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Studies of the R-phase transformation in a Ti–51at.%Ni alloy by transmission electron microscopy
Scripta Materialia 47 (2002) 363–369 www.actamat-journals.com
Studies of the R-phase transformation in a Ti–51at.%Ni alloy by transmission electron microscopy Danuta Str oz_
*
Institute of Physics and Chemistry of Metals, University of Silesia, Bankowa 12, 40-007 Katowice, Poland Received 2 April 2002; received in revised form 2 April 2002; accepted 9 April 2002
Abstract The R-phase transformation in a Ti–51at.%Ni shape memory alloy was studied by transmission electron microscopy including high resolution microscopy and in situ observations on cooling. It was found that prior to this transformation there is a wide temperature range at which an incommensurate phase exists in the alloy. The incommensurability decreases with the temperature decrease reaching finally the lock-in R-phase state. 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Shape memory alloys; Martensitic transformation; TEM; HREM
1. Introduction Presence of the R-phase in NiTi is either related to the chemical composition of the alloy (addition of Fe or Al) or to the stress fields generated in the two-component alloy by introduction a proper dislocation structure or by coherent precipitates. Such a material undergoes on cooling two transformations which both are of thermoelastic martensitic type. The enthalpy of the R-phase transformation is very small and since its temperature hysteresis is also very low it has attracted significant attention, as it is useful for many industrial applications. Many studies have been carried out in order to understand the transformation mechanism. It has been found out that the transformation sequence is as follows: B2 parent
*
Tel.: +48-32-359-1617; fax: +48-32-259-6929. E-mail address: [email protected] (D. Str oz_ ).
phase ! incommensurate ðICÞ ! commensurate ðRÞ ! B190 martensite. The R-phase transformation was explained in terms of a charge density wave [1] or modulated lattice relaxations [2]. Shapiro et al. [3] made a detailed studies by X-ray diffraction technique using a linear position sensitive detector and found out that the incommensurability is neither regular nor periodic and the incommensurate wave vector depends on the Brillouin zone studied. Thus he denied the charge density wave as a mechanism of the R-phase formation. Yamada [2] proposed a modulated lattice relaxation model for the incommensurate state of the parent phase. Cai et al. [4] studied the R-phase transition by means of in situ transmission electron microscopy. They claimed that the incommensurate phase exists in a wide temperature range above the R-phase transition, but the electrical resistance rise starts at the temperature when the ‘‘lock-in’’ that is the R-phase is formed. This result is different from that obtained by authors of
1359-6462/02/$ - see front matter 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 6 2 ( 0 2 ) 0 0 2 6 4 - 6
364
D. Stroz_ / Scripta Materialia 47 (2002) 363–369
[1]. Tamiya et al. [5] obtained results similar to Cai and his group, moreover, they claim that the Rphase undergoes further lattice distortion that is reflected in the a angle (rhombohedral angle). Thus, it can be seen that there exist discrepancies between the presented results and the origin of the R-phase transformation is still not clear. So, in the present work this transformation was studied in an aged Ti–51%Ni alloy by transmission electron microscopy including high resolution imaging and in situ cooling in the microscope.
2. Experimental procedure Ti–51%Ni alloy was solution treated at 1123 K/ 1 h followed by quenching into iced-water. Then it was cold rolled with the deformation degree 5% and aged at 573 K for 1 h. Specimens for electron microscopy studies were prepared by jet electropolishing with the solution of 1p. H2 SO4 and 5p. CH3 OH. The JEOL JEM3010 high resolution transmission microscope was used for observations. Images were recorded using GATAN CCD slow scan camera and then analysed by DigitalMicrograph software that allows subtracting the diffraction pattern background as well as measurements of the exact peak positions. The electrical resistance measurements were carried out with the use of a linear four point probe and with a cooling/heating rate 5 K/min.
The DSC curves were registered with the use of a Perkin Elmer DSC 7 calorimeter with cooling and heating rate of 10 K/min.
