Effects of Hot Isostatic Pressing (HIP) on Microstructure and Mechanical Properties of K403 Nickel-Based Superalloy

Effects of HIP on casting defects

Figure 1 shows the cross-sectional SEM images of the specimens, which indicates the casting defects in non-HIPed and HIPed specimens. As we can see from Figure 1(a), the as-cast K403 superalloy contains larger area of microporosity and a certain number of micropores. After HIP, however, the microporosity significantly decreases and the proportion of micropores is reduced largely as shown in Figure 1(b). It seems that the elimination of casting defects was more pronounced as the HIP pressure rises from 135 to 145 MPa, as Figure 1(c) shows. The porosity of as-cast and as-HIPed specimens was measured using ImageJ. The result shows that the porosity of K403 superalloy is significantly reduced from 1.51% (Cas) to 0.51% (H35) and 0.32% (H45) through HIP.

Figure 1:

Casting defects in non-HIPed and HIPed K403 superalloy(not etched): (a) Cas; (b) H35 and (c) H45.

As the most common defects in as-cast K403 superalloy, micropores are generally formed during solidification. Two mechanisms contribute to the formation of pores: the microporosity is caused by the shrinkage of the melt and the micropores are generated due to the gas trapped in the melt [15]. The micropores are distributed mainly between the interdendrites and at the grain boundaries. The fluid exhibits an isotropic pressure in a closed tank according to Pascal’s law. Creep and diffusion soften the alloy at high temperature and pressure which HIP provides [16]. Therefore, the microporosity and micropores in K403 superalloy is eliminated during HIP, and accordingly, the alloy is consolidated and densified. The elimination of casting defects through HIP reduces the scatter of mechanical properties and improves mechanical properties of the alloy significantly [17].

Effects of HIP on the morphology of grains

Figure 2 shows the grain morphology in non-HIPed and HIPed specimens. It can be seen in Figure 2(a) that the grain size of as-cast K403 superalloy is about 90–120 μm and the size distribution is dispersed. Through HIP, the grain size increases slightly and size range is about 110–130 μm as shown in Figure 2(b). However, the number of coarse grains decreases and the grain size is more uniform as well as the grain shape. Homogenization of grains is particularly evident through 145 MPa HIP as shown in Figure 2(c).

Figure 2:

The grain morphology in non-HIPed and HIPed K403 superalloy: (a) Cas; (b) H35 and (c) H45.

Previous studies have shown that, with the decrease of the grain size, the number of grain boundaries in the path of the mobile dislocations increases, and accordingly, the number of dislocation tangles increases, which enhances the strength of the alloy [18]. During HIP, the pinning effect of γ’ phase on the grain boundary decreased because of the solution of γ’ phase, which may lead to the coarsening of grains. However, the morphology of grains tends to be uniform during HIP and the adverse effect of the increase in grain size can be offset by the beneficial effect of homogenization of the grain morphology. It has been demonstrated that the morphology of grains is closely related to HIP temperature and proper HIP temperature is required to achieve the finest grain morphology [19].

Effects of HIP on carbides at grain boundaries

Figure 3 shows the morphology of carbides at grain boundaries in non-HIPed and HIPed specimens. As shown in Figure 3(a), most of the carbides are thread-like and distributed at grain boundaries in as-cast K403 superalloy. After HIP, in contrast, carbides take on a discontinuous and particle-like morphology at grain boundaries as Figure 3(b)(c) shows. Figure 4 shows the energy dispersive x-ray spectrometer (EDS) spectrum of carbides at grain boundaries of non-HIPed and HIPed specimens and Table 3 shows the mass fraction of relevant elements in carbides at grain boundaries measured using EDS. It can be obviously seen that the carbides of HIPed specimens are richer in chromium and poorer in titanium than are the carbides of non-HIPed specimens. Specifically, after HIP the content of chromium increases by 87.00% (135 MPa HIPed) and 88.34% (145 MPa HIPed), while the content of titanium decreases by 112.1% (135 MPa HIPed) and 137.7% (145 MPa HIPed).

Figure 3:

morphology of carbides at grain boundaries in non-HIPed and HIPed specimens: (a) Cas; (b) H35 and (c) H45.

Figure 4:

EDS spectrum of carbides at grain boundaries in non-HIPed and HIPed specimens: (a), (b) Cas; (c), (d) H35; (e), (f) H45.

