J.M.K. Wiezorek

Approaches for ultrafast imaging of transient materials processes in the transmission electron microscope.

The growing field of ultrafast materials science, aimed at exploring short-lived transient processes in materials on the microsecond to femtosecond timescales, has spawned the development of time-resolved, in situ techniques in electron microscopy capable of capturing these events. This article gives a brief overview of two principal approaches that have emerged in the past decade: the stroboscopic ultrafast electron microscope and the nanosecond-time-resolved single-shot instrument. The high time resolution is garnered through the use of advanced pulsed laser systems and a pump-probe experimental platforms using laser-driven photoemission processes to generate time-correlated electron probe pulses synchronized with laser-driven events in the specimen. Each technique has its advantages and limitations and thus is complementary in terms of the materials systems and processes that they can investigate. The stroboscopic approach can achieve atomic resolution and sub-picosecond time resolution for capturing transient events, though it is limited to highly repeatable (>10(6) cycles) materials processes, e.g., optically driven electronic phase transitions that must reset to the materials ground state within the repetition rate of the femtosecond laser. The single-shot approach can explore irreversible events in materials, but the spatial resolution is limited by electron source brightness and electron-electron interactions at nanosecond temporal resolutions and higher. The first part of the article will explain basic operating principles of the stroboscopic approach and briefly review recent applications of this technique. As the authors have pursued the development of the single-shot approach, the latter part of the review discusses its instrumentation design in detail and presents examples of materials science studies and the near-term instrumentation developments of this technique.

Activation of slip in lamellae of α2-Ti3Al in TiAl alloys

The activation of slip at room temperature in lamellae of α2-Ti3Al in a Ti- 48 at.% Al alloy has been studied using transmission electron microscopy. In common with other studies, it appears that slip activity occurs mainly at, or near to, the intersection of certain cross-lamellar twins, formed by deformation in γ-TiAl, with the α2 lamellae. In polycrystalline samples, heat treated to a duplex microstructure with a significant lamellar component, only dislocations with Burgers vectors given by b = 1/3 have been activated, and essentially no dislocations with b = 1/3 , that is c -component dislocations, have been observed after deformation in tension at room temperature. It has been found that, for these samples slip activity in α2-Ti3 Al may occur either by a slip transmission process or by stress-induced activation of sources in the interfaces between the two phases (α2-Ti3Al and γ-TiAl). The resultant defects are superdislocations with Burgers vectors b = 1/3 gliding on prismatic {1100...

Revealing the transient states of rapid solidification in aluminum thin films using ultrafast in situ transmission electron microscopy

Using high time resolution transmission electron microscopy, we have observed rapid solidification dynamics in 80 nm thick Al thin films after pulsed laser melting. The nanometer spatial and 15 nanosecond temporal resolution of the dynamic transmission electron microscope (DTEM) at Lawrence Livermore National Laboratory allowed us to study the morphology and dynamics of the transformation front moving at speed of 0.1–10 m/s during rapid solidification. Additionally, we used an automated orientation imaging system in the TEM for the post-mortem analysis of grain orientations of the solidified microstructure near the position of the solid liquid interface at the start of solidification.

Deformation mechanisms in a binary Ti-48 at.%Al alloy with lamellar microstructure

The deformation mechanisms active in the eta- and alpha-lamellae of a Ti2 48 at.%Al alloy during room-temperature and 800 C tensile loading have been determined by transmission electron microscopy. A marked change in the plastic behaviour of the alpha-phase has been detected between room and elevated temperature, whereas the deformation mechanisms active in the eta-phase remained the same at both temperatures. At room temperature, plasticity in the alpha-phase occurred only locally by prism plane slip of dislocations with Burgers vectors parallel to 1120 where cross-lamellar twins in the eta-plane impinged on the eta alpha-interface. At 800 C more homogeneous plastic behaviour of the alpha/ phase resulted from slip and climb activity of dislocations with Burgers vector components parallel to 1120 and parallel to both 1120 and [0001]. The increased contribution to strain accommodation at 800 C by the alpha-lamellae coincided with a change in predominant fracture mode.

