Metastable b.c.c. and amorphous Ni produced by mechanical alloying and chemical leaching

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Materials Science and Engineering, A I 81/A 182 (1994) 1184-1189

Metastable b.c.c, and amorphous Ni produced by mechanical alloying and chemical leaching Salah A. Makhlouf*, K. Sumiyama, E. Ivanov**, H. Yamauchi, T. Hihara and K. Suzuki Institute for Materials Research, Tohoku University, Sendai 980 (Japan)

Abstract

B.c.c. and amorphous Ni are obtained by a technique combiningmechanicalalloyingof Ni and A1elementalpowders, and chemical leaching of AI in basic solution. The as-milled and milled-and-leachedspecimens are characterized by X-ray diffraction, transmission electron microscopy, high pressure differential thermal analysis, differential scanning calorimetry and magnetization measurements. B.c.c. and amorphous Ni phases exhibit paramagnetic characters at low temperatures and transform to the ferromagnetic f.c.c. Ni phase at high temperatures. Low temperature, specific heat measurements indicate the enhancement of its electronic coefficient in the paramagnetic b.c.c, and amorphous Ni in comparison with that of annealed f.c.c.Ni.

1. Introduction

Recently, there has been a rapidly increasing interest in the mechanical alloying (MA) process. This attention arises from the possibility of preparing a considerable number of metastable crystalline and amorphous alloy powders of potentially interesting systems [1]. In particular, new materials whose structures and magnetic and catalytic properties are quite different from the original materials have emerged upon combining MA with topotactic chemistry techniques [2]. The formation of new Ni phases by combining MA and chemical leaching processes arises from the following facts: (i) metastable b.c.c, and amorphous Ni-AI alloys can be obtained by MA, in the form of very fine powders [3]; (ii) the chemical leaching process of A1 from its 3d metal aluminides, using a basic solution, is facilitated by the relatively small grain size of the deformed structures of MA specimens [4, 5]; (iii) the lattice constant of B2-type ordered Ni35A165 alloy obtained by M A is about 2.86 A, which is very close to that of the b.c.c. Ni phase epitaxially grown on an Fe surface. Since the topotactic leaching process occurs very slowly, we cannot anticipate considerable changes in the structure as a consequence of this process. Therefore, the outlook for obtaining metastable b.c.c. *Permanent address: Department of Physics, Faculty of Science,Assiut University,Assiut, Egypt. **Present address: Tosoh SMD, 3515 Grove City Road, Grove City, OH 43123, USA. 0921-5093/94/$7.00 SSD1 0921-5093(93)05678-1

and amorphous Ni phases by this combined technique is very promising. Motivated by the above-mentioned aspects, we have carried out MA of Ni-AI powders and chemical leaching of A1 from mechanically alloyed specimens, in order to obtain new Ni phases. Mechanically alloyed Ni35AI65 with the metastable b.c.c, structure can be topotactically transformed into a nanocrystalline b.c.c. Ni phase after leaching A1 in basic solutions [4, 5]. Similarly, leaching of mechanically alloyed Co40A160 with the b.c.c, structure has yielded a nanocrystalline b.c.c. Co phase [6]. B.c.c. Ni exhibits a paramagnetic character, whereas b.c.c. Co exhibits a ferromagnetic character. Our experimental results have been found to be in agreement with the results of sophisticated experiments and with theoretical predictions [7-9]. More recently, the present authors have attempted to obtain amorphous metals by leaching the major portion of A1 in basic solutions from sputter-deposited amorphous NiA1 and mechanically alloyed Ni40Al60 amorphous alloys [10, 11]. In this paper, we present the structural and magnetic properties in connection with the measurements of the low temperature, specific heat for the paramagnetic b.c.c., amorphous and annealed f.c.c. Ni.

