**1. Introduction**

140 Polycrystalline Materials – Theoretical and Practical Aspects

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Agents and Their Influences on Crystallization Behavior and Mechanical Properties of Isotactic Polypropylene. *J Polym. Sci.: Part B: Polym Phys*, Vol.48, No.6, (March The one of actual problems of radiation material science is to reveal plastic deformation laws, hardening and fracture ones of materials under intense external action, particularly irradiation. Herein we imply different kinds of irradiation, for instance, such a (*e*, ) beam irradiation, ion or neutron irradiation and so on. Evolution of a construction material microstructure at a high temperature trial operation is substantially conditioned by free migrating defects [1]. The processes of interaction of point defects with each other, with dislocations and interface underlie all of metal radiation hardening mechanisms [2]. In the section 1, the nonlinear model of dose dependence saturation of the yield strength is proposed on the base of the vacancy and interstitial barrier interaction. Processes of mutual recombination of vacancy and interstitial barriers and formation of vacancy and interstitial clusters are taken into consideration.

A series of different radiation defects (retardation barriers of dislocations), and their sizes, and a form of their volume distribution contribute into a yield strength increment for all of sorts of irradiation. The contribution of a barrier type is determined by conditions of irradiation and tests. At the low temperature irradiation (at the test temperatures up to 0.3 Tm, Tm is melting temperature), interstitial atoms, and vacancies, and their clusters contribute mainly into the hardening. In the section 2, evolution of radiation barrier (vacancies and interstitials) clusters is analyzed under low temperature radiation in the presence of the most important secondary effectes: recombination and formation of divacancy complexes. It is proposed a barrier hardening model in that mechanisms of mutual annihilation of the vacancy and interstitial barriers and their clusterization play a main role.

In the section 3 unlike two preceding sections where the dose dependences are considered, the phenomenological model is formulated to describe a yield strength temperature dependence of polycrystalline materials that have undergone irradiation and mechanical experiences in a wide temperature interval including structure levels of plastic deformation. In this section, a new phenomenological model is proposed to give a suitable description of yield strength temperature dependence of some of irradiated materials in a temperature interval including plastic deformation structure levels.

Influence of Irradiation on Mechanical Properties of Materials 143

defects of certain their concentration, creation round the volume defects of defect-free zones, the beginning of the dislocation channeling and surmounting obstacles processes and so on. In the Ref [5], the model is proposed to describe the dose dependence of the copper yield strength increment where the saturation is explained by a decreasing velocity of the forming clusters with increasing irradiate dose due to interaction between the available clusters and

Here the model is proposed to describe the dose dependence of the yield strength increment

We consider that vacancies and interstitial atoms make a main contribution to the yield strength increment of a certain material at some of irradiation conditions. They are barriers to play the main role in the hardening at the low temperature irradiation. Therefore, in the proposed model *N*=2; the index values of *i*=1 and 2 correspond to the vacancies and

> 

For all of the obstacle types, the metal yield strength increment conditioned by dislocation

where *i* is the parameters characterizing *i*' barrier intensity (a fixed quantity for some of barrier types, material and irradiation condition), is the shear modulus, *b* the Burgers vector length, *Сi* the volume density of *i*' barrier type, *di* their average size. For instance, the vacancy and interstitial have the average size ~ 10 nm, and the parameters characterizing

The present model is based on the system equations for the volume densities of the

1 1 2 1 12 1 2 1 1

*dC <sup>С</sup> K CC C*

*dC <sup>С</sup> K CC C*

where = Ф*t*, Ф is density of particle flux, *t* irradiation time, *Ki*, *i* = 1, 2, the intensities of forming the radiation - induced vacancy and interstitial barriers, *i* are the coefficients of barrier recombination and characterize forming the clusters of acceptable barrier type (it can be named as clusterization coefficients), 12 the coefficient of mutual recombination of the

The first terms of the equation system (2.5) describe the intensity of increasing the volume barrier densities of the acceptable type, the second ones correspond to decreasing the volume barrier densities due to absorbing the barriers on natural sinks: voids, dislocations,

2 2 2 2 12 1 2 2 2

= *KiVi*, where *Vi* are the effective volumes of interaction of the certain barriers with

1

 

2

 

 

1 2 . (2.3)

*bCd* , *i* = 1, 2, (2.4)

,

 

> 

.

*i* 

(2.5)

can be represented by the

interstitial atoms (and their clusters). Then in this model Eq (2.1) takes the form:

1/2 ( ) *i i ii*

 

radiation- induced nonequilibrium vacancies and interstitial barriers *C*1, *C*2:

*d*

 

*d*

annihilating vacancy and interstitial barriers, the coefficients <sup>1</sup>

taking into account of vacancy and interstitial barrier interaction.

newly forming ones.

deceleration is described as [2]:

form: <sup>1</sup> *i* 

each other.

barrier intensity has the value about 0.2 [2].
