M. AYBEY et al.: EFFECT OF HOLDING TIME ON THE PRODUCTION OF Nb-NbAl3 ...
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EFFECT OF HOLDING TIME ON THE PRODUCTION OF Nb-NbAl3
INTERMETALLIC COMPOSITES VIA
ELECTRIC-CURRENT-ACTIVATED SINTERING
VPLIV ^ASA ZADR@ANJA NA IZDELAVO Nb-NbAl3
INTERMETALNIH KOMPOZITOV Z ELEKTRI^NIM TOKOM
AKTIVIRANIH S SINTRANJEM
Muhammed Aybey, Tuba Yener, Mediha Ipek, Sakin Zeytin
Sakarya University, Engineering Faculty, Department of Metallurgy and Materials Engineering, Esentepe Campus, 54187, Adapazari,
Sakarya, Turkey
tcerezci@sakarya.edu.tr
Prejem rokopisa – received: 2015-07-01; sprejem za objavo – accepted for publication: 2016-02-05
doi:10.17222/mit.2015.179
A recently developed powder metallurgy processing technique – Electric Current Activated (Assisted) Sintering (ECAS) was
employed to produce intermetallic Nb-NbAl3 composites. In this study, to produce Nb-NbAl3 in-situ intermetallic composites,
Nb (99.8 % purity, less than 44 μm) and Al (99.5 % purity, less than 44 μm) elemental powders were mixed in the stoichiometric
ratio corresponding to the Nb-Al phase diagram. The effect of different processing times, for (10, 30, 60) s, under maximum of
2000 A and 1.5-2.0 V, was investigated. Scanning electron microscopy and X-ray diffraction analysis were used to characterize
the produced samples. X-ray diffraction studies revealed that the dominant phases are NbAl3 and Nb. Scanning electron
microscopy examinations showed a dense microstructure with a very low amount of porosity and also a trace amount of residual
aluminium. The microhardness of the test materials sintered for 60 s via electric-current-activated sintering was about 405
HV±46 HV0,05.
Keywords: in situ composites, NbAl3 aluminides, electric current activated sintering (ECAS)
Pred kratkim razvita tehnika v metalurgiji prahov: sintranje, aktivirano z elektri~nim tokom (ECAS), je bilo uporabljeno za in
situ izdelavo intermetalnega kompozita Nb-NbAl3. V {tudiji sta bila za in situ proizvodnjo Nb-NbAl3 intermetalnega kompozita
zme{ana elementna prahova Nb (99.8 % ~istost, delci manj{i od 44 μm) in Al (99.5 % ~istost, delci manj{i od 44 μm), zme{ana
v stehiometri~nem razmerju, skladno s faznim diagramom Nb-Al. Preiskovan je bil vpliv razli~nih ~asov izdelave: (10, 30, 60) s
pri toku maksimalno 2000 A in napetosti 1,5-2,0 V. Izdelani vzorci so bili karakterizirani z vrsti~no elektronsko mikroskopijo in
rentgensko difrakcijo. Rentgenska difrakcija je odkrila, da sta prevladujo~i fazi NbAl3 in Nb. Vrsti~na elektronska mikroskopija
je pokazala gosto mikrostrukturo z majhnim dele`em poroznosti in sledovi preostalega aluminija. Mikrotrdota preizkusnega
materiala, sintranega 60 s, s sintranjem aktiviranim z elektri~nim tokom, je bila okrog 405 HV±46 HV0,05.
Klju~ne besede: in situ kompoziti, NbAl3 aluminidi, sintranje aktivirano z elektri~nim tokom (ECAS)
1 INTRODUCTION
Intermetallic compounds have been the focus of
significant research and development efforts during
recent years. Among intermetallic compounds, niobium
aluminides are very important and attractive.1–3 Three
intermetallic compounds are present in the Nb–Al binary
system including Nb3Al (A15 structure), Nb2Al (D8b
structure) and NbAl3 (DO22 structure, TiAl3 type).4–7
Among the various compounds in the Nb-Al system
(Figure 1), NbAl3 with its high melting point (1680 °C),
low density (4.54 g/cm3), is attractive as a potential
material for high-temperature applications.6,8 The appli-
cations of NbAl3 include its use in turbine blades in
aircraft engines or in stationary gas turbines.1,9 However,
despite its attractive features, its usage is limited by in-
adequate ductility at room temperature. So, for optimi-
zation of room-temperature toughness, microstructural
modifications are required.1,6,10 In-situ toughening is an
alternative technique to enable a combination of brittle
intermetallic phase with a ductile metallic phase in one
step production.11,12 Some conventional methods such as
melting, casting and mechanical alloying techniques or
self-propagating high-temperature synthesis (SHS)7–9 can
be used for manufacturing intermetallics. A recently
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Materiali in tehnologije / Materials and technology 51 (2017) 1, 55–58 55
UDK 669.293:621.762:621.79 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(1)55(2017)
Figure 1: Nb-Al phase diagram7
Slika 1: Fazni diagram Nb-Al7
developed electric current activated/assisted sintering
(ECAS) technique has been used in this study. This
system enables the cold formed compact obtained from
uniaxial compression to be inserted into a container,
which is heated by passing an electrical current. In the
present paper we prefer to apply the direct current
resistive sintering technique to the Nb+3Al powder
mixture in order to investigate a new route to determine
the effect of holding time on the production of Nb-NbAl3
in-situ composites.
