Evolution Of Defect Structure Research Articles

Development of defect structures from displacement cascade damage in D-T neutron irradiated gold

Pure gold is adopted as a suitable material for the exposure of important damage and defect processes induced by fusion neutron irradiation. Vacancy type unit defects, typically the stacking fault tetrahedra of 1–2 nm, form groups of a size up to 20 nm containing up to 20 unit defects, reflecting the damage process in the presence of sub-cascades. The three dimensional configuration of sub-cascade damage is measured and illustrated. From the density of the sub-cascade groups, the neutron collision cross-section to create the observable defects is estimated to be 1.8 barn. The observed formation of vacancy clusters at low temperatures implies the existence of the dynamical effect of collisions in the point defect reactions. The mechanism of defect structure evolution at higher temperatures, one defect cluster from one neutron collision, is discussed and the importance of point defect cluster stability and the role of free insterstitials are emphasized.

Defect structure evolution from radiation damage with D-T fusion neutrons

Irradiation of a variety of metals in a rotating target D-T fusion neutron source RTNS-II and observation of defect structures have been performed to obtain a unified understanding of the defect processes involved in the damage structure evolution from irradiations that generate large cascades. The maximum separation of interstitial atoms from the vacancy rich zone is measured. Vacancy type defect clusters form groups reflecting the damage with subcacades, and the three-dimensional configuration of subcascades is disclosed by high resolution stereo electron microscopy. The effective collision cross-section to produce defect clusters is estimated, and the damage efficiency is obtained. The variation of point defect processes is discussed based on the variation of the stability of small clusters in different materials irradiated at different temperatures. The roles of free interstitials released from cascade zones, the elimination of vacancy clustered defects and the formation of their own clusters, are clearly understood from a comparison of the results of irradiation of a bulk specimen with those of thin foils. The necessity of a dynamical effect from collisions on the defect cluster formation is concluded and its mechanism is discussed. Only the descriptions are included in this paper; all the micrographs and figures will appear in another paper in the same journal.

Diffusion and Activation During Rapid Thermal Annealing of Implanted Boron in Silicon

ABSTRACTThe diffusion and activation of implanted boron in silicon during rapid thermal annealing (RTA) has been studied using the analytical techniques of SIMS, TEM, and sheet resistance measurements. Both crystalline and pre-amorphised silicon substrates were investigated. Data analysis in conjunction with a range of numerical models indicates some novel features of boron RTA, as well as accounting for previously observed features. In particular, a large transient diffusion enhancement coupled with an increase of electrical activity, are seen at short anneal times, in the case of crystalline silicon substrates. A non-equilibrium diffusion enhancement of a different type is also seen at much longer times, in both crystalline and pre-amorphised samples implanted to high doses. This second enhancement persists after all the precipitated boron formed on implantation has become substitutional. TEM studies show that the transient enhancement may be associated with the evolution of extended defect structures during the early stages of annealing. Both types of enhancement can be well represented by multiplying the ‘normal’ concentration-dependent diffusivity (with β=0.5) by a factor f>1.

The evolution of a defect structure and its relation to the deformation parameters in TiAlNbZr alloy at 4.2 K

The phase composition of the Ti-(5–6.5) wt.%Al-(2.5–4)wt.%Nb-(1–2.5)wt.%Zr structural alloy and the evolution of the alloy defect structure during its plastic deformation at 4.2 K were studied using transmission electron microscopy. It is shown that according to its phase composition this alloy should be attributed to the category of pseudo α alloys which contain a small (by volume) amount of b.c.c. β phase in an h.c.p. α matrix, the β phase being revealed in the form of fine needles and plates. The high values of the mechanical properties of the alloy at liquid helium temperature are due in part to the presence of strong dislocation pile-ups, pinned at β phase precipitates and creating appreciable internal stresses. In contrast, the high concentration of internal stresses in slip bands containing a high density of dislocations and pinned at the precipitates is the cause of cracking which results in a decrease in the plasticity of the alloy at 4.2 K. The quantitative relationships between the deforming stress σ, the prismatic dislocation density π and the strain ϵ were obtained for an alloy deformed at 4.2 K: σ = σ 0 + αGbϱ 1 2 ϱ = ϱ 0 + Aϵ 3 2 where σ 0 is the stress approximately corresponding to the yield strength, G is the shear modulus at 4.2 K, b is the prismatic dislocation Burgers vector, α (=0.5) is a factor characterizing the contribution from the dislocation elastic interaction to the deforming stress value, ϱ 0 is the dislocation density in an unstrained material and A is the dislocation multiplication factor.

