Martensitic Transformations and Functional Stability in Ultra-Fine Grained NiTi Shape Memory Alloys

Abstract

Martensitic transformations in NiTi shape memory alloys (SMAs) strongly depend on the microstructure. In the present work, we investigate how martensitic transformations are affected by various types of ultra-fine grained (UFG) microstructures resulting from various processing routes. NiTi SMAs with UFG microstructures were obtained by equal channel angular pressing, high pressure torsion, wire drawing and subsequent annealing treatments. The resulting material states were characterized by transmission electron microscopy and differential scanning calorimetry (DSC). The three thermomechanical processing routes yield microstructures which significantly differ in terms of grain size and related DSC chart features. While the initial coarse grained material shows a well defined one-step martensitic transformation on cooling, two-step transformations were found for all UFG material states. The functional stability of the various UFG microstructures was evaluated by thermal cycling. It was found that UFG NiTi alloys show a significantly higher stability. In the present work, we also provide preliminary results on the effect of grain size on the undercooling required to transform the material into B19’ and on the related heat of transformation.

Suppression of Ni4Ti3 Precipitation by Grain Size Refinement in Ni‐Rich NiTi Shape Memory Alloys

“…After HPT, specimens were annealed at 400 8C for 1 h to crystallize them from the amorphous state, producing an NC sample. [11] Samples of the CG, UFG, and NC materials were aged at 400 8C in air for 1, 10, and 100 h and quenched in water for the study of microstructural evolution. A temperature of 400 8C was selected for the long-term annealing based on previous investigations by Prokofiev et al [14] and Prokoshkin et al [15,30] which revealed that the NC grain size formed from the amorphous state can be maintained below 100 nm during annealing up to 400 8C.…”

“…Recently, there has been great interest in the application of severe plastic deformation (SPD) to NiTi materials because the extremely fine grain sizes obtainable by SPD processing have been observed to enhance some mechanical and functional properties as compared to coarse grained (CG) materials. [8][9][10][11][12][13][14][15][16][17] Thus, arterial stent manufacturing presents an exciting and innovative potential application for SPD processing techniques, and the field of shape memory technology is developing strong ties with the research fields of ultra fine grained (UFG) and nanocrystalline (NC) materials. [11] The austenitic high temperature phase of NiTi (which has a B2 strukturbericht designation) can solve small amounts of excess Ni, and solubility decreases with decreasing temperature.…”

“…[8][9][10][11][12][13][14][15][16][17] Thus, arterial stent manufacturing presents an exciting and innovative potential application for SPD processing techniques, and the field of shape memory technology is developing strong ties with the research fields of ultra fine grained (UFG) and nanocrystalline (NC) materials. [11] The austenitic high temperature phase of NiTi (which has a B2 strukturbericht designation) can solve small amounts of excess Ni, and solubility decreases with decreasing temperature. [18,19] The production of shape memory components typically requires thermomechanical processing, [20] and the associated procedures are often performed below the stability range of the B2 phase.…”

On the influence of crystal defects on the functional stability of NiTi based shape memory alloys

“…Internal interfaces have a strong effect on the martensitic transformation, e.g. [9,11], and on the functional stability. Fig.…”

“…Further details on the wire drawing procedure are given elsewhere [9]. In order to investigate the influence of dislocations on the functional stability, a mild cold work (10% deformation) was applied using a rolling mill of type Buehler EW 200 x 120 A.…”