T.E. Mitchell

Dislocation relaxation in single crystals of molybdenum

Abstract The characteristics of the α dislocation relaxation peak in molybdenum have been examined in three orientations of high purity single crystals deformed in tension at 170, 300 and 450 K. For deformation at 450 K, the α peak is absent in crystals oriented for easy glide and deformed into stage 1 of work hardening. For crystals of the same orientation deformed into stages 2 and 3 and for crystals with [001] and [101] tensile orientations, the peak is observed superimposed on a step-like background; it grows rapidly, sharpens and moves to lower temperatures with increasing amounts of plastic deformation. Crystals with the easy glide orientation deformed at 170 and 300 K exhibit rapid work hardening and display small, broad, asymmetrical α peaks. Mild strain aging at 500 K does not greatly affect the α peak, but drastically reduces the step-like background damping. The overall response of the α peak to changes in dislocation substructure is very similar to that of the Bordoni peak in f.c.c. metals. Three widely divergent models are considered: double kink generation on non-screw dislocations, kink diffusion on screw dislocations, and breakaway of dislocations from immobile pinning points. It is concluded that the major features of both peaks are better explained by the breakaway models. For the α peak, a variation of the Hasiguti trapped kink model seems most appropriate.

Effect of normal stress on yield asymmetry in high purity tantalum crystals

Resolved yield stresses were determined in tension and compression for ultra high vacuum degassed tantalum crystals as a function of orientation at 77 K and 197 K. The yield stress at 77K was a minimum for specimens oriented to slip on the (112) [111] system, and a maxi-mum for specimens oriented to slip on the (–112) [1–11] system. The yield stress in tension was less than the yield stress in compression for all orientations. However, at 197 K, the yield stress in tension was found to be greater than in compression for specimens oriented to slip on the (121) [111] system in agreement with the results of Lachenmann and Schultz,1 who also found that, for specimens oriented to slip on the (•112) [1–11] system, the yield stress in compression was higher than the yield stress in tension, as occurs at 77 K. This behavior may be explained by the influence of a normal stress on a lattice expansion which occurs when the leading 112 stacking fault of a dissociated core dislocation as described by Dues-beryet al2 begins to move.

Effects of electron irradiation on precipitation in Al-3.5% Cu

The effects of electron irradiation on precipitation in Al-3.5 % Cu has been studied in the temperature range 20–200°C by high voltage electron microscopy. G.P. zones gradually disappear during irradiation and are replaced by θ″ precipitates. The intermediate stage of θ″ precipitation is inhibited by irradiation. If θ′ precipitates are introduced by preaging, they are also gradually replaced by θ′ precipitates during irradiation, even in the temperature range where they would normally grow. The growth of θ′ precipitates is observed to be enhanced at all temperatures between 20 and 200°C (above 200°C the growth rate is normal), especially at the lower temperatures. In fact, the growth rate is approximately constant between 20 and 120°C where essentially no growth is observed under normal aging. These results are explained in terms of a model of enhanced diffusion resulting from the annihilation at fixed sinks of the vacancies and interstitials created by irradiation; the fixed sinks are the precipitates themselves. The irradiating conditions in the microscope are shown to give rise to a constant contribution to the diffusion coefficient, Drad ∼- 2 × 10−16 cm2/sec for the temperature range 20–120°C. Irradiation also gives rise to small dislocation loops in either the quenched or the aged condition, but these are only observed at room temperature. Voids are never observed; apparently, the precipitates act as sinks for both the vacancies and interstitials.

Defect aggregation in irradiated Ni3Al and NiAl

Defect aggregation has been investigated by high-voltage electron irradiation at temperatures from 300 K to 810 K in Ni3Al and NiAl, which are Ll2 and B2 ordered alloys respectively. For both alloys, the number densities of radiation-induced dislocation loops decrease but the size of the loops increase with increasing irradiation temperature. In Ni3Al, both extrinsic 13<111/111 Frank loops and interstitial type 12<011/011 anti-phase loops are created. Transformation from Frank loops to anti-phase loops have been observed in Ni3Al during irradiation. In NiAl, only interstitial type perfect <001/001 prismatic loops are created. Under the same irradiation conditions, interstitial loop growth rates in NiAl are much smaller than those in Ni3Al, implying that the migration energy of vacancies in NiAl must be much higher than that in Ni3Al.

Irradiation damage of nickel in a high-voltage electron microscope

Abstract Electron irradiation damage in high-purity annealed and 20% deformed nickel has been studied using a high-voltage electron microscope (HVEM) operating at 650 kV. The effects of temperature of irradiation, electron dose and cold work on point-defect clustering in general and void formation in particular have been investigated. Both faulted and unfaulted dislocation loops were observed during irradiation at 240 to 500°C; multilayer dislocation loops were observed at the higher temperatures. Voids exhibited a cubic shape at low dose with a nearly homogeneous distribution in annealed and an inhomogenous distribution in 20% deformed nickel. The average void size for annealed nickel was larger than that for 20% deformed nickel and the void growth rate was found to be higher for annealed nickel. In annealed nickel, the void concentration increased up to ≈14 dpa and then decreased, while in 20% deformed nickel it increased up to ≈35 dpa. Swelling was considerably reduced by cold work compared to annealed nickel. These observations are discussed with emphasis on the role of dislocation density in the nucleation and growth of voids and swelling.

Radiation-induced homogeneous precipitation in Ni-1 at% Be alloys

Abstract High voltage electron microscopy has been used for in-situ observations of precipitation in undersaturated solid solutions of Ni-Be alloys under electron irradiation. The results show that the precipitation follows the normal ageing sequence of saturated solid solutions without irradiation: G.P. zones → γ → γ. At the beginning of irradiation, the precipitation is homogeneously induced. Later on dislocation loops of interstitial type nucleate at precipitates. Most of the loops lie on > {100} planes, having a 〈 100 〉 Burgers vector. Longer irradiations produce dislocation tangles containing precipitates. In thin specimen regions, however, faulted Frank loops form after 〈 100 〉/{100} loops disappear. These results are discussed in terms of migration of interstitial-solute complexes to pre-existing solute clusters as well as specimen surfaces.

Structure of dislocations in tantalum and tantalum-nitrogen alloys

Abstract Weak-beam electron microscopy of Ta and Ta-N crystals deformed at 77 K shows that the image widths of 1 2 〈111〉 screw dislocations are approx. 10–20 A, regardless of the nitrogen concentration between ~ 5 and 15,000 at.ppm. Annealing treatments designed to decorate screw dislocations with nitrogen atmospheres have no effect on the widths. Furthermore, no dissociations are observed at the nodes of the dislocation reactions 1 2 〈111〉 + 1 2 〈1 1 1 〉 = 〈100〉 . It is concluded that the splitting of screw dislocation cores of ~50 A observed by field ion microscopy is enhanced by the local electric field; actual dissociations must be much smaller. Models of interstitial hardening in b.c.c. metals which rely on interstitial-enhanced core expansion of screw dislocations are also not supported by the present results.