2. Experimental details

Using MA, we synthesized a partially ordered B2type NiAI alloy from pure elemental powders of Ni and © 1994 - ElsevierSequoia.All rights reserved

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Ni phases by MA and chemical leaching

A1, with the composition Ni3sA165, using a planetary high energy ball mill with a rotating speed of 700 rev min- i. In contrast, a conventional low energy vibrating ball mill was utilized to obtain amorphous Ni40A160 alloy from its elemental Ni and AI powders. The conventional process for obtaining Raney Ni catalysts [12] was utilized to leach away A1 atoms. In this process, the Ni-A1 powders were slowly placed in an excess of 20-25 wt.% KOH solution at about 370 K, in order to give a gentle reaction and to keep the aluminate in solution. The chemical compositions of both the as-milled and milled-and-leached samples were determined by conventional chemical analysis and electron probe microanalysis (EPMA). These specimens were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential thermal analysis (DTA) under 2.0 MPa of hydrogen gas, and differential scanning calorimetry (DSC) measurements. A conventional vibrating sample magnetometer was employed to measure the magnetization between 290 and 770 K, and a torsion balance magnetometer was used to measure the magnetization between 4.2 and 290 K in magnetic fields up to 17 kOe. An adiabatic heat pulse method was utilized to measure the low temperature, specific heat between 2 and 20 K.

3. Resultsand discussion In Fig. 1, curves a-c show the XRD patterns of

Ni35A165powder produced by high energy ball milling, while curves d-f show those of Ni40A160produced by low energy milling respectively. A partially ordered B2-type (almost b.c.c.) Ni35A165 alloy is obtained using the high energy ball mill, whereas an amorphous Ni40Al60alloy is obtained with the low energy vibrating ball mill. Leaching of A1 atoms from b.c.c. Ni35A165 (curves a-c) and amorphous Ni40A160 (curves d-f) does not induce any significant change in the diffraction patterns, even though most of the AI atoms have been removed. After being annealed at 770 K for 1 h, both the b.c.c, and amorphous phases of Ni transform to the f.c.c, phase. The estimated lattice constant of the f.c.c. Ni phase is slightly larger than that of elemental Ni, suggesting that less than 5 at.% A1 still remains in the Ni skeleton. TEM apparatus equipped with energy-dispersive X-ray (EDX) analysis facilities was employed to confirm the structure and check the chemical composition of the specimens. The chemical compositions of the asmilled samples are close to the nominal compositions. The AI contents of the leached samples were much less than those of the milled samples, suggesting that most of the AI is removed during the leaching process.

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Figures 2(a), 2(c) and 2(e) show electron micrographs of as-milled Ni35AI65 and leached b.c.c. Ni particles. As shown in Fig. 2(e), heating the b.c.c. Ni particle in the TEM apparatus, by increasing the electron beam current, forms f.c.c. Ni with relatively large crystallites. Figure 2(b) shows a TEM micrograph of the asmilled Ni40Al60 alloy. As shown in this figure, a featureless uniform image is observed, indicating the amorphous structure of this alloy. As shown in Fig. 2(d) the leached specimen is aiso characterized by an almost uniform image with slight contrast, which is attributed to the amorphous phase. Figure 2(f) shows the micrograph of the same particle as in Fig. 2(d) but after strong in situ annealing in the electron beam column. The images show the course of the crystallization of the amorphous particle, exhibiting the formation of crystals of Ni 10-30 nm in size. Thus, leaching the A1 atoms introduces almost no change to the structure of the as-milled samples. The electron diffraction patterns taken for these specimens are consistent with the structural information obtained by XRD. As shown in Fig. 3(a), the DTA trace of the leached b.c.c. Ni powder reveals two exothermic effects. Since X-ray and magnetization measurements do not show any considerable change up to the temperature corresponding to the first peak, the low temperature peak is attributed to structural relaxation. The second peak at around 450 K corresponds to the structural transformation from the b.c.c, to f.c.c, structure accompanied

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by the change in the magnetic properties. The XRD pattern shows the f.c.c. Ni fines just after heating the specimen up to the temperature of the second DTA peak. Figure 3(b) illustrates the DSC curve of the leached amorphous Ni powder. Two broad but distinct exothermic peaks are observed. The first peak at about 500 K can be attributed to the crytallization of the amorphous Ni phase. Figures 4(a) and 4(b) show the temperature dependence of the magnetization o of the leached b.c.c, and amorphous Ni specimens in an applied magnetic field H of about 10 kOe. The o value for amorphous Ni is very small (0.8 emu g- 1), whereas it is about 6 emu g- 1 for the leached b.c.c. Ni. This suggests that the unprocessed f.c.c. Ni impurity content is lower in the original amorphous NiA1 alloy prepared by the low energy ball mill than the content in the b.c.c. NiA1 alloy prepared by the high energy mill. As illustrated in both figures, o decreases with increasing temperature and disappears at about 630 K. On cooling the specimens from high temperatures, o increases sharply as a result of the transformation of the paramagnetic b.c.c, and amorphous Ni phases to the ferromagnetic f.c.c, phase. Here, the room temperature saturation magnetization of annealed f.c.c. Ni is still lower than the value 54 emu g-I of pure Ni, suggesting the presence of nonmagnetic impurities of either AI or A1 hydroxide, as detected by EPMA. The Curie temperature Tc of the f.c.c, phase obtained by annealing amorphous Ni matches that of pure Ni, indicating that A1 is physically

