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Rare Metals2018年第5期

Forming and growing mechanisms of homogenization-solution pores in a single crystal superalloy

Xiao-Dai Yue Jia-Rong Li Xiao-Guang Wang Zhen-Xue Shi

Science and Technology on Advanced High Temperature Structural Materials Laboratory,Beijing Institute of Aeronautical Materials

收稿日期：29 November 2015

基金：financially supported by the Foundation of Beijing Institute of Aeronautical Materials (No. 150109);

Forming and growing mechanisms of homogenization-solution pores in a single crystal superalloy

Xiao-Dai Yue Jia-Rong Li Xiao-Guang Wang Zhen-Xue Shi

Science and Technology on Advanced High Temperature Structural Materials Laboratory,Beijing Institute of Aeronautical Materials

Abstract：

The forming and growing mechanisms of homogenization-solution pores in a single crystal superalloy were investigated. The microstructures were observed with optical microscope(OM) and field emission microscope(FEM) after homogenization-solution heat treated at1328 ℃ and 1350 ℃ for 2 h, 6 h and 9 h. Results indicate that when heat treated at 1328 ℃, pores appear at the interface of eutectic and matrix at first and then leave in the matrix with the shrink of eutectic. When heat treated at1350 ℃, incipient melting happens at first, and some of them have a pore in the center. After that, with the homogenization-solution process, incipient melting micros tructure fades away gradually. By analyzing the results with thermodynamics and kinetics methods, it is concluded that some pores nucleate during directional solidification and then become larger and visible during homogenization-solution heat treatments; some pores are generated by incipient melting, yet such pores are difficult to be distinguished from other pores; imbalanced elements cross-diffusion induces to the forming and growing of pores too, and such imbalanced diffusion also plays an important part on the growth of all preexisting pores.

Keyword：

Single crystal superalloy; Pore; Homogenization; Solution; Heat treatment; Incipient melting;

Author： Xiao-Dai Yue,e-mail:yuexiaodai0126@126.com;

Received： 29 November 2015

1 Introduction

Nowadays,single crystal superalloys are the major material of advanced aero-engines turbine blades [ 1] .To improve the high temperature capability of turbine blades,more refractory elements,including Mo,Re,Ta,W and Nb,are added to advanced single crystal superalloys.However,other than enhanced solid solution strengthen,those dense refractory elements also lead to severe dendrite segregation [ 2, 3] .Therefore,as-cast single crystal superalloys have to undergo homogenization-solution heat treatments to promote chemical and microstructural homogeneity before service [ 4] .Particularly,because of higher refractory elements contents,higher generations of single crystal superalloys need longer time and more energy for homogenization-solution heat treatments [ 5] .Consequently,another dangerous defect,homogenization-solution pore,becomes severer and has to be brought to the forefront [ 6] .

Although the studies of the forming and growing mechanisms of pores are quite limited until now,it has been accepted that the pores in single crystal superalloys can be pided into two types [ 6, 7, 8, 9] :pores forming during solidification process and pores forming during homogenization-solution heat treatments.For above-mentioned two types,the solidification pores are caused by non-satisfactory feeding effect,as well as alloy shrinkage during solidification.However,the forming mechanism of homogenization-solution pores is far from clear,and viewpoints about such pores mainly concentrate on Kirkendall-Frenkel effect:because of imbalanced elements cross-diffusion,vacancies flow to interdendrite region and generate pores there [ 5, 10] .In addition,Pang et al. [ 5] mentioned another three mechanisms:some pores are created by internal pressure due to the release of dissolved gases or carbon reaction products;some are produced by the change of atomic density via phase transformation;and some homogenization-solution pores actually evolve from crack-like shape pores at the eutectic-matrix interface.

In the present study,the homogenization-solution pores refer to the pores that only can be observed after homogenization-solution heat treatments.In order to study the forming and growing mechanisms of such pores,a single crystal superalloy containing high amount of high refractory elements was investigated.After observing the alloy microstructures with different homogenization-solution heat treatments,the forming and growing mechanisms were analyzed by aid of dynamic and thermodynamic methods.

2 Experimental

To cater for the characteristic of newer generation single crystal superalloys [ 11, 12, 13, 14, 15] ,the experimental single crystal superalloy was added with 21.0 wt%refractory elements,including W,Mo,Ta and Re.3.0 wt%-4.0 wt%Ru was also employed [ 16, 17] .Single crystal bars ofΦ15 mm×170 mm were casted with crystal selection method in the directionally solidified furnace and then were cut intoΦ15 mm×10 mm for different homogenizationsolution heat treatments.

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Table 1 Homogenization-solution heat treatments conducted in experiment

Based on previous thermodynamic tests,the incipient melting temperature of as-cast alloy was identified at1345℃.The specimens are pided into two groups and heated up to 1328℃and 1350℃,respectively.After preserved at above two temperatures for 2 h,6 h and 9 h,the specimens were air-cooled to room temperature.The parameters of homogenization-solution heat treatment are presented in Table 1.After homogenization-solution heat treatments,all specimens were fabricated,polished and etched with CuSO₄+HCl+H₂SO₄+H₂O solution.Microstructures were examined by optical microscope(OM,LEICA DM-4000 M) and field emission scanning electron microscope (SEM,JEOL S-4800).The results were analyzed by aid of differential scanning calorimetry (DSC)method using a STA409CD simultaneous thermal analyzer.