3. Experimental results The applied treatment of the TiNi alloy suppressed the B190 martensite transformation to such a low temperature that it was not observed even after cooling down to the liquid nitrogen temperature. Thus the only transformation observed in these samples was the R-phase transition. The DSC measurements showed that the transformation starts at 309 K (Fig. 1a). Fig. 1b shows electrical resistance of the alloy as a function of temperature, it can be seen that the rise in the resistance starts at 320 K. Moreover, in the temperature range 302–320 K there is no visible hysteresis between the cooling and heating curve. The curves show hysteresis starting from about 300 K and it is not larger than 10. The TEM images of the sample at room temperature do not reveal presence of the R-phase, the only visible details is some number of dislocations. The corresponding diffraction patterns show only the spots of the matrix in the [1 1 1] orientation. However, if this pattern is registered with the slow scan camera and then the background is cut off it reveals very week superlattice reflections situated at 1/3 of 1 1 0 spots that are neither visible with the
Fig. 1. DSC (a) and electrical resistance (b) curves for the studied NiTi alloy annealed at 850 C/1 h, deformed by 5% and then aged at 300 C/1 h.
D. Stroz_ / Scripta Materialia 47 (2002) 363–369
naked eye at the fluorescent screen nor registered at films. This was the case with every place of this specimen when at the first look the diffraction patterns show existence of the parent phase only. Additionally, there were areas where the extra spots were visible directly at the microscope screen. These were used to obtain high resolution images of the samples. Two different orientations of the modulated structure are shown in Fig. 2, one in [1 1 1] zone axis of the matrix (which is [0 0 1] of the R-phase notation) and the other in [0 0 1] zone axis of the B2 phase ([0 1 1]R orientation). Both regions show at each place the same orientation, which was confirmed by calculating the FFT from small image areas. By computing the FFT from the image and then filtering the diffracted beams the processed images were obtained (Fig. 2c and d). They clearly show that the rows of atoms do not form straight lines of atoms of the same brightness. This may be caused by
365
local differences in the foil thickness, orientation but also by differences in atom shifts occurring when the transition takes place. According to the DSC and X-ray experiments the specimen does not undergo the martensite transformation even after cooling down to the liquid nitrogen temperature. This allowed for the in situ observations of the R-phase transition in the microscope. The alloy structure was studied after cooling down the specimen and stabilising the temperature, so that no drift of the sample was observed. The obvious difference in the structure was the appearance of lot of plates in every place of the sample that form sets of different orientation (Fig. 3). They were not uniform in sizes, there were regions with very tiny plates (Fig. 3a) and also those with thick and long ones (Fig. 3c). The diffraction patterns of the cooled sample showed also changes comparing the ones recorded at room temperature. First of all the intensities of the extra
Fig. 2. High resolution images of the modulated structure in [1 1 1]B2 (a) and [0 0 1]B2 orientations (b) and the processed images (c,d).
366
D. Stroz_ / Scripta Materialia 47 (2002) 363–369
Fig. 3. The alloy structure after cooling down to the liquid nitrogen temperature (a,c) and corresponding dark field images taken in the reflections indicated by arrows (b,d).
spots significantly increased and at the same time the main reflections became more sharp and less diffuse. Moreover, if the diffraction pattern was taken from the region with thick large plates additional spots at 1/2 of the 1 1 0B2 were visible accompanied by strong diffuse streaks along some h1 1 0i directions (Fig. 3d). Thus, the dark field observations became necessary in order to explain the new feature of the images. It must be pointed out here, that the dark field experiments are not easy in case of the Rphase studies. The extra spots lie so close to the main B2 phase reflexes that by using the standard objective aperture is not possible to separate the spots. On the other hand, if a smaller aperture is used the intensities are too small to be seen at the screen and record the image on a photo plate. Thus, the slow scan camera and longer exposition times were used to register dark field images obtained with the smallest objective aperture. The
results of these observations are shown in Fig. 3b and d. Surprisingly the plate-like morphology is not the R-phase, which even in this low temperature stays in form of relatively small domains. It was also found out that the 1/2 1 1 0 reflections come from martensite-like plate morphology (Fig. 3d). After heating the specimen in the microscope up to the room temperature and even higher the plate-like features did not disappear from the thin foil. Also measurements of the spot position were carried out for a specimen at room temperature, during cooling down the specimen and after stabilising the temperature in the microscope. It must be pointed out here, that lack of the proper facilities (only one-tilt cooling holder available) made the measurements of the spot positions versus temperature impossible in these studies. However, interesting results were also obtained. The measurements
D. Stroz_ / Scripta Materialia 47 (2002) 363–369
367
Fig. 4. Diffraction pattern of the sample at room temperature (a), the same pattern after subtracting the background (b) and the line profile along [1 1 0] row (c).
