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interaction between dislocation and nitride precipitates

جواهری: مجتبی گردآورنده : پور حسینی دکتر درس استاد

91دی

theory

One way to increase the hardness and yield strength of a metal alloy is a solid solution.

The effect of adding impurities depends on the size difference and adding.

If the solute atoms , are larger than solvent atoms , the compressive strain field is created, otherwise the tension field is formed. In both of them , Dislocation mobility is reduced and the strength increases

If atoms are concentrated topical solution that creates a low-range order alloy. The local atomic order can deterrent slip and increase strength., This phenomenon is known as the Fisher effect.

Between carbon and nitrogen atoms in the atmosphere created by the settlement dislocations (usually Dislocation lines) are and Cottrell atmospheres may form in the area that is locked Dislocation

Dislocation released from restriction of movement within the network and need to be very large yield stress

theory

In high temperature deformation of austenite steels containing Ti orTa elements, internal stress is considered to be one of the important factors, which control creep deformation

the value-variation of effective stress and internal stress in Ti-add samples is more susceptible than Ta-add samples at 873 K

Ti nitrides increase high temperature tensile and creep strengths but Ta nitrides decrease them

Experimental procedures They were melted about 10–15 kg in a vacuum induction furnace

and hot rolled after homogenizing treatment at 1423–1473 K A solution treatment was performed at 1373 K aging treatment for the precipitation of nitrides within grains was

carried out at 873 K The interaction between nitride and dislocation during creep

deformation at 873 K was observed by a weak-beam dark-field technique of TEM

Three millimeter discs for TEM observation were cut off from the specimen which was rapidly cooled by air blowing in the state of loading of applied stress equal to the internal stress after the strain dip test

TEM foils were thinned using the twin jet-type electro-polisher with a solution of 10% HClO4 and 90% CH3COOH and the voltage was 200 keV

Results

The picture shows the TEM micrographs of as-prepared 0.1 wt% Ti-add(A-Ti) and 0.55 wt% Ta-add(A-Ta) samples aged at 873 K.

Results

They are composed of fine(20 nm) TiN precipitates imply a finely dispersed insoluble second phase in a metallic matrix and coarser(100 nm) and spherical TaN precipitates within grains.

The density of fine TiN precipitate is higher than the TaN precipitates in the same resolation

that a lot of fine Ti-nitrides are pinning and incorporating the bowtype moving dislocation

dislocations activated by creep deformation on TiN and TaN precipitated samples have the same Burgers vector.

the dislocation is clearlydissociated into two partials. Contrast experiment shows that theyare the typical Shockley partials

The picture shows the interaction between dislocation and TiN precipitates at deformed sample using weak-beam technique with the electron beam direction

Results

the fine precipitates are newly formed during 873 K creep deformation by applied stress of 236 Mpa

the applied stress contributes to accelerate the formation of new-type precipitate

the coarser TaN precipitate is lopped by dislocation

Discussion

the internal stress of precipitation and/or dispersion strengthened alloys is generally higher than that of solution strengthened alloys because of the disturbance of dislocation motion by particles

It is expected that the size and distribution of these precipitates are critical to optimize the mechanical properties

Inside the grain, the precipitates may either be cut by the dislocations or resist cutting and the dislocations are forced to bypass them. When the precipitates are small and/or soft ,dislocations cut and deform the precipitates.

Discussion

A critical diameter of precipitates for cutting the dislocation can be calculated using the following equation: Dc=4πGmb/Gp

The shear modulus of thestainless matrix, Gm, can be approximately considered as 8.3 GPa and shear modulus of TiN Gp can be measured as 190 GPa, respectively. The critical diameter is quite small compared with the precipitates observed in this study.

Thus,it is reasonable that precipitate is not readily cut but is pinned and/or bypassed by moving dislocation during deformation. It depends on the degree of size and distribution of the precipitates

Discussion

For example in the coarser TaN precipitated sample , dislocations introduced by applied stress are supposed to be able to rather easily bypass without interaction with the precipitates.

in the fine and dense TiN precipitated sample , dislocation motion would be more readily disturbed compared with the coarser TaN precipitated sample.

the difference of value-variation and values of internal stress in high Mn austenite steel can be well explained by the existence of different type of precipitates; one has the nitrides of fine size and the other has coarser ones.

Discussion

the Shockley partial dislocation is energetically stable compared to perfect one and it is impossible the two partials have the screw character simultaneously.

they have to be recombined as perfect one before they move to another {111} slip planes to slip over the precipitates

Therefore, extra stress proportional to repulsion energy of dissociated dislocations is needed to move the dislocation compared to perfect one.

This extra stress should contribute to increase the tensile strength of samples and this effect is more prominent when the nitrides of fine size and dense distribution are formed compared to the coarser ones.