Effect of grain microstructure on thermal helium desorption from pure iron

Experiments were carried out to evidence the role played by grain boundaries on He/Fe interactions. Three microstructures involving grain sizes from 2 to 100 μm in average diameter were irradiated with 8-keV 4He+ ions at 298 K and helium desorption was then studied during constant rate heating by thermal desorption spectroscopy (TDS) from room temperature to 1330 K. The two larger microstructures (40 and 100 μm) could be characterised with the same multiple desorption groups, combining helium involved in small and large clusters as well as in bubbles. The finer microstructural scale (2 μm) was peculiar due to the existence of a broad split of desorption peak (at 856 and 917 K). Such a peak is ascribed to the movement of grain boundaries during recrystallization which drains helium towards surface. The high density of grain boundaries in the finer microstructure also explains the thermal shift and the larger desorption at low temperatures. As a first step towards the quantitative assessment of this effect, cluster dynamics simulation have been performed in order to give the quantity of helium trapped in grain boundaries for the different microstructures.

► Fe-samples with grain sizes from 2 to 100 μm are implanted with 8-keV He+ beam.
► It appears that helium desorption is affected by recrystallisation.
► Helium desorbs at lower temperatures in the finer microstructure.
► Cluster dynamics confirms the role played by GB density on He partitioning in iron.