Superhard silicon nanospheres

Successful deposition and mechanical probing of nearly spherical, defect-free silicon nanospheres has been accomplished. The results show silicon at this length scale to be up to four times harder than bulk silicon. Detailed measurements of plasticity evolution and the corresponding hardening response in normally brittle silicon is possible in these small volumes. Based upon a proposed length scale related to the size of nanospheres in the 20- radii range, a prediction of observed hardnesses in the range of 20- is made. The ramifications of this to computational materials science studies on identical volumes are discussed.     

A fourfold generalization of Peaucellier's inversion cell

New inversors are proposed which are generalizations of the well-known inversor of Peaucellier. It appears that a kite in the Peaucellier cell is replaceable by an arbitrary 4-bar linkage (abhk) whereas the direction and length of the straight line, produced by the inversor, can be manipulated through the particular choice of the relative polar coordinates of a vertex A of the triangular input link. Formulas are derived for practical inversors with a revolving input link. The ones selected are basically governed by the choice of two transmission angles, ₁ and ₃, by the length L of the acquired line, as well as by its direction represented by the angle /2- comprised between the line L and the frame.

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Sommario Vengono proposti nuovi inversori quali generalizzazioni del ben noto inversore di Peaucellier. Si mostra che un quadrilatero isoscele nella cella di Peaucellier è sostituibile da un'arbitraria connessione a quattro barre, mentre la direzione e la lunghezza della retta, prodotta dall'inversore, possono essere manipolate con la particolare scelta delle relative coordinate polari di un vertice A del collegamento triangolare di input. Vengono derivate formule per inversori funzionali con un collegamento di input rotante. Quelli solezionati sono principalmente governati dalla scelta di due angoli di trasmissione, ₁ e ₃, dalla lunghezza L della linea ottenuta, così come dalla sua direzione rappresentata dall' angolo /2- compreso tra la linea L ed il riferimento.

     

Mode I fracture of sheet metal

The perceived wisdom about thin sheet fracture is that (i) the crack propagates under mixed mode I & III giving rise to a slant through-thickness fracture profile and (ii) the fracture toughness remains constant at low thickness and eventually decreases with increasing thickness. In the present study, fracture tests performed on thin DENT plates of various thicknesses made of stainless steel, mild steel, 6082-O and NS4 aluminium alloys, brass, bronze, lead, and zinc systematically exhibit (i) mode I “bath-tub”, i.e. “cup & cup”, fracture profiles with limited shear lips and significant localized necking (more than 50% thickness reduction), (ii) a fracture toughness that linearly increases with increasing thickness (in the range of 0.5-). The different contributions to the work expended during fracture of these materials are separated based on dimensional considerations. The paper emphasises the two parts of the work spent in the fracture process zone: the necking work and the “fracture” work. Experiments show that, as expected, the work of necking per unit area linearly increases with thickness. For a typical thickness of , both fracture and necking contributions have the same order of magnitude in most of the metals investigated.A model is developed in order to independently evaluate the work of necking, which successfully predicts the experimental values. Furthermore, it enables the fracture energy to be derived from tests performed with only one specimen thickness. In a second modelling step, the work of fracture is computed using an enhanced void growth model valid in the quasi plane stress regime. The fracture energy varies linearly with the yield stress and void spacing and is a strong function of the hardening exponent and initial void volume fraction. The coupling of the two models allows the relative contributions of necking versus fracture to be quantified with respect to (i) the two length scales involved in this problem, i.e. the void spacing and the plate thickness, and (ii) the flow properties of the material. Each term can dominate depending on the properties of the material which explains the different behaviours reported in the literature about thin plate fracture toughness and its dependence with thickness.     

电弧喷涂不锈钢涂层耐磨性的研究

研究了电弧喷涂３Ｃｒ１３和１Ｃｒ１８Ｎｉ９Ｔｉ不锈钢涂层的耐磨性，分析了喷涂工艺参数对３Ｃｒ１３涂层耐磨性的影响及涂层中孔隙的作用，并且深入探讨了涂层的磨损机理。     

Mechanism-based strain gradient crystal plasticity—II. Analysis

In part I of this series (Mechanism-based strain gradient crystal plasticity—I. Theory. J. Mech. Phys. Sol. (2005), accepted for publication), we have proposed a theory of mechanism-based strain gradient crystal plasticity (MSG-CP) to model the effect of inherent anisotropy of a crystal lattice on size-dependent non-uniform plastic deformation at micron and submicron length scales. In the present paper, several example problems are investigated to show how crystal anisotropy is reflected by the MSG-CP theory.     

