Plastic deformation in polycrystalline Nb 3 Sn
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Nb3Sn
L. R. EISENSTATT AND ROGER N. WRIGHT A study of the high temperature plastic deformation of polycrystalline Nb3Sn has been undertaken on hot isostatically pressed material having grain sizes in the 12 to 60/zm range. Through compression testing and load-relaxation testing deformation has been studied over a strain rate range from 10 -6 to 10-2/s and a temperature range from 1150 to 1650 ~ Plastic deformation can be observed in compression at 1400 ~ and above and extensive deformation is possible at 1650 ~ Except for the lowest strain rates at 1650 ~ load-relaxation stress-strain rate relationships are consistent with "power law creep". Analysis of stress-strain rate-temperature relationships projects an activation energy for creep of very roughly 500 kJ/mol. Observations on yield point behavior and fracture mode transition are presented. A comparison to monocrystalline V3Si behavior is made, and the role of the sub-structure during testing is considered. U N D E R ambient conditions, the A15 compounds generally do not exhibit plastic deformation. Thus it is that practical A15 superconductor fabrication techniques are somewhat tedious and do not involve deformation of the A 15 compound, p e r se. Nonetheless, the compounds can be plastically deformed at high homologous temperatures and/or under high hydrostatic pressures, and some potential for deformation processing certainly exists. The general question of the deformation processing of A 15 compounds has recently been reviewed by one of the authors. 1Nb3Sn, V3Si and V3Ga polycrystals have been deformed at room temperature under hydrostatic pressures in the range from 1790 to 6000 MPa. 2,3,4 V3Si single crystals have been plastically deformed at temperatures in the range from 1200 to 1800 ~ 5,6,7and exploratory indentation studies have been undertaken on Nb3Sn. 7 While the micromechanisms of deformation have been given only limited consideration, it seems clear that (100} (010) slip occurs, 5,6and the presence of dislocations with cube direction Burgers vectors has been confirmed,s While slip plane dislocation arrays have been suggested by the etch pitting of deformed V3Si,5 as-grown structures display the bulk of the dislocations assembled in cell boundaries. 9,1~ Of the A15 compounds, Nb3Sn has seen the broadest technological application and yet little information exists concerning its plastic deformation. A number of plastic deformation studies have focused on V3Si because of the relative ease of growing single crystalsJ ,6 Along these lines, the subject of polycrystalline deformation seems to have been neglected as well, even though fine grained structures are required for practical conductors. Lastly the subject of strain rate sensitivity has been given little attention. In this context, the present study of the high temperature plastic deformation of polycrystalline Nb3Sn has been undertaken.
L. R. EISENSTATTis AssociateEngineer,AdvancedEnergy SystemsDivision,WestinghouseElectricCorporation,Pittsburgh,PA 15236. ROGERN. WRIGHTis AssociateProfessor,Mate
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