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included in Ni rather than forming a dilute solid solution. However, Tc of the f.c.c, phase obtained by annealing b.c.c. Ni is lower, suggesting that about 5 at.% is A1 retained in the f.c.c. Ni matrix. Figures 5(a) and 5(b) show the magnetization curves at room temperature, 77 K and 4.2 K for leached b.c.c. and amorphous Ni specimens respectively. The magnetization does not saturate up to H = 16 kOe at room temperature and 77 K, and the high field susceptibility becomes larger at 4.2 K: o increases almost linearly with increasing magnetic field between 5 and 17 kOe. The increase in o(H, T ) with decreasing temperature and the pronounced high field susceptibility at 4.2 K are attributable to the paramagnetic character of these Ni phases. According to recent band theoretical calculation [8], the b.c.c. Ni should be paramagnetic and should become ferromagnetic as its atomic radius expands above a critical value (rc). Since the atomic radius of the present b.c.c. Ni is smaller than rc, it is strongly paramagnetic. Theory has also shown that paramagnetism would also be achieved in amorphous Ni under an appropriate degree of structure disordering [13]. Figure 6 shows the low temperature, specific heat CF for the mechanically milled and chemically leached b.c.c, and amorphous Ni powders, and annealed f.c.c.

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Ni powders in the form of C p / T vs. T 2. The electronic specific heat coefficient 7, is larger in the b.c.c, and amorphous Ni powders than that in the annealed f.c.c. Ni powder, where the 7 value for the f.c.c. Ni powder is comparable with that for the bulk f.c.c. Ni metal. At the low temperature ferromagnetic state, the majority spin d band is fully occupied and only the minority spin d band has holes, giving rise to the small magnetic moment of about 0.6pB. However, since the present b.c.c, and amorphous Ni powders are paramagnetic down to 4.2 K, both the majority and minority spin d bands have holes, leading to a large density of states at the Fermi level. We also anticipate b.c.c, and amorphous Ni to be exchange-enhanced paramagnets, similar to Pd metal and dilute Pd alloys. In this context, a paramagnon effect becomes significant and the effective electron mass is strongly enhanced at low temperatures. Therefore, both the large density of states at the Fermi level and the paramagnon effect give rise to the enhanced 7 value in the b.c.c, and amorphous Ni powders. Moreover, the gradient of the C p / T - T 2 curves should be noted in the high temperature range, because it is inversely proportional to the cube of the Debye temperature O D. The lower O D value for amorphous Ni indicates that the amorphous Ni lattice is very soft in comparison with crystalline b.c.c, and f.c.c. Ni.

4. Conclusions

We have succeeded to produce metastable b.c.c, and amorphous Ni containing at most 5 at.% AI by leaching the major portion of AI atoms from the b.c.c. Ni35AI65 and amorphous Ni40A160 alloys obtained by high or low energy mechanical milling. Both the b.c.c, and amorphous Ni phases are paramagnetic and transform to the ferromagnetic f.c.c. Ni phase at high temperatures. The results are in good agreement with theoretical predictions based on band structure calculations of 3d metals. The b.c.c, and amorphous Ni phases display enhancement of the electronic specific heat coefficient as a result of the higher density of states at the Fermi level compared with that of f.c.c. Ni.

Acknowledgment

S.A.M. appreciates financial support from the Egyptian Ministry of Higher Education.

References

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Ni phases by MA and chemical leaching

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9 V.L. Moruzzi, E M. Marcus, K. Schwarz and E Mob_n, Phys. Rev. B, 34(1986) 1784. 10 S. A. Makhlouf, E. Ivanov, K. Sumiyama and K. Suzuki, J. Alloys Compounds, 187 (1992) L1. 11 S. A. Makhlouf, K. Sumiyama and K. Suzuki, J. Alloys Compounds, 199 (1993) 119. 12 P. Fouilloux, Appl. Catal., 8 (1983) 1. 13 Y. Kakehashi, Phys. Rev. B, 43(1991) 10820.