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Characterisation of phase transformation in Ti50+xPd30Ni20−x alloys
Intermetallics 10 (2002) 675–682 www.elsevier.com/locate/intermet
Characterisation of phase transformation in Ti50+xPd30Ni20x alloys Qingchao Tian*, Jiansheng Wu Key Laboratory of the Ministry of Education for High Temperature Materials and Tests, School of Materials Science and Engineering, Shanghai Jiao Tong Univ., Shanghai, 200030, PR China Accepted 26 March 2002
Abstract Generally, a B2–B19 one-stage phase transformation exists in Ti50PdxNi50x alloys when Pd content is higher than 20 at.%. However, in Ti rich Ti50+xPd30Ni20x alloys, a B2–B19–B190 two-stage transformation can also take place. The alloys were firstly heated to high temperature of solid solutions, and then the one-stage transformation can be retained by subsequent furnace-cooling as well as the twostage transformation by quenching. Annealing at lower temperature does not change the nature of the phase transformations. Differential scanning calorimetry (DSC), X-ray diffractometry (XRD), dynamic mechanical thermal analysis (DMTA) and transmission electron microscopy (TEM) were used to ascertain the characteristics of the phase transformations. Discrepancies between the transformation elastic modulus of B2-B19 and B19-B190 have been found, a trough and a step can be employed for a description, respectively. The damping properties of the materials were also investigated, all the results provide valuable information for designs of intelligent materials and structures aimed for vibration control. # 2002 Published by Elsevier Science Ltd. Keywords: B. Martensitic transformation; C. Heat treatment
1. Introduction Shape memory alloys (SMAs) are one of the most important intermetallics, and have been used to develop intelligent actuators and composites for active shape and vibration control [1]. The design was based on the fact that the elastic modulus is various during phase transformations. It was observed that the modulus curves for some SMAs exhibit a trough during both heating and cooling cycles [2]. However, little work has been carried out to study the dependence of the elastic modulus and damping properties upon the change of temperature. In Ti50PdxNi20x alloys, the martensitic transformation has been studied by many authors [3–5]. It is known that the transformation sequence is gradually changed from B2–B190 through B2–R–B190 , to B2–B19– B190 , and then into B2–B19 as the Pd content increases. Here, B2 refers to the high temperature austenite phase * Corresponding author at current address: Testing Centre, Shanghai Baosteel Research Institute, Baoshan Iron & Steel Co. Ltd., Shanghai 201900, PR China. Fax: +86-21-5254-0011. E-mail address: [email protected] (Q. Tian).
(CsCl type structure) while R, B19 and B190 correspond to the rhombohedral phase, the orthorhombic and monoclinic martensite, respectively. Ti50Pd30Ni20 alloy has received much attention because its martensitic transformation temperature is high enough for some engineering application [6–12]. In this paper, Ti rich Ti50+xPd30Ni20x alloys were prepared for the investigation on the phase transformation behaviour. Differential scanning calorimetry (DSC), X-ray diffractometer (XRD), dynamic mechanical thermal analyzer (DMTA) and transmission electron microscope were employed in the experiments. It was found that a twostage phase transformation occurs in this alloy system, and the variation of the elastic modulus for the alloys in different heat-treatment states is different.
2. Experimental procedure Three kinds of Ti50+xPd30Ni20x alloys, i.e., Ti51 Pd30Ni19, Ti50.6Pd30Ni19.4 and Ti50.2Pd30Ni19.8, had been made by arc melting on a water-cooled copper mold. The ingots were melted four times. After homogenisation treatment in vacuum at 1273 K for 5 h, the ingots were
0966-9795/02/$ - see front matter # 2002 Published by Elsevier Science Ltd. PII: S0966-9795(02)00048-1
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subsequently furnace-cooled to room temperature. Then, it was hot-rolled into 1.1-mm thick plate at 1073 K, and finally cold-rolled into 1 mm thick. Specimens for DSC, TEM, XRD with dimensions 12 mm12 mm1 mm and DMTA with dimensions 35 mm2 mm1 mm were spark cut along the rolling direction. All the specimens were mechanically polished, and then annealed in argon filled quartz capsule at either 673 K for 3.6 ks or 1273 K for 18 ks. After annealing, the specimens were quenched into ice water by crushing the capsule. Thus specimens were prepared in two kinds of state, i.e., recovery state (673 K), and solution state (1273 K). A TA instrument 2910 MDSC, a ‘D/max IIIA’ automatic diffractometer, a Rheometric Scientific DMTA IV, a JEOL-200CX and a JEOL-2010 high resolution TEM were employed in the experiments. In both the DSC and DMTA measurement, a heating/cooling rate of 5 k/min was adopted. Nitrogen was used as protective atmosphere and liquid nitrogen was used in the cooling process. TEM specimens were first mechanically polished, then prepared by a twin jet electro-polishing apparatus in an electrolytic solution with an approximate composition in volume percent of CH3COOH:HClO4=92.5:7.5.