were performed using the DigitalMicrograph 2.5.8 program that gives the possibility to obtain the line profiles and to measure the exact positions of the diffraction peaks. The measurements of the FFT patterns calculated from the high resolution images of the modulated regions were also carried out. The results obtained were similar. At room temperature most of the measured diffraction patterns showed that the position of the extra spots are shifted from the exact 1/3 or 2/3 lock-in positions towards lower values. An example of the diffraction pattern recorded at room temperature is shown in Fig. 4a. After subtracting the background (Fig. 4b) the superlattice peaks are clearly visible. Fig. 4c presents the line profile along h1 1 0i direction. The measured positions of the peaks indicate that the 1/3 spots are shifted towards lower positions. Intensities of the extra reflections grow during cooling and the incommensurability along h1 1 0i directions decreased although it existed down to very low temperatures. The diffraction patterns taken at liquid nitrogen temperature showed that at this state the extra spots have reached the exact 1/3 or 2/3 positions.
4. Discussion The obtained results show that the electrical resistance of the studied NiTi alloy starts to rise at
about 318 K which is much above room temperature. At this temperature practically nothing happens on the DSC curve. It declines from the basic line at 308 K, that is very close to the temperature at which the hysteresis between resistance cooling and heating curves occurs. This might indicate that the rise of the electrical resistance is connected with the incommensurate phase forming which is of the second order transition and thus gives no thermal hysteresis. These results are in agreement with the ones obtained by Wayman’s group [1,6] but differ from the more recent results shown by Cai et al. [4], who claim that the R-phase i.e. the commensurate phase transformation starts at the temperature where the electrical resistance starts to increase. The transmission electron microscopy studies presented here show that at ‘‘room temperature’’ which for normal operating conditions cannot be higher than 308 K, the specimen is mostly in the incommensurate state. However, these results also show that there coexist different regions in the sample, these where a large area is already transformed and these where very small nuclei of the new phase were only formed. Thus, comparison of the results obtained for bulk sample and thin foil is difficult, if not impossible. But such a comparison can be made for the X-ray diffraction studies. These, in case of this alloy, showed that the first splitting of the 1 1 0 reflection occurs at about 303 K. Hence, it is believed that the electrical resistance starts to rise at
368
D. Stroz_ / Scripta Materialia 47 (2002) 363–369
the temperature when the incommensurate phase is formed in the sample. It is also believed that the observed high resolution images are the images of the modulated phase rather than of the locked-in R-phase. This is confirmed by both electron diffraction patterns from these regions and FFT’s images calculated for the areas. They show that the positions of the ‘‘superlattice’’ reflections are deviated from the exact 1/3 of the h1 1 0i spots. As the intensities of the extra spots were not very high the observed modulations were weak. Moreover, unlike in the case reported by Ogawa and Kajiwara [7], whose examples of high resolution images of the R-phase showed that the lines corresponding to the 1/3 1 1 0 spots are not distributed over a fairly large region, these lines in the images presented here are quite wavy and of intensities changing from place to place. (This is well revealed in the processed images.) This can be caused by local changes of the foil thickness, by slight changes in the foil orientation. However, as such an effect was never so strong for the foils containing the B2 matrix only, and was always observed in this sample, it is tempting to ascribe it to the nature of the modulations. Indeed, if we look at the overall field of view we can distinguish contiguous domains each exhibiting its own, slightly different then the others, intensity modulation. The sizes of these domains are in the range of 5–7 nm. Murakami and Shindo [8] assumed presence of variants of modulated regions, each subjected to a transverse displacement wave along one of the equivalent hh h 0i-type vectors. If the transformation starts in different regions of the sample by their modulation along a single h1 1 0i direction, then as for one of four {1 1 1} planes there are three h1 1 0i symmetrically equivalent directions, there would be three possible modulations visible in the images of the foil in this orientation (and two for the h1 0 0i zone axis). When the transformation proceeds, the modulated regions grow and ‘‘overlap’’. This could cause the effects observed in the high resolution images of the sample. The strains associated with lattice change are released rather by different amount of shifts of single atom planes than by formation of coupled domains of different modulation vector.
