Comparative Study on Thermodynamical and Electrochemical Behavior of Al88Ni6La6 and Al86Ni6La6Cu2 Amorphous Alloys

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Comparative Study on Thermodynamical and Electrochemical Behavior of Ab8N&& and Ab,N&La$u, Amorphous Alloys Wang Xiufeng (-€%%)', Wu Xueqing ( X + k ) ' 3 2 M , a M o (4 % ) I , Tan Chaogui (@-$A$&)', Lin Jianguo (+k&Q)'* (1. Faculty of Material & Photoelectronic Physics, Ximgtm University, Ximgtm 411105, China; 2. Department of Physics md Information Engineering, Hunm Institute of Hummities, Science md Technology, Loudi 41 7000, China 1 Received 26 July 2006; revised 21 September 2006

Abstlact: Two amorphous ribbons with the compositions of AlssNi,La6and Al,,Ni,La,Cu, were made using the meltspun method, and their thermal response and electrochemical behavior were studied comparatively. Differential scanning calorimetry (DSC) and electrochemical polarization measurements indicated that Al,,Ni,La,Cu, exhibited slightly higher crystallization temperature ( T J , lower melting point ( T , ) and better corrosion resistance in 0.01 mol * L - ' NaCl alkaline solution. These results demonstrated that Cu (2%) addition could sli&tly promote the glass forming ability, but it could greatly improve the corrosion resistance of Als8Ni6La6alloy in 0.01 mol L - ' NaCl alkaline solution. Key words: Al-based amorphous; glass forming ability; corrosion resistance; rare earths Document code: A Alticle ID: 1002 -0721(2007)03 -0381 -04 CLX: number: TG166

Al-based amorphous alloys such as A1-TM-RE (TM =transition metal, RE =rare earth) systems have attracted much attention during the past two decades because of their extraordinary high strength and good ductility",". By partially crystallizing the asquenched Al-based amorphous alloys, a number of fcc-A1 phases with nanometer size can precipitate homogeneously in the amorphous leading to further improvement of the mechanical proper tie^'^]. However, owing to the limited glass forming ability (GFA) in A1-TM-RE alloy systems, the aforementioned good mechanical properties can usually only be achieved in thin ribbons. Thus, application of Albased amorphous alloys as a structural material is restricted. Hence, it is necessary to explore new Albased glassy alloy systems, with higher GFA and

good stability against devitrification. Of late, it was found that minor alloy addition or microalloying technology was an effective way to promote the GFA and thermal stability of many all o y ~ [ ~In- the ~ ~meantime, . alloy additions could have a dramatic influence on the corrosion behavior of amorphous alloys, which has been proved in literat u r e ~ [ ~ " So, ~ ' . it can be expected that alloy additions can promote GFA and corrosion resistance of the alloys simultaneously. In light of this, the thermodynamic and electrochemical behavior of the two amorphous alloys, Al,, Ni,La, and Al,, Ni,La,Cu,, were studied in the present study, aiming to investigate the influence of Cu addition on the GFA and corrosion resistance of Al-Ni-La alloys.

* Comsponding author(E-mail: [email protected]; [email protected]) Foundation item: Projcct supported by the National Natural Science Foundation of China (50371072) Biogtaphy: Wu Xueqing (1973-), Male, Master candidate

Copyright 0 2 0 0 7 , by Editorial Committee of Journal of the Chinese Rare Earths Society. Published by Elsevier B.V. All ri&ts reserved

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1 Expeiimental Al,,Ni,La, and Al,, Ni,La,Cu, alloys were prepared by arc melting of metals with high purity. The alloy ingots were remelted several times to achieve homogeneity in composition. Ribbons of the alloys were made by rapid solidification in a single roller melt, spinning at a wheel speed of 35 m * s - ' . X-ray diffraction (XRD) analysis of the ribbons was carried out by using Philips diffractometer with Cu K a radiation, The thermal response of the samples was investigated with power compensation DSC at a heating rate of 20 K min ' under a flow of argon. Electrochemical measurements were conducted in a typical three-electrode system by using a Solartron model 1287 potentiostat. The electrodes were the same as reported in Refs. [ 11 13 3 . The potential between the working electrode and the reference electrode was measured through a Luggin capillary, which was placed as close as possible to the sample. The samples were tested at a scan rate of 10 mV s I , in 0.01 mol * L - NaCl electrolytes, adjusted initially to a pH value of 12 using 1 N NaOH solution. All polarization curves were obtained at 298 f 1 K under atmospheric pressure, and the samples were equilibrated by immersing them in electrolytes for 60 min, to obtain a near steady state corrosion potential. The cathodic polarization scans were started at the potential of - 2.5 V, setting approximately 1.3 V below the E,,,. Fresh solution was used for each sample. After the corrosion tests, the samples were washed and dried, and the morphology of the surface was observed by scanning electron microscopy (SEM, X-650 ).

