Microstructural variations induced by gravity level during directional solidification of near-eutectic iron-carbon type
- PDF / 3,208,577 Bytes
- 10 Pages / 594 x 774 pts Page_size
- 46 Downloads / 231 Views
I.
INTRODUCTION
PROCESSING of materials in the low-gravity environment of space is emerging as a challenging new field for the materials scientist. Nevertheless, experimental d~,ta are accumulated rather slowly because of the high cost involved and limited access to orbital experimentation facil: ties. This paper presents and interprets some results of experiments in directional solidification of iron-carl'on alloys of the cast iron type during aircraft multiple low-gravity maneuvers. Before detailing the experimental procedure and the results, it is useful to summarize the possible effects of low gravity during the solidification of alloys, and the rationale for the selection of Fe-C alloys for this type of experiment. Perhaps the most important effect of the elimination of gravity during alloy solidification is the disappearance of buoyancy-driven convection and sedimentation. In other words, in a low-gravity (low-g) environment, the gravityinduced thermal and compositional convections are negligible, although some convection may still result from surface tension-driven flow and from the pushing of the liquid by the solidifying phases at the interface. Some of the liquid phase properties also seem to be affected by a low-g environment. It has been suggested that the wetting properties of the substrates are modified, which in turn will change the interface curvature with possible effects on heterogeneous nucleation. ~ Also, it was found that the value of the liquid diffusion coefficient (Dr) is greatly reduced in a low-g environment. 2'3 There are many advantages in using Fe-C alloys in low-g experiments and only one major disadvantage. To start with this negative aspect, one must point out the complexity of the solidification process of these alloys. On the other hand, Fe-C alloys of the cast iron type have industrial relevancy and solidify with a variety of eutectic structures depending on composition and solidification conditions, as follows: continuous lamellar eutectic (austenite-cementite and austenite-lamellar graphite), continuous degenerate eutectic (austenite-compacted graphite), and discontinuous DORU M. STEFANESCU, Professor, and MICHAEL R. FISKE, Graduate Research Assistant, are with the Department of Metallurgical Engineering at The University of Alabama. PETER A. CURRERI is Materials Scientist with NASA/Marshall Space Flight Center, AL 35812. Manuscript submitted September 23, 1985. METALLURGICAL TRANSACTIONS A
(divorced) eutectic (austenite-spheroidal graphite). The eutectic phases in the stable austenite-graphite eutectic have a significant difference in density, and the primary hypereutectic phase (graphite) is much lighter than the liquid. Thus, many buoyancy-driven phenomena are likely to be eliminated by low-g processing of these types of alloys. II.
EXPERIMENTAL METHOD
The basic experimental unit used in this research was a Space Applications Rocket Program (SPAR) prototype automatic directional solidification (ADS) furnace of the Bridgman type, equipped with a platinum rhodium resistanc
Data Loading...