Observations of 1/2 h1 1 0i reflections were reported infrequently in the past. Tamiya et al. [5] concluded that these reflections as not visible in Xray diffraction are not the bulk effect but the thin foil effect. In our X-ray experiments such reflection did not exist either. However, Lotkov et al. [9] observed such reflections in a bulk sample studied by neutron diffraction. The sample was a single crystal of a composition for which only B2 ! B190 transformation occurred. The extra 1/2 h1 1 0i reflections appeared on the neutron diffraction patterns in vicinity of the B2 ! B190 transformation, they showed some amount of incommensurability and were interpreted as the development of a short range followed by the long-range order of transverse atomic displacement waves. In case of the presented here experiments, the extra 1/2 h1 1 0i spots appear at temperatures much lower than the temperature of the R-phase formation and clearly come from the regions much thicker than just the foil edge. This would prove that an additional transition could appear before the martensite transformation. The absence of these reflections at the X-ray spectra may be due to too small volume fraction of the new phase. The phase takes a form of plates with well developed interfaces and its thermal hysteresis is large (the extra spots do no disappear after reheating the sample up to the room temperature). However, further studies are needed in order to determine the nature of the transition and its significance for the two other transformations.
5. Conclusions • The R-phase transition was investigated by in situ TEM and HREM observations. The incommensurate phase was found in a large temperature range prior to the R-phase transition. The high resolution images of the incommensurate phase show domains of different modulations. • The electrical resistance starts to rise at the temperature when the incommensurate phase forms. • Dark field observations of the in situ cooled sample show that also the commensurate
D. Stroz_ / Scripta Materialia 47 (2002) 363–369
R-phase occurs in form of relatively small domains. • Another atomic displacement waves of 1/2 h1 1 0i vector were found at low temperatures. The thermal hysteresis of this transition is large as the extra reflections do not disappear after reheating the sample to the room temperature. References [1] Hwang CM, Meichle ME, Salamon MB, Wayman CM. Phil Mag 1983;A47:9.
369
[2] Yamada Y. In: Proceedings of ICOMAT-86, JIM, Sendai, 1986. p. 89. [3] Shapiro SM, Noda Y, Fujii Y, Yamada Y. Phys Rev 1984;B30:4314. [4] Cai W, Murakami Y, Otsuka K. Mater Sci Eng 1999;A273– 275:186. [5] Tamiya T, Shindo D, Murakami Y, Bando Y, Otsuka K. Mater Trans, JIM 1998;7:714. [6] Salamon MB, Meichle ME, Wayman CM. Phys Rev 1985; B31:7306. [7] Ogawa K, Kajiwara S. Mater Trans, JIM 1993;34:1223. [8] Murakami Y, Shindo D. Mater Trans, JIM 1999;40:1092. [9] Lotkov AI, Dubinin SF, Teplouchov SG, Grishkov VN, Scorobogatov VP. J De physique IV 1995;5:C8551.
Studies of the R-phase transformation in a Ti–51at.%Ni alloy by transmission electron microscopy Danuta Str oz_
*
Institute of Physics and Chemistry of Metals, University of Silesia, Bankowa 12, 40-007 Katowice, Poland Received 2 April 2002; received in revised form 2 April 2002; accepted 9 April 2002
Abstract The R-phase transformation in a Ti–51at.%Ni shape memory alloy was studied by transmission electron microscopy including high resolution microscopy and in situ observations on cooling. It was found that prior to this transformation there is a wide temperature range at which an incommensurate phase exists in the alloy. The incommensurability decreases with the temperature decrease reaching finally the lock-in R-phase state. 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Shape memory alloys; Martensitic transformation; TEM; HREM
1. Introduction Presence of the R-phase in NiTi is either related to the chemical composition of the alloy (addition of Fe or Al) or to the stress fields generated in the two-component alloy by introduction a proper dislocation structure or by coherent precipitates. Such a material undergoes on cooling two transformations which both are of thermoelastic martensitic type. The enthalpy of the R-phase transformation is very small and since its temperature hysteresis is also very low it has attracted significant attention, as it is useful for many industrial applications. Many studies have been carried out in order to understand the transformation mechanism. It has been found out that the transformation sequence is as follows: B2 parent
*
Tel.: +48-32-359-1617; fax: +48-32-259-6929. E-mail address: [email protected] (D. Str oz_ ).
