Evolution of Microstructure and Intergranular Glass Chemistry in Plastically Deformed Nanocrystalline Si₃N₄ Ceramics

Abstract

Nanograined Si3N4 ceramics with Y2O3–Al2O3–MgO as sintering additive exhibited superplastic elongation of >300% at 1923 K with an initial strain rate of 5 × 10−4 s−1. Flow stress was less than 4 MPa up to elongation of 130%. In the later stage of deformation, the flow stress increased with strain due to the grain growth and the alignment of elongated grains. The volume fraction of glassy phase significantly reduced due to vaporization of glassy phase. The chemical analysis by TEM/EDS revealed that the chemical composition of the near Si3N4 grain surface was different from that of the interior of the glass pocket at multigrain junctions due to absorption of additive cations on the Si3N4 grain. The chemistry of intergranular glassy phase changed significantly in the later stage of deformation accompanying the microstructural evolution. It is suggested that the flow stress, which depends on the viscosity of intergranular glassy phase, varies with the change in glass chemistry during deformation.

High-strain-rate superplasticity in nanocrystalline silicon nitride ceramics under compression

“…At strain rates lower than 5 Â 10 À3 s À1 , the peak was not observed, and the strain hardening occurred due to the grain growth together with a to b phase transformation. However, the strain hardening in compression test was much less than that observed in tension test at slower strain rates (10 À4 -10 À5 s À1 ) in Si 3 N 4 ceramics [23]. The major cause of strain hardening in Si 3 N 4 ceramics is the microstructural evolution during deformation at elevated temperature; grain growth [3,6,7,10,24,25], grain elongation associated with a to b phase transformation [11,[25][26][27][28], and grain alignment along the tensile direction [25,27,28].…”

“…The major cause of strain hardening in Si 3 N 4 ceramics is the microstructural evolution during deformation at elevated temperature; grain growth [3,6,7,10,24,25], grain elongation associated with a to b phase transformation [11,[25][26][27][28], and grain alignment along the tensile direction [25,27,28]. For example, at a low strain rate of 5 Â 10 À4 s À1 and at 1650°C, a large superplastic elongation more than 300% was achieved for a nanocrystalline Si 3 N 4 in tension [23]. But, the extensive strain hardening was observed at later stage of deformation due to the microstructural evolution and also the drastic change in glass chemistry at grain boundary and glass pocket due to vaporization of glass phase.…”

Contribution of boundary non-stoichiometry to the lower-temperature plasticity in high-pressure sintered boron carbide

“…The lower temperature plasticity of ceramics combined with high strength might improve their in-service reliability through metal-like behavior. Two strategies are generally chosen to improve plasticity at relatively low temperatures [7][8][9][10]: (1) decreasing grain size to several nanometers to promote grain boundary sliding; (2) accelerating grain boundary diffusion with low melting point sintering additives. However, it is di cult to sinter dense nanoceramics with grains of less than 10 nm.…”

“…In addition, secondary phases such as glass and oxide impurities [7,8] in structural ceramics decrease high-temperature strength signi cantly, thus degrading a key mechanical property. As examples, Y 2 O 3 -Al 2 O 3 -MgO was added to nano-grained Si 3 N 4 ceramics as a sintering additive, with the as-prepared ceramics exhibiting superplasticity at 1650°C, with a ow stress of only 4 MPa [7]. Nanocrystalline silicon carbide doped with boron and carbon was found to show a superplastic elongation of > 140% at 1800°C with a yield stress of 80 MPa [8].…”

“…However, it is di cult to sinter dense nanoceramics with grains of less than 10 nm. In addition, secondary phases such as glass and oxide impurities [7,8] in structural ceramics decrease high-temperature strength signi cantly, thus degrading a key mechanical property. As examples, Y 2 O 3 -Al 2 O 3 -MgO was added to nano-grained Si 3 N 4 ceramics as a sintering additive, with the as-prepared ceramics exhibiting superplasticity at 1650°C, with a ow stress of only 4 MPa [7].…”