A study of collision cascade collapse in Cu³Au
This thesis describes an ion irradiation study of ordered Cu3M/sub>Au. Argon, Copper and Krypton ions of 50 and 100keV were implanted to nominal doses of 1011 ions cm-2 both at room temperature and at liquid helium temperature. The damage induced at low temperature was observed in-situ and after subsequent warming to room temperature. The collapse of collision cascades to vacancy dislocation loops was observed to occur with moderately high probability under all irradiation conditions at low temperature, with no subsequent increase in the number of loops on warming. Comparison of the collapsed fraction of cascades at low temperature with that produced by room temperature irradiation revealed that the collapse process is more efficient in the latter case. An increase in collapse probability with ion mass was also observed, although no dependence on ion energy was found over the range investigated. Quantitative analysis of the disordered zones which result in ordered Cu3Au at individual cascade sites revealed that cascades which had collapsed to loops generally produced larger disordered zones. Additionally, increases in disordered zone size with ion mass, ion energy and irradiation temperature were identified. The observation that disordered zone sizes exceed the theoretically predicted cascade dimensions for the case of the highest energy density in this study (50keV Kr) is interpreted as evidence for leakage of energy from the cascade region to the surrounding lattice. A model is proposed which attributes the observed cascade collapse behaviour to vacancy migration during the cooling of the cascade region being systematically directed toward its centre, and accounts for the observed disordered zones in terms of extensive atomic rearrangement during the cooling. The major factors identified as controlling this vacancy and atomic motion are the initial spatial distribution of energy and vacancies within the region, and the rate at which the region cools.