Effect of Li Addition on Synthesis of Mg-Ti BCC Alloys by means of Ball Milling

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1 Materials Transactions, Vol. 48, No. 2 (07) pp. 121 to 126 #07 The Japan Institute of Metals Effect of Li Addition on Synthesis of - BCC Alloys by means of Ball Milling Kohta Asano, Hirotoshi Enoki and Etsuo Akiba National Institute of Advanced Industrial Science and Technology (AIST), AIST Central-5, Tsukuba , Japan The effect of Li addition to on synthesis of - BCC alloys by means of ball milling was studied. x at%li (x ¼ 4; ; 46) alloys prepared by the induction melting method consisted of the HCP phase (-type), the HCP phase + the BCC phase (Li-type) and the BCC phase, respectively. Lattice parameters of the HCP phase decreased with the increase in x, whereas those of the BCC phase at x ¼ and 46 were smaller than that of Li. x at%li (x ¼ 4; ; 46) alloys and were milled for 0 h in an argon atmosphere. Formation of 96 Li BCC alloy was confirmed by X-ray diffraction (XRD) measurements after h milling. By Li addition to, the synthesis time of the BCC phase was shortened. Li made deformation facile by the activation of the non-basal slips in, and raw materials were efficiently milled. The crystallite size of was readily reduced and formation of the BCC phase was accelerated. The crystallite size and lattice parameter of 96 Li BCC alloy were similar to those of BCC alloy. [doi:10.23/matertrans ] (Received October 17, 06; Accepted December 4, 06; Published January 25, 07) Keywords: magnesium-based alloys, BCC phase, mechanical alloying, ball milling, deformation 1. Introduction is light weighted (1.74 g cc 1 ) and H 2 hydride contains 7.6 mass% hydrogen. Therefore, and -based alloys have been accepted as one of the most suitable materials for on board hydrogen storage and transportation. In particular, the intermetallic compound 2 Ni is known as a typical hydrogen storage material and the hydrogen storage capacity of 2 NiH 4 is 3.6 mass%. 1) However, to introduce -based alloys into practical applications as hydrogen storage materials, these alloys are required to absorb and desorb hydrogen rapidly at moderate temperatures. The thermal stability of hydrides and the kinetics of hydrogen are closely related to the crystal structure of the alloys because hydrogen atoms occupy the interstitial sites of crystal lattice. -based alloys with a body centered cubic (BCC) structure, as typified by -V-Mn and -V-Cr, 2 4) absorb and desorb hydrogen rapidly at moderate temperatures. A BCC structure is a coarse packing structure in comparison with the closest packing structures as a face centered cubic (FCC) and a hexagonal close packed (HCP). The thermal stability of hydrides can be controlled by size of lattice. 4) Furthermore, the diffusion of hydrogen in BCC metals is significantly faster than that in FCC and HCP metals. 5) In recent years, many attempts to synthesize -based alloys with a BCC structure have been made and novel - Co 6,7) and -Tm-V (Tm: transition metals) 8) alloys have been successfully synthesized by means of ball milling. In our previous works, - alloys with a BCC structure have been successfully synthesized by means of ball milling. 9,10) The lattice parameter of - BCC alloys was nm and it was suggested that this lattice is too capacious for absorption and desorption of hydrogen at moderate temperatures. In the ball milling process, the repetition of deformation and mix of raw materials yields the alloys. During ball milling of and, and were deformed mainly by the basal plane slip and the twinning deformation, respectively. The morphology and crystal structure of alloy were changed with the increase in milling time. By the investigation of the synthesis process of BCC alloy, it has been concluded that the difference in the mechanical properties among raw materials is an essential factor for synthesis of -based alloys by means of ball milling ) The structure of and is a HCP structure. The main slip systems in HCP metals are a f0001gh1210i basal plane slip system with a-axis slip direction and a f1010gh1210i prismatic plane slip system with a-axis slip direction ) is deformed mainly by the basal plane slip at moderate temperatures because the value of the critical resolved shear stress (CRSS) of the basal plane slip system is about two orders magnitude smaller than that of the prismatic plane slip system ) On the other hand, in the case of the deformation of, the value of the CRSS of the prismatic plane slip system is slightly smaller than that of the basal plane slip system 14 16) and is known to deform by the twinning deformation much more readily than. 17) By Li addition to, is readily deformed because the non-basal slips as a prismatic plane slip and a f1122gh1123i second order pyramidal plane slip are activated. 