Microstructure and Stress Rupture Properties of K417G Superalloy at Different Hot Isostatic Pressing Temperatures

,


Introduction
Since the 1940s, casting superalloys have been rapidly developed in the aerospace feld and are widely used in aeroengine turbine blades.However, the service condition of the aero-engine gas turbine is extremely severe, and superalloys play a crucial role in the high-temperature condition [1][2][3][4].K417G, a nickel-based precipitation-hardened cast superalloy, has excellent high-temperature mechanical properties and hot-corrosion resistance after being modifed on the basis of K417 superalloy and is used for turbine blades and guide blade components of aircraft engines.[5,6] With the continuous development of advanced blades, micrometallurgical defects, such as pores, shrinkage, and slag patches, inevitably appear in the casting process [7].Although the casting process has been constantly improved and micrometallurgical defects in superalloy castings have been reduced to a certain extent, it cannot completely eliminate the micrometallurgical defects such as shrinkage and the volume fraction of shrinkage content that can generally reach more than 0.3% [8,9].
Hot isostatic pressing (HIP) technology is applied to solve the problem of shrinkage metallurgical defects.It uses the combined efect of high temperature and high pressure to densify the internal structure of metal castings and forms a circular creep raft structure around the pores, resulting in efective healing of metallurgical defects such as pores and segregation in the alloy [10][11][12].As HIP is carried out under high-temperature and high-pressure conditions, it can effectively reduce the eutectic structure, homogenize the composition, and improve grain size and dendrite segregation [13,14].After HIP of the as-cast K403 superalloy, Zhang et al. [15] found that the size and the volume fraction of c′ precipitates were signifcantly improved and the morphology of carbides at the grain boundary changed from linear to granular.After the HIP treatment of the as-cast IN718 superalloy, Chang found that the c″ phase nucleates at the c′/c interface and merges with the c′ phase to form the c′/c′′ eutectic phase.In addition, the δ phase in the microstructure is signifcantly reduced [16].Te globular pore defects formed in the laser powder bed fusion (LPBF) process of IN718 alloy studied by Rezaei et al. [17] were mostly closed after HIP, and the relative density increased from 99.50% of the heat-treated condition to 99.96% of the heat-treated condition after HIP.
In terms of mechanical property, Xuan et al. [18] found that the decrease in the size and the volume fraction of micropores in nickel-based single crystal superalloys after HIP made it difcult to initiate and propagate cracks, thus improving the medium temperature plasticity of the alloy.Petrovskiy et al. [19] found that the HIP process increased the tensile strength from 30 MPa, 70 MPa, and 20 MPa to 211 MPa, 480 MPa, and 560 MPa for stainless steel, pure titanium, and Ti6Al4V, respectively.In addition, the compressive strength of Al-Mg-Sc-Zr alloy also increased by about 25% after HIP.Li et al. [20] found that HIP did not signifcantly improve the tensile strength of K452 alloy but reduced the dispersion of tensile properties.Liu et al. [21] studied the isothermal fatigue of polycrystalline nickel-based alloy IN718 in as-cast and HIP.Te fatigue life after HIP is higher than that of as-cast alloys, which is attributed to the elimination of microporosity, the dissolution of the brittle Laves phase, and the precipitation of the strengthening phase.After HIP treatment of the second-generation nickelbased single crystal superalloy in as-solid-solution sates, He et al. [13] signifcantly eliminated micropores and interdendritic eutectic structures and reduced the crack source in the process of high-temperature stress rupture/creep.Compared with the standard heat-treated alloy without HIP treatment, the stress rupture life is signifcantly improved.At present, the research on improving the microstructure and properties of as-cast superalloy by HIP has drawn more attention, but the microstructure and property of the K417G as-cast superalloy need to be further studied.
In this work, the efects of diferent HIP processes on the microstructure evolution of the as-cast K417G superalloy were studied, and then, the stress rupture properties at 760 °C and 645 MPa were tested.Te infuence mechanism of diferent HIP on the microstructure of the alloy was compared and analyzed, and an optimal HIP process of the K417G alloy was explored.Tis work provides reference for the study of the K417G alloy and the HIP process.