phase ! incommensurate ðICÞ ! commensurate ðRÞ ! B190 martensite. The R-phase transformation was explained in terms of a charge density wave [1] or modulated lattice relaxations [2]. Shapiro et al. [3] made a detailed studies by X-ray diffraction technique using a linear position sensitive detector and found out that the incommensurability is neither regular nor periodic and the incommensurate wave vector depends on the Brillouin zone studied. Thus he denied the charge density wave as a mechanism of the R-phase formation. Yamada [2] proposed a modulated lattice relaxation model for the incommensurate state of the parent phase. Cai et al. [4] studied the R-phase transition by means of in situ transmission electron microscopy. They claimed that the incommensurate phase exists in a wide temperature range above the R-phase transition, but the electrical resistance rise starts at the temperature when the ‘‘lock-in’’ that is the R-phase is formed. This result is different from that obtained by authors of
1359-6462/02/$ - see front matter 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 6 2 ( 0 2 ) 0 0 2 6 4 - 6
364
D. Stroz_ / Scripta Materialia 47 (2002) 363–369
[1]. Tamiya et al. [5] obtained results similar to Cai and his group, moreover, they claim that the Rphase undergoes further lattice distortion that is reflected in the a angle (rhombohedral angle). Thus, it can be seen that there exist discrepancies between the presented results and the origin of the R-phase transformation is still not clear. So, in the present work this transformation was studied in an aged Ti–51%Ni alloy by transmission electron microscopy including high resolution imaging and in situ cooling in the microscope.
2. Experimental procedure Ti–51%Ni alloy was solution treated at 1123 K/ 1 h followed by quenching into iced-water. Then it was cold rolled with the deformation degree 5% and aged at 573 K for 1 h. Specimens for electron microscopy studies were prepared by jet electropolishing with the solution of 1p. H2 SO4 and 5p. CH3 OH. The JEOL JEM3010 high resolution transmission microscope was used for observations. Images were recorded using GATAN CCD slow scan camera and then analysed by DigitalMicrograph software that allows subtracting the diffraction pattern background as well as measurements of the exact peak positions. The electrical resistance measurements were carried out with the use of a linear four point probe and with a cooling/heating rate 5 K/min.
The DSC curves were registered with the use of a Perkin Elmer DSC 7 calorimeter with cooling and heating rate of 10 K/min.
3. Experimental results The applied treatment of the TiNi alloy suppressed the B190 martensite transformation to such a low temperature that it was not observed even after cooling down to the liquid nitrogen temperature. Thus the only transformation observed in these samples was the R-phase transition. The DSC measurements showed that the transformation starts at 309 K (Fig. 1a). Fig. 1b shows electrical resistance of the alloy as a function of temperature, it can be seen that the rise in the resistance starts at 320 K. Moreover, in the temperature range 302–320 K there is no visible hysteresis between the cooling and heating curve. The curves show hysteresis starting from about 300 K and it is not larger than 10. The TEM images of the sample at room temperature do not reveal presence of the R-phase, the only visible details is some number of dislocations. The corresponding diffraction patterns show only the spots of the matrix in the [1 1 1] orientation. However, if this pattern is registered with the slow scan camera and then the background is cut off it reveals very week superlattice reflections situated at 1/3 of 1 1 0 spots that are neither visible with the
Fig. 1. DSC (a) and electrical resistance (b) curves for the studied NiTi alloy annealed at 850 C/1 h, deformed by 5% and then aged at 300 C/1 h.
D. Stroz_ / Scripta Materialia 47 (2002) 363–369
naked eye at the fluorescent screen nor registered at films. This was the case with every place of this specimen when at the first look the diffraction patterns show existence of the parent phase only. Additionally, there were areas where the extra spots were visible directly at the microscope screen. These were used to obtain high resolution images of the samples. Two different orientations of the modulated structure are shown in Fig. 2, one in [1 1 1] zone axis of the matrix (which is [0 0 1] of the R-phase notation) and the other in [0 0 1] zone axis of the B2 phase ([0 1 1]R orientation). Both regions show at each place the same orientation, which was confirmed by calculating the FFT from small image areas. By computing the FFT from the image and then filtering the diffracted beams the processed images were obtained (Fig. 2c and d). They clearly show that the rows of atoms do not form straight lines of atoms of the same brightness. This may be caused by
365
local differences in the foil thickness, orientation but also by differences in atom shifts occurring when the transition takes place. According to the DSC and X-ray experiments the specimen does not undergo the martensite transformation even after cooling down to the liquid nitrogen temperature. This allowed for the in situ observations of the R-phase transition in the microscope. The alloy structure was studied after cooling down the specimen and stabilising the temperature, so that no drift of the sample was observed. The obvious difference in the structure was the appearance of lot of plates in every place of the sample that form sets of different orientation (Fig. 3). They were not uniform in sizes, there were regions with very tiny plates (Fig. 3a) and also those with thick and long ones (Fig. 3c). The diffraction patterns of the cooled sample showed also changes comparing the ones recorded at room temperature. First of all the intensities of the extra
Fig. 2. High resolution images of the modulated structure in [1 1 1]B2 (a) and [0 0 1]B2 orientations (b) and the processed images (c,d).
