Scripta METALLURGICA Vol. I0, pp. 477-480, 1976 Pergamon Press~ Inc, Printed in the United States VARIATIONS IN GRAIN BOUNDARY LEDGE STRUCTURE WITH THERMO- MECHANICAL TREATMENT IN HIGH-PURITY ALUMINUM Eswarahalli Venkatesh and L.E. Murr Department of Metallurgical and Materials Engineering New Mexico Institute of Mining and Technology Socorro, New Mexico 87801 {Received March 15, 1976) It is well known that thermal (annealing) treatments which alter grain size have a marked effect on flow stress and related mechanical properties of metals and a l l o y s . It has also been shown that different residual mechanical proper- ties can also be achieved for many metals and alloys by an appropriate schedule of thermo-mechanical treatment (1,2). Based upon earlier theoretical work of Li (3), Murr (4,5) has recently shown that the flow stress can be altered not only by variations in grain size, but also by changing the grain boundary struc- ture; in particular the density of grain boundary ledges. Such changes in the density of grain boundary ledges have been shown to be dependent upon quenching rate (6) and related thermo-mechanical treatment (7). The present paper is a preliminary report of the effects of thermo-mechanical treatment on the grain boundary ledge structure (density) in high-purity aluminum. It is shown that, similar to previous work on iron (6) and nickel (7), the density of grain bound- ary ledges can be significantly altered by appropriate schedules of thermo- mechanical treatment. High-purity (99.9999%) aluminum sheet, mill rolled to 0.004" thick was used in the so-called flash annealed condition. Batch samples were cut from the bulk- sheet and given a 50% reduction in thickness before annealing at 903°K for a few seconds. These annealed samples were subsequently quenched to different temper- atures and brought to room temperature. The total time spent in this process was less than one second. Another set of samples was cut and deformed 6, 18, and 50% by rolling; annealed in air at 903°K for about one second; quenched in liquid nitrogen and upquenched to room temperature immediately. A third set of samples was cut, and annealed in air at 903°K and finally given a creep treat- ment at 653°K at different constant loads. These treatments were assumed to test l.) the thermal (quenching) effect on grain boundary ledge density, 2.) the mechanical effect on ledge density, and 3.) the thermo-mechanical effect on ledge density. Several thin foil specimens for each thermo-mechanical schedule were pre- pared for examination by transmission electron microscopy using the technique of Westmacott and Peck (8), after ageing the treated samples at room temperatures for greater than 20 hrs. The specimens were examined in a Hitachi-Perkin Elmer H.U. 200F electron microscope operating at 200 KV. A large number of grain boundaries were examined and representative micrographs were recorded. Changes in grain boundary structure, in particular the ledge density, were evaluated. The grain boundary ledge density, which is defined here as the number of ledges per unit length of grain boundary, was measured directly from enlarged prints of grain boundary sections. Ledges were distinguished from other defects such as grain boundary dislocations or matrix dislocations by the presence of steps or kinks which produced slight to exaggerated interruptions in the boundary fringe contrast image. 477