The change of transformation temperature on NiTi shape memory alloy by pressure and thermal ageing

We have investigated the effect of pressure and thermal ageing on transformation behavior of Ni-%45.16Ti shape memory alloy. 70-320 MPa pressure was applied and annealed at 500 o C for 2 hour to see the effect of applied pressure on transformation temperature. It was observed that the starting and final temperature of austenit increased, the martensite start temperature almost remain constant and final temperature decreased with the increase of pressure. The interval stress value of NiTi alloy was determined as 13.77 MPa/ o C during austenite transformation. The transformation temperature of Ni-%45.16Ti alloys after thermal ageing was also investigated. No significant variation in these parameters was obtained. According to XRD measurements, there are two phases namely R (rhombohedral) and B19 (monoclinic) martensite were obtained. We did not see any change on the peaks after ageing. 1.Introduction NiTi alloy with near-equiatomic composition is the most useful shape memory alloy (SMA) among many shape memory alloys. The application fields of these alloys are production of various medical devices such as orthopedic surgical implant, artificial prosthesis [1]. There are three different phases that have been observed throughout the martensitic transformation of NiTi: B2 (cubic), R (rhombohedral) and B19’ (monoclinic). The high temperature austenitic B2 phase (CsCl) undergoes a displacive structure transformation to intermediate R phase about 293 K and transforms to the low temperature martensite B19’ phase. It is not clear in which condition this unusual behavior occurs. This can be explained as another formation of martensitic behavior [2,3]. In summary, there are two series of phase transitions in NiTi: B2→B19 and B2→R→B19’. The first series of transformations is observed in NiTi samples containing less than 50 %at Ni; the second occurs in samples with higher Ni concentration [4]. The shape memory behavior of NiTi alloys is influenced by their chemical composition, heat treatment (annealing and ageing) and mechanical deformation [5]. Effect of ageing treatment on the transformation behavior of NiTi alloy was studied by Y. Zheng and co-workers [6]. They chose various ageing temperature for 1.8 ks (30 min.) and investigated variation on martensitic transformation temperature and crystal structure. It was seen that the multistage phase appeared or disappeared by variable temperatures. Lin et al. were focused on 9th International Conference on Magnetic and Superconducting Materials (MSM15) IOP Publishing Journal of Physics: Conference Series 667 (2016) 012011 doi:10.1088/1742-6596/667/1/012011 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 ageing effect on V doped NiTi alloy. They aged this alloy at 400 o C for 1-10 h and examined martensitic transformation temperature and crystal structure changes. It was found that R phase appeared with residual ageing time [7]. K. Allafi and co-workers investigated the mechanism of multistage martensitic transformations in aged NiTi shape memory alloys. According to their study, annealed NiTi alloys at 850 o C for 15 minutes showed one step on both heating and cooling. When alloys exposed ageing at 400 and 500 o C for different times, multistage transformation was seen [8]. Chu et al. were focused on DSC study of aged porous Ni-rich NiTi shape memory alloy which fabricated by combustion synthesis. Their experimental study indicated that shape memory properties of porous NiTi alloy could be improved by optimizing the ageing technology [9]. The understanding of strain and phase transformation behavior under constant applied pressure is essential, because of improving recent application of NiTi shape memory alloys, including smart structures, intelligent controllers and memory device [10]. There is few study about effect of constant pressure on shape memory alloys. Kröger et al. studied pressure-induced martensitic transformation in Ni rich NiTi alloys [11]. They measured DSC pressure-free and pressure-assisted aging for 4 ks (almost an hour) at 550 o C. It was seen that martensitic transformation temperature of NiTi alloys increased after pressure. The aim of this study is to investigate the effect of pressure and thermal aging on the martensitic transformation of Ni-%45.16Ti shape memory alloys. 2. Experimental NiTi shape memory alloy with nominal composition of Ni-%45.16Ti (atomic) was supplied by Nimesis technology. All samples were annealed for 2 hour at 500 o C and rapidly quenched in ice brined. DSC measurements were done in order to investigate the effect of pressure and thermal ageing on transformation temperature. These temperatures namely Martensite finish-Mf , Martensite start-Ms, Austenite start-As and Austenite finish-Af of alloys were measured by Perkin Elmer Sapphire differential scanning calorimetry with 10 o C/min heating and cooling rate at the temperature range of 0 and 150 o C in Nitrogen atmosphere. X-ray analysis system (Bruker D8 Advance) using Cu Kα radiation was used to determine crystal structure of thermal aged alloys. The X-ray diffraction (XRD) measurements were carried out in the range of 20-80° at the rate of 2°. 3. Result and discussion The DSC charts on heating and cooling of the NiTi shape memory alloy annealed at 500 o C were shown in Figure 1. According to Figure 1, two phase transformation occur during cooling. One of these phase is martensite other is rhombohedral (R) phase. Figure 1. DSC thermogram of NiTi alloy annealed at 500 o C 9th International Conference on Magnetic and Superconducting Materials (MSM15) IOP Publishing Journal of Physics: Conference Series 667 (2016) 012011 doi:10.1088/1742-6596/667/1/012011