
1430—VOLUME 28A, JULY 1997 METALLURGICAL AND MATERIALS TRANSACTIONS A
ders was examined using a DU PONT* 2000 differential
*DU PONT is a trademark of Du Pont de Nemours & Co., Inc.,
Wilmington, DE.
thermal analyzer, where the sample was heated from room
temperature to 1300 7C in a purified argon atmosphere at
a rate of 40 K/min.
III. RESULTS AND DISCUSSION
A. Crystalline-to-Amorphous Transformation
Ni
50
Ta
50
was chosen as a prototype to investigate the
amorphization behavior of Ni-Ta system during MA. The
powder morphology (Figures 1(a) through (d)) and particle
cross sections (Figures 1(e) through (h)) of mechanically
alloyed powders vs milling time were examined using
SEM. More specifically, the MA process for preparing
Ni
50
Ta
50
involves three stages: cold welding (0 to 1 hours),
fracturing (1 to 5 hours), and amorphization (5 to 20 hours).
During the cold welding stage, the mean particle size
sharply increased (starting from ,20 to ;1000
m
m, Fig-
ures 1(a) and (b)) and a typical lamellar structure formed
(Figures 1(e) and (f)). This phenomenon may be attributed
to the predominance of repetitive cold welding of Ni and
Ta elemental powders at the early stage of milling. The
individual layer thickness measured from Figure 1(f) (i.e.,
1 hour of milling) ranged from 0.4 to 4
m
m. As the milling
time increases, the particles are stressed continuously.
Eventually, fracturing dominates, thereby causing a decrease
in particle size and lamellar thickness; the fracturing stage
lasted 1 to 5 hours. Interestingly, during the fracturing stage,
the morphology of the milled powders becomes more spher-
ical (Figure 1(c), 5 hours of milling) than the original plate-
like powders (Figures 1(a) and 1(b)). A balance between cold
welding and fracturing of powders is gradually achieved,
leading to a relatively constant particle size (17 5 5
m
m,
measured from Figure 1(d)) at the end of the amorphization
stage. In addition, the refined lamellar structure obtained af-
ter 5 hours of milling (Figure 1(g)) became indistinguishable
at the end of processing (20 hours, Figure 1(h)).
Figure 2(a) depicts the variation in particle sizes (mean
5 one standard deviation) for mechanically alloyed
Ni
50
Ta
50
powders as a function of milling time. In addition
to the change in particle size and the refinement of layer
thickness, microhardness measurements can be used to
monitor the progress of MA.
[12]
Figure 2(b) shows that mi-
crohardness increases throughout the cold welding and frac-
turing stages, and no significant change occurs during the
amorphization stage. This shows the same trend as reported
in the literature.
[12,13]
The refinement of layer thickness and
the increase in hardness have been used to model the MA
processing.
[13]
X-ray diffraction analysis is another conventional tech-
nique for monitoring the progress of amorphization. Figure
3 displays the X-ray diffraction patterns of as-milled
Ni
50
Ta
50
powders as a function of milling time. As this fig-
ure reveals, most Bragg peaks from pure Ni already dis-
appeared and the intensities of the Bragg peak from Ta
decreased after 5 hours of milling. The peaks of bcc Ta
also broadened asymmetrically toward the high-angle side
because of the dissolution of the smaller Ni atoms in the
bcc lattice. A decreasing intensity and broadening diffrac-
tion peaks are common during the early stage of the MA
process.
[10]
After 20 hours of ball milling, only a broad dif-
fraction peak appears around 2
u
5 18.7 deg, indicating that
fully amorphous powders have formed.
Furthermore, the crystalline-to-amorphous transforma-
tion behavior of Ni
50
Ta
50
was examined by differential ther-
mal analyses (DTA). Figure 4 presents the DTA traces of
the as-milled powders vs milling time, where two or three
exothermic peaks (indicating phase transformations) ap-
pear. Interestingly, as the milling time increases, the first
exothermic peak (850 to 1000 K) decreases and vanishes
after 5 hours; meanwhile, the second exothermic peak
(1000 to 1100 K) continuously increases. To further ex-
amine the implications of these peaks, two Ni
50
Ta
50
speci-
mens milled for 5 and 20 hours were annealed for 5 minutes
at 1000 and 1100 K, respectively, quenched to room tem-
perature, and then subjected to X-ray diffraction analysis.
Figure 5 shows the X-ray diffraction patterns of Ni
50
Ta
50
as milled for 5 hours and annealed powders. This figure
indicates that after annealing at 1000 K (marked as ‘‘I’’ in
Figure 4; i.e.,Ni
50
Ta
50
powders milled for 5 hours and an-
nealed at 1000 K for 2 minutes), the intensities for Ta (110)
and other high angle peaks decrease. This observation im-
plies that the percentage of amorphization increases after
annealing at 1000 K. Recall the refined lamellar structure
in Figure 1(g) after 5 hours of milling; the crystalline-to-
amorphous transformation has not yet been completed at
this stage. This layer structure, however, is still too ‘‘thick’’
for the interdiffusion of Ni or Ta to be completed. In ad-
dition, a previous study noted that the diffusivity of Ni
through amorphous Ni
50
Zr
50
film is at least 10,000 times
lower than that for Ni in Zr.
[14]
Therefore, the formation of
thin amorphous films at the layer boundaries may inhibit
any further diffusion of Ni or Ta. Annealing of the as-
milled (5 hours) powders at a temperature of 1000 K, which
is below the crystallization temperature of 1048 K of amor-
phous Ni
50
Ta
50
powder, may enhance the interdiffusion of
Ni-Ta layers and subsequently increase the proportion of
amorphous phase. Consequently, the phase transformation
of the first exothermic peak is a crystalline-to-amorphous
transition. This thermally activated solid-state amorphiza-
tion reaction exhibits the same trend as several rod-milled
Al
50
TM
50
(TM: transition metals) alloys.
[15,16]
On the other hand, crystalline phases form with powders
heat treated at 1100 K (marked as ‘‘II’’ in Figure 4), and
the second exothermic peak denotes an amorphous-to-crys-
talline transition.
Similar procedures were followed for mechanically al-
loyed Ni
50
Ta
50
powders after 20 hours of processing. As
Figure 6 indicates, the X-ray diffraction patterns of as-
milled (20 hours) and annealed (at 1000 K, marked as
‘‘III’’ in Figure 4) powders exhibit an amorphous phase;
however, crystallization occurs after annealing at 1100 K
(marked as ‘‘IV’’ in Figure 4). This finding reconfirms that
an amorphous-to-crystalline transition occurs during the
second exothermic peak.
B. Mechanical Alloying of Ni-Ta System
1. Amorphization reaction
Weeber and Bakker
[10]
reported three different types of
amorphization reactions by MA of binary elemental powder
mixtures (A
x
B
y
). Type I denotes that the ‘‘effective crystal-