Fabrication of Ti-Ni-Zr Shape Memory Alloy by P/M Process

Transcription

1 Materials Transactions, Vol. 5, No. 1 (29) pp to 245 #29 The Japan Institute of Metals Fabrication of Ti-Ni-Zr Shape Memory Alloy by P/M Process Akira Terayama 1, Koji Nagai 2; * and Hideki Kyogoku 2 1 Hiroshima Prefectural Technology Research Institute, Kure 737-4, Japan 2 Department of Mechanical Engineering, Faculty of Engineering, Kinki University, Higashihiroshima , Japan This work focuses on the fabrication of Ti-Ni-Zr high-temperature shape memory alloy by powder metallurgy (P/M) process. The effects of fabrication conditions on the microstructure and shape memory characteristics of Ti-5.2 mol%ni-5 mol%zr alloy were investigated. In this research, elemental Ti, Ni and Zr powders were used. These powders were mixed by a planetary ball mill at a rotational speed of 5 rpm for milling times of.6 ks (mixed powder) and 72 ks (MAed powder). The mixtures were sintered by a pulse-current pressure sintering equipment at 1153 K for sintering times of 1.8 ks and 1.2 ks. The solution treatment was carried out at various temperatures between 173 K and 1273 K to homogenize the microstructure of the as-sintered alloy. The microstructure of the alloy became more homogeneous with an increase in solutiontreatment temperature. In the case of the mixed powder, however, Zr-rich phases were observed in the microstructure of the solution-treated alloy. The alloy solution-treated at 1173 K showed a yielding behavior in the stress-strain curve, and the tensile strength and elongation of the alloy were more than 35 MPa and 2.5%, respectively. On the other hand, in the case of the MAed powder, the microstructure of the as-sintered alloy was homogeneous. The P/M alloy showed higher transformation temperatures than those of the wrought alloy. But, the alloy showed no shape memory effect and poor tensile property due to contamination of the MAed powder. [doi:1.232/matertrans.m29213] (Received June 23, 29; Accepted July 3, 29; Published September 25, 29) Keywords: titanium-nickel-zirconium shape memory alloy, mechanical alloying, pulse-current pressure sintering, tensile properties, shape memory characteristics 1. Introduction Ti-Ni shape memory alloy (SMA) has been applied to various industrial fields because of unique shape memory effect and superelasticity. Ternary alloying addition can improve the shape memory characteristics of the alloy. Especially, substitution of zirconium for titanium makes the martensitic transformation temperatures more than 373 K. Thus Ti-Ni-Zr alloy is promising to apply to high-temperature SMA. Hsieh et al. 1) reported that the transformation temperatures of Ti-Ni-Zr alloys with various Zr content fabricated by vacuum arc melting increase with increasing Zr content and the alloys with more than 15 mol%zr have the martensitic transformation finished point (M f ) of more than 373 K. Laves phase crystallized out at grain boundaries of B2 phase during solidification leads to brittleness of the alloy. Therefore, the application of powder metallurgy (P/M) process that is a solid-state process has been tried to fabricate Ti-Ni-Zr SMA. Monastyrsky et al. 2,3) reported that the P/M processed Ti-Ni-Zr alloys have many types of secondary phases such as Ti-Ni alloy (Ti 2 Ni, TiNi 3 ) and Ni-Zr alloy (NiZr 2, Ni 5 Zr). Bertheville 4) reported that almost singlephase ternary Ti-Ni-Zr alloys can be obtained using finer TiH 2 and ZrH 2 powders, although a few Zr-rich secondary phases exist in the alloy. There are no reports that the wrought Ti-Ni-Zr SMA showed sufficient tensile strength and shape memory characteristics. It has been reported, however, that sputtered Ti-Ni-Zr thin film shows superior tensile properties and shape memory characteristics because sputtering process can be obtained submicron-order grain sizes. Sawaguchi et al. 5) reported that the sputtered Nipoor Ti-Ni-Zr thin films have higher critical stress for slip deformation of more than 32 MPa and reducing in volume fraction of Laves phase is quite significant for improvement of recoverable strain. *Graduate Student, Kinki University The authors reported that the P/M Ti-Ni and Ti-Ni-Cu SMAs fabricated by a pulse-current sintering using Ti, Ni and Cu elementally powders have superior shape memory characteristics comparable to those of the wrought alloy. 