An Electron Microscope Study of the Featureless Zone Obtained during Rapid Solidification
- PDF / 770,376 Bytes
- 3 Pages / 594 x 774 pts Page_size
- 25 Downloads / 195 Views
at a velocity of 105 mm s -1. The melt depth and the width of the laser melted zone were 0.4 mm and 1.1 mm, respectively. The sample was subsequently sectioned on a plane normal to the beam direction and normal to the surface of the block so that the laser melted zone could be examined in cross-section. Specimens were prepared for examination in the transmission electron microscope by ion beam milling to electron transparency using 5 keV Ar ยง ions. The specimens were cooled to liquid nitrogen temperatures during the milling to minimize the damage introduced and to reduce the heating effect of the ion beam. The samples do not appear to have undergone any observable aging during preparation, i.e., no precipitates were visible in the solution treated region. As all the available plane front zone would have similar heat flow conditions, 4 most of the measurements were restricted to areas near the bottom of the melted zone. Concentration profiles were determined using a combination of energy dispersive X-ray spectroscopy (EDS) and scanning transmission microscopy (STEM). In this way the effects of the spreading of the electron beam as it propagates through the sample (resulting in loss of spatial resolution) are minimized by the use of a thin section of material. The featureless zone is seen in the TEM as a region of slightly different contrast from the solution treated region. An area of high dislocation density occurs across the interface with the unmelted region which supports the previous observations of high strain in this area. 3 A micrograph of this interface is shown in Figure 1. A compositional profile across the interface was determined by taking a series of analysis points along the line marked on the micrograph. The X-ray yield from samples of pure A1 and pure Cu were used as standards to quantify the X-ray data. The results, shown in Figure 2, show a fall in the Cu concentration at the interface with the unmelted region and an increase in Cu concentration toward the featureless zone-cell boundary. The figure also shows an inset of a schematic of part of the A1-Cu phase diagram. Assuming global nonequilibrium but local equilibrium at the transformation interface, the dashed lines in the schematic indicate that a plane front solidifying interface would show an increase in the Cu concentration with distance solidified. The featureless zone is then mostly comprised of the plane front region formed during the solidification and a solute gradient zone formed during the melting process. The solidification process commences after the high solute gradient zone has been established during the melting process and the temperature gradients in the solid and liquid adjust to cause a reversal in the direction of movement of the solid-liquid interface. This naturally happens because the VOLUME 15A, JANUARY1984--29
of a/u - 0 . 0 1 seconds, where u is the velocity of the beam and a is the beam radius) and the region has an immediate adjacent high solute gradient, some attenuation of the composition may be seen, i.e., the
Data Loading...
