72 73 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 Understanding the complex 3D structures ofmetal carbides for optimising the manufacture of Ni superalloys Related publication: Zhang Z., Khong J. C., Koe B., Luo S., Huang S., Qin L., Cipiccia S., Batey D., Bodey A. J., Rau C., ChiuY. L., Zhang Z., Gebelin J.-C., Green N. &Mi J. Multiscale characterization of the 3D network structure of metal carbides in a Ni superalloy by synchrotron X-raymicrotomography and ptychography. Scr. Mater. 193 , 71–76 (2021). DOI: 10.1016/j.scriptamat.2020.10.032 Publication keywords: Synchrotron X-raymicrotomography; Ptychography; Metal carbides; Solidification; IN713LC Ni superalloy M etal carbides are important microstructure constituents in polycrystalline Ni-based superalloys. They play a dominant role in determining the formation of cast defects and the strength of the alloys. However, the nucleation and growth dynamics of metal carbides, especially the true 3D network structure and morphology formed in different solidification conditions, have not been fully understood. Researchers focused on studying the 3D network structures and distribution of the metal carbides in a widely used IN713LC Ni superalloy. Their objective was to reveal the true 3D spatial information of the metal carbides and to understand the nucleation and growth dynamics of the metal carbides in different solidification conditions. The microtomography capability at the Diamond-Manchester imaging branchline (I13-2) offers a spatial resolution down to ~1 µm, while the ptychography capability at the Coherence branchline (I13-1) provides a spatial resolution down to ~30 nm. The research demonstrated clearly the advantage and technical potential of using the two complementary tomography techniques. The team found individual MC carbides distributed on the grain boundary between the gamma and gamma prime phases. The dominant growth directions of carbide branches were mainly determined by the local composition of the remaining liquid phase and geometric constraints. The metal carbides exhibited spherical, strip or network structure depending on the solidification cooling rates. The 3D characteristics of those structures were quantified for the first time. The findings can assist in optimising the manufacture of Ni superalloys to reduce casting defects. Scientists can use the rich 3D datasets to understand the initiation and propagation of crack at metal carbides during plastic deformation and service more quantitatively. Nickel based superalloys have been widely used for producing high- temperature structural components in aircraft and land turbine engines, rocket engines, nuclear power and chemical processing plants 1 . These superalloys present a unique microstructure where the ordered intermetallic γ’(Ni3Al) precipitates distribute coherently within a continuous γ(Ni) matrix. In addition, metal carbide is another important microstructure constituent in the polycrystalline Ni-based superalloys due to its excellent high-temperature stability. The carbide is designed to improve the high-temperature strength of the superalloy through strengthening the grain boundary and preventing grain boundary sliding 2 . However, metal carbides can cause casting defects during the solidification process as well as crack initiation and propagation during plastic deformation. Until now, the nucleation and growth dynamics of metal carbides, especially the true 3D network structure and morphology of metal carbides formed in different solidification conditions have not been fully understood. In this research, synchrotron X-ray microtomography and ptychography were used complementarily to characterise, across the micro- and nanolength scale, the true 3D network structure, morphology and distribution of metal carbides in a widely used IN713LC Ni superalloy. The microtomography experiments were carried out on branchline I13-2. Figure 1 shows the 3D network structure of the carbides in a relatively large volume for the two samples with solidification cooling rates of 0.27 K/s and 1.12 K/s respectively. It is clear that the carbide network distributed along the edge of the secondary dendrite arms in 3D space and within the interdendritic region.Atahighercoolingrateof1.12K/s(Fig.1b),the intervalofthesecondary dendrite arms became smaller, leading