Quenched-in Excess Vacancies Research Articles

昇温焼鈍法による等原子比合金CuPtの規則化過程の検討

The ordering processes of equiatomic alloy CuPt by anisothermal annealing were investigated by means of electrical resistivity measurements, X-ray diffraction and optical microscopic observation.The main results are as follows:(1) Disordered CuPt attains an equilibrium ordered state via the coexistence state of disordered and ordered phases.(2) There are three stages on the ordering process in CuPt alloy; the first stage corresponds to the migration and annihilation of quenched-in excess vacancies (stage Ia), the second to the annihilation of secondary lattice defects (Ib) and the third to the diffusion of equilibrium vacancies (II).(3) Discontinuous ordering corresponding to the boundary reaction occurs at stage Ia followed by continuous ordering to perfect order at stages Ib and II.(4) The activation energies for stages Ia, Ib and II are 0.77, 1.05, 2.42×105 J/mol, respectively.

Application of heterogeneous nucleation theory to precipitate nucleation at GP zones

The accelerated nucleation of precipitates at GP zones is explained using heterogeneous nucleation theory. Nucleation at zone : matrix boundaries is encouraged by several factors: 1) the chemical interfacial energy of zone : matrix boundaries can significantly decrease the interfacial energy barrier to nucleation; 2) destruction of quenched-in excess vacancies at incoherent portions of the nucleus surface may make the change in the volume free energy significantly more negative; 3) the crystal structures of the zone and matrix are identical and parallel which permits the nucleus to be faceted in both phases. Some additional assistance to nucleation at GP zones is provided by: 4) the accelerated diffusivity resulting from the presence of excess vacancies and 5) the large area of zone : matrix boundary per unit volume of matrix. These factors can more than compensate for the decreased solute supersaturation due to the formation of GP zones and provide an explanation for the enhanced nucleation of precipitates in the presence of GP zones.

The effect of direct electric current on precipitation in a bulk Al-4 wt % Cu alloy

The effect of the stress of direct electric current on precipitation in a quenched Al-4.15 wt % Cu alloy is investigated by measurement of the change in electrical resistance of a bulk specimen as a function of current density up to about 3000 A cm−2 at a given annealing time. A similar effect of d.c. stress is observed at 50, 75 and 100‡ C. For a lower current density, a constant or slightly increasing resistance is observed, while for a current density higher than a critical value, the resistance decreases with an increase in density. On the assumption that the temperature increase of a specimen due to the Joule heating is proportional to the square of the current density, it is concluded that there exists some effect of d.c. stress per se to prevent the precipitation reaction, aside from the effect of the temperature increase due to the Joule heating, and it is suggested that this retarding effect is related to the sweeping out of quenched-in excess vacancies into grain boundaries by electromigration.

The role of quenched-in excess vacancies in the coarsening of structures developed by spinodal decomposition

The role of quenched-in excess vacancies in the coarsening of structures developed by spinodal decomposition

Pb-0.038 wt%As合金の時効析出過程

Ageing Characteristics in a supersaturated Pb-0.038 wt%As alloy have been investigated resistometrically with particular emphasis on the quenching method, to make clear the anomalous enhancement effect of As in Pb-Sb alloys.Three stage are found in the isochronal resistivity curves. The rate of resistivity decrease in the first stage (∼−50°C) cannot be explained by the diffusion of As atoms under the equilibrium vacancy concentration. The activation energy for migration of As atoms in this stage is found to be 0.51±0.10 eV. This is near the activation energy for migration of a vacancy in lead. It has been shown that this stage is accelerated and rate controlled by the diffusion of quenched-in excess vacancies. The second stage having activation energy 1.21±0.04 eV is the precipitation of intermediate phase. The third is the precipitation of equilibrium phase, rhombohedral arsenic, which gives the activation energy 1.18±0.07 eV. A faint resistivity increase between the second and the third in the isochronal curves is due to the reversion reaction of intermediate precipitates.Anomalous enhancement effect in Pb-Sb alloys with small As addition may be due to the large binding energy between an As atom and a vacancy, and small As precipitates formed homogeneously with large rate in the early stage of ageing act as nuclei of Sb precipitates. The inevitability of affinity between As and Sb atoms is also discussed.

