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.
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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