Influence of Gravity Driven Convection on the Directional Solidification of Bi/MnBi Eutectic Composites
- PDF / 1,018,037 Bytes
- 9 Pages / 414.72 x 648 pts Page_size
- 54 Downloads / 253 Views
INFLUENCE OF GRAVITY DRIVEN CONVECTION ON THE DIRECTIONAL SOLIDIFICATION OF Bi/MnBi EUTECTIC COMPOSITES RON G. PIRICH and D.J. LARSON Metals Science Laboratory, Research and Development Center, Grumman Aerospace Corporation, Bethpage, New York, USA ABSTRACT The role of gravity on Bridgman-Stockharger directional solidification of eutectic Bi/MnBi has been studied in reduced gravity aboard NASA soundinq rocket SPAR flight experiments and contrasted with normal gravity investigations. The directional solidification of eutectic Ri/MnBi results in a low volume fraction, faceted/nonfaceted aligned rod eutectic whose MnRi rod size, interrod spacing, thermal and magnetic properties are sensitive functions of solidification processing conditions. The morphology of the low-gravity samples showed striking differences compared with identically processed, normal gravity samples grown in the same apparatus. The MnBi rod diameter and interrod spacing distributions were significantly smaller, approximately 50%, for the low gravity samples compared with identically processed one gravity samples. Accompanying the smaller MnBi rod diameters observed in the flight samples, was an increase in permanent magnet properties which reached greater than 97% of the theoretical maximum. Gravitationally induced thermal instabilities in one-gravity which result in irregular interface movement and associated difficulty of the faceted Mn~i phase to branch are suggested to explain the morphological differences between one and low gravity solidification. INTRODUCTION Considerable effort has focused on controlled directional
solidification of binary eutectics in which the microstructure was related
to conditions of solidification, for example growth rate and imposed thermal gradient [1] - [5]. More recently, convection that occurs when the fluid density depends on temperature and solute concentration has been investigated [6] and [7]. To assess the role of solidification conditions and gravitationally induced convective transport on the crystal growth of eutectic binary alloys, we have investigated the directional solidification of eutectic Bi/MnBi in a low gravity, 10-4 ge, environment and compared these results with normal gravity, one-ge, investigations. The Bi/MnBi eutectic was chosen because its microstructure, when grown by Bridgman-Stockbarger directional solidification, results in an aligned ensemble of faceted rods of highly magnetic MnBi dispersed in a non-faceted Bi terminal solid solution with the structure presumably growing in a regular manner. The morphological, thermal and magnetic properties have been shown [8] to be affected by solidification variables and solidification orientation with respect to the direction of gravity in one-ge [9]. Unfortunately, nearly all eutectic growth models have neglected convection in the fluid and assumed a completely diffusioin controlled solidification process. These models also assume nearly equiatomic volume fractions which make their application to a low volume fraction eutectic like Bi/MnBi som
Data Loading...
