FABRICATION AND MECHANICAL PROPERTY CHARACTERISATION OF Al-A1203-MgO. CAST.-. PARTICULATE -iV1MCcCONTAINING HIGH VOLUME FRACTION OF DISPERSIOD A1203 PARTICLES AND SQUEEZED IN THE PRESSURE RANGE OF 80-140 MPa

Abstract

In the present work an effort has been made to disperse relatively coarser A1203 particles (105-150 pm) in commercially pure Aluminium base matrix using "modified MgO coating technique" and "squeeze casting route". The objective was to attain a high oVf retention of A1203 in the matrix thin the usual 17-22oVf retention achieved so far. It was found that big clusters of A1203 particles tend to form in this attempt. A uniform dispersion of A1203 particles .with proper intervening matrix resulting in adequate interparticle spacing and mean free path was therefore not realised. It was thus concluded that "liquid metallurgy route" is not a suitable technique for the fabrication of MMPCs containing high %Vf retention of A1203. However, upon squeezing, the density of composite is found to increase from 2.90 gm/cc for 80 MPa to 2.94 gm/cc for 140 MPa composite. Similarly, the Vicker's hardness and microhardness of composite was found to increase considerably upon squeezing. Also, the composites display excellent tensile properties at ambient temperature in a progressively increasing manner from gravity chill cast till 140 MPa squeeze pressure. The UTS of composites also improve upon squeezing. The UTS of gravity chill cast composite was found to be 99.9 MNm2 while the composite squeezed at 140 MPa displayed the UTS value of 123 MNm(2. The 0.2% proof stress also similarly resulted an improvement from 64.95 for gravity chill cast to 78.16 MNm-2 for 140 MPa squeezed composite. The fracture surfaces of composite squeezed at 140 MPa displayed fully ductile fracture features upon examination by SEM. The above observations of the nature of fracture are also supported by the data of % elongation and % crosssectionAIin area. But the mechanical properties and fracture features of the composite squeezed at 80 MPa presented considerable deviation from the normal trend. This was attributed to the presence of areas in the gauge length of tensile specimens showing huge coagulation of A1203 particles. It is primarily because of this reason that this composite suffered premature failure and also displayed typical brittle fracture features