A crystalline-amorphous transition in CuTi induced by high energy electron irradiation

Scripta

METALLURGICA

V o l . 18, pp. 9 5 7 - 9 6 2 , 1984 Printed in t h e U . S . A .

Pergamon P r e s s Ltd. All rights reserved

A CRYSTALLINE-AMORPHOUS TRANSITION IN CuTi INDUCED BY HIGH ENERGY ELECTRON IRRADIATION D.E. Luzzi*, H. Mori and H. Fujita Research Center for Ultra-High Voltage Electron Microscopy Osaka University, Yamada-oka, Suita, Osaka, 565, Japan M. Meshii Materials Research Center and Department of Materials Science and Engineering Northwestern University, Evanston, Illinois, U.S.A. (Received (Revised

May June

4, 1 9 8 4 ) 5, 1 9 8 4 )

Introduction The occurrence of a crystalline to amorphous (C--A) transition in NiTi alloys by electron irradiation has been well established in recent publications [1-4]. This is of interest not only from the engineering viewpoint that the alloys exhibit excellent shape memory properties [5] and could have application in radiation environments, but also from the scientific viewpoint that displacement cascades are proved not to be necessary to produce sufficient disorder to induce a C-A transition. Quite recently, the study of the electron-irradiation induced C-A transition has been extended to a wide range of binary transition metal compounds in an effort to establish the generality of the C-A transition [4,6]. Twenty compounds were studied and it was found that a C-A transition was produced in twelve of the compounds. These results indicate that the C-A transition is a common feature among intertransition metal compounds whose constituent elements are separated by a minimum number of groups in the periodic table. Therefore it is of interest to focus attention on the nature of this C-A transition. As a first step in the analysis of the C-A transition, it is useful to compare the nature of the resultant amorphous materials produced by electron irradiation with those produced by liquid phase quenching (LQ). For that purpose, NiTi would seem to be ideally suited since much data exists regarding its electron-irradiation induced C-A transition and it has the simplest crystal structure, B2, of the twelve aforementioned compounds. Unfortunately, it is very difficult to produce NiTi amorphous ribbon free from crystallites by LQ. Therefore, Y-CuTi was selected as the material here because it has the second simplest crystal structure, BII, and the alloy is well known to be easily made amorphous by LQ [7]. In the present work, the C-A transition in titanium-rich Y-CuTi is studied and the electron diffraction patterns of the resultant amorphous CuTi are compared with published X-ray diffraction data. Also, the total doses of electrons necessary to cause the transition are measured as a function of irradiation temperature. T h e n the specimens containing the amorphous regions are annealed to produce their crystallization and the resultant structure is analyzed to determine if the original crystal structure is regained. All of the experiments mentioned above are done in-situ in a Hitachi HU-3000 type ultra-high voltage electron microscope at Osaka University. Experimental

Details

99.99% pure copper and 99.9% pure titanium supplied by Ishizu Pharmaceutical Co., Ltd. and Johnson-Matthey Co., Ltd., respectively, were arc-melted in an argon atmosphere in the proportion of Cu - 52 atomic percent Ti. This alloy is within the titanium-rich solubility limit of Y-CuTi and diffraction analysis indicated a resultant single phase structure of Y-CuTi. *Permanent address: Evanston, Illinois,

Department U.S.A.

of Materials

Science and Engineering,

0036-9748/84 Copyright (c) 1 9 8 4

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To ensure complete mixing, an anneal was carried out for 2.59 x 105 s at '1150 K in a vacuum of better than 6.7 x 10-4 Pa. Specimens were prepared by slicing with a spark cutter, hand grinding to a thickness of approximately I00 ~m and jet and elee~troehemical polishing. Jet polishing was done using a nitric acid and methanol solution in the proportions 1:4 by volume at 293 K and i00 volts for twenty seconds. Finally, the specimens were electropolished by a plate cathode method in a similar solution at 223 K and 16-20 volts until a hole appeared. The C-A transition was produced by in-situ irradiation in the Hitachi HU-3000 UHVEM operatlng at 2 MV with an electron flux of 1.05 x 1024 e/m2.s over temperatures ranging from 95 K to 181 K. In order to study the temperature dependence of the required dosage to produce the C-A transition, the temperature was varied over this range and higher temperatures using a liquid nitrogen cold stage with an attached heater. This stage was carefully calibrated to ensure that the stated temperatures accurately represent the specimen temperatures. Annealing experiments were done by heating the specimen to 524 K. All tions were made after cooling the specimen to a constant temperature near room avoid the effects of high temperature electron irradiation. The heating stage periment was also carefully calibrated. Contamination of the specimens during was not a problem as the vacuum in the UHVEM was always better than 1.3 x 10 -4

observatemperature to used in this exthe experiments Pa.

