Precursor phenomena in a quenched and aged Ni52Ti48shape memory alloy

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We measured the electrical resistivity R(T) and specific heat C P (T) between room temperature (RT) and 4.2 K as well as the microstructure by transmission electron microscopy (TEM) of a Ni 52 Ti 48 SMA quenched from 1000°C (B2-Phase range) to RT and then annealed for 1h at T=380°C, 550°C and 650°C. In the as quenched and the 650°C annealed state no martensitic transformations (MT's) occur. The diffraction patterns show faint reflections originating from coherent Ni 4 Ti 3 precipitates in an early state of formation. Additional reflections of the type 1/2 , 1/2 and 1/3 result from various lattice displacement waves, which are precursors of the MT's to the B19' and R-phase, respectively. Indeed, high resolution TEM micrographs of the [001] zone of the as quenched sample reveal transverse 1/2 lattice displacement waves, precursors of the B19' martensite. The coherent Ni 3 Ti 3 precipitates, homogeneously distributed on a small length scale, hinder the MT's in the as quenched and the 650°C annealed state, and thus only the precursors appear. When annealed at T=380°C, however, coherent Ni 4 Ti 3 precipitates with a length of lOnm are clearly visible in TEM. These precipitates trigger the MT from the B2 to the R-phase on cooling, as evidenced also by anomalies in R(T) and C p (T). Annealing at T=550°C leads to the well known two step MT's from the B2 to the R-phase and then into the B19'-phase. These martensitic transitions are clearly seen as additional peaks in the specific heat and anomalies in the resistance, while the as quenched and 650°C annealed samples show weak features in R(T) and C p (T).