P. NOVÁK et al.: POWDER-METALLURGY PREPARATION OF NiTi SHAPE-MEMORY ALLOY ...
141–144
POWDER-METALLURGY PREPARATION OF NiTi
SHAPE-MEMORY ALLOY USING MECHANICAL ALLOYING
AND SPARK-PLASMA SINTERING
UPORABA METALURGIJE PRAHOV ZA PRIPRAVO NiTi ZLITINE
S SPOMINOM S POMO^JO MEHANSKEGA LEGIRANJA IN
SINTRANJA Z ISKRILNO PLAZMO
Pavel Novák1, Hynek Moravec1, Vladimír Vojtìch1, Anna Knaislová1,
Andrea [koláková1, Tomá{ Franti{ek Kubatík2, Jaromír Kope~ek3
1Institute of Chemical Technology, Department of Metals and Corrosion Engineering, Technická 5, 166 28 Prague 6, Czech Republic
2Institute of Plasma Physics AS CR, v.v.i., Za Slovankou 3, 182 00 Prague 8, Czech Republic
3Institute of Physics of the ASCR, v. v. i., Na Slovance 2, 182 21 Prague 8, Czech Republic
panovak@vscht.cz
Prejem rokopisa – received: 2016-01-13; sprejem za objavo – accepted for publication: 2016-02-02
doi:10.17222/mit.2016.011
In this work a combination of mechanical alloying and spark-plasma sintering was tested as a promising route for the
preparation of a nanocrystalline NiTi shape-memory alloy. The mechanism of mechanical alloying was investigated. Results
revealed that the Ti2Ni phase forms preferentially, being followed by the NiTi phase (austenite B2 structure) and a small amount
of Ni3Ti. During spark-plasma sintering, only minor changes occurred in the phase composition, i.e., precipitation of the Ni4Ti3
phase and the partial transformation of NiTi to monoclinic martensite. The selected technology leads to a very high compression
strength (approx. 2300 MPa), but also high brittleness.
Keywords: mechanical alloying, spark plasma sintering, NiTi, shape memory alloy
V delu je bila preizku{ena kombinacija mehanskega legiranja in sintranja z iskrilno plazmo, ki predstavlja obetajo~ na~in za
pripravo nanokristalne NiTi spominske zlitine. Preiskovan je bil mehanizem mehanskega legiranja. Rezultati so odkrili, da se
najprej tvori faza Ti2Ni, ki ji sledi faza NiTi (avstenitna B2 struktura) in manj{i dele` Ni3Ti. Med sintranjem z iskrilno plazmo se
pojavijo le manj{e razlike v sestavi faz, to je izlo~anje faze Ni4Ti3 in delna pretvorba NiTi v monoklinski martenzit. Izbrana
tehnologija povzro~i veliko tla~no trdnost (okoli 2300 MPa) in tudi veliko krhkost materiala.
Klju~ne besede: mehansko legiranje, sintranje z iskrilno plazmo, NiTi, zlitina z oblikovnim spominom
1 INTRODUCTION
The approximately equimolar Ni-Ti alloy, called
nitinol, is the most widely used shape-memory alloy. The
shape-memory effect in this alloy is connected with the
transformation between high-temperature cubic austenite
(B2 structure) and low-temperature monoclinic marten-
site (B19’ structure).1 Due to its exceptional properties,
the NiTi alloy is applied in both medical (dental im-
plants, stents, scaffolds)2 and technical applications
(actuators, robotics, etc.).3
As an alternative to conventional production methods
for the NiTi alloy (vacuum induction melting and va-
cuum arc remelting), powder-metallurgy processes start-
ing from pre-alloyed NiTi powder have been developed.4
An alternative powder-metallurgy process for the
production of ceramics or intermetallics is reaction syn-
thesis. In this process, the compressed mixture of
elemental powders is transformed to intermetallic, ther-
mally activated exothermic reactions. During the reac-
tions, the heat is generated, which sustains and pro-
pagates the reaction through the body of the reactants.
Therefore, the process is called self-propagating
high-temperature synthesis (SHS).5 Mechanical alloying
is one of the techniques used for the production of nano-
structured powders.6 Mechanical alloying is in fact
high-energy ball milling. In this process, the high kinetic
energy of balls causes the following phenomena: crush-
ing of particles leading to the reduction of the particle
size, local welding of particles by plastic deformation,
friction forces and diffusion, structure refinement due to
enormous plastic deformation and the formation of solid
solutions and chemical compounds (intermetallics).6
Spark-plasma sintering (SPS) is the modern compac-
tion method, which uses uniaxial pressing accompanied
by the passage of the electric current through the sample.
