PHASE-TRANSFORMATION INDUCED BY SWIFT HEAVY-ION IRRADIATION OF PURE METALS

It is now unambiguously established that high electronic energy deposi tion (HEED), obtained by swift heavy ion irradiation, plays an importa nt role in the damage processes of pure metallic targets: (i) annealin g of the defects created by elastic collisions in Fe, Nb, Ni and Pt, a nd (ii) creation of additional defects in Co, Fe, Ti and Zr. For Ti, w e have recently evidenced by transmission electron microscopy observat ions that the damage creation by HEED is very important and leads to a phase transformation. Titanium evolves from the equilibrium hcp alpha -phase to the high pressure omega-phase, We studied the influence of t hree parameters on this phase transformation: ion fluence, electronic stopping power and irradiation temperature, The study of Ti and the re sults concerning other metals (Fe, Zr, etc.) and the semi-metal Bi all ow us to propose criteria to predict in which metals HEED could induce damage: those which undergo a phase transformation under high pressur e. As a matter of fact, beryllium is strongly damaged when submitted t o HEED and seems to behave very similarly to titanium. The fact that s uch phase changes from a crystalline form to another form were only ob served in those metals in which high pressure phases exist in the pres sure-temperature diagram, strongly supports the Coulomb explosion mode l in which the generation of (i) a shock wave and (ii) collective atom ic movements are invoked to account for the observed damage creation.