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Ultrafine-grained steel with ferrite grain size less than 4μm, has attracted more and more attention in recent years, for grain refinement is the only way to improve strength and toughness simultaneously [5].
Sampling point and serial number of samples are shown in Fig.1.Hot strip thickness is 2mm, and reduction of each pass from F1 to F6 is 55, 54, 46, 34, 32, 20 %, respectively.
Grain size of the ultrafine-grained steel has been measured and the average grain size is around 3μm.
Large strain accumulation in austenite comes from the large plastic deformation of the stock in the unrecrystallized region, which increases the effective austenite grain boundary area per unit volume and the number of nuclei per unit area of effective austenite grain boundary.
That not only accelerates the nucleation of ferrite grains, but also postpones the growth of ferrite grains, which lead to ferrite grain refinement.

The HPT processing was conducted at RT under quasi-constrained conditions [3] under a pressure, P, of 6.0 GPa and a rotational speed of 1 rpm and through numbers of revolutions, N, up to 5 turns.
The initial microstructure of the as-extruded material before HPT consists of a bi-modal grain distribution with a fraction of >50% of coarse grains having ~25 µm surrounded by several finer grains having ~4-5 µm as shown in Fig. 1(a).
Thus, the evolution in grain structure depends on the initial grain size, the size of the new grains and the amount of imposed strain within magnesium alloys.
Fig. 4 Variation in strain-rate sensitivity for the Zn-Al alloy before and after HPT for increasing numbers of turns [13].
By contrast, the inset shows the HEA exhibited significant increase in hardness after HPT with increasing numbers of turns.

Compared with pure aluminum without any inoculant, although the macrostructure of the alloy only with 0.2%Sc can obtain slightly refined, the refining effect is not ideal due to the existence of large number of coarse columnar crystal.
Tnsile strength Rm /MPa Prcentage elongation A/% Hardness /HV 1# 43.53 42.30 22.40 2# 61.79 39.90 33.93 3# 51.13 40.07 31.66 4# 68.83 46.76 28.03 The grain refinement of aluminum alloys mainly depends on the number of the nucleus during nucleation and the speed of crystal growth in solidification process.
By analysis that the addition of Sc elements can increase the composition undercooking of the alloy and reduces the nucleation temperature in the center region of the melt to some extent, which prompts the nucleation rate increase and makes the number of equiaxed grains increased slightly in the center area.
So the grains of the alloy with 0.2%Sc can achieve a little refined, but the refinement effect is not desirable for the existence of a large number of columnar crystals.
Grain Refinement of Aluminium.

In other words: cyclic stress and number of cycles together determine whether a PSB is formed or not
The resulting grain size was approx. 60 ± 10 mm.
The resulting grain size was indeed 60 ± 10 mm.
The number of cycles was 1.59 x 1010 (same specimen as used in [4]).
For stress/plastic strain amplitudes far below the conventional PSB threshold of 63.0 MPa and 6.2 x 10-6, such as Ds/2 = 36 MPa and Depl/2 ≈ 3.0 x 10-6, only few grains exhibit a changed dislocation structure, i.e. small loosely connected dislocation patches (not shown in this paper), whereby the number of grains decreases the lower the amplitudes are.

This temperature range is also where the number of carbide particles per unit area at grain boundaries reaches its maximum.
The high number of particles per unit area increases the rate of crack initiation at grain boundaries under rapid loading; linking of microcracks along grain boundaries which are already weakened by impurity segregation results in TE and intergranular fracture. 1.
Fig. 1c shows numerous small carbide particles, which have reprecipitated along grain and lath boundaries at 520 °C; the number of these particles per unit area is greatest for this tempering temperature.
The high number of particles means that there are many microcracks, and these microcracks can become linked during further loading along the grain boundaries, which are already weakened by the impurity segregation.
In the temperature range where temper embrittlement occurs, the number of carbide particles per unit area reaches a maximum

The borderline of the minerals (grains) is obviously, the shapes of mineral grains are most of ellipse, but abnormity, the size of the mineral grains are different.
And some small cracks in grains gradually disappear.
And in this phase, cracks basically develop along the mineral grains border.
The number of cracks determined by length is shown in Table 2.
We call Mn as ratio of cracks number.