3. Results 3.1. Phase transformation characteristics Fig. 1 shows the DSC measurement results. For specimens of the three kinds of alloys in recovery state, there was only one peak or valley in the heat flow curves, indicating the occurrence of the B2–B19 onestage phase transformation. The transformation temperatures were represented in Fig. 1a, it was found that the temperatures decrease with the increase of Ti content deviation from 50 at.%. As for specimens in solution state, apparent overlapping peaks/valleys appear in the heat flow curves during heating/cooling process (Fig. 1b), which means a two-stage phase transformation has taken place. As discussed below, we have assumed the possibility of a martensite to martensite (B19–B190 ) transformation if the alloy was quenched from high temperature. Then, a two-stage transformation sequence B2!B19!190 on cooling and B190 ! B19!B2 on heating may exist. Fig. 1c shows the evolution of heat flow curves upon the times of thermal cycling, which can provide information on the stability of the two-stage phase transformation. It
Fig. 1. The DSC measurement results, (a) transformation temperatures of Ti50+xPd30Ni20x, (b) overlapping peaks in the solution specimens, and (c) evolution of heat flow curves with thermal cycling.
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has been found that in the heating process of the third cycling, the phase transformation occurs much earlier and the valley corresponding to B190 !B19 becomes much deeper. The phase transformation becomes stable afterwards. Since the two-stage phase transformation generally exists in the off-stoichiometric Ti50+xPd30Ni20x alloys, only Ti51Pd30Ni19 specimens were used in the following experiments. 3.2. XRD analyses The XRD spectrum can be indexed with references to the lattice parameters, for B2: a=0.31 nm, for B19: a=0.278 nm, b=0.459 nm, c=0.489 nm [13], and for B190 : a=0.278 nm, b=0.433 nm, c=0.464 nm, b=90.4 [3], respectively. Fig. 2 represents the XRD results of Ti51Pd30Ni19 specimens in recovery and in solution states, respectively. It can be found that the room temperature (RT) XRD profiles of the two specimens are different, which means that the martensites in the two kinds of specimens are not of the same kind. The martensites can be identified as B19 and B190 , respectively. When the specimens were heated to high temperature austenitic state, they show similar XRD patterns (curve 3 in Fig. 2). It can be also seen that the specimens exhibit apparent texture [4].
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Fig. 3 shows the measurement results for specimens in recovery and in solution state, respectively. It can be clearly seen that for the former specimens, the elastic modulus curve exhibits a trough during both heating and cooling processes, and the valleys correspond well to the damping peaks (Fig. 3a). As for the latter specimens in solution state, the sight is different. The elastic modulus curve exhibits an obvious step, which corresponds to the broad damping peaks (Fig. 3b). Fig. 4 represents the variation details of the elastic modulus as a function of temperature with different vibration frequencies. It can be found that as the frequency increases, the trough flattens (Fig. 4a), and the fluctuation in the step becomes alleviated (Fig. 4b), indicating the insensitivity of the material to high frequency. For specimens in recovery state, the values of modulus for both austenite and martensite may close to 60 GPa, and the modulus of austenite is a little higher than that of martensite (B19). However, for the specimens in
3.3. Dynamic characteristics 3.3.1. Variation of the elastic modulus It is known that the modulus of a viscoelastic material can be expressed by a complex modulus, E*=E0 +iE00 , where the real part, E0 , is the storage (elastic) modulus, while the imaginary part E’’ is the loss (viscous) modulus. When an oscillatory deformation is applied to a sample, there will exist a phase angle d between the deformation and the response. The tangent of d is the ratio E00 /E0 , a measure of the damping ability. DMTA can give all the information simultaneously.
Fig. 2. XRD patterns of Ti51Pd30Ni19 alloy at different phase states.
Fig. 3. DMTA measurement results of Ti51Pd30Ni19 alloy indicating the relationship between elastic modulus and damping properties, specimen (a) in recovery state and (b) in solution state.
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splits into two peaks as the frequency increases (Fig. 5b). The overlapping peaks can be separated by using the fitting of Gauss profile function, as shown in Fig. 5c. Therefore, the separating result corresponds to a twostage phase transformation. It should be noted that the results of dynamic mechanical measurement may provide valuable information for designs of intelligent materials and structures aimed for vibration control. 3.4. TEM observation
Fig. 4. The variation details of elastic (storage) modulus under different vibration frequencies showing the insensitivity of the material to high frequency, specimen (a) in recovery state and (b) in solution state.