15) According to measurements of the tensile stress elongation curves of -based alloys, 18) by 4 at% Li addition to, the yield stress becomes lower and the value of the break elongation increases about four times. Furthermore, it is expected that lattice parameter of decreases by Li addition because an atomic radius of Li is smaller than that of. In the present work, to control the lattice parameter and to shorten the synthesis time of - BCC alloys, 100 x Li x 100 (x ¼ 4; ; 46; 100) alloys were synthesized by ball milling of -Li alloys and. The effect of Li addition on synthesis of - BCC alloys by means of ball milling has been discussed. 9 13) 2. Experimental Ingots of x at%li (x ¼ 4; ; 46) alloys were prepared by induction melting pellets of (99.9 mass%, Japan Pure Chemical) and Li (99.9 mass%, Furuuchi Chemical) in a

2 122 K. Asano, H. Enoki and E. Akiba Table 1 Chemical composition of x at%li (x ¼ 4; ; 46) alloys. Alloy Li O N C Ca Pb Fe Si Cu Mo Cr Zn /mass% (/at%) /mass ppm 4 at%li 98.9 (96.3) 1.1 (3.7) < <10 <100 <10 <10 at%li 93.5 (.4) 6.5 (19.6) < <10 <100 <10 <10 <10 <10 46 at%li.4 (53.9) 19.6 (46.1) <10 <100 <10 <10 <10 < : -type HCP : Li-type BCC x = 4 a-axis / nm c-axis a-axis c-axis / nm Intensity x = x = 46 a-axis / nm Atomic percent of Li Fig. 2 Lattice parameters of HCP and BCC phases of x at%li alloys. Fig. 1 XRD patterns of x at%li (x ¼ 4; ; 46) alloys. helium atmosphere. The chemical composition of the alloys is shown in Table 1. The ingots of the alloys were shaved into small chips with the size of 1 mm or less. The chips of x at%li (x ¼ 4; ; 46) alloys, powder (<45 mm, 99.9 mass%, Wako Pure Chemical) and stainless balls with a diameter of 10 mm were set in the stainless pot in a globe box filled with purified argon. The amount of the chips of x at%li alloys and powder was 2 g in total. The initial ball to raw materials weight ratio was : 1. The ball milling of 100 x Li x 100 (x ¼ 4; ; 46; 100) alloys was performed in a Fritsch P5 planetary ball-mill for 0 h. The rotation speed was fixed as 0 rpm. The X-ray diffraction (XRD) of the prepared alloys was measured using a diffractometer (Rigaku 20V) with Cu K radiation. The morphology of the alloys was observed by a scanning electron microscope (SEM, Hitachi S-N) and a transmission electron microscope (TEM, JEOL JEM-00FX II). Chemical analysis was performed by an energy dispersive X-ray spectrometer (EDX, EDAX Genesis00). 3. Results and Discussion Figure 1 shows the XRD patterns of x at%li (x ¼ 4; ; 46) alloys prepared by the induction melting method. At x ¼ 4, the HCP phase (-type) was only observed. With the increase in Li content x, the BCC phase (Li-type) was formed. The HCP and the BCC phases were found at x ¼. Finally, at x ¼ 46, the BCC phase was only observed. Figures 2 and show lattice parameters of the HCP and the BCC phases of x at%li alloys synthesized, 19) respectively. With the increase in x, lattice parameters of the HCP phase decreased, whereas that of the BCC phase had a minimum around x ¼. ) The chips of x at%li (x ¼ 4; ; 46) alloys and powder were milled for 0 h in an argon atmosphere. The XRD patterns of 100 x Li x 100 (x ¼ 0; 4; ; 46; 100) alloys milled for 0 h are shown in Fig. 3. At x ¼ 4, the BCC phase was synthesized as at x ¼ 0. However, at x ¼, 46 and 100, raw materials were observed and the BCC phase could not be identified. Figure 4 shows the XRD patterns of 96 Li alloy milled for 1 h in comparison with those of BCC alloy, 11 13) as shown in Fig. 4. With the increase in milling time, and 4 at%li alloy disappeared in the XRD patterns and peaks were significantly broadened. This indicates that and 4 at%li alloy dissolved into. After h milling at x ¼ 4, as shown in Fig. 4, the BCC phase was observed, whereas the BCC phase did not appear after h milling at x ¼ 0, as shown in Fig. 4. The synthesis time of the BCC phase was shortened by Li addition to. The crystallite

