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The stress-rupture properties of two heats of Hastelloy N tubing have been determined at 760°C inand ex-reactor. Irradiation reduced the rupture life and the rupture strain, but no effects on the creep rate were detectable. Small variations in behavior of tubular specimens tested during irradiation and small rod specimens tested after irradiation are explained on the basis of differences in stress states and sizes of test sections. The effects of irradiation are rationalized on the basis of the behavior of helium which occurs in the metal as a result of the thermal 10B(n,a) transformation. INTRODUCTION Numerous cases have been reported where the high-temperature mechan ical properties of nickeland iron-base alloys deteriorated under neutron irradiation.1-9 This deterioration manifests itself as both a reduction •'•D. R. Harries, "Neutron Irradiation Embrittlement of Austenitic Stainless Steels and Nickel Base Alloys," J. Brit. Nucl. Energy Soc. 5, 74 (1966). 2G. H. Broomfield, D. R. Harries, and A. C. Roberts, "Neutron Irra diation Effects in Austenitic Stainless Steels and a Nimonic Alloy," J. Iron Steel Inst. (London) 203, 502 (1965). 3N. A. Hughes and J. Caley, J. Nucl. Mater. 10, 60 (1963). 4F. C. Robertshaw et al., Am. Soc. Testing Mater. Spec. Tech. Publ. 341, 372 (1963). 5N. E. Hinkle, Am. Soc. Testing Mater. Spec. Tech. Publ. 341, 344 (1963). 6P.C.L. Pfeil and D. R. Harries, "Effects of Irradiation in Austenitic Steels and Other High-Temperature Alloys," Am. Soc. Testing Mater. Spec. Tech. Publ. 380, 202 (1965). 7W. R. Martin and J. R. Weir, "The Effect of Irradiation Temperature in the creep-rupture life and in the rupture ductility. This effect has been correlated with the thermal neutron dose and has been attributed to the helium produced by the transmutation of 10B to 7Li and ^He.6'9'10 However, the sensitivity of different materials to helium produced by this process varies greatly1 and our present state of understanding of this problem necessitates that we study each material individually. We have studied Hastelloy N, an alloy developed at Oak Ridge National Laboratory specifically for use with molten fluoride salts.11 It is nickel base, solid solution strengthened with about 16$ Mo, and contains 7$ Cr for moderate oxidation resistance. However, this alloy has become a candidate material for use in several other reactors and we are attempting to learn as much as possible about this material in nuclear environments. Recent studies have shown that the mechanical properties of this material are indeed altered by irradiation.12-'13 In this study we compared the inand ex-reactor properties of two vacuum-melted heats of Hastelloy N tubing at 760°C. The testing techniques used in this program will be described and the resulting test data pre sented. These data will be compared with those obtained for the same material in uniaxial postirradiation stress-rupture tests. on the Post-Irradiation Stress-Strain Behavior of Stainless Steel," Am. Soc. Testing Mater. Spec. Tech. Publ. 380, 251 (1965). 8J. T. Venard and J. R. Weir, "In-Reactor Stress-Rupture Properties of a 20 Cr-25 Ni Columbium-Stabilized Stainless Steel," Am. Soc. Testing Mater. Spec. Tech. Publ. 380, 269 (1965). 9P.C.L. Pfeil, P. J. Barton, D. R. Arkell, Trans. Am. Nucl. Soc. 8, 120 (1965). = 10P.R.B. Higgins and A. C. Roberts, Nature 206, 1249 (1965). 1:LW. D. Manly et al. , "Metallurgical Problems in Molten Fluoride Systems," Progress in Nuclear Energy, 2 (IV), Technology, Engineering, and Safety, pp. 164-79, Pergamon Press (1960). 12W. R. Martin and J. R. Weir, "Effect of Elevated Temperature Irra diation on the Strength and Ductility of the Nickel-Base Alloy, Hastelloy N," Nucl. Appl. 1(2), 160-67 (1965). 13W. R. Martin and J. R. Weir, "Postirradiation Creep and Stress Rupture of Hastelloy N," Nucl. Appl. 3, 167 (1967).

IN- AND EX-REACTOR STRESS-RUPTURE PROPERTIES OF HASTELLOY N TUBING.