The morphology of carbides at grain boundaries influences the mechanical properties of K403 superalloy. Generally filiform carbides have a negative effect on performance of the alloy since they can accumulate lots of dislocations, causing stress concentration. Nevertheless, granular carbides are beneficial for the mechanical properties, for they can prevent grain boundaries sliding, thus improving creep rupture resistance of the alloy [20, 21]. It is demonstrated by EDS analyses that the thread-like carbides of non-HIPed K403 superalloy are Molybdenum carbides (MoC) which are rich in titanium, while the particle-like carbides of HIPed K403 superalloy are M₂₃C₆ carbides which are rich in chromium. According to Sims [22], the reaction may occur in nickel-based superalloy during the aging treatment:

Through this reaction, MC carbides can decompose into M₂₃C₆ carbides. Aging temperature adopted in this study (980°C) is too low to make stable MC carbides decompose completely. However, HIP used herein provides high temperature and high pressure. Increase of pressure results in equilibrium temperature decreasing. Therefore, some of the stable thread-like MC carbides decompose in matrix during HIP. Meanwhile, carbon atoms combine with chromium atoms and form discontinuous particle-like M₂₃C₆ carbides. Therefore, carbides at grain boundaries in K403 superalloy are spheroidized and refined during HIP.

The effect of HIP on the morphology of β′ phase

The morphology of γ′ phase in K403 superalloy is clearly shown in Figure 5, involving both non-HIPed and HIPed specimens. As we can see from Figure 5(a), the microstructure of as-cast K403 alloy comprises matrix of γ phase and precipitation of γ′ phase. Spherical as it is, the size of γ′ phase is approximately 2 μm, of which the shape, size and distribution are nonuniform and the area fraction is about 57.98% measured by ImageJ. Masses of γ′ phase turn to be united and form coarse γ′ phase particles. The γ′–γ eutectic phase appears in partial regions, although not shown in the figure, of which the morphology presents petaloid and plate. Seen from Figure 4(b), two sizes of γ′ phase appear after 135 MPa HIP, among which the size of larger γ′ phase particles grows slightly bigger and the number of coarse γ′ phase particle decreases obviously, which contributes to a more homogeneous size distribution compared with the one of as-cast alloy (Figure 5(a)). Cubic γ′ phase begin to emerge in part of areas and the γ′ particles tend to be distributed in matrix regularly. The size of finer γ′ phase particles is nearly 0.3 μm. The total area fraction of γ′ phase is about 59.77%. There has been evident reduction in the number of γ′–γ eutectic phase. From Figure 4(c), the size of γ′ phase becomes more homogeneous, as well as the shape and distribution after 145 MPa HIP. Large numbers of cubic γ′ phase particles can be observed almost in all fields of view, among which the amount of smaller γ′ phase is increased. The area fraction of γ’ phase reaches to 60.67% and γ′–γ eutectic phase presents granular shape.

Figure 5:

The morphology of γ′ phase in non-HIPed and HIPed specimens (a) Cas; (b) H35 and (c) H45.

Figure 6 shows the EDS spectrum of γ′ phase in non-HIPed and HIPed specimens. The content of relevant elements in γ′ phase is demonstrated in Table 4. Mass fraction is founded to be raised of both aluminum and titanium after HIP. After 135 MPa HIP, the content of aluminum is raised by 17.75%; meanwhile, the content of titanium is enhanced by 69.96%. The content of aluminum and titanium is increased by 38.12% and 51.60%, respectively, after 145 MPa HIP.

Figure 6:

EDS spectrum of γ′ phase in non-HIPed and HIPed specimens (a), (b) Cas; (c), (d) H35; (e), (f) H45.

