Extreme precipitation strengthening in ion-implanted nickel

Precipitation strengthening of nickel was investigated using ion-implantati on alloying and nanoindentation testing for particle separations in the nan ometer range and volume fractions extending above 10%. Ion implantation of either oxygen alone or oxygen plus aluminum at room temperature was shown t o produce substantial strengthening in the ion-treated layer, with yield st rengths near 5 GPa in both cases. After annealing to 550 degrees C the oxyg en-alone layer loses much of the benefit, with its yield strength reduced t o 1.2 GPa, but the dual ion-implanted layer retains a substantially enhance d yield strength of over 4 GPa. Examination by transmission electron micros copy showed very fine dispersions of 1-5 nm diameter NiO and gamma-Al2O3 pr ecipitates in the implanted layers before annealing. The heat treatment at 550 degrees C induced ripening of the NiO particles to sizes ranging from 7 to 20 nm, whereas the more stable gamma-Al2O3 precipitates were little cha nged. The extreme strengthening we observe is in semiquantitative agreement with predictions based on the application of dispersion-hardening theory t o these microstructures. (C) 1999 American Institute of Physics. [S0021-897 9(99)09123-9].