Results and Discussion Figure i shows a series of micrographs depicting the transition of the Y-CuTI from the crystalline to the amorphous state. The arrows indicate a fixed position. Initially, the transition can be detected by a reduction in the contrast and a broadening of the images of dislocations as well as a shift of the visible bend contour as seen in Fig. lb. This shift of the bend contour is considered to be a result of a dilatation of the specimen in the transition region caused by a reduction in density due to amorphisation. This is followed by the disappearance of the dislocation and bend contour contrast as the irradiated volume completes the transition (Figs. lc and d). During these events, the diffraction pattern gradually changes from a spot pattern corresponding to Y-CuTi to a diffuse ring pattern typical of amorphous strue, tures. The sequence of changes begins with the appearance of multiple rings around several low index diffraction spots and the disappearance of high index spots (Fig. ib'). This is followed by an increase in the intensity of the center ring coupled with a decrease in the intensity of the double diffraction rings and a further reduction in the number of spots. Also, the appearance of the split second ring can be detected in Fig. ic'. A complete transition is indicated by the total absence of contrast in the amorphous region as seen in the bright field image (Fig. id), and a diffraction pattern typical of amorphous materials (Fig. id'). All selected area diffraction patterns were taken using a constant selected area aperture size which is depicted in Fig. id. Using a microphotometer, the amorphous diffraction patterns produced by irradiation were analyzed. By combining the measured radius of the rings with an accurate value of the microscope's camera length, the value of the scattering vector, K, corresponding to the first peak could be calculated. Also, the ratios of the radii of the split second ring and the third ring to the radius of the first ring were calculated. These results are presented in Table i with a tabulation of published results obtained by X-ray diffraction of amorphous CuTi alloys produced by liquid phase quenching. Since the first halo (electron diffraction) and the first peak (X-ray diffraction) are the sharpest and have the highTABLE i est ratio of intensity to Data on Peak Positions of a-CuTi Formed by background, their positions Electron Irradiation and Liquid Phase Quenching can be measured with accuracy. As can be seen, the Method KI(A-I)* K2/KI K2'/KI K3/KI tabulated values of the corresponding scattering vecLQ [9] 2.93 1.71 1.92 2.57 tors show excellent agreeLQ [i0] 2.92 1.70 1.94 2.59 ment. The measure of the e- irr. 2.93 1.68 1.95 2.58 ratios of ring radii and (present study) peak separations is subject * Scattering vector, K, calculated using K = 4~sine/% to more scatter due to the KI: first peak (halo) broader peaks and a reduced K2: inner peak of split second peak ratio of signal to backK~: outer peak of split second peak ground, but overall, the K3: third peak

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FIG. 1 Successive stages of the amorphisation of Y-CuTi by electron irradiation with corresponding diffraction patterns. The arrow on each micrograph shows a fixed position. Irradiation temperature and flux are 152 K and 1.05 x 1024 e/m2.s, respectively. a) Before irradiation, b) after 30 s irradiation, c) after 75 s irradiation and d) after 130 s irradiation. The outlined area in (d) depicts the size and position of the selected area aperture which was kept constant for all diffraction patterns.