solution state, the modulus value of martensite (B190 ) is 70 GPa, abnormally higher than that of austenite (60 GPa). Carballo [2] pointed out that the modulus of austenitic phase is approximately 120% that of martensite. It can be seen that from our experimental results, the obtained conclusion needs some conditions. The variations of modulus in different alloy systems may be different, even in the same alloy; different heat treatment may also make the modulus vary. 3.3.2. The damping behaviour Fig. 5 shows the curves of damping dependence of temperature under different vibration frequencies. The damping properties, Tand, of specimen in recovery state (Fig. 5a) is lower than that in solution state (Fig. 5b). Tand demonstrate a maximum peak during both heating and cooling processes, and the values in austenite are larger during heating while smaller during cooling than that in martensite. As vibration frequency decreases, the damping peak grows. Another unusual phenomenon is that the damping peaks become broad and one peak
Fig. 6a shows a typical microstructure of specimen in recovery state at room temperature, in which the martensite plate can be distinguished. In the specimens of such heat treatment state, recrystallization has not taken place, and a high density of dislocations still remains in the specimens after thermal rearrangement [2]. The selected-area diffraction (SAD) patterns taken from the region A indicated in Fig. 6a is shown and indexed in Fig. 6b. The martensite was confirmed to be of a B19 structure. Many large scattered precipitates have been found during the TEM observation, as shown in Fig. 6c. The composition of the precipitate was determined by using energy dispersive analyser, as was that of the matrix around the precipitates. Five positions are selected as shown in Fig. 6c, and the measurement results are listed in Table 1. It can be seen that the atomic composition of the precipitate is Ti2(Ni, Pd)1. The SAD pattern of the precipitate (Fig. 6d) can be successfully indexed to be of a cubic structure. It is known that the crystal lattice of Ti2Ni is cubic while Ti2Pd is tetragonal [13]. Pd substitution for Ni in Ti2Ni will inevitably make the lattice unstable. With the increase of Pd content, the cubic lattice would eventually transform into tetragonal ones. In the alloys studied, the crystal lattice of the precipitate is still of a cubic structure with Pd partially substituting for Ni, which can be written as Ti2Ni(Pd). The effect of precipitation on the matrix is significant, the content of Ti of the matrix is lower than the nominal content 51 at.%, and decreases from 47.35 to 46.17 at.% as the measured spot moves towards the precipitates. The changes of Ti content are an evidence of diffusion process
Table 1 Composition of both the precipitate and the matrix around Measurement position
Ti (at.%)
Ni (at.%)
Pd (at.%)
Matrix
1 2 3 4
47.35 47.13 46.6 46.17
18.96 18.9 18.04 17.95
33.68 33.97 35.36 35.88
Precipitate
5
64.72
22.46
12.82
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Fig. 5. The damping characteristic of Ti51Pd30Ni19 alloy, specimen (a) in recovery state, and (b) in solution state, and (c) the separation of the overlapping peaks indicating a two-state phase transformation.
during the precipitation. The content variation may cause a various transformation temperature distribution in the region around the precipitates, resulting in wide or overlapping even two individual peaks/valleys in the DSC curves. According to the DSC measurement results, the expected phenomenon does not happen, which can be deduced that the transformation temperatures are not sensitive to the Ti content variation from 47.35 to 46.17 at.%. Fig. 7 represents the TEM observation on the specimen in solution state. In this specimen, a long plate of martensite is the main characteristic at room temperature (Fig. 7a). The martensite is confirmed to be of B190 type by the corresponding SAD patterns (Fig. 7b). After the specimen was heated to 873 K and followed by cooling to room temperature in vacuum, high resolution TEM observation was carried out. Fig. 7c shows that much precipitation has occurred, and the precipitates grew to some strips along a certain direction in the martensite plate. Fig. 7d and e are the SAD patterns taken from the area without (B) and with (C) precipitate as indicated in Fig. 7c. The matrix patterns (Fig. 7d) can still be indexed to be of B190 martensite. The SAD patterns of the precipitates (Fig. 7e) is not clear in the figure, but according to the precipitates in the alloy in
recovery state and an investigation on the precipitation of the Ti50.6Pd30Ni19.4 alloy [7], the precipitates should be of a Ti2Ni(Pd) type structure.
4. Discussion 4.1. B19$B190 transformation According to the phase diagram of TiPd–TiNi alloys [14], it is impossible to obtain B190 martensite at low temperature for alloys whose Pd content is higher than 20 at.%. However, it is confirmed that in the Ti-rich Ti50+xPd30Ni20x alloy, two kinds of phase transformation may occur, i.e., the B2–B19 one-stage and the B2–B19–B190 two-stage phase transformation. B190 martensite has not been observed for alloys of which Pd content is higher than 20 at.%. In the past, many researchers focused their attention on the equiatomic Ti50(PdNi)50 alloy, for example, Ti50Pd30Ni20, in which no precipitates have been observed [5]. But for off-stoichiometric Ti50+x (PdNi)50x alloys, a large amount of precipitates can be produced in the process of alloy preparation [10], which is also observed by our experiments. The changes of matrix composition affected by
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Fig. 6. Typical structures of the specimen in recovery state, (a) TEM bright field image at room temperature, (b) SAD pattern in the [110]B19 zone, taken from the region A in (a), (c) a typical precipitate and showing the measurement position (1–5), and (d) the corresponding SAD pattern in [123]Ti2Ni zone.
