Preferential etching of ion bombarded GaAs

Abrupt changes in the near-band-edge luminescence of n-type undoped GaAs after implantation with 400 keV Zn ions and vacuum annealing at 580°C are reported. The good agreement of the spectral position, half-width and temperature dependence of the emission obtained after implantation and annealing with that of melt-doped GaAs: Zn indicates that implanted Zn ions have been incorporated at Ga lattice sites. The larger number of Zn substitutions obtained when bombardment is made on the Ga face than for an equivalent fluence on the As face demonstrates the existence of a polar implant effect.     

Direct measurement of precipitate induced strain in an Al-Zn–Mg-Cu alloy with aberration corrected transmission electron microscopy

Precipitates and their associated strain fields significantly influence mechanical properties and, consequently, the industrial performance of aluminum alloys. In this work, we present a direct measurement of strains induced by η′ and η precipitates in an Al-Zn-Mg-Cu alloy using aberration-corrected high-resolution transmission electron microscopy and quantitative strain analysis. The results demonstrate that the strain induced by precipitates in the Al-Zn-Mg-Cu alloy shows significant tensile strains perpendicular to the longitudinal direction of the precipitate discs on the side of the discs and along the longitudinal direction at both ends of the η′ and η precipitates. This strain field can be described by an equivalent dislocation model, in which the lattice mismatch between the precipitate and the matrix is equivalent to a series of dislocation pairs along the precipitate/matrix interfaces.     

Evaluation of femtosecond LIBS for spectrochemical microanalysis of aluminium alloys

The analytical performance of femtosecond laser-induced breakdown spectroscopy (LIBS) for elemental microanalysis of aluminium alloys and for mapping precipitate distribution on the sample surface has been studied in detail. A Ti–sapphire laser system producing pulses of 130 fs at 800 nm was used to generate the laser-induced plasma. Multi-element microanalysis of commercially available aluminium alloys was performed in air at atmospheric pressure. Crater characteristics such as diameter and crater morphology were characterized by optical and scanning-electron microscopy. Scaling of plasma emission and limit of detection as a function of laser pulse energy was also investigated. Current experimental results are presented and are compared with previous nanosecond microLIBS measurements.     

Luminescence studies of Co implanted sapphires

Sapphire single crystals implanted with different fluences of cobalt and subjected to annealing treatments in vacuum and in air during 1 v h at 1000 v C were studied by optical and structural techniques. At room temperature the absorption mainly occurs in the ultraviolet (UV) being the luminescence dominated by a UV/blue broad emission band and a highly structured red emission. Structural analysis indicates that metallic precipitates, CoAl 2 O 4 and CoO phases are present in the implanted and annealed samples depending either on the fluence and on the annealing atmosphere. The observed luminescence cannot be correlated with intra-ionic Co 2+ and Co 3+ in an Al 3+ site in the sapphire lattice and a tentative assignment of the observed emission lines to the presence of aggregates is made.     

Size distribution and volume fraction of T1 phase precipitates from TEM images: Direct measurements and related correction

Transmission Electron Microscopy (TEM) can be used to measure the size distribution and volume fraction of fine scale precipitates in metallic systems. However, such measurements suffer from a number of artefacts that need to be accounted for, related to the finite thickness of the TEM foil and to the projected observation in two dimensions of the microstructure. We present a correction procedure to describe the 3D distribution of disc-like particles and apply this method to the plate-like T₁ precipitates in an Al–Li–Cu alloy in two ageing conditions showing different particle morphologies. The precipitates were imaged in a High-Angular Annular Dark Field Microscope (HAADF-STEM). The corrected size distribution is further used to determine the precipitate volume fraction. Atom probe tomography (APT) is finally utilised as an alternative way to measure the precipitate volume fraction and test the validity of the electron microscopy results.     

Solubility and microstructure in the pseudo-binary PbTe-Ag2Te system

The solvus lines of the PbTe and Ag₂Te phases in the pseudo-binary PbTe-Ag₂Te system have been determined using diffusion couples and unidirectional solidification by the Bridgman method. The solubilities of both Ag₂Te in PbTe and PbTe in Ag₂Te decrease with decrease in temperature. For the former, this change is from 14.9 at% Ag (694 °C) to 0.5 at% Ag (375 °C), while for the latter it is from 12.4 at% Pb (650 °C) to 3.1 at% Pb (375 °C). The decrease in solubilities leads to the formation of precipitates of Ag₂Te in PbTe and PbTe in Ag₂Te. In particular, fast atomic diffusion in Ag₂Te results in the precipitation of PbTe even in quenched samples. From the temperature dependence of these solubilities, heats of solution have been determined. In the diffusion couple, the phase boundary moves toward PbTe. In the region between the phase boundary and the initial interface, PbTe transforms to β-Ag₂Te (cubic) retaining the cube-on-cube orientation relationship.     

