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Among them, an increasing number of researchers have claimed that the alloys with retrogression and re-aging (RRA) treatments possess the lowest fatigue crack propagation rates for cuttable matrix precipitates, discontinuously distributed grain boundary precipitates with small size and relatively narrow precipitate free zones [11,12].
With the increase of cycle number, the gap shows an incremental trend and the gap values are 0.99 mm, 1.08 mm, 1.29 mm and 1.65 mm at the cycle numbers of 35000, 46000, 58000 and 65000.
This indicates that under a same cycle number, the LZ alloy possesses a larger crack length augment than the HZ alloy and this condition becomes obvious with the increase of cycle number.
Fig. 7 (a-d) exhibits the bright field TEM images of matrix precipitates and grain boundary precipitates.
(a,d) Matrix precipitates, (b,e) Grain boundary precipitates and (c,f) Matrix precipitate size distribution.

The average WC grain size of ultrafine WC-Co cemented carbides are 0.2~0.5μm [1], they have both high hardness and high toughness and are widely applied into the metal cutting field of difficult-to-cutting materials, they have great market demand [2].
A large number of studies showed that, in addition to the component itself, the microstructure of cemented carbide played a important role on its strength, hardness and tenacity [5].
The average grain size of WC was measured by line-intercepted method on SEM micrographs shown in Fig.1, and calculation result was that WC average size of YG10UF-P was 0.50 μm and YG10UF-D was 0.45μm.
The SEM micrograph of YG10UF-P in Fig.1(a) shows that WC grain size is relatively homogeneous, adjacency of WC grains is uniform, and there are little Co phase gathered phenomenon between WC grains, most of the WC grains are polygon and triangular shape, grains surface was smoother, grains edges are more rounded.
But the SEM micrograph of YG10UF-D in Fig. 1(b) shows that WC grains appear obvious uneven growth phenomenon, some of the WC grain directly adjacent and some distance large, grain shapes irregular, edges sharp, grain surface roughness, and there are more serious Co phase gathered phenomenon between WC grains.

A part of β phases dissolve in the α matrix, there are some β phases existing in grain boundary yet.
Fig.2 (c) shows that the grain boundary deepens, but when the temperature surpassed eutectic temperature, the magnesium alloy is overheatning and then forms big black dot MgO2 which distributes in grain boundary and grain.
The aluminum atoms spread to grain boundary because there are more defects in the grain boundary than that in the grain.
The β phases are hard and distribute in the grain boundary, which can block the slide of grain boundary.
Research and development of the as-cast grain refinement technology of magnesium alloys.

The Relative-Grain-Size model (D0-Zc and Drex-Z) adequately predicts the type of hot flow behavior before reaching a stable dynamically recrystallized grain size.
More recently Montheillet and Jonas [5] re-examined experimental evidence and have proposed the Relative-Grain-Size model to predict (1) the conditions of single peak DRX and grain refinement, (2) multiple peak DRX and grain refinement and (3) multiple peak DRX and grain coarsening.
An initial grain size will tend vertically to Drex-Z.
A high grain boundary area on the newer smaller grains increases the nucleation rate locally, but the nucleation rate for the remaining initial grain size is slower, thus two behaviors need to be considered.
Figure 4 shows the values obtained for σr, s and v after performing a number of iterations.

The more the impurity content is, the more crystallization nucleus are, and thus the more grains are formed and the smaller the grains are.
Grains interlace as branches, namely dendrite.
The cooling velocity is slow, coagulation time is long, after crystallization, the amount of the grains is small and the grains are thick and big[6].
Compare the average grain size of brass strip and pure copper guide line under the same current: according to Table 3, under the same current conditions, because No. 1 material consists of a large number of Zn of which the melting point is low and ionic bond is formed due to the existence of trace elements such as the chloride, therefore, the melting point is low and the grain size is much smaller than that of the pure copper guide line.
Average grain size of the same material under different current: the bigger the current is, the bigger the average grain size is, and the thicker the grain boundary is.

