Microstructural Evolution During Laser Resolidification of Fe-25 Atom Percent Ge Alloy
- PDF / 934,128 Bytes
- 12 Pages / 594 x 792 pts Page_size
- 22 Downloads / 181 Views
Microstructural Evolution During Laser Resolidification of Fe-25 Atom Percent Ge Alloy KRISHANU BISWAS and KAMANIO CHATTOPADHYAY The microstructural evolution of concentrated alloys is relatively less understood both in terms of experiments as well as theory. Laser resolidification represents a powerful technique to study the solidification behavior under controlled growth conditions. This technique has been utilized in the current study to probe experimentally microstructural selection during rapid solidification of concentrated Fe-25 atom pct Ge alloy. Under the equilibrium solidification condition, the alloy undergoes a peritectic reaction between ordered a2 (B2) and its liquid, leading to the formation of ordered hexagonal intermetallic phase e (DO19). In general, the as-cast microstructure consists of e phase and e–b eutectic and a2 that forms as a result of an incomplete peritectic reaction. With increasing laser scanning velocity, the solidification front undergoes a number of morphological transitions leading to the selection of the microstructure corresponding to metastable a2/b eutectic to a2 dendrite 1 a2/b eutectic to a2 dendrite. The transition velocities as obtained from the experiments are well characterized. The microstructural selection is discussed using competitive growth kinetics.
I. INTRODUCTION
TECHNIQUES of rapidly melting and subsequent cooling of the surface of a material using high-power laser or electron beams offer a unique way of achieving rapid solidification. In the case of laser resolidification, which usually involves solidification at high rates (as high as 18 ms–1[1]), the conditions of equilibrium at the moving solid– liquid interface can no longer be assumed, and thus the formation of metastable phases and microstructures are possible.[2] These non-equilibrium effects make surface treatment processes attractive. Surface treatments using high-energy beams generally overcome three major difficulties, which are encountered in interpreting the rapid solidification microstructures resulting from other techniques, such as melt spinning or atomization. First, the liquid pool that is created during this process is contained by its own solid and generally no nucleation is involved. The microstructural evolution, therefore, can be treated purely as a growth phenomenon.[3] Second, it is possible to measure local growth velocity directly and correlate it with the corresponding microstructure. This is an important aspect because of the fact that growth velocity of the solid–liquid interface is found to be the single most important variable controlling microstructural selection during the process of laser resolidification. Third, the intimate contact of the surface-melted layer with the substrate makes it easier to compare the results with numerical heat flow models.[4] Because microstructural evolution is strongly influenced by the solidification velocity (V), it is imperative to determine KRISHANU BISWAS, Graduate Student, and KAMANIO CHATTOPADHYAY, Professor, are with the Department of Metallurgy, India
Data Loading...
