Mechanical Properties of Ti-Base Shape Memory Alloys

Shape memory alloys (SMA) such as Ti-Ni are widely used in t he biomedical field due to their unique functions of shape memory effect (SME) and superelast icity (SE). However, a drawback of Ti-Ni SMAs in the biomedical applications is the t oxicity of the constitute element Ni, in other words, Ni hypersensitivity. Thus we have conducte to develop new Ni free SMAs composed of non-toxic elements as a new class of biomedical funct ional materials. In the present study, the mechanical and shape memory properties of some Ti-base shape memory alloys we have found are described. The alloy systems s lected were Ti-Nb-Al and Ti-Nb-Ga. Both the alloys were made by Ar arc melting m ethod followed by a homogenization treatment at 1273K for 7.2ks, cold rolling and then a solution treatment at 1273K for 1.8ks. Tensile tests were done to evaluate SME and SE. It w as found that both TiNbAl and TiNbGa alloys exhibit SE at room temperature, and that the SE behavior changes depending on the deformation strain. 2.4% superelastic strain obtained for the TiNbAl alloy after the cyclic deformation of 6%. Besides the SE behavior become s stable after two cyclic deformation under the constant maximum strain. It is summarized t hat these Ti-base SMAs exhibit large potential for practical biomedical use. Introduction Shape memory alloys (SMAs) are fascinating materials for bi medical applications. Although only Ti-Ni SMAs have been used for biomedical practical a pp ications because of their superior shape memory properties as well as biocompatibilit y [1], Ti-Ni SMAs contains Ni atoms which possess the probability of Ni hypersensitivity. T herefore, in order to reduce the risk of toxics of elements, new biomedical SMAs consisting of non-toxic elements only, especially Ni-free, are required to replace Ti-Ni SMAs. Ti-base alloys have been used practically as implant materia ls due to their high biocompatibility, good mechanical properties and high biocorrosion resista nce [2-4]. Then Ti alloys must be also applicable for the biofunctional use if s hape memory effect is improved. Then our group has systematically investigated new Ti SMAs [5-11]. It has been reported that some Ti alloys have a martensitic transformation from β-phase (disordered bcc) to α’-phase (hcp) or α’’-phase (orthorhombic) by quenching [12]. Generally, α’-phase is formed in the alloys containing a small amount of additional element s which stabilize β-phase. On the other hand, α’’-phase can transform to β-phase via reverse martensitic transformation and is formed in the alloys containing a relatively large amount f additional elements which Materials Science Forum Online: 2003-08-15 ISSN: 1662-9752, Vols. 426-432, pp 3121-3126 doi:10.4028/www.scientific. net/MSF.426-432.3121 © 2003 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific. net. (Semanticscholar.org-10/03/20,17:11:27) stabilize β-phase. Thus, in order to create new SMAs based on Ti alloys, the c omposition control of the βand α-stabilizers is important as well as to induce thermoelastic martensite transformation. In this study, shape memory properties of Ti-NbAl and Ti-Nb-Ge alloys were investigated. Experimental Procedure The nominal compositions of Ti-Nb-Al and Ti-Nb-Ge alloys investiga ted were chosen to be 24mol%Nb, and 3mol%Al and Ga, and the alloys are called TiNbAl and TiNbGa. These alloys were prepared by Ar arc melting method using high pur ity elemental Ti, Nb, Al and Ga. The ingots were homogenized at 1273K for 7.2ks in vacuum, and then cold-rolled to b 95% reduction in thickness. The reduction corresponded to 0.4mm in thickness. A olution treatment at 1273K for 1.8ks in vacuum was carried out. Mechanical and s h pe memory properties after solution treatment were evaluated through tensile te ts. Specimens for the mechanical tests were cut from the sheet and damaged surfac e was removed by polishing. Tensile tests were conducted at room temperature (RT) under the stain rate of 5x10 /s. The gauge length of specimens was 10mm. Besides, two kinds of cyclic loading and unloading deformation tests were carried out to investigate the effect of deformation on superelastic behavior: (1) constant strain increment test in which the applied str ain increases by 2% per cycle with increasing the number of loading and unloading cycles, a nd (2) constant maximum strain test in which the maximum engineering strain was fixe d to be 4% and cyclic loading and unloading deformation was done. Results and discussion Tensile properties Figure 1 shows the stress-strain curves of TiNbAl and TiNbGa. Both alloys exhibit good tensile properties: large elongation of 21% f or TiNbAl and 26% for TiNbGa, and ultimate tensile strength (UTS) of 630MPa for TiNbAl and 510MPa for T iNbGa. 0.2% flow stress is 155MPa for TiNbAl and 105MPa for TiNbGa. The diff rence in 0.2% flow stress is not explained by solid solution hardening but by the dif f rence in martensitic transformation temperature. The increase in flow stress is considere d to b simply caused by decreasing Ms: the stress to induce martensitic transformation at a tempera tur becomes higher by lowering Ms due to the Clausius-Clapeyron relationship. In other words, higher stress is required to induce martensitic transformation when Ms becomes lower. The reduction of Ms by addition of Al seems larger as compared with Ga addition, sinc e the 0.2% flow stress is higher in TiNbAl than in TiNbGa if the flow str ess corresponds to the stress to induce martensitic transformation. Constant strain increment test Figures 2 and 3 show stress-strain curves obtained for the constant strain increment tests for TiNbAl (Fig.2) and TiNbGa (Fig.3). In these experiments, the specimens were firstly deformed up to 2% and unloa ded. Then, the specimens were again loaded up to 4% (previous strain of 2% plus addit ional strain of 2% constant) strain and unloaded. And also the specimens were loaded again up to 6% (=4%+2%) and unloaded. Such cyclic loading-unloading deformation was repea ted up to 12%. It is clear that both alloys exhibit superelasticity. Mor eover, with repeating the cyclic deformation, the superelastic behavior becomes clear for TiNbAl in Fig.2 but the hysteresis of superelastic curves between loading and unloading becomes small for TiNbGa in Fig.3. The effect of cyclic deformation on superelasticity is drawn in Fi gure 4. The superelastic strain is small for the solution treated alloys because plastic deform ation due to dislocations is also introduced during deformation. However, the superelasticity becomes s table by the cyclic deformation due to work hardening: this is called the training eff ect. Consequently, the stress to induce martensitic transformation ( σSIMT) should become lower because of higher internal stress introduced. Figure 5 shows relationship between σSIMT and number of cycles (N). σSIMT becomes lower with increasing the number of cycles, as expected. 3122 THERMEC'2003