hot rolled and then solution heat treatedat 968C with a grain size of 18 m) was aged at 910C for5 h in order to precipitate a large volume of phase prior tocompression. Following the compression tests, the micro-structure resulting from the conventional process and initialspecimen were compared with those of the two specimensprepared using the proposed method, namely re-solutionheat treatment at 1030 and 1060C, respectively, prior tocompression. In every case, the specimens were compressedto a reduction ratio of 66% and then heat treated at 950C for15min to prompt recrystallization.Figures 8(a) and 8(b) present SEM micrographs of the

    aged conventional process specimen and the initial specimen(i.e., hot rolled and then solution heat treated at 968C),respectively. Meanwhile, Figs. 8(c) and 8(d) present micro-graphs of the specimens re-solution heat treated at temper-atures of 1030 and 1060C, respectively. As shown inFigs. 8(a) and 8(b), the phases in the conventional processand initial samples had a rod-like morphology. The phaseresulted in the formation of Nb-depleted zones in thecompressed microstructure. Consequently, the precipitationof uniform ne phase is limited in the subsequentrecrystallization process. As a result, the grain-boundarypinning effect of the phase is also reduced, and thus themicrostructure around the residual phases has a coarse grainsize.In contrast, in the samples re-solution heat treated at

    1030C [Fig. 8(c)] and 1060C [Fig. 8(d)], a large volumefraction of ne phase precipitated from the Nb over-saturated matrix during the recrystallization process. Theseprecipitates suppressed the growth of the recrystallizedgrains, and thus the nal microstructure was characterizedby a ne and uniform grain size.Figure 9(a) shows the phase density and grain size

    distribution in the four samples shown in Fig. 8. Meanwhile,Fig. 9(b) shows the distribution of the phase length in thefour samples. In general, the results show that the specimenre-solution heat treated at 1060C had the greatest number ofne precipitates and the highest phase density of the foursamples, and therefore had the nest grain structure. Inaddition, the specimens re-solution heat treated at 1030 and1060C, respectively, precipitated a greater number of ne-spherical phases than the conventional processed and initialspecimens (i.e., hot rolled and then solution heat treated at968C), and therefore achieved a ner grain structure. Hence,the effect of grain renement was affected by the size,density, and distribution of phase.Since the proposed grain renement method involves

    performing re-solution heat treatment at a temperature higher

    (a)

    (b)

    (c)

    Fig. 7 Relationship between starting grain size, compression ratio, andrecrystallized grain size in specimens heat treated at: (a) 968; (b) 1030;and (c) 1060C.

    H.-T. Lee and W.-H. Hou720

  • than the phase solvus temperature, the residual phase isdissolved into the matrix, and thus the amount of Nbavailable for ne phase formation is increased. Further-more, a higher compression reduction ratio generates agreater number of dislocations, and therefore produces morenucleation sites for phase precipitation. Therefore, in thesubsequent recrystallization treatment performed at 950C(i.e., a temperature lower than the solvus temperature), ahigh volume fraction of ne phase precipitates, which pinsthe recrystallized grains and results in a ne grain structure.Besides, the sizes of the rene phase are mostly less than0.5 m and two times that of the aged conventional process;it is thus inferred that the effect of precipitation strengthwould result.In contrast, in the conventional process and initial

    condition, the presence of a large number of residual rod-like phase precipitates results in Nb-depleted regions in thecompressed microstructure, and therefore limits the formationof ne phase in the recrystallization process. As a result,the grain structure is coarser and less uniform than that in thespecimens processed using the proposed approach.

    3.5 Mechanical test resultsThe compressed specimens, as stated in section 3.3, were

    heated at 950C for 15min, followed by standard aging heattreatment at 718C for eight hours, cooling at a rate of 55Cper hour to 620C, holding at 620C for eight hours, and

    (a) (b)

    (c) (d)

    Fig. 8 Microstructures of various samples compressed to a reduction ratio of 66% and recrystallized at 950C: (a) solution heat treated at968C and then aged at 910C for 5 h, average grain size was 3.6 m; (b) solution heat treated at 968C, average grain size was 2.8 m;(c) re-solution heat treated at 1030C, average grain size was 2.2 m; and (d) re-solution heat treated at 1060C, average grain size was2.0 m.

    (a)

    (b)

    Fig. 9 Comparison of phase characteristics in conventional processspecimen, initial specimen, and specimens processed using proposedmethod: (a) phase precipitation density and grain size; and (b) lengthdistribution of phase.

