Band 103, Heft 3

It has recently been observed that micro- and nano-sized pillars under uniaxial compression and tension attain a higher flow-stress than samples of macroscopic size. Here it is hypothesized that such a mechanical size-effect is driven by the interaction of dislocations with the free surfaces. An extended continuum plasticity theory is presented to account for such an effect by considering the free-surface imagestresses on the dislocation motion immediately following dislocation nucleation. The model employs only fundamental material parameters, Young's modulus, Poisson's ratio, bulk yield-strength, and the Burgers vector. The model predicts the magnitude of the size-effects observed in the compression of Au and Cu micro- and nano-pillars.

A procedure for the modelling of microstructural changes induced by non-uniform large plastic deformation of metals is developed. For a given plastic working process, a set of deformation paths for different initial locations of a material element is generated first for a non-hardening material by using the finite element method. Next, changes of a cellular microstructure and related hardening effects along each path are calculated by using a recently proposed model. The procedure is applied to a non-conventional process of cold extrusion assisted by cyclic rotation of the die. The evolution of microstructural parameters, their effect on strain hardening and a distribution map over the specimen cross-section are calculated.

Phase transformations of freestanding nanoparticles are well analyzed. The situation is different for particles embedded in a second matrix. The special case of melting and crystallizing of nanoparticles in a rigid matrix is the topic of this study based on the energy balance, resulting in a size dependency of these phase transformations primarily on variation of the materials properties with the particle size. Therefore, an important outcome of this study is the result that thermodynamic data of bulk materials are insufficient to describe phenomena connected to nanoparticles. The differences may be significant.

Internal oxidation experiments using two Ag–Sn–Bi alloys with Bi contents of 2.6 wt.% and 5.0 wt.% were carried out. The presence and spatial distribution of Bi 2 O 3 , SnO 2 and Bi 2 Sn 2 O 7 in the oxidized microstructure was characterized using a field emission gun secondary electron microscope, energy dispersive X-ray analysis and X-ray diffraction techniques. It is shown that the addition of Bi prevents both external oxidation and internal oxide bands in Ag–Sn alloys with Sn content above 4 wt.%. Conditions are defined under which these alloys develop instabilities at the internal oxidation front. A result of the unstable growth of the reaction front is a dendritic morphology consisting of different oxide phases. Coupled growth of Bi2Sn2O7 and SnO2 inside the dendrites determines the local arrangement and distribution of the phases.

The Sb–Te–I system was investigated by using differential thermal analysis, X-ray diffraction analysis and electromotive force measurements with an antimony electrode. A number of polythermal sections and the projection of the liquidus surface were constructed. The fields of the primary crystallization, as well as the types and coordinates of nonand monovariant equilibriums were determined. It is shown that the quasi-binary sections SbI 3 –Sb 2 Te 3 , SbI 3 –Te, and SbI 3 –TeI 4 triangulate the Sb–Te–I system, leading to four independent subsystems. From the electromotive force measurements, the partial molar functions of antimony as well as the standard integral thermodynamic functions of SbTeI were calculated. The latter were found to have the following values: ΔG 0 f,298 = −55.77 ± 1.7 kJ · mol −1 ; ΔH 0 f,298 = −55.72 ± 1.39 kJ · mol −1 ; S 0 298 = 153.5 ± 2.8 J · K −1 · mol −1 .

The co-sintering of model bimaterial {steel/cemented carbide} is studied. The cemented carbide consists of WC grain cemented by an Fe-rich binder. The steel is prepared from a powder mixture (Fe + WC + C g ). The experimental approach involves two steps: (1) the study of the single materials sintering (dimensional changes and weight losses during sintering, microstructure and hardness after sintering), (2) the study of the co-sintering of a two layer material which differs from the average behaviour of single materials. This difference arises from chemical interaction between the two materials such as M 6 C formation in the interfacial region. M 6 C formation is closely related to liquid formation in steel and liquid migration into the WC base material. A mechanism is proposed; it is based on WC dissolution and asymmetric diffusion of Wand C in the steel layer.

When solution treatment was carried out at 1553 K, a rafting structure formed even above the dissolution temperature of the γ' phase in a liquid metal cooling directionally solidified nickel-based superalloy. This abnormal coarsening behavior may be induced by the phase transformation stress due to the dissolution of non-uniform distribution of the γ/γ' eutectic. Electron backscatter diffraction and transmission electron microscopy were employed to analyze the stress.

