Quantitative investigation of the all round shape memory effect in a
Ni51Ti49
alloy by
TEM
orientation imaging
Authors: Yao Xiayang , Li Yuanyuan , Cao Shanshan , Ma Xiao , Zhang Xin‐ping , Schryvers Dominique 
Published: December 2016

Ni‐Ti shape memory alloys are being investigated for a new application which focuses on the artificial sphincter for human implantation. In this application, the all round shape memory effect (ARSME) [1], which describes the effect of a dramatic and repeatable shape change from the parent (austenite) phase to an opposite shape in the product (martensite) phase, controls the performance of the alloy for its utilization.

According to the Ni‐Ti phase diagram, several precipitates can appear during thermal treatment, with the metastable Ni
4
Ti
3
type being the most relevant one for controlling the SME. These precipitates can be produced by appropriate ageing and normally have a lenticular shape and remain coherent with the matrix if they do not grow too large, leading to small but important strain fields around the precipitates [2]. For the alloy with ARSME, the way of training is by constrained ageing. The different strain state on the outside and inside of the bended alloy ribbon will cause the formation of different variants of the Ni
4
Ti
3
precipitates, and this difference leads to the final result of the ARSME. Although Nishida et al. have already clarified the origin of this effect, their conclusions are mainly based on direct acquisition of images from different conditions and then matching those with the macroscopic performance of the alloy [3]. Obtaining quantitative and statistically relevant data on these precipitates, however, remains a problem due to their small size and the four orientation variants in which they can appear.

In this study, a Ni
51
Ti
49
alloy was fabricated by arc melting from Ni and Ti powders with fast solidification, followed by different sets of treatment which include constrained aging and stress free aging. The ARSME varies when aging conditions are changed in both time and temperature. DSC was used to investigate the thermal behavior and TEM was used for studying the microstructure of the alloys. The new technique of automated crystal orientation and phase mapping (ACOM‐TEM from NanoMEGAS
®
) is used to investigate the precipitates at nanoscale resolution. The electron diffraction spot patterns are collected with an external CCD camera, capturing the diffraction pattern on the small retractable phosphorous screen inside the column, while the sample area of interest is scanned by a nanoprobe electron beam. Local crystallographic orientation and/or phase are identified through an algorithm that compares the recorded electron diffraction spot patterns with pre‐simulated kinematical templates for all possible orientations and/or phases of a number of pre‐selected structures. This technique provides clear orientation and phase distribution information yielding quantitative information on the precipitates' dispersion.

Since this is the first attempt to apply this technique to nanosized Ni
4
Ti
3
precipitates, a parameter optimization was performed before the quantification and based on reliability maps and shape and size measures of detected precipitates when compared to BF TEM images. The best conditions include an incoming beam precession of 0.2°, a spot size 9 and a 30 μm C2 aperture (Tecnai G2 microscope equipped with field emission gun). The template generation for the Ni
4
Ti
3
structure should be performed with an excitation error of 0.2, intensity scale 14 and step count 120 while for the template generated for the Ni‐Ti B2 matrix an excitation error of 0.5 and step count 120 is preferred.

Figure 1 gives one example on one of the stress free samples to show the result of orientation and phase mapping of Ni
4
Ti
3
precipitates in the Ni‐Ti B2 matrix. Figure 2 shows the result of orientation and phase mapping of a stress assisted sample. The precipitate density in figure 1b is calculated to be 17%, while density in figure 2b is increased to 21%. From these images, such quantitative comparisons between different fabrication processes can be obtained, which will lead to better understanding and improvement of the ARSME in Ni‐Ti.