Precursor phenomena in a quenched and aged Ni ₅₂Ti ₄₈ shape memory alloy

Abstract
We measured the electrical resistivity R(T) and specific heat C (T) between room temperature (RT) and 4.2 K as well as the microstructure by transmission electron microscopy (TEM) of a Ni ₅₂Ti ₄₈ SMA quenched from 1000°C (B2-Phase range) to RT and then annealed for 1h at  C, 550°C and 650°C. In the "as quenched" and the "650°C annealed" state no martensitic transformations (MT's) occur. The diffraction pattems show faint reflections originating from coherent Ni ₄Ti ₃, precipitates in an early state of formation. Additional reflections of the type 1/2 <110 >, 1/2 <111 > and 1/3 <110 > 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 <110 > <110 > lattice displacement waves, precursors of the B19' martensite. The coherent Ni ₄Ti 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 C, however, coherent Ni ₄Ti ₃ precipitates with a length of 10 nm 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 (T). Annealing at 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 (T).