In-situ TEM observations of martensitic transformations in Ni-rich NiTi single crystals with coherent and aligned precipitates

摘要

Single crystal ingots were drawn using a Bridgman technique starting from polycrystalline NiTi alloys with a nominal Ni concentration of 50.7 at.-%. Compression aging in the [111]B2-direction for different aging times results in the formation of only one family of coherent, lenticular Ni_4Ti_3 precipitates which align perpendicular to the [111]_(B2)-direction. DSC measurements show three step transformations after short and two step transformations after longer aging times. An increase of M_s and R_s is observed with an increase in aging time. The formation of thermally-induced martensite was observed during in-situ TEM-cooling experiments. The first DSC peak of the phase transformation can be explained with the homogeneous formation of R-phase in precipitate/matrix-interfaces. With decreasing temperature, B19'-phase grows into the matrix and fills the channels between the precipitates (second peak). During in-situ TEM straining, small scale burst like transformation events from B2 to B19' in the channels between the precipitates are observed. No evidence for stress induced formation of R-phase was found during in-situ straining in the TEM.