UDK 621.762:621.762.5:669.018.29	ISSN 1580-2949
Original scientific article/Izvirni znanstveni članek	MTAEC9, 49(5)721(2015)
EFFECT OF ELECTRIC CURRENT ON THE PRODUCTION OF NiTi INTERMETALLICS VIA ELECTRIC-CURRENT-ACTIVATED
SINTERING
VPLIV ELEKTRIČNEGA TOKA PRI IZDELAVI INTERMETALNE ZLITINE NiTi S SINTRANJEM, AKTIVIRANIM Z ELEKTRIČNIM
TOKOM
Tuba Yener1, Shafaqat Siddique2, Frank Walther2, Sakin Zeytin1
1Sakarya University, Engineering Faculty, Department of Metallurgy and Materials Engineering, Esentepe Campus, 54187 Adapazari, Sakarya,
Turkey
2TU Dortmund University, Faculty of Mechanical Engineering, Department of Materials Test Engineering (WPT), Baroper Str. 303,
44227 Dortmund, Germany tcerezci@sakarya.edu.tr
Prejem rokopisa - received: 2014-08-01; sprejem za objavo - accepted for publication: 2014-10-09
doi:10.17222/mit.2014.161
This study focuses on investigating the fabrication of in-situ intermetallic NiTi composites from a powder mixture containing the mass fractions 50 % nickel powder and 50 % titanium powder. The elemental powders were mixed in the stoichiometric ratio corresponding to the NiTi intermetallic molar proportion of 1 : 1, ball-milled and uniaxially compressed under a pressure of 170 MPa. Sintering was then carried out for 15 min in a steel mold using the electric-current-activated sintering method. Electric-current values of 1000 A, 1300 A and 2000 A were used for the sintering while keeping the voltage in the range of 0.9 V to 1.2 V. The phases in the samples were analyzed with XRD and their Vickers hardness was measured as (701 ± 166) HVo.o5. Energy dispersive X-ray spectroscopy carried out with a scanning electron microscope (SEM-EDS) showed that the micro-structures of the samples consist of different phases such as Ti, Ni2Ti3, NiTi2, Ni3Ti and TiO2 as a function of electric current. The XRD analysis also supported the SEM-EDS results. The nano-indentation technique was used to determine the elastic modulus of different phases.
Keywords: NiTi intermetallics, electric-current-activated sintering (ECAS), nano-indentation
Ta študija je usmerjena v preiskavo in situ izdelave intermetalnega NiTi-kompozita iz mešanice prahov z masnima deležema 50 % niklja v prahu in 50 % titana v prahu. Obe vrsti prahu sta bili zmešani v stehiometričnem razmerju, ki ustreza molskemu razmerju NiTi 1 : 1, zmleti v kroglastem mlinu in enoosno stisnjeni pri tlaku 170 MPa. Petnajstminutno sintranje je bilo izvršeno v jeklenem kalupu s sintranjem, aktiviranim z električnim tokom. Pri sintranju so bili uporabljeni električni tokovi 1000 A, 1300 A in 2000 A, medtem ko je bila napetost v območju med 0,9 V in 1,2 V. Faze v vzorcih so bile določene z rentgenom (XRD) in izmerjena je bila trdota po Vickersu (701 ± 166) HV0.05. Energijska disperzijska rentgenska spektroskopija (EDS), izvršena na vrstičnem elektronskem mikroskopu (SEM-EDS), je pokazala, da je mikrostruktura vzorcev sestavljena iz različnih faz, kot so Ti, Ni2Ti3, NiTi2, NisTi in TiO2, odvisno od električnega toka. XRD-analiza je podprla rezultate, dobljene s SEM-EDS. Elastični modul različnih faz je bil določen z nanovtiski.
