Effect of beam current on defect formation by high-temperature implantation of Mg ions into GaN

We evaluated the beam current dependence of defect formation during Mg ion implantation into GaN at a high temperature of 1100℃ with two beam currents. Photoluminescence spectra suggested that low-beam-current ion implantation reduced the vacancy concentration and activated Mg to a greater extent. Moreover, scanning transmission electron microscopy analysis showed that low-beam-current implantation reduced the density of Mg segregation defects with inactive Mg and increased the number of intrinsic dislocation loops, suggesting a decrease in the density of Ga and N vacancies. The formation of these defects depended on beam current, which is an important parameter for defect suppression.

Nucleation centers for martensite with habitus {110} in the shape memory alloys

The dynamic theory of martensitic transformations (MT) considers the formation of habit planes of martensite crystals as a consequence of the propagation of a controlling wave process (CWP). The general ideology makes it possible, by comparing the observed habits with calculations of the elastic fields of defects (as a rule, dislocations), to identify nucleation centers. In a number of cases (In-Tl alloys, Ni50Mn50 alloys, Heusler alloys …) under MT in the shape memory alloys, {110} habits are observed (in the basis of the initial cubic phase), which often have a fine twin structure with twin boundaries of the same type. This highly symmetric structure is described by the CWP containing longitudinal waves (both relatively long-wavelength ℓ and short-wavelength s) propagating along the 4-order symmetry axes. In this paper, it is shown that such habits are associated with rectilinear segments of dislocation loops with directions Λ along <001> and Burgers vectors along <010> (or <110>) orthogonal to Λ, both for sliding and for prismatic loops. The tetragonality, the relative volume change during the MT, and the dependence of the start temperature M s on changes in the concentration of alloy components are also briefly discussed.

Effects of Fe-Ions Irradiation on the Microstructure and Mechanical Properties of FeCrAl-1.5wt.% ZrC Alloys

Fe-13Cr-3.5Al-2.0Mo-1.5wt.% ZrC alloy was irradiated by 400 keV Fe+ at 400 °C at different doses ranging from 6.35 × 1014 to 1.27 × 1016 ions/cm2 with a corresponding damage of 1.0–20.0 dpa, respectively, to investigate the effects of different radiation doses on the hardness and microstructure of the reinforced FeCrAl alloys in detail by nanoindentation, transmission electron microscopy (TEM), and atom probe tomography (APT). The results show that the hardness at 1.0 dpa increases from 5.68 to 6.81 GPa, which is 19.9% higher than a non-irradiated specimen. With an increase in dose from 1.0 to 20.0 dpa, the hardness increases from 6.81 to 8.01 GPa, which is an increase of only 17.6%, indicating that the hardness has reached saturation. TEM and APT results show that high-density nano-precipitates and low-density dislocation loops forme in the 1.0 dpa region, compared to the non-irradiated region. Compared with 1.0 dpa region, the density and size of nano-precipitates in the 20.0 dpa region have no significant change, while the density of dislocation loops increases. Irradiation results in a decrease of molybdenum and carbon in the strengthening precipitates (Zr, Mo) (C, N), and the proportionate decrease of molybdenum and carbon is more obvious with the increase in damage.

Dislocation Loop Generation Differences between Thin Films and Bulk in EFDA Pure Iron under Self-Ion Irradiation at 20 MeV

In the present investigation, high-energy self-ion irradiation experiments (20 MeV Fe+4) were performed on two types of pure Fe samples to evaluate the formation of dislocation loops as a function of material volume. The choice of model material, namely EFDA pure Fe, was made to emulate experiments simulated with computational models that study defect evolution. The experimental conditions were an ion fluence of 4.25 and 8.5 × 1015 ions/cm2 and an irradiation temperature of 350 and 450 °C, respectively. First, the ions pass through the samples, which are thin films of less than 100 nm. With this procedure, the formation of the accumulated damage zone, which is the peak where the ions stop, and the injection of interstitials are prevented. As a result, the effect of two free surfaces on defect formation can be studied. In the second type of experiments, the same irradiations were performed on bulk samples to compare the creation of defects in the first 100 nm depth with the microstructure found in the whole thickness of the thin films. Apparent differences were found between the thin foil irradiation and the first 100 nm in bulk specimens in terms of dislocation loops, even with a similar primary knock-on atom (PKA) spectrum. In thin films, the most loops identified in all four experimental conditions were b ±a0<100>{200} type with sizes of hundreds of nm depending on the experimental conditions, similarly to bulk samples where practically no defects were detected. These important results would help validate computational simulations about the evolution of defects in alpha iron thin films irradiated with energetic ions at large doses, which would predict the dislocation nucleation and growth.