precipitation are so significant that the transformation behaviour should be influenced without question. After homogenisation at 1273 K, the alloy becomes a homogeneous solution. The eutectoid decomposition can proceed completely if the alloy was furnace cooled. Only a large amount of dislocations and internal stress were introduced when the alloy was rolled, after recovery and recrystallization, the nature of the phase transformation cannot be changed. Through the solution treatment, the precipitates are dissolved and the alloy becomes a homogeneous solution again. The process of precipitation can be suppressed on the consequent quenching, causing a more serious distortion of the crystal lattice, and resulting in the appearance of the B190 phase. Thus, B19 becomes a media for the lattice deformation during a martensitic transformation. However, it can be deduced that an alloy in such a solution state is metastable, TEM observation did reveal that much precipitation has occurred when the specimen was heated to high temperature. With the growth of the precipitates, the Ti content of the matrix would be greatly lowered, and a change of phase transformation from the obtained B2–B19–B190 to B2–B19 would happen. In our experiment, fine precipitates were produced in some microregions. The effect of precipitation on the matrix composition is not high enough to
affect the transformation sequence, therefore, the twostage transformation characteristics has not been changed. Another question may put forward, that is, is the medial phase a new phase like R-phase? It can not be, because the Pd content is very high, according to the transformation sequence and the phase diagram, it should be the B19 phase. 4.2. Soft mode and the elastic modulus The trough in the elastic modulus curve means that the elasticity softens as the temperature changes. This phenomenon is easily related to the theory of a soft mode in the structural phase transformation. The soft mode usually connects with a certain shear elastic constant term [15]. The soft mode is a universal phenomenon in thermoelastic transformations involving the deformation of the crystal lattice. It is confirmed by the experimental results of the specimen in recovery state, in which only the one-stage phase transformation (B2– B19) exists. As for the specimen in solution state, there does not appear a trough but a step in the modulus curves. It seems to be a fallacy. However, if attention is focused on the fluctuation on the modulus steps, a trough occurs in the B2$B19 transformation, and a step is caused by the B19$B190 transformation.
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Fig. 7. Typical structures of specimen in solution state, TEM bright field images at RT (a), and cooling to RT (c), and SAD pattern in [110]B190 zone (b) [taken from the region A in (a)] and in [100]B190 zone (d) and (e) [taken from the region B and C in (c)].
5. Conclusion 1. Ti51Pd30Ni19, Ti50.6Pd30Ni19.4 and Ti50.2 Pd30 Ni19.8 alloys in different heat treatment states may show different phase transformation behaviours. For specimens in recovery state, there exists the B2–B19 one-stage phase transformation, with the transformation temperature decreasing with the increase of Ti content deviation from 50 at.%. The B2–B19–B190 two-stage phase transformation occurs for the alloys in solution state. The reason lies in that much precipitation occurs in the alloys in recovery state, resulting in a lowered Ti content of the matrix. 2. The variation of elastic modulus as a function of temperature is different for Ti51Pd30Ni19 alloys under
different heat treatment conditions. There exists a trough in the modulus curves for specimens with a B2– B19 transformation, while apparently steps appear for B2– B19–B190 transformation. The steps are originated from B19–B190 transformation. The B190 phase exhibits a much higher elastic modulus than B19. A trough can be observed clearly under lower vibration frequency when B2-B19 transformation takes place in B2–B19–B190 process. 3. Phase transformation leads to a maximum peak in the damping curves both in the heating and cooling processes. The damping properties, Tand, of specimen in recovery state are lower than that in solution state, and show a higher value after phase transformation than before. It is true for both the martensitic and its reverse transformations.
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Acknowledgements This work is sponsored by the Science and Technology Commission of the Shanghai Municipal Government, No. 00JC14055. The authors wish to express thanks to the reviewer for his/her valuable comments on this paper. References [1] Choi SB, Hwang JH. J Sound Vibration 2000;231(4):1168–74. [2] Carballo M, Pu ZJ, Wu KH. J Intell Mater Syst Struct 1995; 6:557–65. [3] Matveeva NM, Kovneristyi YuK, Savinov AS, Sivokha VP, Khachin VN. J Physique 1982;43:C4–249. [4] Mathews S, Li J, Su Q, Wuttig M. Philosophical Magazine Letters 1999;79(5):265–72.
