D. Schryvers

Advanced three-dimensional electron microscopy techniques in the quest for better structural and functional materials

Abstract After a short review of electron tomography techniques for materials science, this overview will cover some recent results on different shape memory and nanostructured metallic systems obtained by various three-dimensional (3D) electron imaging techniques. In binary Ni–Ti, the 3D morphology and distribution of Ni4Ti3 precipitates are investigated by using FIB/SEM slice-and-view yielding 3D data stacks. Different quantification techniques will be presented including the principal ellipsoid for a given precipitate, shape classification following a Zingg scheme, particle distribution function, distance transform and water penetration. The latter is a novel approach to quantifying the expected matrix transformation in between the precipitates. The different samples investigated include a single crystal annealed with and without compression yielding layered and autocatalytic precipitation, respectively, and a polycrystal revealing different densities and sizes of the precipitates resulting in a multistage transformation process. Electron tomography was used to understand the interaction between focused ion beam-induced Frank loops and long dislocation structures in nanobeams of Al exhibiting special mechanical behaviour measured by on-chip deposition. Atomic resolution electron tomography is demonstrated on Ag nanoparticles in an Al matrix.

On the interpretation of high resolution electron microscopy images of premartensitic microstuctures in the Ni-Al β2 Phase

The premartensitic microstructures of quenched Ni-Al β2 (B2) phase have been examined at room temperature by high resolution electron microscopy. The alloy of Ni63Al37 (which transforms to a close-packed 7R martensite at Ms ≅ 200 K) exhibits a fine-scale mosaic assembly of non-uniformly distorted and micromodulated domains. The atomic structure simulated for these domains involves {110} 〈110〉 shear-plus-shuffle displacements. This inhomogeneous microstructure is shown to be directly related to the ensuing transformation deriving from the unusually low energy of the ∑4-TA2 phonon mode of the β2 parent phase, and its anomalous temperature-dependent incomplete softening as T → Ms. Analysis of the microstructural development provides important insights into the nature of heterogeneous nucleation of the martensitic transformation.

Unit cell determination in CuZr martensite by electron microscopy and X-ray diffraction

As several other binary alloy compounds, stoichiometric CuZr has a B2 phase with a CsCl type bcc based structure. In the present system this phase appears as a line compound between 715 C and 935 C. Rapid cooling to below 140 C transforms this phase into at least two monoclinic structures which have been shown to have martensitic characteristics, not unusual for B2 phase alloys, including shape memory behavior. Unit cell dimensions for both monoclinic phases, one about twice the size of the other, were previously suggested on the basis of powder X-ray diffractometry and limited electron microscopy results. The aim of the current investigation was to confirm or correct these unit cells by extensive selected area electron diffraction (SAED), high resolution electron microscopy (HREM) and improved fitting procedures of the existing X-ray diffraction data. The crystallographic relation between parent and product phases will also be discussed.

On the crystal structure of TiNi-Cu martensite

Electron Microscopy for Materials Research (EMAT), University of Antwerp, RUCA,Groenenborgerlaan 171, B-2020 Antwerp, Belgium *Moscow Engineering Physics Institute,Kashirskoe Shosse 31, 115409, Moscow, Russia(Received June 7, 2000)(Accepted July 28, 2000)Keywords: TiNi; TiNi-Cu; Martensitic transformation; Martensite; X-ray diffraction; CrystalstructureIntroductionTiNi-based shape memory alloys attract much attention for their unique properties associated with thereversible martensitic transformation. For a comprehensive understanding of the mechanism of thetransformation and its related macroscopic behaviour, it is important to know the crystal structure of thephases involved. In binary TiNi, the transformation is known to proceed from the parent B2 structureto the martensitic B199 structure. The latter has been interpreted as a monoclinic distortion of the B19orthorhombic structure. After Otsuka and Ren [1], the lattice changes during the B2-B199 transforma-tion can be described as follows (Fig. 1a). A straining of the B2 structure along [001]

Nano-structures at martensite macrotwin interfaces in Ni65Al35

Abstract The atomic configurations at macrotwin interfaces between microtwinned martensite plates in Ni 65 Al 35 material are investigated using transmission electron microscopy. The observed structures are interpreted in view of possible formation mechanisms for these interfaces. A distinction is made between cases in which the microtwins, originating from mutually perpendicular {110} austenite planes, enclose a final angle larger or smaller than 90°. Two different configurations, a crossing and a step type are described. Depending on the actual case, tapering, bending and tip splitting of the smaller microtwin variants are observed. The most reproducible deformations occur in a region of approximately 5–10 nm width around the interface while a variety of structural defects are observed further away from the interface. These structures and deformations are interpreted in terms of the coalescence of two separately nucleated microtwinned martensite plates and the need to accommodate remaining stresses.