Experimental
Te K417G superalloy was smelted in a 25 kg semicontinuous vacuum induction furnace.Te nominal chemical composition of the K417G alloy is shown in Table 1.In order to investigate the efect of HIP treatment on the as-cast alloy, it was decided to compare the HIP process at diferent temperatures with the ordinary as-cast alloy.Te K417G alloy was analyzed by diferential scanning calorimetry (DSC) before HIP treatment.Te results are shown in Figure 1.During the heating process, the solidus temperature was 1290 °C and the liquidus temperature was 1321 °C.Te dissolution temperature of the c′ phase was 1197 °C, and the HIP process temperature should be below this temperature.It is well known that the higher the temperature of HIP, the better the efect on eliminating defects.Terefore, 1195 °C was set as the highest hot isostatic pressing process temperature, and then, two groups of HIP processes were selected at diferent temperatures.Te interval between each group is 10 °C, and the three treatments are 1175 °C/170 MPa, 1185 °C/170 MPa, and 1195 °C/ 170 MPa, respectively.Te specifc HIP process parameters are shown in Table 2.
Samples for metallographic analysis were cut on the standard sections of the as-cast and three diferent HIP specimens.Te samples were sliced, ground, and polished, and then, the micropore was observed using an optical microscope (OM).Ten, the dendrite morphology of the samples was observed by chemical etching.Te composition of the etching solution was 20 mL HCl + 5 g CuSO 4 + 25 mL H 2 O.After regrinding and polishing, the samples were placed in a solution of H 3 PO 4 , HNO 3 , and H 2 SO 4 at a ratio of 1 : 3 : 5 for electrolytic corrosion, the corrosion voltage was 3.5 V, and the corrosion time was 5 s.Te microstructure was observed by using the TESCAN CLARA feld emission scanning electron microscope (SEM).Te composition of the dendrite cores and interdendritic regions was characterized by using the JXA-8100 electron probe microanalyzer (EPMA).Six test points were selected from each sample for characterization, and the points in the test process were as far as possible to avoid interdendritic carbides and (c + c′) eutectic phases.Te micropore and area fraction of the (c + c′) eutectic phase and the average size, area fraction, and size distribution of the c′ precipitates were measured by using Image-Pro Plus software.Each set of values was calculated using at least three representative images and averaged.
Te test bar was machined into a specifed standard stress rupture sample, and the size of the stress rupture sample is shown in Figure 2. Te stress rupture test was performed at 760 °C and 645 MPa.After the stress rupture test, the dislocation confgurations were observed on an FEI Talos F200X transmission electron microscope (TEM).Te thin slice of the sample was cut from the specimen and ground manually to a thickness of 60 μm for electrochemically thinning in a twin-jet polisher with a solution of 10% perchloric acid and 90% ethanol at −20 °C and 40 mA to prepare the thin foils for the TEM.

Results
As shown in Figure 3, for the K417G alloy, the micropore and carbide morphology can be observed inside the microstructure without corrosion using an optical microscope.Te micropore in the as-cast alloy is clearly shown in Figure 3(a), and the porosity was about 0.33%.Te statistical results of the micropore area fraction are shown in Figure 4. Ten, after HIP at diferent temperatures, the micropores of the alloys are signifcantly reduced compared with those of

