Wide Hysteresis Shape Memory Alloys Research Articles

Microstructure and Phase Transformation Temperature of NiTiNb Shape Memory Alloy Prepared by Laser Solid Forming Using Mixed Powder

NiTiNb is a wide-hysteresis shape memory alloy. The Laser Solid Forming (LSF) technology can overcome the shortcomings of the traditional long cycle processing to prepare NiTiNb. In this work, we studied the microstructure and phase transformation temperature of the NiTiNb prepared by LSF., in which the Ni + Ti + Nb mixed powder was melted under different laser power P, scanning speed v, layer thickness t, and energy density EV. The results show that the combination of LSF process parameters with P = 2000 W and v = 900 mm/min can obtain a good metallurgical bond. As the laser power increases, the grain size increases, and the proportion of equiaxed crystals increases, the martensite transformation temperature increases. The inhomogeneity of the LSF-NiTiNb microstructure results in different phase transformation temperatures even in the same sample. The subsequent heat treatment at 850 °C for 3 h increases the phase transformation temperature and hysteresis of LSF-NiTiNb. The tensile properties of the LSF-NiTiNb samples with different building heights are significantly different. The maximum elongation reaches 8% and the minimum elongation is only 0.8%. The LSF parameter combination in this work has reference value for the parameter selection of subsequent preparation of NiTiNb.

ECAP based regulation mechanism of shape memory properties of NiTiNb alloys

NiTiNb is a kind of wide hysteresis shape memory alloy with great application potential in aerospace. However, the mechanism of improving the shape memory properties of the NiTiNb alloy by tailoring grain size and grain boundary type has not been well identified. Accordingly, taking equal channel angular pressing (ECAP) as the grain refinement method, this paper focused on the regulation mechanism of grain size and grain boundary type on the shape memory properties. First, grain refinement structures with a special distribution of low- and high- angle grain boundaries (LAGBs and HAGBs) were prepared by ECAP with different passes. Then, the mechanism of the grain boundary on the shape memory properties was first revealed. It was found that many LAGBs appeared in NiTiNb alloy after ECAP, and with increasing of ECAP passes, the LAGBs were transformed into the HAGBs, achieving grain refinement. The stress-induced transformation produced (001) compound twins in the dense sub-microcrystalline region surrounded by LAGBs, leading to a non-monotonic variation of transformation strain with decreasing grain size. Meanwhile, with the decrease of grain size (or the increase of grain boundary content), the transformation hysteresis temperature and recovery stress of the NiTiNb alloy increased simultaneously. To obtain better shape memory characteristics, we can tailor the type and distribution of specific grain boundaries through the design of ECAP.

Tailoring of microstructure and shape memory properties of hot-forged NiTiNb alloy by multi-pass ECAP at high temperatures

NiTiNb is a wide-hysteresis shape memory alloy, and the microstructure directly affects the shape memory properties. In this study, the microstructures and shape memory properties of hot-forged NiTiNb alloys were tailored by multi-pass equal-channel angular pressing (ECAP) at high temperatures. First, grain refinement structures with severely deformed grains and fully dynamic recrystallized equiaxed grains were prepared by ECAP at 650 °C and 800 °C (denoted as ECAP-650 and ECAP-800, respectively). The correlation between the two types of ECAP-based microstructures and shape memory properties of the NiTiNb alloy was established. The results indicated that there was no direct relationship between the grain size and thermally induced martensite (TIM) transformation. However, the dislocations in the deformed grains hindered the TIM transformation, resulting in a lower martensite transformation temperature. The greater β-Nb deformation during the stress-induced martensite (SIM) increased the martensite transformation hysteresis of the ECAP-processed samples. Simultaneously, the recrystallized grains were more beneficial to the stability of the SIM. Compared with the initial sample, the transformation hysteresis temperatures of the ECAP-650 and ECAP-800 samples increased by 29 °C and 34 °C, respectively. The refined grains were not conducive to the recovery of transformation strain, but they were beneficial for increasing the recovery stress. Compared to the initial sample, the recoverable transformation strain of the ECAP-650 and ECAP-800 samples decreased by 18.3% and 6.6%, respectively, and the corresponding maximum recovery stresses increased by 23.7% and 12.2%. Similar to the recovery stress, the thermomechanical cyclic stability of the ECAP-processed samples was also improved because the dislocation content introduced by lattice mismatch was reduced, and the sample with deformed grains had better stability than the sample with recrystallized grains.

Feasibility of self-pre-stressing concrete members using shape memory alloys

Shape memory alloys are a class of smart materials that recover apparent plastic deformation (∼6%–8% strain) after heating, thus “remembering” the original shape. This shape memory effect can be exploited for self-post-tensioning applications, and NiTi-based shape memory alloys are promising as shape memory effect is possible at elevated temperatures amenable to practical application compared to conventional NiTi. This study investigates the feasibility of self-post-tensioned concrete elements by activating the shape memory effect of NiTiNb, a class of wide-hysteresis shape memory alloys, using the heat of hydration of grout. First, the microstructure characterization of the NiTiNb wide-hysteresis shape memory alloys is discussed. Then, the tensile stress-induced martensitic transformations in NiTiNb shape memory alloy tendons are studied. Next, the temperature increase due to the heat of hydration of four commercially available grouts is investigated. Pull-out tests are also conducted to investigate the bond between the grout and shape memory alloy bar. Results show that the increase in temperature due to hydration heat can provide significant strain recovery during a free recovery experiment, while the same temperature increase only partially activates the shape memory alloys during a constrained recovery.