A computational method for dislocation-precipitate interaction

A new computational method for the elastic interaction between dislocations and precipitates is developed and applied to the solution of problems involving dislocation cutting and looping around precipitates. Based on the superposition principle, the solution to the dislocation-precipitate interaction problem is obtained as the sum of two solutions: (1) a dislocation problem with image stresses from interfaces between the dislocation and the precipitate, and (2) a correction solution for the elastic problem of a precipitate with an initial strain distribution. The current development is based on a combination of the parametric dislocation dynamics (PDD) and the boundary element method (BEM) with volume integrals.The method allows us to calculate the stress field both inside and outside precipitates of elastic moduli different from the matrix, and that may have initial coherency strain fields. The numerical results of the present method show good convergence and high accuracy when compared to a known analytical solution, and they are also in good agreement with molecular dynamics (MD) simulations. Sheared copper precipitates (2.5 nm in diameter) are shown to lose some of their resistance to dislocation motion after they are cut by leading dislocations in a pileup. Successive cutting of precipitates by the passage of a dislocation pileup reduces the resistance to about half its original value, when the number of dislocations in the pileup exceeds about 10. The transition from the shearable precipitate regime to the Orowan looping regime occurs for precipitate-to-matrix elastic modulus ratios above approximately 3-4, with some dependence on the precipitate size. The effects of precipitate size, spacing, and elastic modulus mismatch with the host matrix on the critical shear stress (CSS) to dislocation motion are presented.     

Superelasticity in a New BioImplant Material: Ni-rich 55NiTi Alloy

With the drive towards minimally invasive procedures, the medical industry is looking towards ‘avant-garde’ materials, with 50NiTi currently being the prime choice for many critical components/applications. This paper examines a new Ni-rich NiTi alloy that exhibits superelasticity (SE) and shape memory (SM) properties. Superelastic (SE) properties of 55NiTi* [all compositions are quoted in atomic% throughout the paper. The reader should note the following conversions: 50NiTi (at.%)≈55NiTi (wt.%) and 55NiTi (at.%)≈60NiTi (wt.%)] are studied here as a function of heat-treatment between 400–800°C, and compared with the corresponding response of 50NiTi*, with an aim to develop and optimize thermal treatment procedures to maximize recoverable elastic strains. While optimal tuning of the SE properties in 50NiTi necessitates cold working in conjunction with specific heat treatment/aging, 55NiTi does not require cold work to achieve its optimal SE behavior. Moreover, it can be heat treated to produce strong, stable SE and SM response from the same ingot, with transformation temperatures being a strong function of heat treatment. The main difference between the two alloys is that Ni–Ti alloys with Ni content greater than 50.6 at.% are sensitive to heat treatment; aging in these materials leads to precipitation of several metastable phases. The initial work focuses on SE properties relevant to biomedical use, such as: plateau stress, recoverable strains and strength, as a function of heat treatment and microstructure. ¹All compositions are quoted in atomic% throughout the paper. The reader should note the following conversions: 50NiTi (at.%)  ≈  55NiTi (wt.%) and 55NiTi (at.%)  ≈  60NiTi (wt.%)     

Evaluation of particle size distributions of precipitates in a 9% chromium steel using energy filtered transmission electron microscopy

Energy filtered transmission electron microscopy (EFTEM) has been used to evaluate particle size distributions of different phases in the creep resistant 9% chromium steel P92, aged at 600 or 650°C for up to 26,000 h. Precipitates of type VN, M₂₃C₆ and Laves phase were visualised by forming so-called jump ratio images of the elements vanadium, chromium and tungsten. Digital image analysis was used to measure the size of the precipitates at different ageing times. As a complement to EFTEM, atom probe field ion microscopy (APFIM) has been used to monitor the composition of the ferritic matrix. This indirectly gives information about precipitated volume fraction of different phases. In the case of VN precipitates the volume fraction was also determined from EFTEM images and the result was in good agreement with the data obtained with APFIM. The high-temperature stability of the secondary phases present in steel P92 and their relation to creep properties are discussed. To further study the applicability of EFTEM, steel P122 (with a composition similar to steel P92, but with an addition of Cu) was investigated and a Cu-rich phase was found.     

Energy loss of protons in thin films of carbon,aluminium and silver

The energy loss of keV protons, in thin films of C, Al and Ag is determined by a method in which the strong energy dependence of the cross section for X-ray excitation by protons, in a material, on which the films are evaporated, is used as an indicator of the proton energy.