Nucleation of misfit dislocations in strained-layer InGaAs on GaAs

Abstract

Austenitic 316L stainless steel alloys annealed at 550°C for 100 h present a few ferrite precipitates surrounded by a new interfacial phase, here called the I-phase, that develops at the level of the austenite/ferrite interface. The I-phase presents the typical patterns of an icosahedral quasicrystal with a primitive hypercubic lattice of parameter A = 0.63 nm. The marked orientation relationships between the I-phase and the ferrite precipitates strongly suggest that this phase results from a decomposition of the ferrite and not of the austenite. The I-phase is metastable and transforms eventually after annealing at 700°C to the stable crystalline σ-phase.     

Recrystallization of amorphous ion-implanted silicon carbide after thermal annealing

Single crystals of 6H–SiC were implanted at room temperature with 4-MeV Au ions to a fluence of 10^15?cm^?2. Raman spectra showed that full amorphization was achieved. The recrystallization process was studied by micro-Raman spectrometry after isochronal thermal annealing between 700 and 1500°C. The spectra permitted the evolution upon annealing of Si–C bonds, and also of Si–Si and C–C bonds, to be followed. Amorphous phase relaxation takes place below 700°C; then recrystallization of the 6H polytype sets in at 700°C. At 900°C crystallites with different crystalline states are formed. Moreover, Raman spectra provide evidence of graphitic nanocluster formation at 1500°C.     

Defect analysis of Fe3(Al1– x Si x ) alloys by positron-lifetime spectroscopy

The recovery behaviour of vacancy-type defects created at 1000°C in Fe₃(Al_{1– x} Si _x ) alloys has been investigated by positron-lifetime measurements in samples cooled to room temperature by water quenching, air quenching and furnace cooling. In addition, a set of samples was aged for one week at 520°C. After water quenching, measurements reveal a high concentration of thermal vacancies, and from the decrease of the average positron lifetime it is inferred that vacancy recovery is strongly composition-dependent. Greater decreases of the positron lifetime were found for Si-rich samples, suggesting diffusion enhancement as the Si concentration increases. For the Fe₃Al alloy, the retained vacancy concentration depends strongly on the cooling rate. This is related to atomic ordering and a rebuilding of the D0₃ structure.     

Severe-to-mild wear transition at higher temperatures in Al-17Si-5Cu alloy after short-duration isothermal heat treatment

ABSTRACT

The wear behaviour of hypereutectic Al–Si alloys, which are suitable for use in automobile engines as a replacement for cast iron, has not been explored at higher temperatures. In the present study, the wear behaviour of Al-17Si-5Cu alloy (AR alloy) has been studied in as-received condition as well as after applying a short-duration isothermal heat treatment developed by the present research team. It was found that AR alloy exhibits severe wear, whereas heat-treated alloy (HT alloy) shows mild wear at higher temperatures (up to 300°C) at all applied loads. This severe-to-mild wear transition is addressed here.     

Thermal-equilibrium processes and electronic transport in undoped hydrogenated amorphous silicon

Abstract

The temperature and time dependence of the d.c. conductivity of undoped hydrogenated amorphous silicon is presented. Measurements of the electronic transport are reported, with particular emphasis on the effects of annealing and cooling the samples. Two regimes of behaviour are observed. When samples are rapidly cooled from 200°C below a temperature T _E～145°C a non-equilibrium dark conductivity, higher than that corresponding to slow cooling, is observed. The electronic and atomic structure then slowly relax and the time dependence of the excess conductivity is well described by a stretched exponential function. The second regime above T _E corresponds to a relaxation time short compared to experimental times and the conductivity is independent of which order the annealing temperature is chosen. Thus the thermal equilibrium processes observed in undoped samples are qualitatively very similar to those observed in doped samples as recently reported in the literature.     

Effects of annealing temperature on structural,optical, and electrical properties of antimony-doped tin oxide thin films