Ključne besede: intermetalna zlitina NiTi, sintranje, aktivirano z električnim tokom (ECAS), nanovtiski
1 INTRODUCTION	Nickel titanium is a near-equiatomic intermetallic exhibiting distinctive and desirable thermo-mechanical To meet the increasing demand of the aerospace and properties, namely, the thermal shape-memory effect and
automobile industries for lightweight structural materials	super elasticity.8 Up to now, a number of processes in-
suitable for high-temperature applications, researchers1	cluding self-propagating high-temperature synthesis
focus on lightweight and high-strength intermetallics.1	(shs), thermal explosion, laser-melting deposition, cast-
Intermetallic compounds are promising materials for	2,5,9,10
, , , ,•	ing and mechanical alloying techniques2,5,9,10 have been structural and non-structural high-temperature applications (heat resistance, corrosion resistance, electronic	used for manufacturing intermetallics ln the electric-
devices, magnets, super conductors).2,3 Especially NiTi	current-activated/assisted sintering (ECAS) technique, a
alloys are some of the most technologically important cold-formed compact obtained with uniaxial compres-
shape-memory alloys4,5 which find extensive applications	sion is inserted into a container heated by the passing in the mechanical, medical, electronic, chemical and electrical current. A sintering pressure of 50 MPa is
aerospace industries due to their excellent shape-memory	applied and maintained throughout the sintering.11 The
effect, high erosion resistance, high damping capacity	present study aimed to determine the effect of the current
and biocompatibility. These properties make them suit-	on the production of a NiTi intermetallic from the Ni and
able candidates for various engineering applications.2-7	Ti elemental powders.
2 EXPERIMENTAL PROCEDURE
2.1 Materials and methods
Powder materials from titanium (99.5 % purity, 35-44 ^m) and nickel (99.8 % purity, 3-7 ^m) were used as the starting materials to manufacture a NiTi inter-metallic compound. The Ni and Ti powders were mixed in the stoichiometric ratio corresponding to the Ni-Ti phase diagram (Figure 1), in a molar proportion of 1 : 1.
The powder mixture was cold-pressed before the sintering to form a cylindrical compact in a metallic die under a uniaxial pressure of 170 MPa. Dimensions of a compact were 15 mm in diameter and 5 mm in thickness. The production of the NiTi intermetallic compounds was performed with the electric-current-activated sintering technique in an open atmosphere at 1000-2000 A for 15 min as shown in Figure 2. The process parameters are listed in Table 1.
Table 1: Process parameters for the samples Tabela 1: Procesni parametri pri vzorcih
Sample code	x/%	Current (A)	Voltage (V)
NT1	50Ti-50Ni	1.000	0.9-1.2
NT2	50Ti-50Ni	1.300	0.9-1.2
NT3	50Ti-50Ni	2.000	0.9-1.2
After the sintering, the specimens were unloaded and air-cooled to room temperature. After metallographic preparations, the resulting microstructures and phase constitutions were characterized.
2.2 Characterization
The morphologies of the samples were examined with scanning electron microscopy (SEM-EDS) in terms of the resulting phases. An X-ray diffraction (XRD) analysis was carried out using Cu-Ka radiation with a wavelength of 0.15418 nm over a 20 range of 10-80 The micro-hardness of the investigated samples was
Figure 2: Schematic of electric-current-assisted sintering (ECAS) process
Slika 2: Shematski prikaz sintranja z električnim tokom (ECAS)
measured employing the Vickers-indentation technique with a load of 50 g using a Struers Duramin hardness instrument. For determining the elastic modulus of the samples, nano-indentation device DUH-211S by Shi-madzu was used at a load of 98 mN and a dwell time of 10 s. The ImageJ programme was also used to determine the porosity of fractions of the samples.
