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18.1. IMPRQVING THE RESISTANCE
are also initiating the procurement of 1500-lb coni-
OF MASTEbbQY N TO RADIATION ~ A ~ mercial A melts ~ of. some ~ of the more attractive alloys.
BY COMPOSITI089 MODIFICATIONS
The properties of the modified Hastelloy N in the
unirradiated condition seem very attractive.
1-1. E. McCoy
Strengths are slightly better than standard Hastelloy
N, and fracture ductilities aw about double.
Although Hastelloy N has suitable properties for
long-term use at elevated temperatures, we have
found that the propertiks deteriorate when it is ex-
posed to neutron irradiation. This type of radi-
ation damage manifests itself through a reduction
in the creep-rupture life and the rupture ductility.
This damage is a function of the thermal negtron
dose and is thought to be associated with the
helium that is produced by the 1°B(n,a) transmu-
tation. Hnwcver, the threshold helium content
required. for damage is so low that the property
deterioration cannot be prevented by reducing the
'OB level in the alloy. We have found that slight
modifications to the composition offer considerable
improvement.
Our studies have shown that the normal massive
precipitate, identified ' as M,C, can be eliminated
by reducing the molybdenum level to the 12 to 13%
range. * The strength is not reduced significantly,
and the grain size is more uniform and more easily
controlled. The addition of small amounts of ti-
tanium, zirconium, or hafnium reduces the irradi-.
ation damage problem significantly. Figure 18.1
illustrates the fact that several alloys have been
developed with postirradiation properties that are
superior to those of unirradiated standard Hastelloy
N. \Ne are beginning work to optimize the compo-.
sitions and heat treatments of these alloys. We
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18.2. AGING STUDIES ON TITANIUM-
MODIFIED HASTELB-OY N
C. E. Sessions
Although normal Hastelloy N is not known to
suffer from detrimental aging reactions, the addition
of titanium to increase the resistance to
radiation damage introduces another variable
which might possibly affect the aging tendencies.
Thus, aging studies are in progress to evaluate
the creep and tensile properties of small cominercial
heats of modified Hastelloy N containing
varying amounts of titanium.
Initial tests on heat 66-548, which contains
0.45% Ti and 0.06% (2, have been coinpleted. Specimens
of this alloy were fabricated by two different
procedures aid were then annealed at either 2150
or 2300°F. They were aged for 500 hr at temperatures
of 1200 and 1400OF and tested at 1200OF.
The results of these tests are given in Table 18.1.
These results indicate that there are no deleterious
effects of aging up to 500 hr on the tensile
properties of modified Hastelloy N containing
0.5% Ti. There ate significant changes in the
strength; the changes in the yield stress do not
seem to follow a pattern, but the ultimate strength
is consistently increased by aging. The elonga-
'R. E. Gehlbach and H. E. McCoy, Metals and Cerarnics
Div. Ann. Proar. Reut. lune 30. 1967. ORNL-
I . "-
41 70. tion at fracture is increased by aging in most
LH. E. McCoy and J. R. Weir, Materiafs Developnenf
for Molten Salt Breeder Reactors, OKNL-TM-1854 (June
1967).
cases, remains unchangcd in a few cases, but is
never reduced significantly.
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