Search

High quality Van der Waals chalcogenides are important for phase change data storage, thermoelectrics, and spintronics. Using a combination of statistical design of experiments and density functional theory, we clarify how the out-of-equilibrium van der Waals epitaxial deposition methods can improve the crystal quality of Sb2Te3 films. We compare films grown by radio frequency sputtering and pulsed laser deposition (PLD). The growth factors that influence the crystal quality for each method are different. For PLD grown films a thin amorphous Sb2Te3 seed layer most significantly influences the crystal quality. In contrast, the crystalline quality of films grown by sputtering is rather sensitive to the deposition temperature and less affected by the presence of a seed layer. This difference is somewhat surprising as both methods are out-of-thermal-equilibrium plasma-based methods. Non-adiabatic quantum molecular dynamics simulations show that this difference originates from the density of excited atoms in the plasma. The PLD plasma is more intense and with higher energy than that used in sputtering, and this increases the electronic temperature of the deposited atoms, which concomitantly increases the adatom diffusion lengths in PLD. In contrast, the adatom diffusivity is dominated by the thermal temperature for sputter grown films. These results explain the wide range of Sb2Te3 and superlattice crystal qualities observed in the literature. These results indicate that, contrary to popular belief, plasma-based deposition methods are suitable for growing high quality crystalline chalcogenides., Comment: 24 pages, 8 figs

This study involved fully pearlitic wires of seven different diameters (5.5-1.6 mm). All samples were laboratory annealed to re-austenitize and were then air-cooled to reform the pearlite structure. Morphological alignment of the pearlite, along the wire axis, improved significantly, 32% to 93%, as the wire diameter decreased. This improvement coincided with increases in the < 110 > ferrite fiber texture, and falls in the axial residual stresses. In all the wires, the majority of the pearlite lamellae appeared to align, in a 2-D analysis, with minimum elastic stiffness (E-Min, under simple compression) for the ferrite (alpha). This correlation increased from 80% to 98% with decrease in wire diameter and fall in axial residual stresses. 3-D microstructures by serial sectioning, 3-D rotations seeking E-min and observations on coarse pearlite, indicated that {011}(alpha) and < 001 > (alpha) were, respectively, the pearlite interface (or habit plane) and growth direction. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Discrete dislocation plasticity simulations are performed to investigate the static frictional behavior of a metal asperity on a large single crystal, in contact with a rigid platen. The focus of this study is on understanding the relative importance of contact slip opposed to plasticity in a single asperity at the micrometer size scale, where plasticity is size dependent.Slip of a contact point is taken to occur when the shear traction exceeds the normal traction at that point times a microscopic friction coefficient. Plasticity initiates through the nucleation of dislocations from Frank-Read sources in the metal and is modeled as the collective motion of edge dislocations.Results show that plasticity can delay or even suppress full slip of the contact. This generally happens when the friction coefficient is large. However, if the flattening depth is sufficiently large to induce nucleation of a large dislocation density, slip is suppressed even when the friction coefficient is very small. This study also shows that when self-similar asperities of different size are flattened to the same depth and subsequently loaded tangentially, their frictional behavior appears size independent. However, when they are submitted to the same contact pressure, smaller asperities slip while larger asperities deform plastically. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The microstructure of hot dip galvanized zinc coatings on dual phase steel was investigated by electron microscopy and the coating adhesion characterized by tensile testing. The zinc coating consists of a zinc layer and columnar zeta-FeZn13 particles on top of a thin inhibition layer adjacent to the steel substrate. The inhibition layer is a thin compact and continuous layer that consists of eta-Fe2Al5-xZnx fine and coarse particles. The coarse faceted particles are on top and fine faceted particles are at the bottom. The steel surface is covered with small fraction manganese oxides, which may impair adhesion of the zinc coating. The adhesion at various interfaces that exist in zinc-coated steel was quantitatively estimated using a so-called "macroscopic atom" model. In addition, the adhesion at the interfaces in zinc-coated steel was qualitatively assessed by examining the fracture and delamination behavior upon tensile testing. In accordance with this model, fracture along zinc grain boundaries preceded fracture along the zinc layer/inhibition layer and zeta-FeZn13 particle/inhibition layer interfaces. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The effect of strain rate on the inelastic properties of nanocrystalline Au films was quantified with 0.85 and 1.76 mu m free-standing microscale tension specimens tested over eight decades of strain rate, between 6 x 10(-6) and 20 s(-1). The elastic modulus was independent of the strain rate, 66 +/- 4.5 GPa, but the inelastic mechanical response was clearly rate sensitive. The yield strength and the ultimate tensile strength increased with the strain rate in the ranges 575-895 MPa and 675-940 MPa, respectively, with the yield strength reaching the tensile strength at strain rates faster than 10(-1) s(-1). The activation volumes for the two film thicknesses were 4.5 and 8.1 b(3), at strain rates smaller than 10(-4) s(-1) and 12.5 and 14.6 b(3) at strain rates higher than 10(-4) s(-1), while the strain rate sensitivity factor and the ultimate tensile strain increased below 10(-4) s(-1). The latter trends indicated that the strain rate regime 10(-5)-10(-4) s(-1) pivotal in the mechanical response of the particular nanocrystalline Au films. The increased rate sensitivity and the reduced activation volume at slow strain rates were attributed to grain boundary processes that also led to prolonged (5-6 h) and significant primary creep with initial strain rate of the order of 10(-7) s(-1). (C) 2010 Acta Materialia Inc.

