Shape Memory Characteristics Of Ti Research Articles

Shape Memory Characteristics of Biomaterial Ti-(14-22)Zr-6Nb-2Mo-2Sn(at.%) Alloys

Since the recently reported biotoxicity of Ni, the use of Ti-Ni base shape memory alloys as biomaterial has been reduced. For this reason, Ti–Nb base superelastic alloys consisting of only non-toxic elements are being actively studied for potential use as biomaterials. In this study, the effects of Zr content and annealing temperature on the shape memory characteristics of Ti-(14-22)Zr-6Nb-2Mo-2Sn alloys were investigated using tensile tests, X-ray diffraction measurement and Electro Backscatter Diffraction (EBSD). Results showed that the maximum total recovery strain and critical stress for slip increased with increasing Zr content. This is due to the change in lattice constant with the addition of Zr. In all annealed conditions XRD analysis of the Ti-22Zr-6Nb-2Mo-2Sn(at.%) alloy revealed only β phase at room temperature, and outstanding superelasticity was observed. This means that the α/β transus temperature of this alloy is below 650oC. Maximum total recovery strain and critical stress for slip increased with decreasing annealing temperature. A maximum of 592MPa and 5.83% were obtained in the Ti-22Zr-6Nb-2Mo-2Sn(at.%) alloy annealed at 650°C for 1hr. The {001}β<110>β texture intensity increased with decreasing annealing temperature. The maximum recovery strain in the Ti-22Zr-6Nb-2Mo-2Sn(at.%) alloy annealed at 650°C for 1hr may have been caused by the well-developed {001}β<110>β type recrystallization texture.

Shape memory characteristics of Ti 49.5Ni 25Pd 25Sc 0.5 high-temperature shape memory alloy after severe plastic deformation

A Ti 49.5Ni 25Pd 25Sc 0.5 high-temperature shape memory alloy is thermomechanically processed to obtain enhanced shape-memory characteristics: in particular, dimensional stability upon repeated thermal cycles under constant loads. This is accomplished using severe plastic deformation via equal channel angular extrusion (ECAE) and post-processing annealing heat treatments. The results of the thermomechanical experiments reveal that the processed materials display enhanced shape memory response, exhibiting higher recoverable transformation and reduced irrecoverable strain levels upon thermal cycling compared with the unprocessed material. This improvement is attributed to the increased strength and resistance of the material against defect generation upon phase transformation as a result of the microstructural refinement due to the ECAE process, as supported by the electron microscopy observations.

Transformation Behavior of Ti-(45-x)Ni-5Cu-xCr (at%) (x = 0.5-2.0) Shape Memory Alloys

Transformation behavior and shape memory characteristics of Ti-(45-x)Ni-5Cu-xCr (x=0.5-2.0) alloys have been investigated by means of electrical resistivity measurements, differential scanning calorimetry, X-ray diffraction and thermal cycling tests under constant load. Two-stage B2-B19-B19' transformation occurred in Ti-(45-x)Ni-5Cu-xCr alloys. The B2-B19 transformation was separated clearly from the B19-B19' transformation in Ti-44.0Ni-5Cu-1.0Cr and Ti-43.5Ni-5Cu-1.5Cr alloys. A temperature range where the B19 martensite exists was expanded with increasing Cr content because decreasing rate of Ms (85 K / % Cr) was larger than that of Ms' (17 K / % Cr). Ti-(45-x)Ni-5Cu-xCr alloys were deformed in plastic manner with a fracture strain of 68% ~ 43% depending on Cr content. Substitution of Cr for Ni improves the critical stress for slip deformation in a Ti-45Ni-5Cu alloy due to solid solution hardening.

Effect of annealing conditions on microstructures and shape memory characteristics of Ti 50–Ni 30–Cu 20 alloy ribbons

Ti 50–Ni 30–Cu 20 alloy ribbons without any crystal phases have been fabricated at the wheel velocities of 55 m/s and the melt spinning temperature of 1500 °C by melt spinning process. Their microstructures and shape memory characteristics were investigated by means of optical microscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The crystallization temperature was measured to be 440 °C. The crystallized ribbon exhibited fine microstructure after annealed at 440 °C for 10 min and was deformed up to about 6.8%. During thermal cycle with the applied stress of 220 MPa, transformation hysteresis and elongation associated with a B2–B19 transformation were observed to be 4 and 3.6%. The ribbons showed a perfect superelasticity and their stress hysteresis was as small as 20 MPa which was attributed to the single pair martensite variants in small grains. This controlled microstructure was achieved by a proper heat treatment of amorphous ribbons in Ti 50–Ni 30–Cu 20 alloy.

The Effect of Rapidly Solidified Microstructures on the Martensitic Transformation in Ti<sub>50</sub>-Ni<sub>45</sub>-Cu<sub>5</sub> Shape Memory Alloys

Transformation behaviors and shape memory characteristics of Ti–Ni45-Cu5 alloy ribbons prepared by melt spinning were investigated by means of DSC, XRD and OM. In these experiments particular attention has been paid to change the ejection temperature of the melt from 1400°C to 1600 °C. As the results, the thickness of ribbons could be controlled. An increase of the superheat of the melt leads to a reduced ribbon thickness and a refinement of grains. The microstructural refinement and the increased internal strains achieved by controlling the melt-spinning temperature decreased Ms significantly. It was also found that two-step transformation (B2-B19-B19’) occurred in the ribbons fabricated at higher melt-spinning temperatures than 1450°C.

Phase transformation behavior and shape memory characteristics of Ti−Ni−Si alloys

Transformation behavior, microstructures and shape memory characteristics of Ti−(50−X)Ni−XSi (X=2, 4, 6 at.%) and (50−X)Ti−Ni−XSi (X=2, 5, 7, 10 at.%) alloys were investigated by means of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, differential scanning calorimetry, electrical resistivity measurements and constant load thermal cycling tests. Ti 5 Si 3 , Ni 16 Ti 6 Si 7 and Ni 4 Ti 4 Si 7 were formed in Ti−(50−X)Ni−XSi alloys, while Ti 5 Si 4 , Ni 3 Si, Ni 3 Ti 2 and Ni 3 Ti 2 Si were found in (50−X)Ti−Ni−XSi alloys. The total amount of silicides increased with increasing Si content, irrespective of Si content. The B2→B19 transformation occurred in Ti−(50−X)Ni−XSi alloys, and their transformation temperatures appeared to be almost constant. Transformation elongation associated with the B2→B19 transformation decreased with increasing Si content. In contrast to Ti−(50−X)Ni−XSi alloys, a transformation accompanied with structural change did not occur in (50−X)Ti−Ni−XSi alloys.