Effect of Deformation and Thermal Treatment of NiTi Alloy on Transition Sequence

Transformation sequences in a NiTi shape memory alloy after deformation and subsequent annealing were studied using various methods. Low temperature annealing of the deformed alloy caused that the R-phase preceeded the martensitic transformation. The R-phase formation during both cooling and heating depends strongly on the deformation degree and temperature of annealing. On DTA curves an additional peak in the martensitic transformation range was observed for specimens annealed at 500 and 550°C. It is suggested that this peak was caused by presence of dislocation cell structure and subgrains whose boundaries interact with growing martensite plate. 1 .INTRODUCTION Deformation and subsequent annealing of NiTi alloys offers great possibilities of controlling temperatures and sequences of transformations. Todoroki and Tamura [I] showed that, depending on annealing temperature, changes in transformation sequences in deformed alloys occur during both heating and cooling. Abujudom et. al. [2] found that the deformation degree significantly influences both course and characteristic temperatures of the transformations. The authors of [3-81 studied the effect of both deformation degree and annealing temperatures on the course of transformations. In [4,5] and [8] the influence of deformation and annealing on shape recovery phenomenon was investigated. It was shown [8] that large deformations have a positive influence on shape recovery. However, the results mentioned above show certain discrepancies, especially in variations of characteristic temperatures versus annealing temperature and in sequences of transformations. The aim of this paper was to establish the influence of dislocation density and configuration on the characteristic temperatures and sequences of transformations. EXPERIMENTAL PROCEDURE A commercial NiTi shape memory alloy containing 50.6 at % Ni was used in this study. Specimens in shape of strips were quenched from 800°C after annealing for l h and then cold-rolled by 5 %, lo%, 20% or 25 % reduction. The deformed samples were then annealed at various temperatures between 300-550°C. The transformation behaviour and the characteristic temperatures were determined using the DTA method on the TA1 Mettler instrument and the internal friction method on an acoustic relaxator with the logarithmic decrement of damping. The phase composition of the specimens was established using the X-ray phase analysis on a Philips diffractometer equipped with a temperature attachment. CuKa radiation was used. The alloy structure was studied on thin foils using JEM 200B transmission electron microscope. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1995232 JOURNAL DE PHYSIQUE IV 3. EXPERIMENTAL RESULTS 3.1. Effect of deformation The studied alloy, after quenching from 800°C, consisted of the parent B2 phase only as was shown by X-ray studies. Deformation by cold-rolling caused strengthening of the alloy as evidenced by increase in its hardness. The 110 B2 diffraction line widened due to increase in lattice strain and density of dislocations (Fig. 1). Fig. 1 Hardness, M, temperature and half-width of the 110 B2 versus deformation degree Fig. 3 The DTA curves obtained during cooling of the alloy deformed by 10% Fig. 2 Changes of enthalpy of reverse transformation and amount of martensite versus deformation degree