As the main strengthening phase in K403 superalloy, γ′ phase affects mechanical properties of the alloy importantly, especially under high temperature. The γ′ phase in cast structure, the so-called primary γ′ phase, is generated during the solidification process, of which the morphology, size and distribution are irregular. During HIP, the second γ′ phase is precipitated with the dissolution of the primary γ′ phase and the non-equilibrium γ′–γ eutectic phase, which makes the volume fraction of γ′ phase increased [23]. The size and morphology of second γ′ phase are closely associated with the temperature as well as cooling rate. Generally, the size of second γ′ phase particles decreases with the augment of cooling rate. When the temperature of solid solution is lower than dissolution temperature, the γ′ phase won’t be dissolved with the growth in size. However, the γ′ phase turns to a single size and grows in the following cooling process when experiencing solid solution above dissolution temperature. The primary γ′ phase will be partially dissolved and form second γ′ phase when experiencing solid solution around the dissolution temperature, thus generating two sizes of γ′ phase [24]. Known as it is, the temperature of HIP adopted in this study was 1,200°C, close to the dissolution temperature of γ′ phase in K403 superalloy. Therefore, two kinds of γ′ phase were observed (see Figure 5(b) and (c)). Study has shown that fine size (smaller than 0.5 μm) of second γ′ phase contributes to stress rupture strength at high temperature and rupture life at high temperature extended with the augment of volume of second γ′ phase in fine size [25].

Diffusion delay and less segregation of alloy element may be in relation to pressure of HIP [26]. The reduction of γ′–γ eutectic phase and granulation of its shape demonstrate that alloy segregation has been decreased and distribution becomes more homogeneous. The augment of the content of titanium and aluminum in the precipitation on one hand makes the number of γ′ phase increase, and on the other hand contributes to forming regular cubic γ′ phase. Additionally, pressure is helpful to delay the growth of γ′ phase as well as the nucleation of γ′ phase during cooling process [24, 27].

The effect of HIP on nanohardness and elastic modulus

In nanoindentation experiment, 15 points were randomly selected on the surface of each specimen, respectively. Since the size of grain is large enough (>100 μm) and the area at (or near) the grain boundary (within 1 μm of the grain boundary) is relatively small, the probability of the test points falling at (or near) the grain boundary is very small (<2%). The distance between adjacent test points exceeded 50 μm for the reason that when indentation spacing was 25 times greater than the maximum indentation depth, the test results do not influence each other. After the experiment, nanohardness and elastic modulus of each point was calculated according to the theory proposed by Oliver and Pharr [28]. Eliminating abnormally large values resulting from grain boundary hardening and abnormally small values producing larger surface roughness, 10 legitimate values were then chosen from 15 sets of datas and averaged, resulting in the nanohardness and elastic modulus of each specimen as shown in Table 5. Figure 7 shows the changes of nanohardness and elastic modulus of K403 superalloy after HIP, from which we can find a large boost in both nanohardness and elastic modulus of the alloy after HIP. More details are represented as follows: compared with the as-cast alloy, nanohardness and elastic modulus were increased by 10.76% and 5.56% after 135 MPa HIP, respectively. Under the condition of 145 MPa HIP, nanohardness was raised by 12.99%, meanwhile, elastic modulus was enhanced by 3.35%. Attention should be paid that the elastic modulus of the alloy at 145 MPa shows the signs of reduction compared with the one under the terms of 135 MPa HIP.

Figure 7:

The variation of nanohardness and elastic modulus after HI.

Microporosity and micropores in as-cast K403 alloy are effectively eliminated through HIP (Figure 1), which leads to densification of the alloy and then makes positive contribution to the improvement of nanohardness and elastic modulus of the alloy. Additionally, nanohardness and elastic modulus are in relation to morphology of γ′ precipitation in the alloy. After HIP, γ′ morphology turns to cuboidal and the size becomes uniform (as Figure 5 shows). The calculational formula for volume fraction of γ′ phase in the precipitation hardening of nickel-based superalloys [29] is as follows:

where f represents the volume fraction of γ′ phase and C denotes wt% of its own alloy element. Quoting the data listed in Table 4, volume fraction of γ′ phase in each group of specimens can be obtained (Table 6). Evidence may lead to the conclusion that the volume fracture of γ′ phase in K403 superalloy was greatly increased after HIP. In a word, the densification of the alloy and formation of better γ′ phase jointly promote a remarkable increase to nanohardness and elastic modulus of the alloy due to the treatment of HIP. With the increase of HIP pressure (from 135 to 145 MPa), the elastic modulus of the alloy decreases, which may be in correlation with grain coarsening (Figure 2). During HIP, a little increase of grain size may affect the elastic modulus of the alloy.

In general, material hardness and elastic modulus show positive correlation relationship to material strength. And yet, till now we don’t totally figure out the specific contact between the data of nanohardness with elastic modulus got from nanoindentation experiment and macrohardness, young modulus as well as macroscopic mechanical properties of K403 superalloy. As a consequence, more details need to be studied hereafter.