Effect of helium flux on near-surface helium accumulation in plasma-exposed tungsten.

We report results of object kinetic Monte Carlo (OKMC) simulations to understand the effect of helium flux on the near-surface helium accumulation in plasma-facing tungsten, which is initially pristine, defect-free, and has a (100) surface orientation. These OKMC simulations are performed at 933 K for fluxes ranging from 1022to 4 × 1025He/m2 s with 100 eV helium atoms impinging on a (100) surface up to a maximum fluence of 4 × 1019He/m2. In the near-surface region, helium clusters interact elastically with the free surface. The interaction is attractive and results in the drift of mobile helium clusters towards the surface as well as increased trap mutation rates. The associated kinetics and energetics of the above-mentioned processes obtained from molecular dynamics simulations are also considered. The OKMC simulations indicate that in pristine tungsten, as the flux decreases, the retention of implanted helium decreases, and its depth distribution shifts to deeper below the surface. Furthermore, the fraction of retained helium diffusing into the bulk increases as well, so much so that for the flux of 1022He/m2 s, almost all of the retained helium diffused into the bulk with minimal/negligible near-surface helium accumulation. At a given flux, with increasing fluence, the fraction of retained helium initially decreases and then starts to increase after reaching a minimum. The occurrence of the retention minimum shifts to higher fluences as the flux decreases. Although the near-surface helium accumulation spreads deeper into the material with decreasing flux and increasing fluence, the spread appears to saturate at depths between 80 and 100 nm. We present a detailed analysis of the influence of helium flux on the size and depth distribution of total helium and helium bubbles.

Cascade morphology transition in bcc metals.

Energetic atom collisions in solids induce shockwaves with complex morphologies. In this paper, we establish the existence of a morphological transition in such cascades. The order parameter of the morphology is defined as the exponent, b, in the defect production curve as a function of cascade energy (N(F) ~ E(MD)(b)). Response of different bcc metals can be compared in a consistent energy domain when the energy is normalized by the transition energy, µ, between the high- and the low-energy regime. Using Cr, Fe, Mo and W data, an empirical formula of µ as a function of displacement threshold energy, E(d), is presented for bcc metals.

Ab initio study of H, He, Li and Be impurity effect in tungsten Σ3{1 1 2} and Σ27{5 5 2} grain boundaries.

Density functional theory calculations were performed to investigate the effect of H, He, Li and Be impurities on the intergranular cohesion of W Σ3〈1 1 0〉{1 1 2} and Σ27〈1 1 0〉{5 5 2} grain boundaries (GBs). A rigorous search of unique interstices was performed to reveal a range of impurity behaviors. It was found that He exhibits the largest defect formation energy, however it is Li that shows the strongest tendency to segregate towards the GBs. The embrittling potency was studied using the ideal GB fracture energy (Rice-Wang) criterion (Rice and Wang 1989 Mater. Sci. Eng. A 107 23). The potency was found to depend significantly on the local atomic environment. A site-dependent concentration was used as the weighing factor to obtain an average potency as a function of temperature. It was found that the potency depends only weakly on temperature. In Σ3 H, He, Li and Be decrease fracture energy by 4%, 27%, 30% and 14%, respectively. In Σ27 H, He and Li also decrease cohesion, albeit less severely: 3%, 14% and 7%. On the other hand, Be increases cohesion in Σ27 by 3%.

Multislice simulation of transmission electron microscopy imaging of helium bubbles in Fe.

Formation of nanoscale helium (He) bubbles in reduced activation ferritic/martensitic steels may lead to degradation of mechanical properties of materials. Transmission electron microscopy (TEM) has commonly been used to image the Fresnel contrast of He bubbles, using an underfocus of 0.5-1 µm. This paper presents our study of multislice simulation of the size correlation between imaged Fresnel rings and the actual He bubbles. It was found that for bubbles equal to or >3 nm in diameter, the imaged bubble size, represented by its inner diameter of the first dark Fresnel ring (D(in)) in underfocused imaging conditions, increases with increasing electron-beam incoherency, but decreases with increasing underfocus. The electron-beam accelerating voltage, bubble size, bubble position and TEM sample thickness were found to have no significant influence on the deviation of D(in) from the actual bubble size (D(0)). However, for bubbles equal to or <2 nm, D(in)/D(0) increases dramatically with increasing underfocus when it is above a threshold limit (e.g. Δf = -1 µm for a 2-nm bubble). The results of this study also suggested that He bubbles can be differentiated from argon (Ar) bubbles by contrast differences.