No abstract

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

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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

This paper describes the dependence of microstructure of NiTi shape memory alloy on the conditions of nonconventional powder metallurgy processing routes -reactive sintering of compressed Ni+Ti powder mixture and mechanical alloying with consequent compaction by Spark Plasma Sintering. First method was chosen as the process enabling to yield the high-purity NiTi alloy, while the second one aimed to reach the ultrafine-grained microstructure. The microstructure and phase composition of the products are compared in this work. The positive effects of high heating rate (> 300 K.min-1) and high temperature (at least 900 °C) on the reactive sintering process were recognized. Microstructure of the product is composed by NiTi matrix with dispersed Ti2Ni particles. Similar microstructure can be also obtained by mechanical alloying for at least 120 min and consequent compaction by Spark Plasma Sintering.