Trevor Evans. 26 April 1927 — 10 October 2010

Trevor Evans was responsible for revealing the main physical processes which take place in natural diamond both in the upper mantle of the earth, where it is stabilized by high pressure and temperature, and as it is ejected by volcanic action to the surface. By measuring the activation energies required for graphitization, he clarified the reason for its very long life as a metastable crystal, valuable both as a gemstone and as an industrial abrasive. He learned how to make diamond specimens for examination in the transmission electron microscope, which enabled his discovery of dislocation loops and platelet precipitates in nitrogen-containing (type 1) stones. In a series of exacting laboratory experiments under geologically relevant conditions he pioneered the study of the emergence of nitrogen from solution to precipitation during the ejection process. In synthetic diamonds, using high-energy electron irradiation, he was able to reproduce the sequence of all the various types of nitrogen aggregation found in natural diamond. His work played a major role underpinning the characterization of gemstones, explaining many features of their colour. For many years he led diamond research in the UK, supported by De Beers. His work stimulated and has been confirmed by research in many other laboratories around the world.

Electron Microscopy of Bubbles and Dislocations in Helium Implanted Metals

<p>The theoretical contrast in transmission electron microscope of a superlattice of helium gas bubbles in copper is computed using the two-beam and many-beam dynamical theories of electron diffraction with the aim the aim of measuring the density and size of dislocation loops associated with the bubble array. A wide range of parameters (foil thickness, diffraction vector, excitation error, defocus, and depth, radius, and strain-field of the bubble) is considered to considered to construct a library of theoretical images and intensity profiles for a single, isolated bubble. Various criteria are applied to obtain a measurement of the bubble radius from the simulations but the results are inaccurate because of the sensitive dependence of the intensity profile on the imaging parameters. A better measurement is profiles from a single stack of bubbles are modeled and electron diffraction from superlattices simulated. The results obtained suggest that the bubble ordering is of limited extent. A library is made of the theoretical contrast when imaging a system of dislocation loops punched out along the <110> directions by the growth of gas bubbles ordered on a superlattice aligned with the host fcc matrix. These image simulations use the displacement fields surrounding loops and bubbles predicted by isotropic elasticity theory. For a variety of structures involving loops and bubbles, the following imaging parameters were investigated: beam direction, foil normal, diffracting vector, excitation error, number of beams, and defocus, These simulations indicate that it should be possible to image the small dislocations at high density thought to be present in the bubble lattice, provided well focused micrographs taken under strong two-beam conditions can be obtained. In Practice it proved difficult to tilt specimens containing superlattices to strong two-beam conditions because of the deterioration in crystallinity resulting from the implantation. However, the lower concentrations by low dose implantations.</p>

Electron Microscopy of Bubbles and Dislocations in Helium Implanted Metals

<p>The theoretical contrast in transmission electron microscope of a superlattice of helium gas bubbles in copper is computed using the two-beam and many-beam dynamical theories of electron diffraction with the aim the aim of measuring the density and size of dislocation loops associated with the bubble array. A wide range of parameters (foil thickness, diffraction vector, excitation error, defocus, and depth, radius, and strain-field of the bubble) is considered to considered to construct a library of theoretical images and intensity profiles for a single, isolated bubble. Various criteria are applied to obtain a measurement of the bubble radius from the simulations but the results are inaccurate because of the sensitive dependence of the intensity profile on the imaging parameters. A better measurement is profiles from a single stack of bubbles are modeled and electron diffraction from superlattices simulated. The results obtained suggest that the bubble ordering is of limited extent. A library is made of the theoretical contrast when imaging a system of dislocation loops punched out along the <110> directions by the growth of gas bubbles ordered on a superlattice aligned with the host fcc matrix. These image simulations use the displacement fields surrounding loops and bubbles predicted by isotropic elasticity theory. For a variety of structures involving loops and bubbles, the following imaging parameters were investigated: beam direction, foil normal, diffracting vector, excitation error, number of beams, and defocus, These simulations indicate that it should be possible to image the small dislocations at high density thought to be present in the bubble lattice, provided well focused micrographs taken under strong two-beam conditions can be obtained. In Practice it proved difficult to tilt specimens containing superlattices to strong two-beam conditions because of the deterioration in crystallinity resulting from the implantation. However, the lower concentrations by low dose implantations.</p>