    Fine Grains Forming Process, Mechanism of Fine Grain Formation and Properties of Superalloy 718 721

  • cooling in air. Because hardness could represent and betransferred to strength, the micro-hardness was tested to studythe effect of the ne grains on strength. The micro-hardnesstest results are presented in Fig. 10. As shown in Fig. 10, inthe initial condition, the original hardness of the material washigher than in the other conditions due to the ner grains.According to the results in section 3.3 of Fig. 7, it was clearthat the hardness was elevated by the rened the grain size ascompression reduction increased. Therefore, as compressedreduction increased to 66%, the hardness values convergedbecause the grain size was rened to a similar grade.As described above, the proposed grain renement

    process results in the formation of a high volume fractionof phase in the recrystallized structure. The presenceof this phase reduces the amount of Nb available for BBphase formation, and may therefore reduce the strength ofthe recrystallized microstructure.11) However, the resultsshow that the micro-hardness increased as the compressedreduction increased.In addition, tensile tests were performed to evaluate the

    mechanical properties of the recrystallized specimens underroom temperature and elevated temperature conditions. Thetests were performed using specimens produced by theproposed method (re-solution at 1030C) with a ne grainstructure obtained by compression deformation at roomtemperature followed by recrystallization to acquire auniform grain structure. A ne recrystallized structure witha grain size of 23m was obtained by heat treating thespecimens at 950C for 15min. Uniform grain structures ofthe tensile specimens at lower magnication are shown inFig. 11. The tensile test specimens were also precipitationhardening treated by standard aging treatment. Tensile testswere then carried out at temperatures of 25C (at a loadspeed of 3 kg/s) and 650C (at a strain rate of 5 103 and5 104 s1 respectively).Figure 12 presents the tensile test results obtained at

    temperatures of 25 and 650C for the ne recrystallized grain

    test specimens and the initial test specimens (i.e., hot rolledand then solution heat treated at 968C). The 25C testresults show that the ultimate tensile strength (UTS) ofthe recrystallized specimens was around 125MPa (18KSI)higher than that of the initial test specimens. In addition, therecrystallized specimens retained a similar degree of ductilityas the initial specimens. From the high temperature tensiletest results, the UTS of the recrystallized specimens wasfound to be around 28MPa (4KSI) and around 76MPa(11KSI) higher than that of the initial specimens at strainrates of 5 103 and 5 104 s1, respectively. Further-more, the elongation was improved by more than 66% atthe strain rate of 5 104 s1.Kirman and Warrington reported that the application of

    external strain increases the dislocation density and numberof stacking faults,27) and therefore facilitates the nucleation of phase and BB phase.25) In the present study, the volumefraction of phase in the specimens compressed at roomtemperature and then recrystallized at 950C for 15min

    Fig. 10 Effects of various solution conditions and compressed reduction onhardness.

    Fig. 11 Microstructure of tensile test specimen.

    Fig. 12 Stressstrain curves of initial and recrystallized test specimens at25 and 650C.

    H.-T. Lee and W.-H. Hou722

  • increased with increasing strain (as shown in Fig. 6), therebycausing a reduction in the amount of Nb available for BBphase formation. Furthermore, during the recrystallizationprocess, the dislocations and extrinsic stacking faults wereannihilated; therefore, the strength could not be elevated byBB phase.However, as stated above, the enhanced grain-boundary

    pinning effect produced by the large volume of ne phaseprecipitates results in a signicant grain rening effect,which not only improves the ductility of the microstructure athigher temperatures but also compensates for the loss instrength caused by the absence of BB phase in the recrystal-lized structure. Thus, even though a large volume fractionof rene phase is precipitated in the recrystallized matrix,the ne grain structure and the rened phase result ina signicant enhancement in the mechanical strength andductility.

    4. Conclusions

    The experimental ndings support the following majorconclusions:(1) The volume fraction of phase precipitates increases

    signicantly as the compression reduction ratio isincreased to 50%, and is an important manufacturingparameter of the proposed method.

    (2) Even though the proposed method will cause the grainstructure to transform to coarse grains, as the re-solutionheated specimens reach a reduction ratio of 66%, thenal grain structure is insensitive to the initial grain sizeand acquires the nest grain structure.

    (3) In the proposed method, the phase precipitationdensity and the number of precipitates with a length ofless than 0.5 m are around twice that of the specimensprepared using the conventional process. Therefore, theproposed method results in an improved grain-boundarypinning effect, and yields a more rened and uniformly-distributed grain structure, resulting in the ne grainsize of 23m.

    (4) The ultimate tensile strengths of the ne grain samplestested at 25 and 650C, respectively, are higher thanthat of the initial sample, despite the greater volumefraction of phase, and have premium ductility.

    Acknowledgements

    The authors gratefully acknowledge the nancial supportfor this research by the ITRI South and the Ministry ofEconomic Affairs of Taiwan under Grant No. A301ARY730.

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    Fine Grains Forming Process, Mechanism of Fine Grain Formation and Properties of Superalloy 718 723