METALLURGICAL AND MATERIALS TRANSACTIONS A U.S. GOVERNMENT WORK VOLUME 28A, AUGUST 1997—1705 NOT PROTECTED BY U.S. COPYRIGHT In Situ Studies of Precipitate Formation in Al-Pb Monotectic Solidification by X-Ray Transmission Microscopy WILLIAM F. KAUKLER, FRANZ ROSENBERGER, and PETER A. CURRERI Al-1.5 wt pct Pb monotectic alloys were unidirectionally solidiﬁed. X-ray transmission microscope (XTM) observations, both during and after solidiﬁcation, revealed various new morphologi- cal/compositional features in the melt and solid. In the melt, nonuniform lead-rich interfacial seg- regation layers and droplets were observed to form well ahead of the interface. In the solid, periodic striae formed at translation/solidiﬁcation velocities as low as 6  10 -6 m/s. The striae shape does not replicate that of the interface. The striae spacing decreases from 4 to 2  10 -4 m with an increasing solidiﬁcation rate between 6 and 16  10 -6 m/s. High resolution postsolidiﬁcation XTM examination reveals that these striae consist of Pb-rich particles of 2 to 3  10 -6 m diameter. At translation/solidiﬁcation velocities below 6  10 -6 m/s, Pb incorporation into the solid occurs in the form of continuous ﬁbers and strings of particles of about 5  10 -6 m diameter. Bands, parallel to the interface, in which these ﬁbers were aligned in the solidiﬁcation direction, alternated with bands of poor ﬁber alignment. The width of these bands is comparable to the striae spacings obtained at the high solidiﬁcation rates. 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 thermophys- ical and transport properties differ signiﬁcantly from me- tallic systems. Thus, there is a need for experimental techniques that allow real time determinations of the dy- namics and morphology in metal systems. X-ray transmission (or shadow) microscopy can image concentration gradients in the solid and liquid through dif- ferences 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 con- vection caused by dissolving gold and silver wires in liquid sodium [24] with a resolution of 3 to 5  10 -4 m. However, the imaging of microstructural features requires resolutions of 1 to 100  10 -6 m. Only recently have X-ray sources and detectors been ad- vanced enough in resolution and contrast to allow system- WILLIAM F. KAUKLER, Assistant Research Professor, and FRANZ ROSENBERGER, Professor, are with the Center for Microgravity and Materials Research, University of Alabama in Huntsville, Huntsville, AL 35899. PETER A. CURRERI, Metals and Alloys Group Lead, Space Science Laboratory, NASA, Marshall Space Flight Center, AL 35812. Manuscript submitted October 2, 1996. 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 pro- jects. [8,9] We have imaged the solidiﬁcation of alloys in real time with resolutions of up to 3  10 -5 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 ob- served, in real time, the formation of the interfacial solute boundary layer in the liquid, interfacial morphologies, drop- let coalescence, droplet incorporation, and particle/void en- gulfment 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 res- olution 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 strings of Pb precipitates. Bands, parallel to the interface, in which these ﬁbers were aligned in the solidiﬁcation di- rection, alternated with bands of poor ﬁber alignment. In the following sections, after an outline of the experimental approach used, we will describe these observations in greater detail. II. EXPERIMENT APPARATUS AND SAMPLE PREPARATION The apparatus and details of exposure (X-ray energy and ﬂux) employed for this study have been described in detail elsewhere. [1,2,3] Here, we will only outline the basic concept and essential features. In the projection radiography used, the specimen (in a furnace) is placed between the X-ray