M. L. Jenkins

High-resolution electron microscopy studies of the structure of Cu precipitates in α-Fe

Abstract Transmission electron microscopy and high-resolution electron microscopy investigations of the structure of Cu precipitates in the size range 4–30 nm were carried out as part of a wider investigation into Cu precipitation from thermally aged Fe-Cu and Fe-Cu-Ni ferritic model alloys. A twinned 9R structure was found to be present in precipitates slightly larger than 4nm in diameter, following transformation from b.c.c. Two twin-related 9R variants were observed in all the smallest 9R particles studied. The 9R precipitates were observed to grow subsequently as spherical, multiply twinned particles up to approximately 17 nm, indicating that further twinning must occur during growth in the 9R phase. At sizes larger than 17 nm, a second transformation to the more stable 3R structure takes place. Observations on these 3R particles indicate that, following transformation from 9R, the precipitates are untwinned and have a distorted f.c.c. structure. The particle-matrix orientation is close to, but not ex...

Transmission electron microscope investigations of the structure of copper precipitates in thermally-aged Fe—Cu and Fe—Cu—Ni

Abstract Conventional transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM) experiments have been carried out on thermally-aged binary Fe 1·30 wt% Cu and ternary Fe 1·28 wt% Cu 1·43 wt.% Ni alloys to study the structure of small (7–15 nm) copper precipitates in the α-Fe matrix. The experiments show that the precipitates have a twinned 9R close-packed structure, rather than the expected f.c.c. structure. It is believed that this structure is generated initially by a martensitic transformation from the metastable b.c.c. phase.

The measurement of stacking-fault energies of pure face-centred cubic metals

Abstract Using standard imaging techniques of electron microscopy it is not possible to resolve the partial dislocations having the equilibrium separation for dissociated dislocations in the pure face-centred cubic metals. Consequently the stacking-fault energy γ cannot be determined from direct measurements of the separations of the partials. In this paper it is demonstrated that such determinations are possible using the weak-beam method of electron microscopy. The separations of Shockley partial dislocations in silver and copper have been measured, as a function of dislocation line orientation, using this method. From these measurements, the values γsilver= 16·3+1·7 erg cm−2 and γcopper= 41±9 erg cm−2 have been obtained from analyses based on anisotropic elasticity theory.

The bcc-to-9R martensitic transformation of Cu precipitates and the relaxation process of elastic strains in an Fe-Cu alloy

Abstract High-resolution electron microscopy experiments have been performed to explore the bcc—9R transformation and the subsequent elastic relaxation of Cu precipitates in an Fe—Cu alloy aged at 550°C. It was found that both electron irradiation (at an electron energy of 400 kV) and thermal annealing caused rotation of the close-packed (009)9R planes in twinned 9R Cu precipitates. For 400 kV electron irradiation, such rotations were observed in precipitates smaller than about 12nm in diameter. For specimens cooled from the ageing temperature of 550°C to a given temperature up to −60°C, and then annealed at 400°C, the rotation of (009)9R planes was found to occur only in precipitates above a size which depended on the temperature to which the specimen had been cooled. This critical size ranged from about 9 nm for specimens cooled to 400°C, to 4 nm for specimens cooled to −60°C. It is argued that these critical sizes are indicative of the sizes at which coherent bcc precipitates transform martensitically at different temperatures. At the ageing temperature of 550°C, the transformation to 9R takes place when precipitates reach a size of about 12nm. The number of twin segments in the transformed 9R precipitates is determined by the transformation, depending on the precipitate size. The annealing-induced plane rotations are shown to be connected with the diffusional relaxation of elastic strains, which are created upon the martensitic transformation. From the precipitate size and annealing time dependence of the rotations, it is concluded that the elastic strains relax by atomic diffusion along the interfaces between the Fe matrix and Cu precipitates. The activation energy for the interfacial diffusion is evaluated to be 1.7eV.