[5] Lo YC, Wu SK. Script Metall Mater 1992;27:1097. [6] Golberg D, Xu Y, Murakami Y, Morito S, Otsuka K, Ueki T, Horikawa J. Mater Lett 1995;22:241. [7] Golberg D, Xu Y, Murakami Y, Morito S, Otsuka K. Intermetallics 1995;3:35. [8] Golberg D, Xu Y, Murakami Y, Morito S, Otsuka K, Ueki T, Horikawa J. Scripta Metall Mater 1994;30:1349. [9] Cai W, Otsuka K, Asai M. Mat Trans JIM 1999;40(9):895. [10] Shimizu S, Xu Y, Okunishi E, Tanaka S, Otsuka K, Mitose K. Mat Lett 1998;34(1–2):23. [11] Xu Y, Shimizu S, Suzuki Y, Otsuka K, Ueki T, Mitose K. Acta Mater 1997;45(4):1503. [12] Lo YC, Wu SK, Wayman CM. Script Metall Mater 1990; 24:1571. [13] Quandt E, Halene C, Holleck H, Feit K, Kohl M, Schlossmacher P, Skokan A, Skrobanek KD. Sensors Actuators A (Switz) 1996; A53:434–9. [14] Otsuka K, Ren X. Intermetallics 1999;7:511. [15] Feng D. Phys Metals, vols 2 and 3. Beijing: Science Press; 1999.
Characterisation of phase transformation in Ti50+xPd30Ni20x alloys Qingchao Tian*, Jiansheng Wu Key Laboratory of the Ministry of Education for High Temperature Materials and Tests, School of Materials Science and Engineering, Shanghai Jiao Tong Univ., Shanghai, 200030, PR China Accepted 26 March 2002
Abstract Generally, a B2–B19 one-stage phase transformation exists in Ti50PdxNi50x alloys when Pd content is higher than 20 at.%. However, in Ti rich Ti50+xPd30Ni20x alloys, a B2–B19–B190 two-stage transformation can also take place. The alloys were firstly heated to high temperature of solid solutions, and then the one-stage transformation can be retained by subsequent furnace-cooling as well as the twostage transformation by quenching. Annealing at lower temperature does not change the nature of the phase transformations. Differential scanning calorimetry (DSC), X-ray diffractometry (XRD), dynamic mechanical thermal analysis (DMTA) and transmission electron microscopy (TEM) were used to ascertain the characteristics of the phase transformations. Discrepancies between the transformation elastic modulus of B2-B19 and B19-B190 have been found, a trough and a step can be employed for a description, respectively. The damping properties of the materials were also investigated, all the results provide valuable information for designs of intelligent materials and structures aimed for vibration control. # 2002 Published by Elsevier Science Ltd. Keywords: B. Martensitic transformation; C. Heat treatment
1. Introduction Shape memory alloys (SMAs) are one of the most important intermetallics, and have been used to develop intelligent actuators and composites for active shape and vibration control [1]. The design was based on the fact that the elastic modulus is various during phase transformations. It was observed that the modulus curves for some SMAs exhibit a trough during both heating and cooling cycles [2]. However, little work has been carried out to study the dependence of the elastic modulus and damping properties upon the change of temperature. In Ti50PdxNi20x alloys, the martensitic transformation has been studied by many authors [3–5]. It is known that the transformation sequence is gradually changed from B2–B190 through B2–R–B190 , to B2–B19– B190 , and then into B2–B19 as the Pd content increases. Here, B2 refers to the high temperature austenite phase * Corresponding author at current address: Testing Centre, Shanghai Baosteel Research Institute, Baoshan Iron & Steel Co. Ltd., Shanghai 201900, PR China. Fax: +86-21-5254-0011. E-mail address: [email protected] (Q. Tian).
(CsCl type structure) while R, B19 and B190 correspond to the rhombohedral phase, the orthorhombic and monoclinic martensite, respectively. Ti50Pd30Ni20 alloy has received much attention because its martensitic transformation temperature is high enough for some engineering application [6–12]. In this paper, Ti rich Ti50+xPd30Ni20x alloys were prepared for the investigation on the phase transformation behaviour. Differential scanning calorimetry (DSC), X-ray diffractometer (XRD), dynamic mechanical thermal analyzer (DMTA) and transmission electron microscope were employed in the experiments. It was found that a twostage phase transformation occurs in this alloy system, and the variation of the elastic modulus for the alloys in different heat-treatment states is different.
2. Experimental procedure Three kinds of Ti50+xPd30Ni20x alloys, i.e., Ti51 Pd30Ni19, Ti50.6Pd30Ni19.4 and Ti50.2Pd30Ni19.8, had been made by arc melting on a water-cooled copper mold. The ingots were melted four times. After homogenisation treatment in vacuum at 1273 K for 5 h, the ingots were
0966-9795/02/$ - see front matter # 2002 Published by Elsevier Science Ltd. PII: S0966-9795(02)00048-1
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subsequently furnace-cooled to room temperature. Then, it was hot-rolled into 1.1-mm thick plate at 1073 K, and finally cold-rolled into 1 mm thick. Specimens for DSC, TEM, XRD with dimensions 12 mm12 mm1 mm and DMTA with dimensions 35 mm2 mm1 mm were spark cut along the rolling direction. All the specimens were mechanically polished, and then annealed in argon filled quartz capsule at either 673 K for 3.6 ks or 1273 K for 18 ks. After annealing, the specimens were quenched into ice water by crushing the capsule. Thus specimens were prepared in two kinds of state, i.e., recovery state (673 K), and solution state (1273 K). A TA instrument 2910 MDSC, a ‘D/max IIIA’ automatic diffractometer, a Rheometric Scientific DMTA IV, a JEOL-200CX and a JEOL-2010 high resolution TEM were employed in the experiments. In both the DSC and DMTA measurement, a heating/cooling rate of 5 k/min was adopted. Nitrogen was used as protective atmosphere and liquid nitrogen was used in the cooling process. TEM specimens were first mechanically polished, then prepared by a twin jet electro-polishing apparatus in an electrolytic solution with an approximate composition in volume percent of CH3COOH:HClO4=92.5:7.5.
