Wim Tirry

Linking a completely three-dimensional nanostrain to a structural transformation eigenstrain

Ni-Ti is one of the most popular shape-memory alloys, a phenomenon resulting from a martensitic transformation. Commercial Ni-Ti-based alloys are often thermally treated to contain Ni(4)Ti(3) precipitates. The presence of these precipitates can introduce an extra transformation step related to the so-called R-phase. It is believed that the strain field surrounding the precipitates, caused by the matrix-precipitate lattice mismatch, lies at the origin of this intermediate transformation step. Atomic-resolution transmission electron microscopy in combination with geometrical phase analysis is used to measure the elastic strain field surrounding these precipitates. By combining measurements from two different crystallographic directions, the three-dimensional strain matrix is determined from two-dimensional measurements. Comparison of the measured strain matrix to the eigenstrain of the R-phase shows that both are very similar and that the introduction of the R-phase might indeed compensate the elastic strain introduced by the precipitate.

High-quality sample preparation by low kV FIB thinning for analytical TEM measurements.

Focused ion beam specimen preparation has been used for NiTi samples and SrTiO3/SrRuO3 multilayers with prevention of surface amorphization and Ga implantation by a 2-kV cleaning procedure. Transmission electron microscopy techniques show that the samples are of high quality with a controlled thickness over large scales. Furthermore, preferential thinning effects in multicompounds are avoided, which is important when analytical transmission electron microscopy measurements need to be interpreted in a quantitative manner. The results are compared to similar measurements acquired for samples obtained using conventional preparation techniques such as electropolishing for alloys and ion milling for oxides.

Microstructure of adiabatic shear bands in Ti6Al4V

Abstract Microstructural deformation mechanisms in adiabatic shear bands in Ti6Al4V are studied using traditional TEM and selected area diffraction, and more advanced microstructural characterisation techniques such as energy dispersive X-ray spectroscopy, high angle annular dark field STEM and conical dark field TEM. The shear bands under investigation are induced in Ti6Al4V samples by high strain rate compression of cylindrical and hat-shaped specimens in a split Hopkinson pressure bar setup. Samples from experiments interrupted at different levels of deformation are used to study the evolution of the microstructure in and nearby the shear bands. From the early stages of adiabatic shear band formation, TEM revealed strongly elongated equiaxed grains in the shear band. These band-like grains become narrower towards the centre of the band and start to fraction even further along their elongated direction to finally result in a nano-crystalline region in the core. In fully developed shear bands, twins and a needle-like martensite morphology are observed near the shear band.

Electron-diffraction structure refinement of Ni4Ti3 precipitates in Ni52Ti48

The atomic coordinates of the crystal structure of nanoscale Ni4Ti3 precipitates in Ni-rich NiTi is refined by means of a least-squares method based on intensity measures of electron-diffraction patterns. The optimization is performed in combination with density functional theory calculations and has yielded an R\bar 3 symmetry with slightly different atomic positions when compared with the existing structure. The new unit cell offers a better understanding of the lattice deformation from the B2 matrix.

Optimization of a FIB/SEM slice-and-view study of the 3D distribution of Ni4Ti3 precipitates in Ni–Ti

The 3D morphology and distribution of lenticular Ni4Ti3 precipitates in the austenitic B2 matrix of a binary Ni51Ti49 alloy has been investigated by a slice‐and‐view procedure in a dual‐beam focused ion beam/scanning electron microscope system. Due to the weak contrast of the precipitates, proper imaging conditions need to be selected first to allow for semi‐automated image treatment. Knowledgeable imaging is further needed to ensure that all variants of the precipitates are observed with equal probability, regardless of sample orientation. Finally, a volume ratio of 10.2% for the Ni4Ti3 precipitates could be calculated, summed over all variants, which yields a net composition of Ni50.27Ti49.73 for the matrix, leading to an increase of 125 degrees for the martensitic start temperature. Also, the expected relative orientation of the different variants of the precipitates could be confirmed.

