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In this paper, a theoretical model is established to describe the effect of nanoscale amorphization in nanocrystalline bimaterials on the dislocation emission from the tip of a collinear crack at the interface. In the description, nanoscale amorphization is formed by the splitting transition of the Grain Boundary (GB, the disclination of GBs caused by the movement of GBs). The analytical solution of the model is obtained by the elasticity complex potential solution method. In addition, the effects of nanoscale amorphization, dislocation emission angle, interfacial crack length and material constants of nanocrystalline bimaterials on the critical stress intensity factor of interfacial crack tip corresponding to dislocation emission are discussed through numerical analysis. The analysis shows that the influence of nanoscale amorphization in nanocrystalline bimaterials on the critical stress intensity factor (SIF) corresponding to dislocation emission depends on the dislocation emission angle, the position and size of the nanoscale amorphization, interface crack length and relative shear modulus. With the increase in relative shear modulus and dislocation emission angle, the normalized critical SIF decreases at first and increases afterwards. When the nanoscale amorphization size is small, the critical SIF of the dislocation is less affected, but when the size is larger, the impact becomes great. The influence of nanoscale amorphization on the dislocation emission from collinear interface crack tip is related to nanoscale amorphization and relative shear modulus. There is a critical relative shear modulus that the increase in dislocation intensity has little effect on dislocation emission. Appropriate selection of materials for the upper and lower planes can reduce the critical stress intensity factor corresponding to dislocation emission, thereby promoting the dislocation emission from interface cracks and improving the toughness of the nanocrystalline bimaterials. [ABSTRACT FROM AUTHOR]

Shear-coupled grain boundary migration (SCGBM) has already been observed in extensive experiments and serves as an effective mechanism for the grain growth in nanocrystalline materials. Meanwhile, it is well known that precipitates strongly inhibit the motion of GBs. However, the effect of precipitates on SCGBM is not clear. In this paper, a theoretical model is established to investigate the role of precipitates on SCGBM. The effects of the characteristics of the precipitate and the GB are studied in detail. It is found that precipitates can significantly inhibit SCGBM. Interesting, the results show that the pinning effect exerted by precipitates on SCGBM is strongly relevant to their positions. Moreover, the resistance for SCGBM hindered by precipitates can be controlled by tailoring the characteristics of both the precipitates and GBs. In addition, it is discussed that a possible recrystallization occurs when the GB is hindered and it is found that the SCGBM process also facilitates it. [ABSTRACT FROM AUTHOR]

The presence of granular matter between the sliding blocks (such as for example gouge in rock joints and faults) considerably affects friction. The effect of grain movement and rolling is often modelled by considering them as spherical. This paper investigates the influence of the grain/particle shape. We found that rolling of non-spherical particles leads to an apparent friction reduction related to the effect of the moment equilibrium about a point of contact. The collective behaviour of rolling non-spherical particles is affected by their interaction by the elasticity of the material of the sliding blocks. In the case of identical and symmetrical particles, there exists a uniform solution whereby all particles undergo identical displacements. This solution is, however, unstable; at certain magnitudes of the friction force and the corresponding values of the displacement the solution bifurcates such that different particles show different displacements. When many particles of different sizes are rolling without interaction, the friction force fluctuates, and as the displacement increases, the fluctuations reach a steady state. The amplitudes of the fluctuations depend upon the shape factor; they are the smallest when the particles have a square shape and considerably increase as the period of particle rolling approaches the diameter. [ABSTRACT FROM AUTHOR]

In this paper, a gradient crystal plasticity model in a polycrystalline grain structure is investigated. Hereby, the focus is on the influence of the grain boundary conditions. A new type of grain boundary conditions is introduced, the so-called micro-flexible boundary condition. In particular, it is compared to existing grain boundary conditions of plastic slip. Numerical results are given for the stress-strain response as well as for the plastic slip field in the grain structure. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experimental study for nano-indentation size effects for different face centered cubic metals with different purities. The selected materials are: Silver, Copper, Aluminum, Lead, and Nickel. Nano-indentation tests are run using a Berkovich indenter with continuous stiffness measurement procedure where hardness is measured continuously with indentation depth. The results show three distinctive regions for the indentation size effects for the material, where hardness could increase or decrease with increasing indentation. This behavior is modeled through a proposed simple power law model, which includes the effect of grain boundaries. [ABSTRACT FROM AUTHOR]