Microstructure formation in rapidly solidified eutectic Nb-59.5 at % Al alloys

Abstract

The wide variety of parameters, such as melt undercooling, growth velocity, temperature gradient ahead of the solid-liquid interface and post-solidification cooling rate render the prediction of microstructures in rapid solidification processes difficult [1]. For various eutectic alloys such as Ag-Cu [2–4], Ni-Sn [5–7], Co-Sb [8], Co-Sn [9], Co-Mo [10], a transition from lamellar eutectic microstructure to an anomalous morphology beyond a critical undercooling level in containerless solidification experiments has been reported. The transition occurred in rapidly solidified Al-Cu samples, also [11]. There is still some controversy about the mechanism of this change of microstructure. In a previous paper [12] the solidification behaviour of bulk undercooled melts of eutectic and hypoeutectic Nb-Al alloys was revealed by containerless electromagnetic levitation experiments. Evidence of an intermediate metastable tetragonal o -Nb2Al phase beyond the critical undercooling level of ,135 K was given by in situ observations of the solidification process and position resolved X-ray diffraction patterns of the quenched samples [12]. The analysis of solidification routes suggests that the anomalous eutectic morphology results from the decomposition of the metastable phase on postsolidification cooling. The aim of the present letter is a comparison of the predictions from containerless solidification experiments with microstructures of eutectic Nb-Al alloys subjected to different rapid solidification methods. One major difference between solidification of bulk undercooled melts and rapidly solidified samples is the post-solidification cooling rate, which can exceed _ T . 104 K sy1 for small sample dimensions. The eutectic Nb-59.5 at % Al master alloy was prepared from 99.999% pure Nb and Al elements. This choice of eutectic composition, instead of Nb55 at % Al given in [13], is based on a re-assessed phase diagram derived from recent thermodynamic data [14, 15]. Melt portions of 1.5 g weight were forced by an applied pressure through an orifice of a quartz crucible and gas atomized. The free falling drops, with diameters typically in the range of 50 im to 1 mm, were cooled and solidified in an Hefilled stainless steel drop tube of 6 m length, which enables cooling rates of 103 to 105 K sy1. Furthermore, ribbons of 20 to 40 im thickness and 2 to 6 mm width were melt spun by ejecting the melt through a quartz nozzle onto a rotating copper wheel. The microstructure of as-solidified samples was revealed by scanning electron microscopy (SEM). Electron probe microanalysis was performed in the energy-dispersive mode (EDX). Selected regions of the melt spun ribbons, which displayed a fine microstructure, were subjected to Ar ion beam thinning. The Ar‡ bombardment was carried out towards the substrate-side surface. The foils were investigated by the transmission electron microscope (TEM) CM20 FEG. Structure and composition of microstructure elements were revealed by selected area electron diffraction and EDX. Supplementary X-ray diffraction measurements were accomplished with melt spun ribbons. Although in rapidly solidified specimens regions of different microstructures co-exist, there is an apparent particle size dependence of the microstructure of as-solidified gas atomized Nb-Al droplets. Large particles (diameter d 900 im) display a lamellar two-phase microstructure (Nb2Al ‡ NbAl3) with cellular appearance and some coarsening near the cell boundaries (Fig. 1a). The average lamellar spacing is 0.53 im. The solidification of drops exceeding d . 1 mm cannot be completed on falling. Consequently, splats with co-existing Nb2Al dendrites and residual lamellar eutectic result from the impact on the substrate walls. The microsegregation between the Nb-rich core and the Al-rich surface of dendrites reduced with decreasing particle size. Medium size particles (d 250 im) exhibit rosettes of the anomalous (granular) eutectic microstructure (A) surrounded by NbAl3-streaks (B) (see Fig. 1b). In some regions of medium size particles small volume fractions of a metastable phase were retained. The small diameter fraction of particles (d , 125 im) displayed a preferentially dendritic appearance (Fig. 1c). The dendrites consisted of the metastable supersaturated o -Nb2Al phase. This metastable phase was previously detected in quenched bulk undercooled samples in levitation experiments [12]. There is an Al enrichment in the interdendritic region as compared to the dendrite centre, as proven by EDX investigations. The X-ray diffraction pattern of melt spun ribbons