On the applied stress dependence of the subgrain size

The present work studies the collection of experimental data from which Raj and Pharr (Mater. Sci. Eng., 81 (1986) 217) deduced a universal empirical dependence of the subgrain size on the applied stress. In accord with their result and some theoretical predictions the normalized subgrain size d_s/b was ssumed to be proportional to G/σ (G is the shear modulus, b the Burgers vector length, σ the applied stress). The evaluated factor of proportionality K₁, having the value within the interval from 0.76 to 180 in the inspected data sets, was discussed from the point of view of various factors which can influence the experimental data.     

Work hardening behaviour and subgrain size distribution of hot worked aluminium

Abstract

The subgrain structure of hot worked aluminium was found to remain geometrically self similar for various deformation conditions, undergoing only alteration in size as the flow stress changes. The stress and temperature related alterations of the subgrain size distributions have been described mathematically by the two parameters of the lognormal probability function which fits the experimental data. It has been found that the rate of scaling of the structure changes in stage IV work hardening. Experimental observations and statistical considerations suggest that the lognormal character of the subgrain size distribution arises as a result of a multiplicative interaction of recovery processes.     

Sintering of aluminium nitride: Particle size dependence of sintering kinetics

The effect of particle size (0.78 4.4 m) on the sintering kinetics of AIN powder was investigated in the temperature range from 1600 to 2000° C and the results were analysed on the basis of vacancy diffusion models. The mechanisms of sintering are discussed.Fractional shrinkage is proportional to the nth power of soaking time with n = 0.20 for 4.4 m and 1.5 m powders and 0.33 for 0.78 m powder. For the 0.78 m powder at 1900° C, however, n decreases gradually as grain growth proceeds. The experimental activation energy for sintering is between 92 kcal/mole for 4.4 m and 129 kcal/mole for 0.78 m powder. Unlike this activated energy, the rate of sintering and the diffusion constant calculated from it increase drastically with decrease of particle size; the derived diffusion constant for 1.5 m powder is 10¹ to 10² times larger than that of 4.4 m powder, and for 0.78 m powder the diffusion constant is estimated to be still higher.The particle-size dependence of parameter n and the diffusion constant seems to be caused by a variation in predominant diffusion mechanisms; namely, bulk diffusion in coarse powder and surface or grain-boundary diffusion in fine powder.     

Particle size dependence of fatigue crack propagation in SiC particulate-reinforced aluminium alloy composites

Fatigue crack propagation (FCP) and fracture mechanisms have been studied for two orientations in powder metallurgy 2024 aluminium alloy matrix composites reinforced with three different sizes of silicon carbide particles. Particular attention has been paid to make a better understanding for the mechanistic role of particle size. The FCP rates of the composites decreased with increasing particle size regardless of orientation and were slightly faster in the FCP direction parallel to the extrusion direction. After allowing for crack closure, the differences in FCP rate among the composites and between two orientations were significantly diminished, but the composites showed lower FCP rates than the corresponding unreinforced alloy. Fracture surface roughness was found to be more remarkable with increasing particle size and in the FCP direction perpendicular to the extrusion direction. Taking into account the difference in the modulus of elasticity in addition to crack closure, the differences in FCP rate between the unreinforced alloy and the composites were almost eliminated.     

The influence of residual gas pressure on the stress in aluminium films

The paper presents the results of an experimental study of the influence of residual gas pressure on the internal stress in vapour-deposited thin Al films. The results of specific resistivity measurements were used for a quantitative evaluation of the degree of Al film oxidation. The curve of stress versus degree of oxidation is interpreted as due to relaxation forces generated on the crystallite boundaries.     

Residual stress relief in the aluminium alloy 7075

The residual stresses in heat-treated 7075 aluminium alloy blocks have been characterised using two neutron diffraction strain scanning instruments. The influence of uniaxial cold compression on relieving the residual stress has been determined. Systematically increasing the magnitude of cold compression from 1 to 10% has been shown to have a small beneficial effect on the final residual stress distribution, by reducing the magnitude of the range in the triaxial residual stresses. The effect of an overaging precipitation treatment on the residual stress has also been characterised, and this was found to have a significant stress relieving influence (25–40%). A relationship between the width of the {311} diffraction peaks and the amount of cold compression was also observed.     

超薄Al膜可见、红外光区吸收率的尺寸效应

超薄Al膜吸收率随薄膜厚度变化的关系表明：金属薄吸收率具有尺寸效应,并具有极大值。对比超薄Al膜生长过程的结构变化,结构特征变化是导致吸收率尺寸效应的重要原因。     

The stress dependence of the crack growth rate during creep

An equation is derived for the crack growth rate under creep conditions. In the model, the propagation of a grain boundary crack is controlled by the plastic growth of cavities located in the grain boundaries ahead of the crack. It is assumed that the cavities grow by power law creep in the elastic crack tip stress field. Hence, the stress dependence of the crack velocity is provided through the elastic stress intensity factor, i.e., dC/dt=BK _I ^p .The cavity spacing, , appears as an important factor in the coefficient,B ^–(p–2)/2. At large values of , corresponding to less severe creep damage in the grain boundaries, the above equation would predict very low values for the crack velocity. Under such conditions, we suggest that another mechanism, whose stress dependence is provided through the net section stress, becomes active, i.e., dC/dt=B _net ^p . Since increases with decreasing applied stress, one should observe the _net correlation at low stresses. The results of recent creep crack growth experiments which tend to support this hypothesis are presented.

