Recovery stress of Ni-Ti-Nb wide-hysteresis shape memory alloy under constant strain and thermomechanical cycling

Abstract

Ni-Ti-Nb alloy is a new type of shape memory alloy with a wide transformation hysteresis (A's-M~ = 130-150 °C) [1]. Parts made of the alloy can be stored and shipped at ambient temperature without keeping in liquid nitrogen, which is very convenient for engineering. Therefore, this new alloy has great potential usage as mechanical components for joining, fastening and sealing. The microstructure and effect of deformation on transformation hysteresis of the alloy have been studied [2][3][4][5]. However, the recovery stress of Ni-Ti-Nb shape memory alloy has not been reported. Therefore, the purpose of the present work was to investigate the effect of deformation and thermomechanical cycling on the recovery stress of Ni-Ti-Nb alloy.The composition of the alloy was 44.7 at % Ni, 46.3 at % Ti and 9 at % Nb. The Mr, Ms, As and Af temperatures of the alloy were determined to be -36, 14, 45 and 60 °C, respectively, by means of electrical resistance versus temperature measurement. The resultant ingot was hot-swaged and rolled at 850 °C to strips of approximately 0.5 mm thickness. Specimens for all tests having a tensile gauge length of 40 mm and a width of 5 mm were cut from the strips and annealed at 850°C for 1.8 x 103s under 0.1 MPa vacuum and then furnace cooled. For measuring recovery stress, tensile tests were performed on a Shimadzu Autograph DDS-10T-S machine with fixture to allow heating the specimen for temperature control with a strain rate of 4.1 x 10 -4 s -1. The schematic stress-strain curve in the experiments is shown in Fig. 1. The tensile specimens were deformed at various temperatures (Ta) to different total strain (eT) and then the load was removed. The specimen was then slowly heated from Td to T 1 above Af and the stress was increased to OR (recovery stress). In order to study the effect of thermomechanical cycling on recovery stress, following the above process, the specimens were cooled to T2 below Mf under constant strain and then heated to T 1 again. These heating and cooling processes were repeated 50 times and (Xg was recorded as a function of the cycling number N. The values of T 1 and T2 were 300 and -40 °C, respectively. Fig. 2 shows the relationships between tr R and eT for specimens deformed at room temperature (20 °C). It can be seen that when e T is < 9% o R increases with increasing eT and reaches a maximum value at eT = 9%, then decreases with increasing eT.It was previously found that the microstructure of Ni-Ti-Nb alloy consists of NiTi matrix and fi-Nb phase [2]. When the Ni44.7Ti46.3Nb 9 alloy was deformed at room temperature (a little higher than Ms) the stress-induced martensitic transformation occured in the Ni-Ti matrix [7]. Thus, Fig. 2 can be easily understood. When the total strain is < 9% no true plastic deformation occurs for the NiTi matrix of the specimen. The strain of the NiTi matrix is mainly attributed to the stress-induced martensitic transformation and martensite reorientation. The value of aR depends on the amount of reversible strain of the Ni-Ti matrix constr...

Mechanical behavior in NiTiNb shape memory alloys with low Nb content

“…So far, most of the work on mechanical behavior in NiTiNb alloys was taken the alloy with 9 at.% Nb [4,5]. However, recent research focus more eyes on the NiTiNb alloys with low Nb.…”

Characterization of shape memory alloys for safety mechanisms.

“…Rod 3 was cooled with liquid nitrogen to −50 °C and subsequently heated at 2 °C/min in a thermomechanical analyzer. The rod contracted negligibly during the cooling to −50 °C, but the pre-treatment resulted in a lowering of the transformation temperature to 40 °C, which indicates the sensitivity of the NiTiNb behavior to processing and its initial condition [1,2,4,6,12,20]. The effect of cooling NiTiNb on its heating transformation behavior has not been studied; therefore more testing is required to better understand this observed phenomenon.…”

Constant-Strain Thermal Cycling of a Ni50.3Ti29.7Hf20 High-Temperature Shape Memory Alloy

“…The magnitude of the recovery stress is dependent on the amount of oriented martensite present in the material; thus maximizing the amount of oriented martensite such as prestraining [17][18][19] will magnify these stresses. Plastic deformation, as with other shape memory properties, can supersede this process.…”