Part IX – September 1968 - Papers - Grain Boundary Sliding, Migration, and Deformation in High-Purity Aluminum

Grain boundary sliding and migration were studied in pure aluminum bicrystal and polycrystal samples with two-dimensional grain structure. Scratches, 50 P apart, were used for measurement of sliding and migration distanceso. Samples were deformed at constant rate at 315C and events recorded continuously on wrotion picture film. Electron micrograPhs of boundary-scratch intersections were obtained. Yield and flow stress values were measured. The sequence of sliding and migration events for a three-grain junction is described in detail. Sliding depended only on the resolved shear stress imparted to the boundary. Sliding was accowmodated by formation of shear zones in grains opposite triple points and adjacent to curved boundaries. These shear zones provided the driving force for grain boundary migration. Migration caused rumpling of the boundaries, decreasing the sliding rate. Sliding and migration generally began at the same time, occurred simultaneously and ended at the same time. In the bicrystal, sliding and migration rates were proportional. Initial sliding rules of 5 X joe cm per sec. were measured for the polycrystal and bicrystal samples. These sliding rates agree wilh the internal friction experirnents of K;. The observations seem consistent with a viscous boundary sliding nzechanism. GRAIN boundary sliding is the translation of one grain relative to its neighbor by a shear motion along their common boundary. Sliding is thought to be an important mode of deformation at elevated temperatures and at low strain rates such as prevail in creep,' and perhaps in the area of superplastic behavior.2"4 Although much work has been done to investigate grain boundary sliding, the effort has not led to the identification of a mehanism. KG showed that grain boundaries in aluminum exhibit a viscous nature under very small displacements of internal friction measrements. Various dislocation mechanisms have been proposed but are without conclusive experimental support. Attempts to relate sliding to 6's viscous boundaries have been unsuccessful in that measured rates of sliding are always several orders of magnitude lower than KG'S results would predict.= In bi crystals7and polycrystalsR of aluminum tested under constant load, the grain boundary sliding was found to be proportional to the total creep elongation which indicated that sliding might be controlled by deformation of the grains. Shear zones were observed to extend beyond grain boundaries at triple points to accommodate the sliding.8 Surface observations brought forth the opinion that sliding and migration occurred alternately, in sequence.' Measurements of sliding at the surface have been criticized because they might not be representative of the interior of the sample. Generally speaking, it seemed that much of the previous work and knowledge was based on observations made at relatively low magnification and examination of samples after deformation had been accomplished. Thus, it was the purpose of the present study to continuously record, at high magnification, the events occurring during the deformation of pure aluminum. Samples with two-dimensional grain structures were used to simplify interpretation of the results. The sliding and migration of small areas of many samples were continuously recorded by time-lapse motion pictures. Replicas of the surface were used to provide high-resolution electron micrographs. These observations, coupled with tmsile strength data, provide sufficient information to arrive at an understanding of the phenomenon. EXPERIMENTAL PROCEDURE An ingot of 99.999 pct A1 was rolled to sheet, 0.127-cm thick. Tensile specimens, with a gage length of 0.85 cm, were machined from the sheet. Bicrystal tensile specimens, of the same dimensions, were spark cut from a large bicrystal ingot. The grain boundary was oriented at 45 deg to the tensile axis. The surfaces of the tensile samples were ground flat on fine metallographic paper and were then electropolished in a solution of 75 parts absolute alcohol and 25 parts of perchloric acid. The solution was cooled in an ice-water bath. Using a weighted sewing needle suspended from a small pivot on a precision milling machine, a grid of fine scratches, 50 p apart, was scribed on one surface of the sample. The polycrystalline samples were then annealed in hydrogen for 15 min at 350" to 400°C to produce a two-dimensional grain structure of about 0.2-cm average grain diameter which would not undergo further growth at the test temperature, 315OC. Examination of both surfaces of the samples showed that the grain boundaries were perpendicular to the surface of the polycrystal and bicrystal samples. A hot-stage tensile machine was constructed for use with an optical microscope as shown in Fig. 1. The specimen is shown mounted in the grips. The grips ride in V-ways so that the sample can be mounted without damage. The rear grip is free to slide so that when the sample expands during heating it is not put under a compressive stress. When the grips and samples are at temperature, the rear grip is locked in place by two set-screws. The other grip is connected to a synchronous drive motor which, through a worm gear and a fine-threaded rod, deforms the