3 Effect of Li Addition on Synthesis of - BCC Alloys by means of Ball Milling 123 x = 0 : : : Li : BCC h : : : BCC x = 4 x = 75 h 100 h x = 46 x = h Fig. 3 XRD patterns of 100 x Li x 100 (x ¼ 0; 4; ; 46; 100) alloys milled for 0 h. size of in 96 Li and alloys was estimated by using of Scherrer s equation, 21) as shown in Fig. 5. With the increase in milling time, the crystallite size of was reduced. The reduction in crystallite was accelerated by Li addition to. After 1 h milling, the crystallite size and lattice parameter of 96 Li BCC alloy were estimated to be 3 nm and nm, respectively. These values were similar to those of BCC alloy. Figures 6 and show the SEM images of 96 Li and BCC alloys milled for 1 h, respectively. The particle size of these alloys was about 10 mm. Figure 7 shows the TEM image (the dark-field and the selected electron diffraction (SAD) images) of 96 Li BCC alloy milled for 1 h in comparison with that of BCC alloy milled for 1 h, 11 13) as shown in Fig. 7. The grain size of 96 Li BCC alloy was under 10 nm. The SAD image of the alloy exhibited a series of co-axial rings, which correspond to a BCC structure as that of BCC alloy. To elucidate the effect of Li addition to on synthesis of BCC alloy, 96 Li and alloys milled for h were observed by SEM. In the previous works, alloy was synthesized by ball milling of and powder with the size of under 1 mm ) To investigate the effect of Li addition on mix of and, ingot of was shaved into small chips with the size of 1 mm or less, as x at%li alloys. After h milling of chips and powder, the disc-like particles with the size of under 1 mm were obtained. Only and were found in the XRD patterns, as alloy synthesized by h h 75 h 100 h 1 h milling of and powder in Fig. 4. Figures 8 and show the SEM (back scattered electron (BSE)) images of cross section of 96 Li and alloys milled for h, respectively. As shown in Fig. 8 and, stuck : : : BCC Fig. 4 XRD patterns of 96 Li and alloys milled for 1 h.

4 124 K. Asano, H. Enoki and E. Akiba Crystallite size of / nm 100 Fig Li Milling time / h 100 on the 4 at%li and particles. Figures 9 and show the SEM (BSE) images of cross section of the surface of 96 Li and alloys milled for h, respectively. It was found that particles penetrated into the 4 at%li and particles. The penetration depths of into 4 at%li and particles were about 0 mm and mm, respectively. In an extended scale of the SEM (BSE) images and the EDX mapping results as shown in Fig. 9(d), which stuck on the surface of the particles was expanded along the surface. On the other hand, as shown 75 Crystallite size of in 96 Li and alloys. in Fig. 9(c), particles which penetrated into 4 at%li particles were hardly expanded. Except for the bright area identified to be and the dark area to be 4 at%li alloy, the intermediate brightness area was identified to be the composite of 4 at%li alloy and. In the XRD pattern of 96 Li milled for h, as shown in Fig. 4, the BCC phase was found. Therefore, the composite of 4 at%li alloy and in Fig. 9(c) corresponds to the BCC phase formed by dissolution of 4 at%li alloy into, as shown in Fig. 4. On the other hand, by milling of and for h, was hardly mixed with as shown in Fig. 9(d) and formation of the BCC phase was not confirmed as shown in Fig. 4. By Li addition to, Li made deformation facile and raw materials were efficiently milled. The crystallite size of was readily reduced and dissolved into. In consequence, the synthesis of - BCC alloys by means of ball milling was accelerated. The XRD patterns of 100 x Li x 100 (x ¼ ; 46) alloys milled for 0 h are shown in Fig. 10. The broadened peaks of were observed and the patterns of and Li were not identified. By ball milling for 0 h, x at%li (x ¼ ; 46) alloys dissolved into in comparison with that milled for 0 h, as shown in Fig. 3. However, the BCC phase was not found in Fig. 10. Because x at%li (x ¼ ; 46) alloys are significantly softer than and 4 at%li alloy, formation of the BCC phase was delayed by alleviation of collisions between balls and milling pot during ball milling. Fig. 6 SEM images of 96 Li and alloys milled for 1 h. Fig. 7 TEM image (dark-field and selected electron diffraction (SAD) images) of 96 Li and alloys milled for 1 h.

