Intragranular Distributions Research Articles

Subgrain geometrically necessary dislocation density mapping in spalled Ta in three dimensions

In this work, the intragranular distributions of the densities of geometrically necessary dislocations (GNDs) generated in a polycrystalline tantalum (Ta) sample loaded in shock compression to spall are investigated. The sample studied is a highly resolved three-dimensional (3D) map of the orientations, grain boundary morphology, and voids across a 0.276mm3 (with sample dimensions of 688.5µm × 669µm × 600µm) region of the microstructure about the spall plane obtained by TriBeam tomography. We present a formulation for calculating the crystallographic screw and edge character GND densities at a material point within body centered cubic (BCC) crystals when provided any number of potential slip systems or modes. Combining the 3D characterization, GND formulation, and a sample containing ≈ 6000 grains, we sought statistically relevant correlations between GND density per grain with grain size, grain orientation, void formation, and proximity from the spall plane. We show that GND density per grain increases with decreasing distance from the spall plane. Most grains had heterogeneously distributed GND densities but were free of any detectable voids, at least within the ≈ 1.5µm resolution of the characterization technique. It is found that the GND density for most grains is insensitive to grain size and grain orientation. Only the fraction of grains with the highest GND densities tended to have a [001] parallel to shock-direction orientation. Intragranular 3D maps of GNDs reveals that grains that contain voids have high GND density concentrations in the intragranular region immediately surrounding the void. The local intragranular GND mapping also showed voids that are connected to grain boundaries preferred to grow into the grain with the larger size, regardless of orientation.

An internal-oxidation-based strategy induced high-density alumina in-situ nanoprecipitation and carbon nanotube interface optimization for co-reinforcing copper matrix composites

Achieving high strength and ductility for copper matrix composites reinforced by carbon nanofillers remains challenging due to secondary agglomeration, few intragranular distributions, and poor interfacial bonding in carbon/copper system. Here, we report an internal-oxidation-based fabrication strategy to simultaneously facilitate high-density in-situ nanoprecipitation into matrix grain interior as well as interfacial optimization between carbon nanotubes (CNTs) and copper matrix. Comprehensive experimental results show that in-situ generation of high-density coherent γ-Al2O3 nanoparticles with intragranular distribution and formation of amorphous copper oxides (CuxOy) between CNTs and Cu are attributed to oxygen diffusion and in-situ solid-reaction ascribing to thermodynamic driving force during internal oxidation. The co-strengthening effect of high-density γ-Al2O3 nanoparticles and interface optimized CNTs is corroborated to contribute to the high mechanical properties of the composite, which exhibits a high tensile strength of up to 510 MPa and excellent ductility of 20.2%. Mechanism analysis indicates that both high-density intragranular γ-Al2O3 nanoparticles and interface optimized CNTs co-render high strength of the hybrid composite with regard to Orowan strengthening and highly effective load transfer strengthening respectively, which greatly agrees with theoretical quantitative and experimental values of corresponding strength contributions. Meanwhile, cooperation of the dislocation accumulation caused by intragranular γ-Al2O3 nanoparticles and cracking resistance enhanced by interfacial bonding optimized CNTs collectively maintains tensile ductility. The present design concept may provide a promising strategy for achieving the remarkable co-strengthening effect of nanoscale dual-reinforcements, contributing to high strength and ductility in other carbon nanofillers reinforced metal matrix composites.

Thermal Properties of Mg–Al/AlN Composites Fabricated by Powder Metallurgy

Magnesium matrix composites reinforced with AlN particles were fabricated by the powder metallurgy technique. Different mixing methods were used in this study to control the distribution of AlN particles. The microstructure, thermal diffusivity and thermal expansion of the Mg–Al/AlN composites using different mixing methods were investigated. The results showed that the intergranular and intragranular distributions of AlN particles were obtained, respectively, by controlling the mixing methods. The composite with intragranular particles exhibited lower thermal diffusivity because of the existences of more interfaces, defects and grain boundaries, which acted as scattering centers and reduced the mean free path of electrons and phonons. The existence of AlN particles lowered the coefficient of thermal expansion (CTE) and enhanced the dimensional stability of the composites. And the use of the improved mixing method further reduced the CTE of Mg–Al/AlN composites.

A method for intragranular orientation and lattice strain distribution determination

A novel approach to quantifying intragranular distributions is developed and applied to the α → ∊ phase transition in iron. The approach captures both the distribution of lattice orientation within a grain and the orientation dependence of the lattice strain. Use of a finite element discretization over a ball in Rodrigues space allows for the efficient use of degrees of freedom in the numerical approach and provides a convenient framework for gradient-based regularization of the inverse problem. Application to the α → ∊ phase transition in iron demonstrates the utility of the method in that intragranular orientation and lattice strain distributions in the α phase are related to the observed ∊ orientations. Measurement of the lattice strain distribution enables quantitative analysis of the driving forces for ∊ variant selection. The measurement and analysis together indicate quantitatively that the Burgers mechanism is operative under the experimental conditions examined here.

Grain-size dependent accommodation due to intragranular distributions of dislocation loops

A grain-size dependent accommodation law for polycrystals is deduced from an inclusion/matrix problem (i.e., each grain is seen as embedded in a homogeneous equivalent medium) where plastic strain inside the inclusion is given as a discrete distribution of circular coaxial glide dislocation loops. The loops are assumed constrained at spherical grain boundaries. From thermodynamic considerations specific to a process of identical plastification in all the loops (considered as “super-dislocations”), an average back-stress over the grain is derived. In order to compute the very early stages of plastic deformation in a face-centred cubic polycrystal, this back-stress is incorporated into a diluted model in terms of concentration of plastic grains. Contrary to conventional mean-field approaches, a grain-size effect is obtained for the initial overall strain-hardening behaviour. This size effect results from an intrinsic contribution of intragranular slip heterogeneities on the kinematical hardening.

Compositional and valent state inhomogeneities and ordering of oxygen vacancies in terbium-doped ceria

Intragranular distributions of composition and valent state in sintered Tb-doped ceria have been systematically investigated. Through detailed studies of electron energy loss spectroscopy and energy filtering transmission electron microscopy, both compositional and valent state inhomogeneities of Ce and Tb were confirmed, which are related to the existence of nanosized domains in Tb-doped ceria. Compared with their matrix, the domains have higher Tb concentration and Ce and Tb cations in the domains tend to be trivalent. Furthermore, ordering of oxygen vacancies in the domains, which increases with increasing doping concentration, has been determined by EELS.

Compression characteristics of granulated materials. VII. The effect of intragranular binder distribution on the compatibility of some lactose granulations.

Two sets of lactose-polyvinylpyrrolidone granulations (95:5) of different intragranular binder distributions were produced. The intragranular binder distribution was controlled by a two-step granulation procedure. The compactibility as well as the volume reduction behavior of the granulations was evaluated. Granulations with a more homogeneous distribution of binder in the granules generally produced tablets of a higher mechanical strength than granulations with a peripheral localization of binder. The tablet strength of the latter granulations was also comparatively more reduced by the addition of magnesium stearate. Thus, it is suggested that high granule porosity in combination with homogeneous intragranular binder distribution is advantageous for the compactibility of a granulation. The results of this study therefore contradict earlier suggestions in the literature regarding the preferred intragranular binder distribution.