3 RESULTS AND DISCUSSION 3.1 SEM-EDS analysis
The morphologies of the as-received Ni and Ti powders are shown in Figure 3. The metallic Ni-powder
Figure 1: Ni-Ti phase diagram3 Slika 1: Fazni diagram Ni-Ti 3
Figure 3: SEM micrographs of: a) Ni powder, b) Ti powder Slika 3: SEM-posnetka prahov: a) Ni, b) Ti
Figure 4: SEM-EDS analyses of: a) NT1, b) NT2 composites Slika 4: SEM-EDS-analizi kompozitov: a) NT1, b) NT2
particles were generally spherical with a diameter of 4-7 pm. The Ti-powder particles had sharp corners and were less than 40 pm in size.
SEM-EDS analyses of the NT1 and NT2 interme-tallic compounds are shown in Figure 4. The micro-structure in Figure 4a shows that a low current intensity results in separately formed Ni and Ti areas. When increasing the values of the current for NT2 (Figure 4b), new phases like NiTi2 (Figure 1) start to form, but these microstructures are still far from the stoichiometric composition of the main NiTi phase.
When increasing the current to 2000 A for the NT3 sample (Figure 5), the main phases in the microstructure such as NiTi, NiTi2, Ni3Ti are evident. Besides, there are also a small amount of the residual Ti phase and a small oxidation problem due to the open atmosphere. It can be
Figure 6: SEM-EDS analyses of NT3 composite Slika 6: SEM-EDS-analize kompozita NT3
inferred that a high current intensity leads to a high amount of intermetallic phases. According to the ImageJ image-analysis programme, a porosity of 0.2 % can be found in the microstructures.
As can be seen from the SEM and EDS analyses presented in Figure 6, the reaction of the NiTi-compound synthesis was not completed. It is assumed that the applied voltage was insufficient for a complete transformation of the NiTi phases during the sintering.
3.2	XRD analysis
The XRD analysis (Figure 7) shows that the main phase of the composite is NiTi2. The NiTi phase is also seen as a small peak. Additionally, Ni3Ti, Ti and TiO are the other phases existing in the NT3 composite. These results support the observations from the SEM-EDS analysis.
3.3	Hardness and the elastic modulus
The measured HV0.05 hardness values for the NT1, NT2 and NT3 samples are (250 ± 27), (405 ± 71) and
Figure 5: SEM-EDS analyses of NT3 composite Slika 5: SEM-EDS-analizi kompozita NT3
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P^"' - "
«hK^ V .y/:^, 5 urn V \ ' , /
Figure 7: XRD analysis of NT3 sample Slika 7: Rentgenogram vzorca NT3
(701.6 ± 166), respectively. The hardness values are in good agreement with the literature.112 The elastic-modulus values for the NT1, NT2 and NT3 samples determined with the nano-indentation technique are (98.3 ± 9) GPa, (120 ± 23) GPa and (132 ± 25) GPa, respectively. Nano-indentation is an important technique for probing the mechanical behavior of materials at small-length scales.13 The results for the elastic modulus cannot be compared to the literature because of the contrasting reported values.14 However, the obtained value of 135 GPa is between the elastic-modulus values for the Ni and Ti materials.
4 CONCLUSIONS
The reaction of the NiTi-compound synthesis was not completed. It is inferred that the applied voltage was insufficient for a complete transformation of the NiTi phases. When increasing the current intensity from 1000 A to 2000 A, the fractions of intermetallic phases NiTi, NiTi2 and Ni3Ti increased as well. There was also a small amount of retained metallic Ti in the microstructure. The hardness of the intermetallic composites was increased from 250 HV to 701 HV by increasing the current intensity, due to the formation of higher amounts of the inter-metallic phases. The elastic modulus of the composites was between the elastic-modulus values for the Ni and Ti materials.
Acknowledgements
This paper was completed within the framework of the PhD work of Mrs. Tuba YENER during a research visit at the TU Dortmund University, the Department of Materials Test Engineering (WPT) between 06-09/2014. The host institution and the home institution, the Sakarya University, the Department of Metallurgy and Materials Engineering, gratefully acknowledge the excellent scientific exchange and the financial support of the PhD student exchange.
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