3. Results 3.1. Phase transformation characteristics Fig. 1 shows the DSC measurement results. For specimens of the three kinds of alloys in recovery state, there was only one peak or valley in the heat flow curves, indicating the occurrence of the B2–B19 onestage phase transformation. The transformation temperatures were represented in Fig. 1a, it was found that the temperatures decrease with the increase of Ti content deviation from 50 at.%. As for specimens in solution state, apparent overlapping peaks/valleys appear in the heat flow curves during heating/cooling process (Fig. 1b), which means a two-stage phase transformation has taken place. As discussed below, we have assumed the possibility of a martensite to martensite (B19–B190 ) transformation if the alloy was quenched from high temperature. Then, a two-stage transformation sequence B2!B19!190 on cooling and B190 ! B19!B2 on heating may exist. Fig. 1c shows the evolution of heat flow curves upon the times of thermal cycling, which can provide information on the stability of the two-stage phase transformation. It
Fig. 1. The DSC measurement results, (a) transformation temperatures of Ti50+xPd30Ni20x, (b) overlapping peaks in the solution specimens, and (c) evolution of heat flow curves with thermal cycling.
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has been found that in the heating process of the third cycling, the phase transformation occurs much earlier and the valley corresponding to B190 !B19 becomes much deeper. The phase transformation becomes stable afterwards. Since the two-stage phase transformation generally exists in the off-stoichiometric Ti50+xPd30Ni20x alloys, only Ti51Pd30Ni19 specimens were used in the following experiments. 3.2. XRD analyses The XRD spectrum can be indexed with references to the lattice parameters, for B2: a=0.31 nm, for B19: a=0.278 nm, b=0.459 nm, c=0.489 nm [13], and for B190 : a=0.278 nm, b=0.433 nm, c=0.464 nm, b=90.4 [3], respectively. Fig. 2 represents the XRD results of Ti51Pd30Ni19 specimens in recovery and in solution states, respectively. It can be found that the room temperature (RT) XRD profiles of the two specimens are different, which means that the martensites in the two kinds of specimens are not of the same kind. The martensites can be identified as B19 and B190 , respectively. When the specimens were heated to high temperature austenitic state, they show similar XRD patterns (curve 3 in Fig. 2). It can be also seen that the specimens exhibit apparent texture [4].
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Fig. 3 shows the measurement results for specimens in recovery and in solution state, respectively. It can be clearly seen that for the former specimens, the elastic modulus curve exhibits a trough during both heating and cooling processes, and the valleys correspond well to the damping peaks (Fig. 3a). As for the latter specimens in solution state, the sight is different. The elastic modulus curve exhibits an obvious step, which corresponds to the broad damping peaks (Fig. 3b). Fig. 4 represents the variation details of the elastic modulus as a function of temperature with different vibration frequencies. It can be found that as the frequency increases, the trough flattens (Fig. 4a), and the fluctuation in the step becomes alleviated (Fig. 4b), indicating the insensitivity of the material to high frequency. For specimens in recovery state, the values of modulus for both austenite and martensite may close to 60 GPa, and the modulus of austenite is a little higher than that of martensite (B19). However, for the specimens in
3.3. Dynamic characteristics 3.3.1. Variation of the elastic modulus It is known that the modulus of a viscoelastic material can be expressed by a complex modulus, E*=E0 +iE00 , where the real part, E0 , is the storage (elastic) modulus, while the imaginary part E’’ is the loss (viscous) modulus. When an oscillatory deformation is applied to a sample, there will exist a phase angle d between the deformation and the response. The tangent of d is the ratio E00 /E0 , a measure of the damping ability. DMTA can give all the information simultaneously.
Fig. 2. XRD patterns of Ti51Pd30Ni19 alloy at different phase states.
Fig. 3. DMTA measurement results of Ti51Pd30Ni19 alloy indicating the relationship between elastic modulus and damping properties, specimen (a) in recovery state and (b) in solution state.
