Composition gradients surrounding Ni4Ti3 precipitates in a Ni-rich NiTi alloy studied by EELS, EFTEM and EDX

Zhiqing Yang 1Dominique Schryvers 

1. University of Antwerp, Department of Physics, Antwerp, Belgium

Abstract

NiTi alloys with near-equiatomic composition can exhibit shape memory and superelastic properties resulting from a temperature or stress induced austenite-martensite phase transformation. The behaviour and characteristics of this transformation are strongly influenced by the presence of Ni4Ti3 precipitates in the B2 austenite matrix and which can be obtained by appropriate annealing procedures. The formation of Ni4Ti3 precipitates not only introduces a strain field in the surrounding matrix, it also affects the composition of the retained matrix since the precipitates are enriched in Ni comparing with the original material with a nominal composition of Ni51Ti49. Both the strain field and changes in compsition can affect the phase transformation behaviour. The strain field was recently studied by high-resolution electron microscopy. In the present investigation, nanoprobe electron energy loss spectroscopy (EELS), Energy Filtered TEM (EFTEM) and energy dispersive X-ray (EDX) analysis are used to determine the presence of a possible variation in Ni concentration in close proximity of a Ni4Ti3 precipitate. A Ni depleted zone in the surrounding matrix up to 150 nm away from the matrix-precipitate interface was detected. The size and concentration change of the depletion zone perfectly compensate for the excess Ni in the precipitate. Moreover, since different precipitates are often found in nanoscale proximity, depletion zones from different precipitates can enforce one another in certain regions. These effects, next to the lattice strains

studied earlier, are very important with respect to the potential

martensitic transformation and shape memory behaviour of the matrix.

 

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Presentation: oral at E-MRS Fall Meeting 2005, Symposium C, by Zhiqing Yang
See On-line Journal of E-MRS Fall Meeting 2005

Submitted: 2005-03-25 16:01
Revised:   2009-06-07 00:44