This paper reports several new findings on the breakdown of dynamic roughening in thin film growth. With increasing energy flux of concurrent ion impingement during pulsed DC sputtering, a transition from dynamic roughening to dynamic smoothening is observed in the growth behavior of TiC/a-C nanocomposite films. The nanocomposite films show a negative growth exponent and ultra-smoothness (RMS roughness similar to 0.2 nm at a film thickness of 1.5 mu m). Based on high-resolution cross-sectional transmission electron microscopy observations we conclude that during growth an amorphous front layer of 2 nm covers the nanocomposite film and suppresses the influence of nanocrystallites on the roughness evolution of the nanocomposite films. We were able to predict the evolution of surface roughness based on a linear equation of surface growth which contains two diffusivity parameters that control the atomic mobility along the growing outer surface. The model is in good agreement with atomic force microscopy measurements of roughness evolution. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

In this paper, we report some striking results on the microstructural control of TiC/a-C nanocomposite coatings with pulsed direct current (DC) magnetron sputtering. The interface morphology and microstructure evolution as a function of pulse frequency and duty cycle were scrutinized using atomic force microscopy, scanning electron microscopy and high-resolution transmission electron microscopy techniques. It is shown that, with increasing pulse frequency, the nanocomposite coatings exhibit evolutions in morphology of the growing interface from rough to smooth and in the microstructure from strongly columnar to fully columnar-free. In addition, the smoothly growing interface favors the formation of a tailor-made multilayered nanocomposite structure. The fundamental mechanisms are analyzed with the assistance of plasma diagnostic experiments. Ion mass/energy spectrometry measurements reveal that, depending on the frequency and duty cycle of DC pulses, pulsing of the magnetrons can control the flux and energy distribution of Ar+ ions over a very broad range for concurrent impingement on the growing interface of deposited coatings, in comparison with DC sputtering. The significantly enhanced energy flux density is thought to be responsible for the "adatom transfer" in interface smoothening and thus the restraint of columnar growth. (C) 2007 Acta Materialia, Inc. Published by Elsevier Ltd. All rights reserved.

This paper reports a study of the indentation of a model polycrystal using two-dimensional discrete dislocation plasticity. The polycrystal consists of square grains having the same orientation. Grain boundaries are modelled as being impenetrable to dislocations. Every grain has three slip systems.. with a random distribution of initial sources and obstacles, and edge dislocations that glide in a drag-controlled manner. The indenter is wedge shaped, so that the indentation depth is the only geometrical length scale. The microstructural length scale on which we focus attention is the grain size, which is varied from 0.625 to 5 mu m. While the predicted uniaxial yield strength of the polycrystals follows the Hall-Petch relation, this grain size dependence couples to the dependence on indentation depth. Polycrystals with a sufficiently large grain size exhibit the same "smaller is harder" dependence on indentation depth as single crystals, but an inverse indentation depth dependence occurs for fine-grained materials. For sufficiently deep indentation, the predicted nominal hardness is found to scale with grain size d according to H-N = H-N(infinity) (1 + d*/d)(1/2), where H-N(infinity) is the single-crystal nominal hardness and d* is a material length scale. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The mechanical response to nanoinclentation near grain boundaries has been investigated in an Fe-14%Si bicrystal with a general grain boundary and two Mo bicrystals with symmetric tilt boundaries, In particular, the indentations performed on the Fe-14%Si show that as the grain boundary is approached, in addition to the occurrence of a first plateau in the load versus depth nanoindentation curve, which indicates grain interior yielding, a second plateau is observed, which is believed to indicate dislocation transfer across the boundary. It is noted that the hardness at the onset of these yield excursions increases as the distance of the tip to the boundary decreases, providing thus a new type of size effects, which can be obtained through nano indentation. The energy released during an excursion compares well to the calculated interaction energy of the piled-up dislocations. Hall-Petch slope values calculated from the excursions are consistent with macroscopically determined properties, suggesting that the Hall-Petch slope may be used to predict whether slip transmission occurs during indentation. No slip transmission was observed in the Mo bicrystals; however, the staircase yielding commonly found during initial loading was suppressed in the proximity of a grain boundary due to preferential dislocation nucleation at the boundary. An estimate for the nucleation shear stress at the boundary was obtained from the measured interaction range. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.