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AlN epilayers were grown directly on sapphire (0001) substrates using a combined growth scheme, a low temperature nucleation layer and a high temperature pulsed-atomic-layer-epitaxy layer obtained via metal organic chemical vapor deposition. The effects of growth temperature on properties of AlN films were investigated by atomic force microscopy, high resolution X-Ray diffraction and transmittance spectral. Due to the strong influence of growth temperature on the mobility of Al adatoms and parasitic reaction, a different surface morphology and growth rate for AlN films were obtained by varying the growth temperature.

GaN epilayers were grown on the AlN buffer layer obtained by pulsed atomic-layer epitaxy (PALE). The influence of PALE-AlN buffer thickness on the quality of GaN epilayers were investigated by atomic force microscopy, high resolution X-Ray diffraction, and photoluminescence (PL) spectrum. The strain states of the GaN epilayers were studied by Raman spectrum. It was found that the thickness of PALE-AlN buffer layer is a key parameter that affects the quality of GaN epilayer, and a proper growth period of PALE-AlN buffer layer leads to excellent surface morphology, crystal quality and optical properties of the GaN epilayer.

GaN and GaN/Al0.25Ga0.75N multiple quantum wells (MQWs) over c- and r-plane sapphire substrates have been grown by metal-organic chemical vapor deposition. A comparative study of photoluminescence (PL) in GaN epitaxial layers and AlGaN/GaN MQWs on these two types of substrates is reported. At low excitation levels, the measured room temperature PL signal in GaN layers grown over r-plane sapphire was more than order of magnitude lower than in GaN on c-plane substrates. In contrast, the emission intensity from AlGaN/GaN MQWs grown over r-plane substrates was almost 30 times stronger than in the structures grown over c-plane sapphire. Furthermore, with excitation power density up to 1 MW/cm², the PL peak position for the non-polar MQWs kept completely stable whereas the one for the c-plane structures exhibited a blue shift as large as 250 meV. We attribute this large difference in the ultraviolet emission intensity to the suppression of a strong quantum Stark effect in the AlGaN/GaN MQWs on the r-plane sapphire. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The deformation and fracture behaviours in fully lamellar TiAl alloys were investigated by unixial compression and an in situTEM technique. The results indicated that the deformation and fracture behaviours in fully lamellar TiAl alloys were closely related to the relative orientation of the lamellae to the loading axis. When the lamellae were parallel to the loading axis, the slip in the a2phase occurred along its prism plane, and microcracks nucleated preferentially along its prism plane. When the lamellae were perpendicular to the loading axis, slip became difficult, slip in the a2phase occurred along its pyramidal plane, microcracks nucleated and propagated preferentially along its lamellar interface.

Static bifurcation of electromagnetic wave transmission differential equation for multilayered structural composite materials has been studied to explain the transformation effect of absorption and reflection. Experimental result is given to illustrate the absorption and reflection transformation caused by variation of inherent impedance of the composite.

A polycrystalline Ti-52at.% Al alloy has been deformed in compression at -196°C and its microstructure has been examined by transmission electron microscopy. The <101] superdislocations were frequently observed to decompose into two inequivalent units with smaller orthogonal Burgers vectors of 1/2 <112] and - <110] respectively. Observation of local decomposition of <101] superdislocation dipoles and dislocation triple nodes formed by the three dislocations are presented. Arguments based on the observation have ruled out the possibilities that several alternative dislocation reactions are responsible for the observation. Possible factors whichMayinfluence the decomposition are discussed. It is concluded that the decomposition could occur in Ti-Al deformed over a wide temperature range if the <101] superdislocation lies in a favourable orientation.

] Some new geometrical features of 1/6<112] faulted dipolar loops have been observed in a Ti-52 at.% Al alloy deformed at 77 K. It was observed that the 1/6<112] dipole could switch its alignment from <101] to <011] directions, and vice versa; also the loops in alignment along both <101] and <011] could be created by the same parent superdislocation along its line and, on further movement of the parent dislocation, the two loops could attach to one another at the meeting point. No extra dislocation debris or segments were created at either the turning point or the attachment site. These particular features have been discussed on the basis of the available models for the formation mechanism of the dipoles. It is predicted that the particular features can be created only under a favourable stress and no such features are expected for 1/3<211]faulted dipolar loops.

Tensile tests of notched and unnotched Al–6Zn–2·3Mg–1·7Cu alloy bars have been carried out. The underaged and peak-aged unnotched specimens exhibit macroscopic shear localization during testing, followed by shear failure, whereas the overaged unnotched specimens exhibit the conventional ‘cup–cone’ mode of failure. The introduction of a notch changes the deformation pattern and the fracture mode of the specimens. Deformation is constrained within the notched region, and the triaxial tensile stress developed at the notch results in grain boundary splitting, which complicates the deformation pattern and failure mode. The fracture behaviour of these specimens is explained by an adaptation of crystal slip theory.MST/197

The fast fracture of high-strength aluminium alloys appears to be closely involved with the fracture or cavitation of dispersoid particles, in the size range 0·1–0·5 μm, which are present in high volume fraction and which contribute to grain refinement. In the present study, two high-strength A1–Zn–Mg–Cu (7000 series) alloys have been examined: one treated with Zr (7010) and one treated with Cr (7475). It was found that the crack paths in these alloys were of a ‘zig-zag’ nature due to a ‘fast shear’ fracture process, in which the critical step appears to be the decohesion of dispersoids within narrow shear bands. A model has been used to relate the toughness of the alloys to the basic tensile properties of yield stress and strain-hardening exponent, and to dispersoid parameters such as size, shape, distribution and dispersoid/matrix interfacial strength. The model has also been employed to try to explain the difference between the effects of dispersoids on toughness in the Zr-treated alloys and the Cr-treated alloys.