Effects of Ni/Ti Ratio and Heat-Treatment on Transformation Temperature, Mechanical Properties and Shape Recovery Strain of Ni-Ti-Nb Alloy

Ni-Ti-Nb wide hysteresis shape memory alloys of three Ni/Ti ratio components were manufactured by vacuum induction melting. The transformation temperature, mechanical properties and recovery strain were studied by using differential scanning calorimeter and material testing machine. It shows that with Ni/Ti ratio increase, the transformation temperature and mechanical properties decrease. Shape recovery strain is higher when Ni/Ti ratio is 1.068, with recovery strain range from 6.8 to 7.5. The faster the cooling rate after annealing, the higher is the transformation temperature, and the lower are the mechanical properties and recovery strain.

Study of Ni–Mn–Ga–Cu as single-phase wide-hysteresis shape memory alloys

Abstract Ni 50 Mn 25 Ga 25− x Cu x ( x = 3–10) alloys were studied as single-phase wide-hysteresis shape memory alloys, with the microstructure, martensitic transformation temperatures and hysteresis, mechanical properties, and shape memory behaviors. Single phase of martensite with tetragonal structure was detected for x x ≤ 10. The martensitic transformation start temperature M s was linearly increased from 477 K for x = 5 to 673 K for x = 8, then remained constant for x > 9. Simultaneously, the transformation hysteresis A s – M s , between the austenitic start and the martensitic start temperatures, was monotonically expanded from 6.6 K for x = 5 to 40 K for x = 8. The ductility of Ni–Mn–Ga alloys was effectively improved by Cu addition via strengthening the grain boundaries. Shape memory effect of 4.4% was obtained in Ni 50 Mn 25 Ga 17 Cu 8 polycrystal. The wide transformation hysteresis, the achieved shape memory effect and the improved ductility indicate Ni–Mn–Ga–Cu alloys a promising candidate for single-phase wide-hysteresis shape memory alloys.

A single-phase wide-hysteresis shape memory alloy Ni 50Mn 25Ga 17Cu 8

A single-phase wide-hysteresis shape memory alloy Ni50Mn25Ga17Cu8 is reported. This alloy undergoes martensitic transformation at 264 °C, and has transformation hysteresis of 40.4 °C, shape memory effect of 6.2%, compressive plasticity of 22%, and compressive strength of 878 MPa. The transformation hysteresis is further expanded by deformation in the martensite state.

Manufacturing Technology and Material Characterization of Ti<sub>44</sub>Ni<sub>47</sub>Nb<sub>9</sub> Alloy Fastener Ring by Tungsten Inert Gas Welding

The advanced manufacturing technology of TiNiNb wide hysteresis shape memory alloy fastener ring was performed by using the tungsten inert gas (TIG) welding technique. It was found that transformation temperatures of the joint were about 20 °C higher than these of the base alloy, which should be ascribed to the heat treatment in the joint caused by welding. The strain recovery behavior of the ring after the diameter expanding process was studied. For the ring with the diameter expansion of 16% and without the heat treatment after welding, the strain recovery began at 55°C and ended at 75°C. It is concluded that, by appropriate expanding diameter process, TIG welding technique is suitable for the production of TiNiNb wide hysteresis shape memory fastener ring.

Study of the thermal physical properties of Ti 47Ni 44Nb 9 wide hysteresis shape memory alloy

Dilatation characteristics of Ti 44Ni 47Nb 9 wide hysteresis shape memory alloys were measured in the temperature range of −60 to 350 °C. The coefficient of thermal expansion (CTE) undergoes obvious variations when a phase transformation occurs in the transformation temperature range. The results show the average coefficient of thermal expansion of Ni–Ti–Nb alloy is about 11.4 × 10 −6 °C −1 in the temperature range of 25–350 °C. And the heat capacity ratio, thermal diffusion ratio and coefficient of heat conductivity in the temperature range of room temperature to 400 °C of Ti 47Ni 44Nb 9 wide hysteresis shape memory alloys were also measured and analyzed. It is found that the heat capacity ratio, thermal diffusion ratio and coefficient of heat conductivity in function of the temperature curves are similar and their variation trends are also similar in the tested temperature range, which is increased with increasing of the temperature.

Two-Way Shape Memory Effect and Its Stability in a Ti–Ni–Nb Wide Hysteresis Shape Memory Alloy

The effect of training strain, deformation temperature and number of training cycles on the two-way shape memory effect (TWSME) and its stability have been investigated systematically in a Ti 46.3 N1 44.7 Nb 9 (at%) wide hysteresis shape memory alloy (SMA). The experimental results indicate that two-way shape memory strain increases with increasing the training strain up to 14.5% and then decreases with further increasing the training strain. When the training strain is less than 12%, the two-way shape memory strain increases with the increase of number of training cycles in the Ti 46.3 Ni 44.7 Nb 9 alloy deformed at the room temperature. Compared with TiNi binary alloy, the TWSME is smaller and the training strain to get the maximum TWSME is larger in the Ti-Ni-Nb alloy due to the deformation of β-Nb soft particles during training. However, the Ti-Ni-Nb alloy exhibits excellent stability of TWSME during the thermal cycling.

Temperature memory effect of Ni47Ti44Nb9 wide hysteresis shape memory alloy

Abstract The temperature memory effect (TME) phenomenon of Ni 47 Ti 44 Nb 9 wide hysteresis shape memory alloy was studied. It was found that TME occurred during the reverse transformation for the thermally-induced martensite (TIM) but not for the stress-induced martensite after incomplete transformation cycling. The reverse transformation temperature interval of TIM can be doubly broadened after 10 incomplete transformation cycles.

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

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 [2-5]. 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

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

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