2
Advances in Materials Science and Engineering the as-cast alloy.Te porosity of HIP1175 was 0.06%, that of HIP1185 was about 0.05%, and that of HIP1195 was only 0.02%.In addition, the morphology, size, and distribution of carbides in four diferent conditions have no obvious change.According to the study of as-castnickel-based superalloys, carbide is mainly the MC type rich in Ti and Mo, and its morphology has a small particle, strip, and block [22][23][24].
Figure 5 shows the dendrite morphology image of the K417G alloy after diferent treatments.Figure 5(a) shows that the dendrite cores and interdendritic regions inside as-cast grains can be distinguished by the depth of chemical corrosion.Te dendrite morphology in Figures 5(b) and 5(c) is not clear compared to that in Figure 5(a), while the dendrite cores and interdendritic regions in Figure 5(d) are difcult to distinguish by observing the depth after chemical etching.Terefore, it can be inferred that the as-cast microstructure after HIP treatment can relieve the segregation between dendrite cores and interdendritic regions, and with an increase in the HIP temperature, from 1175 °C to 1185 °C, element segregation was further relieved.When the temperature increases to 1195 °C, the segregation of the dendrite core and the interdendritic region has been improved compared with that of the as-cast alloy.In addition, it can be seen from the OM after corrosion that the (c + c′) eutectic was mainly distributed in the interdendritic region and the grain boundary.

Advances in Materials Science and Engineering
Te statistics of the eutectic content of the four conditions of the alloy are shown in Figure 4.It can be seen that the eutectic content was about 3.99% on average in the ascast alloy.After HIP treatment, part of the (c + c′) eutectic in the interdendritic region and the grain boundary dissolves back to reduce its content, and with gradually increasing the adopted HIP temperature from 1175 °C to 1195 °C, the eutectic content also decreases.Te eutectic content decreased from 3.27% in HIP1175 to 2.99% in HIP1185 and then to 2.95% in HIP1195.Te microstructural evolution trend of eutectic in three diferent HIP conditions was not signifcant, which was similar to the evolution of porosity in diferent HIP conditions.It is indicated that the resolution efect of eutectic is not obvious during HIP treatment.Te healing efect of HIP on micropores is limited and cannot completely eliminate the micropore of the K417G alloy.If the HIP treatment temperature continues to increase, the micropore will heal slightly more.
Te morphology of the c′ precipitates in the dendritic region of the K417G alloy in as-cast and HIP conditions is shown in Figure 6, and the size and the area fraction of the c′ phase in diferent conditions were measured, as shown in Figure 7. Te c′ precipitates in the as-cast dendrite core were regular cubes, as shown in Figure 6(a), and the average size and the area fraction of the c′ precipitates were 0.65 μm and 63.9%, respectively.After diferent HIP temperatures, the c′ phase exhibits a diferent degree of morphological evolution.After HIP1175 treatment, the size of the c′ precipitates in dendritic regions increased obviously, in which the average size and the area fraction were about 0.84 μm and 66.1%, respectively, and the edge of the c′ precipitates was diferent from that in the as-cast alloy; the edge morphology of the c′ precipitates was uneven and similar to that of the serrated c′ precipitates, as shown in Figure 6(b).After HIP1185 treatment, the size of the c′ precipitates at the dendrite core position was further increased compared with that of HIP1175, with a size and an area fraction of 1.19 μm and 67.2%, respectively.After HIP1195, the average size and the area fraction of the c′ phase reached 1.34 μm and 69.3%, respectively.Te cubicity of the dendritic c′ precipitates in HIP1185 and HIP1195 was also lower than that of the as-cast c′ phase, and the edge morphology was similar to the serrated edge morphology, as shown in Figures 6(c   Te stress rupture test of as-cast and three diferent HIP conditions was carried out at 760 °C and 645 MPa.At least three standard test samples were prepared for each condition alloy for mutual verifcation.Te stress rupture properties data are shown in Figure 8. Te stress rupture life of the ascast alloy was 107 h, and elongation was the lowest in the four conditions, only 3.0%.Te stress rupture life of HIP1195 was slightly lower than that of the as-cast alloy, which was only 103 h, but elongation was improved to 4.4%.Te stress rupture life of HIP1185 was 127 h, and elongation was 4.4%.Compared with those of the as-cast alloy, the stress rupture properties of HIP1185 were improved to a certain extent.When as-cast K417G was treated with HIP1175, the stress rupture life and elongation reached the highest values in the four conditions, the stress rupture life reached 197 h, and elongation was 5.2%.Terefore, from the above experimental results, it can be concluded that compared with the as-cast and three diferent HIP treatments, the 1175 °C/ 170 MPa HIP treatment of K417G as-cast can signifcantly improve the stress rupture properties of the alloy at 760 °C and 645 MPa. Te dislocation confguration near the fracture of the ascast alloy after the stress rupture test is shown in Figure 9(a).Te TEM morphology is taken from the area about 3 mm from the fracture.A typical dislocation network can be seen in the c matrix, and wide stacking faults (SFs) are shown in the c′ precipitates.Te existence of SFs in the c′ precipitates indicates that the shear mechanism of dislocations in the matrix interacts with the c′ precipitates.Among them, SFs caused by a/3<112> Shockley partial dislocations were widely recognized.Te a/2<110> dislocations in two different Burgers vectors break down into the c/c′ interface and transform into a/3<112> and a/6<112> two partial dislocations and SFs.Te a/6<112> partial dislocations stay at the c/c′ interface [25].
Te dislocation confguration in the HIP1175 condition is shown in Figure 9(b).Compared with the as-cast alloy, it can be seen that a large number of dislocations accumulate in the c matrix and the width of SFs inside the c′ phase becomes narrower.Tis phenomenon indicates that the dislocation generated in the matrix changes when it moves to the c/c′ interface and that the movement mechanism of the dislocation changes, which makes the dislocation no longer shear into the c′ phase by breaking down into partial dislocations.As the stress rupture life of the HIP1175 condition is longer, it can be inferred that the dislocations at the c/c′ interface may change the original motion direction of the dislocations when the process of shearing into the c′ phase is hindered.Te dislocations reduce the stress concentration at the interface by bypassing the c′ phase.When the extended dislocation is in the mechanical equilibrium, the relationship between the width of the stacking fault d 0 and the stacking fault energy F � c SFE is as follows: where b is the Burgers vector length of a partial dislocation, ] is Poisson's ratio, μ is the shear modulus, and φ is the angle between the Burgers vector and the dislocation line.

