L.A. Charlot

Amorphous phase formation in irradiated intermetallic compounds

Abstract A variety of intermetallic compounds have been irradiated with high energy ions to determine the criteria for an amorphous transformation. Those compounds with limited compositional range or solubility tend to become amorphous during irradiation, whereas those compounds with wide solubility tend to remain crystalline. This solubility criterion is consistent with the concept that a critical defect density which will result in a greater free energy of the crystal phase than the free energy of the amorphous phase is necessary for the amorphous transformation. This critical defect density is lower in those compounds that become amorphous. The ionicity criterion of Naguib and Kelly does not work for these intermetallic compounds, but the temperature criterion is probably valid and is, in fact, directly related to the proposed solubility criterion.

A method for evaluating radiation-induced grain boundary segregation using ion-irradiated microcrystalline stainless alloys

Abstract A new method to evaluate radiation-induced segregation (RIS) at internal interfaces is described. In this method bulk damage is produced in microcrystalline stainless alloys by ion irradiation, and grain boundary composition changes are measured using analytical electron microscopy. Material composition is controlled and modified by high-rate sputter-deposition synthesis of stainless alloys from composite targets. Nickel-rich 304 and 316 stainless steels doped with phosphorus and silicon were deposited and a microcrystalline grain size established by subsequent heat treatment. Irradiation damage was produced, and irradiation parameters varied, using nickel-ion bombardment from a tandem accelerator. Microstructural and microchemical changes during irradiation were documented by transmission electron microscopy (TEM) examination of near surface regions and by high-resolution composition measurements across grain boundary regions with a scanning TEM. The combination of high-rate sputtering to deposit microcrystalline alloys of controlled composition and heavy-ion irradiation enabled parametric studies of RIS to be performed in a timely and inexpensive manner.

Microstructure of irradiated Ti-70a and Ti-6Al-4V

Abstract Titanium alloys have several properties which make them attractive materials for fusion reactor applications; however, knowledge of their response to radiation is very sparse. Irradiation experiments on two titanium alloys were undertaken to determine if void swelling would occur and, if so, under what conditions and to determine the stability of the α and β phases in Ti-6Al-4V. This paper reports on the results of this study. A commercial purity titanium alloy, Ti-70A, and Ti-6Al-4V have been irradiated to 50 dpa over the temperature range of 400 to 630°C simultaneously with 5 MeV Ni2+ and 300 keV He+. Dislocation loops were induced in Ti-70A at 400 to 500°C with very little apparent damage occurring at 630°C. A radiation-induced precipitate was observed in Ti-6Al-4V at all temperatures as was a dislocation network. The radiation-induced phase was indexed as having a bcc crystal structure with a lattice parameter close to that of Ti-V β phase. A habit plane relationship of (011)bcc” (0001)hcp was observed. Voids were not observed in the Ti-70A or the Ti-6Al-4V at any temperatures. This was true of both the α and β phase of the Ti-6Al-4V. Voids or cavities have been reported by Ayrault and Nolfi [3] in Ti-6Al-4V irradiated at 540 to 660°C. Comparisons between the present work and their work are made.

14 MeV Neutron damage in molybdenum

Abstract The radiation damage in molybdenum produced by 14 MeV neutrons has been compared with that produced by fission reactor neutrons. The increased damage level from 14 MeV neutrons compared to fission neutrons as measured by lattice parameter and microstructural changes is much greater than predicted from displaced atom calculations. The damage level measured by electrical resistance changes nearly agrees with the displaced atom calculations. The results are interpreted in terms of an increase in the ratio of clustered defects to free defects after 14 MeV neutron irradiation compared to fission reactor irradiations.