Our recently developed model to describe the viscosity of silicate melts is extended to describe and predict the viscosities of alkali-rich silicate melts. The model requires one additional binary parameter for each M 2 O–SiO 2 system, where M is an alkali metal, to a total of three binary parameters per binary system alkali oxide – silica. In addition to unary and binary parameters, the model requires two ternary parameters for each alumina-containing ternary system M O x –Al 2 O 3 –SiO 2 , where M O x is a basic oxide, to describe the viscosity maxima in these ternary systems due to the Charge Compensation Effect. The viscosity of multicomponent melts and of ternary melts M O x – N O y –SiO 2 , where M O x and N O y are basic oxides, is predicted by the model solely from the unary, binary and ternary parameters. The available viscosity data for the alkali-containing subsystems of the Al 2 O 3 –CaO–MgO–Na 2 O–K 2 O–SiO 2 system are reviewed. The model reproduces the experimental data for binary and ternary melts and predicts the viscosities of multicomponent melts within experimental error limits. In particular, the viscosities of glass melts and melts of importance for petrology are well predicted by the model.

A quantitative structure –property relationship model was constructed to correlate the glass transition temperatures ( T g ) of aromatic heterocyclic polymers with four molecular descriptors calculated from the repeating units. Multiple linear regression analysis was used to develop the model. The data set of 52 polymers was randomly divided into a training set, and a testing set. The training set was used to develop the model, which was evaluated externally using the test set. Even with the diversity of the data set, the optimal model exhibited some predictive power in estimating the T g , with the mean relative errors of 3.299% ( R = 0.963, n = 28) for the training set and 3.064% ( R = 0.959, n = 23) for the test set.

The relationship between γ ( T x /( T g + T l )) of Fe-based bulk metallic glasses (BMGs) and three parameters (i. e., electronegativity difference, atomic size parameter and heat of mixing) is studied in this paper. The conclusion can be drawn from the available experimental data in the literature that parameter γ has a close correlation to three parameters in Fe-based BMGs. It can be expressed as follows: γ = 0.40334 − 0.20242 × Δ x 2 − 0.1209 × δ 2 + 3.7395 × 10 −6 × Δ H 2 . The function can be obtained by a nonlinear regression analysis program and it can be used to find new Fe-based BMGs alloys.

Epitaxial films of rare-earth zirconates, RE 2 Zr 2 O 7 (RE = La, Nd, Sm and Gd) were grown on yttrium stabilized zirconia (100) single crystal substrates using metal-organic deposition. A precursor solution of 0.25 '0.40 M concentration of total cations was spin-coated on yttrium stabilized zirconia substrates and crystallized at 1 000 8C for 3 h in Ar-4%H 2 after calcination at 500 8C for 1 h. X-ray diffraction studies showed that the resulting pyrochlore RE 2 Zr 2 O 7 films were highly textured with cube-on-cube epitaxy. Atomic force microscopy investigations revealed that the surfaces of La 2 Zr 2 O 7 , Nd 2 Zr 2 O 7 and Sm 2 Zr 2 O 7 films had a fairly dense and smooth microstructure without cracks and porosity, but that voids could be seen on the surface of the Gd 2 Zr 2 O 7 film. Optical microscopy measurements confirmed that the Gd 2 Zr2O 7 precursor solution showed poor wetting behavior on the substrate. It was concluded that the Nd 2 Zr 2 O 7 and Sm 2 Zr 2 O 7 films could be potentially used as buffer layers for YBa 2 Cu 3 O 7 -d coated conductors.

Low temperature nanocrystalline titanium dioxide particles were prepared via the sol-gel method using titanium(IV) butoxide for fabrication of flexible dye sensitized solar cells. The prepared sample was characterized using X-ray powder diffraction, specific surface area, transmission electron microscopy, scanning electron microscopy and atomic force microscopy. Results confirm that the synthesized sample was highly crystalline anatase TiO 2 with an average particle size of 35 nm. The measured specific surface area for TiO 2 powder and average surface roughness for TiO 2 film were 61.11 m 2 g −1 and 17.25 nm respectively. The prepared nano-scale TiO 2 was then employed to fabricate flexible dye sensitized solar cell via the low temperature sintering and operated with a platinum counter electrode. The solar energy conversion efficiency for the cell fabricated with TiO 2 nanoparticles was about 0.85%.

Zn 0.9 Co 0.1 O green pigments, with crystallite sizes from 29 to 181 nm, were synthesized via solid state reaction and solution combustion using different fuels. The effects of experimental conditions on the color properties of the pigments were investigated by means of the spectral diffuse reflectance and the color coordinates determination. The results showed that the maximum perceptible color depth was obtained for the combustion method with citric acid. The calculated color properties as well as the hue angle values around 1578 confirmed a similar green shade with lightness ranging from 34 to 62. A good accordance was demonstrated between the results of the color properties in terms of the adsorption parameter of K/S and the crystallite size variations. K and S are the coefficients of absorption and scattering of monochromatic light of an opaque substrate. The optimum color characteristics were obtained for the combustion synthesized sample by using citric acid as fuel.