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splits into two peaks as the frequency increases (Fig. 5b). The overlapping peaks can be separated by using the fitting of Gauss profile function, as shown in Fig. 5c. Therefore, the separating result corresponds to a twostage phase transformation. It should be noted that the results of dynamic mechanical measurement may provide valuable information for designs of intelligent materials and structures aimed for vibration control. 3.4. TEM observation
Fig. 4. The variation details of elastic (storage) modulus under different vibration frequencies showing the insensitivity of the material to high frequency, specimen (a) in recovery state and (b) in solution state.
solution state, the modulus value of martensite (B190 ) is 70 GPa, abnormally higher than that of austenite (60 GPa). Carballo [2] pointed out that the modulus of austenitic phase is approximately 120% that of martensite. It can be seen that from our experimental results, the obtained conclusion needs some conditions. The variations of modulus in different alloy systems may be different, even in the same alloy; different heat treatment may also make the modulus vary. 3.3.2. The damping behaviour Fig. 5 shows the curves of damping dependence of temperature under different vibration frequencies. The damping properties, Tand, of specimen in recovery state (Fig. 5a) is lower than that in solution state (Fig. 5b). Tand demonstrate a maximum peak during both heating and cooling processes, and the values in austenite are larger during heating while smaller during cooling than that in martensite. As vibration frequency decreases, the damping peak grows. Another unusual phenomenon is that the damping peaks become broad and one peak
Fig. 6a shows a typical microstructure of specimen in recovery state at room temperature, in which the martensite plate can be distinguished. In the specimens of such heat treatment state, recrystallization has not taken place, and a high density of dislocations still remains in the specimens after thermal rearrangement [2]. The selected-area diffraction (SAD) patterns taken from the region A indicated in Fig. 6a is shown and indexed in Fig. 6b. The martensite was confirmed to be of a B19 structure. Many large scattered precipitates have been found during the TEM observation, as shown in Fig. 6c. The composition of the precipitate was determined by using energy dispersive analyser, as was that of the matrix around the precipitates. Five positions are selected as shown in Fig. 6c, and the measurement results are listed in Table 1. It can be seen that the atomic composition of the precipitate is Ti2(Ni, Pd)1. The SAD pattern of the precipitate (Fig. 6d) can be successfully indexed to be of a cubic structure. It is known that the crystal lattice of Ti2Ni is cubic while Ti2Pd is tetragonal [13]. Pd substitution for Ni in Ti2Ni will inevitably make the lattice unstable. With the increase of Pd content, the cubic lattice would eventually transform into tetragonal ones. In the alloys studied, the crystal lattice of the precipitate is still of a cubic structure with Pd partially substituting for Ni, which can be written as Ti2Ni(Pd). The effect of precipitation on the matrix is significant, the content of Ti of the matrix is lower than the nominal content 51 at.%, and decreases from 47.35 to 46.17 at.% as the measured spot moves towards the precipitates. The changes of Ti content are an evidence of diffusion process
Table 1 Composition of both the precipitate and the matrix around Measurement position
Ti (at.%)
Ni (at.%)
Pd (at.%)
Matrix
1 2 3 4
47.35 47.13 46.6 46.17
18.96 18.9 18.04 17.95
33.68 33.97 35.36 35.88
Precipitate
5
64.72
22.46
12.82
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Fig. 5. The damping characteristic of Ti51Pd30Ni19 alloy, specimen (a) in recovery state, and (b) in solution state, and (c) the separation of the overlapping peaks indicating a two-state phase transformation.
during the precipitation. The content variation may cause a various transformation temperature distribution in the region around the precipitates, resulting in wide or overlapping even two individual peaks/valleys in the DSC curves. According to the DSC measurement results, the expected phenomenon does not happen, which can be deduced that the transformation temperatures are not sensitive to the Ti content variation from 47.35 to 46.17 at.%. Fig. 7 represents the TEM observation on the specimen in solution state. In this specimen, a long plate of martensite is the main characteristic at room temperature (Fig. 7a). The martensite is confirmed to be of B190 type by the corresponding SAD patterns (Fig. 7b). After the specimen was heated to 873 K and followed by cooling to room temperature in vacuum, high resolution TEM observation was carried out. Fig. 7c shows that much precipitation has occurred, and the precipitates grew to some strips along a certain direction in the martensite plate. Fig. 7d and e are the SAD patterns taken from the area without (B) and with (C) precipitate as indicated in Fig. 7c. The matrix patterns (Fig. 7d) can still be indexed to be of B190 martensite. The SAD patterns of the precipitates (Fig. 7e) is not clear in the figure, but according to the precipitates in the alloy in
recovery state and an investigation on the precipitation of the Ti50.6Pd30Ni19.4 alloy [7], the precipitates should be of a Ti2Ni(Pd) type structure.
