Narrow Results

Gd-doped ceria nano powder(~5 nm) was shock-loaded by a plate impact experiment using a single stage light gas gun. A computational model was used to simulate the shock state (pressure and density changes along time) of the sample in two dimensional Eulerian code. The predicted density of compacted sample from the simulation was about 90%. To reveal the effect of shock compaction on sintering behavior, the recovered sample was heat-treated and the microstructure was compared with that of a conventionally compacted and heat-treated sample.

The cubic (Ll₂) titanium trialuminide intermetallics obtained by alloying a tetragonal (DO₂₂) Al₃Ti compound with Cu, Ni, Fe, Cr, and Mn have low densities, excellent oxidation resistance up to 1000°C, good mechanical strength up to fairly high temperatures and a substantial apparent ductility in compression. All these attributes make them potential candidates for engineering applications as high temperature materials. Unfortunately, despite their cubic crystallographic structure they are brittle in tension and exhibit rather limited fracture toughness at ambient temperature (~4 MPam^1/2). This paper is an overview of our efforts to improve mechanical properties of Ll₂-stabilized titanium trialuminides by producing a nanostructured material employing a controlled ball milling of powders and subsequent consolidation into bulk form by various methods with the major goal of retaining their nanocrystalline microstructure. The nanostructured, disordered (most probably FCC solid solution) ball milled powders of Mn – stabilized titanium trialuminide (Al₃Ti (Mn)) were successfully consolidated into low – porosity, ordered (Ll₂) ordered bulk compacts by hot pressing at ~900°C for ~15 min under continuous pressure (210 – 430 MPa). In order to retain the nanostructure the heating time to the hot consolidation temperature must be minimized (< 4 – 5 min). The as – milled disordered powders can also be consolidated into low – porosity compacts by a triple shock – wave loading. Shock consolidated compact made from the as – milled disordered Al₃Ti(Mn) exhibits a web of fine microcracks indicating its brittleness and the powder particles in the compact do not exhibit plastic deformation during shock - wave consolidation. The indentation fracture toughness of the hot consolidated, fully ordered powder compact of cubic (Ll₂) titanium trialuminide with possibly nanocrystalline structure, is only ~2 MPa m^0.5 compared to 4 – 5 MPa m^0.5 for a coarse-grained, homogenized Ll₂Al₃Ti (Mn). It seems that the nanostructure does not automatically guarantee high toughness of nominally brittle intermetallics.