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

Authors: Yuanhong Lin; Lihui Zhang; Hongmin Guo
Journal: Advances in Materials Science and Engineering
 (2023)

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## Abstract

The effect of hot isostatic pressing (HIP) on the stress rupture property of K417G nickel-based superalloy is studied experimentally. The results show that the stress rupture property of the alloy is improved by the appropriate HIP process. After HIP, the average microporosity of alloy after HIP is 0.04%, which is reduced by nearly 90% compared with the as-cast microstructure. The cubicγ′ precipitates grow up, accompanied by the precipitation of the fine γ′ phase in the matrix. In addition, the area fraction of (γ + γ′) eutectic decreases, dendritic segregation is improved, and the element distribution is homogenized. As the HIP temperature increased from 1175°C to 1195°C, the microporosity and eutectic are further reduced slightly. The stress rupture life at 760°C and 645 MPa of alloys after HIP at 1175°C and 1185°C increased by 90 h and 20 h, respectively, compared with the as-cast alloy, but it is not the case for HIP at 1195°C. In addition, the elongation after HIP is increased by about 60% compared to the as-cast alloy, with an average of 4.7%. The changes in the γ′ phase and the deformation mechanism after HIP are discussed.


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## Body

## 1. Introduction

Since the 1940s, casting superalloys have been rapidly developed in the aerospace field and are widely used in aero-engine 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 [1–4]. K417G, a nickel-based precipitation-hardened cast superalloy, has excellent high-temperature mechanical properties and hot-corrosion resistance after being modified 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 effect 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 effective healing of metallurgical defects such as pores and segregation in the alloy [10–12]. 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 γ′ precipitates were significantly 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 γ″ phase nucleates at the γ′/γ interface and merges with the γ′ phase to form the γ′/γ′′ eutectic phase. In addition, the δ phase in the microstructure is significantly reduced [16]. The 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 difficult 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 significantly 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. The 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 nickel-based single crystal superalloy in as-solid-solution sates, He et al. [13] significantly 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 significantly 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 effects of different 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. The influence mechanism of different HIP on the microstructure of the alloy was compared and analyzed, and an optimal HIP process of the K417G alloy was explored. This work provides reference for the study of the K417G alloy and the HIP process.


## 2. Experimental

The K417G superalloy was smelted in a 25 kg semicontinuous vacuum induction furnace. The nominal chemical composition of the K417G alloy is shown in Table1. In order to investigate the effect of HIP treatment on the as-cast alloy, it was decided to compare the HIP process at different temperatures with the ordinary as-cast alloy. The K417G alloy was analyzed by differential scanning calorimetry (DSC) before HIP treatment. The results are shown in Figure 1. During the heating process, the solidus temperature was 1290°C and the liquidus temperature was 1321°C. The dissolution temperature of the γ′ 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 effect on eliminating defects. Therefore, 1195°C was set as the highest hot isostatic pressing process temperature, and then, two groups of HIP processes were selected at different temperatures. The 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. The specific HIP process parameters are shown in Table 2.Table 1
Nominal composition of the K417G superalloy (wt%).
CCrCoMoAlTiMnVBZrNi0.179.010.03.05.254.40.20.750.0180.07Bal.Figure 1
DSC results for the K417G alloy during the heating process.Table 2
The HIP regimes applied in this study.
NumberSpecimen no.Temperature (°C)Pressure (MPa)Soaking time (h)1As-cast———2HIP1175117517043HIP1185118517044HIP119511951704Samples for metallographic analysis were cut on the standard sections of the as-cast and three different HIP specimens. The samples were sliced, ground, and polished, and then, the micropore was observed using an optical microscope (OM). Then, the dendrite morphology of the samples was observed by chemical etching. The composition of the etching solution was 20 mL HCl + 5 g CuSO4 + 25 mL H2O. After regrinding and polishing, the samples were placed in a solution of H3PO4, HNO3, and H2SO4 at a ratio of 1 : 3 : 5 for electrolytic corrosion, the corrosion voltage was 3.5 V, and the corrosion time was 5 s. The microstructure was observed by using the TESCAN CLARA field emission scanning electron microscope (SEM). The 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 (γ + γ′) eutectic phases. The micropore and area fraction of the (γ + γ′) eutectic phase and the average size, area fraction, and size distribution of the γ′ precipitates were measured by using Image-Pro Plus software. Each set of values was calculated using at least three representative images and averaged.The test bar was machined into a specified standard stress rupture sample, and the size of the stress rupture sample is shown in Figure2. The stress rupture test was performed at 760°C and 645 MPa. After the stress rupture test, the dislocation configurations were observed on an FEI Talos F200X transmission electron microscope (TEM). The 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.Figure 2
Stress rupture test specimen of the K417G alloy.