Heavy-ion irradiations of Fe and Fe–Cr model alloys Part 2: Damage evolution in thin-foils at higher doses

A study of heavy-ion damage in Fe and Fe–Cr alloys started in Part 1 1 was continued with an investigation of damage development in UHP Fe and Fe–8%Cr at higher doses up to 2 × 1019 ions m−2 (∼13 dpa). In thin-foil irradiations with 150 keV Fe+ ions at 300°C and room temperature (RT), more complex microstructures started to develop in thicker regions of the foils at doses greater than about 2 × 1018 ions m−2, apparently involving cooperative interaction, alignment and coalescence of smaller loops. First strings of loops all with the same ½⟨111⟩ Burgers vectors formed. In UHP Fe irradiated at 300°C the damage then developed into colonies of resolvable interstitial loops with ½⟨111⟩ Burgers vectors. By a dose of 2 × 1019 ions m−2, large (several hundred nanometre) finger-shaped loops with large shear components had developed by the growth and subsequent coalescence of smaller loops. Similar but finer-scale damage structures developed in UHP Fe irradiated at RT and in Fe–8%Cr irradiated at both RT and 300°C.

Heavy-ion irradiation of α-iron

Abstract In this paper we report the results from a transmission electron microscope study of α-iron after irradiation with heavy ions. No visible damage was observed following a low dose of self-ions in the energy range 40–240 keV. This is in contrast to the results from other pure metals where visible damage is produced after self-ion irradiation. Damage, in the form of small vacancy loops, was observed as the incident ion mass was increased, and quantitative results are presented on the dependence of both the defect yield and the defect size distribution on the ion mass. The Burgers vectors of the vacancy loops formed from 80 keV W+ ions are analysed and it is shown that both of the possible perfect loop Burgers vectors are present in the foils, i.e. a/2〈111〉 and a〈100〉. Finally, the sensitivity of the defect yield on ion mass and the nucleation of the a〈100〉 loops are discussed.

Transmission electron microscopy studies of displacement cascades in Cu3Au

Abstract Displacement cascades produced in ordered Cu3Au by the impact of Cu+ions of energy 5–100 keV are studied using the method described in Part I. It is found that each incident ion of energy ≳ 10 keV creates, at the core of the resulting displacement cascade, a zone of reduced long-range order which can be imaged in the electron microscope. The sizes of displacement cascades in Cu3Au are well described by the analytical theory of Sigmund and co-workers. The best evidence for this comes from a comparison of experimental and theoretical image contrast profiles at an ion energy of 10 keV, when reasonably direct physical arguments can be used to link experimental and theoretical parameters. At ion energies ≳ 30 keV many of the disordered zones develop extrusions, and the formation of some discrete sub-cascades is observed. The various mechanisms which might give rise to the disordering are discussed.

Characterisation of radiation-damage microstructures by TEM

Abstract A critical review is given of the conventional imaging techniques used in transmission electron microscopy to characterise radiation damage microstructures. Particular emphasis is given to the limitations of some of these techniques. The techniques are illustrated by examples.

The collapse of defect cascades to dislocation loops

We describe a number of experiments that we have recently performed to investigate the collapse of defect cascades to dislocation loops. This important ion and neutron irradiation phenomenon has been studied with in situ ion bombardment using the High Voltage Electron Microscope-Ion Accelerator Facility at Argonne National Laboratory in Cu3Au, Cu, and Fe at temperatures of 30 and 300 K and in Ni at 30, 300 and 600 K. These experiments have demonstrated that individual defect cascades collapse to dislocation loops athermally at 30 K in some materials (Ni, Cu and Cu3Au), while in Fe overlapping of cascades is necessary to produce dislocation loops. A slight sensitivity to the irradiation temperature is demonstrated in Cu3Au and Fe, and a strong dependence on the irradiation temperature is seen in Ni. This phenomenon of cascade collapse to dislocation loops in metals at 30 K provides an understanding for previous neutron irradiation data. The more detailed dependencies of the collapse probability on material, temperature, bombarding ion dose, ion energy and ion mass contribute much information to a thermal spike model of the collision cascade which we will describe.

Experimental studies of cascade phenomena in metals

Abstract We review recent ion-irradiation experiments which have been performed to investigate the collapse of displacement cascades to dislocation loops in a range of metals and alloys. Many of the results including the dependencies of the collapse probabilities on irradiation temperature, and ion dose, energy and mass, can be explained within the framework of a thermal spike/cascade melting model which has been suggested by computer molecular dynamics simulations. Other aspects, such as the dependence of collapse probabilities on the crystal structure and the effects of alloying and impurities, are less well understood.