Grain boundary sliding and its relation to ductility and fracture in fine-grained polycrystalline materials with a particular focus on γ-TiAl

Damage-Tolerant, Hard Nanocomposite Coatings Enabled by a Hierarchical Structure

“…According to the simulations and experimental observations, when the grain size of conventional materials decreases from the micrometer scale down to the nanometer scale, a shift occurs in the governing deformation mechanism from a dislocation-mediated process in the coarse-grained materials to a grain boundary-mediated process in the nanocrystalline materials. − The beneficial effects of grain boundary sliding on the toughness of nanocrystalline intermetallics and ceramics has been observed by other researchers, ,, which is in agreement with the experimental results from the MM coating. However, numerous investigations on single-phase nanocrystalline metals indicated that a grain boundary-mediated process is insufficient, compared to that for a dislocation-controlled process, which results in low toughness at room temperature. − The reduced toughness in nanostructured metals is presumably due to their inability to resist excessively large local strains responsible for plastic flow instabilities, such as necking and shear banding. , Interestingly, nature has provided excellent examples for the design of hard, yet tough, materials.…”

“…4 Indeed, there has been considerable effort to understand and control the mechanical behavior of nanostructured metals and alloys, 5À7 but less so for intermetallic compounds. 8 Our previous studies have shown evidence that nanoscrystalline Cr 3 Si and Mo 5 Si 3 , with an average grain size smaller than 10 nm, have superior hardness and toughness compared with their coarse-grained counterparts. 9,10 In this paper, we present a novel hierarchical structured coating that consists of a nanostructured MoSi 2 outer layer supported by a compositionally graded, bimodal MoSi 2 /Mo 5 Si 3 nanocomposite layer.…”

“…For example, the introduction of a second phase to form a composite microstructure and alloying with other silicides have been applied to enhance the damage resistance of monolithic MoSi 2 , since MoSi 2 is thermodynamically stable with a wide variety of ceramics and high melting point silicide reinforcements. , From the standpoint of fracture mechanics, a reduction in grain size down to nanoscale levels is another potential avenue to toughen these materials . Indeed, there has been considerable effort to understand and control the mechanical behavior of nanostructured metals and alloys, − but less so for intermetallic compounds . Our previous studies have shown evidence that nanoscrystalline Cr 3 Si and Mo 5 Si 3 , with an average grain size smaller than 10 nm, have superior hardness and toughness compared with their coarse-grained counterparts. , In this paper, we present a novel hierarchical structured coating that consists of a nanostructured MoSi 2 outer layer supported by a compositionally graded, bimodal MoSi 2 /Mo 5 Si 3 nanocomposite layer.…”

Engineering grain boundary sliding and cavitation effects in superplastic alloys employing thermodynamics

“…For a strain increment de in the specimen, the Gibbs free energy (per unit volume) to form a cavity (dG) depends on the competition between the strain energy build-up at the grain boundaries (dH), and the energy dissipation by grain boundary sliding (TdS) 6…”

Plastic deformation in impure nanocrystalline ceramics

“…The problem of improving the ductility of brittle materials through the refinement of the grain size was treated theoretically by DiMelfi. 30,31 He determined a criterion based on ratios of gb to matrix strain rates, and grain size to gb thickness.…”