Dear all,
I am studying the dendrite growth and the corresponding orientation evolution within the platform region of single crystal superalloy turbine blade.
After directional solidification, the interested region was chosen for orientation measurement via Electron Back-scattered Diffraction (EBSD). It was found along the dendrite growth direction, misorientation gradient was about 1°/mm, with the weak linear relationship between misorientation accumulation and measurement length.
After solution heat treatment (SHT), it was found along the dendrite growth direction, misorientation gradient was about 4°/mm, with the strong linear relationship between misorientation accumulation and measurement length.
I know that SHT could relieve residual stress which generate during solidification, but I do not know how could SHT induce the increased misorientation gradient along dendrite growth direction.
Does anybody have suggestion on the topic?
Best
Dejian Sun.
Dear Dejian,
From regular practice of alloy development I understand after solutionizing heat treatment you quenched the sample. In general, gamma prime precipitation in super alloy leads to reduce the volume of the crystal and/or grain. Therefore in DS alloy -ve creep is observed caused by rafting (Coarsening of gamma prime). Basically when you do solution treatment you increases the volume of the crystal and/or grain. As I understand in DS less number of grains are present, therefore to accommodate this change in volume crystal has to gone through certain misoriantaion..
With best regards
PS Manoranjan Jena
Solutionizing heat-treatment causes lattice strain (due to different atomic size). The strain become even larger across the grain boundaries. The lattice tries to minimize the strain across the grain boundary by reorienting themselves (of course not by physical rotation but by atomic diffusion assisted lattice plane generation).
Dear Chandra Bhanu Basak,
Thanks for the valuable explanation. Do you have any reference (article) of this topic?
@ Ps Manoranjan Jena, Thank you all the same.
Actually, after directional solidification and after solution heat treatment (SHT), misorientation accumulation mainly occur along every single dendrite arm (not mainly across the grain boundaries). This phenomena could be described clearly via the corresponding pole figures. Particularly, after SHT, the continuous rotation of dendrite arm orientation could be identified obviously.
With respect to the misorientation induced by gamma prime precipitation, there are already related articles (for example, the report of N. Siredey et al. J. Cryst. Growth 130 (1993) 132-146), and it is suggested that this misorientation is random, not accumulated.
Best regards,
Dejian
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