6 8) Using a mechanically alloyed (MAed) powder has an advantage in homogeneous microstructure of the sintered alloy, 9) moreover, MA is a favorable process for refinement of grain size in sintered Ti-Ni-Zr alloy. In this research, Ti-Ni-Zr high-temperature shape memory alloy was fabricated by P/M process. The effects of fabrication conditions on the microstructure and shape memory properties of Ti-5.2 mol%ni-5 mol%zr alloy were investigated. 2. Experimental Procedure In this research, elementally Ti, Ni and Zr powders were used. The chemical composition and mean particle diameter were shown in Table 1. The powder was milled using a planetary ball mill at a composition of Ti-5.2 mol%ni- 5 mol%zr under two conditions; at a rotation speed of 5 rpm for.6 ks (mixed powder) and 72 ks (MAed powder). Figure 1 shows X-ray diffraction patterns of these powders. In the case of the mixed powder, peaks of the Ti, Ni and Zr are observed on X-ray diffraction pattern. The powder is just mixed and not alloyed. On the other hand, the Ti, Ni and Zr peaks vanishes completely in the case of the MAed powder. These powders were filled into graphite die and sintered in vacuum using a pulse-current sintering equipment. The mixed powder was sintering at a temperature of 1153 K for 1.8 ks. To homogenize the microstructure of the alloy, the solution treatment was performed in argon atmosphere at various temperatures between 173 K and 1273 K because the powder was inhomogeneous as already reported. 6) The microstructure of the MAed powder may consist of finer grains, and a large amount of dislocations stored during MA. It is important not only to release dislocations but also

2 Fabrication of Ti-Ni-Zr Shape Memory Alloy by P/M Process Table Powder characteristics and chemical compositions of Ti, Ni and Zr powders. Chemical compositions (mass%) Mean particle diameter (mm) Fe O N H Ca Mg C Cu Ni Zr Ti to restrain the formation and grain growth of Laves phase during sintering to obtain sufficient shape memory characteristics of the sintered alloy. Therefore, sintering time of the MAed powder is selected a shorter time of 1.2 ks. X-ray diffraction, Energy dispersive spectrometry (EDS) analysis and tensile test were performed to examine the properties of the as-sintered alloy and the solution-treated alloy. The tensile test specimens were prepared by cutting and grinding the alloys into the dimensions of 55 mm in length, 5.5 mm in width and 1.5 mm in thickness. The gauge length of the specimens was 25 mm. Intensity (a.u.) MAed powder (milled for 72 ks) Mixed powder (milled for.6 ks) 3. Results and Discussions Fig. 1 ( ) X-ray diffraction patterns of the mixed and MAed powders. SEI Ti-Kα Effect of mechanical alloying on the microstructure of the alloys Figure 2 shows the secondary electron image (SEI), Ti-K image, Ni-K image and Zr-L image of (a) the as-sintered alloy by the mixed powder, (b) the solution-treated (1173 K for ks) alloy by the mixed powder and (c) the assintered alloy by the MAed powder. In the case of the as- Ni-Kα Zr-Lα (a) As-sintered alloy by the mixed powder (b) Solution-treated (1173 K for ks) alloy by the mixed powder (c) As-sintered alloy by the MAed powder Fig. 2 EDS analysis of the alloys fabricated under various fabrication conditions.

