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Microestructural Characterization of equiatomic NiTi alloy prepared by high energy
milling
M.C.A. da Silva1, A.C.R. Veloso2, R.M.Gomes2, S.J.G. de Lima2, T.A. A de Melo2, F.Ambrozio
Filho3
1-
Departamento de Metalurgia e Materiais da Universidade de são paulo-USP
Laboratório de Solidificação rápida da universidade Federal da Paraíba-DEM-UFPB
3Instituto de pesquisa Energeticas e Nucleares-IPEN
2-
[email protected]
Keywords: NiTi, Mechanical alloying, Characterization
Abstract: NiTi alloys with equiatomic composition of NiTi have the highest technological
interest for its potencial application in differents areas such as biomedical, naval, aerospace,
nuclear, automobilist , robotic,etc. In this work , it was used a 50Ni50Ti at % powder mixture,
comercially pure, prepared by mechanical alloying in a Attritor with the following conditions: the
milling speed and the ball charge were 1500 rpm and 10:1 respectively. The milling time was
2,4,8 and 16h, under an argon atmosphere at room temperature. After milling it was determined
the particle size distribution, the phases by X-ray diffractions (XRD) and the powder morphology
by scanning electron microscopy (SEM). The milling promotes dissolution of Titanium in Nickel
and continuous amorphization by increasing the milling time. The powders after milling were
compacted and heat treated at high temperature and microstructural evolution was characterized.
In the heat treated samples were detected different phases showing heterogeneity in the alloy.
Contamination by milling was detected in the powder after milling and in the heat treated
samples.
Introduction
The NiTi alloy has attracted much interest fot it potencial application in diferents areas. It is most
common used as shape memory alloys (SMA). The poor workability and problems associated
with casting, such as segregation of alloying elements and excessive grain growth, enhance the
interest in powder metallurgical(PM) and Mechanical alloying(MA).as an alternative to prepare
the alloy.[1]
The challenges of PM production of NiTi SMA have been the production of alloys with
properties competitive with those of cast alloys[1]. The powder metallurgy mimimize problems
related to casting and moreover to obtain an accurate chemical composition control( essencial in
martensitic tranformation) and offers the ability to produce a variety of components shapes.
Howerver, the powder metallurgy route introduces impurities, such as, Carbon, Oxygen and iron.
There has been considable interest in recent years in the synthesis of amorphous structures by
high-energy milling of either elemental powder mixtures or powder of intermetallics. Yamada
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and Kock [2] describing of mill energy and temperature under which the amorphization of NiTi
can occur, have concluded that deformation of nanocrystalline structure drives the amorphization
process. Koike et al [3] observed partial amorphization on the TiNi intermetalic which they
attribute to the large dislocation density (1013_1014/cm2).
In the present work was characterized an equiatomic NiTi alloy prepared by mechanical alloying
and heat treated compacted samples.
Experimental procedures
Commercially elemental powders of Ni and Ti were used in this study. The chemical
composition and milling parameter are shown in table 1
Table 1: Milling parameters
Alloy NiTi at%
Jar volume (l)
Powder mass (g)
Ball mass (g)
Ball diameter (mm)
Shaft speed (rpm)
Flowing gas
Cooling media
Processing time (h)
Attitor with horizontal
shaft (ZOZ )
Coordenação
50-50
2
70
1500
7
1500
Argon
water
0,2,4,8,16 h
The milling was performed at room temperature and conducted in a attitor mill (Zoz model). No
process control agent was used in the processing. Milled powders were compacted and heat
treated at different temperatures in a tubular furnace under argon atmosphere.
The following techniques were used to characterize the powders after milling and heat treatment:
Granulometric analysis: to determine the effect of milling time on powder particle size. The
results were obtained as a cumulative mass versus particle size. The mean particle size was
obtained from the accumulated mass of 50%. The powder dispersed in water with a detergent
was analysed in a laser scattering Cilas 1064.
X-ray diffraction: To identify the phases in the powders after milling and after heat treatment. Xray diffraction measurements were carried out in a Siemens 5000 diffratometer with CuKα
radiation (wavelength of 15,42 ηm) and with scanning angles from 30 to 100˚.
The morphology of the powders at various milling durations was examinad using a Philips XL-30
scanning electron microscope(SEM) operated at 20Kv equipped with an energy dispersive
spectroscopy (EDS) unit. The powders were dispersed over a conducting tape placed on the
sample holder.
Results and discussion
The morphology of 50Ni-50Ti powders at some milling times, examined by SEM, are shown in
the fig.1
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(a) Ni powder
(b) Ti powder
(c) NiTi 2h
(d) 4h
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(e) 8h
(f) 16h
Fig 1 . SEM morphology of elemental powders and mechanically alloyed 50NI-50Ti powders.
The titanium powder has an angular shape, while the nickel powder has a spherical shape (fig 1 a
and b). In the beginning of the milling, some particle powders are crushed and flattened as result
of the interaction between the powder, balls and jars fig (1c and d). For higher milling time (8
and16 h), the shape is more rounded
The fig.2 and table 2 it is observed a considerable increase in particle size with increasing milling
time up to 4h, remaining itself constant after that.
