K.T. Aust

Solute induced hardening near grain boundaries in zone refined metals

Abstract Microhardness studies of zone refined metals (Pb, Sn, Zn) doped with various solutes indicate that a certain class of solute atoms results in excess hardening near the grain boundary, while another class of solute atoms gives a region which is softer near the grain boundary than in the grain. As a result of the experimental observations, a solute-clustering model is proposed for grain boundary quench-hardening based on preferential solute-vacancy interactions and non-ideal thermodynamic behavior of the binary system. The present mechanism may have important implications to impurity-segregation problems at interfaces in both metallic and non-metallic systems.

Effect of solute and orientation on the mobility of near-coincidence tilt boundaries in high-purity aluminum

Abstract The motion of 37° 〈100〉 tilt coincidence boundaries and 39° 〈100〉 and 42° 〈100〉 tilt boundaries in bicrystals of high-purity aluminum and 37° 〈100〉 and 42° 〈100〉 tilt boundaries in aluminum containing 40 atom ppm copper was studied in the temperature range between 300 and 660°C by the Sun and Bauer technique. Two distinct regimes of grain boundary motion were observed, characterized by differences in activation energy and temperature range over which they occurred. It is proposed that the change from one mode of grain boundary behavior to the other indicates that a transformation in grain boundary structure occurs. This transformation is dependent upon both the structure of the boundary and upon the solute content of the material. In the regime of high temperature and low activation energies for pure aluminum, the rate of boundary motion was found to decrease with increasing deviation from the coincidence misorientation. The activation energy for boundary motion increased with increasing deviation from coincidence. These results support the interpretation of grain boundary structures in terms of both boundary coincidence and structural periodicity models. The experimental data provide further support for the Turnbull theory of grain boundary migration in pure materials, and for the Cahn and Lucke and Stuwe theories of impurity-controlled grain boundary migration.

Diffusion induced grain boundary migration in Ni-Cu Diffusion couples

Abstract Diffusion induced grain boundary migration (DIGM) was investigated in the Ni substrate of Ni-Cu diffusion couples. After annealing at 615°C for 26 h, TEM/STEM revealed: 1. (1) dislocation walls at the initial grain boundary positions, 2. (2) a small misorientation (0.1–0.5°) between the DIGM zone and the Ni matrix, 3. (3) highest Cu concentrations in the DIGM zones adjacent to the original grain boundary positions, 4. (4) steps and line defects at the DIGM boundaries and 5. (5) grain boundary faceting caused by DIGM. After 92 h of annealing at 615°C, two major changes were observed as compared with the shorter 26 h anneal at the same temperature: 1. (1) a decrease in the dislocation densities in the DIGM zones, in the matrix and at the original grain boundary positions 2. (2) a change in Cu concentration profiles across the DIGM zones. The experimental results are interpreted in terms of current DIGM theories.

Plastic deformation of aluminium bicrystals having ∑7 and ∑21 coincidence tilt boundaries

Abstract Three types of isoaxial summetrical bicrystals and their component single crystals were tested in tension to clarify the effect of the grain boundary on plastic properties. The 〈111〉 oriented bicrystal was deformed by the mode similar to that of the 〈111〉 oriented single crystal. Fine multiple slip which appeared from the early stage of plastic deformation was suppressed at the boundary. It was found that the flow stress of this bicrystal was almost equal to that of the component single crystal and the effect of grain boundary on the flow stress did not appear. Two bicrystals having the ∑7 and ∑21 coincidence tilt boundary were deformed by single slip at the early stage of plastic deformation. The flow stress of each bicrystal was increased by the presence of the boundary. The boundary strength rapidly increased from the yield point to 0.5% strain by the interaction between primary slip and additional slip in the vicinity of the boundary; only a slight increase was observed from about 1.0–5.0% strain. The effect of the grain boundary on the flow stress of the bicrystal is prominent by introducing multiple slip near the grain boundary in the stage 1 region of the component single crystal but not prominent in the stage II region. The effect of the grain boundary on the flow stress is negligible after multiple slip takes place away from the boundary in the adjoining grains.

