Short bar (and short rod) fracture toughness of an aluminium 6061 metal matrix composite

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...

Toughness and fatigue behaviour of eutectic and hypereutectic Al–Si–Cu–Mg alloys produced through lost foam and squeeze casting

“…The toughness measured on the heat-treated specimens at room temperature (13-15 MPa m 1/2 ) is at the lower end of the toughness values range given for AMCs with a similar volume fraction of ceramic particles. [42][43][44][45] The fracture toughness obtained in the SQT6 alloys, is higher than that reported in other hypereutectic Al-Si alloys 46 and very similar to that reported in Al-Si alloys with lower silicon contents (8-10%Si). 47,48 The fine size of the eutectic silicon particles after the heat treatment is responsible for the high toughness values obtained.…”