3 Results

Figure 1 shows as-cast microstructure of the alloy.Because of severe segregation,a large scale of eutectic exists,and theγ/γ'microstructure is quite heterogeneous at different regions.

Figure 2 shows the microstructures of specimens after homogenization-solution heat treated at 1328℃.As can be seen,eutectic still exists,and no incipient melting occurs.With preservation time increasing,the dendriteinterdendrite boundaries get fuzzy,and the eutectic amount decreases apparently.After 9 h,about 0.5%eutectic remains.

Other than eutectic,a number of pores appear when persevered at 1328℃(Fig.3).After 2 h,pores appear on the eutectic-matrix boundaries;after 6 h,all pores are located in the matrix,and there becomes some distance between eutectic and pores;after 9 h,the distances turn to be even father.No pore appears at new eutectic-matrix boundaries throughout the process.Therefore,it can be assumed that those pores nucleate on the eutectic-matrix boundaries of the as-cast alloy,grow larger and are left in the matrix with the homogenization-solution heat treatment process.

Fig.1 Micro structures of as-cast alloy:a OM image and b SEM image of heterogeneousγ/γ'microstructure

Fig.2 OM and SEM images of specimens homogenization-solution heat treated at 1328℃for a,d 2 h,b,e 6 h and c,f 9 h

Fig.3 SEM images of pores in specimens homogenization-solution heat treated at 1328℃for a,b 2 h,c,d,e 6 h and f,g 9 h

Figure 4 shows the microstructures of specimens homogenization-solution heat treated at 1350℃.For this group,a very interesting phenomenon is noticed:Massive incipient melting occurs after 2 h of preservation;after 6 h,incipient melting micros truc ture reduces,which not only refers to the decrease in incipient melting points number,but also to the shrinkage of a inpidual incipient melting point;after 9 h,all incipient melting disappears,leaving some pores behind.

Further observations of the pores in this group of specimens are shown in Figs.5-7.For the specimens heat treated at 1350℃for 2 h,massive incipient melting emerges,and some of them have a pore in the center.Here,the pores can be pided into two kinds:with or without incipient melting borders,which are obvious different in radial size.After 6 h,the incipient melting boundaries become narrower,and the number of melting points decreases.After 9 h,the incipient melting micros true ture disappears completely,and the edges of previous incipient melting microstructure become the same as the matrix.

Fig.4 OM and SEM images of specimens homogenization-solution heat treated at 1350℃for a,d 2 h,b,e 6 h and c,f 9 h

Fig.5 SEM images of incipient melting and pores in specimen homogenization-solution heat treated at 1350℃for 2 h:a image with different pores,b incipient melting without a pore in center,c incipient melting with a pore and incipient melting bonder and d a pore without incipient melting bonder

Fig.6 SEM images of incipient melting and pores after homogenization-solution heat treated at 1350℃for 6 h:a-c pores and d,e incipient melting

Fig.7 SEM image of pores in specimen homogenization-solution heat treated at 1350℃for 9 h

4 Discussion

With the increase in refractory elements,the homogenization-solution pore problem becomes severer and has to be taken more seriously.In spite of their small sizes,they deteriorate the mechanical properties significantly,especially the fatigue lifetime [ 8] .According to the experimental results,clues of three forming and growing mechanisms can be found.

4.1 Pores evolving from invisible cracks at eutectic matrix interface

During the solidification of single crystal superalloys,dendrites solidify earlier than interdendrites.As a result,it is unavoidable to have some uncompacted microstructure existing at the eutectic-matrix interface.For the experimental single crystal superalloy,no crack-like shape pore can be found before homogenization-solution heat treatments.However,after 2 h of preservation at 1328℃,some small pores appear at the matrix-eutectic interface.With the homogenization process,eutectic shrinks gradually and those pores become farther and farther from eutectic relatively.Meanwhile,no pore appears on the interface of matrix and contracted eutectic.As a result,it can be deduced that such pores nucleate at the matrixeutectic interface of as-cast alloy;with the homogenization process,those pores become larger;concurrently,as the range of eutectic becomes smaller,they are left in the matrix.

In another word,although the'crack-like shape pores'cannot be observed directly,such pores do exist.They can be recognized as invisible pore nucleus generating on the eutectic-matrix interfaces of as-cast alloy,and then they evolve to visible pores during the homogenization-solution heat treatment process.

4.2 Pores evolving from incipient melting

In this research,another kind of homogenization-solution pore is noticed:pores evolving from incipient melting.

