In situ studies of precipitate formationin Al-Pb monotectic solidification by X-ray transmission microscopy
- PDF / 166,130 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 108 Downloads / 141 Views
I.
INTRODUCTION
DUE to their inherent complexity, monotectic reactions have been investigated in less detail than the formation of eutectics or solid solutions. In particular, when the density of the second liquid, that forms on solidification, is higher than that of the original melt, sedimentation in the Earth’s gravity field complicates detailed studies of such systems. Hence, there has been considerable interest in microgravity solidification of monotectic alloys.[1–17] Metals are optically opaque. However, some insight into metallic solidification phenomena and their microstructural evolution has been obtained from transparent organic model systems.[14,18–20] But the details of the phase morphologies of the optically transparent systems and their thermophysical and transport properties differ significantly from metallic systems. Thus, there is a need for experimental techniques that allow real time determinations of the dynamics and morphology in metal systems. X-ray transmission (or shadow) microscopy can image concentration gradients in the solid and liquid through differences in absorption. Traditional X-ray sources have been used to examine the homogeneity of thick specimens (of the order of millimeters),[21] to image shrinkage porosity during aluminum solidification,[22] melt-solid interface shape during Bridgman growth of germanium,[23] and convection caused by dissolving gold and silver wires in liquid sodium[24] with a resolution of 3 to 5 3 1024 m. However, the imaging of microstructural features requires resolutions of 1 to 100 3 1026 m. Only recently have X-ray sources and detectors been advanced enough in resolution and contrast to allow system-
atic studies of the relationship between melt dynamics and resulting microstructure. During our development of an X-ray transmission microscope (XTM) for solidification studies, we examined a number of alloys with which we had experience from prior microgravity research projects.[8,9] We have imaged the solidification of alloys in real time with resolutions of up to 3 3 1025 m, employing a state-of-the-art 10 to 100 keV source, featuring a submicron focal spot.[1,2,3] Using solidifying aluminum alloys, we observed, in real time, the formation of the interfacial solute boundary layer in the liquid, interfacial morphologies, droplet coalescence, droplet incorporation, and particle/void engulfment by the advancing interface. Our preliminary studies of monotectic alloy solidification with the XTM revealed various unexpected morphologies. Kaukler and Rosenberger[3] and Curreri and Kaukler[1,2] first demonstrated striation formation in such systems. In the present work, using recent improvements in the image resolution and contrast, we have further quantified striation formation in Al-Pb. The Al-1.5 wt pct Pb monotectic alloy is particularly well suited for X-ray solidification studies due to the good contrast provided by the large difference in X-ray absorptance between the immiscible phases. At lower solidification velocities, we found fine fibers and string
Data Loading...
