Binary Ni-Ti alloys have a wide application in industry and medicine due to their shape memory effect and superelasticity properties. These mechanical properties are known to be caused by a martensitic transformation of which the characteristics are strongly dependent on Ni₄Ti₃ precipitates formed during aging.

In this study a Ni-Ti alloy which is quenched immediately after the production and aged at room temperature is investigated. No precipitation is expected to form in the sample, which is confirmed by conventional TEM images. However, the alloy still shows a changing shape memory effect with ageing time at room temperature, which indicates there must exist some small structural changes not visible by conventional TEM. These are also expected from observed diffuse intensities (Fig. 1) arranged around particular geometrical loci in reciprocal space. Different techniques are proposed and used to identify these microdomains. The Cluster Model [1] that assigns the shape of the diffuse reciprocal intensity to that of microdomains is applied to analyze the results. In the present case the diffuse intensity can to a first order be approximated by {111}* reciprocal planes, which can be translated into atomic rows along the [111] crystallographic directions in the cubic Ni-Ti lattice. Such rows of pure Ni are also present in the crystal structure of Ni₄Ti₃ precipitates, as seen in Fig. 2. In other words, the diffuse intensity can be correlated with contiguous strings of Ni atoms in the cubic directions of the B2 matrix, which normally reveals a …-Ni-Ti-Ni-Ti-… sequence along these directions. In order to observe such atomic strings in real space, aberration corrected HAADF-STEM has been performed along a cubic direction. Simulations indicate that the clustering of heavier Ni atoms can be seen as an increment of appr. 2% of intensity of a single atomic column due to the Z-contrast nature of the HAADF-STEM imaging concept. The experimental image shown in Fig. 3a indeed shows some random but coagulated fluctuations in intensity of the columns, as can be seen from the line trace in Fig. 3b (as well as by slightly defocusing your eyes when looking at the picture). However, to what extend these can be attributed to the atomic clustering is still not clear. In the near future, also other advanced TEM techniques will be applied in order to further identify the columns containing contiguous strings of Ni atoms.

[1] D. van Dyck, R. de Ridder, G. van Tendeloo, and S. Amelinckx, “A cluster model for the transition state and its study by means of electron diffraction. III. Generalisations of the theory and relation to the SRO parameters,” Phys. Status Solidi A, vol. 43, no. 2, pp. 541–552, Oct. 1977.

Figures:

Fig. 1: Diffuse intensities along different directions in the diffraction pattern of a quenched Ni-Ti alloy

Fig. 2: Ni4Ti3 crystal structure revealing pure Ni atoms along the [001]4:3 direction.

Fig. 3: (a) <111> HAADF STEM image revealing some intensity fluctuations. (b) Line profile along the indicated line, revealing differences of 10% in intensity.

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EMC Abstracts - https://emc-proceedings.com/abstract/short-range-order-sro-in-quenched-ni-rich-ni-ti-alloys/