Observation of Nb filaments in Cu-Nb in-situ composites

This paper reports on in-situ composites that are two-phase structures produced by heavy deformation of a coarse initial structure, such as a cast ingot, blended powders, or stacked sheets. After deformation the structure consist of a matrix phase and a strengthening phase, which is present in the form of extremely fine filaments of sheets depending upon the type of deformation processing method used. The most widely studied in-situ composites are the Cu-Nb alloys, however Ta, Fe, and Cr have also been used as the strengthening phase for filament formation. The filaments are believed to be responsible for blocking dislocation motion through the composite material and are extremely important in producing the high strengths noted in these alloys; however, considerable controversy exists as to the exact role they play in strengthening the matrix. Due to the importance of these filaments, several studies have been conducted using transmission electron microscopy (TEM) to measure the size and spacing of these filaments as a function of deformation and relate the results to the observed strengths. Obtaining statistically valid data using TEM is often questioned since only small areas are typically examined in TEM foils and these regions may not be representative of the bulk sample. Thus, many samples must be examined in order to obtain reliable data. It is in these studies that conical-scan dark-field (CS-DF) has proven to be an extremely useful tool and has enabled more accurate information to be obtained from thicker regions of the foils examined, and by allowing observation over large areas of the sample