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An appropriate deletion criteria was used to attain minimum energy grain boundary.
Mis-oreintation angle Cell size (Å) Number of atoms W (Å) Crack size (2a) (Å) 1. 44.8° 145x340x121 250148 120 50 3.
In the absence of crack, the grain boundaries are comparatively at higher stress than the crack tip.
Roles of grain boundaries in improving fracture toughness of ultrafine-grained metals.
Blunting of a brittle crack at grain boundaries: An atomistic study in BCC iron.

Due to the increased complexity of steel microstructures, when considering the application of available Hall-Petch type equations for yield strength prediction, a number of difficulties raises.
FATT which is the temperature at which the surface of a Charpy test piece has a 50% ductile appearance has been expressed for ferrite-pearlite through an equation that takes into account a number of contributions [[] K.J.
However, for more complex microstructures, this formulation faces a number of problems addressed by the authors recently [[] A.
Optical grain size was determined by MLI on specimens etched with Nital 2%.
Conclusions -The grain size determined by EBSD depends on the tolerance angle applied for grain boundary definition.

Processing by HPT is proven to improve the mechanical properties and microstructural behavior of materials by refining the grain size to ultrafine grains [11-15].
This processing route is proven to cause equiaxed ultrafine grains having high angle grain boundaries [23].
High-angle grain boundaries denote misorientations across the interface of 15° or more and the low-angle grain boundaries are defined as measurements having misorientations between 2° to 15°.
The average grain size of the Al-7075 disk processed by HPT through a total number of 5 turns was ~ 250 nm in the center and ~ 500 nm around the peripheral regions as reported earlier [26].
These curves are typical of ultrafine grained materials at high temperatures.

The higher hardness was attributed to fine-grained microstructures in the as-rolled CSP strips.
As CSP process has a number of its own distinct features and its products have a much difference between conventional continuous casting process products.
TEM analyses of carbide precipitation show that a large number of small carbide precipitates are distributed in the tempered specimens, as illustrated in Fig. 4.
However, for the specimen produced by CSP, carbides were precipitated not only in the matrix, but also along grain boundaries and sub-grain boundaries because it has much more boundaries owing to its ultra-fine grains.
Therefore, CSP products have more fine-grained and uniform microstructures compared with conventional process [2-5].

Fig.1 Schematic precipitate microstructure in Al-Li alloys Schematic precipitate microstructure in Al-Li alloys shows in Fig. 1[4],main precipitates include δ’ (Al3Li), β’, θ’ (Al2Cu), T1 (Al2CuLi), S’ (Al2CuMg), Al20Cu2Mn3, etc..One of the effective method of Improve Al-Li alloys’ properties is regulating the corresponding microstructure by multi-micro-alloying.A large number of researches have shown that trace amount of certain elements to add will significantly affect the microstructure and mechanical properties of Al-Li alloy, For example, changing the size,shape,distribution of the precipitated phase and the volume fraction or generating new strengthening phase,and refining grains, controlling recrystallization and grain orientation,etc. [5,6].
Fig. 2(a) shows 2A97T3 Al-Li alloy base metal microstructure, the major phase is α-Al, and the precipitates distribute and in the grains and grain boundaries.The heat affected zone(HAZ) is presented in Figure 2(b).Comparing the grain size between Fig. 2(a) and Fig.2(b),it shows that the grain size of HAZ is slight larger than of base metal(BM).
From the HAZ to BM,grain size shows no obvious difference,and the HAZ is rather narrow.
in this area,as well as the high surface activity of lithium,making nucleation rate very high, namely a large number of fine spherical equiaxed grains are formed in EQZ[8].Specifically,on the one hand, those suspended particles can reduce the wetting angleθ,on the other hand,the high surface activity of lithium may lower the surface tension coefficient σ,subsequently reduce the energy required for non-spontaneous nucleus,EQZ are developed in the end.
Fig.4 Microstructure of FZ（×500） (a) non-dendritic equiaxed grain zone（b）columnar crystal zone （c）columnar dendrite zone(d)equiaxed dendrite zone Fig.5 Schematic of crystallization of the welding joint A narrow band of EQZ which consists of spherical equiaxed grains is located between the partial melted zone (PMZ) and FZ,as shown in Fig. 4(a).

The microstructure is homogeneous, the average grain size of the matrix was ~1.2 mm, the average size of the second phases or precipitates which homogeneously dispersed within grains was <50 nm, and the size of the precipitates observed at grain boundaries was ~0.25 mm.
（1） where n is the total number of valid fatigue tests, m is the number of stress levels, is the maximum stress at i level, is test number at i stress level.
Effect of fine-grain strengthening.
It is well known that grain boundaries can strengthen the metallic materials, and the yield strength of an alloy with fine grain size is higher than that with coarse-grain structure.
Crack can propagate more easily through the coarse-grained structure, so the fine-grained alloys have higher fatigue limit.

Usually these shape factors are dimensionless quantity, independent from the size of the grains and their position if the object contains a sufficient number of pixels [1].
Since during termomechanical rolling the ferrite grains form from rolled austenite grains it is expected that more of these ferrite grains more will have an orientation close to the rolling direction than the ferrite grains in the normalising rolled steels.
First the number of the intersections of the studied structure and the parallel test lines having different angles (Θ) has to be determined.
The number of the intersections as a function of the Θ angle gives a rose diagram.
In the picture numbers the 0 means the smallest while 5 the largest orientation of the pearlite.

The review stresses concepts rather than voluminous facts and a number of theories.
Liquid precipitates form within grains and along grain boundaries during heat treatment.
New forms of grains appear at grain interior while new grains form at grain boundaries.
Brittleness stems from a fairly high ordering energy of the compound, which does not permit enough number of independent active slip systems to operate to fulfill the compatibility requirement of grain boundaries of B2-type crystal structure.
It also leads to reduction of electrical conductivity by reduction of ferrous ion numbers at the octahedral sites.

Due to the difference in the number of nucleation sites, martensitic transformation is greatly promoted in cold-drawn specimen rather than cold-rolled one. 1.
The mean austenite grain size was approximately 60 μm.
As a result, the austenite grains observed in the DD section keep an equiaxial shape.
This suggests that deformation twinning in multiple directions provides a number of nucleation sites for deformation-induced martensite over a whole grain, and this leads the promotion of martensitic transformation in cold-drawing rather than in cold-rolling.
Since a large number of nucleation sites are prepared for deformation-induced martensite by intersecting twin boundaries, martensitic transformation is promoted in cold-drawing rather than cold-rolling

One option to produce an ultrafine grained microstructure is severe plastic deformation (SPD).
This is a top down method which begins usually with an initial coarse grained (CG) microstructure and leads to a break up of former grain boundaries and a production of very small new grains with high angle grain boundaries.
The softening annealed state offers spheroidal carbides in a homogeneous coarse grained microstructure with well-defined grain boundaries (Figure 2 a)).
After 6 rotations the grain boundaries have broken up and some new grain boundaries are visible in Figure 2 b).
It is obvious that the hardness increases with the number of rotations for both carbide morphologies.