Leontios Christodoulou

The role of equiaxed particles on the yield stress of composites

Possible explanations are investigated for the yield strength enhancement of discontinuously reinforced Al alloy matrix MMCs, for the case of low temperature yield behavior where deformation occurs by dislocation slide. The Al alloys contain 0.1-10 micron diameter equiaxed particle discontinuous reinforcements of TiB2, Al2O3, and TiC. Attention is given to a single dislocation-particle interaction model, and both dislocation pile-up and forest-hardening multiple-dislocation particle interaction models.

Creep deformation of TiB2-reinforced near-γ titanium aluminides

The deformation behavior under creep conditions of several unreinforced and participate (TiB2)-reinforced binary near-gamma titanium aluminide alloys has been examined. Material has been produced using both ingot-and powder-processing techniques, yielding material of varying microstructures and grain sizes. It has been found that the rate of deformation is strongly dependent on matrix grain size and microstructural stability. The presence of the dispersed phase dramatically impacts the microstructural character which evolves following isothermal forging, and this has been observed to strongly influence the creep behavior. Constitutive equations which describe the observed behavior indicate that the mechanism of creep deformation is the same for both the reinforced and unreinforced variants and is independent of processing route.

Investment-cast processing of XDTM near-γ titanium aluminides

Abstract Investment-cast processing of γ-based titanium aluminide alloys represents a viable means to manufacture near-net-shaped components. Alloys containing TiB 2 introduced via XD TM processing are particularly attractive because of the substantial grain refinement and improved microstructural uniformity in the as-cast condition. It has been demonstrated that such alloys can be produced and components fabricated using only minor modifications to conventional titanium alloy melt practice and cast processing. Examples of microstructures and representative mechanical properties, demostrating the potential of these materials for high temperature structural applications, will be presented.

Effect of phase morphology on the mechanical behavior of two titanium aluminide composites

Two binary titanium aluminide alloys, Ti-43A1 and Ti-47A1 (atomic percent), were discontinuously reinforced with 6 vol pct titanium diboride, resulting in a two-phase Ti3Al and TiAl (α2 and γ, respectively) matrix with a dispersion of TiB2 particulate. Cast material was successfully ex-truded and subjected to a series of single-step and duplex-step heat treatments. Thermo-mechanical processing was correlated with microstructural changes, and the ambient temperature mechanical properties were measured for the various heat-treated conditions using tensile and hardness testing. Yield stress and plastic elongation to failure and hardness were found to cor-relate well with the fraction of proeutectoid, or primary, TiAl formed during heat treatment within the α/γ phase field. Precipitation of y within proeutectoid α grains during subsequent aging treatments within the α2 phase field was seen to increase the room-temperature ductility with negligible debits in yield stress. Enhanced ductility and decreases in yield stress and hard-ness are associated with morphologically large regions of the TiAl phase. Incompatibility of slip systems across γ/α2 and the inherent resistance to slip in hyperstoichiometric Ti3Al are suggested as possible explanations for the observed phenomena.