P. NOVÁK et al.: FORMATION OF Ni-Ti INTERMETALLICS DURING REACTIVE SINTERING AT 800–900 °C 679–685 FORMATION OF Ni-Ti INTERMETALLICS DURING REACTIVE SINTERING AT 800–900 °C OBLIKOVANJE NiTi INTERMETALNIH ZLITIN MED REAKTIVNIM SINTRANJEM PRI 800–900 °C Pavel Novák 1 , Vladimír Vojtìch 1 , Zuzana Pecenová 1 , Filip Prù{a 1 , Petr Pokorný 1 , Davy Deduytsche 2 , Christophe Detavernier 2 , Adriana Bernatiková 1 , Pavel Salvetr 1 , Anna Knaislová 1 , Kateøina Nová 1 , Lucyna Jaworska 3 1 University of Chemistry and Technology, Department of Metals and Corrosion Engineering, Technická 5, 166 28 Prague 6, Czech Republic 2 Ghent University, Department of Solid State Sciences, Krijgslaan 281, S1 9000 Gent, Belgium 3 Institute of Advanced Manufacturing Technology, 37a Wroclawska St., 30-011 Krakow, Poland panovak@vscht.cz Prejem rokopisa – received: 2016-08-17; sprejem za objavo – accepted for publication: 2016-11-16 doi:10.17222/mit.2016.257 In this work the formation of intermetallics in the Ni-Ti system by reactive sintering at 800–900 °C was studied. The mechanism and kinetics of the reactions, which led to Ni-Ti phases, were determined by thermal analysis, in-situ XRD and the application of an experimental model consisting of nickel-plated titanium. It was found that the formation of Ni-Ti phases below the transformation temperature of titanium is controlled by diffusion. Above this temperature, the reactions switch to the rapid Self-propagating High-temperature Synthesis (SHS) mode. Keywords: reactive sintering, powder metallurgy, NiTi V delu je bil raziskan nastanek intermetalnih zlitin v sistemu NiTi pri reaktivnem sintranju na 800-900 ° C. S termi~no analizo, XRD-in situ analizo in uporabo eksperimentalnega modela, nikljanega s titanom, sta bila dolo~ena mehanizem in kinetika reakcij, ki sta vodila k NiTi fazam. Ugotovljeno je bilo, da je tvorba NiTI faze pod transformacijsko temperaturo titana, nadzorovana z difuzijo. Nad to temperaturo se reakcije spremenijo na hitro rasto~i temperaturno -sintezni na~in (SHS). Klju~ne besede: reaktivno sintranje, metalurgija prahov, NiTi 1 INTRODUCTION The Ni-Ti alloy called nitinol, in approximately equi- molar proportions, is the most widely known shape- memory alloy. The shape-memory effect in this alloy is connected with the transformation between high-tempe- rature cubic austenite and low-temperature monoclinic martensite. 1,2 For the practical application of these alloys, superelasticity is very important. This phenomenon occurs when the NiTi alloy is deformed slightly above the martensite ® austenite transformation temperature. Deformation induces the formation of the martensite phase, which is continuously transformed to austenite during unloading. Due to this phenomenon, this alloy behaves like an enormously elastic material. 1,2 In addition, the NiTi alloy is also a corrosion-resistant material. 3 Due to its exceptional properties, the NiTi alloy is applied in both medical (dental implants, stents, scaffolds) 4,5 and technical applications (actuators, robo- tics, etc.). 6–8 The most commonly applied techniques in the indus- trial production of nitinol alloy are melting metallurgy processes – vacuum induction melting (VIM) and vacuum arc remelting (VAR). 9,10 In the VIM of Ti-con- taining alloys there is a serious danger of a strong contamination of the melt due to the high reactivity of molten titanium. 11 The VAR technique makes it possible to prepare alloys of higher purity, but there is a problem with homogeneity. To obtain a sufficiently homogenous product, the VAR process has to be repeated even more than 4 times. 10 This implies that it is costly and relatively problematic to obtain a NiTi shape-memory alloy. If a simple production technology would be developed, the NiTi alloy could be more frequently applied, not only in specific areas requiring the shape-memory effect, but also in other technical branches as a corrosion-resistant alloy. It will be beneficial for European economy, because this alloy does not contain any elements listed as critical raw materials. 12 A promising alternative to melting metallurgy production routes is powder metallurgy (PM). A simple non-conventional PM production technology is reactive sintering. In general, the reactive sintering is a densifi- cation process, where initial components in powder form are transformed to a compact product via thermally-acti- vated chemical reactions. 13 These reactions are mostly exothermic when intermetallics are formed. The route from powders to the compact usually contains powder blending, cold pressing and sintering. 13,14 When pure powders and an efficient protective atmosphere are MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS Materiali in tehnologije / Materials and technology 51 (2017) 4, 679–685 679 UDK 621.763:621.762.3:620.172.22 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(4)679(2017)