When a single crystal super alloy contains high amount of refractory elements,it is likely to be highly segregated after solidification,and theγ'solution temperature,solidus temperature and liquidus temperature are quite different in different regions.Sometimes,the solution temperature ofγ'in the dendrites is even higher than the solidus temperature of interdendrites.Under such circumstances,if the as-cast alloy is kept at a temperature slightly higher than the interdendrite solidus temperature of as-cast alloy,but lower than the incipient melting point of homogenized alloy,this will happen:in the beginning,incipient melting happens;afterward,with low melting point elements diffusing from interdendrites to dendrites and refractory elements diffusing in the opposite direction,the solidus temperature of interdendrites increases gradually;accordingly,the melted region is re-solidified and turns to the same microstructure as the matrix gradually.

To further analyze the effect of homogenization-solution progress on the thermal characteristic of single crystal superalloys,the thermal characteristics of the single crystal superalloy with different conditions were measured using DSC measurements.Figure 8 shows DSC curves of the ascast alloy and the specimen after homogenization-solution heat treated at 1318℃for 3 h.It can be seen that the whole endothermic region rises about 20℃.That is to say,the homogenization process does exert great influence on the phase transforming temperatures of the single crystal super alloy.

According to above analysis,when the as-cast alloy is held at 1350℃which is higher than the solidus temperatures of the low melting point regions,incipient melting happens.If the incipient melting is severe enough,the liquid molten bathe will collapse,and a pore generates.Then,with the homogenization process,the solidus temperatures of interdendrites increase,the molten regions resolidify and are homogenized to the same microstructure with the matrix gradually.

Nowadays,most homogenization-solution heat treatments of advanced single crystal superalloys own a preservation time at the peak temperature for no less than 4 h,some even more than 10 h [ 2, 3, 4, 5] .For such a long time,if slight incipient melting occurs and turns back to the same microstructure with the matrix afterward,it is quite difficult to be perceived,which is very dangerous for engine blade manufacture.Consequently,such phenomena must get enough recognition for single crystal superalloys,especially for those new generation ones.

4.3 Pores caused by Kirkendall-Frenkel effect

Kirkendall-Frenkel effect is the most reported mechanism for homogenization-solution pores until now.The main idea is that in the multiple diffusion system of single crystal superalloys,the composition is equalized by two crossdiffusion fluxes during homogenization:W,Re,Cr and Co flow from the dendrites to interdendrites;Al,Nb and Ta flowing in the opposite direction.The former flux is much weaker than the later one because of the different diffusivities of alloying elements.Such imbalanced crossdiffusion fluxes result in an increase in vacancies in the interdendrite region,and the vacancy supersaturation leads to the generation of new pores in the interdendrites. [ 5, 7, 10, 13, 14] .

Fig.8 DSC curves of experimental alloy a at as-cast condition and b after homogenization at 1318℃for 3 h

Fig.9 Segregation characteristic of A1 and Re after homogenization for 2 h,6 h and 9 h:a Al,1328℃;b Re,1328℃;c Al,1350℃;d Re,1350℃

Fig.10 The schematic diagram of atomic and vacancy currents in the single crystal superalloy

Figure 9 shows the calculation results of the segregation characteristics of Al and Re,which are considered to be the fast and the slowest diffusing elements in the single crystal superalloy,respectively [ 6, 17] .The calculation tool is a widely used material simulation software named JMatPro.It can be seen that Al basically achieves homogenous after9 h of homogenization-solution at both temperatures.Even at the lower temperature of 1328℃,it only takes 6 h for Al to reduce the concentration difference between dendrites and interdendrites to less than 1.0 wt%,and 10 h to less than 0.5 wt%.By contrast,Re concentration hardly changes.Under such unbalanced diffusion system,the atomic flow of Al,Nb and Ta from interdendrites to dendrites is much stronger than those of W,Re,Mo,Co,etc.,from dendrites to interdendrites.Correspondingly,vacancies flow from dendrites to interdendrites and condense there.As a result,new pores generate and grow in interdendrites.Figure 10 shows atomic and vacancy currents in the single crystal superalloy during homogenization.Actually,such unbalanced diffusion also plays an important part in the growth of all existing pores.

5 Conclusion

For advanced single crystal superalloys,some invisible cracks exist at the interfaces of eutectic and matrix after solidification.Such cracks may evolve to visible pores during homogenization-solution heat treatments.When the single crystal superalloy is heat treated at a temperature slightly higher than the interdendrite solidus temperature of as-cast alloy but lower than the incipient melting point of homogenized alloy,incipient melting happens at first,and some of the melting points induce to pores;then,with the homogenization process,the molten regions re-solidify and are homogenized to the same microstructure with the matrix gradually.When the single crystal superalloy contains high amount of refractory elements,obviously imbalanced element cross-diffusion induces to the gathering of vacancies in the interdendrites during homogenization-solution heat treatments.Such effect causes the generation and growth of pores,and it also plays an important role in the growth of all pre-existing pores.Although the forming and growing mechanisms are different,the pore microstructures are too similar to distinguish their mechanism type.

Acknowledgements This study was financially supported by the Foundation of Beijing Institute of Aeronautical Materials (No.150109).