Effect of micro-grain casting process on the microstructure and high cycle fatigue properties of K492M alloy at 700 °C

A nickel base polycrystalline superalloy K492M was used as the material in the present study. The samples of K492M alloy were prepared by conventional casting process, fine-grain casting process and micro-grain casting process, respectively, to study the effects of micro-grain casting process on the microstructure and high cycle fatigue property of K492M alloy at 700 °C. The experimental results show that micro-grain casting process can significantly refine grains, reduce solidification segregation and increase γ′ volume fraction. The micro-grain casting K492M sample exhibited a significantly higher high cycle fatigue life at 700 °C, which was improved by 412% and 269% from that of conventional and fine-grain casting samples, respectively. Analysis for fracture sample indicated that fatigue cracks initiated from the persistent slip bands on the surface or subsurface of the sample. In addition, the cracked carbides accelerated the crack propagation and fatigue fracture. The refined grains and carbides by micro-grain casting process significantly slowed down the propagation of fatigue cracks, and thereby improves the fatigue life.


Introduction
Nickel-based polycrystalline superalloys have been widely used in aero-engines and gas turbines due to their excellent medium-low temperature performance, microstructural stability and relatively low manufacturing costs [1][2] .With the continuous development of aero-engine and auxiliary power unit (APU), there is a desire for higher service performance of superalloy castings.Grain refinement is an effective approach to strength polycrystalline castings and delay their failures in intermediate temperature, which has become one of the research focuses in recent decades.Since 1980, first generation integral fine grain process (GX ® ) and second generation integral fine grain process (MX ® ) have been successively developed by Howmet Corporation.The grain size could be refined from ASTM M6 (4 mm) to ASTM 2~ M8.5 (0.18 mm~1.89mm) by GX ® , and even to ASTM 3~ 5 (0.065 mm~0.125 mm) by MX ® [3][4] .Bouse et al. [5][6] used MX ® to prepare IN718C alloy and improved its ultimate tensile strength and fatigue life significantly.This has driven the further engineering application of MX ® , since it could achieve high mechanical properties while ensuring low cost and high formability.In 1990s, the first fine grain casting vacuum furnace was successfully developed in China.The research on integral fine grain technology has been carried out since then, and has been applied in many products [7][8][9] .K492M alloy has excellent hot corrosion resistance and strength at medium and high temperatures, and especially exceptional strength and low cycle fatigue property at low temperature.It is widely used in the production of blades of industrial gas turbines, integral turbines on APU and other parts.A series of investigation have been finished involved in solidification behavior [10] , heat treatment and microstructure stability [11] , hot isostatic pressing and mechanical properties [12][13] of K492M alloy.
However, the existing studies mainly focused on conventional casting alloys, and there are few reports about the influence of integral fine grain process on microstructure and mechanical property.In this work, the effects of different fine grain casting processes on the microstructure and high cycle fatigue properties of K492M alloy, which would provide data support for the microstructure control and performance optimization.

Materials and experimental methods
The nominal composition of K492M alloy is shown in table 1.After vacuum remelting, the specimens with varies of grain sizes were prepared by conventional casting, fine-grain casting and micro-grain casting, respectively.Hot isostatic pressing (HIP) and heat treatment (HT) were carried out to eliminate the microporosity and optimize the microstructure.High cycle fatigue tests were carried out at 700 ℃ with maximum stress amplitude of 500 MPa, with the stress ratio of R=-1 and frequency of 130 Hz.Considering the data dispersion, 4~5 samples of each casting process were tested to obtain the statistic average fatigue life as the performance data. is the concentration of element  in interdendrite regions.  <1 indicates that the element is enriched in the interdendrite regions, and   >1 indicates that the element is enriched in dendrite regions.

