Evidence of void nucleation and growth on planar slip bands in a Nb-Cr-Ti alloy

Self Cite

This article describes a new computation-based approach for designing ductile Nb-Ti-Cr-Al solidsolution alloys. The proposed approach is based on computation of the surface energy and the Peierls-Nabarro (P-N) barrier energy as a function of alloy composition. The surface energy is used as a measure of the resistance to cleavage fracture, while the P-N barrier energy is used as a measure of dislocation mobility. The ratio of the surface energy to the P-N barrier energy is utilized as a material index which can be adjusted by alloying additions. Analytical relations are developed for computing (1) the elastic constants in terms of the d ϩ s electrons per atom in the alloys and (2) the lattice parameter, surface energy, and P-N barrier energy in terms of alloy composition. Design of a ductile solid-solution alloy is achieved by manipulating the number of d ϩ s electrons, through alloying additions, to obtain a high value of the ratio of the surface energy to P-N barrier energy by reducing the misfit energy of the dislocation core. Applications of the methodology to designing binary, ternary, and quaternary Nb-based solid-solution alloys with Ti, Cr, and Al alloying additions are illustrated with promising results, demonstrating that the proposed methodology is a viable approach for alloy design.

Section: Resultsmentioning

confidence: 99%

A computational approach to designing ductile Nb-Ti-Cr-Al solid-solution alloys

Chan

2001

Self Cite

“…The absence of ductile-phase toughening in Nb-based ininduced cleavage fracture. [2][3][4][5]28,[13][14][15][70][71][72] Although accomsitu composites with a high intermetallics content is the panied by plasticity, plasticity-induced cleavage in the Nb ss consequence of a high plastic constraint in the Nb solidphase limits the fracture-toughness level that could otherwise solution phase in the composites. be attained if the ductile fracture process of void nucleation, 5.…”

Section: A Unconstrained Fracture Toughnessmentioning

confidence: 99%

“…A Ti addition increases the fracture toughness of Nballoys. [72] The amount of Ti addition required to attain a based in-situ composites through its influence on the ductile fracture process by void nucleation, growth, and fracture toughness of the Nb solid-solution phase. coalescence, however, might be high.…”

Section: A Unconstrained Fracture Toughnessmentioning

confidence: 99%

“…Presently, there is no information on the minimum amount of Ti addition that is required to facilitate a fracture transition from cleavage to ACKNOWLEDGMENTS ductile void nucleation, growth, and coalescence, but it This work was supported by the Internal Research Proappears to be Ͼ35 pct. [72] gram of Southwest Research Institute (SwRI). The cleriThe optimum microstructure for fracture toughness has cal assistance of Ms. P. Soriano and Ms. L. Salas, SwRI, in not been established for Nb-based in-situ composites.…”

Section: A Unconstrained Fracture Toughnessmentioning

confidence: 99%

See 1 more Smart Citation

Delineating brittle-phase embrittlement and ductile-phase toughening in Nb-based in-situ composites

Chan¹,

Davidson²

2001

Self Cite

The fracture toughness of Nb-based in-situ composites typically decreases with increasing volume fractions of hard intermetallic phases, despite the presence of a ductile niobium solid-solution phase in the microstructure. For composites with a continuous intermetallic matrix, the fracture toughness can be more than double that of the monolithic intermetallics, but is still low in absolute terms, indicating that the solid-solution phase is not very effective in inducing ductile-phase toughening. The lack of enhancement of the fracture resistance appears to arise from an embrittlement effect instigated by the brittle phases in the microstructure, whose nondeformability results in a high plastic constraint acting on the ductile phase. In this article, an analytical model is developed for treating both brittle-phase embrittlement and ductile-phase toughening in terms of constituent properties and microstructural variables. The model is then used to (1) delineate brittle-phase embrittlement and ductile-phase toughening in Nb-based in-situ composites, and (2) design fracture-resistant in-situ composites based on Nb-Ti-Cr, Nb-Ti-Al, and Nb-Ti-Si systems.

Effects of Ti addition on cleavage fracture in Nb-Cr-Ti solid-solution alloys

Chan

Davidson

1999

Self Cite

The fracture toughness of Nb-Cr-Ti solid-solution alloys has been shown to be greatly improved by Ti addition, but the mechanism of toughness enhancement has not been established. In this study, critical experiments were performed on the tough Nb-Cr-Ti alloy to characterize the crack-tip fracture process and to investigate the origin of fracture toughness. In addition, theoretical calculations of the unstable stacking energy (USE) and the Peierls-Nabarro (P-N) energy and stress were performed as a function of Ti content in the Nb-Cr-Ti alloys. The experimental results indicate that the fracture toughness in the tough Nb-Cr-Ti alloy originates from extensive dislocation emission that suppresses cleavage crack propagation from the crack tip. The theoretical calculation indicates that Ti addition lowers the P-N energy and stress, but has little effect on the USE. These results are used to elucidate the effects of Ti addition on cleavage fracture in Nb-Cr-Ti alloys by considering the influence of the P-N energy and stress values on (1) dislocation mobility, (2) crack-tip dislocation emission, (3) fracture toughness, and (4) brittle-to-ductile fracture transition. It is concluded that dislocation emission in the Nb-Cr-Ti alloys appears to be controlled by the P-N energy, which influences dislocation mobility, rather than by the USE, which influences dislocation nucleation. Ti increases the fracture toughness of Nb-Cr-Ti alloys by increasing dislocation mobility and dislocation emission from the crack tip through a reduction of the P-N energy and stress.