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BMGs"~],T,, T , and the ratio of T , and T , ( a = T,/ T , ) of the two alloys can be obtained from their DSC traces. These values are summarized in Table 1. It can be seen that, in comparison to Al,,Ni,La,, T , of Al,,Ni,La,Cu, increases by 3 K, but T I decreases by 6 K, resulting in a slight increase of (Y =T,IT,, from 0.55 to 0.56. According to a new GFA parameter proposed by K. Mondal, etc.[I5],a =T,IT, can be used to access the GFA for those glass forming alloys that do not show a clear glass transition temperature, and a higher value of (Y leads to higher GFA. The result in the present study reveals that the addition of 2% Cu to A18,Ni,La6 alloy can enhance T , and lower T,, leading to a slightly higher value of (Y or higher GFA of the alloy. Fig. 3 shows the potentiodynamic polarization curves of the as-spun samples of A1,,Ni,La6 and Al,, Ni, La, Cu, in 0.01 mol * L - NaCl alkaline solution ,

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XRD patterns of as-spun AlggNi,L% and AI,,Ni,La,Cu, alloys I

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2 Results and Discussion Fig1 shows XRD patterns of the as-spun samp les of Al,, Ni, La, and Al,, Ni, La, Cu, . The amorphous structures were obtained in the two ribbons, as evidenced by a broad diffraction hump in their XRD patterns. Fig2 shows DSC traces of the two amorphous ribbons obtained at a heating rate of 20 K min- I . Both of them exhibit no distinct glass transition events, but show two characteristic exothermal heat release events, indicating successive stepwise transformations from the metastable undercooled liquid state to the equilibrium crystalline phases, at different temperatures. At about 900 K, an endothermic peak appears in both DSC curves, corresponding to the melting points of the two samples respectively. According to the common definitions of characteristic temperatures, that is, the onset temperature of crystallization ( T , ) and the offset melting point ( T , ) , in the field of

450 500 550 600 650 700 750 800 850 900 950 1000 TcmpcraturdK

Fig2 DSC curves of as-spun amorphous Alg6Ni,L%Cu2and Ab,NaLa, alloys at a heating rate of 20 K min- '

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T h e r m o d m a l and E€ectmchemicd Behavior of Al&i&a, and Al8Pi&a&?u2

respectively, at 298 K, in air. For comparison, the corrosion behavior of pure crystalline A1 was also tested under the same conditions. The corrosion data, including corrosion potential (E,,,,), corrosion current density (I,,,,), critical passive potential ( E p ) ,and critical passive current density (Ip)are congregated in Table 2. It can be seen that the two amorphous alloys exhibit high corrosion potential and low corrosion current density with respect to pure Al, indicating excellent corrosion resistance of the glassy materials. It can also be seen that, in 0.01 mol L - ' NaCl alkaline solution, the two amorphous alloys, Al,, Ni,La, and Al,, Ni,La,Cu,, exhibit distinct self-passive transition, with extremely low passive cment density of the order of A c m - 2 and a wide passive region between pitting potential and corrosion potential (E,,,), indicating that the alloys have high pitting resistance"61. With increasing anodic voltage, the passive film on the surface of the alloys is dissolved into a porous film, that is, transpassive dissolution occurs. As compared with Al,,Ni,La,, although the open potential of Al,,Ni,La,Cu, is identical to that of Al,,Ni,La, ,the critical passive current density (Ip)of Al,,Ni,La,Cu, is much lower, which indicates that 2% Cu addition can promote the formation of a more stable passive film,with respect to Al,,Ni,La,. It can also be seen that the passive region of A&, Ni,La,Cu, is narrower, but the passive dissolution current is lower than that of Al,, Ni,La,, indicating that the passive film on the surface of Al,,Ni,La,Cu,

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Fig3 Potentiodynamic polarization curves of as-spun samples of AIRHN&&and AI,Ni,La&u, in 0.01 mol * L - ' NaCl akaline solution Table 2 Results of potentiodvndc polarization masmmnts

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is more stable than that of Al,,Ni,La,. The SEM observations were performed after potentiodynamic polarization, to see where the pitting attack occurred, and the SEM images of the corroded surfaces of these two alloys are shown in Fig.4. The surface morphology of A&,NibLa&CuZalloy is very different from that of Al,, Ni6La6 alloy. The surface morphology is not even for the Al,,Ni,La, alloy after polarization in NaCl solution, and some pits are dispersed at the surface because of the breakdown of passive film (Fig.4(a)). Compared to the Al,,Ni,La, alloy, the surface morphology of the Al,,Ni,La,Cu, alloy is much more homogeneous and smooth. No obvious pitting attacks occur after polarization in NaCl solution. The observation results are in good ageement with the potentiodynamic polarization results. From the above-mentioned results, it can bc concluded that the Al,,Ni,La,Cu, alloy has a higher corrosion resistance than the Al,, Ni,La, alloy because of the addition of Cu.

1.0

Potcntial, E N vs SCE

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383

Fig4

SEM micrographs of corroded surfaces of as-spun ribbons after potentiodynamic polarization test in 0.01 mol * L ' ' NaCl alkaline solution (a) Al,,N&La,; (b) Al,,Ni,La,Cu,

3 Conclusions 1. Completely amorphous A188 Ni6La6 and AI,, Ni, La, Cu, ribbons were successfully prepared using the melt-spun method. Al,, Ni,La,Cu, exhibited a slightly higher crystallization temperature ( T , ) and a

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lower melting point ( T , ) , indicating that it GFA than A1,,Ni,La6. 2 . For the Al,,Ni,La,CU, day, the open potential was identical to that of A1,,Ni,La6, but the critical passive current density (Ip)was much lower than that of Al,, Ni6La6, indicating that Al,, Ni,La,Cu, had a better corrosion resistance in 0.01 mol * L - ' NaCl alkaline solution than Al,,Ni,La,Cu,. 3. Addition of 2% Cu could not only slightly promote GFA, but also could greatly improve the corrosion resistance of the Al,,Ni,La, alloy in 0.01 mol * L - ' NaCl alkaline solution. Acknowledgments: The authors would like to thank the State Key Lab for Powder Metallurgy of Central South University for the preparation of samples.

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