## 3. Results

As shown in Figure3, for the K417G alloy, the micropore and carbide morphology can be observed inside the microstructure without corrosion using an optical microscope. The micropore in the as-cast alloy is clearly shown in Figure 3(a), and the porosity was about 0.33%. The statistical results of the micropore area fraction are shown in Figure 4. Then, after HIP at different temperatures, the micropores of the alloys are significantly reduced compared with those of the as-cast alloy. The 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 different 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 [22–24].Figure 3
Images of micropore and carbide morphology of K417G in as-cast (a), HIP1170 (b), HIP1185 (c), and HIP1195 (d).
(a)(b)(c)(d)Figure 4
Area fraction of the microporosity and eutectic of the K417G alloy in as-cast and HIP conditions.Figure5 shows the dendrite morphology image of the K417G alloy after different 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. The 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 difficult to distinguish by observing the depth after chemical etching. Therefore, 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 (γ + γ′) eutectic was mainly distributed in the interdendritic region and the grain boundary.Figure 5
Dendrite morphology of the K417G alloy under as-cast (a), HIP1170 (b), HIP1185 (c), and HIP1195 (d).
(a)(b)(c)(d)The statistics of the eutectic content of the four conditions of the alloy are shown in Figure4. It can be seen that the eutectic content was about 3.99% on average in the as-cast alloy. After HIP treatment, part of the (γ + γ′) 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. The eutectic content decreased from 3.27% in HIP1175 to 2.99% in HIP1185 and then to 2.95% in HIP1195. The microstructural evolution trend of eutectic in three different HIP conditions was not significant, which was similar to the evolution of porosity in different HIP conditions. It is indicated that the resolution effect of eutectic is not obvious during HIP treatment. The healing effect 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.The morphology of theγ′ 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 γ′ phase in different conditions were measured, as shown in Figure 7. The γ′ 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 γ′ precipitates were 0.65 μm and 63.9%, respectively. After different HIP temperatures, the γ′ phase exhibits a different degree of morphological evolution. After HIP1175 treatment, the size of the γ′ 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 γ′ precipitates was different from that in the as-cast alloy; the edge morphology of the γ′ precipitates was uneven and similar to that of the serrated γ′ precipitates, as shown in Figure 6(b). After HIP1185 treatment, the size of the γ′ 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 γ′ phase reached 1.34 μm and 69.3%, respectively. The cubicity of the dendritic γ′ precipitates in HIP1185 and HIP1195 was also lower than that of the as-cast γ′ phase, and the edge morphology was similar to the serrated edge morphology, as shown in Figures 6(c) and 6(d)). In addition, fine γ′ precipitates can be seen in the γ matrix channel between the dendritic γ′ precipitates after HIP.Figure 6
Comparison ofγ′ precipitates in as-cast (a), HIP1175 (b), HIP1185 (c), and HIP1195 (d) dendrite cores of the K417G alloy.
(a)(b)(c)(d)Figure 7
Average size and area fraction ofγ′ precipitates in different conditions of the K417G alloy.The stress rupture test of as-cast and three different 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 verification. The stress rupture properties data are shown in Figure8. The stress rupture life of the as-cast alloy was 107 h, and elongation was the lowest in the four conditions, only 3.0%. The 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%. The 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%. Therefore, from the above experimental results, it can be concluded that compared with the as-cast and three different HIP treatments, the 1175°C/170 MPa HIP treatment of K417G as-cast can significantly improve the stress rupture properties of the alloy at 760°C and 645 MPa.Figure 8
Stress rupture properties of K417G alloy in different conditions at 760°C and 645 MPa.The dislocation configuration near the fracture of the as-cast alloy after the stress rupture test is shown in Figure9(a). The TEM morphology is taken from the area about 3 mm from the fracture. A typical dislocation network can be seen in the γ matrix, and wide stacking faults (SFs) are shown in the γ′ precipitates. The existence of SFs in the γ′ precipitates indicates that the shear mechanism of dislocations in the matrix interacts with the γ′ precipitates. Among them, SFs caused by a/3<112> Shockley partial dislocations were widely recognized. The a/2<110> dislocations in two different Burgers vectors break down into the γ/γ′ interface and transform into a/3<112> and a/6<112> two partial dislocations and SFs. The a/6<112> partial dislocations stay at the γ/γ′ interface [25].Figure 9
TEM images showing the configuration of SFs and dislocations in as-cast (a) and HIP1175 (b).
(a)(b)The dislocation configuration in the HIP1175 condition is shown in Figure9(b). Compared with the as-cast alloy, it can be seen that a large number of dislocations accumulate in the γ matrix and the width of SFs inside the γ′ phase becomes narrower. This phenomenon indicates that the dislocation generated in the matrix changes when it moves to the γ/γ′ interface and that the movement mechanism of the dislocation changes, which makes the dislocation no longer shear into the γ′ 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 γ/γ′ interface may change the original motion direction of the dislocations when the process of shearing into the γ′ phase is hindered. The dislocations reduce the stress concentration at the interface by bypassing the γ′ phase. When the extended dislocation is in the mechanical equilibrium, the relationship between the width of the stacking fault d0 and the stacking fault energy F=γSFE is as follows:(1)d0=μb28πγSFE.2−v1−v1−2vcos2φ2−v,where b is the Burgers vector length of a partial dislocation, ν is Poisson’s ratio, μ is the shear modulus, and φ is the angle between the Burgers vector and the dislocation line. According to the formula, it can be concluded that the stacking fault width is related to the stacking fault energy. The narrower the stacking fault width, the higher the stacking fault energy. The stacking fault width in the γ′ 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 difficult to form inside the alloy and SFs are reduced. In addition, the complete dislocations in the γ matrix decompose into incomplete dislocations at the γ/γ′ interface and enter the γ′ phase. When the extended dislocations entering the γ′ phase increase, the dislocation density in the matrix decreases [26–28].


