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Huang J.Y.,Southwest Jiaotong University | Lu L.,Anhui University of Science and Technology | Fan D.,The Peac Institute of Multiscale science | Sun T.,Argonne National Laboratory | And 4 more authors.
Scripta Materialia | Year: 2015

Dynamic compression experiments are conducted on micron-sized SiC powders of different initial densities with a split Hopkinson pressure bar. Digital image correlation is applied to images from high-speed X-ray phase contrast imaging to map dynamic strain fields. The X-ray imaging and strain field mapping demonstrate the degree of heterogeneity in deformation depends on the initial powder density; mesoscale strain field evolution is consistent with softening or hardening manifested by bulk-scale loading curves. Statistical analysis of the strain probability distributions exhibits exponential decay tail similar to those of contact forces, which are supposed to lead to the grain-scale heterogeneity of granular materials. © 2015 Acta Materialia Inc.

Wang J.,The Peac Institute of Multiscale science | Wang J.,University of Utah | Wang J.,Southwest Jiaotong University | Seifert S.,Argonne National Laboratory | And 6 more authors.
Journal of Physical Chemistry C | Year: 2015

It has previously been demonstrated that organopalladium precursors can break down under combustion conditions, forming nanoparticles that catalyze ignition. Here, we use in situ small-angle X-ray scattering (SAXS) to probe the formation and growth of palladium nanoparticles in an ethylene flame doped with 28 ppm (mol) of Pd(acetate)2. The particles appear to nucleate in the flame front and are observed by SAXS to grow in size and mass in the high-temperature region of the flame (∼2200 K) with median diameters that evolve from 1.5 to 3.0 nm. Transmission electron microscopy of particles collected on a grid located outside the flame shows that the particles are metallic palladium with sizes comparable to those determined by SAXS. Molecular dynamics simulation of particles of selected sizes indicates that at the flame temperature the particles are molten and the average mass density of the particle material is notably smaller than that of bulk, liquid Pd at the melting point. Both experimental and computational results point to homogeneous nucleation and particle-particle coalescence as mechanisms for particle formation and growth. Aerosol dynamics simulation reproduces the time evolution of the particle size distribution and suggests that a substantial fraction of the particles must be electrically charged during their growth process. © 2015 American Chemical Society.

Dharmendra C.,City University of Hong Kong | Rao K.P.,City University of Hong Kong | Zhao F.,The Peac Institute of Multiscale science | Zhao F.,City University of Hong Kong | And 3 more authors.
Materials Science and Engineering A | Year: 2014

The effect of silicon (0.2-0.8wt%) addition on the hot working behavior and deformation mechanisms of the Mg-3Sn-2Ca-0.4Al (TX32-0.4Al) alloy has been evaluated by generating processing maps in the temperature and strain rate ranges of 300-500°C and 0.0003-10s-1. The processing map for the base TX32-0.4Al alloy exhibited two dynamic recrystallization (DRX) domains in the ranges (1) 300-360°C and 0.0003-0.001s-1 and (2) 400-500°C and 0.003-0.7s-1. While 0.2% Si addition did not result in any significant change in the processing map of the base TX32-0.4Al alloy, 0.4% Si addition has enhanced hot workability by widening the processing window(s) and by reducing flow instability. The rate controlling mechanism in Domain 1 is identified as climb, whereas it is cross-slip in Domain 2. When the Si content is increased to 0.6 and 0.8%, the volume fraction of hard intermetallic particles has increased nearly two fold. The processing map for the alloy with 0.6% Si addition exhibited an additional Domain 3 at higher temperatures and high strain rates (475-500°C and 0.01-10s-1). However, cracking has occurred in Domain 1 due to void formation at hard particles. In Domains 2 and 3, DRX occurred predominantly by basal slip with climb as a recovery process, as confirmed by the resulting basal texture and tilt type sub-boundary structure. This is attributed to the large back stress generated by the increased volume fraction of intermetallic particles due to which the extensive activation of basal slip required considerably high temperatures. Increase in the volume fraction of hard particles due to higher Si content reduces the flow instability by generating a high rate of entropy production through increasing the nucleation sites for power dissipation and enhances the occurrence of void formation and/or ductile fracture. © 2014.

Zhong T.,City University of Hong Kong | Rao K.P.,City University of Hong Kong | Prasad Y.V.R.K.,City University of Hong Kong | Zhao F.,City University of Hong Kong | And 2 more authors.
Materials Science and Engineering A | Year: 2013

Processing map for hot working of hot extruded AZ31-1.5 vol% nano-alumina magnesium composite (AZ31-NAL) prepared by disintegrated metal deposition (DMD) technique has been developed in the temperature range of 250-500°C and strain rate range of 0.0003-10s-1. The starting composite microstructure is fine grained and is much less textured compared with the base AZ31 material prepared by similar technique (AZ31-DMD). The processing map for the composite exhibits three domains in the temperature and strain rate ranges: (1) 250-350°C/0.0003-0.01s-1; (2) 375-500°C/0.0003-0.01s-1; (3) 300-400°C/1-10s-1, which are all similar to those exhibited by the base alloy. In Domains #1 and #3, dynamic recrystallization occurs and results in fine grained microstructure, while in Domain #2, grain boundary sliding leading to wedge cracking in compression and intercrystalline cracking in tension have been identified. In comparison with the base alloy AZ31-DMD, the efficiency in the first domain has decreased, the third domain moved to lower temperatures and the effect on second domain is marginal. These differences are attributed to the lower intensity of starting texture in the nano-composite compared with the base alloy. A study of texture changes in the deformed specimens revealed that nano-alumina additions are helpful in reducing the preferred orientation in AZ31 alloy. © 2013 Elsevier B.V.

Wang M.,Sichuan University | Wang M.,Southwest Jiaotong University | Wang M.,The Peac Institute of Multiscale science | Lu L.,The Peac Institute of Multiscale science | And 8 more authors.
Materials Science and Engineering A | Year: 2016

We investigate deformation and damage of a magnesium alloy, AZ91, under high strain rate (∼105s-1) loading via planar impact. The soft-recovered specimens are examined with electron back-scatter diffraction (EBSD). EBSD analysis reveals three types of twinning: (1012) extension, (1011) contraction, and (1011)-(1012) double twinning, and their number density increases with increasing impact velocity. The extension twins dominate contraction and double twins in size and number. Dislocation densities of the recovered specimens are evaluated with x-ray diffraction, and increase with increasing impact velocity. X-ray tomography is used to resolve three-dimensional microstructure of shock-recovered samples. The EBSD and tomography results demonstrate that the second phase, Mg17Al12, plays an important role in both deformation twinning and tensile cracking. Deformation twinning appears to be a common mechanism in deformation of magnesium alloys at low, medium and high strain rates, in addition to dislocation motion. © 2016 Elsevier B.V.

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