Search results

Superplastic behavior and cavitation were investigated for WC-15 mass % Co cemented carbides with the WC grain sizes of 0.7 µm (A) and 5.2 µm (B), WC-10 mass % Co cemented carbide with the WC grain size of 1.5 µm (C) and WC-5 mass % Co cemented carbides with the WC grain sizes of 0.5 µm (D) and 2.5 µm (E) by tensile tests at 1473 K.
The WC grain size, dWC, and the mean free path of Co phase, λCo, are given by [9] s L WC N N d ⋅= π 4 (1) L Co N f− = 1 λ (2) where NL is the number of the carbides intercepted per unit length, Ns is the number of the carbides included per unit square and f is the volume fraction of carbide.
Values of the WC grain size, the free path of Co phase and the WC contiguity for the cemented carbides.
WC grain size, µm Mean free path of Co phase, nm WC contiguity Fine-grained WC-15 mass. % Co cemented carbide (A) 0.7 55.7 0.51 Coarse-grained WC-15 mass. % Co cemented carbide (B) 5.2 154.4 0.31 Medium-grained WC-10 mass. % Co cemented carbide (C) 1.5 533.4 0.27 Fine-grained WC-5 mass. % Co cemented carbide (D) 0.5 23.4 0.56 Coarse-grained WC-5 mass. % Co cemented carbide (E) 2.5 109.8 0.49 Results and Discussion SEM photograph of A annealed at 1473 K for 1.8 ks are shown in Fig. 1, where the specimens were polished mechanically and then etched with Murakami reagent.
The WC contiguity, which is one of parameters representing the number of the WC grain in direct contact with the WC grain without the Co phase, is given by [10] WCWC CoWC WCWC WC NN N C / / / 2 2 − = (3) where CWC is the WC contiguity, NWC/WC is the number of interfaces between WC grains intercepted per unit length and NWC/Co is the number of interfaces between WC grains and Co phase intercepted per unit length.

The samples were pressed for various numbers of passes up to a maximum of 24 corresponding to a maximum imposed strain of ~24.
The retention of a constant grain size with increasing numbers of passes is consistent with a model for grain refinement in ECAP [19].
The grains were essentially equiaxed after 4 or more passes although a slightly more uniform distribution of the Zn-rich and Al-rich phases was achieved after pressing through large numbers of passes.
N is number of ECAP passes.
Grain refinement in both alloys showed saturation after 4 passes of ECAP. 2.

The varistor voltage increases with increasing number of ZnO grain boundaries between the electrodes.
Therefore, to fabricate varistors with low breakdown voltages, it is necessary to reduce the number of ZnO grain boundaries between the electrodes.
Adding only Ba to Bi-based ZnO varistors promotes grain growth, which enables large ZnO grains to be obtained [2].
This is because compounds containing both Ba and Mn do not form at grain boundaries between ZnO grains.
Excess Zn2+ ions at interstitial sites in ZnO grains have been reported to diffuse from inside the grains to the grain boundaries during annealing at approximately 700 °C [7].

To illustrate the effects, the average grain stack model was introduced.
The breakdown electrical field EB of the varistors is determined by barrier density n per unit length and barrier voltage Vb [11]: EB=n ·Vb (1) where n also presents the average grain number per unit length.
The resistivities and capacitances of grains are much lower than those of grain boundary layers.
The capacitance Cgb of single grain boundary Cgb=εBε0 d 2/t (4) where εB is the relative permittivity of the grain boundary material, ε 0 is vacuum permittivity. d and t are mean grain size and grain boundary thickness, respectively.
The thickness of grain boundary dielectric layer is normally in the range of 10~100nm, while the grain size d is in the magnitude order of µm.

(a) strong axial orientation columnar grain, and (b) equiaxed grain Rolling experiment.
When the cold deformation further increased to 61.86%, a number of fine bending deformation bands formed with the axial of the tube to be 15- 45° in the columnar grain, as shown in Fig. 2e.
Grain boundary strengthening effect depends on the grain boundary misorientation.
When the cold deformation further increased to 61.86%, a number of deformation bands were clearly observed, as shown in Fig.3e and 3f.
Axial Radial Axial Axial Radial Radial There were a large number of grain boundaries in the pure copper with equiaxed grain than columnar grain, leading to a more obviously prevent effect on dislocations during plastic deformation.

This topic was then restudied in USSR just after World War II but it is only after the seventies that a large number of investigations were focused on superplastic metallic alloys.
What is the contribution of grain boundary sliding?
In the case of HPT processing, strain gradient are of course observed from the disk center to the periphery but appropriate strategies for extracting samples have been developed [36] and reasonable homogeneity can be obtained after a large number of turns [37].
This is not straightforward since it requires to follow a large number of cavities and to deal with local strains and not only with macroscopic ones.
Ma, Superplasticity governed by effective grain size and its distribution in fine grained aluminum alloys, Mater.

Fig. 5a-b shows the histograms of the internal stresses σ in grains with simple and complex grain bending for the deformation degree ε = 14 %.
The distribution analysis showed that both at simple and complex grain bending of a grain one sees the inhomogeneous polycrystal grain deformation.
Consequently, the number of more stressed sample sections (σ > 2 GPa) is insignificant.
It should be also pointed out that the total area of the second and the third mode for a grain with complex bend is greater than for a grain with simple bending, i.e. the number of sample sections with the internal stress increasing 2 GPa is greater at complex grain bending than in grains with simple bending.
This is explained by considerable relaxation of the internal stresses in steel caused by appearance at ε > 20% of a great number of microtwin packages in the deformed material.