Nucleation of θ′ precipitates in an Al4% Cu alloy

Investigations were made on the effects of G-P zones and quenched-in excess vacancies on the nucleation of θ′ precipitates in an Al4%Cu alloy. It was found that the size of G-P zones has no effect on the number of θ′ precipitates and that excess vacancies increase the number of θ′ precipitates when quenched to near 0°C. It is considered that excess vacancies form vacancy clusters at near 0°C and that these clusters enhance the θ′ nucleation. The size of the vacancy clusters does not affect the number of θ′ precipitates. Even very small vacancy clusters are equally effective as grown-up clusters in enhancing the θ′ nucleation. The mechanism of θ′ nucleation is discussed, (a) when vacancy clusters are very small and (b) when vacancy clusters are fairly large, about 50∼100 åA in diameter.

Modulated structures in AuNi alloys

A modulated structure has been shown to develop on {100} planes in certain gold-nickel alloys on aging at temperatures in the vicinity of 100–200°C. The critical composition is at 45 at.% Ni and the maximum temperature at which these structures occurs is 220°C. The boundary of the modulated structure region is nearly symmetric, with extreme compositions at approximately 20 and 60 at.% Ni. This structure has been observed by means of electron diffraction and is characterized by the formation of satellite reflections on 〈100〉 axes around each fundamental peak. The modulated structures exhibited a composition amplitude of approximately 5 at. % and characteristic wavelengths which varied from 6.5 Å at 54 at.% Ni to 13 Å at 22 at.% Ni. An activation energy of 13,500 cal. per mol. was calculated for the rate limiting step in the low temperature region, corresponding to the energy of migration for vacancies in this composition range. The satellite reflections disappeared on aging just above the critical temperature ; on reaging at a lower temperature, the satellites did not reappear. This is taken to indicate that the presence of a large concentration of quenched-in excess vacancies is required for the development of the modulated structure. The formation of the modulated structure may be associated with the postulated spinodal decomposition in this system, although the theories of spinodal decomposition do not yet adequately account for the role of vacancies.

DIRECT OBSERVATION OF RAPID SELF-DIFFUSION ALONG DISLOCATIONS IN ALUMINUM

Voids were produced by the precipitation of excess quenched-in vacancies in aluminum. The annealing of these voids in thin foil specimens by a self-diffusion mechanism was studied using transmission electron microscopy. The annealing rates of both isolated voids and voids which were connected to the foil surfaces by dislocations were observed. The voids which were hooked up to the surfaces by dislocations annealed out at appreciably higher rates than the isolated voids. The results indicate that the dislocations acted as rapid self-diffusion pipes for the transport of atoms into the voids from the surfaces.

Precipitation and Generation of Dislocations in an Al-Mg Alloy

Some features of precipitation in an Al-8.5 wt.% Mg alloy were investigated by means of transmission electron microscopy. Needle-like (or rod-like) precipitates of various orientations and plate-like precipitates on both {100} and some planes deviated from {100} towards {130} were present in the specimens aged at 150°C and 200°C. At an early stage of ageing, dislocation loops were observed around some small precipitates, which were formed by the annihilation of the excess quenched-in vacancies. During the growth of precipitates, a large number of dislocations were generated around the precipitates. Some possible mechanisms of the dislocation generation in the last case were discussed, considering the nature of stress-field around the precipitates. Also, some movements of dislocations during growth of precipitates were observed by heating the thin foils in the electron microscope. The distribution of dislocations observed in the aged bulk specimens can be explained from such movements of dislocations durin...

VACANCIES AND DISLOCATION LOOPS IN GRAPHITE

Electron microscope studies were undertaken to determine concentrations and mobilities of vacancies in graphite. Crystals were quenched and annealed to various temperatures. Crystals under strain (some deliberately bent) before heat treatment contained larger numbers of loops while crystals treated under strain- free conditions showed no loops. Crystals quenched from 3000 deg C showed no loops even when bent or irradiated with neutrons, indicating that loops are not produced by coalescing of excess quenchedin vacancies. (D.C.W.)

Inhomogeneous ordering due to excess vacancies: An order-strengthening mechanism

It is noted that quenched-in excess vacancies decay more rapidly in the vicinity of dislocations, and this gives rise to a lower long range ordering rate near dislocations. It is shown that in partially ordered systems, dislocations will be looked-in by an order gradient and that a yield point is to be expected. The magnitude of the effect is calculated for β-brass on the basis of quasi-chemical binding and a yield stress comparable with the observed is obtained. It is also shown that this model of inhomogeneous ordering yields a simple explanation of an anomalous resistivity effect observed in Cu 3Au by Dugdale.