Search results

It has been reported by previous work that ultrafine-grained (UFG) steels with grain sizes smaller than a few micrometers have super mechanical properties such as high strength, enhanced superplasticity [1-3].
The size and number of carbide become smaller and lesser with decreasing the rolling reduction.
There is no formation of ultrafine ferrite grain.
They stated that the yield drop phenomenon was observed due to the ferrite grain size was smaller than 1 µm and the yielding behavior was slightly different if the grain size was larger than 1 µm.
This means that significant strengthening by grain refinement was achieved in the present ultrafine grained steel (Fig. 4c).

Combining with SEM images (Figs. 1 and 2), it is known that as fZ is below 27%, the ceramics is mainly made up of lath-shaped α-Al2O3 grains and rod-shaped grains.
SEM images show that the crack generally propagates along rod-shaped grains as fZ is below 27% (Fig. 5).
Therefore, as the crack-propagating direction is quite different from both the growth orientation of nano-micron fibers and the axial of the rod-shaped grain, the crack is forced to deflect along the rod-shaped grain due to high elastic modulus of the rod-shaped grain and pinning effect of nano-micron monocrystal fibers around the crack tip, and deflection-induced toughening is properly initiated.
However, the ceramics is comprised of lamina-shaped grains as fZ exceeds 27%, resulting in introducing of a large number of interfaces between the alternating Al2O3/ZrO2 layers into the ceramics, so interface debonding is inclined to take place and deflection-induced toughening is weakened.
As for Al2O3/18%, 21% and 24% ZrO2 composites, the matrix comprised of lath-shaped and rod-shaped α-Al2O3 grains, and within rod-shaped grain the nano-micron fibers are oriented in essentially the same direction.

The ferrite grains are elongated to a certain extent along the cold-rolling direction.
It is clear from Fig.5 that the grain size distribution of sample A is much more uniform than that of sample B in which there is a large number of fine grains about 2µm.
While for sample A, since it suffers from CSCR, the average grain size of martensite is close to that of ferrite, and thus there are not a large number of fine grains, which agrees with the observation of SEM shown in Fig. 3a.
Fig.5 Grain size distributions determined by EBSD.
The fine average grain sizes for both samples A and B result from the addition of Nb refining the original austenite grains.

This deformation is caused by a number of creep mechanisms, such as dislocation glide and climb, grain boundary sliding and diffusional processes.
These particles and defects can be depicted by their type, shape, arrangement and number densities [7].
Grain boundaries have a distinct influence on these reacting internal stresses, resulting from the discontinuity of elastic properties across the grain boundary [9].
In this work, a special focus is set on the interaction of grain interior, grain boundaries and triple points.
Adjacent grains are separated by grain boundaries.

Table 3 Change the parameters of one-side welding Serial number Current (A) Voltage (V) Welding Speed（mm/s） Gas Flow （L/min） Root Gap (mm) 1-1 170 22 8 15 1 1-2 190 22 8 15 1 1-3 210 22 8 15 1 2-1 220 22 6 15 1 2-2 220 22 7 15 1 2-3 220 22 8 15 1 3-1 220 22 8 15 0 3-2 220 22 8 15 0 3-3 220 22 8 15 2 Table 4 Change the reverse current Serial number reverse current （A） Positive current（A） Voltage (V) Gas Flow （L/min） Root Gap (mm) welding speed（mm/s） 4-1 160 140 22 15 2 8 4-2 180 4-3 200 Experimental results and analysis The weld penetration is as a finger shown in Fig1.
There are equiaxed grains in the middle of the joint,and columnar crystal near the fusion line as shown in Fig2.The main structure of the joint is α (Al) + β (Al3Mg2), containing a small amount of Mg2Si on the grain boundary.From 1-1, 1-3 and 2-2 can be seen the line of energy will increase, whether increases the current or decreases the welding speed.It keep the molten pool at high temperature for a long time and slow to cool the weld area[2].
The base material has been in the preheating temperature when welding plank opposite,which makes the weld temperature at the peak time is longer, the cooling speed is slow, so the tissue morphology of equiaxed grain.
On the grain size, 4-1 to 4-3 is increasing, the preheating current of sample 4-3 is larger than4-1,so the grain of 4-3 is more bulky than 4-1.
The organization has no obvious change with changing of the root gap, but the grain size has a slight change.

