THE IMPACT OF SPEED OF DRAWINGCARBON FIBER TOW THROUGHALUMINUM MELT ON THE TENSILESTRENGTH AND FIBER PULLOUTLENGTH DISTRIBUTIONSON FRACTURE SURFACES OFTHE UNIDIRECTIONAL CF/ALCOMPOSITES

We study tensile strength and fracture of the new high-strength unidirectional composites with an aluminum matrix alloyed with bismuth and continuously reinforced with carbon fibers by means of tensile tests, scanning electron (SEM), confocal laser microscopy (CLSM), and automated analysis of microstructure and fracture surface topography of specimens. The composite wires were produced at the experimental facility of the Institute of Solid State Physics by drawing 1000 filament carbon fiber tows through the matrix melt at varying speeds ranging from 2 to 16 mm/s with simultaneous ultrasonic treatment to accelerate impregnation. The diameter of the wire was 300 μm, the volume fraction of fibers was 58-65%, and the density was 2270 kg/m3. To analyze the distributions of fiber cross-sectional areas, local heights, and lengths of pulled-out fibers on fracture surfaces, we applied algorithms we developed for automated processing and quantitative analysis of SEM and CLSM images of fracture surfaces with complex topography obtained in bending or tensile tests to failure. Combined use of two complementary high-resolution microscopy techniques and in the joint analysis of fracture surfaces from both complementary halves of a specimen allow to obtain higher accuracy in recognizing all contours of structural elements and to extract a significantly larger amount of data than from individual images: to detect fiber boundaries, to determine fiber areas, centers, and diameters, to characterize the nonuniform distribution of fibers across the cross-section, to reconstruct the topography of fracture surfaces with large height gradients, and to calculate almost all of their quantitative characteristics, including the lengths of pulled-out fibers and their distribution density. These characteristics are useful for analyzing mechanical test data and sample fracture mechanisms, for evaluating and adjusting the properties of fiber-matrix interface, the components of fracture work, the effective physical and mechanical characteristics of composites, and for modeling their behavior. The influence of the time (speed) of drawing carbon fiber tow through the matrix melt on the stress-strain curves of CF/Al-0.5Bi wire samples obtained in tensile tests and on the distribution of lengths of pulled-out fibers on fracture surfaces were studied. It was found that the elastic modulus of the wire remained stable and equal to 125±10 GPa at all drawing speeds from 2 to 16 mm/s. The tensile strength increased monotonically from 950 to 1750 MPa when the drawing speed was enhanced from 2 to 8 mm/s, it was equal to 1700±100 MPa in the speed range of 6÷10 mm/s, and decreased with a further increase in speed.