Creator:K.E. Horton and R.L. Carter Date Created:August 28, 1956 Place Created:Canoga Park, California Keywords:radiation-induced hardness in graphite Context:article reprinted from Nuclear Science and Enigneering ************************************************** Reprinted from Nuclear Science and Engineering, Vol. 2, No. 4, July 1957 Academic Press Inc. Printed in U.S.A. nuclear science and engineering: 2, 513-515 (1957) Radiation-Induced Hardness Changes in Graphite K. E. Horton and R. L. Carter ,1 lomics International, Division of North American Aviation, Inc., Canoga Park, California Received August 28, 1956 Changes in Knoop Hardness of artificial graphite from the range 9 to 12 to the range 30 to 75 are found to accompany fast neutron irradiation at 30°C. The irradiation of graphite is accompanied by large changes in its electrical, thermal, and mechanical properties (/). The magnitude of most of these has been measured quantitatively, and in many cases has been at least qualitatively correlated with a microscopic physical model of radiation damage (2, 3). To the writers' knowledge, no quantitative reports of the large changes in hardness which accompany irradiation have been made, although these are well known to laboratory mechanics who have prepared experimental samples of irradiated • graphite. Quantitative measurements of graphite hardness have been made on J microscopic scale with a Tukon Microhardness Tester. This machine was hosen in order to permit hardness readings to be taken over an area com- * prised of a small number of grains. Initially, microhardness measurements were made upon large crystallite graphite obtained by the cooling of a supersaturated solution of carbon in molten nickel to room temperature. The nickel was etched away leaving graphite particles of large size. Samples were mounted by "floating" the platelets on a. redissolved surface of acrylic mounting material.2 Microhardness readings obtained upon these samples were erratic due to the fact that graphite platelets w'ithin the grains were not parallel to the mount surface. .•* Better reproducibility was obtained with finer grained graphite. Extruded .artificial graphite (AGOT-KC) was sectioned, mounted, and polished. Similar samples of neutron-irradiated AGOT-KC graphite were also prepared. As certain property changes induced by irradiation in graphite are known to an- ^ 1 This work was performed under the auspices of the U. S. Atomic Energy Commission. •t 2 Material used was Acrylic Denture Powder mixed with Fast Repair Liquid manufactured by Wm. Getz Corporation, Chicago, Illinois. 513 514 HORTON AND CARTER neal out at temperatures near 100°C, the curing temperature of the mounting material was kept below 100°C. The Knoop Hardness Numbers of the specimens measured are shown in Table I. There were 16 to 30 hardness readings taken on each specimen. One-half of the number of readings on each specimen were taken with the indentor moving in a direction parallel to that on which the graphite had been extruded, the other half, in a direction perpendicular to the extrusion axis. From the table it may be seen that the irradiated specimens have a larger spread between high and low values than the unirradiated specimens at a low (50 g) indentor load. This is a result of the exponential curve relating Knoop Hardness to the length of the indentations on each of the various specimens. The latter varied only by approximately 1 Filar unit under a 10.25 lens. To simulate the reproducibility of macrohardness readings the Tukon Tester was also used with 100 and 300 g loads. As seen by the Knoop Hardness Numbers under the appropriate headings in the table the spread between high and low values is less than with a 50 g load. The data in the table show a definite increase in hardness in irradiated graphite specimens, due presumably to irradiation. However, the small difference in hardness between specimens irradiated to 5 X 1019 nvt and to 1.5 X 1020 nvt (146 Mwd/T and 460 Mwd/ T, respectively, in units of reference 1) suggests strongly that, radiation effects on graphite hardness have saturated at some lower irradiation level. During earlier work, in which thermal setting mounting plastics were used, it was observed that most of the hardness change had an- TABLE I Knoop Hardness Numbers of Graphite Samples Measured parallel to Measured perpendicular extrusion axis to extrusion axis 50 g 100 g 300 g 50 g 100 g 300 g Unirradiated (KC) Average 11 12 12 11 10 12 High 13 15 14 13 14 15 Low 9 10 11 9 8 9 Irradiated 5 X 1019 nvt (fast) at 30°C Average 40 41 43 37 34 38 (Kc Sample 207) High 66 72 58 53 45 40 Low 31 19 38 29 23 25 Irradiated 1.5 X 1020 nvt (fast) at 30°C Average 50 47 58 71 68 44 (KC Sample 202) High 81 65 67 81 81 58 Low 37 33 49 37 48 35 Irradiated 1.5 X 1020 nvt (fast) at 30°C Average 44 42 47 40 43 49 (KC Sample 293) High 77 59 74 71 72 70 Low 29 35 37 29 29 40 radiation hardening of graphite 515 nealed out during a one-hour 100°C oven treatment. While the detailed mechanism relating radiation induced hardness increases to microscopic changes in the graphite remains to be propounded, the observed trends are similar to those reported for other properties of graphite after neutron irradiation. REFERENCES /. W. K. Woods, L. P. Bupp, and J. F. Fletcher, Geneva Conference Paper No. 740 (1955). 2. G. R. Hennig and J. E. Hove, Geneva Conference Paper No. 751 (1955). S. G. Montet, G. R. Hennig, and A. Kurs, Nuclear Sci. and Eng. 1, 33-52 (1956).