## 4. Discussion

### 4.1. Effect 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 and creep behavior to the internal pores of the structure, and the edge of the internal pore position heals under the influence of a diffusion mechanism, resulting in a reduction in porosity [29, 30]. After HIP, the number of pores in the alloy is significantly reduced, as shown in Figure 3. The average porosity in the original as-cast 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 significantly 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 significantly [31].


### 4.2. Effect of HIP on Eutectic

The (γ + γ′) eutectic of Ni-based superalloys is formed in the final stages of solidification, 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 (γ + γ′) eutectic was mainly in the form of a blocky sunflower and plate shape. The as-cast sunflower-shaped eutectic is usually composed of a tiny dense γ/γ′ phase. After HIP, the internal γ matrix channels of some sunflower-shaped eutectic are separated into points and strips, and most of the regions become coarse γ′ precipitates. It can be inferred that, under the condition of the HIP at high temperature and high pressure, the γ′ phase part inside the (γ + γ′) eutectic grows up and combines with the surrounding γ′ phase and finally transforms into a coarse γ′ phase with a small number of points and strips γ 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]. Therefore, the low content of the (γ + γ′) eutectic can improve the stress rupture life.Figure 10
Comparison of the (γ + γ′) eutectic structure between as-cast and HIP1185.
(a)(b)