In this investigation attention was focused on the fracture behaviour of some high strength aluminium alloys in the as-cast and heat treated condition. Fracture is initiated in constituent particles located at grain or dendrite boundaries, and propagates in an intergranular manner. Crack initiation occurs independently of temper condition at a critical stress of about 1000 MN m−2. Fracture toughness varies between 17 and 25 MN m−3/2depending upon aging condition. The measured toughness can be explained in terms of the critical decohesion stress, the yield stress, and the volume (or distance) over which the critical decohesion stress is exceeded.

The short transverse fracture toughness of an Al–Li–Cu–Mg–Zr extrudate was determined as a function of aging condition and testing temperature. To elucidate the underlying micromechanisms, the short transverse fracture surfaces of the extrudate were characterised via scanning electron microscopy, grain boundary precipitates and precipitation free zones were identified via transmission electron microscopy, and segregation of elements to grain boundaries was analysed using secondary ion mass spectrometry. Three principal observations were made as follows. First, with increasing aging time, the short transverse toughness of the extrudate increased when tested at room temperature, but decreased at liquid N2temperature, whereas with decreasing testing temperature, it remained essentially constant for the underaged condition, and decreased sharply for the peak aged and overaged tempers. Second, in addition to regions exhibiting shallow dimples, smooth ‘featureless’ zones were revealed on the short transverse fracture surfaces, which are intergranular in nature for all the specimens tested. The area fraction of the featureless regions decreased noticeably with increasing aging time when tested at room temperature, and increased markedly with decreasing testing temperature for the peak aged and overaged conditions. Third, segregation of Li, Si, Na, and H was detected for both the underaged and overaged specimens, and also of K for the underaged specimens only. In general, the enhancement of the room temperature short transverse toughness with aging and the negative effect of cryogenic temperature on fracture toughness are in obvious contrast to the in plane toughness behaviour reported in the literature, the featureless character of the short transverse fracture and its connection with poor toughness seldom having been emphasised. Based upon the present study, segregation induced brittleness is proposed as the critical micromechanism responsible for grain boundary weakness, and thus for the poor short transverse fracture toughness.MST/1829

The fracture behaviour and JIcfracture toughness of a commercial Al–Zn–Mg–Cu alloy plate has been investigated. Based on the experimental results and available theoretical analyses, the following results were obtained. Secondary grain boundary cracks appeared ahead of the main crack, which served as a triggering mechanism for small scale shearing. Shear failure facets on the fracture surface of single edge notched bend specimens represent the same type of fracture as the fast shear failure that occurred during tensile tests on notched specimens. The grain boundary cracking–small scale shearing mechanism is essentially a type of shielding event that not only makes the fracture appearance obviously anisotropic, but also, to a considerable extent, accounts for the strong anisotropy of fracture toughness.MST/1111

ARALL (Aramid Aluminium Laminate) is a super-hybrid composite, and there are many factors influencing the performances of the laminate. In this paper, fuzzy theory is applied to deal with this composite. Based on experimental results, the factors which influence the properties of ARALL, including the residual stresses, adhesive properties and adhesive content, are analysed by fuzzy multi-criteria. The results show that the state of the residual stress is the key factor which influences the performance of ARALL. The adhesive properties followed by the adhesive content in the composite also play a role in determining the properties of the composite.

Vector quantisation (VQ) is a well known data compression technique which maps an ordered set of real numbers into a single integer. However, it is difficult to achieve accurate compression with the use of unconstrained Voronoi VQ when the codebook level and vector dimensionality are very large, due to the extremely high real-time computational complexity involved in full codebook search. To overcome this difficulty, a classified pre-selection method is proposed. Compared to the conventional full search method, the algorithm reduces the computational complexity involved in the code vector selection procedure by 70% ~ 90% with almost no loss in coder performance, at the cost of only a slight increase in the storage requirement.

A novel codebook generation scheme for vector quantisation is presented. The proposed scheme is of comparable computational complexity to the Linde-Buzo-Gray (LBG) algorithm, but its performance is shown to be superior.

Electromagnetic wave (EM wave) functional composite materials containing metal–coated carbon fibers (MCFs) and lossy dielectrics have been investigated. Electromagnetic wave decay model of the composites was set up by transmission line simulation and field analysis. Transformation of absorption to reflection of electromagnetic wave of the composite materials are also concerned.