Helium Bubbles In Grain Boundaries Research Articles

Cracking behavior of helium-irradiated small-volume copper

Radiation-induced grain-boundary helium bubbles are the cause of the significant helium embrittlement. However, the influence of grain-interior helium bubbles on the cracking behavior of metals has not been explored. Here, we investigate the cracking propensity of helium-irradiated small-volume Cu single-crystals, and compare with those of Cu single-crystals and Cu bicrystals. We found that the irradiation-induced helium bubbles only slightly increased the cracking propensity of the single-crystal Cu, while the cracking resistance of the helium-irradiated Cu single-crystals was superior to that of the Cu bicrystals, both with and without helium bubbles.

Helium-induced weld cracking in austenitic and martensitic steels

Helium was uniformly implanted into type 316 stainless steel and Sandvik HT-9 (12Cr-1 MoVW) to levels of 0.18 to 256 and 0.3 to 1 a.p.p.m., respectively, using the “tritium trick” technique. Autogenous bead-on-plate, full penetration, welds were then produced under fully constrained conditions using the gas tungsten arc welding (GTAW) process. The control and hydrogen-charged plates of both alloys were sound and free of any weld defects. For the 316 stainless steel, catastrophic intergranular fracture occurred in the heat-affected zone (HAZ) of welds with helium levels ≥2.5 a.p.p.m. In addition to the HAZ cracking, brittle fracture along the centreline of the fusion zone was also observed for the welds containing greater than 100 a.p.p.m. He. For HT-9, intergranular cracking occurred in the HAZ along prior-austenite grain boundaries of welds containing 1 a.p.p.m. He. Electron microscopy observations showed that the cracking in the HAZ originated from the growth and coalescence of grain-boundary helium bubbles and that the fusion-zone cracking resulted from the growth of helium bubbles at dendrite boundaries. The bubble growth kinetics in the HAZ is dominated by stress-induced diffusion of vacancies into bubbles. Results of this study indicate that the use of conventional GTAW techniques to repair irradiation-degraded materials containing even small amounts of helium may be difficult.

A possible solution of the problem of helium embrittlement

From the great number of various suggestions how to overcome the problems of helium embrittlement, one proposal might offer a practicable solution: Introducing precipitates within the matrix as trapping centers for helium. This is concluded from the results obtained in post-helium implantation high temperature creep tests. It is shown that (1) TiC precipitates in austenitic steels very efficiently trap helium, (2) number density and size of helium bubbles in grain boundaries are strongly reduced when TiC precipitates in the matrix are fine and homogeneously distributed, and (3) losses in creep ductility and creep strength are dependent on the size and spatial distribution of TiC precipitates; helium embrittlement effects are minimized when fine dispersive TiC precipitates have been introduced into the steel specimens.

The effect of implanted helium on the high temperature mechanical properties of a model austenitic Fe-17% Cr-17% Ni alloy

Foils of the ternary alloy Fe-17wt%Cr-17wt%Ni were cyclotron-implanted with 160 appm helium and then creep rupture tested at 1023 K together with unimplanted controls. The range of the applied stress was 9.8 MPa till 39.2 MPa. In addition, a few tensile tests (/. ϵ = 1.5 × 10 −3/ s) were performed with implanted and unimplanted specimens at the same temperature. The mechanical properties of the material are changed by the presence of helium — characteristically for high temperature helium embrittlement: Reduced ductility, reduced creep rupture time, increased yield stress (matrix hardening) and intergranular fracture. The changes in the mechanical properties are accompanied by a change in the deformation mode from “climb and glide” (mixed with some grain boundary sliding) in the unimplanted samples to predominantly grain boundary sliding in the helium implanted ones. This can be correlated with high densities of helium bubbles in grain boundaries. Initiation of grain boundary cracks seems to be strongly influenced by very large helium bubbles at grain boundary triple point junctions.