### 4.3. Effect of HIP on theγ′ Phase

In general, theγ′ phase is the main strengthening phase in nickel-based superalloys; the size, morphology, and area fraction of the γ′ phase have a direct effect on the properties of the superalloy [34–36]. The size and the area fraction of the γ′ phase in as-cast and three different HIP temperatures were compared, as shown in Figures 7 and 11. The area fraction increases slightly after HIP, with an average increase of 3.6%, which is related to the increase in the γ′ phase size. It can be seen that the size of the γ′ phase increases obviously after HIP. As shown in Figure 7, the sizes of the γ′ 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 γ′ phase increases after HIP and that the γ matrix is accompanied by fine γ′ precipitation of the phase. With an increase in the HIP temperature, the primary γ′ phase and the fine γ′ phase continue to grow. It is well known that when the γ′ phase size is too coarse, it has an adverse effect on performance improvement. Therefore, the proportion of different sizes of the γ′ phase on the dendrite core is calculated, as shown in Figure 11. It can be seen that the size of the γ′ phase in the as-cast 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%. The proportion of the γ′ phase size <0.5 μm in HIP1185 is 16%, which is higher than that in the as-cast alloy. The proportion of the γ′ phase size <0.5 μm in HIP1175 is highest in four different conditions, reaching 20%. The γ′ 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. The greater the critical resolved shear stress, the greater the hindrance to dislocation movement. It was reported that higher creep deformation occurs in the alloy microstructure with a larger γ′ phase and hence deteriorates the creep properties of the superalloy [37–40]. The 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. Therefore, when the size of precipitates is no more than 0.5 μm in the alloy microstructure, it will be beneficial 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 effectively improved. In the three HIP conditions, the proportion of the γ′ phase with <0.5 μm of HIP1175 is higher than that of the other two HIP conditions and the stress rupture life is significantly extended. Moreover, with an increase in the HIP temperature, the fine γ′ phase and the primary γ′ phase further grow up and the smaller size of the γ′ phase decreases. Therefore, the overall size increases, and the improvement of its stress rupture life gradually decreases. When the overall size of the γ′ phase is too large, the stress rupture life is shortened.Figure 11
Size distribution ofγ′ precipitates in different conditions of the K417G alloy.Figure 12
Schematic of morphology evolution of the as-cast microstructure after HIP.In addition to the abovementionedγ′ phase size, the serrated morphology at the γ′ phase edge is also found in this study, as shown in Figures 6(b)–6(d)). This phenomenon is mainly due to interface diffusion at the γ/γ′ interface during the thermal process, and the Al element in the Ni lattice is driven by the elastic strain energy of dislocations to interdiffuse, making the γ/γ′ interface uneven [41]. The diffusion of the interface can improve the stress concentration between the two phases, and elongation is increased during the mechanical test after HIP.


### 4.4. Effect 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 configuration of the primary and fineγ′ precipitates was studied by TEM, as shown in Figure 13. A large number of dislocations are mainly distributed in the γ matrix, which are blocked by the relative slip of the γ′ precipitates. With the continuous deformation process, dislocations in the matrix will take place in a series of reactions at the γ/γ′ 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 different. 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 γ′ precipitates. When the stress required for the dislocation to bypass the precipitate is less than the stress required to shear precipitates, an Orowan loop is formed around the precipitates, as shown in Figure 13(b), where the Orowan loop formed by the dislocation bypassing the precipitates was exhibited around the smaller γ′ phase in the matrix. This phenomenon is consistent with the previous analysis of the effect of the γ′ precipitate size on the dislocation mechanism.Figure 13
TEM micrographs of the K417G alloy after the stress rupture test at 760°C and 645 MPa. Primaryγ′ phase particles sheared by dislocations (a) and (b) fine γ′ phase particles surrounded by the Orowan loop.
(a)(b)


