Search results

This work is funded by the National Science Foundation under Grant Number DMR-0517351.
Titanium alloys have a number of properties that make them desirable for many applications.
Large-grained alloys (>200 µm) show extensive slip and twinning, while small-grained alloys (<100 µm) deform solely by slip.
%V alloy at 95% YS as a function of grain size.
It has been found that a number of factors affect twinning during low temperature creep.

However, even for this material, significant changes of the porosity and grain structure are observed, where original grains of several µm diameter are divided into smaller grains of around 0.1 to 0.3 µm.
Pati of Combustion Engineering (C-E) and from Windscale hot laboratory on KWU fuels, observing significant number of µm size porosities at pellet rim [4].
Here "grain sub-division" is defined by observed fine grain formation and the definition does not include bubble formation nor swelling.
The sizes of the sub-divided grains are 150 to 200 nm which are the size range of the grains at the Cauliflower structure.
Number of cycles could be order of 104 to develop microstructures.

A considerable number of dislocations exist in the grains and grain boundaries, which may accommodate a high density of strain energy.
The grain size distribution inserted was measured from a number of bright field and dark field TEM images by number-averaging the diameters of 200 grains.
The grain size ranges from 15 nm to 300 nm, and the average grain size is about 100 nm.
The SMAT sample shows smaller value as compared with coarse-grained one.
Figure 4 is the worn surface morphologies of SMAT Ti and coarse-grained counterpart.

In many cases there are deep incursions of the secondary grain into the primary grain structure behind the macro-boundary.
In the vast majority of cases, no special character was evident for grain boundaries around the periphery of the secondary grains.
The secondary grains are marked with ‘S’.
The small triangular grains appear to be separated from the secondary grain but with a very acute junction.
The strongest selectivity of the Goss orientation will occur when the pinning strength is high since this will increase the number of neighbouring grains that must cooperate.

However, the prior austenite grain size reduced with increasing the cooling rate.
In fact, the microstructure is not stable and may degenerate during the weld processing, especially in the coarse grain heat affected zone (CGHAZ).
The prior austenite grain size was measured by Digital Micrograph Software.
a b c Fig. 3 Optical microstructures of testing steel formed via various cooling rates applied, (a) 0.02˚C/s, (b) 7.5˚C/s, (c) 60˚C/s A mixture of tempered martensite with different amounts of δ-ferrite was obtained in all conditions, and the prior austenite grain size at low cooling rate is extremely uniform, and display a very irregular shape, the average austenite grain size is large, large grains were also mixed with small grains.
On the other hand, martensite lath had been coarse greatly, and δ-ferrite of slender strip was observed, a large number of precipitate particles inside the δ-ferrite were also seen.

There are some recrystallized grains mixed in coarse deformed grains.
At 400°C, a large number of small recrystal grains generate in deformation structure, most of them are round and of uniform distribution.
The average grain size increases as strain rate bigger than 1s-1, at higher strain rate, the number of recrystallization grains is smaller for some deformed grains have not triggered recrystallization.
Thus increasing the number of dislocation sources per unit volume in a grain, aggravating the trend of dislocation tangle simultaneously.
The growth of recrystallization grains is finished by combining the grains.

Fibrous tissue is caused by severe plastic deformation (SPD), long strip grains tell us that regional plastic deformation occurs, but a large number of equiaxed grains are still located in the inner of specimen mainly, which is in stage of rapid elastic deformation yet.
We can see that a large number of small grains appear nearby fibrous tissue, which should be recrystallization structure.
The large number of long strip grains in Figure 5(d) confirms that the second outer layer of rotating band is still subject to large compressive stress.
(a) Long strip grain in second outer layer of specimen (b) Equiaxed grain in inner layer of specimen Figure 6.
It was reflected as macroscopic plastic deformation and increasing of hardness, while in the micro scale there were a large number of dislocation and dislocation tangles.

The change of the damping properties of graphitic steel caused by thermomagnetic treatment is relatively small, due to a large number of diamagnetic inclusions of graphite and quite fine-grained ferrite matrix.
This is explained by high density of diamagnetic graphite inclusions in a relatively fine-grained ferrite matrix (the average grain diameter equals to about 20 μm).
The sources of enrichment can be, firstly, segregation of Cr atoms at the grain boundaries, which are diffused throughout the grain body under thermomagnetic treatment, which is most likely in the fine-grained alloys; and, secondly, inclusions dissolve under thermomagnetic treatment.
The numbers on the distribution function of ultra-fine magnetic fields (Fig. 3) in the order 1–9 correspond to atomic interactions: Fe-Fe, Fe-Cr2, Fe-Cr1, Fe-(Cr1,Cr2), Fe-(Cr1,2Cr2), Fe-(2Cr1,Cr2), Fe-(2Cr1,2Cr2), Fe-(2Cr1,3Cr2), Fe-(3Cr1,2Cr2), where the index 1 or 2 is the number of the coordination sphere in which Cr atom is located, and the numerical coefficients are the quantity of Cr atoms in these coordination spheres.
In graphitized high carbon steels with ferrite-graphite structure, the change in damping properties as a result of thermomagnetic treatment is quite insignificant, which is explained by a large number of diamagnetic graphite inclusions in a relatively fine-grained ferrite matrix. 3.

Introduction Nanostructured (ns) and ultra fine-grained materials exhibit higher strength and hardness, as well as excellent tribological properties compared to their coarse grained counterparts [1, 2].
Mishra et al. [6] compared the fretting behaviour of ns Ni (grain size of 8 nm) prepared by means of electrodeposition with that of bulk coarse grained polycrystalline Ni (grain size of 61 μm).
A gradient in microstructure i.e. finer grains in the surface and near surface regions and coarse grains in the bulk may be seen.
Figure 6 shows the variation of TFC with the number of fretting cycles for untreated and treated samples at 4.9 N normal load.
They reported that at a particular grain size (32 nm) the wear resistance was maximum and grain sizes above and below this size resulted in poor wear resistance.

The effective output of heat is beneficial for the supercooling of melt, and then a large number Fig. 1.
When the whole melt reaches a certain degree of supercooling, a large number of grains will be produced in an instant.
In this stage, a large number of nucleation makes the grain density increase, and further makes the grain spacing smaller.
When a certain degree of supercooling is achieved, a large number of nucleation will occur in the melt.
It greatly reduces the size of grains in the melt and increases the number of critical nuclei when the melt reaches the uniform solid fraction in a shorter time, 4.