100
Particle Size (µc)
Coordenação
80
60
40
20
0
0
2
4
6
8
10
12
14
16
18
Milling Time (H)
Fig. 2 everage diameter of the particles size with milling time for 50Ni50Ti % at . The mean
particle size was obtained from the accumulated mass of 50%
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Table 2- Mean particle size (µm) –particle size corresponding to accumulated mass of 50% after
different milling times
Time
Ni powder Ti powder 0
2
4
8
16
(h)
18,6
17,7
19,0 46,6 87,6 79,6 80,1
Mean diameter (µm)
The high energy milling process normally causes an initial increase in the particle size and then a
decrease due to hardening until an equilibrium, where the particle size remain constant. In this
particular case, the absence of a process control agent during the milling process, accelerated the
agglomeration by welding until a certain limit in which occurs the welding and break of the
particles and the average diameter remains constant. However, there is a continuous reduction of
the crystallite size and subsequent formation of amorphous phase.
The x-ray of the 50Ni-50Ti powder mixture and mechanical alloying are shown in fig.3. It can be
observed that the diffraction lines become broader and smaller, as the milling time is increased.
The increase in the lattice parameter of nickel clearly indicates that the titanium atoms, which
have a larger atomic radius (1,45 Å) than nickel atoms (1,24 Å), dissolve into f.c.c nickel,
forming a disordered f.c.c solid solution of titanium in nickel [4]. Surynarayana [5] mentioned
that the line profile become broader with increase the milling duration because of the
fragmentation of crystallites and/or disorder effects. The broadening of the XRD lines also
indicates the amorphization. After just 8h milling it shows amorphous and crystalline phases.
This is better visualized in an schematic drawing presented in fig 4. After 16h milling the
amorphization is almost completed. It is suggested that the disordering and nanocrystalline grain
boundaries may increase the free energy and therefore act as the driving force for the order-todisorder and crystalline-to-amorphous transitions [7].In comparison with another works it was
observed the formation of a amorphous phase with very short milling time. This fact can be
explained due to absence of process control agent and to the high speed of the shaft during the
milling, submitting the alloy to severe deformation. It was observed powder weldings in balls and
jar after 8h of milling.
Fig 3 XRD patterns of 50Ni-50Ti powders after mechanical alloying for various milling
durations
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Fig 4 schematic drawing of the X-ray diffraction for 8 hour milled powder based of the [6]
Analysis was made of the powder samples for Fe contamination at various milling times. This
was done in the SEM with EDS attachment. The elemental powders did not show any detectable
contamination, while an estimated contamination of about 0,98% at Fe was observed after 8h of
milling (fig 5).
a)
Fig 5 EDS for the a) mixture and b) powder milled 8h
b)
Figure 6 presents X-Ray diffraction of high energy milled powders after compaction and
sintering. The detected phases are: Ni3Ti, NiTi, Ni2Ti and oxide.
The lines of Ni3Ti and NiTi are in the same position and are not separated by this technique. The
presence of crystalline oxide confirms the contamination of the material in the high energy mill.
It was expected only the formation of the phase NiTi at the high temperatures utilized. Even
considering oxidation, as the alloy is a binary alloy, it would be expected only two phases in
equilibrium. So the conditions of heat treating was not enough to obtain the equilibrium phases of
the alloy, for any alloy composition. So the alloy is heterogeneous and to obtain a homogeneous
microstructure the conditions should be changed.
The results also show a difference of phases for different time of milling. This suggests a strong
influence of the initial state of the mixture Ni and Ti.
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Fig. XRD patterns of 50Ni-50Ti powders after heat treatment for various milling durations
Conclusions
-
The milling in the attritor promotes dissolution of Titanium in Nickel and continuous
amorphization by increasing the milling time.
The processing in the High energy mill contaminates the alloy with iron and oxides
In the heat treated samples were detected different phases showing heterogeneity in the
alloy. The phases are: Ni3Ti, NiTi, Ni2Ti and Ni2Ti4O
References
[1] L. Krone, E. Schuller, M.Bram, O, Hamed, H.-P. Buhkremer, D. Stöver, Materials Science
and Engineering A 378 (2004) 185-190
[2] K. Yamada, C.C Koch journal of materials research 8 (1993) 1317-1325
[3] J.Koike, D.M.Parkin, and M.Nastasi, J. materials research 5 (1990) 1414
[4]Y.W.Gu, C.W.Goh, L.S.Goi, C.S.Lim, C.S.Lim, A.E.W.Jarfors, B.Y.Tay, M.S.Yong materials
Science and Engineering A 392 (2005) 222-228
[5] C. Suryanarayana, Progress Materials Science 46 (2006) 1.
[6] LPS dos Santos,Caracterização óptica e estrutural de PbTiO3 nanoestruturado obtido por
moagem mecânica de alta energia, Thesis Masters, USP-São Carlos-Brazil
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