No abstract

This note serves to report some short bar and short rod fracture toughness results for COMRAL-85 TM [ 1], a metal matrix composite consisting of aluminium alloy 6061 reinforced with alumina-based ceramic microspheres.The COMRAL-85 TM composite was fabricated by a liquid metallurgy route and extruded to 19mm diameter rod. The matrix was commercial A1 6061 alloy and the reinforcement comprised 20 volume percent of alumina-based polycrystalline ceramic microspheres with average diameter 20pm [2]. This material was heat-treated to a T6 condition, that is, solution treated at 530°C for 1 1/2 hours followed by pre-aging at room temperature for 20 hours and aging at 175°C for 8 hours. Figure 1 shows the optical microstructure of COMRAL-85, from which it is apparent that the particle distribution is nearly homogeneous. The mechanical properties of both composite and matrix alloy are given in Table 1 [3]. Fracture toughness testing was conducted in accordance with ASTM E 1304-89 [4]. Figure 2 shows the geometry of the chevron-notched specimens and the orientations selected for this study. Fracture surfaces were examined using scanning electron microscopy.Short rod (and bar) fracture toughness test results are contained in Tables 2-4. Three specimens of R-L orientation with 19mm diameter section, six specimens of L-R orientation of 8ram square section, and six specimens of C-R orientation also of 8mm square section, were tested by the standad test method. K_ M and K_ were measured for each. Out of 15 specimens, only 4 valid test R14These results are comparable with a K~ value of 18.5 MPa m obtained by other workers from 75mm wide by 12.5mm thick plate material using 50mm wide compact tension specimens [6]. By comparison, I~ for monolithic A1 6061-T6 is around 33 MPa m [7]. In the present case, fracture toughness values for COMRAL-85-T6 in the L-R and C-R orientations are similar, but that in the R-L orientation is less than the others. The possible factors contributing to this anisotropy are, fh'stly, the presence of particle-free bands along the extrusion direction and, secondly, anisotropy of the grain structure. Although the particle distribution is relatively homogeneous, there are some localised particle-free areas and these are elongated in cigar shapes along the extrusion direction. In the L-R orientation these particle-free bands are aligned perpendicular to the plane in which the crackpropagates, Thereforc, thc ~-~ . . . . . : . . . . . . '-,~,, , ,~,,.",r, ,., ,~,.~. couldbe-retarded by them since they are much tougher than the bulk composite. In the C-R orientation they are aligned in the same plane as, and parallel to, the advancing crack front. As such, these particle-free bands may constitute a long barrier to the propagatintg crack. However, in the R-L orientation the particle-free bands are aligned perpendicular to, and coplanar with, the advancing crack front, so that they would not serve to retard its propagation. Figure 3 shows an S.E.M. micrograph of the fracture surface. Many broken particles are revea...