FRACTURE BEHAVIOUR OF RAPID SOLIDIFICATION PROCESSED IRON-ALUMINUM-SILICON ALLOYS (DELAMINATION, ANISOTROPY, DISPERSION STRENGTHENING)

Iron-aluminum silicon alloys prepared by the hot consolidation of RSR Fe-Al-Si alloy powders have been found to show dramatic increases in strength and ductility and outstanding high temperature oxidation resistance compared to conventionally processed alloys. The fine grain size (< 1 (mu)m) in these alloys is stabilised by a fine dispersion of extremely fine TiB(,2) particles (0.01-0.1 (mu)m). This investigation is focussed on the evaluation of fracture toughness (J(,IC)/K(,IC)) of RSR Fe-Al-Si alloys and the influence of varying the consolidation techniques on the fracture behavior of these alloys. Blended alloys prepared by the consolidation of blends of a relatively hard alloy powder and a relatively soft Fe-8Al alloy powder were also investigated. Alloys powders were consolidated by (1) hot extrusion and (2) hot isostatic pressing plus hot working. Alloys were tested using three point bend specimens for the L-T orientation and using double cantilever beam specimens for the T-L orientation.; The RSR Fe-Al-Si alloys show a marked improvement in room temperature fracture toughness compared to ingot metallurgy processed alloys. However they exhibit highly anisotropic behaviour. The extruded alloys behave like a fiber composite with a high interfacial strength. The ductile Fe-8Al alloy and blended alloys, when tested in L-T orientation, showed crack tip blunting by delamination at nearly the same J level of about 90 kJ/m('2). In less ductile alloys, no crack tip blunting was observed. Extensive micro-cracking was observed on the fracture surface. The toughness level in these alloys were in the range of 2-4 kJ/m('2).; The HIPed plus hot forged plus hot rolled Fe-8Al alloy behaved like a laminate. When tested in L-T orientation, it showed stable crack extension. Ahead of the crack front, delaminations parallel to the rolling plane occurred and the subsequent deformation occurred under plane stress conditions. The high value of toughness (89 kJ/m('2)) observed at room temperature was retained to a large extent (62 kJ/m('2)) even at temperatures as low as 77 K. The enhanced fracture toughness is due to the fine grain size (about a micron) and the dispersion size (0.01 to 0.1 (mu)m). The anistropy exhibited by these alloys stems from the redistribution of TiB2 particles on planes parallel to the extrusion direction or the rolling plane.