THERMAL NUCLEATION AND GROWTH OF He BUBBLES IN He IMPLANTED STAINLESS STEELS

Nucleation and growth of He bubbles at different annealing temperatures T≤1023 K in stain- less steels HR-2 and SS321,implanted with He^+(40-70 keV.1×10^(16)-6× 10^(17) He^+/cm^2),were ohserved under TEM.T=0.45T_m seems to be a transition temperature.The He bubbles continue mainly their nucleation at 0.3T_m< T<0.45T_m,and grow predominantly at T≥0.45T_m,due probably to migration and coalescence by surface diffusion of metal atoms around the bubbles.The apparent activation energies for the growth are found to he 0.41 and 0.31 eV for HR-2 and SS321 respectively.The bubble density in HR-2 is greater than that in SS321 within whole measuring temperature range,and the onset temperature of rapid swelling is also higher.It is believed that the resistance of HR-2 to He is superior to that fo SS321.

High-Temperature Annealing Induced He Bubble Evolution in Low Energy He Ion Implanted 6H-SiC

Bubble evolution in low energy and high dose He-implanted 6H-SiC upon thermal annealing is studied. The （0001）-oriented 6H-SiC wafers are implanted with 15keV helium ions at a dose of 1×10^17 cm^-2 at room temperature. The samples with post-implantation are annealed at temperatures of 1073, 1173, 1273, and 1473K for 30rain. He bubbles in the wafers are examined via cross-sectional transmission electron microscopy （XTEM） analysis. The results present that nanoscale bubbles are almost homogeneously distributed in the damaged layer of the as-implanted sample, and no significant change is observed in the He-implanted sample after 1073 K annealing. Upon 1193 K annealing, almost full recrystallization of He-implantation-induced amorphization in 6H-SiC is observed. In addition, the diameters of He bubbles increase obviously. With continually increasing temperatures to 1273K and 1473 K, the diameters of He bubbles increase and the number density of lattice defects decreases. The growth of He bubbles after high temperature annealingabides by the Ostwald ripening mechanism. The mean diameter of He bubbles located at depths of 120-135 nm as a function of annealing temperature is fitted in terms of a thermal activated process which yields an activation energy of 1.914＋0.236eV.

Effects of Zr/Mo addition on He bubble formation in Y_(2)O_(3)-added W alloys

To improve the mechanical properties of the Y_(2)O_(3)-added W alloy,Mo and Zr were independently added to a sample of W-Y_(2)O_(3).In this study,the effects of minor additions of elemental Mo and Zr on He bubble formation in the Y_(2)O_(3)added W alloys were investigated at 773 K,973 K,and 1173 K,where the maximum irradiation fluence of the samples using 5 keV He ions was 1.8×10^(21) He m^(−2).He bubbles were observed at all temperatures for both alloys;however,the irradiation fluences at which He bubbles could be observed differed at each temperature.With increase in irradiation temperature,the He irradiation fluence for which the formation of He bubbles could be observed decreased.At the relatively low temperature of 773 K,the void swelling in the W-Mo-Y_(2)O_(3)and W-Zr-Y_(2)O_(3)alloys was slightly lower than that of W-Y_(2)O_(3)alloy.However,at high temperature of 973 K and 1173 K,the He irradiation resistances of both W-Mo-Y_(2)O_(3)and W-Zr-Y_(2)O_(3)were worse than that of W-Y_(2)O_(3).The addition of Zr enhanced the interaction between vacancies and He,enhancing the formation of He bubbles.However,the addition of Mo coarsened the Y_(2)O_(3)particles,reducing the suppression of vacancy cluster formation due to the dispersion of Y_(2)O_(3)particles.It is,therefore,necessary to fully consider mechanical properties and T-retention in addition to the irradiation resistance of He to select an alloy as the plasma-facing material for fusion applications.

Microstructural evolution in W-1%TiC alloy irradiated He ions at high temperatures

To investigate the surface damage of a material by He ions,a dispersion-strengthened W-1wt%TiC alloy was irradiated by 5-keV He ions at 773 K,973 K,and 1173 K up to an ion dose of 1.8×10^(21)He m−2,respectively.No He bubble formation was observed under transmission electron microscopy at any temperature for He doses less than 1.5×10^(20)He m^(−2).When this dose was exceeded,He bubbles grew and void swelling increased with increasing irradiation doses.Naturally,the growth of He bubbles and the void swelling became more pronounced with increasing irradiation temperatures.Compared to the published data on commercially available pure W,the formation of He bubbles was suppressed by TiC particles in the W-TiC alloy,especially at low doses.