Search

A model is suggested that describes enhanced strain rate sensitivity of nanocrystalline and ultrafine-grained metals. Within the model, plastic deformation of such metals incorporates dislocation transmission across grain boundaries (GBs) in the stress fields of dislocation pileups, the emission of individual dislocations from GBs as well as GB sliding accommodated by GB dislocation climb and/or Coble creep. The model predicts a strong increase in the strain rate sensitivity and a decrease in the activation volume with decreasing grain size, in accord with experimental data.We also considered the effect of GB sliding and Coble creep on the anomalous dependence of the activation volume on temperature observed in nanocrystalline Ni. It is demonstrated that although an account for GB sliding and Coble creep leads to the appearance of cusps in the temperature dependence of the activation volume, these mechanisms alone cannot be responsible for the observed anomalous dependence of the activation volume on temperature.

A model is proposed describing the effect of crack bridging on the fracture toughness of ceramic/graphene composites. The dependences of the fracture toughness on the graphene content and the sizes of the graphene platelets are calculated in the exemplary case of yttria stabilized zirconia (YSZ)/graphene composites. The calculations predict that if crack bridging prevails over crack deflection during crack growth, the maximum toughening can be achieved in the case of long graphene platelets provided that the latter do not rupture and adhere well to the matrix. The model shows good correlation with the experimental data at low graphene concentrations.

We provide a survey of the results of recent experiments, molecular dynamics simulations and theoretical models concerning the plastic deformation and fracture processes in metal-matrix composites reinforced with graphene platelets or graphene nanoribbons. We consider homogeneous metal/graphene composites with randomly oriented and aligned graphene platelets, as well as laminated metal/graphene composites. The focus of the review will be on the strengthening and strain hardening mechanisms of the composites and the results of modeling the processes of their plastic deformation and strength properties. We examine in detail the effects of the inclusion dimensions, characteristics of interfaces, and the presence of buffer layers between the metal matrix and graphene on strength and ductility of metal/graphene composites. We critically review various theories of strengthening of such composites and discuss the contradictory results that these theories predict. In addition, various plastic deformation and fracture processes, including dislocation interaction with graphene inclusions, grain and lamella boundaries, self-healing of the composites, and crack generation and growth, will be examined, and the influence of these processes on the mechanical properties of metal/graphene composites will be discussed. We demonstrate that the excellent mechanical properties of metal/graphene composites are related to their unique microstructure and the variety of strengthening and strain hardening mechanisms. We also discuss the effect of the bimodal grain size distribution of the metallic matrix on their strength and ductility. The summary will outline the conclusions and briefly highlight unresolved issues and prospects for further research. [ABSTRACT FROM AUTHOR]

The synchrotron radiation (SR) phase radiography, optical and scanning electron microscopies (SEM) and color photoluminescence are used to study the interaction of micropipes (MPs) with foreign polytype inclusions in 4H-SiC bulk crystals grown on 6H-SiC substrates. A model of a polytype inclusion interactions with MPs near the free surface of a growing crystal is developed, which has helped in understanding the pore formation and expansion in SiC bulk crystals.

Formation of pores at foreign polytype boundaries in bulk SiC crystals is analyzed by using synchrotron radiation phase-sensitive radiography, optical and scanning electron microscopies and color photoluminescence. The studies have shown that the pore can either be a separate micropipe, or grow up to a certain length or occupy the whole facet of the inclusion depending on the inclusion distortion.

The ramification of micropipes is observed using scanning electron microscopy, optical microscopy, optical microscopy, and synchrotron x-ray radiography. It is shown that the ramification of micropipes in two smaller ones is possible only for micropipes with radii that exceed the equilibrium micropipe radius and is associated with a decrease in the total micropipe surface area.

A new mechanism for relaxation of misfit stresses in composite nanowires (quantum wires) is suggested and theoretically examined, namely the formation of misfit dislocation loops. The stress field of a prismatic dislocation loop in a cylinder (nanowire) is calculated. The parameters of two-phase composite nanowires at which the formation of misfit dislocation loops is energetically favourable are estimated. The effect of stress fields of dislocation loops on the formation of compositionally modulated nanowires is discussed. [ABSTRACT FROM AUTHOR]