A study of sintering of spherical silver powder—I. The intermediate stage

Abstract A model is proposed for the shrinkage rate of random packings of equalsized spheres; in this, following Beere, account is taken of the loss in volume due to shrinkage of the framework of particles. A method is presented for calculating the average shrinkage rate when new contacts are formed during sintering and the shrinkage rate varies from neck to neck. It is assumed that the mechanical constraints of the interparticle necks ensure that the compact shrinkage is essentially homogeneous. The experimental means for determining the geometric parameters in the resulting shrinkage rate equation are discussed, and the particular advantage of the pore replica method are examined. Finally the results of an investigation of shrinkage rates in the intermediate stage sintering of specimens of silver spheres in argon and air are reported. The agreement between the proposed theory and the observed shrinkage rates in air is within experimental error and supports the proposed model. The experimental shrinkage rates for the argon atmosphere are lower than those predicted. It is suggested that this discrepancy is due to the interaction between trace impurities in the atmosphere and the silver powder.

Relative energies of grain boundaries near a coincidence orientation relationship in high-purity lead

Abstract The effect of deviations from ideal coincidence orientation relationship on the relative energy of a coincidence grain boundary was studied in tricrystals of high purity lead. Lower relative energies were observed within ±2° of the ideal 36.9° 〈100〉 coincidence relationship, with a maximum energy decrease of about 30 per cent. The present results, combined with previous energy data for different high-density coincidence boundaries in high purity lead, are shown to be related to the calculated number of shared atom sites per unit area (in lattice units) of the grain boundary. These results provide experimental support for the boundary coincidence and “relaxed” coincidence models.

Crystallization behaviour of metglass 2826A by isochronal and isothermal annealing

Abstract Crystallization of Metglass 2826A by isochronal annealing showed a strong dependence on scan rate over the range of 1.4–48.7 K/min. At scan rates less than 9.2 K/min, crystallization of Metglass 2826A occurred by primary crystallization in a two stage process leading to both MS-I and MS-II type phases. A third (SIII) transformation in the crystalline state was also noted at a scan rate of 9.2 K/min. At scan rates of 22.5 and 48.7 K/min, MS-I and MS-II crystallization were concurrent and inseparable. Isothermal annealing revealed the presence of a low temperature polymorphic phase transformation corresponding to a supersaturated solid solution (SS) phase.

A study of the sintering of spherical silver powder—II. The initial stage

Abstract Sintering studies between 700 and 800°C in air and in argon were conducted using spherical silver powder. The technique of three-dimensional replication of the pore structure of silver powder samples has proved a precise means for measuring the sintering parameters, neck size and curvature, for calculation of theoretical neck growth and shrinkage rate. A neck growth model gave a close correlation with measured neck size after sintering in air , showing the important contribution of surface and grain boundary diffusion to neck growth. Grain boundary diffusion was found to be the dominant mechanism for densification in the initial stage of sintering in air of spherical silver powder. However, the experimental data obtained after sintering in argon disagreed with both the neck growth model (based on surface and grain boundary diffusion) and the densification model (based on grain boundary diffusion). The important effect of sintering atmosphere is discussed in terms of its interaction with solute impurities in the silver powder.

Solute hardening at interfaces in high purity lead—II. Free surfaces

Abstract Prolonged annealing of doped crystals of zone-refined lead in dynamic atmospheres of low oxygen partial pressure is observed to result in a cyclic hardening of the surface, the periodicity of which is a function of the annealing temperature. This surface hardening has been shown to be a complex phenomenon involving three different processes: oxidation, solute segregation and vacancy defect complex formation. The results suggest that cyclic surface hardening may be an indicator of the mode of oxidation in metals. Attention is newly focused on vacancy defects as hardening agents in metals and on the role of dilute concentrations of solute in the nucleation and stabilization thereof. The significant differences found in the phenomenology of segregation-induced surface hardening and grain boundary hardening will require further work to elucidate the fundamental mechanisms.

Effect of gaseous HCL on sulphur segregation and intergranular embrittlement in 321 stainless steel

Abstract Investigation of the susceptibility of Ti stabilized austenitic stainless steel to environmental degradation in argon, synthetic flue gas and synthetic flue gas with 0.2 vol.% HC1 at 813 K was performed. The slow strain rate technique was used over the range of strain rates ( \ ge) of 10−4–10−7 s−1. Microvoid coalescence was observed in both argon and synthetic flue gas environments. Auger analysis of TiC precipitates after exposure to the latter two environments revealed high concentrations of S associated with the carbide; no S was detected in the matrix or grain boundary. The addition of HCl promoted intergranular failure at \ ge = 10 −6 and 10−7 s−1. Chlorine was found to accumulate at the metal-oxide interface. Sulphur was now located both in the TiC carbide and the grain boundary. A reaction sequence is proposed in relation to hydrogen effects on grain boundary segregation in austenitic stainless steels.