UDK 621.785.3:669.35'571 ISSN 1580-2949 Original scientific article/Izvirni znanstveni članek MTAEC9, 40(4)153(2006) ISOTHERMAL DECOMPOSITION OF THE ß' PHASE IN Cu-Zn-Al SHAPE-MEMORY ALLOYS IZOTERMNA RAZGRADNJA ß'-FAZE V ZLITINAH S SPOMINOM Cu-Zn-Al Vanja Asanovi}, Kemal Deliji}, Nada Jaukovi} University of Montenegro, Faculty of Metallurgy and Technology, Podgorica,Cetinjski put, bb, 81000 Podgorica, Montenegro vanjaaŽcg.ac.yu Prejem rokopisa – received: 2005-12-20; sprejem za objavo - accepted for publication: 2006-04-16 The decomposition of the/?' phase in Cu-25.7Zn-3.1Al and Cu-17.9Zn-5.4Al shape-memory alloys was studied. The B2-»DO3 order transition was not completely suppressed by quenching in icy water. The aging process at temperatures below 400 °C involved the formation of bainite or a phase. In specimens aged at higher temperatures the a phase was observed. The activation energies of a precipitation in the Cu-25.7Zn-3.1Al alloy and Cu-17.9Zn-5.4Al alloy were determined to be 109.85 kJ mol-1 and 132.12 kJ mol-1, respectively. The formation of y precipitates was not observed. Key words: shape-memory alloys, decomposition, precipitation Raziskana je bila razgradnja/S'-faze v zlitinah s spominom Cu-25,7Zn-3,1Al in Cu-17,9Zn-5,4Al. Premena urejenosti B2-»DO3 nibila popolnoma preprečena s kaljenjem v ledenivodi. Pristaranju prinižjitemperaturiod 400 °C je nastal bainit ali a-faza. V vzorcih, žarjenih pri višjih temperaturah, smo našli a-fazo. Aktivacijske energije za a-izločanje v Cu-25,7Zn-3,1Al in Cu-17,9Zn-5,4Al so bile 109,85 kJ/mol oz. 132,12 kJ/mol. Nastanka/-izločkov nismo opazili. Ključne besede: zlitine s spominom, razgradnja, izločanje 1 INTRODUCTION "Shape memory" is the term used to describe an interesting property by which some materials "remember" their original shape and revert to it at characteristic transformation temperatures. This feature was observed in samples of Cu-Zn-Al alloys in the 1970s. Cu-Zn-Al shape-memory alloys are now being used for various applications. The shape-memory effect is based on a diffusionless phase transformation called the thermoelastic martensitic transformation. In this transformation atoms move cooperatively. In Cu-Zn-Al alloys, the parent phase is the bcc ß phase, and the transformation product is martensite. The high-temperature ß phase has a disordered A2 structure, but cooling induces an ordering process, bcc-»B2, by which the B2 superlattice structure develops. Upon further cooling, the structure goes through a next-nearest-neighbor ordering, i.e., B2-»DO3, and the structure eventually becomes the DO3. The lower-temperature phase (martensite) has a lower symmetry than the parent phase. The structures of martensites in Cu-Zn-Al shape-memory alloys are of a long-period stacking-order type: 6R, 18R and 2H. In order to obtain a good reliable shape-memory effect in Cu-Zn-Al alloys a sufficiently rapid cooling (quenching) from a betatizing temperature is necessary to avoid a eutectoid decomposition, ß' -» a + y. The nature of both the parent ß phase and the martensite in the Cu-Zn-Al alloys is metastable. Consequently, the stability of their shape-memory properties is influenced strongly by aging1-6. This work is concerned with the decomposition of the ß' phase (ordered parent phase) in two ternary Cu-Zn-Al shape-memory alloys. An optical microscope and X-ray diffractometer have been employed to examine the microstructural changes that occur in the alloys during aging. 