Effects of micro-grain casting on the microstructure of K492M
3.1.1.Macrostructure morphology.Figure 1 presents the grain morphology and distribution of samples with three casting process, where the grain boundaries were outlined in yellow.All samples showed equiaxed crystal structure at gauge section.The average grain size of conventional casting K492M was 4.1 mm (figure 1(a)), which is evaluated as ASTM M-6.0 according to ASTM E112.The fine-grain casting process effectively reduced the average grain size to 743 μm (figure 1  Figure 2 shows the SEM images of microstructures of the as-cast and heat-treated samples.Figure 2(a1-c1) show the microstructures of the as-cast samples casted by different casting processes, i.e., conventional casting, fine-grain casting and micro-grain casting processes, respectively.It is obvious that some micropores discretely distributed in interdendrite regions in the as-cast samples.Statistical results show that the porosities in conventional casting and fine-grain casting samples were 0.26% and 0.25%, respectively.While it was relatively higher in the micro-grain casting one, which reached 0.30%.The pores are the main metallurgical defects in castings, which have great impact on the fatigue performance.They will not only reduce the effective bearing area to increase the stress, but also make it easier for fatigue cracks to initiate resulting from the stress concentration at the edge of irregular hole.Especially for the large pore aggregation, it is easy to cause the early initiation and rapid propagation of fatigue cracks [14] .It was found that after HIP and HT, the porosity significantly decreased to 0 ~ 0.01%, as shown in figure 2(a2-c2), indicating that HIP could effectively eliminate pores and improve the density of castings.Apart from micropores, there were carbides dispersed in the interdendrite regions (figure 2).The area fractions of the carbides in conventional casting, fine-grain casting and micro-grain casting samples were 0.08%, 0.08% and 0.05%, respectively.In other word, micro-grain casting process reduced the carbide content in K492M alloy.The results of composition analysis show that the carbides were mainly MC carbides rich in Ti, Ta and M6C carbides rich in Mo, W. It has been reported that the carbides could effectively improve the mechanical properties of casting superalloys, especially the granular discontinuous carbides precipitated at grain boundaries, as they would prevent intergranular sliding and crack propagation to prolong the service life.However, the carbides usually exhibit relatively high strength and poor plasticity.As a result, there are always microcracks formed by carbide cracking or debonding due to the incoordinate plastic deformation between carbides and matrix during fatigue process.Large bulk carbides are particularly easy to become the source of fatigue cracks and the fast propagation paths, which need to be avoided [1] .

Dendritic segregation.
Figure 3 shows the dendrite morphologies of as-cast and heat-treated samples.There were coarse dendrite structures in conventional casting sample (figure 3(a1)), which suggests severe dendritic segregation.This would induce the precipitation of eutectics and coarse carbides, which are harmful for mechanical properties.The microstructure of fine-grain casting sample was significantly refined compared to the conventional one, although there were still dendritic structures (figure 3(b1)).Micro-grain casting sample presented cellular crystal structure (figure 3(c1)), where the dendritic structures were basically eliminated.The EPMA results are shown in figure 4 to compare the composition segregation.The elements such as W, Cr and Co were enriched in the dendrite cores, while the elements such as Ta, Mo, Al and Ti were enriched in the interdendrite regions.For as-cast samples, the conventional casting one had the most serious dendritic segregation, followed by the fine-grain casting one, and the micro-grain casting one was the weakest.This indicates that micro-grain casting process could reduce composition segregation, which is attributed to the fact that the grain refinement hinders the growth of dendritic structure and hereafter shortens the diffusion distance of alloying elements [15] .After HT, the dendritic structures of all samples were degraded (figure 3(a2-c2)), and the composition segregations were obviously weakened (figure 4).

γ/γ' two-phase microstructure.
Figure 5 shows the γ/γ' morphologies in dendrite cores of as-cast and heat-treated samples prepared by different casting processes.The γ' phases in as-cast samples exhibited very small sizes, and their morphologies were spherical or nearly cubical (figure 5(a1-c1)).While in the heat-treated samples, the size of the γ' phases were much larger and they showed regularly cubical shapes (figure 5(a2-c2)).Table 2 lists the volume fraction and size of γ' phases in dendrite cores of all experimental samples.For as-cast samples, the γ' volume fractions in conventional casting, finegrain casting and micro-grain casting samples were 42%, 40% and 35%, respectively, and the sizes of γ' phases were 241 nm, 235 nm and 201 nm, respectively.It is obvious that micro-grain casting process slightly reduced the volume fraction and size of γ' phase in dendrite core of as-cast samples, resulting from the incomplete precipitation caused by accelerated solidification.
For the heat-treated samples, the volume fractions and sizes of γ' phases were bigger than the as-cast samples as the γ' phases fully precipitated and grew during high temperature aging.The volume fractions and sizes of γ' phases in heat-treated conventional casting, fine-grain casting and micro-grain casting samples were 47.2%, 48.9%, 50.0% and 381 nm, 361 nm, 302 nm, respectively.These results indicate that micro-grain casting process slightly increases the volume fraction of γ' phase and reduces its size,

Effects of micro-grain casting on the high cycle fatigue property of K492M alloy
Aero-engine turbine blades are subjected to periodic loads during service, which may cause fatigue fracture and thereby bring challenges to the service safety.Different casting processes could adjust the grain size, and the refined grains would hinder fatigue crack propagation to prolong fatigue life.Therefore, it is of great significance to investigate the influence of micro-grain casting process on fatigue properties, which also has importance of the actual production and application of superalloys.

