R. E. Smallman

Climb kinetics of dislocation loops in aluminium

Abstract The climb kinetics of dislocation loops in thin foils in terms of either the emission of vacancies from the loop or the diffusion of vacancies to the surface has been critically examined. Although both mechanisms give similar rate equations, predictions can be made which allow the rate controlling process to be determined experimentally. The annealing behaviour of faulted and prismatic loops in quenched aluminium has been studied using quantitative electron microscopy techniques and shown to support a theory based on vacancy diffusion. Hence by comparing the annealing rates of faulted loops with those of prismatic loops as a function of temperature the stacking-fault energy of aluminium has been determined to be 135±20 ergs/cm2. This value is shown to be consistent with the physical properties of aluminium in relation to other metals.

Faulted dislocation loops in quenched aluminium

Abstract Dislocation loops containing stacking faults have been observed in quenched aluminium using the electron microscope. It is found that lowering the quenching rate relaxes the stringent purity conditions governing the retention of faulted loops, such that a high proportion of the loops in oil-quenched aluminium (99·97 % purity) contain faults. The results are discussed in terms of the influence of quenching stresses on the stability of the faulted loop. A low quenching rate also favours double-faulting, in which a Frank sessile dislocation loop is nucleated on an existing fault. The stability of this defect is attributed to the lower stacking-fault energy of a double fault compared with that of two intrinsic stacking faults. Under the action of stress the outer fault of the double defect can sometimes be removed to produce a faulted loop within a prismatic loop. The annealing behaviour of the four different types of dislocation loop, i.e. prismatic, single-faulted, double-faulted and faulted prisma...

Vacancy trapping in quenched aluminium alloys

Abstract Foils of some aluminium-based alloys have been given various quenching treatments and the size and distribution of the vacancy clusters observed in an electron microscope. It is concluded that the solute atom trap the vacancies and that their binding energies determine both the male of the clusters and the number of vacancies they contain.

The influence of a dilute magnesium addition on the growth ant shrinkage of dislocation loops in aluminium

Abstract The influence of a dilute magnesium addition to pure aluminium on the climb of dislocation loops has been studied by isothermal annealing of thin foils of Al-0·65% Mg. Faulted loops were observed to shrink at temperatures above 130°C, whereas prismatic loops above a critical size grew at temperature? ≳200°C; in pure aluminium they generally shrink. The various mechanisms previously proposed to account for excess vacancies have been critically examined and shown to be inconsistent with the experimental observations, and an alternative mechanism based on the growth of a surface oxide is proposed. Experimental observations consistent with the oxide growth mechanism are described. The variation of shrinkage rate with temperature for both prismatic and faulted loops over a wide temperature range has been determined. The results, which do not conform to emission controlled kinetics, have been analysed in terms of diffusion controlled behaviour to deduce an accurate value for the intrinsic stacking-faul...

The stacking-fault energy of F.C.C. metals

Abstract The determination of stacking-fault energy from the dimensions of stacking-fault tetrahedra is critically examined and the results compared with those obtained by other methods. It is concluded that the dimensions of stacking-fault tetrahedra do not provide a reliable measure of stacking-fault energy. New values of the stacking-fault energy of Ag and Al are used as calibration points for the method of determination of Dillamore et al. (1964) and their values of γAu, γPt, γTh, γPd, γNi and γRh are revised.

The influence of d-band structure on stacking-fault energy

Abstract The variation of stacking-fault energy across the alloy systems, Cu-Ni, Cu-Pd and Ag-Pd respectively has been estimated by a texture method based on the dependence of cross-slip on the stacking-fault energy. The variations with composition have been correlated with known changes in the electronic specific heats and magnetic susceptibilities of these alloys. It is concluded that the presence of empty d-states in these alloys results in a large increase in the stacking-fault energy.

The plastic deformation of NiAl single crystals between 300°K and 1050°K

Abstract The experimental results detailed in the preceding paper are interpreted in terms of glide and climb of disolcations of b = (100). It is shown that the same slip systems operate both when hard crystals deform uniformly and when kinking occurs. In the case of uniform deformation climb also causes significant plastic strain. The localized deformation associated with kinking is shown to be due to geometrical softening. On the basis of these conclusions the fact that a change of temperature, or change of strain rate, can give rise to kinking in a specimen which would otherwise deform uniformly (as shown in I) can be simply explained.

A determination of the stacking-fault energy of some pure F.C.C. metals

Abstract A method for determining the stacking-fault energy of f.c.c. materials has been developed using an already-established dependence of preferred orientation on stacking-fault energy. An empirically determined parameter, characteristic of the type of rolling texture exhibited by the metal, is related to the stacking-fault energy value obtained by measurements of dislocation node radius, and from the γ value for the three metals, Ag, Cu and ni the stacking-fault energy of Th, Au, Pd, Rh, Pt and Yb has been obtained by interpolation using measured values of the texture parameter.

The observation of a dislocation ‘Climb’ source

Abstract It has been found, using thin film electron microscopy, that aluminium 3.5 and 7.3 atomic per cent magnesium alloys quenched into silicone oil contain concentric diamond-shaped dislocation loops lying upon the cube planes. An examination of the various dislocation interactions which occur together with the electron diffraction contrast behaviour shows that these loops have alternate Burgers vectors of the (a/2)⟨110⟩-type lying in the cube face perpendicular to that in which the loops lie. The source is believed to be a dislocation of the a⟨001⟩-type which accepts vacancies during the quench, but dissociates during climb into two component dislocations. This dissociation can occur with the new Burgers vectors lying in either of two possible cube planes and evidence of both types is frequently seen. The origin of the source is discussed.

Oxidation-induced defects in NiAl

Abstract The annealing of voids and vacancy dislocation loops previously found in slowly cooled crystals of NiAl (Fraser, Loretto, Smallman and Wasilewski, 1971) has been studied using transmission electron microscopy. It is observed that both types of defect grow during annealing even when it is carried out in a vacuum of about 10−7 torr and that the detailed behaviour is consistent with vacancy introduction by surface oxidation. The occurrence of voids in as-grown single crystals, and the formation of vacancy-type loops in quenched NiAl (Ball and Smallman 1968) may now be interpreted in terms of surface oxidation rather than a low energy of formation of vacancies in NiAl.