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D. Stroz_ / Scripta Materialia 47 (2002) 363–369
Fig. 3. The alloy structure after cooling down to the liquid nitrogen temperature (a,c) and corresponding dark field images taken in the reflections indicated by arrows (b,d).
spots significantly increased and at the same time the main reflections became more sharp and less diffuse. Moreover, if the diffraction pattern was taken from the region with thick large plates additional spots at 1/2 of the 1 1 0B2 were visible accompanied by strong diffuse streaks along some h1 1 0i directions (Fig. 3d). Thus, the dark field observations became necessary in order to explain the new feature of the images. It must be pointed out here, that the dark field experiments are not easy in case of the Rphase studies. The extra spots lie so close to the main B2 phase reflexes that by using the standard objective aperture is not possible to separate the spots. On the other hand, if a smaller aperture is used the intensities are too small to be seen at the screen and record the image on a photo plate. Thus, the slow scan camera and longer exposition times were used to register dark field images obtained with the smallest objective aperture. The
results of these observations are shown in Fig. 3b and d. Surprisingly the plate-like morphology is not the R-phase, which even in this low temperature stays in form of relatively small domains. It was also found out that the 1/2 1 1 0 reflections come from martensite-like plate morphology (Fig. 3d). After heating the specimen in the microscope up to the room temperature and even higher the plate-like features did not disappear from the thin foil. Also measurements of the spot position were carried out for a specimen at room temperature, during cooling down the specimen and after stabilising the temperature in the microscope. It must be pointed out here, that lack of the proper facilities (only one-tilt cooling holder available) made the measurements of the spot positions versus temperature impossible in these studies. However, interesting results were also obtained. The measurements
D. Stroz_ / Scripta Materialia 47 (2002) 363–369
367
Fig. 4. Diffraction pattern of the sample at room temperature (a), the same pattern after subtracting the background (b) and the line profile along [1 1 0] row (c).
were performed using the DigitalMicrograph 2.5.8 program that gives the possibility to obtain the line profiles and to measure the exact positions of the diffraction peaks. The measurements of the FFT patterns calculated from the high resolution images of the modulated regions were also carried out. The results obtained were similar. At room temperature most of the measured diffraction patterns showed that the position of the extra spots are shifted from the exact 1/3 or 2/3 lock-in positions towards lower values. An example of the diffraction pattern recorded at room temperature is shown in Fig. 4a. After subtracting the background (Fig. 4b) the superlattice peaks are clearly visible. Fig. 4c presents the line profile along h1 1 0i direction. The measured positions of the peaks indicate that the 1/3 spots are shifted towards lower positions. Intensities of the extra reflections grow during cooling and the incommensurability along h1 1 0i directions decreased although it existed down to very low temperatures. The diffraction patterns taken at liquid nitrogen temperature showed that at this state the extra spots have reached the exact 1/3 or 2/3 positions.
