Search results

Since superplasticity requires a very small grain size, typically <10 µm, it is feasible to process HEAs using severe plastic deformation in order to introduce significant grain refinement.
Furthermore, when the number of alloying elements increases beyond five, the contribution of the configurational entropy to the total free energy becomes sufficiently significant that it can overcome the enthalpies of compound formation and phase separation and thereby stabilize the solid solution state relative to any multi-phase microstructure [1-3].
It is reasonable to anticipate that there may be an opportunity to achieve a combination of high solid solution strengthening and good ductility if the solid solution phase possesses a simple crystal structure, such as a face-centred cubic (fcc) lattice, where there will be a large number of active slip systems [1-4].
Thermo-mechanical processing is generally employed in industry to achieve the requisite small grain sizes but this processing is capable only of producing grain sizes of the order of a few micrometers and it is not possible to achieve exceptional grain refinement into the submicrometer and nanometer range.
Experiments show that HPT has an advantage over ECAP because it produces both smaller grain sizes [12] and a higher fraction of grain boundaries having high angles of misorientation [13].

Then the alloy was annealed in order to obtain samples of a given grain size.
An average grain size is about 100 nm (Fig. 1).
At higher magnifications one could see that the alloy was not fully recrystallized and there was quite a number of defects inside the grains.
Also quite a number of low angle boundaries was found inside some grains.
The average grain size was 100 nm.

A significant number of experimental investigations have now been performed on both columnar and equiaxed growth using a number of different alloy systems [4–6].
Currently, a number of different microgravity platforms are available, and have been used, for solidification research, e.g.
The numbered dots correspond to g-level measurements in Fig. 3.
Numbered dots correspond to the image stills in Fig. 2.
Fig. 3 shows the g-level and temperature measurements recorded during solidification, along with the time-axis locations of the images in Fig. 2 correlated by the numbered dots.

Therefore the used definition “the dynamic superplasticity” reflects consecutive change of states which happens in material with initial varying grain size structure under the changing temperature-rate conditions: initial varying grain size → equiaxed fine-grained structure (4…7 microns) formed in the temperature-rate conditions of superplasticity → coarse-grained at further increase in strain rate.
Formation of fine-grained structure and its dependence on temperature and strain rate in the range of phase transitions, create conditions for realization of the mechanism of grain boundary slipping, characteristic for superplasticity.
Statement of the specified task is based on a large number of the experimental data generalized in [1] and relating to alloys as: AMg5 (AlMg5, 5056), 1561, D18T (2117), 1980, B95 (AlZnMgCu1.5, 7075), AK4 (2618), AK6 (AlCuMg0.5, 2117), AK8 (AlCuSiMn 2014).
Valiev, Grain Boundaries and Properties of Metals, Metallurgiya, Moscow, 1987
Lyashenko, A model of grain boundary sliding during deformation, Technical Physics Letters. 38 (11) (2012) 972–974

In the case of the Zr containing alloys, a very fine columnar grain structure was found over the entire surface, independent of the subsurface grain structure.
In this large Peclet number regime the front advances near the point of absolute stability, above which a planar solidification front is seen [5].
It is likely that the columnar grain growth is selective, with those 'seed' grains that have their fast <100> crystallographic direction well aligned to the maximum thermal gradient outgrowing less well aligned grains.
They mimick the grain size of the unmelted substrate, leading to fine columnar grains forming over FSW nugget regions and coarse grained layers over the parent plate, TMAZ and HAZ.
In Zr containing alloys (2096 and 7150), fine columnar grains were observed, irrespective of the substrate grain structure.

Introduction Wrought aluminium alloys have a number of desirable properties such as low density, high specific strength and excellent formability that make them particularly attractive in various applications such as aviation and automotive [1].
In the chemical refinement, chemical elements such as Ti and Sc are added as grain refiners to Al alloys, reducing the grain size [3].
Grain sizes measured using the linear intercept method (ASTME112-10) [15] and the average grain size of 7 samples reported.
In the case of the ultrasonic agitation, the grains are finer at the centre of the ingot, whilst the grains are larger towards the edge.
Therefore, the grains do not refine uniformly.

Hence, microcracks were evaluated regarding their propagation capabilities according to grain orientation and barrier function of grain boundaries.
Coarse-grain annealing resulted in an average grain size of ~ 410 mm in contrast to the as-received condition with an average grain size less than ~ 120 mm.
In order to reach very high numbers of cycles and hence very low crack growth rates, an ultrasonic fatigue testing system was used.
In case of the coarse-grained condition of Ni201 crack started from a grain boundary triple junction (Fig. 2b).
Red ellipses denote the crack initiation grain boundaries.

The increasing versatility of testing machines, like dilatometry with easily variable temperatures, in addition to the growing expenses that go along with increasing the number of experiments for high cost materials, leads to the question whether performing all those experiments is really justified.
Reducing the number of experiments by 50 % during materials characterization therefore appears feasible.
The grain sizes after cooling are measured metallographically.
Similar spacing is used for the matrices covering grain growth, srx kinetics, grain size after drx and srx respectively.
While in classical models grain growth can only start after full srx, Fig. 8 shows that grain growth starts while srx is still in progress.

China Keywords: Titanium Silicon Carbide, spark plasma sintering, fine grain, layered structure Abstract.
Fine Plate-like grains of the samples with sizes of 2~6µm could be identified by scanning electronic microscope (SEM).
Since the Ti3SiC2 phase was first identified by Jeitschko and Nowotny [1] in 1967, a large number of works on the synthesis of this material have been reported [2-8].
Ti3SiC2 phase with plate-like grains can be observed around the dark speckles.
Fine grain polycrystalline Ti3SiC2 with a purity over 98vol.% can be synthesised by SPS sintering at temperature range of 1100~1300 o C for 5min.

However, there are few reports [12] concerning about the fatigue property of ultrafine-grained steel.
Fig.5 shows relationship between the load ratio of (applied stress)/(tensile strength) on fatigue tests and number of cycles.
It is found that the stronger filamentary microstructures of the ferrite phase and the pearlite phase are developed with increasing number of passes of the pure shear deformation.
From the results of TEM observation, the ferrite phase with a grain size of 1µm is observed in the microstructure of all carbon steels after heat treatment and the grain size becomes larger with decreasing Vickers hardness.
A grain size of the ferrite of the pearlite phase is sub-micrometer due to the presence of the ultrafine spherulitic cementite.