Issue 37

M. S. Raviraj et alii, Frattura ed Integrità Strutturale, 37 (2016) 360-368; DOI: 10.3221/IGF-ESIS.37.47 361 there is sufficient lattice matching (coherency) for the direct nucleation of solid Al grains to occur on the particle surface [2,3] thus resulting in change in microstructure and properties of the composite. The Al-TiC MMCs occupy a unique position in the family of metal matrix composites as they find numerous applications in commercial aerospace, space technology, automobile, general industrial and engineering structures [4, 5]. Investigators [6, 7] showed the stir casting technique is the simplest and most commercial used technique to incorporate the ceramic particulate into liquid aluminum melt and allowing the mixture to solidify. Fatchurrohman [8] has justified that the solidification characteristics of TiC particulate reinforced Al alloy matrix composites are correlated with its hardness by stir casting technique . Some of the basic mechanical properties and micro-structural findings of Al6061 alloy matrix reinforced with various proportions of TiC particulates estimated by stir casting technique have been reported in our earlier publications [9]. However, evaluation of fracture toughness is essential for the reliable use of these composites for structural parts and components. Researchers [10-13] estimated fracture toughness for Al alloy matrix composites with SiC and Al 2 O 3 reinforcement particles. They showed that the reinforcement particles play significant effects in controlling the fracture toughness of the composites. To the best of our knowledge, not much of the work on fracture toughness values of Al-TiC composites are reported in open literature. In the present work an attempt is made to produce Al6061 alloy MMCs with different wt% TiC particles by stir casting method and the experimental work carried out to evaluate the fracture toughness and micro-structural features of fracture surfaces. E XPERIMENTAL WORK he Al-TiC metal matrix composites were produced by stir casting method so as to achieve uniform distribution of TiC particulates in the Al matrix. Commercially available Al6061alloy was melted in the graphite crucible and TiC particulates (2µm average in size) in the form of powder with three different wt% were added while stirring the melt at a speed of 750rpm. The melt at temperature of about 920 o C was poured to rectangular shaped moulds of required size. Composites of various rectangular sheets or blocks with 3wt%, 5wt% and 7wt% of TiC were produced. The chemical composition and the mechanical properties of Al6061-TiC metal matrix composites are reported in our earlier work [9]. The compact tension specimens were machined out by CNC wire EDM from the rectangular blocks of various wt% TiC composites as per ASTM Standard E399 [14]. The detail dimensions and schematic views of the CT specimen are shown in Fig. 1. The CT specimens with variable thickness were machined so as to obtain different B / W of 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7, as shown in Fig. 2(a). The straight through V-notch was machined so as to achieve a / W of 0.5 after fatigue pre- cracking for a length of 2.5mm, as seen in Fig. 2(b). The intended notch in the test specimen is to simulate an ideal crack with essentially zero root radius [15]. The fatigue pre-cracking of the CT specimens was done in a BiSS (Bangalore Integrated Systems Solutions) servo-hydraulic testing machine with tensile cyclic loading. The fatigue loads were maintained at 0.4 times the yield load of the material with the load ratio ( R ) (min. tensile load/max. tensile load) of 0.1maintaining the frequency of 10 Hz to obtain pre-crack length of 2.5mm successfully. The method of pre-cracking is similar to the earlier work [16]. Different load ranges were used depending on the specimen thickness. Once the crack T Figure 1 : CT specimen specifications.