Impact of boron as an alloying addition on the microstructure, thermo-physical properties and creep resistance of a tungsten-free Co-base γ/γ′ superalloy

Sharma, A and Dixit, S and Baler, N and Agrawal, P and Makineni, SK and Chattopadhyay, K (2022) Impact of boron as an alloying addition on the microstructure, thermo-physical properties and creep resistance of a tungsten-free Co-base γ/γ′ superalloy. In: Materials Science and Engineering A, 855 .

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Abstract

While boron is added to many structural alloys primarily as a grain-boundary strengthener, it has myriad other implications on the microstructure and thermophysical properties of these alloys. In the present study, we report these effects by varying boron addition in the range of 0.02 at.% to 0.8 at.% on the microstructure, thermo-physical properties and the creep resistance of tungsten free, γ/γ′ base Co–30Ni–10Al–5Mo–2Ta–2Ti (at.%) polycrystalline superalloy. Without boron addition, the plastic strain during creep at 850 °C is primarily accommodated through grain-boundary decohesion, often enhanced by severe oxygen attack along the grain boundaries. The addition of boron alleviates the poor grain-boundary cohesivity while inhibiting internal oxidation. An increase in the solvus temperature with boron addition was found, leading to an increase in the high-temperature capability of this alloy (∼40 °C). However, a concurrent decrease in the alloy's melting point has been observed. Coupled with improved grain-boundary cohesivity and larger grain size, improvement in creep resistance by almost an order of magnitude could be achieved. While at low amounts of boron (0.02–0.08 at.%) borides do not form, indicating solid solubility of boron within the bulk alloy, at high boron contents (0.2–0.8 at.%), Co and Mo rich boride forms primarily along the grain boundaries. These borides are surrounded by a distribution of fine secondary γ′ precipitates around them. In contrast, borides in Co–Al–W alloys promote precipitate free zones (PDZ), causing deterioration of mechanical properties. The beneficial effect of boron along grain-boundaries, both in elemental form and in the form of borides, has been highlighted in the present study which paves way for optimizing boron addition in other alloys of this class.

Item Type:	Journal Article
Publication:	Materials Science and Engineering A
Publisher:	Elsevier Ltd
Additional Information:	The copyright for this article belongs to Elsevier Ltd.
Keywords:	Borides; Creep resistance; Deterioration; Grain boundaries; Microstructure; Superalloys; Thermodynamic properties, Alloying additions; Boron additions; Cobalt-base superalloys; Cohesivity; Grain boundary strengthening; Grain-boundaries; Thermo-physical property; Tungsten-free; γ/ microstructure; �/γ� microstructure, Creep
Department/Centre:	Division of Mechanical Sciences > Materials Engineering (formerly Metallurgy)
Date Deposited:	06 Oct 2022 08:30
Last Modified:	06 Oct 2022 08:30
URI:	https://eprints.iisc.ac.in/id/eprint/77018

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