to smaller local volume for the carbide network. Moreover, the spherical (blocky type in 2D view) and irregular rod (strip type in 2D view) carbides were predominant in the sample of 1.12 K/s in spite of the formation of some small carbide networks (Fig. 1d). At a lowcooling rate of 0.27 K/s, the spherical and rod carbides interconnected each other and formed compact and complex carbide networks (Fig. 1c). Such a feature is often observed as script-typed carbides in a 2D projection (or sectional) view. The complex carbide network was due to further growth of the spherical and rod carbides and coalescence between each other. The average particle sizes for the rod carbides in the 0.27 K/s sample were 97 μm 3 , confirming the further growth and coalescence of the carbides due to having sufficient diffusion time in the low cooling rate sample. In particular, the average size of the network carbides changed from 288 to 328 μm 3 . The comparison of the shape factor (Fig. 1f) distribution for the samples with different cooling rate reveals that the individual carbides grew from the spherical morphology to rod morphology when reducing the cooling rate. The skeletonisation analysis of the network structure 3 further revealed the growth paths of the carbide branches during solidification. The ptychographic X-ray computed tomography was carried out on the Imaging & Microscopy beamline (I13-1), revealing the detailed morphology and phase relationships of the metal carbides with the γ and γ› phases. Figure 2 shows that these carbides were localised on the grain boundaries between the matrix γ and γ› phase. The typical 2D slices containing carbides extracted from the 3D tomographic dataset (Fig. 2d-h) reveal that the dominant growth directions of carbide branches were mainly determined by the local composition of the remaining liquid phase and geometric constraints. References: 1. Unocic R. R. et al. Mechanisms of creep deformation in polycrystalline Ni-base disk superalloys. Mater. Sci. Eng. A 483 – 484 , 25–32 (2008). DOI: 10.1016/j.msea.2006.08.148 2. Long H. et al. Microstructural and compositional design of Ni-based single crystalline superalloys - A review. J. Alloys Compd. 743 , 203–220 (2018). DOI: 10.1016/j.jallcom.2018.01.224 3. Zhao Y. et al. 3D characterisation of the Fe-rich intermetallic phases in recycled Al alloys by synchrotron X-ray microtomography and skeletonisation. Scr. Mater. 146 , 321–326 (2018). DOI: 10.1016/j. scriptamat.2017.12.010 Funding acknowledgement: Project Funding & Financial Support: UK Engineering and Physical Sciences Research Council (Grant No. EP/L019965/1), the Royal Society Industry Fellowship (for J. Mi in 2012-2016) and the Chinese Scholarship Council (Funding No. 201806785038 for Z. Zhang’s visiting fellowship in the academic year 2019-2020); the relevant synchrotron X-ray beamtime awarded by Diamond Light Source (MT9974, MT13488 and MG22525); access to the University of Hull supercomputer, Viper, and the assistance by the support team (Mr Chris Collins, in particular) for data analysis and visualisation. Corresponding authors: Prof. Jiawei Mi, Department of Engineering, University of Hull,
[email protected]; Dr Zhiguo Zhang, Institute of AdvancedWear & Corrosion Resistant and Functional Materials, Jinan University, China,
[email protected] Imaging andMicroscopy Group Beamline I13 (Branchlines I13-1 and I13-2) Figure 2: X-ray ptychography image and tomography for the carbides in the sample with a cooling rate of 1.12 K/s. (a) a transmission projection; (b) and (c) are the retrieved 2D projections at − 83.5 ° and 45 °, respectively; (d) 3D morphologies for the carbides with a higher resolution of 30 nm; (e) the correlations of carbides, γ and γ’ phases in 3D space; (f) indicates the extract slice positions for (g) and (h); (g) shows the spherical carbide and its correlation with other phases; (h) is the strip carbide and its correlation with other phases. Figure 1: The 3D morphologies, distribution and network of the carbides. (a) and (b) are the 3D morphology and network of the carbides in the samples with cooling rate of 0.27 K/s and 1.12 K/s, respectively; (c) and (d) are the enlarged local carbide morphology for (a) and (b); (e) and (f) are the particle size distribution and the shape factor distribution for the spherical and rod carbides.