Search results

SEM micrograph unveiled that the Jc was induced significantly by continuity of grain formation via grain size.
Generally micro cracks induced by a large number of low angles grain boundaries (GBs) through in melt-textured samples favored to restrict Jc in bulk materials due to parasitic nucleation via fluctuations of thermodynamic parameters during crystallization process.
Larger grain size manifest the lessen of grain boundaries [2,7].
Larger grain size will enhance the grains continuity, lessen the porosity thus enhance the Jc value.
Significant Orthorhombic tendency and larger grain size revealed by Y123 system decreased the number of grain boundries thus increasing grains connectivity and continuity as verified by SEM.

Table 2(a) Processing parameters of pulsed electron beam welding Number Accelerating voltage (kV) Beam current Peak value (mA) Welding speed (mm/s) 1 60 12 10 2 60 12 6.0 3 60 13 6.0 4 60 12 6.0 5 60 12 6.0 6 60 13 6.0 Table 2(b) Processing parameters of pulsed electron beam welding Number focusing Electric current (MA) Pulse frequency (Hz) Duty cycle 1 560 -- -- 2 560 -- -- 3 560 20 0.8 4 560 80 0.8 5 560 160 0.8 6 560 160 0.4 Test Results and Analysis Effect of Heat Input on Microstructure of Joints.
Under the condition of welding thermal cycle, the grains grow up obviously and produce thick beta columnar grains in the weld. 0 By comparing the grain sizes of Fig. 1 (a), (b), it can be seen that the grain size of the joint of No. 2 specimen is larger than that of No. 1 specimen.
By changing the duty cycle of voltage pulse, it is found that the grain size decreases.
After pulsed electron beam welding, the grain size of the weld is still growing, and the structure is growing beta grain.
The duty cycle of pulse decreases and the grain grows

A JMAK based model was applied to describe the nucleation grain growth process.
(a) Recrystallized fraction and (b) average grain size versus annealing time at 700°C.
After nucleation, each grain grows freely until it meets another growing core [8,13].
Grain growth rate (G) as a function of the annealing time (t).
Acknowledgements The authors acknowledge FAPEMIG (TEC process number APQ-3318-5.07/07) and CNPq (process number 476377/2007-2) for the financial support for this research and the contribution of Usiminas for this publication.

Meanwhile, undissolved ferrite inhibits grain growth.
The number of M-A islands is small after intercritical quenching and tempering, and distribute uniformly like small dots in grains, as shown in Fig.2(a) and Fig.2(b).
The M-A islands (marked by red circles) distribute in the boundary of grains by in the shape of point-liner or gather among the grains.
The M-A island is harder phase, and the larger the number of M-A islands is, the higher the strength of the steel becomes.
After quenching, the grains are very fine.

In grains, deformation twins ({111}/<112> type) were often observed.
Finely dispersed precipitates were observed within grains.
Although the growth of grains took place during aging, fine grains were well retained even after prolonged aging at 320°C.
In-grain precipitates were not essentially observed, but a small number of coarse precipitates existed on grain boundaries, indicated by the arrows in Fig. 5.
A higher dislocation density in the HPT specimens may result in a higher number density of the G.P. zones than those of non-deformed and cold-rolled specimens.

It is evident that in Fig. 3(a) initial grains have been elongated and recrystallization took place at the preexisting grain boundaries.
DRX subgrains were formed along it and marked by number.
It is implied that twinning contributed to recrystallization by expanding DRX grains, that is to say, it helped DRX grains to grow.
While DRX took place along preexisting grain boundaries, DRV carried out in the interior of initial grains.
It helped DRX grains to grow

A stochastic nucleation model is established to calculate the nucleus number as follows: (3) Where N is the nucleus density, ΔT is the undercooling, Ns is the maximum nucleus density, ΔTσ is the standard deviation of the distribution, and ΔTN is the average nucleation undercooling.
The grain growth is based on the KGT equation [18], and the growth speed of the grain tip is described as follows: (5) Where α and β are the coefficients.
Previous studies [11, 15] showed that a concave shaped mushy zone might make the grain convergent, and a convex shaped mushy zone makes the grain divergent.
There are some broken grain in the exhaust edge of the blade.
The grain grew very well in the back side of the blade, and some stray grain nucleate in the listrium of the blade.

Depending on the applications, a number of different steel grades, coupled with proper process routes, can be used.
The focus is mainly placed on the role of prior austenite grain size (PAGS).
In the case of MC-B steel, repeated cycles resulted to be useful to reach the fine grain condition.
This result is not surprising because the model, which takes into account recrystallization and grain coarsening, considers very low interpass time and high strain rates (typical of wire rod rolling), which allow for a higher grain refinement.
Yield strength, which is affected by ferrite grain size, increases with PAGS refining, and yield point elongation (YPE) is clearly more pronounced when grain size gets smaller (Fig. 5).

The top layer composed of columnar grains and a transition layer with equiaxed grains was close to the substrate.
The top layer composed of columnar grains and a transition layer with equiaxed grains was close to the substrate.
It was also observed that the average size of the equiaxed grain was very small and the thickness of the equiaxed grain layer was very thin.
As a result, columnar crystals normally grew to the surface from a relatively small number of crystallization centers.
The top layer composed of columnar grains and a transition layer with equiaxed grains was close to the substrate.

It is found that many recrystallized grains are formed from {111}<123> deformed grains at the beginning of recrystallization.
Recrystallization was observed to be initiated at both grain boundaries and within grains, although it was observed that recrystallized grains initiating within grains grew faster than those formed on grain boundaries.
Half of the recrystallized grains form from {111}<123> deformed grains and a quarter of recrystallized grains also have {111}<123> orientations.
Deformed grain texture Recrystallized grain texture Number Fraction [%] → {111}<123> 7 17.5 → {111}<110> 8 20 → {111}<121> 2 5 {111}<123> → Other Orientations 6 15 {111}<121> → {111}<123> 1 2.5 → {111}<110> 1 2.5 → {111}<121> 1 2.5 → {111}<123> 2 5 {111}<110> → {111}<121> 1 2.5 → {111}<123> 1 2.5 {110}<110> → {111}<110> 1 2.5 → {111}<123> 3 7.5 → {110}<110> 1 2.5 {112}<110> → {112}<110> 1 2.5 Other Orientations → Other Orientations 4 10 In comparison, there was a considerable incubation time before the recrystallization of grains of {111}<110> and {112}<110> orientation occurred.
Recrystallization initiates from {111}<123> deformed grains and grains of orientation other than those within the α-fibre. 2.