Y.Q. Huang¹, J. Shen¹, D. Wang¹, Y.Z. Lu¹, L.H. Lou¹, J. Zhang^1*

¹ Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 

^* jianzhang@imr.ac.cn 

Directionally solidified Ni-based superalloys are widely used in aero-engines and industrial gas turbines.[1-2] However, the <001> columnar grains often deviates from the principal stress axis during directional solidification (DS). Besides, blades are inevitably subjected to non-longitudinal stresses due to the complex geometry and thermal stresses. The effect of columnar grain boundary orientation on the stress rupture properties is investigated in present work.

DZ411 was used in the experiments. Bicrystal specimen was achieved by rotating one of the seeds, relative to the other seed (Fig. 1a). The bicrystal slab was machined into stress rupture specimens (10mm×2mm×1.5mm) with stress axis deviated different angles from grain boundary (GB). Some of the creep rupture tests were interrupted for microstructure examination. 

Experimental results showed that stress rupture life decreased significantly when GB orientation angle exceeded 20^o at 850^oC/500MPa. Whilst there was a relatively minor effect on the stress rupture properties when the GB orientation angle was within 30^o at 980^oC/220MPa.

In specimen with stress axis deviated 30^o from GB, a large amount of γ′ phase precipitated from γ phase at 850^oC/500MPa for 20h (Fig. 1b1). Cracks initiated from the micro-pores at GB subsequently (Fig. 1b2). For the specimen with 30^o misorientation angle interrupted at 980^oC/220MPa for 20h, slight rafting and GB precipitation were observed (Fig. 1c1-c2). More plastic deformation occurred in matrix (Fig. 1c3) and cracks formed in matrix (Fig. 1c4). 

The present results indicate that GB misorientation in DS casting is detrimental to stress rupture properties. However, this detrimental effect may alleviate at higher temperature. There are two possible reasons from microstructural point of view: (1) new GB precipitates formed during creep at 980^oC which provides additional GB strenthening effect, and (2) deformation is no longer restricted within the GB regime. Due to the enhanced atomic diffusion ability at high temperature, dislocations may cross GB so that local damge at GB is avoided. 

References

[1] J.T. Guo, C. Yuan, H.C. Yang, V. Lupinc, M. Maldini, Metall. Mater. Trans. A, 2001, 32A, 1103-1110.

[2] L. Mataveli Suave, J. Cormier, P. Villechaise, D. Bertheau, G. Benoit, F. Mauget, G. Cailletaud, L. Marcin, Superalloys 2016, 2016, 747-756.