### 4.5. Effect of HIP on Element Segregation in Microstructures

After HIP treatment, the distribution of different elements in dendrite cores and interdendritic regions also changed. Table3 and Figure 14 show the composition segregation comparison of the K417G alloy in four different conditions. The 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. Therefore, the decrease in the eutectic content makes dislocations or slip bands easier to move inside, and thus, plastic deformation is easy to occur.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 coefficients,k′, the ratio of the concentration (wt%) of an element in the dendrite core to that in the interdendritic region.
SpecimenRegionNiCoCrAlTiMoVAs-castDendrite68.5610.718.715.873.072.080.84Interdendrite66.8510.2810.245.033.832.390.71k′1.031.040.851.170.800.871.18HIP1175Dendrite68.5910.399.015.373.502.290.81Interdendrite67.289.989.905.163.782.390.78k′1.021.040.911.040.930.961.04HIP1185Dendrite67.0910.859.325.443.382.300.83Interdendrite66.9210.559.835.283.422.390.81k′1.001.030.951.030.990.961.02HIP1195Dendrite68.0610.899.155.623.362.680.72Interdendrite67.7410.349.735.403.402.800.71k′1.001.050.941.040.990.961.01Figure 14
Microsegregation coefficient in the dendrite core and interdendritic region of as-cast and different HIP treatments of K417G alloys,k′.


### 4.6. Effect of HIP on Stress Rupture Properties

The results of the stress rupture test in Figure6 show that the appropriate HIP treatment can improve the stress rupture properties at low-temperature/high-stress conditions. Due to the high-temperature 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 difficult during the stress rupture test. It can be seen from the figure that the internal pore content of the as-cast structure is relatively high and that porosity decreases significantly after HIP. Therefore, the stress rupture properties of the K417G alloy in the HIP condition is significantly improved. HIP not only reduces pores inside the as-cast structure and improves performance but also changes the size of the γ′ phase significantly. Appropriate HIP treatment can increase the proportion of the γ′ phase size <0.5 μm and extend the stress rupture life. When the HIP temperature increases from 1175°C to 1195°C, the γ′ phase changes, which makes the stress rupture life gradually decrease. The high proportion of fine γ′ phase makes the stress rupture life of HIP1175 also longer, while the size of the γ′ phase in HIP1185 and HIP1195 is obviously coarse, which makes creep resistance worse. In addition, the low content of the (γ + γ′) 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 (γ + γ′) eutectic in the final solidification 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 as-cast 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 diffusion of elements at the γ/γ′ interface can reduce the stress concentration after HIP. Therefore, compared with those of the as-cast condition, the plasticity and mechanical properties of the K417G alloy after HIP treatment are significantly 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 significantly 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. The increase in elongation is related to the decrease in the eutectic content and the improvement in element segregation.Figure 15
Engineering stress/strain curves of samples without and with HIP at 760°C [18].


## 4.1. Effect 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 and creep behavior to the internal pores of the structure, and the edge of the internal pore position heals under the influence of a diffusion mechanism, resulting in a reduction in porosity [29, 30]. After HIP, the number of pores in the alloy is significantly reduced, as shown in Figure 3. The average porosity in the original as-cast 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 significantly 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 significantly [31].


## 4.2. Effect of HIP on Eutectic

The (γ + γ′) eutectic of Ni-based superalloys is formed in the final stages of solidification, 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 (γ + γ′) eutectic was mainly in the form of a blocky sunflower and plate shape. The as-cast sunflower-shaped eutectic is usually composed of a tiny dense γ/γ′ phase. After HIP, the internal γ matrix channels of some sunflower-shaped eutectic are separated into points and strips, and most of the regions become coarse γ′ precipitates. It can be inferred that, under the condition of the HIP at high temperature and high pressure, the γ′ phase part inside the (γ + γ′) eutectic grows up and combines with the surrounding γ′ phase and finally transforms into a coarse γ′ phase with a small number of points and strips γ 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]. Therefore, the low content of the (γ + γ′) eutectic can improve the stress rupture life.Figure 10
Comparison of the (γ + γ′) eutectic structure between as-cast and HIP1185.
(a)(b)