Advances in Materials Science and Engineering
According to the formula, it can be concluded that the stacking fault width is related to the stacking fault energy.Te narrower the stacking fault width, the higher the stacking fault energy.Te stacking fault width in the c′ phase after HIP treatment becomes narrower, which indicates that the stacking fault energy in the HIP condition is higher than that in the as-cast alloy so that the extended dislocation is difcult to form inside the alloy and SFs are reduced.In addition, the complete dislocations in the c matrix decompose into incomplete dislocations at the c/c′ interface and enter the c′ phase.When the extended dislocations entering the c′ phase increase, the dislocation density in the matrix decreases [26][27][28].

Efect of HIP on Porosity.
As known, the pore is a common crack source of shrinkage defects, so porosity is a very important parameter to characterize the performance of the alloy.HIP causes plastic deformation creep behavior to the internal pores of the structure, and the edge of the internal 6 Advances in Materials Science and Engineering pore position heals under the infuence of a difusion mechanism, resulting in a reduction in porosity [29,30].After HIP, the number of pores in the alloy is signifcantly reduced, as shown in Figure 3. Te average porosity in the original ascast alloy is about 0.33%, while the average porosity after HIP is only 0.04%.It can be inferred from the results that HIP can signifcantly reduce the pore defects of the K417G superalloy.
When the reduction of pores in the microstructure decreases the stress concentration around pores and the numbers of nucleation of crack sources, both life and plasticity for stress rupture properties are improved signifcantly [31].