It causes rapid heating of the sample and discharges
between powder particles that can cause local welding of
the particles. Due to the high sintering rate during SPS
this method is highly suitable for the compaction of
nanocrystalline materials and phases with a lower ther-
mal stability.6
In our previous paper6, we developed a novel mecha-
nical alloying process that allows for the formation of
intermetallics in a much shorter time than in previously
published papers.7 In this work, this improved mecha-
Materiali in tehnologije / Materials and technology 51 (2017) 1, 141–144 141
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UDK 621.762:621.762.5:669.24:669.295 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(1)141(2017)
nical-alloying process on NiTi shape-memory phase
synthesis is studied. The combination of this process
with spark-plasma sintering for the production of a NiTi
shape memory alloy was tested.
2 EXPERIMENTAL PART
In this work, the material based on the NiTi shape
memory phase was prepared from elemental powders by
mechanical alloying and subsequent spark-plasma sint-
ering (SPS) compaction. The mechanical alloying was
carried out in a planetary ball mill (Retsch PM 100 CM)
under the following conditions, optimized in our pre-
vious paper dealing with the synthesis of intermetallics:6
• milling duration: 15–360 min,
• change of rotation direction each 15 min,
• rotation speed: 400 min–1,
• atmosphere: argon
• powder batch: 5 g
• ball-to-powder weight ratio: 70:1.
The powder mixtures for milling contained 54 % Ni
and % Ti (by weight). This composition corresponds to
the equimolar NiTi phase.8 Milled powders were exa-
mined by X-ray diffraction analysis (PANalytical X’Pert
Pro diffractometer, Cu K radiation with the wavelength
of 0.154060 nm) in order to identify the phase com-
position. The XRD patterns were evaluated using
PANalytical HighScore software with the PDF-2 data-
base. XRD patterns were also quantitatively processed
using the Rietweld method. Metallographic samples
were prepared from selected powders. The microstruc-
ture of powder samples was studied after etching by
modified Kroll’s reagent (10 mL HNO3, 5 mL HF,
85 mL H2O). Individual phases in the powders were
identified on metallographic samples by chemical
microanalysis using TESCAN VEGA 3 LMU scanning
electron microscope equipped with OXFORD Instru-
ments X-max EDS SDD 20 mm2 detector (SEM-EDS).
Powder prepared under selected conditions (milling
duration of 2 h) was compacted by SPS at the Institute of
Plasma Physics AS CR. The weight of the batch for
sintering was 5 g. Compaction was carried out using a
Thermal Technology SPS 10-4 device using a pressure of
70 MPa for 5 min at various process temperatures with a
heating rate of 300 K/min. The phase composition of the
prepared compact samples was determined by XRD. The
porosity of compact samples was studied on polished
metallographic samples by image analysis using Lucia
4.8 image analyser. The mechanical properties of the
SPS-consolidated material were tested in compression
using LabTest 5.250SP1-VM universal loading machine
(produced by LaborTech).
3 RESULTS
The dependence of phase composition of the powders
obtained by mechanical alloying on process duration is
presented in Figure 1. During this experiment, a con-
stant rotational velocity of 400 rpm and a ball-to-powder
ratio of 70:1 were applied. After 15 min of mechanical
alloying, a small amount of Ti2Ni phase was formed. In
addition to this phase, the obtained powder contained
only unreacted initial powders of nickel and titanium.
After prolongation of the mechanical alloying process to
30–60 min, the Ti2Ni phase still dominated the phase
composition and the NiTi phase arose in the XRD
patterns. Residual nickel is still present in the powder
mixture. Mechanical alloying for 120 min produced a
powder composed of NiTi (austenite structure) and Ti2Ni
phases (Figure 1). When prolonging the mechanical
alloying to 360 min, the Ti2Ni still remains in the powder
mixture and the new Fe2Ti phase arises as a result of the
contamination by milling in an iron-based vessel (Figure
1). Therefore, long-term milling cannot be recommended
in this system and experimental setup.
The development of the microstructure of the
powders during the milling process is shown in Figure 2.
