Grain boundary imaging, gallium diffusion and the fracture behavior of Al–Zn Alloy – An in situ study

Abstract

Phase contrast radiology using unmonochromatic synchrotron X-ray successfully imaged the grain boundaries of Al and AlZn alloy without contrast agent. Combining the high penetration of X-ray and the possibility of 3D reconstruction by tomorgraphy or stereography method, this approach can be very used for nondestructive characterization of polycrystalline materials. By examine the images with 3D perspective, we were able locate the observed void-like defects which lies exclusively on the grain boundary and identify their origin from last stage of the rolling process. We studied the Ga Liquid metal diffusion in the AlZn alloy, under different temperature and stress conditions. High resolution images, ∼2 μm, of Ga liquid metal diffusion in AlZn were obtained in real time and diffusion paths alone grain boundaries and surfaces were clearly identified. Embrittled AlZn responses to the tensile stress and fractures in a drastic different manner than the pure AlZn. These results, although very much expected from the known weakening effect of the liquid metal embrittlement demonstrated, however, that this particular radiology method is fully capable of dynamic study in the micrometer scale.

Introduction

Liquid metal embrittlement (LME) belongs to a special class of grain boundary diffusion phenomena which often disintegrates strong materials, such as Al metal and Al alloys, by small amount of liquid metal diffusion [1], [2]. This has been attributed to one of the important causes of materials failure and its dramatic effects have intrigued many materials researchers. Not surprisingly, this phenomenon is very widely investigated – with somewhat limited success. For example, the best known case of LME, Ga in Al, was reported more than a century ago [3], but is not yet fully understood [4], [5], [6]. Our successful application of high resolution real-time phase contrast radiography to investigate the dynamic behavior of this phenomenon not only demonstrated that this approach is suitable to study this type of dynamic processes, but already yields many interesting observations.

In polycrystalline materials, difference between grains and grain boundaries is typically small. To visualize grain boundaries, therefore, usually requires sophisticate sample preparations to differentiate grains and grain boundaries. These preparations generally affect dynamic process such as the grain boundary diffusion in an unpredictable fashion. With large variation in the diffusion rate, the static measurements at best provide partial informations which can lead to erroneous interpretation of the dynamic process.

The small sizes of metal grains, typically micrometers or smaller, calls sub-micrometer resolution for microscopy approaches to investigate grain boundary diffusion. This requirement limits most of the microscopy studies to ex situ and static measurements on specially prepared samples [1], [2], [3], [4].

The high contrast and high lateral and time resolution of the newly developed phase contrast radiology seems to provide a good solution. We performed indeed such experiments on Ga diffusion in the polycrystalline AlZn alloy and report in this article the first microscopic visualization of this phenomenon in a real-time, non-destructive fashion.

X-ray radiography has long been a standard method for non-destructive evaluation in industrial application. The specific approach we introduced in the previous works using unmonochromatic synchrotron radiation is capable of performing real-time measurement – with a single snap shot as fast as few millisecond without any sophisticated image intensifier – without noticeable degradation in resolution and contrast.

Due to the relative high X-ray absorption of Ga, it acts not only as a diffusion agent but also as a contrast dye in our radiography study of the diffusion process. Nevertheless, as we found in this work, radiography with phase contrast alone is sufficient to visualize the grain boundary in the pure Al or AlZn alloy. Therefore, the application is not necessary limited to system with high absorption contrast such as Ga and Al.