Positron Annihilation Measurements in Cu-Al-Ni Based Shape Memory Alloys
Katholieke Universiteit Leuven, Dept. Metaalkunde en Toegepaste Materiaalkunde, de Croylaan 2, 3001 Leuven (Heverlee), Belgium
2 Rijksuniversiteit Gent, Vakgroep subatomaire en stralingsfysica, Proeftuinstraat 86, 9000 Gent, Belgium
Positron annihilation lifetime and Doppler-broadening measurements were carried out on Cu-Al-Ni based alloys to which Ti, Mn or B were added. The decrease in the mean lifetime until a final value as a function of both the ageing time and temperature in β-phase is explained by a combination of factors including annealing of excess vacancies, ordering and precipitation. The in situ measurements reveal a drastic effect in the S-lineshape parameter behaviour when the defects anneal out at the transition from martensite to the beta phase. Two stages have been detected in this process for the alloy containing both Ti an B. When crossing the transition region several times, an increase of the S-lineshape parameter value is observed. This is interpreted as due to the formation of dislocations and precipitate-martensite interfaces. In the alloy with both Ti and Mn additions a distinct behaviour in the S-lineshape parameter is observed at low temperatures and attributed to some type of vacancy association or cluster.
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