Local dislocation creep accommodation of a zirconium diboride silicon carbide composite

Abstract

A grain boundary sliding creep mechanism, accommodated by "mantle" dislocation activities, is shown to allow for large strain (ε > 0.08) during the creep of a ZrB 2-20% SiC composite at 1800°C. We characterized the local grain deformation behavior using high resolution electron backscatter diffraction (HREBSD) microscopy and an indentation deformation mapping (IDM) technique. Deformation gradients near grain boundaries ("mantle") produced average geometrically necessary dislocation (GND) densities of 3x10 11-4x10 12 cm-2 , about one order of magnitude above that of the grain interiors ("core"). A deviation from single crystal grain core deformation defines the mantle where excess GND's accommodate the grain deformation gradient. Evidence supporting grain boundary sliding as the primary contribution to the creep strain appears in our earlier publication, but we show here the role of dislocations in the deformation of the grain mantle as the rate-controlling accommodation step.

Prediction of tensile power law creep constants from compression and bend data for ZrB2–20 vol% SiC composites at 1800 °C

“…Furthermore, the stress exponent suggests further contribution from an independent diffusional creep mechanism in pure compression. Therefore, we expect grain boundary sliding and diffusional creep to dominate the deformation in compression creep of ZrB 2 -20% SiC at 1800 ℃ , consistent with the flexure findings [22,23]. The dislocation accommodated sliding attributed to the present material should hold for both compressive and tensile components of the deformation as tensile and compressive regions in four-point bending samples did not show substantial microstructural differences at 1800 ℃ with higher cavitation observed along the tensile surface [18].…”

“…Grain boundary sliding was manifested as grain rotations and translations. Additionally, quantification of geometrically necessary dislocations using electron back scatter diffraction techniques revealed increased activity near the grain boundaries to achieve the necessary accommodation for the sliding grains [22,23]. Presence of limited cavitation in the compressive microstructures indicates grain boundary sliding mechanism operating in the present specimens.…”

“…The dislocation accommodated sliding attributed to the present material should hold for both compressive and tensile components of the deformation as tensile and compressive regions in four-point bending samples did not show substantial microstructural differences at 1800 ℃ with higher cavitation observed along the tensile surface [18]. Consequently, under pure tension, the tensile stress exponent of t 2 n  , implies increased contribution from an independent cavitation mechanism [18,22,23]. Table 2 Power law parameters determined experimentally and using Chuang [ …”

“…Recent research [22,23], conducted in our group, on ZrB 2 -20% SiC creep behavior at 1800 ℃ has shown geometrically necessary dislocation activity increases within grain boundary networks to accommodate the grain boundary sliding event during high temperature deformation. The creep mechanism was supported by direct observation of the microstructure where a series of nano-indents were placed on a polished surface and tracked after the deformation.…”

“…No experimental tensile data are available in the literature for direct comparison. In fact, for ZrB 2 composites, only flexure and compression experiments were carried out in the literature [7,[18][19][20][21][22][23]. Additionally, we propose an enhanced specimen geometry for compression creep testing that allows higher creep strains to be reached prior to the onset of catastrophic specimen instability.…”