Antimony-doped tin oxide (ATO) films, approximately 320 nm in thickness, have been prepared by electron beam evaporation onto glass substrates. The films were annealed at temperatures between 400°C and 550°C in air and their structure and surface morphologies were observed by X-ray diffraction (XRD) and atomic force microscopy (AFM) after the different annealing treatments. XRD patterns of the ATO thin films as-deposited and annealed at 400°C showed that they were amorphous, but annealing beyond 400°C caused the films to become polycrystalline with tetragonal structure and orientated in the (1 1 0) direction. The grain size in the annealed films, obtained from the XRD analysis, was in the range 146–256 Å and this increased with the annealing temperature. The dislocation density, cell volume and strain were found to decrease gradually with increasing annealing temperature. Photoluminescence spectra revealed an intensive blue/violet peak at 420 nm, which increased gradually in height with annealing. It is suggested that an increase in the population of Sb⁺⁵ ions might be the reason for the enhancement of the blue/violet emission. The optical properties of the films were also investigated in the UV-visible-NIR region (300–1000 nm). The optical constants, namely the refractive index n and the extinction coefficient k in the visible region were calculated. The optical energy band gap, as determined by the dependence of the absorption coefficient on the photon energy at short wavelengths, was found to increase from 3.59 to 3.76 eV with annealing temperature.     

Phase-transformation-induced hardening in Zn–22Al alloys

A phase-transformation-induced hardening effect is reported in Zn–22Al (Al: 22?wt.%) alloys. The Zn–22Al specimens were held at 300?°C for 10?h and then quenched in water. A hardening effect took place in subsequent artificial aging at 100–200?°C, which was accompanied by a phase decomposition of a soft α ₂ phase and a grain coarsening. The phase-transformation-induced hardening affects the hardness more than the grain-coarsening-induced softening, which leads to the age-hardening phenomenon.     

Kinetic study of static recrystallization in an Fe–Al–O ultra-fine-grained nanocomposite

A nearly abrupt coarsening of grains is observed in a newly developed Fe–Al–O ultra-fine-grained nanocomposite with a significant volume fraction (4%) of alumina nano-precipitates. The microstructure of the alloy was analysed in different states (as-received and annealed) by means of scanning electron microscopy, transmission electron microscopy (TEM) and hardness. The initial grain size 150–200 nm increases up to 50 μm during annealing 1000 °C/8 h and thereafter demonstrates saturation. A linear correlation between volume fraction of coarse grains and hardness was found. It was identified by TEM that alumina nano-precipitates stabilize the dislocation microstructure against recovery very effectively and the grain coarsening is due to fast growth of very few dislocation free grains. Thus, the observed grain coarsening has the attributes of static recrystallization.     

Nucleation of the C40-to-C11 b transformation in magnetron-sputtered MoSi 2 thin films

Transmission electron microscopy has been used to reveal the microstructure of metastable C40 MoSi 2 thin films produced by annealing amorphous magnetron-sputtered deposits at 700°C. The films contain nanoscale acicular grains elongated parallel to (0001), with extensive basal faulting. The faults are intrinsic with R ´ 1/3[0001] and correspond to thin slabs of the equilibrium C11 b phase. It is proposed that these faults may act as nuclei for the subsequent transformation from C40 to C11 b by a process akin to discontinuous coarsening.     

Dodecagonal quasicrystal in Mn-based quaternary alloys containing Cr,Ni and Si

A dodecagonal quasicrystal showing 12-fold symmetry forms in Mn-rich quaternary alloys containing 5.5 or 7.5 at.% Cr, 5.0 at.% Ni and 17.5 at.% Si. After annealing at 700 °C for 130 h, the quasicrystal precipitated in a matrix of β-Mn-type crystalline phase. The shape of the quasicrystal is needle-like having a length of several tens of micrometres. Electron diffraction as well as powder X-ray diffraction experiments has revealed the following characteristics of the quasicrystal: diffraction symmetry 12/mmm, the presence of systematic extinction for h₁h₂h₂h₁h₅-type reflections with odd h₅ index, and then five-dimensional space group P12₆/mmc. Indexing of the reflections indicated that the dimension of the common edge in the equilateral triangle–square tiling is 4.560 Å, and the periodicity is 4.626 Å along the 12-fold axis. This is the first example of the dodecagonal quasicrystal synthesized by ordinary metallurgical method in 3-d transition-metal alloys.     

Cascade effects on the irradiation stability of amorphous SiOC

We studied the heavy ion radiation tolerance of amorphous silicon oxycarbide (SiOC) alloys by in situ Kr ion irradiation within a transmission electron microscopy. The amorphous SiOC thin films were grown via co-sputtering from SiO₂ and SiC targets on a surface-oxidized Si (100) substrate. These films were irradiated by 1 MeV Kr ions at both room temperature and 300 °C with damage levels up to 5 displacements per atom (dpa). TEM characterization shows no sign of crystallization, void formation or segregation in all irradiated samples. Our findings suggest that SiOC alloys are a class of promising radiation tolerant materials.     