Mechanism-based strain gradient crystal plasticity—I. Theory

We have been developing the theory of mechanism-based strain gradient plasticity (MSG) to model size-dependent plastic deformation at micron and submicron length scales. The core idea has been to incorporate the concept of geometrically necessary dislocations into the continuum plastic constitutive laws via the Taylor hardening relation. Here we extend this effort to develop a mechanism-based strain gradient theory of crystal plasticity. In this theory, an effective density of geometrically necessary dislocations for a specific slip plane is introduced via a continuum analog of the Peach-Koehler force in dislocation theory and is incorporated into the plastic constitutive laws via the Taylor relation.     

Relaxation modes of molten polydimethylsiloxane

Although it is known that chain reptation with contour length fluctuations and tube constraint release are the relevant mechanisms of chain relaxation, only applications of the theory to specific polymers could allow one to determine the relative importance of these relaxation modes. With this purpose we study the relation between the shear relaxation modulus of entangled, linear and flexible polydimethylsiloxane (PDMS) samples with narrow molecular weight distribution (MWD) and some of their blends. The linear PDMS are synthesized by anionic polymerization. Binary blends having different concentrations of the linear PDMS with narrow MWD are also prepared.  A theory developed by some of the authors has been proposed previously and is based on the double reptation mixing rule and a law for the relaxation time of the chains. Since, calculations in the theory must be carried out with rheometric data of linear viscoelasticity, the time-temperature superposition principle is used at a reference temperature of 20°C to obtain the and master curves. We conclude that the PDMS chains in a matrix with 1.5 relax predominantly by reptation while the tube renewal follows Rouse mode, which is proportional to chain molecular weight raised to the square power. It is also shown that dilution effects due to short chains start to be important in the binary blends of PDMS samples due to the presence of Zimm mode of relaxation, which is suggested by the theory for linear polymers of relatively high polydispersity. Received: 27 January 1998 Accepted: 13 August 1998     

Grain boundary versus transgranular ductile failure

The competition between intergranular and intragranular fracture is investigated using a bilayer damage model, which incorporates the relevant microstructural features of aluminium alloys with precipitate free zones (PFZ) nearby the grain boundary. One layer represents the grain behaviour: due to precipitation, it presents a high yield stress and low hardening exponent. The other layer represents the PFZ which has the behaviour of a solid solution: it is much softer but with a much higher strain hardening capacity. In both layers, void growth and coalescence is modelled using an enhanced Gurson-type model incorporating the effects of the void aspect ratio and of the relative void spacing. The effects on the ductility (i) of the flow properties of each zone, (ii) of the relative thickness of the PFZ, and (iii) of the particles spacing and volume fraction in the PFZ are elucidated. Comparisons are made with experimental data. Based on the previous analysis, qualitative understanding of trends in the fracture toughness of aluminium alloys can be gained in order to provide a link with the thermal treatment process.     

Investigation of carrier scattering mechanisms in n-Cd1−xMgxSe single crystals using Fourier Transform Infrared Spectroscopy

This paper presents results of investigations of carrier scattering mechanisms in n-Cd_1−xMg_xSe mixed crystals with magnesium content varying from x = 0 to x = 0.33. Experimental results obtained by means of the Fourier Transform Infrared Spectroscopy (FT-IR) and Hall measurements are discussed in the frame of the Drude and the quantum theories. The character of the wavelength dependence of the optical absorption coefficient in investigated crystals was found to be of the type ∼λ^p, where 2 < p < 3.5. The p = 2 is expected from the Drude theory and the relaxation time approximation. The obtained experimental values of p parameter suggest that the optical phonon and impurity scattering mechanisms are dominating scattering mechanisms in these crystals. The calculated carrier concentration from optical absorption spectrum for a n-CdSe crystal is in a good agreement with this obtained from Hall measurement.