5 Effect of Li Addition on Synthesis of - BCC Alloys by means of Ball Milling at.%li 0 µm 0 µm Fig. 8 SEM (BSE) images of cross section of 96 Li and alloys milled for h. 100 µm 4 at.%li surface 100 µm surface (c) µm (d) µm Fig. 9 SEM (BSE) images of cross section of surface of 96 Li and alloys milled for h; in extended scale of SEM and EDX mapping results of (c) 96 Li and (d) alloys.

6 126 K. Asano, H. Enoki and E. Akiba x = x = Conclusions The synthesis time of - BCC alloys were shortened by ball milling of 4 at%li alloy and. During ball milling, the crystallite size of was reduced. After dissolved into, the BCC phase was formed. By Li addition to, was readily deformed and the reduction in crystallite was accelerated. On the other hand, the BCC phase was not found by milling of x at%li (x ¼ ; 46) alloys and. Because x at%li (x ¼ ; 46) alloys are significantly softer than and 4 at%li alloy, the collisions between balls and milling pot during ball milling was alleviated. The synthesis of -based alloys by means of ball milling is significantly affected by the mechanical properties of raw materials. Acknowledgement : Fig. 10 XRD patterns of 100 x Li x 100 (x ¼ ; 46) alloys milled for 0 h. Technology Development Organization (NEDO) under Basic Technology Development Project for Hydrogen Safety and Utilization. REFERENCES 1) J. J. Reilly and R. H. Wiswall, Jr.: Inorg. Chem. 7 (1968) ) E. Akiba and H. Iba: Intermetallics 6 (1998) ) E. Akiba and M. Okada: MRS Bull. 27 (02) ) H. Iba: Doctor Thesis, Tohoku University, 1993, pp ) J. Völkl and G. Alefeld: Hydrogen in Metals I, Basic Properties, (Springer-Verlag, Berlin, 1978) pp ) Y. Zhang, Y. Tsushio, H. Enoki and E. Akiba: J. Alloys Compd. 393 (05) ) Y. Zhang, Y. Tsushio, H. Enoki and E. Akiba: J. Alloys Compd. 393 (05) ) T. Kuji, S. Nakayama, N. Hanzawa and Y. Tabira: J. Alloys Compd (03) ) Y. Tsushio, S. J. Choi, H. Enoki and E. Akiba: Collected Abstracts of the 02 Spring Meeting of the Japan Inst. Metals (02) pp ) S. J. Choi, Y. Tsushio, K. Asano, H. Enoki and E. Akiba: J. Alloys Compd., submitted for publication. 11) K. Asano, H. Enoki and E. Akiba: Acta mater., submitted for publication. 12) K. Asano, H. Enoki and E. Akiba: Collected Abstracts of the 06 Spring Meeting of the Japan Inst. Metals (06) pp ) K. Asano, H. Enoki and E. Akiba: Collected Abstracts of the 06 Spring Meeting of MRS, (06) pp ) H. Tonda and S. Ando: Metall. Mater. Trans. A 33 (02) ) S. Ando and H. Tonda: Mater. Trans., JIM 41 (00) ) S. R. Agnew, M. H. Yoo and C. N. Tomé: Acta mater. 49 (01) ) M. H. Yoo: Metall. Trans. A 12 (1981) 9. 18) W. Fujitani and Y. Umakoshi: J. Japan Inst. Light Metals 45 (1995) ) Metal Databook Ver. 3, (Japan Institute of Metals, 1993) pp. 32. ) D. W. Levinson: Acta Metall. 3 (1955) ) H. P. Klug and L. E. Alexander: X-ray Diffraction Procedures 2nd ed., (Mellon Institute of Science, Carnegie-Mellon University, 1974) pp This work is supported by The New Energy and Industrial