Powder-metallurgy preparation of NiTi shape-memory alloy using mechanical alloying and spark-plasma sintering

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

Synthesis of Intermetallics in Fe-Al-Si System by Mechanical Alloying

“…From our previous results with our ultra-high energy mechanical alloying process, we can conclude that the temperature localized in the contact point between powder, ball and wall exceeds 650 • C, because this temperature is needed for the formation of some intermetallics as the Ni-Ti and Fe-Al binary phases prepared in our previous paper [19]. The required temperature was determined by DTA heating curve of the powder mixtures of pure metals previously [16]. Hence it can be expected that the energy level including friction and the use of high ball-to-powder ratio is much more than two times higher (approx.…”

“…The MA leads to the reduction of the input powder size thanks to the high kinetic energy of the milling balls. Furthermore, other phenomena also occur during the process, such as local welding of particles by plastic deformation, friction forces and diffusion, structure refinement due to severe plastic deformation, as well as the formation of solid solutions and chemical compounds (intermetallics) [16,17]. Products prepared by MA under appropriate conditions usually possess an ultrafine-grained nanocrystalline or amorphous microstructure [18].…”

Microwave versus Conventional Sintering of NiTi Alloys Processed by Mechanical Alloying

“…This was a remarkable result; lamellar structure has only been observed in the literature for milling times ranging from 4 to 180 h [27][28][29]. However, even with long times, it is not guaranteed to obtain a lamellar structure because there are other process parameters that must be controlled, as has been observed in [24,28,30].…”

Development of TiAl–Si Alloys—A Review

“…The temperature of 650 °C is required for the formation of some intermetallic compounds, such as the binary phases Fe-Al of Ni-Ti prepared in [ 114 , 115 ]. The required temperature for the formation of these phases was determined by the DTA method on pure metal powder mixtures [ 116 ]. Therefore, the energy including friction and the use of a high ball to powder ratio can be expected to be more than twice as high (approximately 400–500 W).…”