Can Amortization be Induced by Ion Mixing in Ag-Cu and Ag-Ni Systems?
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binary alloy systems by ion irradiation(1),thus
demonstrating
that ion mixing is a powerful means of producing amorphous alloys. Among the systems studied so far,only a few systems did not show amorphization and metastable crystalline phases were formed instead.In a previous paper(2),it was demonstrated experimentally that the original "Structural Difference Rule"(3) could be extended into a more general form,i.e. the two-phase region rule.This rule specifies a sufficient condition that an amorphous phase is most lixely to form by ion mixing at sufficiently low temperature if-the alloy composition of a multilayered sample is chosen to be in the twophase region of an equiliorium phase diagram. This criterion is well satisfied by most of tne data available of ion mixing studies in binary metal systems(l). However,there exist a few exceptions remaining to be further investigated and explained, with A6-Cu and Ag-Ni systems being two typical examples. Both of these two systems feature a two-phase region character,but no amorphous phase was observed as reported in the early work of ion mixing by Tsaur et al.(4,5). The Ag-Cu and Ag-Ni systems are of interest also because i) Ag-Cu amorphous alloy has already been obtained by co-evaporating Ag and Cu onto cooled substrates and the transition temperature from amorphase to crystalline phase TA-C was measured to be around 373K(6), and ii) Ag-Ni system has a large positive heat of formation(+22kJ/mol),which,according to Alonso et al.(7), wuuld prohibit ion induced amorphization. The low transition
Mat. Res. Soc. Symp. Proc. Vol. S4. ý1986 Materials Research Society
216
temperature of amorphous Ag-Cu alloy and the large positive heat of formation,hence the large positive heat of mixing in Ag-Ni system inhibit the formation of amorphous phase. Therefore,it is believed that some exact experimental conditions and special precautions must be taken if the attempt to obtain amorphization can be a success. Thus a careful reexamination of these early studied systems seems useful to the modifying of the proposed criterion as well as to the understanding of ion mixing induced amorphization. EXPERIMENTAL PROCEDURE Multilayered samples of Ag-Cu and Ag-Ni were prepared by alternate deposition of the constituent metals onto inert substrate(NaCl) in a vacuum system. The vacuum level during evaporation was about 5XlO-5 Torr.. The relative thicknesses of the metals were designed to desired compositions and the total thickness of the deposited materials was approximately equal to the projected range plus projected range straggling of the irradiation ions(Xe+,300 KeV). The samples were then irradiated at liquid nitrogen temperature by Xe+ ion beam to the doses ranging from 5X1O1 5 Xe jcm 2 to 2X10 1 6 Xe+/cm2 . The vacuum level in the target chamber during irradiation was better than 2 X 10 6 Torr.. The ion bea- current density was carefully controlled to about 1 pA/cm in order to avoid oeam heatin6 effect. After ion beam mixin6,all the samples were analyzed by the TEM to identify the struct
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