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Résumé On dérive une équation décrivant la vitesse de propagation d'une fissure dans les conditions de fluage. Dans ce modèle, la propagation d'une fissure aux frontières de grains est contrôlée par la croissance dans le domaine plastique de cavités situées aux frontières de grains en avant de la fissure.On suppose que les cavités s'étendent dans le champ de contraintes élastiques situées à l'extrémité de la fissure en suivant une loi de fluage parabolique. Dès lors, la dépendance de la vitesse de la fissuration en fonction de la contrainte est fournie par un facteur d'intensité de contrainte élastique, c'est-à-dire dC/dt=BK _I ^p .L'espace entre les cavités, , apparaît être un facteur important dans les coefficientsB. Pour de grandes valeurs de , qui correspondent à un dommage moins sévère par fluage aux frontières des grains, l'équation ci-dessus permettrait de prédire des valeurs très faibles de la vitesse de fissuration.Sous ces conditions, il est suggèré qu'un autre mécanisme, dont la dépendance de la contrainte est fournie par la contrainte agissant sur la section droite, devient plus actif; on a alors dC/dt=B _nette ^p .Comme augmente lorsque la contrainte appliquée diminue, on devrait observer une corrélation de nette à basses contraintes.Les résultats d'essais de croissance de fissure sous des conditions de fluage effectués récemment tendent à supporter cette hypothèse et sont présentés.

     

The dependence of grain-boundary sliding on shear stress

Polycrystalline specimens of four materials: aluminium, a lead/thallium alloy,-brass and Magnox AL80 were subjected to slow creep at a temperature of approximately 0.5T _m (whereT _m is the melting temperature in degrees Kelvin) and measurements made of the dependence of grain-boundary sliding on the orientation of the boundary with respect to the applied stress. The results were analysed in detail on the basis of the hypothesis that sliding is governed by the component of the applied stress resolved in the boundary plane. Agreement between theory and experiment leads to the conclusion that this is a most important factor governing sliding. The component of sliding revealed by steps on a free surface is seen to contain an extra term due to the unbalanced pressure from grains below the surface.     

Temperature dependence of the tensile behaviour of aramid/aluminium laminates

Aramid/aluminium laminates (ARALL^® laminates) are a family of new hybrid composites made of alternating layers of thin aluminium alloy sheets with plies of epoxy adhesive prepreg containing unidirectional aramid fibres. The effect of elevated and cryogenic temperatures on these materials is critical to aerospace applications. ARALL 1, 2, 3, and 4 laminates have been tested in tension at temperatures ranging from –300F–400 °F (–184–204 °C) and at room temperature after exposure. This paper summarizes how tensile properties depend on temperature for these four ARALL laminates under the conditions described. At cryogenic temperatures, no degradation of ultimate tensile strengths, tensile yield strengths and moduli were found for either the longitudinal or transverse directions for ARALL 1–4 laminates. Furthermore, the mechanical properties remained the same or increased slightly as the temperature decreased. Longitudinal and transverse ultimate tensile strengths, tensile yield strengths, and moduli of ARALL 1–3 laminates at room temperature remain nearly constant after the laminates were exposed for 1, 10 and 100 h to temperatures up to 250 °F (121 °C), and up to 350 °F (177 °C) for ARALL 4 laminates. However, these properties determined at the elevated temperatures after 1, 10 and 100 h exposure showed a tendency to decrease with increasing temperature. The properties of ARALL laminates are much better in the longitudinal fibre direction than those of conventional monolithic aluminium alloys. Typical failure modes of the test specimens in the high-temperature range were examined using a scanning electron microscope. The discussions are also described in the paper.     

Mechanistic model for grain size and temperature dependence of flow stress in 316L austenitic stainless steel

Abstract

During plastic deformation of a polycrystalline material, both the grain interior and the grain boundary regions exhibit distinctly different dislocation behaviours at a given strain and temperature. Studying the variation of experimental flow stress with temperature, it seems that the flow stress of a fine grained polycrystalline material is mainly controlled by dislocation dynamics at and in the vicinity of grain boundaries. At low temperatures in a polycrystalline material, the dislocations are piled up at grain boundaries and the density of dislocations increases significantly in the grain boundary region, while at high temperatures the annihilation of dislocations take place at and in the vicinity of the grain boundaries during deformation. Therefore, the flow stress behaviour of a polycrystalline material can be understood in terms of the process of accumulation and annihilation of dislocations at and in the vicinity of grain boundaries at a given strain and temperature.