HARDNESS AND ELASTIC-MODULUS OF ZIRCON AS A FUNCTION OF HEAVY-PARTICLE IRRADIATION DOSE .2. PB-ION IMPLANTATION DAMAGE

Synthetic single-crystals of zircon [(100) growth surfaces] were impla nted at 77K with 540 keV Pb ions with fluences of 3.3 x 10(11) to 5 x 10(15) ions/cm(2)-a dose range that completely spans the crystalline-r o-amorphous transition as determined by Rutherford backscattering/ion- channeling measurements. A chemically unzoned, natural crystal [polish ed (100) section] was similarly implanted. Both the natural and synthe tic crystals, as examined by mechanical-properties microprobe (MPM) me thods, exhibited an overall radiation-induced softening (70% reduction ) and decrease in modulus (42%) through the crystalline-to-amorphous t ransition. This transition occurs over a narrow range of dose, i.e. 1 x 10(13) to 1 x 10(14) Pb ions/cm(2). Moreover, below the transition, a pronounced radiation-induced hardening occurs in the near-surface re gion of the synthetic crystals. This radiation hardening may be due to the accumulation of interstitials. In contrast, the natural zircon sh owed no hardening; however, its initially higher hardness values sugge st that it was already hardened. Despite the continued re-damage proce ss (due to overlapping of displacement cascades) for ion doses beyond the saturation value of 1 x 10(14) ions/cm(2) the hardness remains con stant.