MICROSTRUCTURE AND NANOHARDNESS OF HAFNIUM DIBORIDE AFTER ION IRRADIATIONS

In order to simulate helium and lithium appearance originating from th e B-10 (n,alpha)Li-7 reaction, that occurs under neutron irradiation i n a PWR, hafnium diboride samples have been irradiated with Hei and Li + ions at 350 degrees C. As implanted and annealed materials have been characterised by TEM observations and nanohardness measurements. In s pite of high concentration of implanted helium, no helium bubbles were found whereas the TEM images showed a contrast associated with tiny d islocation loops. Observation on samples annealed at 1600 degrees C al lowed a more detailed analysis of loops which exhibit an interstitial nature (resp. vacancy) for a {11(2) over bar 0} or {10(1) over bar 0} habit plane (resp. (0 0 0 1)). The organisation of these secondary def ects leads to a strongly anisotropic deformation of grains which is th ought to cause intergranular microcracking when conflictual stresses a re met at grain boundaries. (C) 1998 Elsevier Science B.V. All rights reserved.