2 EXPERIMENTAL Two Cu-Zn-Al shape-memory alloys with the nominal compositions given in Table 1 were prepared by melting high-purity copper, pre-alloy Cu-Zn (49.2 % Zn) and pre-alloy Cu-Al (47.8 % Al) in a graphite crucible using a resistance-heated furnace. The two billets were first homogenized at 800 °C for 2 h and quenched in water at room temperature, then extruded into 6-mm-diameter rods at 800 °C. Table 1: The mass fraction w/% of the chemical composition of alloys Tabela 1: Kemična sestava zlitin Alloy Zn Al Cu A 25.7 3.1 rest B 17.9 5.4 rest In order to study the isothermal decomposition of the ß' phase in the alloys, test specimens were cut from the rods, solution treated at 850 °C for 10 min and quenched MATERIALI IN TEHNOLOGIJE 40 (2006) 4 153 V. ASANOVI] ET AL.: ISOTHERMAL DECOMPOSITION OF THE ß' PHASE IN Cu-Zn-Al SHAPE-MEMORY ALLOYS into icy water. Quenched specimens were introduced into a 60 % KNO3 and 40 % NaNO2 salt bath at fixed temperatures: (200, 240, 280, 320, 360, 400, 440, 480, 520, 560, 600 and 640) °C. Then, they were taken one by one, for various times up 2048 min, and quenched into icy water. Afterwards, the analysis of the phases formed was performed using optical microscopy and X-ray diffraction. 3 RESULTS AND DISCUSSION Optical microscopy observations of the quenched specimens revealed the parent-phase grain boundaries and the martensite structure, as shown in Figure 1. From the X-ray diffraction patterns of the quenched specimens, the lattice parameters a, b, c, ß and

B2->DO3 transitions in the Cu-Zn-Al alloys is large. The ordering of the bcc ß phase to the B2 superlattice is so rapid that it is almost impossible to quench-in the disordered bcc w(Cu)/% Figure 1: Microstructure of the quenched specimens Slika 1: Mikrostruktura gašenih vzorcev Figure 2: Isothermal sections of the Cu-Zn-Al phase diagram at 700 °C (a), 550 °C and 350 °C (c)7 Slika 2: Izotermniprerez faze Cu-Al-Zn pri(a) 700 °C, (b) 550 °C in (c) 350 °C 9 structure7. This transformation is second order and occurs at high temperatures (about 550 °C). It is thought8 that the ordering process bcc-*B2 in the Cu-Zn-Al alloys is completely finished during quenching. The Cu-Zn-Al alloys exhibit a secondary B2->DO3 ordering reaction prior to the martensitic transformation. The critical temperature for B2oDO3 varies greatly from one alloy to another. The presence of M18R martensite in quenched specimens indicates that the B2->DO3 order transition cannot be completely suppressed by rapid cooling. On the basis of the isothermal sections of the equilibrium phase diagram of the Cu-Zn-Al ternary alloy system9 shown in Figure 2, the following phase transformations and reactions are possible for the alloys under investigation: ßČa+ß at T = 700 °C ß -> ß' at T < TC (TC is the critical tempera- ture for ordering) ß -Ča+ß' at T = 550 °C ß'Ča+y 350 °C< T<550 °C At temperatures from 350 °C to 550 °C, the alloys are expected to remain within the three-phase field of a, ß and y. Therefore, the eutectoid reaction occurs in the examined alloys at a temperature that is lower than 550 °C but higher than 350 °C. The reaction can be expressed as: ß' -> a +y The a phase is face-centered cubic, while the y phase has a cubic structure. 154 MATERIALI IN TEHNOLOGIJE 40 (2006) 4 V. ASANOVI] ET AL.: ISOTHERMAL DECOMPOSITION OF THE ß' PHASE IN Cu-Zn-Al SHAPE-MEMORY ALLOYS Alloy A Figure 3: Optical micrographs of the microstructure in specimens aged for: a) 512 min at 200 °C, b) 2048 min at 200 °C Slika 3: Optični posnetek mikrostrukture vzorca, ki je bil staran: a) 512 min pri 200 °C, b) 2048 min pri 200 °C with X-ray diffraction as a1 bainite Š9R] with the following lattice parameters: a = 0.469 nm, b = 0.264 nm and c = 1.950 nm. But the a1 bainite transformed ultimately into the fcc a phase (Figure 3b). The rod-like product is observed after annealing at higher temperatures (Figure 4), which was identified as fcc a phase with the lattice parameter a = 0.369 nm. The y phase was not detected in the aged specimens. To determine the amount of a phase that can occur in the Cu-25.7Zn-3.1Al and Cu-17.9Zn-5.4Al alloys as a result of isothermal aging treatments, a microstructural