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51.
C. R. Das S. Dhara H. C. Hsu L. C. Chen Y. R. Jeng A. K. Bhaduri Baldev Raj K. H. Chen S. K. Albert 《Journal of Raman spectroscopy : JRS》2009,40(12):1881-1884
The mechanism of the recrystallization in epitaxial (0001) GaN film, introduced by the indentation technique, is probed by lattice dynamic studies using Raman spectroscopy. The recrystallized region is identified by micro‐Raman area mapping. ‘Pop‐in’ bursts in loading lines indicate nucleation of dislocations and climb of dislocations. These processes set in plastic motion of lattice atoms under stress field at the center of indentation for the initiation of the recrystallization process. A planar defect migration mechanism is evolved. A pivotal role of vacancy migration is noted, for the first time, as the rate‐limiting factor for the dislocation dynamics initiating the recrystallization process in GaN. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
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53.
Nanocrystals of calcium tungstate (CaWO4) of three different grain sizes were synthesized through chemical precipitation technique and the grain sizes and crystal structure were determined using the broadening of X‐ray diffraction patterns and transmission electron microscopy. The microhardness of compacted pellets of nanocrystalline calcium tungstate (CaWO4) with different grain sizes were measured using a Vickers microhardness tester for various applied loads ranging from 0.049 N to 1.96 N. The values of microhardness showed significant reverse indentation size effect at low indentation loads. The microhardness data obtained for samples of different grain sizes showed grain size dependent strengthening obeying normal Hall‐Petch relation. The dependence of compacting pressure and annealing temperature on microhardness of the nanostructured sample with grain size of 13 nm were also studied. The samples showed significant increase in microhardness as the compacting pressure and annealing time were increased. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
54.
In the present work a novel methodology is proposed, based on the combination of the Rockwell and the Vickers tests, to provide
estimates of the mechanical properties of metal substrates. The analysis is based on some novel invariants obtained from the
finite element solution of the Vickers indentation (the imprint diagonal relates to the maximum indentation depth and the
residual indentation depth with the average pressure and the elastic modulus). Several other useful results are discussed
and experiments are performed with a modified Rockwell apparatus on steel and aluminium alloys. The results are important
for the interpretation of micro indentation tests. Inverting the indentation data, reasonably accurate results can be obtained
for strain hardening properties for “power law” behaviour, whereas more complex strain hardening would require further investigation. 相似文献
55.
56.
用压痕法研究玻璃的动疲劳强度 总被引:1,自引:0,他引:1
本文借助扫描电镜在对带压痕的钠钙硅玻璃试件的断面分析中,提出了理想的径向裂纹模型,分析了由压痕引起的表面接触残余应力裂纹长短对裂纹在开裂纹在开裂和亚监界扩展区的疲劳强度的影响,并设计了相应的强度实验进行了比较。 相似文献
57.
A. Boersma V. A. Soloukhin J. C. M. Brokken‐Zijp G. de With 《Journal of Polymer Science.Polymer Physics》2004,42(9):1628-1639
Load and depth sensing indentation has been used to characterize the elastic modulus and hardness of various polycarbonate films. This analytical technique is shown to be extremely suitable for the determination of gradients in these mechanical properties. Furthermore, it is demonstrated that such a gradient exists over a length of micrometers in chemically aged polycarbonate, but it is virtually absent in physically aged polycarbonate. From these results, it is concluded that, although the first 100 nm cannot be probed, physical aging occurs homogeneously throughout the bulk of the sample. However, chemical aging starts at the surface and moves progressively into the bulk of the material. From the study of these films, it appears that for the interpretation of these measurements, knowledge about the amount of creep occurring during the measurements and about the mechanical properties of the substrate on which these films are applied is needed. Creep can be measured with the same indenter through the application of a constant load for a period of time. Load and depth sensing indentation appears to be a powerful method for studying the physical and chemical aging of polymers. It is especially valuable for coatings and films for which conventional tensile testing is problematic. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1628–1639, 2004 相似文献
58.
