Unveiling Nanoscale Coherent Precipitates and their Strain Fields in NiTiHf-based Shape Memory Alloys Using 4D-STEM

NiTiHf-based shape memory alloys (SMAs) have exceptional shape memory and pseudoelastic properties that make them ideal for use in high-temperature applications. With their impressive mechanical properties (e.g., high yield strength), corrosion resistance, and light weight, NiTiHf-based SMAs are a prime candidate for a variety of applications in the aerospace, biomedical, and automotive industries [1]. Compared to conventional binary NiTi SMAs, alloys containing Hf exhibit higher transformation temperatures and improved structural stability during thermal and mechanical loading due to the formation of coherent precipitates in the parent matrix via precipitate strengthening. To further increase the work output and cyclic lifetime of NiTiHf-based SMAs, precipitate strengthening with both primary and secondary phases in the matrix has been predicted to raise the critical shear stress for slip, thereby further improving the mechanical, shape memory, and pseudoelastic properties of the material system [2]. Previous research has shown that the addition of Al to the NiTiHf system will precipitate a coherent Ni2TiAl Heusler precipitate in addition to the Hf-rich precipitate (Han phase) and provide an increase in mechanical strength [3]. However, Al significantly lowers the transformation temperature. Due to the small size (typically < 10 nm) of the Heusler precipitates and the limited contrast in conventional electron microscopy imaging modes, there is a lack of experimental evidence that quantify the characteristics (e.g., strain and orientation) of the Heusler precipitate and elucidating its role in modifying the mechanical and transformation behavior of these SMAs. To address this challenge, this work employs 4D scanning transmission electron microscopy (4D-STEM), otherwise known as scanning electron nanobeam diffraction (SEND), to determine the position and orientation of Heusler precipitates, and quantitatively measure the strain fields across multiple coherent precipitate boundaries in NiTiHf-based SMAs fabricated with varying compositions and heat treatments. We investigate NiTiHf-based SMAs with Al additions using an electron microscope pixel array detector (EMPAD) to collect nanobeam electron diffraction (NBED) data in scanning mode resulting in a 4D-STEM dataset. We achieve an electron probe size close to 1–2 nm to obtain diffraction patterns from the isolated Heusler precipitate in NiTiHf-based SMAs. Figure 1(a) shows a high-angle annular dark-field (HAADF) STEM image of the area scanned with 4D-STEM. The red, blue, and orange cross hairs indicate the locations where the individual NBED patterns in Figs. 1(e-g) were taken representing the matrix, Heusler, and Han precipitate, respectively. The labels “P” and “M” indicate Han precipitate and matrix. Shown in Figs. 1(b-d) are schematics of the matrix, Heusler and Han precipitate, respectively. Figures 1(e-g) show individual NBED patterns collected with an EMPAD at the matrix, Heusler precipitate, and Han precipitate, respectively. The blue rings in Figs. 1 (e-g) indicate the diffraction disks used to construct the virtual annular dark-field (ADF) images in Figs. 1(h-j). Virtual masks were placed over the blue disks in each diffraction pattern and the intensity under each mask was summed. Due to the overlap of some disks across different precipitates, the Heusler precipitate cannot be entirely isolated in the ADF reconstruction, this is illustrated in Figs. 1(i,j) where we can see Heusler ADF intensity inside of the known area of the Han precipitate. Figures 1(k,l) show the calculated strain maps in Exx and Eyy, respectively. For both maps, the reference length used was calculated by measuring the d-spacing in the matrix regions, d1¯10 and d002 and with respect to the matrix crystal structure for Exx and Eyy, respectively. The high strain inside of the Han precipitate in Fig 1(l) is indicative of the large lattice parameter compared to the matrix. The dashed green ovals in Fig. 1(l) correspond to the same location in Fig. 1(i) highlighting the increased strain in regions where the Heusler precipitate is located. The locations and magnitude of these localized high strain regions are consistent with previous work done on single precipitate NiTiHf SMA systems [4]. We plan to incorporate unsupervised machine learning methods to improve precision when distinguishing between precipitates allowing for more accurate strain mapping. Using strain analysis, we will discuss the role of strain fields from coherent coprecipitates on the mechanical and shape memory properties of NiTiHf SMAs. (a) HAADF-STEM image near the boundary between the Han precipitate (P) and matrix (M). (b-d) Schematics of the matrix, Heusler and Han precipitate. (e-g) Individual NBED patterns from the 4D-STEM data for the matrix, Heusler and Han precipitate, respectively. The locations where these NBED patterns were collected are marked as red, blue, and orange crosshairs respectively in (a). The disks used for virtual ADF construction are circled in blue. (h-j) Virtual ADF images constructed by integrating the diffraction spots marked with blue circles in (e-g), representing the matrix, Heusler Precipitate, and Han Precipitate. (k) Exx and (l) Eyy strain maps. The dashed green ovals in (l) and (i) highlight Heusler-Han interface regions.