A short milling duration (15 min) results in the lamellar
structure containing deformed nickel and titanium parti-
cles, which are mechanically bonded (Figure2a). On the
interface between these particles, the layers and/or frag-
mented particles of Ti2Ni and traces of NiTi start to
form. Prolonging the duration of milling to 30–45 leads
to the disappearing of titanium particles (Figure 2b).
The milling duration of 120 min creates the structure
composed of NiTi matrix with dispersed Ti2Ni particles
(Figure2c).
During spark-plasma sintering consolidation of the
material, new phases precipitated from the mechanically
P. NOVÁK et al.: POWDER-METALLURGY PREPARATION OF NiTi SHAPE-MEMORY ALLOY ...
142 Materiali in tehnologije / Materials and technology 51 (2017) 1, 141–144
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Figure 1: Amounts of phases in milled powders and SPS-consolidated
sample (determined by XRD and Rietveld refinement
Slika 1: Koli~ine faz v mletem prahu in vzorcu, konsolidiranem z SPS
(dolo~eno z XRD in Rietveld metodo)
alloyed material, i.e., the Ni4Ti3 and Ni3Ti intermetallic
phases (Figure 1). Most probably, the Ni4Ti3 is a result
of thermal exposure and compressive stress and Ni3Ti
originates from the reaction of residual nickel with NiTi
phase. In addition, the NiTi phase was found in two
crystal modifications: the monoclinic and B2 cubic
phase. The product of spark-plasma sintering contains
only a low amount of pores (below 1 vol. %, Figure 3).
The compressive strength of the mechanically
alloyed and consolidated NiTi material reaches 2200 ±
90 MPa. However, the material exhibits almost brittle
behaviour (Figure 4), without the signs of the deforma-
tion-induced transformation of cubic austenite phase to
monoclinic martensite. The reason probably lies in the
strong deformation strengthening of the powder during
milling. The material did not recover significantly during
SPS consolidation due to the short time applied for
sintering. Due to this fact, the material does not allow for
plastic deformation during loading.
4 CONCLUSIONS
In this paper, the Ni-Ti phase evolution during ultra-
high-energy short-term mechanical alloying was investi-
P. NOVÁK et al.: POWDER-METALLURGY PREPARATION OF NiTi SHAPE-MEMORY ALLOY ...
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Figure 4: Stress-strain curve in compression of the NiTi alloy
prepared by mechanical alloying for 120 min and spark-plasma
sintering at 900 °C with a heating rate of 300 °C min–1
Slika 4: Krivulja napetost-raztezek pri tla~nem preizkusu NiTi zlitine,
pripravljene z mehanskim legiranjem 120 min in sintranjem z iskrilno
plazmo pri 900 °C in hitrostjo segrevanja 300 °C min–1
Figure 2: Microstructure of NiTi50 (in amount fractions, a/%) alloy
powder prepared by mechanical alloying for: a) 15 min, b) 45 min, c)
120 min
Slika 2: Mikrostruktura prahu zlitine NiTi50 (v volumskih dele`ih,
a/%), pripravljene z mehanskim legiranjem, a) 15 min, b) 45 min, c)
120 min
Figure 3: Microstructure of the NiTi alloy prepared by mechanical
alloying for 120 min and spark-plasma sintering at 900 °C with a
heating rate of 300 °C min–1
Slika 3: Mikrostruktura zlitine NiTi, pripravljene z mehanskim
legiranjem 120 min in sintranjem z iskrilno plazmo pri 900 °C, s
hitrostjo segrevanja 300 °C min–1
gated. In this technology, the Ti2Ni phase forms pre-
ferentially, being followed by the NiTi shape-memory
phase with an austenite (B2) structure. During spark-
plasma sintering of the mechanically alloyed powder, the
Ni4Ti3, Ni3Ti and monoclinic NiTi develop. Due to this
fact, the formation of the undesirable Ti2Ni phase cannot
be avoided.
The samples achieve much higher mechanical pro-
perties than a NiTi alloy produced by conventional route,
but they exhibit almost brittle behaviour. This can be
caused by the change of deformation mechanism when
going to nanoscale, or by trace contamination of the
grain boundaries during the mechanical alloying process,
lowering the cohesion of the grains.
Acknowledgement
This research was financially supported by the Czech
Science Foundation, project No. 14-03044S.
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