4. Discussion 4.1. B19$B190 transformation According to the phase diagram of TiPd–TiNi alloys [14], it is impossible to obtain B190 martensite at low temperature for alloys whose Pd content is higher than 20 at.%. However, it is confirmed that in the Ti-rich Ti50+xPd30Ni20x alloy, two kinds of phase transformation may occur, i.e., the B2–B19 one-stage and the B2–B19–B190 two-stage phase transformation. B190 martensite has not been observed for alloys of which Pd content is higher than 20 at.%. In the past, many researchers focused their attention on the equiatomic Ti50(PdNi)50 alloy, for example, Ti50Pd30Ni20, in which no precipitates have been observed [5]. But for off-stoichiometric Ti50+x (PdNi)50x alloys, a large amount of precipitates can be produced in the process of alloy preparation [10], which is also observed by our experiments. The changes of matrix composition affected by
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Fig. 6. Typical structures of the specimen in recovery state, (a) TEM bright field image at room temperature, (b) SAD pattern in the [110]B19 zone, taken from the region A in (a), (c) a typical precipitate and showing the measurement position (1–5), and (d) the corresponding SAD pattern in [123]Ti2Ni zone.
precipitation are so significant that the transformation behaviour should be influenced without question. After homogenisation at 1273 K, the alloy becomes a homogeneous solution. The eutectoid decomposition can proceed completely if the alloy was furnace cooled. Only a large amount of dislocations and internal stress were introduced when the alloy was rolled, after recovery and recrystallization, the nature of the phase transformation cannot be changed. Through the solution treatment, the precipitates are dissolved and the alloy becomes a homogeneous solution again. The process of precipitation can be suppressed on the consequent quenching, causing a more serious distortion of the crystal lattice, and resulting in the appearance of the B190 phase. Thus, B19 becomes a media for the lattice deformation during a martensitic transformation. However, it can be deduced that an alloy in such a solution state is metastable, TEM observation did reveal that much precipitation has occurred when the specimen was heated to high temperature. With the growth of the precipitates, the Ti content of the matrix would be greatly lowered, and a change of phase transformation from the obtained B2–B19–B190 to B2–B19 would happen. In our experiment, fine precipitates were produced in some microregions. The effect of precipitation on the matrix composition is not high enough to
affect the transformation sequence, therefore, the twostage transformation characteristics has not been changed. Another question may put forward, that is, is the medial phase a new phase like R-phase? It can not be, because the Pd content is very high, according to the transformation sequence and the phase diagram, it should be the B19 phase. 4.2. Soft mode and the elastic modulus The trough in the elastic modulus curve means that the elasticity softens as the temperature changes. This phenomenon is easily related to the theory of a soft mode in the structural phase transformation. The soft mode usually connects with a certain shear elastic constant term [15]. The soft mode is a universal phenomenon in thermoelastic transformations involving the deformation of the crystal lattice. It is confirmed by the experimental results of the specimen in recovery state, in which only the one-stage phase transformation (B2– B19) exists. As for the specimen in solution state, there does not appear a trough but a step in the modulus curves. It seems to be a fallacy. However, if attention is focused on the fluctuation on the modulus steps, a trough occurs in the B2$B19 transformation, and a step is caused by the B19$B190 transformation.
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Fig. 7. Typical structures of specimen in solution state, TEM bright field images at RT (a), and cooling to RT (c), and SAD pattern in [110]B190 zone (b) [taken from the region A in (a)] and in [100]B190 zone (d) and (e) [taken from the region B and C in (c)].
5. Conclusion 1. Ti51Pd30Ni19, Ti50.6Pd30Ni19.4 and Ti50.2 Pd30 Ni19.8 alloys in different heat treatment states may show different phase transformation behaviours. For specimens in recovery state, there exists the B2–B19 one-stage phase transformation, with the transformation temperature decreasing with the increase of Ti content deviation from 50 at.%. The B2–B19–B190 two-stage phase transformation occurs for the alloys in solution state. The reason lies in that much precipitation occurs in the alloys in recovery state, resulting in a lowered Ti content of the matrix. 2. The variation of elastic modulus as a function of temperature is different for Ti51Pd30Ni19 alloys under
different heat treatment conditions. There exists a trough in the modulus curves for specimens with a B2– B19 transformation, while apparently steps appear for B2– B19–B190 transformation. The steps are originated from B19–B190 transformation. The B190 phase exhibits a much higher elastic modulus than B19. A trough can be observed clearly under lower vibration frequency when B2-B19 transformation takes place in B2–B19–B190 process. 3. Phase transformation leads to a maximum peak in the damping curves both in the heating and cooling processes. The damping properties, Tand, of specimen in recovery state are lower than that in solution state, and show a higher value after phase transformation than before. It is true for both the martensitic and its reverse transformations.
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Acknowledgements This work is sponsored by the Science and Technology Commission of the Shanghai Municipal Government, No. 00JC14055. The authors wish to express thanks to the reviewer for his/her valuable comments on this paper. References [1] Choi SB, Hwang JH. J Sound Vibration 2000;231(4):1168–74. [2] Carballo M, Pu ZJ, Wu KH. J Intell Mater Syst Struct 1995; 6:557–65. [3] Matveeva NM, Kovneristyi YuK, Savinov AS, Sivokha VP, Khachin VN. J Physique 1982;43:C4–249. [4] Mathews S, Li J, Su Q, Wuttig M. Philosophical Magazine Letters 1999;79(5):265–72.
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