Grain size control of rhenium strip

Pure Re is being used in the SP‐100. To obtain the desired final grain size it is necessary to both control the grain size of the starting Re strip and to avoid excessive grain growth during subsequent fabrication. It was found that the as‐received strip, supplied by commerical vendors, typicall showed a large amount of scatter in its grain size. It was also observed that considerable grain growth often occurred during fabrication. The latter was due to strain induced grain boundary migration. A program was undertaken to develop a procedure to produce Re strip with a small and uniform grain size. In addition pure Re tapered tensile specimens were fabricated and tested to qunatify the effects of the grain boundary migration. Results of the work with Re strip showed that its grain size could be made fine and uniform by following a rolling procedure that employs relatively large reductions between short intermediate anneals. The tapered tensile specimen tests showed that the amount of grain growth due to grain boundary migration increases greatly with higher annealing temperatures. Also the critical strain regime varies inversely with this temperature‐‐ that is the critical strain regime decreases to a lower range of strain values at higher annealing temperatures.