Efect of HIP on Eutectic.
Te (c + c′) eutectic of Nibased superalloys is formed in the fnal stages of solidifcation, mainly distributed in the grain boundary and the interdendritic region, as shown in Figures 5 and 10. Figure 10(a) shows the eutectic structure of the as-cast alloy, and Figure 10(b) shows the eutectic structure of HIP1185.In the as-cast alloy, the (c + c′) eutectic was mainly in the form of a blocky sunfower and plate shape.Te as-cast sunfowershaped eutectic is usually composed of a tiny dense c/c′ phase.After HIP, the internal c matrix channels of some sunfower-shaped eutectic are separated into points and strips, and most of the regions become coarse c′ precipitates.
It can be inferred that, under the condition of the HIP at high temperature and high pressure, the c′ phase part inside the (c + c′) eutectic grows up and combines with the surrounding c′ phase and fnally transforms into a coarse c′ phase with a small number of points and strips c channels.In addition, combined with the change in the eutectic content shown in Figure 4, it can be inferred that, as the temperature of HIP treatment gradually increases, the eutectic content decreases due to the partial dissolution.Previous research has shown that the brittle characteristics of the eutectic at low and medium temperatures make it easy to become a weak region of deformation and fracture [32,33].Terefore, the low content of the (c + c′) eutectic can improve the stress rupture life.

Efect of HIP on the c′ Phase.
In general, the c′ phase is the main strengthening phase in nickel-based superalloys; the size, morphology, and area fraction of the c′ phase have a direct efect on the properties of the superalloy [34][35][36].
Te size and the area fraction of the c′ phase in as-cast and three diferent HIP temperatures were compared, as shown in Figures 7 and 11.Te area fraction increases slightly after HIP, with an average increase of 3.6%, which is related to the increase in the c′ phase size.It can be seen that the size of the c′ phase increases obviously after HIP.As shown in Figure 7, the sizes of the c′ phase in the dendrite cores of HIP1175, HIP1185, and HIP1195 increase from 0.65 μm in the as-cast alloy to 0.84 μm, 1.19 μm, and 1.34 μm, respectively.Combining the morphology evolution shown in Figures 6 and 12, it can be seen that the size of the primary c′ phase increases after HIP and that the c matrix is accompanied by fne c′ precipitation of the phase.With an increase in the HIP temperature, the primary c′ phase and the fne c′ phase continue to grow.It is well known that when the c′ phase size is too coarse, it has an adverse efect on performance improvement.Terefore, the proportion of diferent sizes of the c′ phase on the dendrite core is calculated, as shown in Figure 11.It can be seen that the size of the c′ phase in the ascast alloy is mainly distributed in the range of 0.5 μm-1.0 μm, accounting for 85%, while the proportion of <0.5 μm is only 11%.Te proportion of the c′ phase size <0.5 μm in HIP1185 is 16%, which is higher than that in the as-cast alloy.Te proportion of the c′ phase size <0.5 μm in HIP1175 is highest in four diferent conditions, reaching 20%.Te c′ phase size of HIP1195 is only 7% of the proportion of less than 0.5 μm.In addition, the movement of dislocations is divided into Orowan bypass and dislocation shearing mechanisms, and the movement mode is determined by the shear stress required to pass through particles.As the particle size changes, the maximum critical resolved shear stress required for dislocation movement will also change.Te greater the critical resolved shear stress, the greater the hindrance to dislocation movement.It was reported that higher creep deformation occurs in the alloy Advances in Materials Science and Engineering microstructure with a larger c′ phase and hence deteriorates the creep properties of the superalloy [37][38][39][40].Te alloy containing precipitates with an average diameter of no more than 0.5 μm stores more dislocations locally, which also causes a higher accumulation of average dislocation density, as shown in Figure 9. Terefore, when the size of precipitates is no more than 0.5 μm in the alloy microstructure, it will be benefcial to the extension of the stress rupture life.Compared with the as-cast structure, pore defects have been well eliminated after HIP treatment, and stress rupture properties can be efectively improved.In the three HIP conditions, the proportion of the c′ phase with <0.5 μm of HIP1175 is higher than that of the other two HIP conditions and the stress rupture life is signifcantly extended.Moreover, with an increase in the HIP temperature, the fne c′ phase and the primary c′ phase further grow up and the smaller size of the c′ phase decreases.Terefore, the overall size increases, and the improvement of its stress rupture life gradually decreases.When the overall size of the c′ phase is too large, the stress rupture life is shortened.
In addition to the abovementioned c′ phase size, the serrated morphology at the c′ phase edge is also found in this study, as shown in Figures 6(b)-6(d)).Tis phenomenon is mainly due to interface difusion at the c/c′ interface during the thermal process, and the Al element in the Ni lattice is driven by the elastic strain energy of dislocations to interdifuse, making the c/c′ interface uneven [41].Te diffusion of the interface can improve the stress concentration between the two phases, and elongation is increased during the mechanical test after HIP.