Meanwhile, the Nd-rich grain boundary phase was precipitated at the grain boundary of the main phase to form a distinct phase separated from the main magnetic phase.
Materials and methods The sintered cylinder of Nd-Fe-B permanent magnet with the serial number of N38SH and the size of Φ10mm×8mm was used in this study.
So the main phase grains are more thoroughly isolated from each other.
In this work, the domain wall pinning effect was enhanced in the two aging treated samples, in which a large number of the thinner and more continuous Nd-rich phase formed along the grain boundaries, so as to significantly increase the intrinsic coercive force. 4.
Acknowledgments This work was supported by the national major special project for the rare earth and rare metallic materials with the project approval document number: (2012) 1743.

If the number of boxes of size d with the same probability is ，for multifractal，the following scaling relationship holds： （3） where f（aij）is a kind of fractal dimension of aij subset．The dependence of f（aij） on aij is multifractal spectrum，which is one way of describing the multifractal structure．It usually is smooth bell or hook shape curve．
A series of samples of TB8 alloy（15.3Mo，2.9Nb，2.9Al，0.08Fe，0.18Si，0.03C，0.011N， 0.10O、0.01H，others Ti）were compressed at strain rates of 0.01 and 1s-1 to the reductions of 40% in height at different temperatures and immediately cooled to room temperature in water．Then solution heat treatment at 850for 20min and quench were conducted．Fig.1 illustrates the binary images transformed from microstructures of TB8 alloy samples deformed at different conditions．It can be seen that the obtained microstructures include two parts：the initial coarse deformed grains and the fine recrystallized grains．The recrystallized grain size（dr），recrystallized grain volume percent（r）and deformed grain size（d）vary with the deformation conditions．
（a） （b） （c） Fig.1 Binary images transformed from microstructures of TB8 alloy （a）t=750，=0.01s-1 （b）t=1100，=0.01s-1 （c）t=750，=1s-1 As we know，at higher strain rate and lower temperature，the dislocation density in deformed microstructure increases and deformation energy stored in deformed alloy increases also．Moreover，finer grains are obtained and the grain boundary area per unit volume increases．It will lead to the nucleation sites in deformed microstructure increase．Therefore，in following solution treatment，the driving force for recrystallization increases and the recrystallized grains nucleate more easily．The recrystallized grain volume percent increases and the recrystallized grain size increases also．The microstructure is fine and uniform．
Table 1 Key parameters of the microstructures and multifractal spectra t=750 =0.01s-1 t=1100 =0.01s-1 t=750 =1s-1 amin 1.857 1.889 1.908 f(amin) 0.067 0.327 0.394 amax 2.646 2.621 2.510 f(amax) 0 0.259 0 Da 0.789 0.732 0.602 Df 0.067 0.068 0.394 dr（mm） 34.7 39.6 43.8 r（%） 15 23 36 d（mm） 124 112.3 103.5 Da reflects state of microstructure distribution and is related to locations of grains．So Da describes the physical properties of fractal structure at different locations．Whereas，parameters such as recrystallized grain size，deformed gain size，recrystallized grain volume percent，are statistical average values of microstructure in the whole domain．They are related to grain numbers throughout the whole domain and independent of grain locations．So such parameters can not serve to describe the local microstructure precisely and roundly．Furthermore，although the size of each grain can be calculated，the distribution state of each grain can not be characterized．However multifractal spectrum
Conclusions On the basis of MATLAB software，recrystallization microstructures of TB8 alloy after hot deformation and solution treatment have been studied with multifractal theory．And multifractal spectra of the microstructure images have been calculated．The studies show that in current scaling range，the recrystallization microstructure of TB8 alloy has the characteristic of multifractal．And the microstructure can be reflected more objectively and accurately by the shape and width of the multifractal spectrum．With increase of the recrystallized grain size and decrease of the deformed grain size，the width of spectrum Da decreases．A narrower spectrum curve indicates a uniform microstructure．Otherwise，with increase of the recrystallized grain volume percent，the range of Df increases．Recrystallized grains are the leading distribution，the microstructure is more uniform and finer．

In this study, a fine nanocrystalline structure with an extremely number of α-Fe grains with the similar size of ~ 30 nm was obtained.
Fe-based nanocrystalline powders particularly usually show low power loss on the high frequency band, higher performance with a thinner layer due to higher saturation magnetization because of a large number of α-Fe grain precipitation as well as higher permeability than those of the conventional materials.
An extremely large number of α-Fe grains with very fine grain size of ~ 25 nm were precipitated in the amorphous matrix.
On the other hand, the crystallization degree became larger as a large number of α-Fe nanocrystalline grains precipitated from amorphous phase with increasing Tq.
It can be observed that these samples have a fine nanocrystalline structure with an extremely number of α-Fe grains with the similar size (28, 27 and 20 nm when Tq is 773, 798 and 823 K, respectively) below 30 nm in general dispersed in amorphous phase.