Search results

The X-ray analysis [2] showed that the value of microstress weakly depends on number of cycles.
It is recognized that main role in fracture process [6] play dislocation structure, grain boundary precipitates and grains of δ-ferrite [7-8].
In some grains the deformation twins are found to be grouped.
However, there are some grains without twins, or their number is too small.
The microstructure of the steel becomes more complicated in grain interfaces.

Austenite grain size does not exceed ASTM number 6.
Austenite grain is refined with increasing bismuth content.
Austenite grain size in lead and bismuth steels is the same.
Presence of bismuth refines the austenite grain of the forged steel even if aluminum content is critical and cannot provide grain size of ASTM number 6 (Table 6).
Austenite grain size is not greater than ASTM number 6.

The size of the lamellae does not change significantly with the increasing number of cycles in AA8006 specimens.
Moreover, in contrast to the AA8006 specimens, lND shows a clear decreasing tendency with the increasing number of cycles.
initial, LM lND (ARB processed, TEM) Alloy lRD lND 2 cycles 3cycles 4 cycles 7 cycles AA8006 19 14 - 0,36 0,36 0,33 AA5754 13 9 0,26 0,21 0,09 - AA8011 - 9 - - - 0.17 Grain subdivision by low-angle boundaries (LAGB) and a progressive conversion of LAGB into high-angle grain boundaries occurs with increasing number of cycles in all alloys (Figs. 6a,b).
Thus, the process of grain subdivision and grain refinement is the most intensive in AA5754 alloy.
On the contrary, the alloys AA8011 and AA5754 exhibit a steady increase in hardness with increasing number of ARB cycles.

In a number of areas of engineering practice, alloying may be unacceptable, for example in medical materials for reasons of biological compatibility, or undesirable, as in nuclear materials due to deterioration in functional properties and the need for long-term tests when putting into operation of new material.
In recent decades, great progress has been made in obtaining materials with ultrafine grain.
In an effort to use grain refinement to increase the yield strength and strength of metal materials, various technologies for producing fine grains of polycrystalline aggregate were developed (see, for example, reviews [1-5]).
This made it possible to obtain metal materials with virtually any grain size up to 100 nm or less.
Conclusion The SLM method makes it possible to form an ultrafine-grained structure.

Hence, the region near grain boundary flows plastically prior to the matrix.
Therefore, the dislocations concentrated near the grain boundary.
The propagation of dislocations to the matrix is easiest when the maximum shear stress in grain is parallel to longitudinal grain boundaries by applying the external stress in the 45° direction of elongated grain structure.
The diminishing of yield strength anisotropy in peak-aging condition could come from increased number of operating slip system near the grain boundary regions with PFZ.
Acknowledgements It is a project supported by natural science foundation of shanxi province China (project number: 2009011028-1; 2011011021-1).

This statistical cut off was set to be one plus the average number of different slip systems active in all grains.
The matrix grains with the highest stored energy are associated with the highest number of nuclei.
We assumed the total number of nuclei Ntot as tot rand GB stab trans N N N N N = + + +
Then the ratio of Fstab to Ftrans was related to the absolute numbers of stable grains nstab and grains with transition bands ntrans, which were derived with the nucleation spectra models described above.
For a use in a space resolved growth model like a cellular automaton absolute numbers have to be predicted.

An analysis of strength and ductility of ultrafine grained Al alloys R.
Over the last two decades a number of secondary processing techniques have emerged, with the primary aim of refining the microstructure to characteristic length scales near or below 1 µm.
This may be due to the more elongated grains and the lower fraction of high angle grain boundaries present in ARB alloys.
Micro-shear bands form during deformation and impinge on grain boundaries.
Effect of grain boundary character on ductility In addition to the important influence of grain size on ductility, the nature of grain boundaries also play a major role.

Minimization of heat-affected zone size in welded ultra-fine grained steel under cooling by liquid nitrogen during laser welding Hideki Hamatani 1, a, Yasunobu Miyazaki1, b, Tadayuki Otani1, c and Shigeru Ohkita1, d Environmental Conscious Ultra-Fine-Grained Steel Consortium of JRCM (The Japan Research and Development Center of Metals) 1 Welding and Joining research center, Nippon Steel Corporation, Futtsu, Chiba, 293-8511, Japan a hamatani@re.nsc.co.jp Keywords: Laser welding, YAG, ultra-fine grained steel, heat-affected zone, softening, and liquid nitrogen Abstract Ultra-fine grained steel (UFGS) with an average grain size of less than 1µm has been developed and is expected to demonstrate superior properties.
The employed UFGS consists of Ferrite and Martensite with a grain size of around 1µm.
For this experiment, the occurrence of porosity was expressed as the number of blowholes/pits to a bead length of 100mm.
The Ferrite grain size at the softest position was almost the same with that at base metal, and microstructures of re-crystallized grain and non re-crystallized grain at both positions were, also, the same.
In the case of CO2 laser welding, when the laser had to pass through a liquid nitrogen layer, larges numbers of pit or blowhole were produced.

The size of pores after the irradiation was between 0.2~1.2 μm, and mostly distributed at 0.3μm ~0.6 μm; The pores at grain boundary of two adjacent grains was less, the pores at grain boundary were distributed by the way of triangle or quadrilateral.
Fission gases and fission fragments were produced during fuel assemblies being irradiated, and the modality and number of pores would change due to fission outcome.
a) D2 b) D4 Fig. 2 SEM micrographs of specimens Specimens after etched grain boundaries were visible, grain size was about 5μm~30μm, grain size greater than 20μm and less than 10μm were few, mostly grain size distributed at 10μm ~15μm, The pores at grain boundary of two adjacent grains was less, the pores shape distribute at grain boundary presented either triangular or square (Fig 3 arrow or circle diagramming), this viewpoint correspondence match with José R.A.
Table 1 Measurement results of pore size Number of fuel rod Number of specimens Biggest diameter（μm） Average diameter（μm） Pore density（106/cm2） 1# D1 3.293 0.662 0.248 D2 2.297 0.628 0.168 D3 1.732 0.569 0.120 2# D4 1.218 0.427 0.379 D5 1.714 0.482 0.360 D6 1.870 0.442 0.337 Pre-irradiated fuel rod - 1.299 0.329 2.425 Conclusion a) There were cracks in fuel pellets, most micro-cracks were trans-granular crack.
The size of pores at grain boundary of two adjacent grains was less, the pores at grain boundary were distributed by the way of triangle or quadrilateral.

Grain Boundary Diffusion.
Last 60 years special attention in material science is paid to grain boundaries.
Certainly, the main effect is connected with GB structure, determined by orientation of one grain to another.
The typical distribution of ln P value is presented at Fig. 3 b (N is number of GB with given value of P, step is equal to 0.5).
For chemical reaction at grain boundary: nAb+mBb=AnBmb the equilibrium constant can be written as : or for dilute solution.