## 4.3. Effect of HIP on theγ′ Phase

In general, theγ′ phase is the main strengthening phase in nickel-based superalloys; the size, morphology, and area fraction of the γ′ phase have a direct effect on the properties of the superalloy [34–36]. The size and the area fraction of the γ′ phase in as-cast and three different HIP temperatures were compared, as shown in Figures 7 and 11. The area fraction increases slightly after HIP, with an average increase of 3.6%, which is related to the increase in the γ′ phase size. It can be seen that the size of the γ′ phase increases obviously after HIP. As shown in Figure 7, the sizes of the γ′ 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 γ′ phase increases after HIP and that the γ matrix is accompanied by fine γ′ precipitation of the phase. With an increase in the HIP temperature, the primary γ′ phase and the fine γ′ phase continue to grow. It is well known that when the γ′ phase size is too coarse, it has an adverse effect on performance improvement. Therefore, the proportion of different sizes of the γ′ phase on the dendrite core is calculated, as shown in Figure 11. It can be seen that the size of the γ′ phase in the as-cast 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%. The proportion of the γ′ phase size <0.5 μm in HIP1185 is 16%, which is higher than that in the as-cast alloy. The proportion of the γ′ phase size <0.5 μm in HIP1175 is highest in four different conditions, reaching 20%. The γ′ 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. The greater the critical resolved shear stress, the greater the hindrance to dislocation movement. It was reported that higher creep deformation occurs in the alloy microstructure with a larger γ′ phase and hence deteriorates the creep properties of the superalloy [37–40]. The 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. Therefore, when the size of precipitates is no more than 0.5 μm in the alloy microstructure, it will be beneficial 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 effectively improved. In the three HIP conditions, the proportion of the γ′ phase with <0.5 μm of HIP1175 is higher than that of the other two HIP conditions and the stress rupture life is significantly extended. Moreover, with an increase in the HIP temperature, the fine γ′ phase and the primary γ′ phase further grow up and the smaller size of the γ′ phase decreases. Therefore, the overall size increases, and the improvement of its stress rupture life gradually decreases. When the overall size of the γ′ phase is too large, the stress rupture life is shortened.Figure 11
Size distribution ofγ′ precipitates in different conditions of the K417G alloy.Figure 12
Schematic of morphology evolution of the as-cast microstructure after HIP.In addition to the abovementionedγ′ phase size, the serrated morphology at the γ′ phase edge is also found in this study, as shown in Figures 6(b)–6(d)). This phenomenon is mainly due to interface diffusion at the γ/γ′ interface during the thermal process, and the Al element in the Ni lattice is driven by the elastic strain energy of dislocations to interdiffuse, making the γ/γ′ interface uneven [41]. The diffusion of the interface can improve the stress concentration between the two phases, and elongation is increased during the mechanical test after HIP.


## 4.4. Effect 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 configuration of the primary and fineγ′ precipitates was studied by TEM, as shown in Figure 13. A large number of dislocations are mainly distributed in the γ matrix, which are blocked by the relative slip of the γ′ precipitates. With the continuous deformation process, dislocations in the matrix will take place in a series of reactions at the γ/γ′ 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 different. 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 γ′ precipitates. When the stress required for the dislocation to bypass the precipitate is less than the stress required to shear precipitates, an Orowan loop is formed around the precipitates, as shown in Figure 13(b), where the Orowan loop formed by the dislocation bypassing the precipitates was exhibited around the smaller γ′ phase in the matrix. This phenomenon is consistent with the previous analysis of the effect of the γ′ precipitate size on the dislocation mechanism.Figure 13
TEM micrographs of the K417G alloy after the stress rupture test at 760°C and 645 MPa. Primaryγ′ phase particles sheared by dislocations (a) and (b) fine γ′ phase particles surrounded by the Orowan loop.
(a)(b)