Evidence of carbon ordering and morphology change in a cubic carbide phase

Abstract

The crystallographic and microstructural changes of the ordered cubic carbide (K) phase on ageing have been studied in rapidly solidified Fe-30.9 wt% Mn-8.9 wt% AI-2.8 wt%C ((Fe_0.65, Mn_0.35)_0.83 Al_0.17–12 at.%C) alloy. In the as-rapidly solidified condition, an austenitic phase coexists with the carbide phase which is composed of antiphase domains in the interior of the solidification cell. The antiphase domain boundaries were aligned nearly parallel to the {100} planes. By visually comparing the intensity ratio (I₁₁₀/I₁₀₀) of the selected-area diffraction pattern in the transmission electron microscope and by quantitatively comparing the same X-ray diffraction intensity ratio, we have deduced that the K phase tends to consolidate into the L'1₂ structure on ageing at 450°C but into the L1₂ structure on ageing at 700°C.     

Secondary ageing in Al-Cu-Mg

Secondary ageing, that is microstructural evolution occurring at room temperature after short heating at temperatures above the metastable phase boundary of Guinier-Preston zones, has been studied for an Al-Cu-Mg alloy with a high Cu-to-Mg ratio. Combined data from positron annihilation spectroscopy, Vickers microhardness measurements and differential scanning calorimetry show that, on secondary ageing after 5 or 7min at 190°C, firstly, hardening takes place at a rate nine to 16 times slower than natural age hardening; secondly, vacancies slowly released by Cu-rich aggregates formed during the heat treatment at 190°C promote further formation of solute aggregates, with a time-dependent chemical composition; thirdly, the thermal stability of the structures formed during secondary ageing increases with increasing dwell time at room temperature; and, fourthly, solute aggregates formed at 190°C undergo a structural reorganization and possibly a change in the composition, leading to species with a different thermal stability. The slow release of vacancies from Cu-rich aggregates is proposed as one of the limiting factors of the hardening rate.     

Impact of coherent and incoherent twin boundaries on the microhardness of annealed nanocrystalline Ni–W alloys

The microstructural evolution of nanocrystalline Ni–W alloys with annealing temperature and more specifically grain boundary (GB) character is investigated through several techniques and correlated with the hardening behaviour. It is shown that two distinct regions can be identified in relation to the annealing temperature and the microstructural evolution. At temperatures below 550 °C (Regime I), a small increase in grain size is observed and is accompanied by a significant hardening and an increase in the fraction of Σ3 incoherent twin boundaries. At temperatures above 550 °C (Regime II), the thermal stability is overcome and important grain growth occurs with a decrease in both the volumic fraction of GBs and the microhardness. It is suggested that the microhardness evolution during heat treatment is influenced by two opposing processes: an increase in the fraction of incoherent twin boundaries (hardening effect) and grain growth (softening effect). Both aspects are directly associated with the mean free path of mobile dislocations.     

Experimental and simulated grain boundary groove profiles in tungsten

Grain-boundary grooving has been studied on polished surfaces of polycrystalline tungsten annealed at 1350°C. Atomic force microscopy images were taken in the same area for each groove after different annealing times. Secondary oscillations next to the main groove maxima (predicted for grooving by surface diffusion) were observed, to our knowledge for the first time. The agreement between experimental and calculated groove profiles (using the surface diffusion model of Mullins (1957, J. appl. Phys., 28, 333)) improved when grain-boundary fluxes were introduced.     

Accelerating heterogeneous nucleation to increase hardness and electrical conductivity by deformation prior to ageing for Cu-4 at.% Ti alloy

ABSTRACT

Deformation bands, especially shear bands were intentionally formed in supersaturated Cu-4 at.% Ti alloy by groove-rolling with 75% area reduction. Cold deformation prior to the ageing process changed the precipitation behaviour of the alloy. The deformed structure with shear bands was maintained, without showing recovery and recrystallisation, even after prolonged ageing at 450°C for more than one day. The thermally stable shear bands act as nucleation initiation sites and prevent the expansion of discontinuous cellular precipitates. The hardness reached a value of 305?Hv and an electrical conductivity of 15% IACS (IACS?=?International Annealed Copper Standard. 100% IACS is defined as the conductivity corresponding to a volume resistivity at 20°C of 17.241?nΩ), compared to 283?Hv and 13% IACS for conventional solid-solutioned and peak-hardened alloy. In addition, the ageing time to reach the highest hardness was shortened from 720 to 180?min at 450°C.     