In order to gain an optimum balance between strength, fatigue and creep more recent near-α alloys have been developed to be heat treated just below the β transus to retain a small amount of αp limiting β grain growth, which upon cooling produces a bimodal microstructure of αp in a matrix of fine grained α' [1].
The as-received microstructure consisted of approximately 70% equiaxed αp grains with an average grain size of 30µm, in a matrix of coarse α' lamellar.
Examination of the orientation image maps revealed clustering of similar orienatations in the αp over areas of at least 250µm, which consequently reduced the effective number of orientations analysed in the datasets minimising the likelihood of seeing a fibre texture.
It was assumed that the largest grains observed in the orientation image maps were αp and all small grains were associated with the α' lamellar.
Grain boundaries were defined as any misorientation between neighbouring regions greater than 2.5°, which enabled the area of each grain to be defined and ranked in descending size order.

The mixture for creating three samples 40 × 40 × 160 mm was created from 450 g of fine grained granulated blast furnace slag Štramberk 380 (specific surface area 380 m2/kg), 180 g of sodium silicate (water glass) with modulus 1.6, silica sand with three different grain size 450 g from each grain size and 95 g of water.
Results To evaluate the origin of micro cracks during stress, we focused on the activity of AE, respectively on the most used parameter which is the number of signals overshooting a preset threshold.
Fig. 1 Dependence of the number of AE overshoots and relative loss in weight versus time for specimen 0/w In Fig. 3 we can observe the modification of the dominant frequency during 432 hours and 480 hours from mixing.
Fig. 2 Dependence of number of AE overshoots and relative loss in weight versus time for specimen 2/w Fig. 3 Change of dominant frequency over time for both specimen In Fig. 4 we can see results of ultrasound testing when we monitor the modification of the transit time during 432 hours and 480 hours from mixing.
The rate of moisture release is in good accordance with the number of signals detected by the AE method.

The superior mechanical properties were achieved in the bottom region, attributed to the grain refinement because of high cooling rate.
The overall grain size of the bottom, middle, and top samples is evaluated as 3.39 ± 1.9 µm, 5.32 ± 1.2 µm, and 7.35 ± 1.35 µm.
The increase in build height leads to a corresponding increase in the total grain size.
This may be attributed to the decrease in the cooling rate caused by the rise in inter-layer temperature (with the increase in the layer number) during the deposition process.
Rao, Influence of heat input on the evolution of δ-ferrite grain morphology of SS308L fabricated using WAAM-CMT, Mater.

Zone І which is close to the substrate contains columnar crystal with obvious straight boundary among the grains, and zone Ⅱ which is far from the substrate is composed of equiaxed grains.
Again, it can be seen that the columnar grains in zone І grow up at a certain angle to the horizontal plane.
It can be seen that the grains with a low size-uniformity ranged from 200nm to 500nm distribute on the surface random, and there are a 50μm Ⅰ Ⅱ Fig.4 Cross-section micrograph of as-deposited sample Fig. 3 XRD patterns of Ti-Al samples deposited with different target-substrate distance small number of micropores with a macro gaps along the grain boundaries on the surface which mean a lower density.
There are many slender columnar grains distributed in each island and the average size of the grains is about 300nm.
The main cause to lead to such a small grain size is due to the high cooling velocity of EB-PVD technology.

Press the block lightly to divide the ceramic microspheres to single grains.
The sintered body are composed of glass phase, micro pores and ultra-fine grains which dimension are less than 2µm.
The peak seat of α- Al2O3 (standard card number: 10-173): 2.09, 2.55, 1.60, 3.48, 2.38, 1.74, 1.40, 1.37Å, marked with • in the figure.
And the peak seat of mullite (3Al2O3•2SiO2, standard card number: 15-776): 3.39, 3.43, 2.21, 5.39, 2.89, 2.69, 2.54, 2.42, 2.29, 2.12, 1.84Å, marked with ∆ in the figure.
(3) The sintered body was composed of glass phase, micro pores and ultra-fine grains with dimension less than 2µm.