Evolution of microstructure and defect structure in L10-ordered manganese aluminide permanent magnet alloys

Abstract Defects produced in massively transformed L1 0 -ordered τ-MnAl have been characterized by detailed TEM studies. The defect population in massive τ-MnAl comprises arrays of overlapping octahedral stacking faults, {111}-conjugated microtwins, thermal antiphase boundaries and dislocations. The genesis of these defects has been attributed to atomic attachment faulting on {111}- and {020}-type facets of the essentially incoherent growth interface between the parent and product phases. The features of the defect genesis in τ-MnAl are discussed with respect to the role of atomic level processes at solid-state transformation interfaces in general and growth interfaces in massively transforming materials systems in particular.

Simultaneous determination of highly precise Debye-Waller factors and structure factors for chemically ordered NiAl

Accurate Debye-Waller (DW) factors and several low-index structure factors of chemically ordered β-NiAl at different temperatures have been measured using an off-zone-axis multi-beam convergent-beam electron diffraction method. The temperature dependences of DW factors of Ni and Al atoms are compared with previous experimental measurements and theoretical calculations. The temperature below which the DW factor of Ni becomes smaller than that of Al was found to be lower than previously reported. Structure factors are determined with an accuracy of 0.05% and compared with prior reports.

Simultaneous determination of highly precise Debye-Waller factors and multiple structure factors for chemically ordered tetragonal FePd.

Accurate Debye-Waller (DW) factors and low-index structure factors up to 222 of chemically ordered FePd have been measured at 120 K. Ordered FePd has a simple tetragonal unit cell (tP2, P4/mmm) with Fe and Pd atoms at 0, 0, 0 and at ½, ½, ½, respectively, requiring the measurement of four different DW factors. It was possible to simultaneously determine all four DW factors and several low-order structure factors using different, special off-zone-axis multi-beam convergent-beam electron diffraction patterns with high precision and accuracy. The different diffraction conditions exhibit different levels of sensitivity to changes in DW and structure factors. Here the sensitivity of different off-zone-axis convergent-beam electron diffraction patterns with respect to changes in DW factors and structure factors is discussed.

Determination of Debye-Waller factor and structure factors for Si by quantitative convergent-beam electron diffraction using off-axis multi-beam orientations.

Debye-Waller (DW) factors and structure factors have been measured for Si using convergent-beam electron diffraction (CBED) experiments with a transmission electron microscope equipped with a field-emission gun and a post-column energy-filtering device. Si has been used here to evaluate the accuracy of multi-beam near-zone-axis orientations for the simultaneous refinement of DW factors and multiple structure factors. Strong dynamic interactions among different beams are obtained by tilting the crystal to specific four- or six-beam orientations near major zone axes, which provide sufficient sensitivity to determine accurate DW factors and structure factors. The DW factors of Si were measured using four-beam conditions near the [001] zone axis for temperatures ranging from 96 to 300 K. A comparison of the multi-beam near-zone-axis orientations with other CBED methods for DW and structure factor F(g) refinement is presented.

Microstructural evolution of PST-TiAl during low-rate compressive micro-straining at 1023 K in hard and soft orientations

Scanning and transmission electron microscopy (SEM and TEM) have been combined with quantitative image analyses to document systematically the microstructural evolution of polysynthetically twinned Ti-48at.%Al (PST-TiAl) during low-rate compression in hard and soft orientations at 1023 K to micro-strains (0.2%<e < 0.4%). The PST-TiAl contained excess volume fraction of a2-phase after conventional homogenization. The PST-TiAl maintained a lamellar morphology, the excess a2-phase lamellae transformed to g-phase lamellae and both the populations of a2- and g-phase lamellae coarsened during elevated temperature straining for hard and soft orientations. The microstructural changes were accomplished mainly by the instability mechanism of termination migration. Plastic strain accommodating defects assisted in the formation of lamellar terminations. The rates of the microstructural evolution were faster for the hard orientation tests than for the soft orientation tests, which has been attributed to the more frequent interactions between hard deformation modes and the lamellar interfaces in the former. It has been proposed that microstructural changes similar to those reported here are suitable to rationalize the large primary creep strains characteristic of fully lamellar TiAl alloys. # 2003 Elsevier Science Ltd. All rights reserved.

Large lattice strain in individual grains of deformed nanocrystalline Ni

We have used nanobeam electron diffraction to monitor the behavior of individual grains in nanocrystalline Ni during in situ tensile straining in the transmission electron microscope. The diffraction patterns reveal large variations in the magnitude of grain rotation and indicate that grain interiors can experience large lattice distortions (up to 5.5% linear strain). These large distortions may assist grain rearrangement during grain boundary facilitated deformation and are consistent with the behavior expected of nanocrystalline Ni near its predicted peak in strength.