Search results

The grain size was refined from 800 µm to 15 µm.
The fatigue cracks developed on the surface of the work piece limit the number of corrugation and straightening.
Cyclic extrusion can be used to produce superfine grained metal materials, to locally strengthen a part or to locally refine the grain size in a part.
The as-received Al99.5 has elongated grains (Fig. 3).
The potential applications of cyclic extrusion include producing superfine grained metal materials and locally hardening or grain refining in a part.

Based on reported observations [6-8], massive γ grains are assumed to nucleate heterogeneously at α/α grain boundaries.
The nucleation rate, Js, corresponds to a number of new nuclei per unit time and unit area of grain boundaries.
The volumetric grain density depends therefore on the α grain size, as it is expected for this phase transformation [7].
The grains are assumed to grow as hemispheres.
As growth proceeds, nucleation becomes less effective since the massive grains cover very quickly the grain boundaries.

Fig.1 Schematic diagram of axial ultrasonic honing Fig.2 Velocity diagram of the oilstone Grain distribution on the oilstone surface The distribution of grain is random on the oilstone surface, so the size and distribution situation of grain will directly related the honing force, honing heat and the workpiece surface quality.
According to the volume fraction ω and mean diameter of grain, the average number of abrasive per unit area can be estimated as Eq. 2:
Force model of single grain with ultrasonic vibration In the ultrasonic honing process, the material was enabled to be chip and removed through the interactional function of rowing, ploughing and cutting between grains and workpiece.
Assuming the grain sharp to be cone with a flat-topped (Fig.5), the honing force involved in chip deformation force and friction force of single grain can be represented as Eq. 3:
(11) (a) Schematic of instantaneous velocity of oilstone (b) Schematic of honing force Fig. 6 Schematic diagram of velocity and force in ultrasonic honing process Where, is the total number of dynamic effective grains contacted with the workpiece on the oilstone surface; is the half-angle of honing checker in the ultrasonic honing process; m represents the mass of oilstone and oilstone seat.

The results can be characterized as that a large number of supersaturated Si elements precipitated from α-Al phase during the process of sintering.
From Fig. 5, slight grain growth can be observed in the microstructure of the sintered compact, but the grain sizes still remain nano size.
The amount of segregated primary silicon grains and the grain size increases, but the average grain sizes still keep submicron size (Fig. 5b, c).
The amount of segregated primary silicon grains and the grain size increases, but the average grain sizes are still submicron size.
In the sintering process of alloy power, a large number of supersaturated solidification silicon elements precipitation from α-Al phase distributed homogeneously in matrix

Second, the melting of Cu-Ce intermediate alloy form a large number of small undercooling zones which increases local undercooling and nucleation rate.
At last, the Ce-rich liquid phase is pushed to the grain boundary by the grown crystals and precipitate Ce-rich phase at the boundary of grain in the crystal growth process, which prevents the grain growing by blocking grain boundary migration and diffusion of atoms [7].
Considered the accumulation of Ce in the grain boundary causing the more serious atomic staggered and grain boundary roughening, b¢ phase and grain boundary have been thicken [8].
There are two reasons for this phenomenon. ①a-phase which is corner block-shaped and sheet increased to the stress concentration at the cusp, which makes the strength and ductility decreased. ②Due to the content of RE addition is too large, it produces the large numbers of compounds inclusions which distributed at the grain boundary.
The grain is rounded and small at 0.15% Ce.

Best crystallinity obtained at 1400ºC sintering temperature of 78.18% due to the growing number of crystals formed by the arrangement of atoms in the sample and more regular.
Furthermore perfecting similar crystalline formation, increasing the grain size, and the narrowing of grain boundaries were occurred [12].
Enlarged impact on the grain size reduced electron scattering, which leads to increased electrical conductivity [13] and the growing grains will increase the electrical conductivity due to the increasing number of electrons that can flow more easily pass through the grain boundaries [11].
Enlarged grain size resulted fewer grain boundaries and increased mobility [10, 14].
The ZnAl2O4 phase to electrons scattering centers, separating the crystal grains and grain boundary area becomes large.

Adding rare earth not only reduced the degree of super-cooling, but also increased the number of nucleation and accelerated crystallization rate.
During the solidification of magnesium alloy, Re widely dispersed on the grain boundaries and prevented grain growing and the grain was refined.
As a result, it was difficult for the Mg17Al12 phase to form a network compound on the grain boundaries and the relative number of the phase decreased.
This was because that the network phrase changed into the grainy shape on the grain boundaries and the grain was also refined.
And also, RE refines grain of the alloy.

The number of inclusions has a direct impact on the content of acicular ferrite.
The number of inclusions will be affected by heat input in the austenite grain, the content of acicular ferrite will changed at the same time through two aspects.
On the other hand, due to the increase of heat input, the high temperature station time of liquid metal increased, which made grain size of the weld metal metallurgical structure increase.Grain coarsening will cause the decrease of the number of grain boundaries, making the grain refining strengthening effect is weakened.
Acicular ferrite grain boundary is large angle boundary, when the cleavage fracture occurs.
But acicular ferrite grain boundaries have precipitated carbide which increase the resistance of crack across the grain boundaries, crack extension requires more energy[5].

Kimura, Mechanical properties of ultra fine grained steels, J.
Tsuzaki, On Annealing Mechanisms Operating in Ultra Fine Grained Alloys, in M.
Also, the characteristic length of dislocation becomes grain size dependent in submicrocrystalline/nanocrystalline materials, i.e. the grain size strengthening should vanish as the grain size decreases down to nanoscale [[] E.
This suggests that the grain size strengthening becomes less effective as the grain size decreases to submicron range.
Acknowledgements The financial support received from the Ministry of Education and Science, Russia, under Grant No. 14.575.21.0003 (ID number RFMEFI58414X0003) is gratefully acknowledged.

The layer microstructure is composed of nano-scale grains and some deformation grains, which contains compressive residual stress of -479Mpa.
Especially, a large number of non-equilibrium defects are brought in the surface layer of the materials and the general bulky grains will be refined into nanometer grains [6].
The average grain size is approximately 30nm.
At the depth of about 200μm, the microstructure is elongated band grains with width of 100nm and length of larger than 5μm on metallographic section (Fig.3b), which are nearly mutual parallel within the raw grains.
It is a large number of crystal defects that make them become grain boundaries.