On the grain boundary strengthening effect of boron in γ/γ′ Cobalt-base superalloys
| Autor: | Ivan Povstugar, Dierk Raabe, Julian M. Müller, Erdmann Spiecker, Mathias Göken, Markus Kolb, Lisa P. Freund, Baptiste Gault, Fabian Fischer, Steffen Neumeier, Surendra Kumar Makineni |
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| Rok vydání: | 2018 |
| Předmět: |
010302 applied physics
Materials science Polymers and Plastics Annealing (metallurgy) Alloy Metals and Alloys chemistry.chemical_element 02 engineering and technology engineering.material 021001 nanoscience & nanotechnology 01 natural sciences Electronic Optical and Magnetic Materials Superalloy chemistry 0103 physical sciences Ceramics and Composites engineering Grain boundary Crystallite Composite material 0210 nano-technology Boron Strengthening mechanisms of materials Grain boundary strengthening |
| Zdroj: | Acta Materialia. 145:247-254 |
| ISSN: | 1359-6454 |
| DOI: | 10.1016/j.actamat.2017.12.020 |
| Popis: | Boron is an essential solute element for improving the grain boundary strength in several high temperature metallic alloys especially in Ni- and Co-base superalloys although the detailed strengthening mechanisms are still not well understood. In superalloys, boron leads to the formation of borides and precipitate depleted zones around the grain boundaries and alters the bond strength among the grains directly. In this paper, we explore in detail the role of the boron content in ternary γ/γ′ Co-9Al-9W alloys. Local as well as bulk mechanical properties were evaluated using nanoindentation and compression testing and correlated to near-atomic scale microstructure and compositions obtained from electron microscopy and atom probe tomography. The alloy variant with low B content (0.005 at.% B) reveals an increase in yield strength at room temperature and 600 °C and atom probe tomography investigations show that solute B segregates to the grain boundaries. However, in the bulk B exclusively partitions to the γ′ phase. Additionally, the γ′/γ′ grain boundaries are depleted in W and Al with the concentration locally shifted towards the γ composition forming a very thin γ layer at the γ′/γ′ grain boundaries, which supports dislocation mobility in the γ′/γ′ grain boundary region during deformation. Higher content of B (0.04 at.% B) promotes formation of W-rich borides at the grain boundaries that leads to undesirable precipitate depleted zones adjacent to these borides that decrease the strength of the alloy drastically. However, it was also found that a subsequent annealing heat treatment eliminates these detrimental zones by re-precipitating γ′ and thus elevating the strength of the alloy. This result shows that, if a precipitate depleted zone can be avoided, B significantly improves the mechanical properties of polycrystalline Co-base superalloys by strengthening the γ′ phase and by improving grain boundary cohesion. |
| Databáze: | OpenAIRE |
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