Research Interests:
Research Interests:
The formation and evolution of dislocation patterns in pure polycrystalline aluminum was examined using transmission electron microscopy. The conventional characterization of the deformed samples was combined with in-situ tensile tests of... more
The formation and evolution of dislocation patterns in pure polycrystalline aluminum was examined using transmission electron microscopy. The conventional characterization of the deformed samples was combined with in-situ tensile tests of prestrained samples which were carried out in order to get a better understanding of dislocation motion during deformation. The role of different types of boundaries was studied and it was found that while dense dislocation walls have an ordered structure since they are geometrically necessary, incidental dislocation boundaries can change their configuration from tangled to ordered.
Research Interests:
ABSTRACT The formation and evolution of dislocation patterns in pure polycrystalline aluminum was examined using transmission electron microscopy. The conventional characterization of the deformed samples was combined with in-situ tensile... more
ABSTRACT The formation and evolution of dislocation patterns in pure polycrystalline aluminum was examined using transmission electron microscopy. The conventional characterization of the deformed samples was combined with in-situ tensile tests of prestrained samples which were carried out in order to get a better understanding of dislocation motion during deformation. The role of different types of boundaries was studied and it was found that while dense dislocation walls have an ordered structure since they are geometrically necessary, incidental dislocation boundaries can change their configuration from tangled to ordered.
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
ABSTRACT Electromagnetic levitation was used to determine Cu–Nb phase diagram and to study supercooling effects on solidification characteristics of the alloys containing 5–70wt% Nb. The Cu–Nb stable phase diagram was found to exhibit... more
ABSTRACT Electromagnetic levitation was used to determine Cu–Nb phase diagram and to study supercooling effects on solidification characteristics of the alloys containing 5–70wt% Nb. The Cu–Nb stable phase diagram was found to exhibit near-flat liquidus with a peritectic reaction at 1093°C. Melt separation was found only for specimens containing approximately 20wt% Nb. The results indicate that melt separation in the alloy requires supercooling exceeding 230K combined with high cooling rates during solidification. Some specimens quenched from the solid+liquid zone on a copper chill also show evidence of melt separation which is attributed to minor oxygen impurities. Nb-rich liquid which nucleates below the T 0 curve solidifies as a metastable Nb-bcc lattice containing only 67wt% Nb as compared to 96wt% of the regular Nb dendrites.
Research Interests:
Research Interests:
Research Interests: Materials Engineering, Mechanical Engineering, Condensed Matter Physics, Materials Science, Crystallography, and 11 moreTransmission Electron Microscopy, Copper, Dislocation, Dislocations, Nickel, Temperature Dependence, Microstructures, Plastic deformation, Internal Structure, Crystallite, and Microstructure evolution
Research Interests:
Abstract Compressive creep properties of AlSi10Mg parts produced by additive manufacturing selective laser melting (AM-SLM) were studied using a spark plasma sintering (SPS) apparatus, capable of performing uniaxial compressive creep... more
Abstract Compressive creep properties of AlSi10Mg parts produced by additive manufacturing selective laser melting (AM-SLM) were studied using a spark plasma sintering (SPS) apparatus, capable of performing uniaxial compressive creep tests. Stress relief-treated specimens were tested under an applied stress of 100–130 MPa in the 175–225 °C temperature range. Utilizing two different configurations, the creep tests were conducted either with or without a low-density electric current (∼2.63–3.26 A/mm2) flowing through the test specimens. The results revealed that the creep rate increased under the influence of an applied electric current. The creep parameters (i.e., stress exponent n and apparent activation energy Q), were empirically determined. The stress exponent values were found to be 19.6 ± 1.2 and 16.2 ± 1.4 with and without current, respectively, while apparent activation energy was found to be 142 ± 9 kJ/mol and 150 ± 13 kJ/mol with and without current, respectively. The experimental results, together with microstructural examination of specimens, indicate that plastic deformation was controlled by dislocation activity. Furthermore, it is suggested that the annihilation process of dislocations during creep was enhanced by the electric current.
Research Interests:
Dynamic recrystallization (DRX), driven by the dynamic stored energy of cold work, promotes shear localization. However, the influence of twinning remains to be investigated. We consider two model materials under impact, pure Titanium and... more
Dynamic recrystallization (DRX), driven by the dynamic stored energy of cold work, promotes shear localization. However, the influence of twinning remains to be investigated. We consider two model materials under impact, pure Titanium and a Ti6Al4V alloy, for which the extent of twinning and DRX differs vastly. A grain-scale finite element model shows that twinning delays DRX and consequently strain localization. Moreover, the calculated temperature elevations (local and global) remain very modest until the onset of failure.
