ultrasound action on strength properties of polycrystalline metals

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74 THE ANNALS OF UNIVERSITY “DUNĂREA DE JOS “ OF GALAŢI FASCICLE VIII, 2006 (XII), ISSN 1221-4590 TRIBOLOGY and the nominal input power of 600 W. The acoustic power dissipated by ultrasonic probe in 1000mldeionized water at ambient temperature and pressure as a function of electrical input power was determined by calorimetry. These data were used to allow selection of the appropriate input power to give constant transmitted power. After the completion of measurements, the ultrasonic horn was examined microscopically. No attack of the stainless steel by liquid metal samples was observed in either case, so there was no evidence of contamination of the liquid metal by alloying. Table 1. Chemical composition of sample (ppm). Fe Si Cu Zn 60 60 30 30 The grain size of specimens was determined using the linear intercept method in conjunction with a logarithmic Gauss distribution. To make individual hardness measurements for aluminium, Brinell hardness tests are widely used. 3. RESULTS AND DISCUSSIONS Figures 1 and 2 show the yield stress as a function of grain size for samples solidified under similar conditions both with and without sonication. −1 2 A linear relationship between σ and d can be established and it shows that HP equation is valid. The scatter of experimental values is high and it can be explained by the experimental difficulties in the determination of the yield stress in this very soft material. Substituting the constants obtained for the fitted straight line into eq. (1), we obtained: −12 σ = 9.095 + 0.714 ⋅ d without <strong>ultrasound</strong>, −12 σ = 9.620 + 1.166 ⋅ d with <strong>ultrasound</strong>. The slope of the Hall-Petch plots (K y values) is higher for samples solidified in ultrasonic field presence. This higher value (see figures 1 and 2) it is expected to be to twinning. However, this remark will be followed up by further investigation. The samples solidified in presence and in absence of the ultrasonic field present a different grain size due to various solidification conditions. The samples solidified without <strong>ultrasound</strong>s presence present the grain size significantly larger (fig. 3). When the solidifying process takes place in presence of ultrasonic field, the equiaxed fine-grained aluminum appeared. The average size of aluminum grain is smaller. The grain size was measured on transversal sections with areas of 40 mm 2 after mechanical and electrochemical polishing in optical microscope equipped with image analyzer. Investigations in our laboratory on aluminum revealed superior mechanical properties for samples solidified in ultrasonic field. The mechanical properties obtained for the aluminum are given in table 2. The ultrasonic field presence caused hardness increased unto 28%. σ ( MPa ) 11.5 11.0 10.5 10.0 9.5 9.0 without us 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 d - 1/2 ( mm - 1/2 ) Fig. 1. The yield stress as a function of grain size (without <strong>ultrasound</strong>). σ ( MPa ) 14.5 14.0 13.5 13.0 12.5 12.0 with us 2.5 3.0 3.5 4.0 d - 1/2 ( mm - 1/2 ) Fig. 2. The yield stress as a function of grain size (with <strong>ultrasound</strong>). a) b) Fig. 3. Microstructure of 99.97%Al; (a) without <strong>ultrasound</strong> and (b) with <strong>ultrasound</strong>.
