Search results

The grain morphologies reveal that irregular-type and cubic-type grains coexist, and pores are observable on the surface.
Because vaporization of Bi2O3 (or Bi+3) is unavoidable, the number of vacancies in the stoichiometric NBT-BT3 ceramic increases considerably, causing distortion of the unit cell.
The number of pores and the amount of excess Bi2O3 are believed to be the two most important factors.
From the morphology revealed by the SEM image in Fig. 1, a decrease in the number of pores appears to be the primary cause of this increase.
The excess Bi2O3 segregates at the grain boundary of NBT-BT3 and inhibits grain growth by filling the space between the grains.

At lower temperatures, it is followed by the medium-temperature stage accompanied by precipitation of grain-boundary alpha and the formation of coarse secondary alpha-lamellae.
The low-temperature stage has been established to be characterized by the formation of dispersed alpha-platelets within beta-grains.
In addition to the treatments, the number of specimens were furnace heated and then water quenched.
The second stage includes the formation and subsequent growth of grain-boundary α as well as growth of coarse secondary α platelets from the grain boundaries which are referred to as α**.
With an increase of the heating temperature from 880 to 920 °C, the number of transformation stages was found to increase.

The smaller grain size of the composite microstructure the higher hardness can be achieved.
Conventional vacuum sintering could be reliable and simple technology to produce cermets with average grain size >1 µm, but it becomes quite challenging to obtain submicron grains [4].
To minimize the number of variables hardmetals with same binder volume (24%) and mass (15%) fraction and similar average grain size (<1 µm) will be studied.
Each grain was separated manually by a thin line to avoid errors in the data analysis.
Results and discussion The indentation surface fatigue tests were repeated at least three times for each number of indentation loading cycles, respectively 1000, 10 000 and 100 000.

For this later case, it has been shown that their coercivity strongly depends on grain size which is related to grain boundaries acting as pinning sites for domain walls [2, 6].
A large number of techniques have been designed, most of them being modification of existing metal forming processes, except the High Pressure Torsion (HPT) that was originally designed for geological applications [12, 13].
The bright field image exhibits an ultrafine mixture of recrystallized grains (with a low defect density) and un-recrystallized grains (with strong distortion contrasts typical of crystalline defects) appear.
Because of the lower boundary mobility (i.e. smaller growth rate), this gives rise to a higher number density of small ordered domains in the material aged at 400°C compared to 500°C.
iii) During annealing of the HPT processed alloys, ordered domains nucleate along grain boundaries and grow to fully transform un-recrystallised grains.

A highly heterogeneous sample, containing grains, grain boundaries and weak zones, is employed in the numerical simulation.
A large number of computer codes for flow-stress or flow-strain coupling analysis have been developed in recent years [3, 4, 5].
The numerical modeling is conducted on a numerically rectangle sample that is assumed to be composed of rock aggregate, grains, grain boundaries, weak zones and water.
the boundary of the grains and weak zones.
It can be seen that the onset of failure in the specimen is first indicated by the formation of a large number of isolated fractures (Fig. 5 [a]).

As shown in Fig. 3a, b, c, columnar grains and the proeutectoid ferrite precipitates mainly distribute in the weld center along the grain boundary.
The microstructure of the grain interior is composed of tiny ferrite and pearlite, and.
So the tensile samples were taken from near surface, 1/4 thickness and 1/2 thickness of the start welding position and they were numbered A1, B1, C1.
The same sampling method was used to take tensile samples from the end welding position, and they were numbered A2, B2 and C2.
Moreover, the size of the ferrite grain in these two locations is about 20~35 µm.

The SRM propellant grain is HTPB composite propellant, Assuming that grains as an isotropic linear viscoelastic material, the thermal stress and thermal strain of grain does not considered.
One coupling step contains some sub-iterations, in order to solve the field equation, each solver will obtain the required data from another solver in every coupling sub-iteration. sub-iteration will stop when it reach to a given maximum iteration number or all physical fields converge at this coupling step, then calculation will switch to the next coupling step, ensuring each coupling calculation is implicit.
The front surface of the propellant grain locates in x = 0.022, and the back grain surface locates in x = 0.347.
(2) The cold-flow gas pressure has a great impact on the grain in a short time, especially in head end of the grain
(3) Grains deformation is mainly concentrated on point A and point C, and gradually increases with time; stress is mainly concentrated on the joint of grain front surface and the motor case, the stress concentration generated here may cause grain offsticky

The influence of the plasma parameter on both the melting depth and the grain size was discussed.
Secondly, it prevents grain from growing leading to obtain the fine grain, which is less than 6µm.
Salt spray test exhibits that pits were appear only after 16 h for plasma surface-modified AM60 sample, and the number of pits becomes more and more after 48 h.
Surface micro-hardness is enhanced by grain size strengthening and solid solution strengthening.
The micro-hardness of the melted layer is up to 160-200 HV0.05 by grain size strengthening

Except temperature distributions, the macro-micro models can offer more information about solidification process, such as undercooling, grain size, grain density etc.
Except temperature distributions, the macro-micro models can offer more information about solidification process, such as undercooling, grain size, grain density etc.
For the macro-micro models, since grains are assumed to be spherical, the local volume fraction of solid, fs, can be given by ( ) 3 ( ) ( ) 4/3 ( )   =   sf t N t R tπ (6) where N(t) is grain number per unit volume and R(t) is grain radius.
In general, it is thought that the higher cooling rate (dT/dt) yields the larger grain density since the higher cooling rate leads to the greater undercooling, which causes the larger number of nuclei [18].
From the figure, it can be found the grain sizes are strongly related to the undercooling.

The measurements were done with an accuracy of 0.01 mm and numbers of loading cycles N were registered.
The grains form bands according to the rolling direction: elongated grains of phase α, 5-25 µm wide, and fine grains of phase β, 1-5 µm in diameter.
Fig. 2a shows the specimen surface under Ma = 11.2 N⋅m and the stress ratio R = -1 after a number of cycles Nf = 1753000 together with the final course of the crack about 250 µm in length.
At the fractures, transcrystalline cracks through grains of phase α predominate, but cracks along the grain boundaries are also observed.
The grains form strips arranged according to the direction of plastic working; elongated grains of phase α are to 50 µm in width, grains of phases β and S are fine and their mean diameters are 5-10 µm.