HIGH-RESOLUTION ELECTRON-MICROSCOPY STUDIES OF MARTENSITE AROUND XS PRECIPITATES IN A CU-AL-NI-MN-TI SHAPE-MEMORY ALLOY

Microstructures of beta l' martensite around Xs precipitate in a Cu-Al -Ni-Mn-Ti shape memory alloy have been studied by high resolution elec tron microscopy. Both formation of self-accommodation martensite varia nts and change in 18R stacking sequence to 2H sequence were often obse rved to occur simultaneously around Xs in samples water-quenched from 1223K. A new orientation relationship between 2H and 18R martensite ha s been found: (12 (8) over bar)(18R)//(12 (1) over bar)(2H) and [231]( 18R)//[2 (1) over bar 0](2H). Analysis of composition of martensite ar ound and away from the Xs precipitate shows that enrichment of Al in t he matrix close to Xs account for the change in the martensite stackin g sequence. Meanwhile, Xs was found to have different composition with X-L precipitate and could be termed as (Cu, Ti)(2)NiAl. (C) 1997 Acta Metallurgica Inc.