2 EXPERIMENTAL PART
2.1 Materials and methods
Al and Nb elemental powders with 35–44 μm grain
size and purity of 99.5 % and 99.8 %, respectively, were
mixed to give the nominal composition of Nb40Al60
(w/%) for the formation of a NbAl3 intermetallic based
metallic Nb reinforced in-situ composite. The powder
mixture was ball milled for 15 min then cold-pressed
before sintering to form a cylindrical compact in a
metallic die under a uniaxial pressure of 200 MPa. The
dimensions of the compact were 15–16 mm diameter and
3–4 mm thickness. The production of Nb aluminide-
based intermetallic compound was performed via the
electric current activated sintering technique in an open
atmosphere at 2000 A for (10, 30, 60) s. They are de-
noted as C1 and C2 and C3 in the following section, res-
pectively. Th eprocess parameters are listed in Table 1.
Table 1: Process parameters for the samples
Tabela 1: Procesni parametri vzorcev
Sample code w/% Current (A) Holding time(s)
C1 40Nb-60Al 2000 10
C2 40Nb-60Al 2000 30
C3 40Nb-60Al 2000 60
2.2 Characterization
The morphologies of the samples were examined by
scanning electron microscopy (SEM-EDS) in terms of
the resulting phases. X-ray diffraction (XRD) analyses
were carried out using Cu–K radiation with a wave-
length of 0.15418 nm over a 2 range of 10–80°. The
microhardness of the test materials was measured using a
Vickers indentation technique with a load of 0.98 N
using Leica WMHT-Mod model Vickers hardness
instrument.
3 RESULTS AND DISCUSSION
3.1 SEM-EDS Analysis
Figure 2 shows SEM micrographs of the elemental
Nb and Al powder particles. As seen in Figure 2 the Al
particles are rounded and Nb particles are angular and
sharp cornered in shape.
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Figure 3: SEM micrographs of: a) C1 and b) C2 samples
Slika 3: SEM-posnetka vzorcev a) C1 in b) C2
Figure 2: SEM micrographs of: a) Nb, b) Al powder
Slika 2: SEM-posnetka: a) Nb in b) Al prahu
SEM-EDS analyses of C1, C2 and C3 intermetallic
compounds are shown in Figure 3. The microstructure in
Figure 3a shows that the low holding time results in
separately formed Nb and Al areas. Increasing the pro-
cessing time from 10 s to 30 s in C2, (Figure 3b), it
starts to form a new phase like NbAl3, but these micro-
structures are still far from the desired stoichiometric
composition of the main NbAl3 phase.
When it comes to the C3 sample, as seen in Figure 4,
increasing the process holding time to 60 s, the main
phase in the microstructure is Nb and NbAl3. Besides
this, there is also a little amount of residual Al phase and
a little oxidation problem because of the open atmo-
sphere sintering in ECAS. But that was not detected in
the XRD analyses for being a small amount. In addition
to that, a nearly fully dense microstructure was obtained
after just a minute of holding process time, thanks to the
electric current resistive sintering system.
As can be seen in the SEM-EDS analyses in Figure 5,
the reaction was also not completed in the C3 compound.
It is assumed that the applied voltage or current is
insufficient for the complete transformation of the NbAl3
phases in the sintering.
3.2 XRD Analysis
The XRD analysis, Figure 6, shows that the main
phase of the composite is NbAl3. The Nb phase is also
seen in the XRD analyses, as desired. These results
support the observations from the SEM-EDS analysis
(Figure 5). This can be inferred from this result:
Nb-NbAl3 in-situ composites can be obtained for one
minute and one step electric current activated system;
however, for eliminating the residual aluminium in the
compact. It is obvious that, it can be optimized with
some other parameters such as voltage or current.
3.3 Hardness
The hardness values HV0.05 of the C1, C2 and C3
samples were measured as 165±20, 250±27, 405±46 res-
pectively. The hardness of the intermetallic composites
increases from 165±20 HV to 405±46 HV by increasing
the holding time in the process due to the formation of a
higher proportion of intermetallic phases. The hardness
results for the NbAl3 composite is in agreement with the
literature.13
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Figure 4: SEM micrographs of C3 Sample: a) 1000 ×, b) 5000 ×
Slika 4: SEM-posnetka vzorca C3: a) 1000 ×, b) 5000 ×
Figure 6: XRD Analyses of C3 sample (Nb-NbAl3)
Slika 6: Rentgenogram vzorca C3 (Nb-NbAl3)
Figure 5: SEM-EDS analyses of C3 samples
Slika 5: SEM-EDS-analize na vzorcu C3
4 CONCLUSIONS
Some of the results obtained from this study can be
summarized as follows:
• Niobium aluminide-based composites were fabri-
cated by electric current activated/assisted sintering
at 2000 A in only 60 s.
• Under such conditions, the reaction is nearly com-
pleted within a very short period of time and the
end-product is consolidated to a nearly fully dense
microstructure.
• SEM-EDS and XRD analyses showed that 60 s are
sufficient for obtaining Nb-NbAl3 phases. However,
for eliminating the residual aluminium in the com-
pact it can be optimized some other parameters such
as voltage or current.
• Hardness of the intermetallic composites increases
from 165±20 HV to 405±46 HV by increasing the
holding time in the process by ensuring the formation
of a higher proportion of intermetallic phases
Acknowledgement
This work was supported by Sakarya University
Research Foundation (Project Number: 2013-01-08-034)
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