Cadmium sulfide nanorods were synthesized via a simple non ionic surfactant assisted hydrothermal method. Thioacetamide was selected as the sulfur source to maintain the homogeneity of the reaction environment and condition. Each of the nanorods is single crystalline. The length direction is along the c-axis of the crystal. The composition and purity of the samples were characterized using X-ray diffraction and energy dispersive X-ray analysis. Scanning electron microscopy and transmission electron microscopy images indicate that the surfactant, PEG 400, and reaction media played a key role in the formation of CdS nanorods. The formation of nanorods depends on the reaction media. Nanorods were formed in diluted strong acid solution media such as hydrochloric acid, nitric acid and not in acetic media. PEG assisted nanorods showed large aspect ratios and uniform faces compared with the sample prepared in the absence of PEG. The optical properties of the products were studied using photoluminescence spectroscopy.

This work analyses the wear behaviour and impact toughness of high Cr –Mo white irons alloyed with titanium as related to their microstructure and solidification path. Five alloys were used with different percentages of titanium (0, 0.47, 0.932, 1.31 and 1.78 %). Such additions of titanium resulted in precipitation of small hard TiC particles in the austenitic matrix. These particles were well distributed in the matrix with Ti-additions up to 1.31% and then started to agglomerate with 1.78%Ti. Hardness increased dramatically with increasing amount of titanium addition. The wear behaviour was investigated for each alloy in the as-cast condition using the pin-on-ring configuration for 30 min against a ring made of SiC that rotated at a constant speed of 65 rpm. The wear test was performed under a load range of 30–180 N. The results are discussed in terms of microstructure and the minimum wear rate was obtained for the white cast iron alloyed with 1.31% titanium. The optimum tensile strength, impact toughness and wear resistance were also observed for the iron alloyed with 1.31% titanium. However, these properties started to deteriorate with further increase in titanium addition due to the clustering effect of titanium carbide particles in the austenitic matrix. Therefore, the optimum wear resistance and impact toughness of white cast iron under investigation (16% Cr-1.3% Mo-2.3% Ni) can be achieved by adding 1.31% titanium.

In this paper, a thermo-mechanical model has been used to predict the temperature history and residual stress distribution in gas tungsten arc welding of AA5251 plates. This model has also been utilized to estimate the residual stresses under different welding sequences, while in the model the effect of temperature on material properties were taken into account. In order to verify the predictions, residual stresses within the welded samples and weld pool geometry were experimentally measured employing hole drilling and macro-examination of weld cross-section, respectively. The comparison between numerical and experimental data shows a reasonable agreement. The predictions show that the welding sequence has a major effect on maximum temperature and temperature distribution as well as stress and plastic strain distributions around the weld.

Electrochemical machining has been shown to be a potentially suitable process for the machining of high-strength conductive materials. This paper highlights features of the development of a comprehensive mathematical model for correlating the interactive and higher-order influences of various machining parameters on the dominant machining criteria, i. e. the metal removal rate and the surface roughness ( R a ) phenomena, through response surface methodology, utilizing relevant experimental data as obtained through experimentation. Optimal combination of these parameters can be used in order to achieve maximization of the metal removal rate and the minimum surface roughness.

Nanoclays are a new generation of processed clays of interest in a wide range of high performance composites. Nanoclay is defined as clay that can be modified to make the clay complexes compatible with organic monomers and polymers. Here, it can be said that polymeric nanocomposites are among the most exciting and promising classes of materials discovered recently. A number of physical properties are enhanced successfully when a polymer is modified with a small amount of nanoclay on condition that the clay is dispersed at the nanoscopic level. This research has accomplished a comparative rheological test on unmodified and nanoclay modified bitumen. For this, two types of nanoclay were used: Nanofill-15 and Cloisite-15A. X-ray diffraction has been used to study the nanostructure exfoliation of nanoclay platelets in bitumen. The results show that compared to unmodified bitumen, the nanoclay modification leads to changes in rheological parameters by increasing stiffness and decreasing the phase angle, hence it can also reduce aging effects in bitumen. Further, comparison of the rutting parameter (G∗/sin δ) shows that the nanoclay modification could increase the rutting resistances of bitumen and the improvement is dependent upon the type and amount of nanoclay. The fatigue resistance parameter (G∗/sin δ) at low to medium temperatures shows that the nanoclay modification reduces the fatigue life, however it exhibits the same fatigue life as that of the unmodified bitumen after a particular aging condition.