Photon Factory Activity Report 2005 #23 Part B (2006)
Materials Science

12C/2004G271

EXAFS study on local structure changes of MgNi alloys during
mechanical alloying processToshio NASU1, Yuh NOMURA1, Takeshi USUKI2, Masaki SAKURAI3
1
Faculty of Education, Art and Science, Yamagata Univ., Yamagata 990-8560, Japan2
Department of Material and Biological Chemistry, Yamagata Univ., Yamagata 990-8560, Japan
3
Institute for Materials Research, Tohoku Univ., Sendai 980-8577, Japan
Introduction
Mechanical alloying (MA) of elemental powder
mixtures have been successfully employed and widely
studied as a method for synthesizing amorphous alloys[1]. Mg-Ni system is one of the most attractive materials
for hydrogen storage alloy. In this paper, we reportstructural change around Ni atom during solid state
amorphization in a Mg50Ni50 alloy by x-ray absorption
fine structure (XAFS) and x-ray diffraction measurements
and discuss the mechanism of amorphization.
Experimental procedure
Mg-Ni alloys were prepared by mechanical alloying
using a planetary ball-milling machine. Starting materials
were pure Mg powder (99.8%, <325 mesh) and pure Nipowder (99.8%, 0.3-0.7nm).The x-ray diffraction patterns
were measured using Cu K α radiation to observe phasechange of Mg50Ni50 alloy.
The local structure around Ni atom was observed by xray absorption fine structure (XAFS). Ni K-edge(8339eV) XAFS measurements were carried out at thebeam line BL12C in Photon Factory, KEK, Tsukuba, with
Si(111) double crystal monochromator.
Results and discussion
The radial structure function, F(r), around Ni atom
provided by the Fourier transform of Ni K-edge EXAFS
spectra are shown in Fig.1. Although peak heightdecrease with milling time, F(r) for MA alloys up to 10h
milling exhibit same pair correlation peaks as fcc-Ni up to0.8nm. F(r) for 50h MA alloy is different from fcc-Ni;
position of the first nearest peak at 2.2nm and long
distance correlation peaks at around 0.4 to 0.5 nm areshift to shorter distance. In Fig.1, F(r) for MgNi2 andMg2Ni intermetallic compounds obtained from EXAFS
simulation using FEFF6 program are drawn. It is obviousthat F(r) for MA50h alloy is similar to that of MgNi2
intermetallic compound; i.e. the local structure around Ni
atom change from fcc to MgNi2-like structure between
10h to 50h of milling. The height of the peaks
corresponding to the long range order of the crystalline
phase are reduced with increasing milling time andalmost vanish at 200h. Only the main peak near 0.23nm
which corresponding to the short range order of theamorphous alloy remained.

Fig. 1 Radial structure function (Fourier transformof EXAFS k3χ(k)), F(r), around Ni atom in themechanically alloyed Mg50Ni50 as a function ofmilling time.
SummaryAt early stage of milling (< 10h), a part of Ni atom
diffuse into crystalline Mg and form highly disordered
Mg-rich alloy. At the second stage of alloying (10h ~50h), Mg atoms diffuse into fcc-Ni, resulting in this Nirich Ni-Mg mixture forming MgNi2-like short-range order,
as seen in the EXAFS results. In the third stage (100h ~200h) amorphous phase appear, i.e. the long range orderof crystalline phase disappear in the radial structure
function, F(r) , obtained by XAFS measurements.
Coordination numbers of Ni and Mg around Ni atoms inamorphous state are 4.3 and 7.7, and Ni-Ni and Ni-Mg
atomic distances are 0.244 nm and 0.273 nm, respectively.
References[1] C.C. Koch, O.B. Cavin, C.G. McKamey and J.O.
Scarbrough: Appl. Phys. Lett. vol.43(1983), p. 1017.

*nasu@kekexafs.kj.yamagata-u.ac.jp

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