Diffusion anisotropy and void development under cascade irradiation

Abstract

Cascade irradiation of metals gives rise to swelling as a result of the creation of voids and the evolution of the void ensemble. Under suitable circumstances, the originally disordered void distribution transforms into to a void lattice. As demonstrated previously, the understanding of the evolution and the unique features of the void ensemble requires a difference in the anisotropy of the diffusion (DAD) of vacancies and self-interstitial atoms (SIAs), which is achieved by one-dimensional diffusion of the SIAs. On the other hand, void swelling has been successfully modeled in terms of three-dimensional diffusion of both vacancies and SIAs. In the present paper it is shown that these seemingly contradicting interpretations and all related observations can be quantitatively reconciled by a small DAD created by only ∼1% of SIAs diffusing one-dimensionally. It is also demonstrated that at the initial stage of void-lattice formation, ordering occurs mainly on close-packed crystal planes, which is in contrast to the naïve expectation that one- A difference between the anisotropies of diffusion of vacancies and self-interstitials (henceforth called Diffusional Anisotropy Difference or, briefly, DAD) gives rise to a bias that plays an important role in the irradiation-induced evolution of microstructures in metals. Based on Gösele's work [1] on reaction kinetics, the DAD effect was first introduced by Woo and Gösele [2] to explain irradiation growth. Later irradiation creep and void-lattice formation were interpreted in terms of DAD by Woo [3] and Woo and Frank [4], respectively, before DAD has been promoted to an important concept in the treatment of irradiation damage in non-cubic metals by Woo [5,6].The role played by DAD in the irradiation-induced transformation of a void ensemble from a randomly distributed collection to a highly ordered void lattice was summarized by Woo and Frank [7]: ". . . the one-dimensional migration of the (interstitial) crowdions. . . is in marked contrast to the three-dimensional diffusion of the dumbbell interstitials and the vacancies. This diffusional anisotropy difference between the crowdions and vacancies provides the source of ordering via a Darwinian selection among randomly distributed voids." Hence, the strength of DAD is important to the behavior of a void ensemble under irradiation, and at the same time, the evolution of the void ensemble reflects the

Radiation-induced damage and evolution of defects in Mo

“…However, there are contradicting statements in the previous works about the mechanisms and dimensionality of di-interstitial SIA (di-SIA) and SIAC diffusion. 2,13,14,16,17 At the same time, the comparison with the recent experimental results showed a large discrepancy in the diffusivity of dislocation loops. 4 The understanding of the mechanisms of the production of large clusters of defects is of practical importance for reducing the irradiation embrittlement.…”

“…The resulting data can be deployed as an input for the theoretical models of long time evolution of damaged material. 3,14,33,34 It was shown that both the SIAs and the vacancy loops have high one-dimensional (1D) mobility along close-packed crystal directions in bcc crystals. 10,31,32 The small clusters can switch their propagation direction between equivalent directions, which result in the three-dimensional (3D) diffusion at elevated temperatures.…”

“…Although many features of the radiation damage have been explained, several phenomena are still discussed, including the mechanisms of production of large clusters of defects and dislocation loops, 5,[8][9][10][11][12] the effect of PKA mass on the defect yield, 8,12 the influence of surfaces in the experiments with thin foils, 8 the mobility of self-interstitial atoms (SIAs), clusters (SIACs), and dislocation loops. 2,4,[13][14][15][16] One of the most plausible explanations of the formation of superlattices of bubbles [1][2][3]13,14,16 in several metals under the radiation damage is based on the anisotropic diffusion of SIAs or SIACs. However, there are contradicting statements in the previous works about the mechanisms and dimensionality of di-interstitial SIA (di-SIA) and SIAC diffusion.…”

Phase-field modeling of void lattice formation under irradiation

“…Since the first observation of a void lattice in irradiated materials [1], many theoretical models have been proposed to explain the formation of ordered microstructures [12][13][14][15][16][17]. One important family of the models is based on an elastic energy minimization argument [17][18][19][20].…”

Modeling dislocation structure development and creep–swelling coupling in neutron irradiated stainless steel

“…However, the invariable presence of compositional disorder in the crystal lattice due to alloying and various thermal-mechanical treatment may lead to the trapping or scattering of the clusters [6,[33][34][35], drastically reducing the likelihood of long-range one-dimensional transport of interstitial defects. Moreover, recent investigations [36,37] showed that void lattice formation in ''pure" metals cannot occur if more than $1% of the interstitial atoms undergo one-dimensional diffusion. The development of large-scale heterogeneous void swelling found near grain boundaries and dislocation walls can also be understood without invoking the one-dimensional diffusion [38][39][40].…”