The current work investigates the effects of test temperature (77 to 131 K), grain size (63 to 165 m), and solid solution alloying additions of zirconium on the cleavage fracture stress ( F ) of polycrystalline niobium. Extensive fracture surface analyses of fractured notched-bend specimens revealed the location of the apparent cleavage fracture nucleation sites, while comparisons have been made to peak stress locations using existing finite element models. The effects of such microstructural changes on the magnitude of the cleavage fracture stress are rationalized via comparisons to models for cleavage fracture.

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The critical resolved shear stress for activating the (0001) 〈21‐1‐0〉 slip system of monocrystalline 6H‐SiC has been determined as a function of test temperature and strain rate via constant‐displacement compression tests. Tests were conducted at temperatures between 550 and 1300 °C at strain rates of 1.3×10—4, 6.3×10ü—5 and 3.1×10ü—5 s—1. The current study shows that 6H‐SiC crystals can be plastically deformed via relatively modest resolved shear stresses on the basal plane at temperatures as low as ≈︂550 °C. For temperatures below ≈︂1300 °C for the fast and intermediate strain rates, and for temperatures below ≈︂1100 °C for the slow strain rate, the stress exponent n, and the activation enthalpy H(2.1 ± 0.7) eV, respectively. At higher temperatures at the slowest strain rate, the activation enthalpy was determined to be (4.5 ± 1.2) eV. Subsequent to the deformation tests, transmission electron microscopy (TEM) was used to rationalize some of the results.

Recent deformation experiments on semiconductors have shown the occurrence of a break in the variation of the critical resolved shear stress of the crystal as a function of temperature. These and many other examples in the literature evidence a critical temperature at which a transition occurs in the deformation mechanism of the crystal. In this paper, the occurrence of a similar transition in two polytypes of SiC is reported and correlated to the microstructure of the deformed crystals investigated by transmission electron microscopy, which shows evidence for partial dislocations carrying the deformation at high stresses and low temperatures. Based on these results and data in the literature, the explanation is generalized to other semiconductors and a possible relationship to their brittle-ductile transition is proposed. a)

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The current work investigates the effects of test temperature (77 to 150 K), grain size (63 to 165 m), and solid solution alloying additions of zirconium (Zr) on the fracture toughness (K q , K Ic ) of polycrystalline niobium (Nb). Extensive fracture surface analyses of the fractured specimens revealed the location of the apparent cleavage fracture nucleation sites. Comparisons have been made to models for cleavage fracture toughness as well as to predictions of the peak stress locations using existing finite element models for a crack loaded under plane strain conditions.

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