Based on the radiographs, the variations of equiaxed grain number and dendrite growth rate with time were measured and analyzed.
The dynamic processes of grain nucleation, free growth, grain interaction and impingement are of great importance to further understand the grain refining mechanism.
Fig. 4a shows the number of visible grains in the field of view (6 × 8 mm2) as a function of time.
It can be clearly seen that for the sample without the application of DC, the grain number increases gradually with time in the beginning and reaches a plateau (18) about 385 s later, while with DC treatment, the grains nucleate much later (about 850 s) compared to the condition without DC treatment, which indicates longer incubation time, and the grains number increases sharply to reach a plateau (163) only 175 s later.
Fig. 4 Time evolution of grain number and dendritic growth rate without and with the application of sequence DC. t = 0 reing is applied. presents the time when cool Fig. 5 shows an image of dendrite fragmentation, as denoted by a red square in Fig. 3c.

A further increase in hardness can be achieved by (3) taking advantage of spinodal decomposition for HPTed Al-Li-Cu alloy, in which nano-scale precipitates of δ’ phase are successfully formed within ultrafine grains, irrespective of the higher number density of grain boundaries.
Introduction In general, strengthening mechanisms of aluminum alloys include strain hardening, hardening by grain refinement (e.g. ultrafine-grained hardening), solid-solution hardening and precipitation hardening [1].
Fig.1 illustrates equivalent strain dependence of both the attained hardness and increment/decrement in hardness during aging (i.e. age-hardenability) for a number of age-hardenable aluminum alloys [3, 4].
The encircled numbers represent the improvement of hardness by our proposed three strategies to achieve concurrent strengthening.
The corresponding TEM microstructures reveal that Cu containing precipitates such as Q’ and Q phases are newly formed, resulting in the increased number density (thus volume fraction) of transgranular precipitates.

To determine the rate of inelastic deformation elastoviscoplastic model of polycrystalline metals is used (15): is associated with implicit internal variables of mesolevel characterizing dislocation sliding – shear rate on slip systems , (K is the number of slip systems for the lattice type considered); tensor о described the actual orientation for crystallographic coordinate system of grain relative to the fixed laboratory coordinate system.
To describe the grain boundary sliding intergranular slip systems are introduced in analogy to intragranular dislocation sliding: boundaries are approximated by flat facets, two independent slip systems of grain boundary sliding are introduced for each of them (the double number of slip systems is used), elastoviscoplastic relation with regard to thermally activated motion of grain boundary dislocations describes shears: , (2) where is the (intergranular) grain boundary shear rate under shear stress equal to critical shear stress , Ugb is the energy barrier (for grain boundary shear), is Boltzmann constant, θ is a temperature, H(·) is Heaviside function, is total slip systems of grain boundary sliding.
On the one hand, grains begin to strike on each other under grain boundary sliding realizing that leads to increasing of critical stress.
In the second case more equiaxed grain structure and a grain boundary sliding is facilitated.
Further development of the model is seen in connecting parameters of grain boundary hardening with grain morphology characteristics.

“P2” is expressed the occupation of the mass, which particle size is less than 5mm.To study the influence of fine particle content P2 on creep property, prepare five groups of samples with fine particle contents of 10%, 20%, 30% ,40% and 50% respectively to correspond to sample number G1, G2, G3, G4 and G5.
The number G0 represents the grain composition of the original soil sample.
Coarse and fine grains are evenly distributed and coarse grains form skeleton with 30% fine grain composition, contributing to high shear strength and small creep deformation.
Coarse grains cannot contact very well with fine grains with more fine grains, and more time is required to stable creep with small soil rigidity and α.
Coarse and fine grains are evenly distributed and coarse grains form skeleton with 30% fine grain composition, contributing to stable creep deformation in a short time and large α.