A morphological study of “Premartensitic” effects in TiNi

Abstract

The microstructure of TiNi just above its martensite transformation temperature has been investigated' by various transmission electron microscopy imaging experiments. The extra reflections which occur in diffraction patterns at this temperature have previously been interpreted by the present authors in terms of lattice displacement waves (LDW) in the structure. Dark field microscopy using the extra reflections reveals, by 2 1 2 D analysis, that two types of premartensitic phenomena are present : one associated with small particles (∼80 Å diameter) and the other with larger particles ∼200 Å diameter). The former contain LDW's which can be directly related to the atomic movements occurring upon the phase transformation, whilst the latter contain LDW's not directly related to the transition. The diffraction effects characteristic of these LDW's also behave differently during temperature cycling. Lattice fringe imaging has also been employed to study the premartensitic structure at high resolution. In addition to confirming the 2 1 2 D image results, new information is obtained concerning the incommensurate nature of the 1 2 〈100〉 〈100〉 LDW's. The structure is rather complex. Some particles are incommensurate, others are commensurate but contain a discommensuration whilst a third category appears to be exactly commensurate with the host lattice. Furthermore, adjacent particles may have LDW's out-of-phase with one another (termed anticommensurate). The structural nature of the premartensitic state is considered in these terms and its relationship to previous work and to the phase transformation is discussed.