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Ductile-phase toughening of in situ niobium silicide-niobium composites

Rigney, Joseph David
http://rave.ohiolink.edu/etdc/view?acc_num=case1061304077

Abstract Details

1994, Doctor of Philosophy, Case Western Reserve University, Materials Science and Engineering.
In situ processing techniques were used to incorporate elongated primary niobium (Nb p) and secondary niobium (Nb s) phases in a brittle niobium silicide (Nb5Si3) intermetallic matrix. The presence of these phases increased the fracture resistance of the composite by the ductile-phase toughening mechanism Resistance-curve behavior and peak toughnesses of 28 Mpasurdm at 380 μm crack extension were determined in slow displacement rate tests monitored in real-time. Bridging and deformation of Nb p ligaments between the crack faces in the Nb5Si3, and plasticity in a process zone ahead of the crack tips, provided for the increased fracture resistance. The properties were compared to previously developed fracture models. The work of rupture (χ) of the ligaments were determined to be 1.6 and constrained flow stresses were predicted to be about six times the uniaxial tensile yield stress (σ y). Even under these high constraints, the Nb p ligaments plastically deformed and exhibited dimpled fracture. Experiments were performed over a range of strain rates at 298 K and 77 K to increase the yield stresses of the body-centered cubic Nb p ligaments and increase propensity for cleavage fracture behavior . On each fracture surface created at 298 K, the Nb p phases exhibited a consistent, smoothly varying fracture behavior with respect to the specimen dimensions. The increasing frequency of cleavage fracture was found to be related to global strain rates experienced by the samples, induced through the motion of the loading point in the notched three point bend test. The higher strain rates augmented yield stresses and subsequently produced a transition in fracture mode. The toughness values determined from experiments spanning six orders magnitude in loading rate at 298 K and 77 K, exhibited little change, even in situations when the majority of Nb p phases failed by cleavage. Although χ-values were calculated to drop to fractions of the slow, 298 K value, the constancy of the toughness values illustrated the importance in considering the toughness of the "ductile" phase (σ ycdotχ), or the area beneath the constrained stress-strain curve. The loss in ductility at high strain rates and/or low temperatures was compensated by high yield stresses in order to match the toughening provided by the ligaments when yield stresses were lower and strains to failure were greater
John Lewandowski (Advisor)
223 p.
Engineering, Materials Science
Ductile-phase toughening; In situ processing; Niobium-silicide-niobium composites

Recommended Citations

Citations

  • Rigney, J. D. (1994). Ductile-phase toughening of in situ niobium silicide-niobium composites [Doctoral dissertation, Case Western Reserve University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=case1061304077

    APA Style (7th edition)

  • Rigney, Joseph. Ductile-phase toughening of in situ niobium silicide-niobium composites. 1994. Case Western Reserve University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=case1061304077.

    MLA Style (8th edition)

  • Rigney, Joseph. "Ductile-phase toughening of in situ niobium silicide-niobium composites." Doctoral dissertation, Case Western Reserve University, 1994. http://rave.ohiolink.edu/etdc/view?acc_num=case1061304077

    Chicago Manual of Style (17th edition)

case1061304077
648
© 1994, all rights reserved.
This open access ETD is published by Case Western Reserve University School of Graduate Studies and OhioLINK.