THE ANNALS OF UNIVERSITY “DUNĂREA DE JOS “ OF GALAŢI FASCICLE VIII, 2006 (XII), ISSN 1221-4590 TRIBOLOGY 75 Without <strong>ultrasound</strong> With <strong>ultrasound</strong> Table 2. Hardness test values (HB) 1 2 3 4 5 21.8 21.8 22.0 22.2 22.3 28.0 27.9 28.2 28.2 28.0 It is known that grain boundaries could act as barriers to dislocation motion, but we also viewed a grain boundary as a sort of "film". If the properties of the grain boundary were known then a more complete analysis of their strengthening effect could be made. Grain boundaries are simply planar defects between adjacent grains, each grain having different crystallographic orientations. Low angle grain boundaries occur when the miss-orientation of the lattices of adjacent grains is less than 15°. These boundaries can be modeled as an array of edge dislocations. The structure of high angle grain boundaries is more complex. The other characteristics of grain boundaries are that they tend to attract impurities. In a sense, this might be thought of as a film which can either strengthen or weaken a material. Grain boundary segregation of impurities, however, does not always embrittle a material [9]. Grain boundaries are lattice imperfections and as such increase the free energy of the material. There is a tendency to minimize the contribution from grain boundaries and this involves reducing the grain boundary area to grain volume ratio, something which happens as the grains grow larger. While other factors can cause grains to grow, this is essentially the process involved in normal grain growth (also called ideal grain growth). Non-ideal grain growth occurs when grain growth is inhibited by the presence of a second phase or is restricted by the edges of the specimen. Ultrasonic <strong>action</strong>, cold work, pressure, magnetic fields, etc. also cause grain growth to deviate from the ideal case. The effect of grain size on the hardness properties of polycrystalline metals consists in increasing of hardness for the sample with finegrained structure. It is suggested that this morphology of ingot solidified in ultrasonic filed results from increased growth rate and smaller crystallite size. So, reducing the grain size of a polycrystalline material is an effective way of increasing its strength [4]. Under ultrasonic conditions, the acoustic flow takes place in the liquid metal. It is clearly demonstrated that the acoustic flows are associated with the <strong>ultrasound</strong> absorption, whatever its nature. However, the absorption coefficient is quite small for the liquid metals, so the increase in the temperature of the melt caused by absorption process has been eliminated. In these conditions, the reason for this prominent change in solidification kinetic is assumed to be large-scale acoustic streaming. Its effect is a permanent stirring of the melt so the effects of thermal and mass homogeneity of the melt are quite obvious [3]. The increasing in the intensity of fluid flow can give rise to grain multiplication, which can be attributed to the increased effective nucleation rate caused by the extremely uniform temperature and composition fields in the bulk liquid at early stages of solidification. Also, the forced convection increases the growth rate. The solidification starts by heterogeneous nucleation at the crucible wall through the so- called “big-bang” mechanism. Only a fr<strong>action</strong> of the nuclei formed at this stage contributed to the formation of the chilled zone and the majority of the nuclei are transferred into the hotter bulk liquid and remelted. The final solidified microstructure depends largely on the amount of nuclei surviving after the big-bang nucleation. Under the <strong>ultrasound</strong> <strong>action</strong> both the temperature and composition fields of the liquid metal are extremely uniform. The nuclei formed will survive due to the uniform temperature field, resulting in an increased effective nucleation rate. In addition, the intensive stirring may also disperse the cluster of potential nucleation agents, giving rise to an increased number of potential nucleation sites. Also, under forced convection, the nucleation and the growth at the chilled wall were suppressed, while the nucleation and growth in the bulk liquid were enhanced [1]. It has been suggested that the forced convector fluid flow induced by <strong>ultrasound</strong> may be sufficient to break small dendrite arms and distribute them throughout the melt. If a high energy boundary is formed in a metal in contact with its liquid then the condition indicates that the grain boundary will be wetted by the liquid phase, i.e. replaced by a thin layer of liquid and thus the dendrites break appear. Further these broken dendrites act as nucleants and grow as globular nondendrite structures. The acoustic streaming produced the change in possibility that hydrodynamic force to cause breakage of dendritic arms under the solidification conditions. In the same time, due to supplementary energy contribution, the ultrasonic field presence hinders the long-range ordering processes of atoms. At this moment, they act as nuclei for the growth of more particles and the relatively small dendrite spacing are created. The possibility that fluid flow could disrupt the crystal bonding is also considered [5]. The shear forces resulting from natural convection flow of the melt are too weak to disrupt the crystal bonding during solidification. However under ultrasonic field, these forces are dramatically increasing. The accuracy of sonic measurements is reasonable taking into account the difficulties associated with getting the <strong>ultrasound</strong> into the melted metal. The ultrasonic field presence into a liquid causes cavitation phenomenon [1]. This imposes a sinusoidal variation in pressure on a steady state ambient pressure. One new question of this study is the problem of cavitation and its microstreaming effect. The effect of <strong>ultrasound</strong> increases with increasing power, but not indefinitely since there is an optimum value beyond which the effect diminishes.
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ultrasound action on strength properties of polycrystalline metals

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