J. L. Brimhall

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.

Synthesis of a MoSi2SiC composite in situ using a solid state displacement reaction

Abstract A high strength in-situ composite of MoSi2SiC was synthesized using a solid state displacement reaction between Mo2C and silicon. Diffusion couples between Mo2C and silicon processed at 1200°C revealed the formation of aligned SiC platelets in an MoSi2 matrix. The reaction zone of this couple had a Vickers microhardness of 12.8 GPa (HV 1000). In-situ composites were also synthesized by blending Mo2C and silicon powders and vacuum hot pressing for 2 h at 1350 °C followed by 1 h at 1700 °C. The resulting microstructure consisted of 30 vol.% SiC particles 1 μm in diameter uniformly dispersed in a fine-grained MoSi2 matrix. Densities of 5.53 g cm−3 were obtained together with a microhardness of 14.2 GPa (HV 1000). Bend bars and chevron-notched bars cut from large-diameter, and slightly less dense, hot-pressed disks revealed a strength of 475 MPa had a fracture toughness of 6.7 MPa m 1 2 at room temperature. Bend strengths increased to 515 MPa at 1000 °C and then decreased to 112 MPa at 1200 °C. Measured fracture toughness increased to 10.5 MPa m 1 2 at 1050 °C. Fractography revealed that the MoSi2 grain size was on the order of 1–2 μm, and it was suggested that the observed SiC particle size and aspect ratio could result in ineffective dislocation pinning and relatively rapid recovery at temperatures above the ductile-to-brittle transition temperature of MoSi2. This was substantiated by comparing these results with those obtained for SiC-whisker-reinforced MoSi2 composites.

The amorphous phase transition in irradiated NiTi alloy

Abstract Observed supralinear dose dependence for the amorphous transformation during irradiation of NiTi is compatible with a cascade overlap model for heavy ion (2.5 MeV Ni+, 6 MeV Ta+++) irradiations. A model based on total defect build-up, however, is necessary to explain the amorphous transition induced by electron irradiation and can also be applied to heavy ion irradiation. The cascade effects in this latter model are manifested by non-uniform defect distribution in the lattice. The defect build-up model requires a high activation energy for interstitial migration which is not incompatible with recent findings. The form of the temperature dependence can also be rationalized using a defect build-up model (amorphous phase transition, heavy-ion irradiation, electron irradiation, NiTi, defect build-up, cascade overlap).

MICROSTRUCTURAL ANALYSIS OF NEUTRON-IRRADIATED TITANIUM AND RHENIUM.

Abstract The defect structure in α-titanium and rhenium irradiated with neutrons at 0.4T m (T m = absolute melting temperature) has been analyzed using transmission electron microscopy. In rhenium, the vacancies agglomerate into voids whereas in titanium, vacancy dislocation loops lying on the prism planes are the only vacancy type defects observed. In both metals, dislocation segments and network fragments are the main evidence of interstitial type defects. The presence of dislocation loops rather than voids in titanium irradiated at this temperature is an anamalous result when compared to results on other similarly irradiated pure metals. Possible explanations for the preferential formation of loops rather than voids in the titanium are discussed.

Effect of irradiation particle mass on crystallization of amorphous alloys

The crystallization temperature of amorphous alloys was found to be significantly lowered by heavy ion or electron irradiation during annealing. However, only heavy ion irradiation altered the mode of crystallization. Both a binary and multi-element amorphous alloy showed this type of response to irradiation. Radiation-enhanced diffusion processes in the amorphous state can explain the increased crystallization kinetics during irradiation. Heavy ion irradiation alters the crystallization mode by causing direct transformation to the final equilibrium phase as opposed to intermediate metastable phase formation during thermal annealing or electron irradiation. The equilibrium phase is believed to nucleate directly in the displacement cascades, which only form during heavy ion bombardment.

Effect of irradiation on phosphorus segregation

Abstract Phosphorus strongly segregated to the surface during irradiation of austenitic-type alloys in the temperature range 775–925 K. Much weaker but measurable radiation induced segregation of phosphorus occurred in a Ni + 0.03% P alloy. Irradiation under similar conditions produced no measurable phosphorus segregation in the ferritic HT-9 or Fe + 0.03% P alloy. The lack of segregation in the ferritic alloys was suggested to result from a weak point defect — impurity interaction in the bcc iron structure while a strong interaction was suggested for the fcc iron structure. The slow accumulation of radiation damage in bcc iron alloys is also consistent with a lack of observable segregation. The evidence strongly suggests a radiation induced mechanism but a radiation enhanced, thermally activated equilibrium segregation cannot be ruled out.

Radiation induced phosphorus segregation in austenitic and ferritic alloys

Abstract The radiation induced surface segregation (RIS) of phosphorus in stainless steel attained a maximum at a dose of 0.8 dpa then decreased continually with dose. This decrease in the surface segregation of phosphorus at high dose levels has been attributed to removal of the phosphorus layer by ion sputtering. Phosphorus is not replenished since essentially all of the phosphorus within the irradiation zone has been segregated to the surface. Sputter removal can explain the previously reported absence of phosphorus segregation in ferritic alloys irradiated at high doses 1,2 (>1 dpa) since irradiation of ferritic alloys to low doses has shown measurable RIS. This sputtering phenomenon places an inherent limitation to the heavy ion irradiation technique for the study of surface segregation of impurity elements. The magnitude of the segregation in ferritics is still much less than in stainless steel which can be related to the low damage accumulation in these alloys.

Radiation induced segregation in candidate fusion reactor alloys

Abstract The effect of radiation on surface segregation of minor and impurity elements has been studied in four candidate fusion reactor alloys. Radiation induced surface segregation of phosphorus was found in both 316 type stainless steel and in Nimonic PE-16. Segregation and depletion of the other alloying elements in 316 stainless steel agreed with that reported by other investigators. Segregation of nitrogen in ferritic HT-9 was enhanced by radiation but no phosphorus segregation was detected. No significant radiation enhanced or induced segregation was observed in a Ti-6Al-4V alloy. The results indicate that radiation enhanced grain boundary segregation could contribute to the embrittlement of 316 SS and PE-16.

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.

Deformation microstructures in ion-irradiated stainless steel

Although radiation damage in stainless steel has been studied extensively, the primary focus has been on microstructural development and welling in alloys irradiated to high dose and elevated temperatures. Considerably less effort has been directed to stainless steel irradiated to low doses at low temperatures. In particular, deformation of stainless steel, irradiated to low dose levels (<10 dpa) at lower temperatures (<300 C), has received relatively little attention. Earlier work has shown that the microstructure of stainless steel, neutron irradiated at temperatures <300 C, is comprised of a high density of small dislocation loops. Deformation bands were observed in material deformed at ambient temperature but no analysis of the bands was made. As part of a larger program to understand the underlying causes for irradiation assisted, stress-corrosion cracking (IASCC), the deformation of irradiated stainless steel is being evaluated. In this study, ion irradiation is used to simulate the microstructure expected after neutron irradiation to dose levels [<=] 10 dpa at temperatures near 300 C.