Compression of semi-solid dendritic Sn-Pb alloys at low strain rates
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I.
INTRODUCTION
IT is only recently that attention has been paid to processing alloys in the semi-solid state. All conventional forms of processing are performed on alloys which are fully solid, as in forging, rolling, e t c . , or fully liquid as in all forms o f casting. Processing alloys in the semi-solid state might prove to be useful as a low-power alternative to forging, or a low-temperature alternative to casting. Additionally, totally new processes such as Rheocasting1-'° and fractional melting and solidification H,12 might arise, resulting from some unique property o f semi-solid materials. A recent study by Suery and Flemings~3 was carried out on the deformation behavior o f ordinary dendritic semi-solid tin-lead alloys, using a parallel-plate apparatus. Bulk segregation o f liquid and solid was observed, and the dendrite structure was seen to break down for strains greater than about 0.4. This work is a continuation of these investigations and is an effort to understand better the parameters affecting the observed segregation and breakdown and to develop a physical understanding o f the deformation o f semi-solid dendritic material.
II.
APPARATUS AND PROCEDURE
Tin-lead alloys ranging in composition from 2 to 23 wt pct lead were examined for their properties in the D. A. PINSKY, formerly with the Massachusetts Institute of Technology Department of Science and Engineering, is now a Materials Engineer with T h e Raytheon Company, Bedford, MA. P.O. CHARREYRON, formerly Visiting Scientist at Massachusetts Institute of Technology, is now with Norton Company, Worcester, MA. M.C. FLEMINGS is Chairman, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Room 8-309, Cambridge, MA 02139. Manuscript submitted August 2 4 , 1982.
METALLURGICAL
TRANSACTIONS B
semi-solid state. All alloy compositions were checked by chemical analysis. The alloys were prepared as in the previous work o f Suery and Flemings,13by being cast in cylindrical, top-risered molds which were well insulated to insure that a dendritic, equiaxed microstructure would result throughout each ingot. The average primary dendrite arm spacing was about 70/zm. The cast rods were subsequently machined into cylindrical samples 1.27 c m (0.500 inch) in diameter and 0.63 c m (0.250 inch) in height for parallelplate experiments and 2.54 c m high and 1.90 c m in diameter for extrusion experiments. As in the study of Suery and Flemings, '3 all samples were subjected to compression by the simple parallel-plate apparatus illustrated in Figure l(a). Four chromel-alumel thermocouples were placed in the plates and their output was recorded by a digital multi-channel recorder. A fifth chromel-alumel thermocouple was used to control the power to the heater via proportional control. The temperatures at the thermocouple locations during a typical heat-up cycle and run are shown in Figure 2. The rate o f heating is minimized near the end o f the heat-up to insure that no appreciable thermal gradients existed during deformation and
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