Predicting microstructure formation during rapid non-equilibrium processing of engineering materials is a fundamental challenge of materials science. Prior to advent of the DTEM we could only simulate these transformations on a computer. We hope to discover the mechanisms of how alloy microstructures evolve during solidification after laser melting by direct and locally resolved observation. Thermodynamics provides for the limiting constraints for the transformations of the materials, but it cannot a-priori predict the pathways the microstructures take as they transition from the liquid to the final solid state.

—Jorg Wiezorek

Dr. Wiezorek expects the research to help validate computer models and determine how composition changes and temperature gradients affect the microstructure. The data will assist in providing a stronger scientific underpinning for establishing relationships between the processing conditions, structure and properties of the alloys obtained by laser processing.

Dynamic time-delay sequences of images recorded with higher spatial resolution during rapid solidification in an Al–4Cu thin-film alloy, showing (a) the transition from incubation to growth, (b) the early stages of columnar growth at the perimeter of the melt pool, (c) columnar growth near the center of the melt pool, and (d) instability at the solid–liquid interface and subsequent growth to the end of solidification. Images labeled Re-solidified were acquired minutes after the rapid solidification experiment. In (a), the red lines are meant to highlight the solid–liquid interface. The red arrows in (b) and (d) indicate the direction of growth. Source: McKeown et al. Click to enlarge.

We are hoping to unravel details of the kinetic pathways taken from the liquid to the final solid structure. This research will help us to refine solidification related manufacturing processes and to identify strategies to optimize how materials perform.

—Jorg Wiezorek

Unlike a traditional transmission electron microscope, DTEM uses lasers to achieve high time resolution, allowing it to record nano-sized transformations in materials on a time scale of nanoseconds to microseconds.

DTEM allows you to see the interface between the solid and liquid during rapid solidification, which is extremely hard to do. We can image this process as it’s moving rapidly, and from that we can measure just how fast it’s going. There’s no other technique to do that.

—Joe McKeown, an LLNL materials scientist

The Dynamic Transmission Electron Microscope allows researchers to see how a chemical reaction, structural deformation or phase transformation takes place with an unprecedented combination of spatial and temporal resolution: nanometers and nanoseconds.

To achieve this level of resolution, researchers at LLNL redesigned the standard electron source and operation procedures for a TEM, to enable a large pulse of electrons (>10⁹ electrons) to be generated by photoemission and then manipulated in the microscope to form high-resolution images. It is the generation and manipulation of this short pulse containing a large number of electrons that enables the transient process being studied to be imaged in a single shot, permitting very fast, irreversible phenomena to be studied in detail for the first time.

The ability to observe and characterize these events leads to a fundamental understanding of properties such as reactivity, stability and strength, and allows researchers to define models that aid in the design of new and improved materials and devices.

McKeown said the data collected from the study could improve predictive capabilities for metal additive manufacturing and validate existing computer models.

We want to develop models across a broad range of velocities. If we didn’t have this technology, you could run these rapid solidification experiments but you wouldn’t know the pathway from point A to point B.

—Joe McKeown

Resources

• Joseph T. McKeown, Kai Zweiacker, Can Liu, Daniel R. Coughlin, Amy J. Clarke J. Kevin Baldwin, John W. Gibbs, John D. Roehling, Seth D. Imhoff, Paul J. Gibbs, Damien Tourret, Jörg M.K. Wiezorek, and Geoffrey H. Campbell (2016) “Time-Resolved In Situ Measurements During Rapid Alloy Solidification: Experimental Insight for Additive Manufacturing” JOM Volume 68, Issue 3, pp 985-999 doi: 10.1007/s11837-015-1793-x

• A Kulovits, JMK Wiezorek, TB LaGrange, BW Reed and GH Campbell (2010) “In Situ Transmission Electron Microscopy of Rapidly Solidifying Aluminum. Microscopy and Microanalysis,” 16 (Suppl. 2), pp 490-491 doi: 10.1017/S1431927610053924