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results exhibit a close correlation between the diffraction patterns of amorphous CuTi produced by electron irradiation and LQ. This suggests that within the limits of diffraction experiments, the amorphous structures produced by both methods are similar. In order to further characterize the electron-irradiatlon induced C-A transition in Y-CuTi, the transition was studied at various temperatures. Figure 2 is a plot of the dosage required for complete amorphisation versus temperature. Every measurement was made in the same grain and the criterion used was the complete interruption of a bend contour or the disappearance of dislocation contrast. As can be seen, the required dosage is approximately constant at temperatures ~5 below 170 K and the results are easily reproduced. ~t temperatures above ~-CuTi 170 K, the required dosage increases 2 MV very quickly with temperature and more ~.~ ~=l.05XlOz4e/m2. s scatter in the results occurs. The appearance of this scatter is a common occurrence in experiments of this type %mO O [2]. The arrow on the data point at 186 K indicates that no amorphlsatlon could be detected at dosages up to 1.5 x 1027 e/m2 At this temperature, Ill a large density of secondary defects (2) was produced, a phenomenon that did O not occur at lower temperatures. It a is interesting to note the very narrow ..I 5' temperature range in which the conditions for the C-A transition change drastically. This sharp transition has been observed in several transi~-tion metal compounds including CuZr, Fe2Ti and Cu3Ti 2 [8]. ~ / Figure 3 is a series of micro= graphs depicting the crystallization of the O ' J ' amorphous region of Fig. i a t 524 K. The IOO 2--O arrows mark the same fixed position. As TEMPERATURE (K) can be seen, the crystallization occurs by the shrinkage of the amorphous region and not by the nucleation of crystals within FIG. 2 the r e g i o n . The crystallized region in Temperature dependence of the total dose of Fig. 3a contains a dense radial array of electrons which is required to cause a dislocations which surround the amorphous crystalline-amorphous transition in Y-CuTi. region. It is considered that these dislocations result from !,packlng errors" as atoms at the periphery of the amorphous region are contacted by the encroaching crystalline boundary and are prevented from arranging in perfect lattice rows by the presence of simultaneously crystallized regions in the surrounding areas. With increasing annealing time, the crystallized region continues to absorb the amorphous region (Figs. 3b and c) until eventually the edges join in a long sub-grain boundary as seen in Fig. 3d. This is a natural result of the accumulation of many of the "packing errors". Analysis of the diffraction pattern in Fig. 3d' shows that the amorphous region has completely crystallized into the BII Y-CuTi phase. These results indicate that the original crystal structure is regained upon annealing although the defect microstructure may differ from the original crystalline phase. An interesting effect can he observed by comparing Figs. Id and 3a. It is obvious from these micrographs that an increase in contrast occurs in the irradiated area upon annealing for short times at 524 K. Due to the relatively large objective aperture in the HU-3000 UHVEM (objective aperture angle equal to .44 of the first halo angle), an experiment was completed using a Hitachi HU-12A microscope operating at 125 kV. In this microscope, with an objective aperture angle which is .25 of the first halo angle, a similar result was observed indicating that with annealing, a decrease in intensity occurs within 1.91 x 10-3 rad of the optic axis at 125 kV. Since the objective aperture size is small compared to the first halo, this increase in contrast is considered not to

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FIG. 3 Successive stages of the crystallization of the amorphous CuTi region produced in Fig. i. The arrows mark the same fixed position. Annealing temperature is 524 K. a) After 1.2 ks anneal, b) after 2.4 ks anneal, c) after 3.06 ks anneal, and d) after 4.26 ks anneal, d') is the selected area diffraction patter~ of the fully crystallized region which was obtained using the aperture size and postion depicted in Fig. id.

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be a result of a decrease in the intensity of the first halo tail. Further diffraction experiments are being performed and a high resolution study is planned to elucidate the reason for this observation. In conclusion, initial comparisons based on diffraction data indicate that the amorphous structures resulting from electron irradiation and LQ are similar. It was also shown that the amorphous structure will return to the original crystal structure when crystallized by annealing. The dosage required to produce the C-A transition shows a marked temperature dependence indicating that ~185 K is the maximum temperature at which the transition is possible in Y-CuTi under the present irradiation conditions. Additional results pertaining to the nature of the electron-irradiation induced C-A transition in intertransition metal compounds will be presented in future publications. Acknowledgements The authors would like to express their thanks to Prof. K. Shimizu and Dr. T. Tadaki for the use of their microphotometer. They also acknowledge the assistance of Messrs. K. Yoshida, M. Komatsu and T. Sakata with these experiments. One of the authors (D.L.) acknowledges the financial assistance provided by the Ministry of Education, Science and Culture of Japan through a Monbusho Scholarship. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

G. Thomas, H. Mori, H. Fujita and R. Sinclair, Scripta Met 16, 589 (1982) H. Mori and H. Fujita, Japanese J. Appl. Phys. 21, L494 (1982) H. Mori, H. Fujita and M. Fujita, Japanese J. Appl. Phys. 22, L94 (1983) H. Fujita, H. Mori and M. Fujita, Proc. 7th Int. Conf. on HVEM, Berkeley, Ca, U.S.A. (1983) p. 233 H. Mohamed and J. Washburn, J. Mater. Sci., 12, 469 (1977) H. Mori, H. Fujita, M. Tendo and M. Fujita (submitted to Scripta Met) P. Oelhafen, E. Hauser and H.J. Guntherodt, Solid State Comm. 35, 1017 (1980) H. Fujita, H. Mori and M. Tendo (unpublished data) M. Sakata, N. Cowlam and H.A. Davies, Proc. 4th Int. Conf. on Rapidly Quenched Metals, Sendai, Japan (1981) p. 327 T. Fukunaga, K. Kai, M. Naka, N. Watanabe and K. Suzuki, Proc. 4th Int. Conf. on Rapidly Quenched Metals, Sendai, Japan (1981) p. 347