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Introduction The grain, grain boundary and unit of ceramic micro-structure are in the level of micro-dimension, because its grain is fined, and the number of grain boundary increases greatly, material strength, hardness and super-plastic may be improved, which has important influence on its properties such as mechanics, electrics, optics, calorifics, magnetics and etc..
When the time of laser thermal action is long, the grain is growed up easily, and caloric dispersion also needs longer time, more longer the time of caloric staying, more bigger the heat-affected zone, its hardness is relatively lower.
By analyzing of coating micro-structures, the strengthening mechanisms involve the following two aspects [5]: (1) The fine-grain strengthening mechanisms of re-melting layer involve two sources: (a) Martensitic phase transformation of Cr-substrate results in fine-grain strengthening, and the substrate is strengthened by fine Al2O3 grains that is get by fast cooling during laser re-melting, and the migration of grain boundary is blocked by Al2O3 grains, which causes the substrate grains fined, it is indicated that martensitic phase transition is produced in the coating during its cooling.
The Al2O3 forms are main branch type in Cr+Al2O3 layer by laser re-melting, and the form of primitive Al2O3 is grain, it is indicated that Al2O3 grains produce dissolution and precipitation, which form network constructure and loading frameworks.
Conclusions (1) The crystal grains of Al2O3 coating by CO2 laser re-melting have been fined greatly, and kept the nano-grains that are distributed in its coating, the micro-hardness of Al2O3 coating by laser re-melting have increased two times than that in primitive status

And the surface morphologies of nanocomposite coating showed better smooth surface, finer grain and more compact microstructure.
The cavitation number in unit time of CU is more than single ultrasound, so standing wave was effectively improved, reducing the dead angle caused by standing wave.
Nanoparticles as large number of crystal nucleus, provided the condition for grain refinement.
The effect of the refining organization grain becomes more obvious, as BU power get higher.
As the probe ultrasonic power is 30W, the microstructure of surface is densification, and the degree of surface grain refinement is high, which is showing in figure 4 (c).

In this study, bottom-up type powder processing and top-down type SPD (severe plastic deformation) approaches were combined in order to achieve both full density and grain refinement of metallic powders with least grain growth, which was considered as a bottle neck of the bottom-up method using the conventional powder metallurgy of compaction and sintering.
The main advantage of SPD processed materials, compared to other nanostructured materials processed by gas condensation or ball milling with subsequent consolidation, is the possibility of overcoming a number of difficulties connected with residual defects and powder contaminations in the compacted samples.
Indeed, grain growth, which was considered as a bottle neck of the bottom-up method using the conventional powder metallurgy of compaction and sintering.
In this study, bottom-up type powder processing and top-down type SPD (severe plastic deformation) approaches were combined in order to achieve both full density and grain refinement of metallic powders with least grain growth.
Figure 4 shows low magnification scanning electron micrographs of longitudinal sections of 200 ℃ equal channel angular pressed Al-20 wt% Si bars after various number of route C passes.

It was found that the SLM fabricated samples consisted of long columnar original β grains together with parallel acicular α' martensitic structure dominated in the β matrix, which was observed in the side view, and fully equaixed β grains pattern in the top view.
SLM technology possesses a number of advantages compared with conventional manufacturing methods, such as near-net-shape production, high buy-to-fly ratio, high flexibility and time-saving for manufacturing.
It can be seen that fully equaixed β grains were obtained during SLM process, accompanied with acicular α' martensites formed in the grains in the top view.
On the contrary, in the side view, long columnar β grains appeared along the building direction and acicular α' martensites were formed inside the grains.
After post heat treatment, more dimples emerged while the cleavage surface disappeared, especially for the samples heat treated at 800℃, a great number of dimples were observed.

In this work the nucleation of the Cube recrystallised grains in AA6111 was investigated.
The grains which are above or below this S grain had similar orientations near to B, and their orientations are shown in pole figure 3.
These B grains were extensively necked.
In Fig. 7 (a) and (b), only part of a grain is shown and this grain has deformation bands having alternating laths sandwiching Cube laths of complementary S orientation as shown in Fig. 7 (c).
Acknowledgment Part of this work was supported by a HKSAR RCG grant number HKU/7342/01E.

The fracture toughness and plasticity of nano-scale ceramics can be greatly improved due to its finer grain and a great deal of grain boundary [1].
Experimental Procedure Besides the matrix Ti(C, N) and the additives nano-scale Al2O3, a small amount of bonding metals such as Ni and Co with a grain size of 2.3um, and added particles MgO, VC and Cr3C2 with a grain size of 2.35um are contained in the compositions.
Table 1 Compositions of nano-Al2O3 dispersed micro-Ti(C, N) matrix cermet tool materials [wt%] Number Micro-Ti(C, N) Ni Mo MgO Cr2C3 Nano-Al2O3 JT8 77.4 7.5 7.5 1.0 0.6 6.0 JT9 71.4 7.5 7.5 1.0 0.6 12.0 JT10 58.4 7.5 7.5 1.0 0.6 25.0 The flexural strength, Vickers hardness and fracture toughness of the new cermet tool materials were tested and compared with the other tool materials.
The fractured surface is characterized by rather finer grains and a great deal of intragranular and intergranular fracture pattern.
The grain size of the new cermet tool materials is decreased with an increase in the content of nano-scale Al2O3 due to the grain fining effect of nano-scale Al2O3 and the fracture pattern is changed from intragranular to intragranular- intergranular and then to intergranular.

CA method is widely used to simulate the transformations controlled by short-range diffusion such as recrystallisation and grain growth.
Characteristics: Neumann neighbourhood, calculations with odd number steps were performed on the basis of Eq.2., and with even number steps by using Eq.2.
Characteristics: Moore neighbourhood, calculations with odd number steps were made on the basis of Eq.1. and with even number steps by using Eq.2.
Barkóczy: Simulation of Grain Coarsening in Two Dimensions by Cellular Automaton, Acta Materialia, (2001), Vol 49/4, pp 623-629
Penelle: "Simulation of Normal Grain Growth by Cellular Automata", Scripta Materialia Vol. 34, No 11. pp. 1679-1683

Here is the velocity of molecule i of the fluid and N is the number of molecules.
The porosity formed by packing of hard spheres is naturally related to the grain (sphere) radius of the packing.
It is easy to see that , where is the grain density in the skeleton (number of grains in unit volume).
The Knudsen number of the system was determined as the ratio of the free path length l to the grain radius of the porous skeleton .
Figure 5 - Self-diffusion coefficient versus Knudsen number (Triangles and circles correspond to and 30).

One approach to control cast microstructure and promote fine, equiaxed grains over coarse, columnar grains is to add extrinsic micron-scale solid particle grain refiners to the melt to enhance heterogeneous nucleation [1,2].
The number of fragmentation events for each time step in Fig. 2 is shown in Fig. 4.
Plot of the number of fragments for each time step in Fig. 2.
Tronche, Grain refinement of aluminium alloys by inoculation, Adv.
Quested, Understanding mechanisms of grain refinement of aluminium alloys by inoculation, Mater.

In these figures globular α grains and eutectic phase which surrounds α-grains are evident.
This is because of low temperature that causes the numbers of unwetted grain boundaries become more than wetted ones, but with increasing temperature more globular grains create.
found that the more the temperature, the more the sphericity and grain size of α-grains.
(a) Effect of temperature on sphericity of grains (solid lines) and average grain size (dash lines).
Effect of rolling strain and holding time at 580 ºC on (a) sphericity of grains and (b) average grain size.