Irradiation induced changes in the grain boundary chemistry of high-manganese low activation martensitic steels

Abstract The effects of neutron irradiation (10 dpa at 638 K, FFTF/MOTA) on solute segregation to the grain boundaries in high-manganese martensitic steels were investigated using Auger electron spectroscopy (AES). The AES spectrum obtained from the grain boundaries in martensitic steels was significantly influenced by neutron irradiation. Neutron irradiation caused a marked increase in the amount of intergranular segregation of manganese in 12%Cr-6%Mn-1%W steel, while no significant increases in sulfur and/or phosphorus segregations were found in the steel. In 9%Cr-2%Mn-1%W steel, a large amount of silicon as well as a small amount of manganese segregation at grain boundaries was induced by the irradiation. The following mechanisms of irradiation induced embrittlement are proposed; (1) neutron irradiation induced intergranular segregation of manganese causes a reduction of grain boundary cohesive force, (2) intergranular segregation of silicon reduces carbon concentration at grain boundaries by a site competition mechanism resulting in the weakening of the grain boundary strength. Another possible explanation involves grain boundary precipitation.

Effect of pulsed irradiation on void swelling in nickel

Abstract This study has compared the void microstructure in nickel induced by a pulsed ion bombardment to that induced by a steady-state irradiation. Pulse cycles of 10 seconds on and 10 seconds off produced no measurable difference in the void growth and swelling in the temperature range 775 to 975 K compared to continuous irradiation at the same instantaneous dose rate. Void annealing during the pulse annealing period was minimal due to the large void sizes which were obtained in these irradiations. Hence no measurable effect of pulsing on void growth was observed.

Effect of helium on void swelling in molybdenum

Abstract Simultaneous irradiation of molybdenum with helium and heavy ions (Ta 3+ ) using a dual beam facility resulted in continued void nucleation in molybdenum to high dose levels, but the added helium had no measurable effect on the void swelling or swelling rate when compared with results for heavy ion irradiation without helium. Pretreatment by neutron irradiation or preinjection with helium resulted in no significant microstructural changes compared to no pretreatment. Also the temperature dependence of swelling was essentially unchanged when helium was added to the irradiation. The lack of a strong helium effect was attributed to the high inherent void nucleation rate in molybdenum. The overall swelling rate was similar to that observed for neutron irradiation and correlated well with the microstructural features that were observed. At the highest temperature and dose (1475 K and 40 dpa), simultaneous helium and heavy ion irradiation did result in a very nonuniform void distribution; thus, helium may have a greater effect on the microstructure at temperatures above those reported here.

Dislocation structures in be12nb after high-temperature deformation

Abstract Dislocation structures and faults in vacuum-hot-pressed Be12Nb have been examined after compressive deformation at 1200[ddot]C. Both perfect and partial dislocations were observed in the deformed polycrystalline material. Most dislocations were found to be partials bounding planar faults in the grains characterized. Slip systems for individual partial dislocations were identified as ½ (101]{121), ½⟨101]{101) and ½⟨100]{011). Several faults were bounded by identical ½⟨101] or ½⟨100] dislocations. Extended planar {101) faults created by emission of a single ½⟨101] partial from grain or twin boundaries were seen in both the vacuum-hot-pressed and deformed conditions. Perfect dislocations in the deformed microstructure were ½⟨101]{101) and ½⟨111]{121), with some evidence for ⟨001]{100] and ⟨101]{101) dislocations. The partial dislocations are related to a possible phase transformation from Be12Nb to Be17Nb2 which may enhance dislocation mobility and promote high-temperature deformation.

Void growth in a pulsed irradiation environment

Abstract The shrinkage of voids in pure nickel during the pulse annealing period was found to be significant only for very small voids. Rapid void growth and coarsening become the dominant mechanism early in the irradiation period at high temperatures. Pulsed irradiation produced increased coarsening which could be accounted for by increased time at temperature compared to steady irradiation. Qualitatively the change in microstructure upon changing irradiation temperature agreed with results from other studies.

Effect of He on the swelling/microstructure produced in Mo by ion bombardment

Abstract Pure molybdenum has been bombarded simultaneously with heavy ions and helium in a dual beam accelerator facility to simulate the helium generation and displacement damage expected in a fusion reactor first wall. Compared to the damage produced by the heavy ions (7 MeV Ta +++ ) alone, simultaneous helium injection results in a high void density that continues to increase with dose. The results suggest continuous void nucleation due to the helium. The swelling during dual beam irradiations is essentially independent of dose at high dose levels. At high doses >40dpa, the helium has little effect on the absolute magnitude of the swelling which is approximately 0.5% with or without helium. Comparisons are made to extrapolated swelling data from fission neutron irradiation studies.