Proton radiography peers into metal solidification

Abstract

Historically, metals are cut up and polished to see the structure and to infer how processing influences the evolution. We can now peer into a metal during processing without destroying it using proton radiography. Understanding the link between processing and structure is important because structure profoundly affects the properties of engineering materials. Synchrotron x-ray radiography has enabled real-time glimpses into metal solidification. However, x-ray energies favor the examination of small volumes and low density metals. Here we use high energy proton radiography for the first time to image a large metal volume (>10,000 mm(3)) during melting and solidification. We also show complementary x-ray results from a small volume (<1 mm(3)), bridging four orders of magnitude. Real-time imaging will enable efficient process development and the control of structure evolution to make materials with intended properties; it will also permit the development of experimentally informed, predictive structure and process models.

Figure 1. Equilibrium phase diagram for the Al-In alloy system that undergoes the monotectic reaction L → Al_S + L₂ (denoted in blue) at the temperature 636.5 °C. Two immiscible liquids, L₁ + L₂ (denoted in red), co-exist over a range of temperatures and compositions.

The nominal alloy composition, Al-10 at.% In, studied here is highlighted by the green dashed line.

Figure 2. Selected images from the first ever proton radiography sequence of melting (upper images) and solidification (lower images) in a 6 mm thick Al-10 at.% In section.

The In-rich L₂ liquid phase appears dark. The monotectic reaction is observed during solidification because macro-scale sedimentation of the denser In-rich L₂ liquid phase occurs during melting. The corresponding video (Supplementary Information Video S1) shows the melting and solidification progression.

Figure 3. (a) A proton radiography image (Figure 2) of the solidification structure and (b,c) corresponding post-mortem scanning electron microscopy of the indicated regions that highlight the In-rich boundaries and regions that exist within the colonies of the solidified structure.

In (a), the higher density In-rich boundaries appear dark, whereas the In-rich boundaries and regions appear bright in (b,c). The In-rich boundaries and regions in (b) and (c) contain a fine-scale Al-rich (dark) phase.

Figure 4. Selected images from a synchrotron x-ray radiography sequence of solidification in a 0.2 mm thick Al-10 at.% In section, highlighting In-rich L₂ liquid phase (dark) along the solid-liquid interface and In-rich L₂ channel development.

In addition to the solidification progression, the corresponding video (Supplementary Information Video S2) reveals In-rich droplet motion and coarsening within the majority L₁ liquid phase.