3 2448 A. Terayama, K. Nagai and H. Kyogoku Intensity (a.u.) As-sintered (MAed powder) : monoclinic(b19') : cubic(b2) : Zr : NiZr 2 : Ti 2 Ni : TiNi 3 : TiNi 5 O 7 Solution-treated at 1273 K Solution-treated at 1173 K Solution-treated at 173 K Heat Flow Endotherm 173 K 1273 K 1173 K As-sintered (MAed powder) M*: 28 K M*: 324 K M*: 322 K M*: 364 K Fig. 3 2 ( ) X-ray diffraction patterns of the alloys. sintered alloy by the mixed powder shown in Fig. 2(a), dispersed white zone whose diameters are approximately 3 mm in the SEI corresponds to Zr. It is difficult for the assintered alloy to archive homogeneous structure. In the case of the solution-treated (1173 K for ks) alloy by the mixed powder shown in Fig. 2(b), the interface between Zr and matrix is difficult to observe clearly. This means that dissolving of Zr into the matrix was proceeded by the solution-treatment. In all images, Ni distributes uniformly because Ni atoms diffuse into Ti and Zr particles during the early stage of sintering. 2) In the as-sintered alloy by the MAed powder shown in Fig. 2(c), uniform distribution of three elements is observed. This is different from that of the wrought alloy. It was reported that, in the microstructure of the wrought alloy, Laves phase located around the (Ti,Zr)Ni grain boundaries and distribution of elements was not uniform. 1) Thus, the MA process is effective to obtain homogeneous microstructure of the sintered alloy. To measure the structure of matrix and the transformation temperatures of the alloys, X-ray diffraction analysis and DSC measurement were performed. Figures 3 and 4 demonstrates the X-ray diffraction patterns and the DSC cooling curves of the solution-treated (173, 1173 and 1273 K) alloy by the mixed powder and the as-sintered alloy by MAed powder, respectively. In X-ray diffraction patterns of the alloys solution-treated at 173 K and 1173 K the peaks of many secondary phases in Ti-Ni system (Ti 2 Ni, TiNi 3 ) and Ni-Zr system (NiZr 2 ) are observed. Since the Zr phase is observed, solid-solution of Zr into the matrix is insufficient. The microstructure becomes homogeneous and the secondary phases vanish with elevating the solution-treatment temperature. A peak of the B2 austenite phase becomes higher at a solution-treatment temperature of 1273 K. The B19 martensite phase and the B2 austenite phase are observed in the as-sintered alloy by the MAed powder. Moreover, although the peaks of the secondary phases are not observed, the peak of TiNi 5 O 7 is observed. This phase may be formed during MA because surface of the powder was activated by MA process and easily reacted with oxygen. M indicates the peaks of temperature of the martensitic transformation in Fig. 4. Comparison of M in the solutiontreated alloys, the transformation temperature M falls with Temperature, T/K Fig. 4 DSC cooling curves of the alloys. elevating the solution-treatment temperature. This may be related to dissolving of Zr into the matrix. The as-sintered alloy by the MAed powder has the highest transformation temperature M of 364 K. But the transformation temperature M of the wrought alloy at the same composition was approximately 323 K as already reported. 1) Thus the transformation temperature M of the as-sintered alloy by the MAed powder is much higher than that of the wrought alloy. The authors reported that the alloy fabricated by the MA powder tends to have higher transformation temperatures than those of the wrought alloy. 9) The reason why this phenomenon may be caused by presence of internal stress field formed by storage of dislocation during MA process or decreasing of Ni content in the alloy by forming Ni-rich oxidative product (TiNi 5 O 7 ). The alloy has the narrowest transformation temperature width of 7 K and the smallest heat change of transformation in the DSC curve as shown in Fig. 4. Sandu et al. 1) reported that Ti-52 mol%ni-6 mol%zr wrought alloy has a narrow transformation width and a similar DSC curve to that of our research. This may closely be related to dissolving Zr into TiNi matrix, but, further research is needed. The oxidative product also may interrupt the martensitic transformation and lower the heat change of transformation. 3.2 Tensile properties and shape memory characteristics of the alloys To measure the tensile properties of the alloys, tensile tests were carried out at room temperature. The stress-strain curves of the alloys are shown in Fig. 5. In the solutiontreated alloys by the mixed powder, the deformation resistance increases with an increase in the solution-treatment temperature. The tensile strength and elongation of the alloy solution-treated at 1173 K were more than 35 MPa and 2.5%, respectively. The deformation resistance of the alloy solution-treated at 1273 K is the highest among three alloys, but the elongation is the lowest. This may be due to appearance of Laves phase, which has the solid-liquid transition temperature at 123 K 1) at a solution-treatment temperature of 1273 K. The stress-strain curve of the assintered alloy by the MAed powder is not given in this figure because the alloy fractured at lower stress of approximately 125 MPa. To improve the tensile property of the as-sintered

