Crystallization behavior and microhardness evolution in
Al
92-x
Ni
8
La
x
amorphous alloys
K.L. Sahoo
a)
National Metallurgical Laboratory, Jamshedpur 831007, India
M. Wollgarten
Department of Materials, Hahn–Meitner-Institut Berlin, D-14109 Berlin, Germany
K.B. Kim
Division of Physical Metallurgy, Darmstadt Technical University, D-64287 Darmstadt, Germany
J. Banhart
Department of Materials, Hahn–Meitner-Institut Berlin, D-14109 Berlin, Germany
(Received 20 January 2005; accepted 12 May 2005)
The crystallization behavior of melt-spun amorphous Al
92-x
Ni
8
La
x
(x 4 to 6) alloys
was investigated by means of differential scanning calorimetry, x-ray diffractometry,
and transmission electron microscopy. Crystallization kinetics were analyzed by
Kissinger and Johnson–Mehl–Avrami approaches. Microhardness of all the ribbons
was examined at different temperatures and correlated with the corresponding
structural evolution. The results show that the variation of La content from Al
88
Ni
8
La
4
to Al
86
Ni
8
La
6
has significant influence on the crystallization pathways from
amorphous to stable crystalline phases and on the evolution of microhardness with
temperature. The two stages of crystallization in Al
88
Ni
8
La
4
and Al
87
Ni
8
La
5
alloys
correspond to formation of fcc-Al and Al
11
La
3
, Al
3
Ni, Al
3
La. In Al
86
Ni
8
La
6
, three
stages of crystallization are observed which correspond to formation of a metastable
phase, fcc-Al, Al
11
La
3
, Al
3
Ni, and Al
11
La
3
, Al
3
Ni, Al
3
La, and decomposition of a
metastable phases to stable crystalline phases.
I. INTRODUCTION
Al-based amorphous alloys, especially Al-Ni-RE
(RE Y, or rare-earth elements) with Al contents between
80 and 90 at.% have attracted considerable attention due
to their low-density, high tensile strength (>1000 MPa),
and acceptable ductility.
1,2
Crystallization changes the
properties of amorphous alloys. Partial crystallization in-
volving formation of nano-sized precipitates enhances
mechanical strength. The favorable mechanical proper-
ties of the alloys fall off again if the size of precipitates
exceeds the range of few nanometres and the volume
fraction of the crystallites exceeds the range of 35–40%.
3
Growth of crystallites can be retarded by choosing alloy
compositions which lead to higher activation energies of
crystallization (kinetic constraints) and/or introduce
athermal barriers for crystallization.
3
With respect to al-
loys that are similar to the compositions of the present
work, the formation of a face-centered-cubic (fcc)-Al
phase after the first crystallization stage was reported by
Ye and Lu
4
in Al
89
Ni
5
La
6
and by Sahoo et al.
5
in
Al
89
Ni
6
La
5
, whereas Zhuang et al.
6
reported eutectic
crystallization of fcc-Al and metastable bcc-(AlNi)
11
La
3
-like phase in Al
89
Ni
5
La
6
alloy. Gangopadhyay and
Kelton
7
investigated the crystallization process in Al-Ni-
RE amorphous alloys and found that crystallization prod-
ucts depend on the radius of the rare-earth atoms. They
showed that alloys containing RE elements of smaller
atomic radii form fcc-Al while RE elements with larger
atomic radii first transform into metastable intermetallic
phases. In contrast, Gogebakan et al.
8
reported that the
first crystallization products in Al
85
Y
11
Ni
4
are fcc-Al,
Al
3
Y, and some unidentified metastable intermetallic
phases. Al-based amorphous alloys with higher amounts
of alloying elements exhibit improved glass-forming
ability and thermal stability.
3
In the present paper, ther-
mal stability, crystallization pathways, and development
of microhardness upon annealing of Al-Ni-La amor-
phous alloys containing 8 at.% Ni and varying amounts
of La are investigated. Evolution of the microstructure
during annealing is correlated with microhardness of the
samples.
II. EXPERIMENTAL
Initial ingots with composition Al
88
Ni
8
La
4
(further de-
noted as A84), Al
87
Ni
8
La
5
(A85), and Al
86
Ni
8
La
6
(A86),
were prepared by alloying the pure elements (purity of
a)
Address all correspondence to this author.
e-mail: klsah@nmlindia.org
DOI: 10.1557/JMR.2005.0385
J. Mater. Res., Vol. 20, No. 11, Nov 2005 © 2005 Materials Research Society 2927