Ion Tracks in Metals and Intermetallic Compounds
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MRS BULLETIN/DECEMBER 1995
by high {dE/dx\; and (3) study of {dE/dx),, effects in bulk targets due to the very large range (a few tens to 100 fim) of such projectiles. It was thus very tempting to determine whether in such extreme conditions, it was possible to induce some damage in metallic targets following high electronic-energy deposition. In fact, quite a few unexpected microstructural modifications, to be described in more detail later, could be evidenced. They are seen in materials that exist in various stable or metastable phases. The easiest observation is the transformation from a crystalline state to an amorphous one. (Amorphous latent tracks are seen in some intermetallic compounds, similar to what was observed long ago in some insulators.)1 Microstructural changes can also occur in materials that have various allotropic forms. Defective crystalline "tracks" are observed in a few pure metals and lead, for example, in titanium to a phase transformation from a crystalline phase to another crystalline phase at high irradiation fluences. All these effects that are due to high electronic excitations will result in microstructural modifications localized in the close vicinity of the projectile paths and will hereafter be named "tracks." In most cases, they will be evidenced directly by transmission electron microscopy or indirectly considering their consequences, using, for example, small angle x-ray scattering or differential chemical attack of the damaged zones (as currently used in insulators).
Amorphous Tracks in Metallic Crystalline Compounds The first tracks in bulk metallic materials were observed a few years ago in intermetallic alloys. Two classes of compounds have been studied:
(1) The first class consists of compounds that are known to be easily amorphized by elastic collisions during irradiations with low-energy ions or MeV electrons. 5 Among them, NiZr 2 , NiTi, and Ni3B are of particular interest. All of them are characterized by low local symmetry. Ni3B and NiZr2 have, respectively, orthorhombic and tetragonal crystallographic structures over the complete temperature range whereas NiTi experiences a martensitic transformation at a temperature that depends on the thermomechanical treatments but lies close to 300 K. NiTi is in a cubic B2 phase above the transition temperature and in a monoclinic phase below. (2) The second class consists of compounds that are characterized by high local symmetry (LI2 crystallographic structure). They include Zr3Al, which is rather difficult to amorphize by elastic collisions,6 and Cu3Au, which has never been amorphized by any method, but which can be chemically disordered during low-temperature irradiations in the elastic collision regime. In the second class of compounds, the only observable damage could be accounted for by elastic collisions 7 " 9 whereas sufficiently high levels of electronic-energy deposition induce amorphous track formation in the first class of compounds.
The Simple Cases of NiZr2 and Ni3B
In NiZr 2 , 81011 latent tracks are observed a
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