3D Reconstruction of Ni4Ti3 Precipitates in a Ni51Ti49 Alloy in a FIB/SEM Dual-Beam System

Ni4Ti3 precipitates play an important role in the shape memory and superelastic behaviour of thermo-mechanically treated Ni-Ti material. The 3D morphology and distribution of such precipitates with lenticular shape and rhombohedral atomic structure in the austenitic B2 matrix of a binary Ni-rich Ni-Ti alloy has been elucidated via a slice & view procedure in a Dual-Beam FIB/SEM system. With the sequence of cross-section SE images obtained from the SEM, a 3D reconstruction has been achieved after proper alignment and image processing, from which both qualitative and quantitative analysis can be performed. Careful imaging is needed to ensure that all variants of the precipitates are observed with equal probability, regardless sample orientation. Moreover, due to the weak contrast of the precipitates, proper imaging conditions need to be selected to allow for semi-automated image treatment. Finally, a volume ratio of 10.2% for the Ni4Ti3 precipitates could be calculated, summed over all variants, which yields a net composition of Ni50.36Ti49.64 for the matrix, leading to an increase of 113 degrees for the martensitic start temperature Ms. Also, the expected relative orientation of the different variants of the precipitates could be confirmed. In the near future, other quantitative measures on the distribution of the precipitates can be expected.

Characterization and Modeling of Twinning in a Titanium Alloy Ti-6Al-4V

The twinning behavior of a commercial Ti-6Al-4V alloy is studied using a combined experimental and numerical approach. An extensive microstructural investigation was performed to identify and quantify the active twin systems. The mechanical behavior as a function of initial texture and strain rate was then modeled using a visco-plastic self-consistent crystal plasticity code (VPSC7). Earlier obtained quasi-static and dynamic data served to fit the parameters of the model, giving good agreement. However, even if the model gave qualitatively good predictions of the stress-strain curves and the texture evolution for the different loadings, the calculated twin fractions differed considerably of the experimental results.

Comparison of Experimental and Numerical Results for the Quasi-Static and Dynamic Tensile Behaviour of a Commercial Ti6Al4V Alloy as a Function of Initial Crystallographic Texture

Quasi-static tensile tests and high strain rate split Hopkinson bar tensile experiments were used to mechanically characterize a commercially available Ti6Al4V alloy. The material was tested along different directions as to assess the influence of the initial crystallographic texture on the mechanical behaviour. In order to circumvent the problem of the limited thickness of the base material, special usage was made of the Linear Friction Welding technique. The results of the experiments were afterwards compared to the numerical results of a crystal plasticity code based on the viscoplastic self-consistent approach, taking into account twinning as an active deformation mode.

Advanced TEM and SEM Methods Applied to 3D Nano- and Microstructural Investigations of Ni4Ti3 Precipitates in Ni-Ti (SMA)

Two different kinds of experimental approaches yielding three-dimensional structural information on metastable semi-coherent precipitates are demonstrated. By combining high-resolution images from two independent viewing directions a full description of the strain field surrounding a nano-sized Ni4Ti3 precipitate in Ni-Ti can be obtained. The principal axes and strains correlate well with the transformation strain of the observed R-phase transformation close to the precipitate. Using a slice-and-view procedure in a FIB/SEM dual-beam instrument, a three-dimensional voxel dataset is produced from which morphological and distributional information on the same precipitates can be obtained yielding new insight into the particular transformation paths of these alloys, relevant for their functional behaviour.

3D Reconstruction of Ni4Ti3 Precipitates in Ni-Ti by FIB/SEM Slice-and-View

Ni4Ti3 precipitates with lenticular shape and rhombohedral atomic structure growing in the austenitic B2 matrix of binary Ni-rich Ni-Ti alloys upon proper annealing treatment have an important influence on the shape memory effect which originates from the martensitic transformation of B2 — B19’ [1]. Figure 1 shows a typical 2D distribution of such precipitates as obtained by TEM under conventional imaging conditions.