High cycle fatigue property at 700 ℃.
High cycle fatigue tests under 700 ℃/500 MPa were carried out to heat-treated samples with different casting processes, and the average fatigue lives are shown in figure 6.The average fatigue life of conventional casting sample was 1.3×105 cycles.The fatigue life of fine-grain casting sample extended to 1.8×105 cycles, which is 39% longer than the conventional one.While the micro-grain casting sample exhibited a dramatical increase of fatigue life, reaching 6.5×10 5 cycles.The increments are 412% and 269%, compared to those of conventional and fine-grain samples, respectively.

Discussion.
There are many factors to affect the fatigue properties of superalloys.The factors of material itself include surface roughness, grain size, carbides, inclusions, microporosities, γ' volume fraction, and so on [1] .Among them, surface roughness, carbides, inclusions and microporosities mainly affect the initiation of fatigue cracks.In this work, the longitudinal sections of all samples were further observed, and there are microcracks near or even inside carbides.Since there would be uncoordinated plastic deformation between carbides and matrix during fatigue process because of the high strength and poor ductility of carbides, dislocations tend to pile up at the interface to cause carbide cracking or interface debonding.As a result, microcracks initiate, which would accelerate crack propagation and fatigue failure [2,14] .Micro-grain casting process reduces the volume fraction of carbides, which weakens the impact of carbide on crack propagation and is beneficial to the improvement of fatigue life.Comparing the main metallurgical factors of the samples after different casting processes (table 2), it is shown that the grain size is the main reason for the difference in fatigue properties, which is consistent with previous studies [13] .The fatigue life of the micro-grain sample is higher than that of the conventional one by 412%, indicating that the process is effective for improving fatigue life.This is attributed to the fact that a large number of grain boundaries in micro-grain casting sample could significantly inhibit the propagation of fatigue cracks.In addition, the grain refinement improves the deformation coordination, stimulates more slip systems, and makes the crack propagate alternately along each crystal plane to form more cleavage steps, as shown in figure 9.The increase of cleavage steps indicates higher propagation resistance, which is also beneficial for improving fatigue properties.

Conclusions
(1) The K492M alloy with grain size of 42.6 μm (ASTM-5.5)has been successfully prepared by micrograin casting process.
(2) The microstructure results shows that the micro-grain casting process can significantly refine grains, reduce solidification segregation, and slightly increase the γ' volume fraction.
(3) The high cycle fatigue tests at 700 ℃ / 500 MPa show that the micro-grain casting process significantly improves the fatigue life.The fatigue life of micro-grain casting sample is 412% and 269% higher than those of conventional and fine-grain ones.
(4) The fracture analysis shows that the fatigue crack initiates from the persistent slip bands on the surface, and the cracking of carbides accelerates the crack propagation and fatigue fracture.The significant grain refinement of micro-grain casting process slows down the fatigue crack propagation, thereby prolonging the fatigue life.

Figure 4 .
Figure 4. Segregation coefficient of K492M alloy casted by different casting processes.

Figure 6 .
Figure 6.High cycle fatigue life under 700 ℃/500 MPa of heat-treated K492M alloy.3.2.2.Fatigue fracture surface.The fracture morphologies of each sample are shown in figure7.The fatigue cracks of all samples nucleated on the surface or subsurface, as marked by red circles.Due to the slow propagation of early cracks, the area around the crack source was relatively smooth, showing a bright facet on the fracture, which could be observed in all samples.However, the propagation zones of each sample were obviously different.Conventional casting and fine-grain casting samples exhibited crystallographic propagation characteristic (figure7(a1-b1)), indicating that fatigue cracks rapidly propagate along crystallographic planes after initiation.While the propagation of micro-grain sample was non-crystallographic and perpendicular to the stress axis (figure7c1).Besides, there were more river patterns and cleavage steps on the fracture of micro-grain casting sample than other samples, suggesting lower crack propagation rate.There was a relatively rough area next to the propagation zone, which was formed by unstable propagation and in the area named as transient fracture zone.Bundles slip bands (shown as the red dotted lines) could be observed near the fatigue source in all samples, indicating that the persistent slip bands (PSB) on the surface are the initiation sites of fatigue cracks (figure7(b2-c2)).

Figure 8 .
Figure 8. Microcracks on longitudinal section of fractured samples.Once fatigue crack initiates, it is generally propagated along a certain slip plane inside the grain where the crack is located.As shown in figure7(a), the fracture surface of conventional casting sample is composed of several intersecting planes and there are few cleavage steps on it, indicating fatigue crack propagate rapidly along crystallographic direction with few resistances after initiation.Therefore, the conventional casting sample has the lowest fatigue life.

Table 2 .
Statistical results for the microstructures of as-cast and heat-treated K492M alloy.