Research Interests:
The effect of strain on the microstructure and detailed internal structure of dislocation boundaries in pure polycrystalline fcc metals (aluminum, copper, nickel and gold) was systematically studied and compared as a function of strain... more
The effect of strain on the microstructure and detailed internal structure of dislocation boundaries in pure polycrystalline fcc metals (aluminum, copper, nickel and gold) was systematically studied and compared as a function of strain following compression at room temperature. At low strains all metals form a cellular structure. The dislocations in the cell walls tend to rearrange themselves from tangles
Research Interests:
Research Interests:
ABSTRACT The combined effect of strain and temperature on the microstructure and detailed internal structure of dislocation boundaries was systematically studied in compressed pure polycrystalline copper and nickel and compared to the... more
ABSTRACT The combined effect of strain and temperature on the microstructure and detailed internal structure of dislocation boundaries was systematically studied in compressed pure polycrystalline copper and nickel and compared to the microstructure of ...
Research Interests:
ABSTRACT 〈101〉-oriented cylindrical single crystalline Fe samples with diameters of 100 nm and heights of 1 μm were implanted with 0.36±0.06 at% helium throughout their gauge sections. Uniaxial deformation experiments revealed a 40%... more
ABSTRACT 〈101〉-oriented cylindrical single crystalline Fe samples with diameters of 100 nm and heights of 1 μm were implanted with 0.36±0.06 at% helium throughout their gauge sections. Uniaxial deformation experiments revealed a 40% higher yield and ultimate strengths in tension and a 25% higher yield strength and flow stress at 10% plastic strain in compression for implanted samples compared with as-fabricated ones. Observed tension–compression asymmetry in implanted pillars was attributed to the non-planarity of screw dislocation cores and to twinning-antitwinning deformation typical of bcc metals and the interaction between dislocations and He bubbles. Compressive stress–strain data in both sets of samples had three distinct regimes: (1) elastic loading followed by (2) discrete strain bursts during plastic flow with significant hardening up to strains of 5%, and (3) “steady state” discrete plasticity characterized by nearly-constant average flow stress. Each regime is discussed and explained in terms of competition in the rates of dislocation multiplication and dislocation annihilation.
Research Interests:
A fabrication methodology for 120 nm-diameter,... more
A fabrication methodology for 120 nm-diameter, <111>-oriented single crystalline Cu nanopillars which are uniformly implanted with helium is described. Uniaxial compression experiments reveal that their yield strength is 30% higher than that of their unimplanted counterparts. This study sheds light on the fundamental understanding of the deformation mechanism of irradiated metallic nanocrystals, and has important implications for the interplay between irradiation-induced defects and the external sample dimensions in the nanoscale.
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
ABSTRACT In situ uniaxial tensile experiments on as-fabricated and helium-implanted 100 nm-diameter Cu/Fe bicrystals unearth the effect of individual face-centred-cubic/body-centred-cubic (fcc-bcc) interfaces on improving radiation-damage... more
ABSTRACT In situ uniaxial tensile experiments on as-fabricated and helium-implanted 100 nm-diameter Cu/Fe bicrystals unearth the effect of individual face-centred-cubic/body-centred-cubic (fcc-bcc) interfaces on improving radiation-damage tolerance and helium absorption. Arrays of nanotensile specimens, each containing a single Cu grain in the bottom half and a single Fe grain on top, were fabricated by templated electron-beam lithography and electrodeposition. Helium is implanted at 200 keV to a dose of 1014 ion/cm2 nominally into the interface region. High-resolution, site-specific transmission electron microscopy (TEM) and through-focus analysis reveal that the interfaces are nonplanar and contain ≈5 nm-spaced He bubbles with diameters of 1–2 nm. Nanomechanical experimental results show that the irradiated samples exhibit yield and ultimate tensile strengths more than 60% higher than the as-fabricated ones, while they retain comparable ductility. Tensile failure always occurs gradually, along the interfaces, with no noticeable shape localization. The absence of brittle failure in He-irradiated metals might be explained, in part, by the inability of the small He bubbles to serve as sufficient stress concentrators for cracking. In addition, the non-orthogonal orientation of the interfaces with respect to the loading axes results in the development of both normal- and shear-stress components. Tensile loading along the pillar axes may cause those interfacial regions subjected to normal stresses to detach, while the inclined regions, subjected to shear, to carry plastic deformation until final fracture.