analysis was made. The progress of an isothermal phase transformation is represented by plotting the fraction of transformation as a function of time and temperature. The time-temperature-transformation curves were obtained for the alloys, as shown in Figure 5. The increase in the amount of a phase with aging time at each temperature was observed. The required durations to form an a precipitate in the Cu-17.9Zn-5.4Al alloy is greater than for the Cu-25.7Zn-3.1Al alloy. It has been reported1011 that the solute contents (Zn and Al) in the a phase are less than those in the parent phase. Since both the melting point and atomic size of aluminum exceed those of zinc the diffusion of aluminum atoms will be the dominant factor in the precipitation rate. Therefore, the specimens of Cu-17.9Zn-5.4Al alloy require longer aging times for the aluminum atoms to diffuse away and form a phase than those of the Cu-25.7Zn-3.1Al alloy. The microstructures of all the specimens after various aging treatments were investigated by optical microscopy. It was found that a plate-like product is developed in specimens after aging treatments at temperatures below 400 °C (Figure 3a). The phase was identified Alloy B Alloy B Figure 4: Optical micrographs of the microstructures in specimens aged for: a) 256 min at 440 °C, b) 128 min at 520 °C, c) 64 min 520 °C, d) 128 min 640 °C Slika 4: Optični posnetek mikrostrukture vzorcev, staranih: a) 256 min pri 440 °C, b) 128 min pri 520 °C, c) 64 min pri 520 °C, d) 128 min pri 640 °C 700 600 500 I 400 S3 300 S 200 100 o a) 0 1 2 20% 4 5 6 7 8 9 10 11 12 13 h (t/s) 40% 10% 700 600 500 400 300 200 100 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 h (t/s) Figure 5: The TTT-transformation diagram of a-precipitation in a) Cu-25.7Zn-3.1Al alloy (A alloy), b) Cu-17.9Zn-5.4Al alloy (B alloy) Slika 5: TTT-diagram a-izločanja v: a) zlitini Cu-25,7Zn-3,1Al (zlitina A), b) zlitini Cu-17,9Zn-5,4Al zlitini (zlitina B) b) MATERIALI IN TEHNOLOGIJE 40 (2006) 4 155 V. ASANOVI] ET AL.: ISOTHERMAL DECOMPOSITION OF THEß' PHASE IN Cu-Zn-Al SHAPE-MEMORY ALLOYS 1 1.2 1.4 1.6 1.8 2 2.2 103°C/T Figure 6: Arrhenius plot of the length of aging period vs. temperature to develop a microstructure with the volume fraction 20 % of a-precipitation (alloy Cu-17.9Zn-5.4Al) Slika 6: Arrheniusova odvisnost med trajanjem staranja in temperaturo za nastanek mikrostrukture s prostorninskim deležem «-izločanja 20 % (zlitina Cu-17,9Zn-5,4Al) The effects of aging treatments on shape-memory capacity have not been examined in this study. However, in our recent research work12 we found that the precipitation of the volume fraction 20 % a phase causes a degradation of the shape-memory capacity. An Arrhenius plot of the length of annealing period vs. temperature to develop a microstructure with 20 % a phase in the examined alloys is shown in Figure 6. The activation energies of a precipitation in the Cu-25.7Zn-3.1Al alloy and the Cu-17.9Zn-5.4Al alloy are thus determined to be 109.85 kJ mol1 and 132.12 kJ mol1, respectively. These values are lower than the activation energies for the diffusion in the ordered ß phase of the Cu-Zn alloys that were reported as 135-200 kJ mol1 by several investigators1316. This implies that the processes occurring during aging are diffusional, but enhanced by a superconcentration of quenched-in vacancies2. 4 CONCLUSIONS The directly quenched martensite in the examined alloys was the M18R type. This indicates that the B2h>DO3 order transition cannot be completely suppressed by rapid cooling. During aging at temperatures below 400 °C, the /?' phase decomposed into bainite, which was ultimately transformed into a phase. At temperatures above 400 °C, only a phase was formed. The formation of y precipitates was not observed. 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