4 Fabrication of Ti-Ni-Zr Shape Memory Alloy by P/M Process 2449 Stress, σ /MPa Solution-treated at 1273 K 1173 K 173K Sintering temp K Stress, σ/mpa Solution-treatment temp. 173 K Solution-treatment time 43.2 ks N=1 N=2 N= Fig. 5 Stress-strain curves of the alloys by the mixed powder. Fig. 7 Stress-strain curves of the solution-treated alloy (173 K) by the mixed powder Solution-treatment temp. 173 K Solution-treatment time 43.2 ks N=1 A s A* A f.5 N= Temperature, T/K N=3 Fig. 6 Fracture surface of the as-sintered alloy by the MAed powder. Fig. 8 Strain-temperature curves of the solution-treated alloy (173 K) by the mixed powder. alloy, the solution-treatment was performed at 1173 K for 43.2 ks. But the solution-treatment did not improve the tensile property of the alloy by MAed powder. Figure 6 shows the fracture surface of the as-sintered alloy by the MAed powder. Ductile dimple cannot be observed. This brittle fracture of the alloy is due to excess oxidation of the MAed powder. The as-sintered alloy by MAed powder shows homogeneous microstructure and higher transformation temperatures, but has poor tensile property. The shape memory properties of the alloy by the mixed powder solution-treated at 173 K, which has highest transformation temperatures among the solution-treated alloys, were measured. It is important to investigate stabilizing behavior of shape memory properties of the alloy. The alloy was loaded up to 1.5% strain and unloaded completely at 3 K and then heated up to austenite finished point (A f ), this cycle were repeated. Figure 7 shows the stress-strain curves of the solution-treated (173 K) alloy by the mixed powder. The number in this figure indicates the number of cycles. The residual strain is observed after unloaded completely because the alloy is consisted of martensite phase at 3 K. Figure 8 demonstrates the strain-temperature curves of the alloys. A s and A indicates the reverse transformation start temperature and the reverse transformation peak temperature, respectively. In heating process, the strain gradually recovers due to shape memory effect of (Ti,Zr)Ni phase with insufficient dissolving of Zr. This is because of inhomogeneous microstructure of the alloy. The residual strain is.24% in first cycle, but it recovers after second cycle. 4. Conclusions In this research, we investigated the fabrication conditions of Ti-Ni-Zr high temperature shape memory alloy by P/M process. The effect of fabrication conditions on the microstructure, transformation temperature, tensile properties and shape memory characteristics were investigated. The results obtained are as follows: (1) The microstructure of the as-sintered alloy by the MAed powder was homogeneous, while the microstructure of the solution-treated alloy by mixed powder was inhomogeneous. (2) The reverse transformation peak temperature of the as-sintered alloy by the MAed powder was higher than that of the solution-treated alloys and the wrought alloy. (3) The deformation resistance of the solution-treated alloys by the mixed powder increased with an increase

5 245 A. Terayama, K. Nagai and H. Kyogoku in the solution-treatment temperature. The tensile strength and elongation of the alloy solution-treated at 1173 K by the mixed powder were more than 35 MPa and 2.5%, respectively. The as-sintered alloy by the MAed powder had the lowest tensile strength and elongation because of oxidation of the mixed powder. (4) The alloy solution-treated at 173 K by the mixed powder had shape memory effect. The shape recovery property of the alloy was stabilized due to the cycle of heating after loading. Thus the Ti-Ni-Zr SMA could be fabricated by P/M process using elemental powders. REFERENCES 1) S. F. Hsieh and S. K. Wu: J. Alloy. Compd. 266 (1998) ) G. E. Monastyrsky, V. Odnosum, J. van Humbeeck, V. I. Kolomytsev and Yu. N. Koval: Intermetalics 1 (22) ) G. E. Monastyrsky, J. van Humbeeck, V. I. Kolomytsev and Yu. N. Koval: Intermetalics 1 (22) ) B. Bertheville: J. Alloy. Compd. 398 (25) ) T. Sawaguchi, M. Sato and A. Ishida: Metall. Mater. Trans. A 35A (24) ) H. Kyogoku, F. Yoshida and T. Sakuma: Proc. The Fourth Pacific Rim Int. Conf. on Advanced Materials and Processing (PRICM4, 21) pp ) A. Terayama, H. Kyogoku, M. Sakamura and S. Komatsu: Mater. Trans. 47 (26) ) H. Kyogoku, T. Tanbo, M. Yokota, S. Komatsu, F. Yoshida, T. Sakuma and U. Iwata: Proc. Adv. Powder Metall. Particulate Mater. 21, MPIF (21) pp ) A. Terayama, H. Kyogoku and S. Komatsu: Mater. Sci. Forum (27) ) A. Sandu, K. Tsuchiya, S. Yamamoto, M. Tabuchi, Y. Todaka and M. Umemoto: Mater. Sci. Forum (27)