Kang Jung

Effects of an Electric Field and Current on Phase Transformations in Metals and Ceramics

Abstract Theoretical and experimental results on the effects of an electric field or current on solid state phase transformations in metals and ceramics are reviewed. Examples are given of the effects on both equilibria and kinetics, and the mechanisms are considered. The available results provide a foundation, but indicate the need for more work. They do, however, indicate that electric fields and currents provide additional parameters for controlling microstructure, and offer the potential for reducing processing costs and related environmental pollution.

Effects of an external electric field applied during the solution heat treatment of the Al–Mg–Si–Cu alloy AA6111

Abstract The application of an external dc electric field during the solution heat treatment (SHT) of AA6111 Al alloy increased the as-quenched resistivity ρ and hardness Hv. The increases represented reductions in SHT of 10–20 K required for a constant ρ or Hv. Analysis of the results gave that the field reduced both the enthalpy and entropy of solution. The resulting reduction in Gibbs free energy gave increases of 0.03–0.04 at.% in solubility of Mg2Si (or Al–Mg–Si–(Cu)-vacancy complexes) at 400–600°C, amounting to a factor of 33 % increase in solubility at 400°C and 6% at 600°C.

Microstructure gradient in 60Sn40Pb solder joints annealed under an external electric field

Prior work [1–13] has shown that an electrostatic field of the order of a kV/cm can have an influence on solidstate reactions in metals and alloys. Since no prior studies have been reported on the influence of an external electric field on phase coarsening in a dual-phase alloy, this topic was of interest in the present investigation. Near-eutectic 60Sn40Pb solder joints were employed, because of their extensive use in the electronics industry. Moreover, an investigation of microstructure coarsening in this alloy without and with a field had previously been performed [14, 15]. Of special interest in the present work was the influence of the electric field on the difference in the microstructure at the perimeter (surface) compared to the center of the circular solder disk. The specimens consisted of 60Sn40Pb solder joints approximately 0.2 mm thick between two 20 mm long × 6.4 mm dia. Cu rods. Details regarding the preparation of the joints and metallographic procedures are given elsewhere [14]. The microstructure of the as-reflowed solder was typical of solder joints in electronic packages, consisting of occasional Pb-rich dendrites in a matrix of a fine, globular, degenerate Pb-Sn eutectic with a relatively uniform distribution of the Pb-rich and Sn-rich phases. The solder-joint specimens were annealed without and with an electric field for 70 h in a silicone oil bath at 150 ± 0.2 ◦C. The electrical arrangement for the tests with field is presented elsewhere [15]. The specimen was connected to the positive terminal of the dc power supply and the anneals were performed with an applied voltage V = 5 kV/cm, which gave a nominal external electric field E = 25 kV/cm with an electric current I = 3.1 μA. Duplicate tests were performed to indicate the reproducibility and accuracy of our results. The as-reflowed and annealed specimens were sectioned in half longitudinally, metallographically polished and etched. Optical photomicrographs were taken at 1000× at three locations: (a) the center of the cylindrical solder disk, (b) within 0.2 mm of the right circumference and (c) within 0.2 mm of the left circumference along the same diagonal. The mean sizes of the individual Pb-rich and Sn-rich phases of the eutectic were determined at each location by measuring the linear intercepts (200– 500) of each phase on randomly-drawn lines on the photomicrographs. The mean sizes of the Pb-rich and Sn-rich phases in the as-reflowed condition were 0.65 and 1.38 μm, respectively, independent of the location within the solder joint, giving an average combined phase size D̄o ≈ 1 μm. These values are similar to those for solder joints in electronic packages. The effect of an electric field on the coarsening of the Pb and Sn phases (given by their respective mean size D̄ at the center of the solder disk) at 150 ◦C has been presented previously [15]. The field retarded coarsening at this location, the degree of which increased with field strength. The mean phase sizes resulting form the anneal of 70 h at 150 ◦C without and with the electric field here are presented in Fig. 1 as a function of distance from the center of the solder joint. Included is the volume fraction of Sn given by fSn = D̄Sn/(D̄Sn + D̄Pb) [15]. It is seen that both the Pb and the Sn phase sizes are smaller when a field is applied than without a field at all three locations. However, the retarding effect of the field is greater near the specimen surface compared with the center. In addition, the field has affected the volume fractions f of the respective phases, with fSn at the center being increased by the field, while fSn at surface is decreased. Conversely, fPb is decreased at the center and increased near the surface, in keeping with fPb + fSn = 1. The equilibrium value was calculated from the phase diagram for the Pb-Sn system considering the density of each constitutent. To explain the effects of an external electric field on phase coarsening in 60Sn40Pb shown in Fig. 1 two generally-accepted physical conditions are considered: (a) upon application of an external electric field there occurs an excess surface charge, but no overall electric field exists in the interior of a metal specimen and (b) with no applied field there exist two main sources of local electric field inside the 60Sn40Pb specimen, namely: (1) charged vacancies, solutes or vacancysolute complexes and (2) contact potential between the Pb and Sn phases. The decrease in rate of coarsening observed here for the 60Sn40Pb alloy is in keeping with the retarding effect of an external electric field on the rates of precipitation in Al and Fe alloys [6–8]. Since these phenomena are diffusion-controlled, it appears that a major effect of a field is to reduce the governing diffusion rate. In the prior work [14] it was determined that the coarsening rates of the Sn and Pb phases in 60Sn40Pb without field were given by D̄n − D̄o = Ao Dbt , where n ≈ 4 and Ao = Bγ co w. Db is the grain boundary or interfacial boundary diffusion coefficient and t is the time. B is a parameter which depends on the volume fraction and the geometry of the phase, γ ≈ the grain boundary energy, co = exp(− H/RT ) the equilibrium solute concentration near the grain boundaries,

Effects of an electric field applied during the solution heat treatment of the Al–Mg–Si–Cu alloy AA 6111 on the subsequent natural aging kinetics and tensile properties

Abstract The effects of an external dc electric field of 0–5 kV/cm, applied at 475–550°C during solution heat treatment (SHT) to AA6111, on the subsequent natural aging kinetics and volume fraction of precipitated clusters were determined employing resistivity. The field increased the as-quenched resistivity and that during natural aging, the effect being significant from 0 to ≈0.5 kV/cm and increasing only slightly, if at all, thereafter. An Avrami-type analysis of the natural aging kinetics gave n = 0.59 ± 0.2 and k = (1.2 ± 0.8) · 10−2 min−1, relatively independent of SHT temperature and field strength. The field increased the volume fraction of precipitated clusters which occurred during natural aging. This was attributed to the increase in solubility during SHT produced by the field. Tensile properties equivalent to those obtained by the SHT at 550°C without a field were obtained at 500°C with a field of only 200 V/cm, representing a reduction of 50°C in the nominal SHT temperature.

Effects of grain size from millimeters to nanometers on the flow stress of metals and compounds

The effect of grain size d from millimeters to nanometers on the flow stress σ of metals and compounds is considered. The data for metals indicate three grain regimes: I (d≥1 µm), II (d ≠ 10–1,000 nm), and III (d<∼10 nm). Grain size hardening occurs in I and II and grain size softening occurs in III. The data for compounds suggest similar regimes, but are too limited for positive identification. The physical mechanisms governing each regime are discussed. The data for regime III are in reasonable accord with the mechanism of grain boundary shear proposed by Conrad and Narayan.