Reactive extrusion synthesis of mechanically activated Ti–50Ni powders

Neves, Filipe; Martins, Isabel M.; Correia, J.B.; Oliveira, Manuela; Gaffet, Éric

doi:10.1016/j.intermet.2007.06.011

Cited by 29 publications

(40 citation statements)

References 47 publications

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“…As was previously reported [7], the mechanically activated powders showed a lamellar microstructure consisting of thin alternating lamellae of Ni and Ti. This was the pathway for the decrease of the inter-diffusion distance and hence for the change of reactivity between Ni and Ti.…”

Section: Resultssupporting

confidence: 82%

“…This was the pathway for the decrease of the inter-diffusion distance and hence for the change of reactivity between Ni and Ti. Following the densification by extrusion (in MARES) and by forging (in MARFOS) high density materials were obtained and the layered structure was maintained consisting of thin layers of the intermetallic phases (NiTi, Ti 2 Ni and Ni 3 Ti) and of Ni and Ti [7,8]. With the objective of increasing the compositional homogeneity, solution and ageing treatments were performed.…”

Section: Resultsmentioning

confidence: 99%

“…One of the advantages of these techniques is the densification and synthesis of Ni-Ti intermetallics at a relatively low temperature through a controlled reaction [7,8].…”

Section: Introductionmentioning

confidence: 99%

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Nonconventional production technologies for NiTi shape memory alloys

Neves

Martins

Oliveira

et al. 2009

ESOMAT 2009 - 8th European Symposium on Martensitic Transformations

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Abstract. The development of new production technologies for NiTi Shape Memory Alloys (SMAs) is always challenging. Recently, we introduced two powder metallurgical (PM) processing routes involving mechanical activation of elemental powder mixtures and densification through extrusion or forging. Those processes were named Mechanically Activated Reactive Extrusion Synthesis (MARES) and Mechanically Activated Reactive FOrging Synthesis (MARFOS). Heat treatments were performed in order to adjust the B2-NiTi matrix composition, yielding a microstructure consisting of a homogeneous dispersion of Ni 4 Ti 3 precipitates embedded in nanocrystalline B2-NiTi matrix. In the present study, we demonstrate the viability of those PM processes for producing NiTi SMAs. With insitu X-ray diffraction and differential scanning calorimetry it is shown that B2-NiTi matrix undergo a B2 R two-step phase transformation.

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Section: Resultssupporting

confidence: 82%

Section: Resultsmentioning

confidence: 99%

See 1 more Smart Citation

Nonconventional production technologies for NiTi shape memory alloys

Neves

Martins

Oliveira

et al. 2009

ESOMAT 2009 - 8th European Symposium on Martensitic Transformations

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“…Although Ni 4 Ti 3 is considered as a metastable phase compared with the equilibrium Ni 3 Ti precipitate, it is quite stable at temperatures below 600 ºC and under normal ageing condition only Ni 4 Ti 3 is observed. The ageing temperature and time dependence comes from the evolution of the density and size of Ni 4 Ti 3 .Recently, it was proposed an innovative process to produce NiTi SMAs using mechanical activation of an elemental powder mixture followed by hot extrusion, called MARES (mechanically activated reactive extrusion synthesis) [3][4]. Homogenization treatment at 950 ºC/ 24 h, in argon, followed by water quenching lead to the formation of a microstructure consisting of a NiTi matrix, with a composition range of 55 -56 at.% Ni, and a relatively uniform dispersion of Ti 2 Ni [4].…”

mentioning

confidence: 99%

“…Homogenization treatment at 950 ºC/ 24 h, in argon, followed by water quenching lead to the formation of a microstructure consisting of a NiTi matrix, with a composition range of 55 -56 at.% Ni, and a relatively uniform dispersion of Ti 2 Ni [4]. To adjust the NiTi matrix composition, ageing treatments were carried out on the homogenized samples.…”

mentioning

confidence: 99%

Ni4Ti3 Precipitation during Ageing of MARES NiTi Shape Memory Alloys Studied by FEG-SEM

et al. 2008

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The use of a high-brightness field-emission gun (FEG) in scanning electron microscopy (SEM) is a powerful technique to examine microstructures at very high spatial resolution down to nanometer level and has significantly enhanced our ability to solve challenging materials problems, allowing studies of nanoprecipitates.Martensitic transformation determines almost all important properties of NiTi shape memory alloys (SMAs), including shape memory effect. Moreover, any compositional inhomogeneities in the matrix of NiTi SMAs affect the martensite start temperature, Ms, because it depends on the concentration of Ni in the matrix: for Ni content exceeding 51.5 at.% Ms is lower than -200 ºC [1]. There is a very good way to adjust the transformation temperature of NiTi alloys for Ni-rich alloys even after the alloys have been made. It is possible to use ageing treatment to rectify the transformation temperature due to incorrect initial composition of Ni-rich NiTi SMAs. The principle behind this method is the (metastable)-equilibrium between TiNi and Ni 4 Ti 3 precipitates since the formation of those Ni-rich precipitates affects the composition of the retained matrix [1-2]. Although Ni 4 Ti 3 is considered as a metastable phase compared with the equilibrium Ni 3 Ti precipitate, it is quite stable at temperatures below 600 ºC and under normal ageing condition only Ni 4 Ti 3 is observed. The ageing temperature and time dependence comes from the evolution of the density and size of Ni 4 Ti 3 .Recently, it was proposed an innovative process to produce NiTi SMAs using mechanical activation of an elemental powder mixture followed by hot extrusion, called MARES (mechanically activated reactive extrusion synthesis) [3][4]. Homogenization treatment at 950 ºC/ 24 h, in argon, followed by water quenching lead to the formation of a microstructure consisting of a NiTi matrix, with a composition range of 55 -56 at.% Ni, and a relatively uniform dispersion of Ti 2 Ni [4]. To adjust the NiTi matrix composition, ageing treatments were carried out on the homogenized samples.In order to contribute to a better understanding of precipitation processes in NiTi SMAs, the present work is an to attempt to measure the concentration variations at the precipitate-matrix interfaces by using a Philips XL30 FEG-SEM equipped with a backscattered electron (BSE) detector and an integrated EDAX energy dispersive X-ray microanalysis system. Samples with 2 mm thickness were cut from the 6 mm diameter Ni-50Ti (at.%) MARES rod and specifically heat-treated to form the Ni 4 Ti 3 precipitates: after homogenization treatment the samples were aged in argon at 400 ºC and 500 ºC during 7 and 48 h. Subsequently the samples were water quenched or furnace cooled.The microstructural observations of the aged samples using BSE (figure 1 and figure 2) were able to resolve two levels of contrast in the matrix: light gray areas, corresponding to the lenticular precipitates, and a medium gray area. These two levels of contrast are related to the variation in the backscatte...

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