In situ Transmission Electron Microscopy of Rapidly Solidifying Metals and Alloys

Abstract

We used the unprecedent nanometer spatial and 15ns temporal resolution of the dynamical transmission electron microscope DTEM at Lawrence Livermore National Laboratory to study morphological and structural changes in metallic thin films induced by rapid liquid-solid transformations ( Figure 1a) [1]. The unprecedent spatiotemporal resolution of the DTEM allowed us to study phenomena associated with ultrafast solidification processes in pure Al, Cu, Ag and two AlCu alloys. The in situ experiments were performed by first melting the thin metal or alloy films with a single 12ns, 1064nm laser pulse (Figure 1b). After a pre-selected time delay a 15ns laser pulse (at FWHM) illuminated the previously molten area to acquire either a bright field image or a diffraction pattern (Figure 1b). The acquisition of a series of images and accompanying diffraction patterns (DP) with different time delays enables direct observation of morphological and structural changes during the entire solidification process. Figure 1c shows bright field images of an Al thin film area after successful melting and solidification. Elongated grains formed during solidification stretch from the edge to the center of the melt pool. Figure 2 shows a series of images and diffraction patterns for different time delays for an Al thin film. Before the melting, the image of the as-deposited nanocrystalline thin film shows minimal contrast due to the low, ~4000x, magnification and ~ 20 nm grain size. The DP shows a ring pattern characteristic of the nanocrystalline Al film and the diffuse background from the amorphous Si 3 N 4 substrate. After 0.5 µs time delay the contrast variations disappear and the sharp ring pattern in the DP disappeared, indicating that the Al is in the liquid state. After 5 µs a planar solidification front was in the field of view. Discrete diffraction spots associated with elongated grains, formed by rapid lateral solidification, close to a Bragg condition appear in the DP. After solidification the DP showed a reduced diffuse background and distinct diffraction spots instead of diffraction rings, due to a significantly reduced number of grains in the field of view. Figure 3 shows select images of delay time sequences for Ag, Cu, a dilute Al-5at.%Cu and a concentrated Al33at.%Cu alloy. Observing solid liquid interface dynamics during the transformation using snap shot images enabled determination of the solidification front velocity, e.g. ~ 3-4 m/s in the case of the Al thin films. Post mortem TEM studies revealed differences in defect content and structure for Al, Cu and Ag, three FCC metals. . The morphology and structure of the advancing liquid solid interface of two Al-Cu alloys of two different compositions were investigated and compared to the structure of the liquid solid interface of pure Al. Figure 3 shows that the planar solidification front destabilizes as the Cu content increases. Future work will include detailed investigation of the dependence of the interfacial stability on chemical composition of the Al-Cu alloys, in ...

Determination of crystal growth rates during rapid solidification of polycrystalline aluminum by nano-scale spatio-temporal resolution in situ transmission electron microscopy

“…13,14,16,19 Notably here, unlike in prior reports, no capping layers have been deposited on the Al layer. 8,11,20,21 Regions of the TEM grid sample adjacent to the electron transparent region are supported by a substrate of Si. Relative to solid Al, the thermal conductivity of amorphous Si 3 N 4 is about twenty times smaller, while that of solid Si is comparable.…”

“…5,7,8 The dynamic transmission electron microscope (DTEM), a TEM modified by two complex laser systems, offers nano-scale spatiotemporal resolution, rendering it uniquely suited for studies of irreversible transitions in materials, including those associated with RS transformations. [8][9][10][11][12][13][14][15][16] The DTEM uses intense electron pulses of durations as short as 15 ns for the acquisition of images or diffraction patterns. In its original operational mode, the DTEM was limited to acquisition of a single image or diffraction pattern at a preselected delay time after initiation of the irreversible transition event, i.e., a single-pump/single-probe mode.…”

“…17 This is equivalent to a frame rate $10 6 times higher than those of conventional tv-rate in situ TEM experiments. Compared with prior in situ DTEM studies using the single-pump/single-probe operational mode, 8,11,13,14,16,18 the MM-DTEM experiments offer potential to greatly reduce uncertainties in quantitative measurements of transformation front velocity evolution during RS and increase significantly the data density obtained from direct observations of the dynamics of the irreversible transformation event induced by a single laser pulse. 15,17 The current paper reports and discusses first results of MM-DTEM in situ investigations of the dynamics of pulsed-laser-induced melting and RS in 160 nm thick Al thin films with a focus on establishing experimental methodology for reproducible quantitative measurements of the transformation rates.…”

Capturing Dynamics of Pulsed Laser Induced Melting and Rapid Solidification in Aluminum Polycrystals with Nanoscale Temporal Resolution In-situ TEM

“…In prior work we utilized the DTEM for single-shot nano-second temporal resolution bright field imaging and diffraction studies of rapid solidification (RS) in Al, Cu, Ag and Al-Cu alloy thin films [2][3][4][5]. Recent upgrades to the DTEM instrument enable single-pump/multiple-image-acquisition experiments (image series), which hold prospect to greatly reduce quantitative uncertainties in measurements, e.g.…”