On the internal structure of Guinier-Preston zones in Al-3 at.% Ag

High-angle annular dark-field scanning transmission electron microscopy has been employed to study partially decomposed Al-3 at.% Ag. After solutionizing at 550°C and quenching to room temperature, samples of an Al-3 at.% Ag single crystal were aged for short times at 105, 140 and 180°C. Independent of the ageing temperature for these early stages, Guinier-Preston zones with a diameter of about 3 nm are found. Most of these zones consist of a silver-depleted core surrounded by a silver-rich shell. The shell structure is not uniformly pronounced. Irregularly shaped Guinier-Preston zones are common, but the shell structure dominates. No indication of different structures of the Guinier-Preston zones in Al-3 at.% Ag was found after short ageing at the three different temperatures. The shell model for Guinier-Preston zones in aluminium-rich Al-Ag alloys, which was previously proposed on the basis of X-ray and neutron scattering experiments, is thus appropriate to describe the structure of the Guinier-Preston zones in the early stages of the decomposition.     

An explanation for the low-temperature H evolution peak in hydrogenated amorphous silicon films deposited 'on the edge of crystallinity'

The puzzling existence of a sharp low-temperature (T = 400°C) H evolution peak in compact hydrogenated amorphous silicon (a-Si : H) films deposited 'on the edge of crystallinity' is examined. From infrared absorption and X-ray diffraction (XRD) measurements, we show that none of the standard methods used to explain the existence of this peak in a-Si : H materials is applicable to the present films. From the Si-H wag-mode peak frequency, we postulate the existence of very small Si crystallites contained within the amorphous matrix. While the crystallite volume fraction is too small to be detected by XRD in the as-grown films, crystallization is observed for this material at anneal temperatures as low as 500°C. It is proposed that these crystallites catalyse the crystallization of the remainder of the amorphous matrix upon moderate annealing, enabling H surface desorption and H₂ out-diffusion to the sample surface along newly formed grain boundaries at low anneal temperatures.     

Light emission from ferroelectric barium titanate nanocrystals

Thin laminates (thickness δ = 20–45 nm) of barium titanate, BaTiO₃ (BT), have been synthesised by the sol–gel method followed by heating of the amorphous precursor powder in air. An orthorhombic BT polymorph forms along with a tetragonal phase (t-BT) after 2 h of heating the precursor at 600°C, as evidenced by a well-defined X-ray diffraction pattern. The average crystallite size D ～ 21 nm compares the δ-value obtained from the scanning electron micrograph. On heating at temperatures as high as 750°C, the structure remains mostly t-BT (D ? 44 nm), which emits violet–blue–green light at a wavelength of 380–580 nm with a maximum at around 422 nm. The emission extends to wavelengths shorter than ～300 nm owing to a quantum-size effect in smaller crystals processed at lower temperatures. Electron paramagnetic resonance (EPR), which occurs at a g-value of 1.995 (linewidth ΔH = 1.12 mT) in a sample heated at 400°C, shows an order of magnitude of lower intensity at g = 1.993 (ΔH = 3.69 mT) on annealing out the paramagnetic defects at 600°C in air. No EPR signal arises in t-BT free from such defects in larger crystals.     

Retention and thermal desorption of helium in pure tungsten

Tungsten (W) is cited as a candidate first-wall material in fusion reactors owing to its outstanding thermal properties and erosion resistance. An important issue is that the energetic isotopes of hydrogen, tritium (T) and deuterium (D) and helium (He) particles damage the surface of W in fusion reactors. He particles cause more notable damage than D or T because the binding energy of defects and He is larger than that of defects and D or T. In this study, well-annealed W specimens were implanted with 5 keV He ions at room temperature and irradiation dosages of 1.0 × 10²⁰ and 2.5 × 10²¹ ions/m². Then, thermal desorption spectroscopy analysis was performed by heating the samples to 1973 K at a ramping rate of 0.5 K/s. Thermal desorption of He in the sample irradiated with a low dosage occurred at 1400 and 1960 K, whereas that in the sample irradiated with a high dosage occurred at 740, 1050 and 1500 K. According to the microstructures observed using transmission electron microscopy, both peaks in the former case were attributed to He de-trapping from irradiation to nduced helium-vacancy clusters of different sizes, whereas the peaks in the latter case were attributed to He de-trapping from surface defects, irradiation-induced dislocation loops and tiny helium-vacancy clusters, respectively.