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The paper presents a physico-mechanical model for predicting creep rupture in neutron-irradiated materials. The model is based on the approach whereby damage is described as voids on grain boundaries. The equations for void nucleation and growth which were proposed by the authors earlier are augmented to include neutron irradiation of material. Constitutive equations are derived to describe viscoplastic deformation of material including void propagation. A criterion for plastic stability of a unit cell is employed as a fracture criterion.

We present some results of prediction of creep rupture strength and plasticity for austenitic materials prior to and after irradiation with variable neutron flux rates, based on physicomechanical model as outlined in Part 1. The calculated results are compared with the available experimental data.Keywords: creep rupture strength, plasticity, void, neutron flux rate. Creep Rupture Strength and Plasticity of 1Kh18N10T Steel in Its Initial State.Based on the physico-mechanical model for intercrystalline fracture [1], calculations were performed to predict creep rupture strength and plasticity in 1Kh18N10T austenitic steel under uniaxial loading. For calculating the creep rupture strength and plasticity curves we simulated steady-load tests and therefore creep rupture strength is expressed in terms of stresses active at the initial time of testing, i.e., conventional stresses. It is known that under the creep tension conditions the cross section of a specimen is reduced [2]. The true stress F true (i.e., the stress corrected for the cross-section reduction) was calculated bywhere F c is the conventional stress in the specimen. The calculation involves temperature-dependent and temperature-independent parameters. The last-mentioned ones include Ω, R 0 , k η , ρ max , and d g .* According to [3], the initial radius of a void arisen R 0 is taken 5 10 4 ⋅ − mm, Ω = ⋅ − 1 21 10 29 . m 3 [4]. As per [5] the grain size d g was set equal to 0.1 mm. The k d g η ratio was determined from the data on stress rupture plasticity in a unirradiated material at T =°650 C. We used ρ max and c α as adjustable parameters and found them by the criterion of the best agreement between the experimental and calculated data on creep rupture strength at various temperatures in the life time range up to 10 3 h. Also, it was established that c α parameter can be constant, i.e., independent of temperature. Hence, we obtained c α = 9 and ρ max =1000 m −2 .The temperature-dependent parameters are as follows: D b b δ governs the grain-boundary diffusion, a c , n c , and m c determine creep {Eq. (32) in [1]}, a p controls the strain hardening, and σ Y is the yield stress. To describe the temperature dependence of D b b δ we used (25) [1], where δ b b D 0 13 2 10 = ⋅ − mm 3 /s and Q b =167 kJ/mole [4].* Hereinafter we will use the notation as in Part 1 [1] unless otherwise defined.

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To investigate the effect of swelling upon mechanical properties of irradiated austenitic steel the investigations were conducted with steel 18Cr-10Ni-Ti and its weld irradiated up to same damage doses in two different temperature ranges: at the irradiation temperature of 330÷340°C when swelling is practically absent and at 400÷450°C when a considerable swelling level of 3÷13% is observed. Basing on the investigation results the temperature dependences of tensile properties of irradiated metal were constructed and analyzed. Fracture surfaces for ruptured specimens were examined by SEM. Comparative investigations of magnetization of irradiated metal at different irradiation temperatures were performed. It was concluded from the performed analysis of the results that in highly irradiated austenitic steel with considerable swelling a ductile to brittle transition is observed, which is caused of the Feγ→Feα phase transformation. The investigations of magnetization of metals with different swelling, as well as on the available literature data confirm a possibility of the Feγ→Feα phase transformation under considerable radiation swelling. The criterion is proposed allowing one to determine the radiation conditions under which the Feγ→Feα transformation make it possible a brittle fracture. The mechanism resulting in a sharp decrease of the ultimate tensile strength of highly irradiated metal is considered.

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The mechanical test results and fractographic observations reported in Part 1 are discussed from the standpoint of possible fracture mechanisms in austenitic steels subjected to intensive neutron irradiation. We put forward the mechanisms that relate the γ α → -transformation to the occurrence of a ductile-brittle transition in the irradiated austenitic steels and presents a criterion that defines the irradiation conditions whereby the γ α → -transformation leads to the ductile-brittle transition.Some possible reasons for the deterioration of the material ultimate strength at a very high (above 20%) level of swelling are discussed.Introduction. Part 1 [1] presented the results of experimental investigations of mechanical properties and irradiation swelling of austenitic steel 08Kh18N10T and its weld metal upon intensive neutron irradiation, which were performed in order to clarify the nature of the effect of the swelling on mechanical properties of austenitic steels.For this study some specimens were irradiated at a "low" temperature (T irr = 330-340°C) with a dose of 46 dpa (hereinafter referred to as the LTI material) and the other at a "high" temperature (T irr = 400-450°C) with a dose of 49 dpa (called the HTI material). These irradiation conditions were chosen for the purpose of producing different levels of swelling in the specimens. Specifically, the irradiation swelling is close to zero in the LTI material and ranges between 3 and 13% in the HTI material.The experimental investigations performed demonstrate that for the base metal the temperature functions σ 0 2 . ( ) T test and σ u test T ( ) are almost equidistant for the unirradiated and LTI material. For the HTI irradiated base metal which has a significant irradiation swelling, these functions are not equidistant to those for unirradiated and LTI specimens. However, for the weld metal they are equidistant for all three material states studied. We noted a significant increase in plasticity of the HTI material in comparison to that of the unirradiated material and the HTI material. At T test ≤°200 C the plasticity of the base metal upon HTI is close to zero.It was suggested in [1] that the base metal upon HTI undergoes an γ α → phase transformations.

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