Indentation tests were carried out on a carbon black filled rubber sample on different length scales. The experiments covered
the range from aggregation of particles on the submicron scale up to structures which represent the bulk properties of the
sample on the millimeter scale. The local stiffness was used to visualize the areas investigated; therefore, mechanical images
were obtained for all length scales. So-called “mechanical units” were defined for every scale. The size distribution curves
for the mechanical units were analyzed and they were found to be non-Gaussian-shaped for every scale. Moreover, the distribution
curves of the mechanical units are similar to the distribution curves of particles and aggregates obtained by electron microscopy
reported in the literature. Evaluation by means of fractal analysis led to fractal dimensions for the mechanical units. It
could be shown in the present case that the fractal dimension D≈1.24 of the mechanical units in the range of submicrons up to several hundred microns is in good agreement with that of the
filler aggregates proposed in the literature. Furthermore, D is constant over a wide range of about 6 decades in area scale starting from aggregates up to the size of agglomerates. This
leads to the conclusion that the local arrangement of the filler ensembles seems to be self-similar from the smallest scale
of aggregation of particles up to the largest formation observed by indentation testing on the millimeter scale.
Received: 15 May 2001 Accepted: 18 August 2001 相似文献
59.
It has been reported previously that, for single and polycrystalline copper (fcc), the indentation size effect and the grain size effect (GSE) can be combined in a single length-scale-dependent deformation mechanism linked to a characteristic length-scale calculable by a dislocation-slip-distance approach (X. D. Hou and N. M. Jennett, ‘Application of a modified slip-distance theory to the indentation of single-crystal and polycrystalline copper to model the interactions between indentation size and structure size effects,’ Acta Mater., Vol. 60, pp. 4128–4135, 2012). Recently, we identified a ‘lateral size effect (LSE)’ in scratch hardness measurements in single crystal copper, where the scratch hardness increases when the scratch size is reduced (A. Kareer, X. D. Hou, N. M. Jennett and S. V. Hainsworth ‘The existence of a lateral size effect and the relationship between indentation and scratch hardness’ Philos. Mag. published online 24 March 2016). This paper investigates the effect of grain size on the scratch hardness of polycrystalline copper with average grain sizes between 1.2 and 44.4 μm, when using a Berkovich indenter. Exactly the same samples are used as in the indentation investigation by Hou et al. (‘Application of a modified slip-distance theory to the indentation of single-crystal and polycrystalline copper to model the interactions between indentation size and structure size effects,’ Acta Mater., Vol. 60, pp. 4128–4135, 2012). It is shown that, not only does the scratch hardness increase with decreasing grain size, but that the GSE and LSE combine in reciprocal length (as found previously for indentation) rather than as a superposition of individual stresses. Applying the same (as indentation) dislocation-slip-distance-based size effect model to scratch hardness yielded a good fit to the experimental data, strongly indicating that it is the slip-distance-like combined length-scale that determines scratch hardness. A comparison of the fit parameters obtained by indentation and scratch on the same samples is made and some distinct differences are identified. The most striking difference is that scratch hardness is over four times more sensitive to grain size than is indentation hardness. 相似文献
60.
Variation of the strain rate during CSM nanoindentation of glassy polymers and its implication on indentation size effect 下载免费PDF全文
George Z. Voyiadjis Leila Malekmotiei 《Journal of Polymer Science.Polymer Physics》2016,54(21):2179-2187
The indentation strain rate is currently assumed to remain unvaried during continuous stiffness measurement (CSM) nanoindentation where is imposed to remain constant. To probe the validity of this assumption for the nanoindentation of glassy polymers, a series of experiments have been performed at different set values on poly(methyl methacrylate) and polycarbonate using CSM technique. It is firstly shown that the actual value changes drastically at shallow indentation depths and it takes a considerable depth, which is material independent, for this parameter to attain a stabilized value. Furthermore, the strain rate is measured directly as the descent rate of the indenter divided by its instantaneous depth ( ), and indirectly via considering the variations of the load and hardness during the test. Both of these approaches reveal that the strain rate is considerably larger at shallow depths, and the depth beyond which it becomes constant is material and ratio dependent. Finally, by considering the relationship between the hardness and strain rate, it is observed that although the strain rate variation alters the hardness, its contribution is not able to justify the observed indentation size effect; hence, other contributing factors for this phenomenon are discussed for their possible effects. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2179–2187 相似文献