It should be pointed out that there is a theoretical restriction on the possible time-step (Dt) and mesh-size (Dy) combinations giving a Fourier number of Fo in order to achieve a stable solution, i.e., Fo=∆tλΔy2ρCp<0.5 (9) The heat transfer problem can be solved using an explicit finite difference method [5].
The effective austenite grain size for each grain component can be estimated via the law of mixtures using the recrystallized and un-recrystallized grain sizes together with the fraction of each component.
A summary regarding the average grain size for the deformed and recrystallized grains average strain for the deformed grain are listed as well.
It can be seen after pass 4 that the tendency for the presence of partial recrystallization is clear and grain structure is a mixture of deformed and fully recrystallized austenite grains.
Average grain size for deformed grains = 12.990 µm Average grain size for recrystallized grains = 14.45 µm Average grain size = 14.410 µm Average strain for deformed grains = 0.050 Average strain = 0.001 Average grain size for deformed grains = 14.420 µm Average grain size for recrystallized grains = 12.17 µm Average grain size = 13.370 µm Average strain for deformed grains = 0.064 Average strain = 0.034 Average grain size for deformed grains = 13.280 µm Average grain size for recrystallized grains = 8.290 µm Average grain size = 12.510 µm Average strain for deformed grains = 0.085 Average strain = 0.072 Average grain size for deformed grains = 12.470 µm Average grain size for recrystallized grains = 7.650 µm Average grain size = 12.100 µm Average strain for deformed grains = 0.091 Average strain = 0.084 Fig. 5 Predicted Grain size hierarchy for passes 4, 5, 6 and 7 References [1] C.M.

The grain in the heat-affected zone (HAZ) and weld seam of gas-jet-assisted keyhole laser welding was finer, and the number of columnar grains was also significantly reduced.
The grains of the HAZ, which are close to the base metal, are refined due to the rapid heating and cooling process of laser welding.
The main challenge regarding crystalline solidification of Q235 is to deter overgrowth of columnar grains.
As for enhanced laser welding, introducing the assisted gas jet is beneficial for grain nucleation, grain refinement, and reducing the growth rate of columnar grains in that the maximum temperature and high-temperature residence time of the molten pool decreases, and the cooling rate of the weld increases, while the temperature gradient between the solid and liquid phases further decreases.
The grain in the HAZ and weld seam of gas-jet-assisted keyhole laser welding was finer, and the number of columnar grains was also significantly reduced.

However, the exact variations of the hardness with increasing number of passes were not given.
There aren’t any apparent grain boundaries but a large number of black Mg2Si particulates in the annealed Al alloy.
After one ECAE pass, the grains are elongated and broken, and the grain size is remarkably refined.
Previous studies reveal that there are a large number of equiaxed ultra-fine dislocation cells in the stripped grains, and the formation of dislocation cells or subgrains is the main mechanism of sub-micron microstructure development [16-18].
Then the hardness decreases successively with additional numbers of passes.

During storage, a number of physicochemical and physiological changes occur, this is usually termed ageing.
However, the respiration rate of grain is lower than fruits or vegetables, so MAP was not so significantly action for preserving grain fresh.
At maturity, rice grain was random harvested.
After drying, grain was stored at room temperature for 5 months, maintaining about 14.3% of moisture.
According to the results, the big hermetic plastic tent was good for rice grain flavor preserving.

The Fig.1(a) shows that a number of point-like matter are dispersed on the Al substrate,grains were elongated and extend along the rolling direction and show distribution of fibrous or streamline.
Discussion The main reasons of the grain was elongated and extended that endured the tensile stress in the rolling process.
So these grains in form of fibrous or streamline distribute;Besides,the grain shown a prolate state under the influence of compressive stress,while its in form of needle-like or rod-like shape[3]in Fig.1(a)(b).
Fig.3 shows the SEM and EDS of fracture of RE-7075 Al alloy′s hot rolled sheet.The fracture surface mainly appeared candy-like pattern and crystalline,which presented brittle fracture.Due to impurity atoms segregated or melted off and the second phase separated out in the grain boundary which came into being smooth grain surface.The grains started the slip system starting-up when the microstructure bore external force which lead to smooth grain surface,too.
Discussion (1) There are needle-like or rod-like grains in the SL direction and the fibrous or streamlined grains in the LT direction of hot rolled sheet′s microstructure