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Brittle-ductile transition of titanium aluminides, alloyd by β-phase stabilization elements

M.Remez,
 
Y.Podrezov,
 
V.Danilenko,
 
N.Danilenko,
 
S.Firstov
 

I. M. Frantsevich Institute for Problems of Materials Science of the NAS of Ukraine, Kyiv
Yupodrezov@ukr.net
Usp. materialozn. 2020, 1:86-97
https://doi.org/10.15407/materials2020.01.086

Abstract

The temperature, structural, and rate sensitivity of the plasticity characteristics in γ-titanium aluminides with different Al contents, doped with β-phase stabilization elements, are studied. Particular attention is paid to dislocation mechanisms that control the brittle-plastic transition. The main role of grain boundaries in the formation of plasticity characteristics is demonstrated. At low temperatures, the grain boundaries stop propagation of brittle transgranular cracks and confine the development of the plastic zone beyond the boundaries of an individual grain, creating the prerequisites for fracture in the microdeformation level. At elevated temperatures, the boundaries contribute to the formation of dislocations pile-up in the plastic zone with a stress concentration required to set off the Frank-Reed sources and the displacement of the plastic zone beyond the boundaries of an individual grain, changing its configuration and stress distribution and inhibiting the propagation of cracks. Acceleration of relaxation processes in the vicinity of the crack’s tip creates the prerequisites for the development of macrodeformation. Local relaxation processes at the crack’s tip contribute to high speed sensitivity of the plasticity characteristics. This effect has important practical consequences, since there is a temperature region near the upper working temperature of γ-TiAl alloys, where the stress value remains high (yield strength σ02~ 700 MPa and ultimate stress σul ~ 1200 MPa at bending tests) regardless of the strain rate, while deformation sharply increases at low speeds. As a result, it is possible to achieve a combination of high strength and ductility during creep tests. In samples tested by tension with low speed (10-5 s -1) the neck formation take a place. Deformation occurs by the dislocation-twinning mechanisms. At small deformations (7%) a twinning mechanism is preferable. Concentration of dislocations sharply increases at large deformations (32%) with formation of dislocation clusters. Stress relaxation on the boundary between γ-phase twins and α2-lamella, occurs by macroscopic shift on α2-lamella.


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γ-TITANIUM-ALUMINIDES, BRITTLE-DUCTILE TRANSITION, PLASTICITY, STRENGTH, STRUCTURE, TEMPERATURE AND RATE SENSITIVITY

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