Efect of HIP on the Deformation Mechanism.
After HIP at 1175 °C/170 MPa for 4 h, the K417G alloy was subjected to the stress rupture test and the dislocation confguration of the primary and fne c′ precipitates was studied by TEM, as shown in Figure 13.A large number of dislocations are mainly distributed in the c matrix, which are blocked by the relative slip of the c′ precipitates.With the continuous deformation process, dislocations in the matrix will take place in a series of reactions at the c/c′ interface.However, dislocations do not always stay at the interface but move by bypassing or shearing two types of dislocation mechanisms.Depending on the size of the shear stress of the two mechanisms, the movement is diferent.For example, when the stress required for the dislocation to bypass the precipitate is greater than the stress required to shear the precipitate, dislocations exhibit shearing into the precipitate.As shown in Figure 13(a), the internal morphology of dislocation shearing into the precipitate can be clearly seen in the larger primary c′ precipitates.When the stress required for the dislocation to bypass the precipitate is less than the stress required to shear precipitates, an Orowan

Efect of HIP on Element Segregation in Microstructures.
After HIP treatment, the distribution of diferent elements in dendrite cores and interdendritic regions also changed.Table 3 and Figure 14 show the composition segregation comparison of the K417G alloy in four diferent conditions.Te results are consistent with the inference of dendritic morphology shown in Figure 5.After HIP treatment, the element segregation between dendrite cores and interdendritic regions are obviously improved and the segregation degree of Ti, Mo, Cr, and Al elements is obviously reduced.In addition, HIP for coelement segregation change is not obvious.HIP treatment can relieve the segregation of elements in the dendrite cores and interdendritic regions, also reducing the content of blocky eutectic in the interdendritic region (Figure 4).Usually, during the process of plastic deformation, the eutectic can hinder the movement of dislocations or slip bands.Terefore, the decrease in the eutectic content makes dislocations or slip bands easier to move inside, and thus, plastic deformation is easy to occur.

Efect of HIP on Stress Rupture
Properties.Te results of the stress rupture test in Figure 6 show that the appropriate HIP treatment can improve the stress rupture properties at low-temperature/high-stress conditions.Due to the hightemperature and high-pressure conditions of HIP, micropores begin to close, so as to obtain a smaller pore area fraction, which makes the nucleation of microcracks difcult during the stress rupture test.It can be seen from the fgure that the internal pore content of the as-cast structure is relatively high and that porosity decreases signifcantly after HIP.Terefore, the stress rupture properties of the K417G alloy in the HIP condition is signifcantly improved.HIP not only reduces pores inside the as-cast structure and improves performance but also changes the size of the c′ phase signifcantly.Appropriate HIP treatment can increase the proportion of the c′ phase size <0.5 μm and extend the stress rupture life.When the HIP temperature increases from 1175 °C to 1195 °C, the c′ phase changes, which makes the stress rupture life gradually decrease.Te high proportion of fne c′ phase makes the stress rupture life of HIP1175 also longer, while the size of the c′ phase in HIP1185 and HIP1195 is obviously coarse, which makes creep resistance worse.In addition, the low content of the (c + c′) eutectic after HIP reduces the weak region of deformation fracture, the probability of fracture in the same environmental conditions decreases, and the fracture resistance increases.
Because the alloy is easy to form (c + c′) eutectic in the fnal solidifcation process of the grain boundary and the interdendritic region, the eutectic content is relatively high in the as-cast alloy, with an average content of 3.9%, as shown in Figure 4.After HIP, the eutectic content of the ascast alloy is reduced by about 0.9% on average, which reduces the dislocation hindrance during the performance test, so the decrease in the eutectic content makes dislocations easier to move and is accompanied by an increase in elongation.Subsequently, the mutual difusion of elements at the c/c′ interface can reduce the stress concentration after HIP.Terefore, compared with those of the as-cast condition, the plasticity and mechanical properties of the K417G alloy after HIP treatment are signifcantly improved.In the study of single crystal superalloy, in the comparison of tensile properties at 760 °C with and without HIP, as shown in Figure 15, the yield strength remains the same and the average elongation of HIP is also signifcantly increased by 45.3%.Its research on intermediate temperature plasticity at 760 °C is consistent with the stress rupture test results that elongation is increased by 60% at 760 °C, 645 MPa.Te increase in elongation is related to the decrease in the eutectic content and the improvement in element segregation.Table 3: Dendrite compositions of the K417G superalloy in as-cast, HIP1175, HIP1185, and HIP1195 conditions were determined by EPMA (wt%), as well as microsegregation coefcients, k′, the ratio of the concentration (wt%) of an element in the dendrite core to that in the interdendritic region.Advances in Materials Science and Engineering