## 4.5. Effect of HIP on Element Segregation in Microstructures

After HIP treatment, the distribution of different elements in dendrite cores and interdendritic regions also changed. Table3 and Figure 14 show the composition segregation comparison of the K417G alloy in four different conditions. The 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. Therefore, the decrease in the eutectic content makes dislocations or slip bands easier to move inside, and thus, plastic deformation is easy to occur.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 coefficients,k′, the ratio of the concentration (wt%) of an element in the dendrite core to that in the interdendritic region.
SpecimenRegionNiCoCrAlTiMoVAs-castDendrite68.5610.718.715.873.072.080.84Interdendrite66.8510.2810.245.033.832.390.71k′1.031.040.851.170.800.871.18HIP1175Dendrite68.5910.399.015.373.502.290.81Interdendrite67.289.989.905.163.782.390.78k′1.021.040.911.040.930.961.04HIP1185Dendrite67.0910.859.325.443.382.300.83Interdendrite66.9210.559.835.283.422.390.81k′1.001.030.951.030.990.961.02HIP1195Dendrite68.0610.899.155.623.362.680.72Interdendrite67.7410.349.735.403.402.800.71k′1.001.050.941.040.990.961.01Figure 14
Microsegregation coefficient in the dendrite core and interdendritic region of as-cast and different HIP treatments of K417G alloys,k′.


## 4.6. Effect of HIP on Stress Rupture Properties

The results of the stress rupture test in Figure6 show that the appropriate HIP treatment can improve the stress rupture properties at low-temperature/high-stress conditions. Due to the high-temperature 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 difficult during the stress rupture test. It can be seen from the figure that the internal pore content of the as-cast structure is relatively high and that porosity decreases significantly after HIP. Therefore, the stress rupture properties of the K417G alloy in the HIP condition is significantly improved. HIP not only reduces pores inside the as-cast structure and improves performance but also changes the size of the γ′ phase significantly. Appropriate HIP treatment can increase the proportion of the γ′ phase size <0.5 μm and extend the stress rupture life. When the HIP temperature increases from 1175°C to 1195°C, the γ′ phase changes, which makes the stress rupture life gradually decrease. The high proportion of fine γ′ phase makes the stress rupture life of HIP1175 also longer, while the size of the γ′ phase in HIP1185 and HIP1195 is obviously coarse, which makes creep resistance worse. In addition, the low content of the (γ + γ′) 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 (γ + γ′) eutectic in the final solidification 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 as-cast 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 diffusion of elements at the γ/γ′ interface can reduce the stress concentration after HIP. Therefore, compared with those of the as-cast condition, the plasticity and mechanical properties of the K417G alloy after HIP treatment are significantly 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 significantly 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. The increase in elongation is related to the decrease in the eutectic content and the improvement in element segregation.Figure 15
Engineering stress/strain curves of samples without and with HIP at 760°C [18].


## 5. Conclusions

The effects 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. The main conclusions are as follows:(1)
The average size of theγ′ phase increases after HIP, and the fine γ′ phase reprecipitates in the matrix channel. After HIP treatment, the size of the γ′ phase can be changed. When the proportion of the γ′ phase size <0.5 μm increases, the stress rupture life is extended. With an increase in the HIP temperature, the γ′ phase continues to grow and the proportion of the γ′ phase size <0.5 μm begins to decrease, so the improvement effect on the stress rupture life is relieved.(2)
The dislocation configurations of the K417G alloy changed after HIP, and there were wide SFs in the as-castγ′ phase. After HIP, the SF width in the γ′ phase becomes narrow. Therefore, after HIP, the stacking fault energy increases.(3)
The HIP process can effectively reduce the porosity of the K417G alloy, and with an increase in the HIP temperature, porosity is further reduced, but the effect is reduced. At the same time, HIP can effectively reduce the (γ + γ′) eutectic, relieve endrite segregation,and homogenize the alloy composition. All these changes have a prominent effect on the improvement of stress rupture properties.(4)
Compared with the as-cast alloy, composition segregation is significantly improved after the HIP treatment of high temperature and high pressure and the (γ + γ′) 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 significantly improved.



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*Source: 1786742-2023-02-08.xml*