TEM and HREM study of the interphase interface structure of Ti3Ni4 precipitates and parent phase in an aged TiNi shape memory alloy

The structures of coherent and semi-coherent interphase interfaces between Ti3Ni4 precipitates and parent phase in different aged Ti-51.5Ni (at.%) shape memory alloy have been studied by means of diffraction contrast analyses in transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). When the alloy was aged at 723 K for 3 h, very small Ti3Ni4 precipitates formed in the matrix with a coherent interface to the parent phase. Very strong strain field was observed in this kind of coherent interface using TEM and HREM. When the alloy was aged at 873 K for 20 h, large Ti3Ni4 precipitates formed in the parent phase matrix possessing a semi-coherent interface. The dislocation networks were observed in the semi-coherent interface. The Burgers vectors of these sets of dislocations are determined to be [111](B2) and [200](B2) types. Very weak strain field was observed in this kind of interface.