Conclusions
Te efects of the HIP process on the microstructure and stress rupture properties of the K417G superalloy were systematically investigated.It is concluded that the HIP process at 1175 °C/170 MPa can improve the stress rupture properties of the K417G alloy at 760 °C, 645 MPa.Te main conclusions are as follows: (1) Te average size of the c′ phase increases after HIP, and the fne c′ phase reprecipitates in the matrix channel.After HIP treatment, the size of the c′ phase can be changed.When the proportion of the c′ phase size <0.5 μm increases, the stress rupture life is extended.With an increase in the HIP temperature, the c′ phase continues to grow and the proportion of the c′ phase size <0.5 μm begins to decrease, so the improvement efect on the stress rupture life is relieved.(2) Te dislocation confgurations of the K417G alloy changed after HIP, and there were wide SFs in the ascast c′ phase.After HIP, the SF width in the c′ phase becomes narrow.Terefore, after HIP, the stacking fault energy increases.(3) Te HIP process can efectively reduce the porosity of the K417G alloy, and with an increase in the HIP temperature, porosity is further reduced, but the efect is reduced.At the same time, HIP can efectively reduce the (c + c′) eutectic, relieve endrite segregation,and homogenize the alloy composition.All these changes have a prominent efect on the improvement of stress rupture properties.(4) Compared with the as-cast alloy, composition segregation is signifcantly improved after the HIP treatment of high temperature and high pressure and the (c + c′) eutectic content in the grain boundary and the interdendritic region is reduced, thus reducing the hindrance to the dislocation or slip band generated during the stress rupture test, so the plasticity of the HIP condition is signifcantly improved.

Figure 1 :
Figure 1: DSC results for the K417G alloy during the heating process.

4
Advances in Materials Science and Engineering 6(d)).In addition, fne c′ precipitates can be seen in the c matrix channel between the dendritic c′ precipitates after HIP.

Figure 7 :Figure 8 :Figure 6 :
Figure 7: Average size and area fraction of c′ precipitates in diferent conditions of the K417G alloy.

Figure 9 :
Figure 9: TEM images showing the confguration of SFs and dislocations in as-cast (a) and HIP1175 (b).

Figure 12 :
Figure 12: Schematic of morphology evolution of the as-cast microstructure after HIP.

Figure 13 :Figure 14 :
Figure 13: TEM micrographs of the K417G alloy after the stress rupture test at 760 °C and 645 MPa.Primary c′ phase particles sheared by dislocations (a) and (b) fne c′ phase particles surrounded by the Orowan loop.

Figure 15 :
Figure15: Engineering stress/strain curves of samples without and with HIP at 760 °C[18].

Table 2 :
Te HIP regimes applied in this study.