4. Discussion The obtained results show that the electrical resistance of the studied NiTi alloy starts to rise at
about 318 K which is much above room temperature. At this temperature practically nothing happens on the DSC curve. It declines from the basic line at 308 K, that is very close to the temperature at which the hysteresis between resistance cooling and heating curves occurs. This might indicate that the rise of the electrical resistance is connected with the incommensurate phase forming which is of the second order transition and thus gives no thermal hysteresis. These results are in agreement with the ones obtained by Wayman’s group [1,6] but differ from the more recent results shown by Cai et al. [4], who claim that the R-phase i.e. the commensurate phase transformation starts at the temperature where the electrical resistance starts to increase. The transmission electron microscopy studies presented here show that at ‘‘room temperature’’ which for normal operating conditions cannot be higher than 308 K, the specimen is mostly in the incommensurate state. However, these results also show that there coexist different regions in the sample, these where a large area is already transformed and these where very small nuclei of the new phase were only formed. Thus, comparison of the results obtained for bulk sample and thin foil is difficult, if not impossible. But such a comparison can be made for the X-ray diffraction studies. These, in case of this alloy, showed that the first splitting of the 1 1 0 reflection occurs at about 303 K. Hence, it is believed that the electrical resistance starts to rise at
368
D. Stroz_ / Scripta Materialia 47 (2002) 363–369
the temperature when the incommensurate phase is formed in the sample. It is also believed that the observed high resolution images are the images of the modulated phase rather than of the locked-in R-phase. This is confirmed by both electron diffraction patterns from these regions and FFT’s images calculated for the areas. They show that the positions of the ‘‘superlattice’’ reflections are deviated from the exact 1/3 of the h1 1 0i spots. As the intensities of the extra spots were not very high the observed modulations were weak. Moreover, unlike in the case reported by Ogawa and Kajiwara [7], whose examples of high resolution images of the R-phase showed that the lines corresponding to the 1/3 1 1 0 spots are not distributed over a fairly large region, these lines in the images presented here are quite wavy and of intensities changing from place to place. (This is well revealed in the processed images.) This can be caused by local changes of the foil thickness, by slight changes in the foil orientation. However, as such an effect was never so strong for the foils containing the B2 matrix only, and was always observed in this sample, it is tempting to ascribe it to the nature of the modulations. Indeed, if we look at the overall field of view we can distinguish contiguous domains each exhibiting its own, slightly different then the others, intensity modulation. The sizes of these domains are in the range of 5–7 nm. Murakami and Shindo [8] assumed presence of variants of modulated regions, each subjected to a transverse displacement wave along one of the equivalent hh h 0i-type vectors. If the transformation starts in different regions of the sample by their modulation along a single h1 1 0i direction, then as for one of four {1 1 1} planes there are three h1 1 0i symmetrically equivalent directions, there would be three possible modulations visible in the images of the foil in this orientation (and two for the h1 0 0i zone axis). When the transformation proceeds, the modulated regions grow and ‘‘overlap’’. This could cause the effects observed in the high resolution images of the sample. The strains associated with lattice change are released rather by different amount of shifts of single atom planes than by formation of coupled domains of different modulation vector.
Observations of 1/2 h1 1 0i reflections were reported infrequently in the past. Tamiya et al. [5] concluded that these reflections as not visible in Xray diffraction are not the bulk effect but the thin foil effect. In our X-ray experiments such reflection did not exist either. However, Lotkov et al. [9] observed such reflections in a bulk sample studied by neutron diffraction. The sample was a single crystal of a composition for which only B2 ! B190 transformation occurred. The extra 1/2 h1 1 0i reflections appeared on the neutron diffraction patterns in vicinity of the B2 ! B190 transformation, they showed some amount of incommensurability and were interpreted as the development of a short range followed by the long-range order of transverse atomic displacement waves. In case of the presented here experiments, the extra 1/2 h1 1 0i spots appear at temperatures much lower than the temperature of the R-phase formation and clearly come from the regions much thicker than just the foil edge. This would prove that an additional transition could appear before the martensite transformation. The absence of these reflections at the X-ray spectra may be due to too small volume fraction of the new phase. The phase takes a form of plates with well developed interfaces and its thermal hysteresis is large (the extra spots do no disappear after reheating the sample up to the room temperature). However, further studies are needed in order to determine the nature of the transition and its significance for the two other transformations.
5. Conclusions • The R-phase transition was investigated by in situ TEM and HREM observations. The incommensurate phase was found in a large temperature range prior to the R-phase transition. The high resolution images of the incommensurate phase show domains of different modulations. • The electrical resistance starts to rise at the temperature when the incommensurate phase forms. • Dark field observations of the in situ cooled sample show that also the commensurate
D. Stroz_ / Scripta Materialia 47 (2002) 363–369
R-phase occurs in form of relatively small domains. • Another atomic displacement waves of 1/2 h1 1 0i vector were found at low temperatures. The thermal hysteresis of this transition is large as the extra reflections do not disappear after reheating the sample to the room temperature. References [1] Hwang CM, Meichle ME, Salamon MB, Wayman CM. Phil Mag 1983;A47:9.
369
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