Low loss surface imaging and transmission microscopy of growth of some thin films

The low-loss surface imaging STEM technique together with conventional TEM and SEM have been applied to study surface morphology and the evolution of microstructure in vapor deposited thin films of gold principally but also of copper and silver. Such films are shown to have cellular surfaces which are not related to the film grain size. Surface relief is observed where twins emerge. Islands of gold are deduced from TEM observations to be cubo-octahedra which can be physically aligned by cleavage steps. These observations give a new insight into the evolution of preferred orientations and defect structures in thin films.

The effects of nucleation and growth on epitaxy in the CoSi 2/Si system

The epitaxial perfection and microstructure in thin CoSi 2(111)(and NiSi 2) films on silicon has been examined using transmission electron microscopy, including high resolution cross section techniques. We find that, under ultrahigh vacuum preparation conditions only, genuine single-crystal epitaxy can occur for both reacted and codeposited CoSi 2 films. A unique interfacial defect structure is associated with this epitaxy, in which the silicide films are rotated through 180° about (111) with respect to the silicon substrates. Under less than perfect clean conditions epitaxial perfection is never obtained for CoSi 2. Moreover such perfection has never been attained under any conditions for NiSi 2 on Si(111). By studying the evolution of epitaxy and defect structure in thin and thick films we have proposed a model to explain the unique quality of ultrahigh vacuum CoSi 2 films. The model invokes a defect pinning mechanism to explain the dominance of the 180°-rotated epitaxy during silicide growth.

The microstructure of “triple-beam” ion irradiated Fe and Fe-Cr alloys

The development of defect structures has been studied in Fe-10% Cr, “triple-beam” (He +, D + 2 and 4 MeV Fe ++) ion irradiated to 10 dpa at temperatures from 725 to 950 K. Limited cavity formation was observed. Peak swelling of ∼0.02% occurred at irradiation temperatures of ∼850 K. Similar irradiations at this temperature led to ∼0.22% swelling in iron, while no cavities were observed in Fe-5% Cr. Calculations have shown that cavities formed at 875 K and above are more bubble-like than those at lower temperatures. Dislocation structures varied from interstitial dislocation loops with b = a〈100〉 at low temperatures to a coarse dislocation network at 850 K. Large dislocation loops had a convoluted shape which suggested that loop growth had occurred preferentially in 〈110〉 directions. The results are discussed and related to current swelling suppression mechanisms for ferritic steels.

The Effects of Nucleation and Growth on Epitaxy in the CoSi2/Si System

The epitaxial perfection and microstructure in thin CoSi2 (111) (and NiSi2) films on silicon has been examined using transmission electron microscopy, including high resolution cross section techniques. We find that, under ultrahigh vacuum preparation conditions only, genuine single-crystal epitaxy can occur for bothreacted and codeposited CoSi2 films. A unique interfacial defect structure is associated with this epitaxy, in which the silicide films are rotated through 180° about (111) with respect to the silicon substrates. Under less than perfectly clean conditions epitaxial perfection is never obtained for CoSi2. Moreover such perfection has never been attained under any conditions for NiSi2 on Si(111). By studying the evolution of epitaxy and defect structure in thin and thick films we have proposed a model to explain the unique quality of ultrahigh vacuum CoSi2 films. The model invokes a defect pinning mechanism to explain the dominance of the 180°-rotated epitaxy during silicide growth.

Dynamic studies of defect mobility using high voltage electron microscopy

The large variety of observed defect-structure development by electron irradiation in fcc and bcc metals is classified from the view point of point-defect mobilities. Vacancy mobility is obtained from the defect structure change caused by the annihilation of vacancies accumulated by irradiation, and their motion activation energy is obtained from the interstitial cluster growth at high temperatures. Mobility of interstitial atoms and their interaction with impurities are obtained from the variation of interstitial cluster formation. Various observed effects of electron radiation induced motion of point defects are explained by the displacement of atoms by the transfer of energy comparable with the migration activation energy of each point defect. The observed superficial temperature dependence of the induced diffusion of vacancies in fcc metals is attributed to divacancies created by the induced diffusion. An efficient method to obtain the self-diffusion energies is proposed with results for some metals.