Nanoscale inhomogeneities in melt-spun Ni-Al

Abstract Ni x Al 100− x material with x =62.5 or 65 was rapidly quenched to room temperature by the melt-spinning technique and studied using X-ray diffraction, different transmission electron microscopy (TEM) modes and calorimetry measurements. Similar to bulk material, the initial B2 structure undergoes a martensitic transformation to the L1 0 or 14M structure. However, the transformation proceeds very inhomogeneously and results in a mixed microstructure consisting of transformed and untransformed regions. The structure of the transformed regions varies from faulted L1 0 to faulted 14M and shows a variety of morphologies and features like wave-like interfaces and curvature of twin planes. The potential factors responsible for such an inhomogeneous behaviour, i.e. internal stresses, lattice defects, incomplete atomic ordering and compositional variations, are investigated and discussed. Finally, we conclude that the special structural state of the melt-spun material is explained mainly by solute segregation appearing during the crystallisation process. Thus, contrary to most other melt-quenched materials, in Ni–Al, solute segregation cannot be suppressed by the rapid quenching procedure.

Hepatocellular transport and gastrointestinal absorption of lanthanum in chronic renal failure

Lanthanum carbonate is a new phosphate binder that is poorly absorbed from the gastrointestinal tract and eliminated largely by the liver. After oral treatment, we and others had noticed 2-3 fold higher lanthanum levels in the livers of rats with chronic renal failure compared to rats with normal renal function. Here we studied the kinetics and tissue distribution, absorption, and subcellular localization of lanthanum in the liver using transmission electron microscopy, electron energy loss spectrometry, and X-ray fluorescence. We found that in the liver lanthanum was located in lysosomes and in the biliary canal but not in any other cellular organelles. This suggests that lanthanum is transported and eliminated by the liver via a transcellular, endosomal-lysosomal-biliary canicular transport route. Feeding rats with chronic renal failure orally with lanthanum resulted in a doubling of the liver levels compared to rats with normal renal function, but the serum levels were similar in both animal groups. These levels plateaued after 6 weeks at a concentration below 3 microg/g in both groups. When lanthanum was administered intravenously, thereby bypassing the gastrointestinal tract-portal vein pathway, no difference in liver levels was found between rats with and without renal failure. This suggests that there is an increased gastrointestinal permeability or absorption of oral lanthanum in uremia. Lanthanum levels in the brain and heart fluctuated near its detection limit with long-term treatment (20 weeks) having no effect on organ weight, liver enzyme activities, or liver histology. We suggest that the kinetics of lanthanum in the liver are consistent with a transcellular transport pathway, with higher levels in the liver of uremic rats due to higher intestinal absorption.

Measuring the absolute position of EELS ionisation edges in a TEM

Measurements of absolute positions of electron energy loss spectroscopy (EELS) core-loss edges in a transmission electron microscopy (TEM) are hampered by noticeable errors caused by instabilities of the primary energy of the incident electrons. These instabilities originate from a continuous drift and random ripple of the high tension and are unavoidable in the present generation of TEM and scanning TEM microscopes. However, more precise measurements are desired, for instance, to study the shift of the edge onset between atoms of different valency or chemical environment, the so-called chemical shift. A solution to this problem is presented by collecting a series of short low-loss acquisitions immediately followed by core-loss ones. To ensure a minimal time lapse between core-loss and low-loss acquisitions, all operations must be computer controlled. Accumulation of a number of acquisitions and their summation corrected for energy drift allows to cancel the energy instabilities and to relate the core-loss EELS spectra to the absolute energy scale. A practical algorithm is presented as well as the necessary calibrations for such a procedure. Also, examples of spectra collected using this principle and the resulting measured chemical shifts in several metal-oxides are presented.

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.

MICROSTRUCTURE AND FUNCTIONAL PROPERTY CHANGES IN THIN Ni-Ti WIRES HEAT TREATED BY ELECTRIC CURRENT — HIGH ENERGY X-RAY AND TEM INVESTIGATIONS

High energy synchrotron X-ray diffraction, transmission electron microscopy and mechanical testing were employed to investigate the evolution of microstructure, texture and functional superelastic properties of 0.1 mm thin as drawn Ni–Ti wires subjected to a nonconventional heat treatment by controlled electric current (FTMT-EC method). As drawn Ni–Ti wires were prestrained in tension and exposed to a sequence of short DC power pulses in the millisecond range. The annealing time in the FTMT-EC processing can be very short but the temperature and force could be very high compared to the conventional heat treatment of SMAs. It is shown that the heavily strained, partially amorphous microstructure of the as drawn Ni–Ti wire transforms under the effect of the DC pulse and tensile stress into a wide range of annealed nanosized microstructures depending on the pulse time. The functional superelastic properties and microstructures of the FTMT-EC treated Ni–Ti wire are comparable to those observed in straight annealed wires.