Ductile Phase Toughening Research Articles

Strength and ductile-phase toughening in the two-phase Nb/Nb5Si3 alloys

The effect of heat treatment on the mechanical properties of Nb-Nb5-Si3 two-phase alloys having compositions Nb-10 and 16 pct Si (compositions quoted in atomic percent) has been investigated. This includes an evaluation of the strength, ductility, and toughness of as-cast and hot-extruded product forms. The two phases are thermochemically stable up to ∼1670 °C, exhibit little coarsening up to 1500 °C, and are amenable to microstructural variations, which include changes in morphology and size. The measured mechanical properties and fractographic analysis indicate that in the extruded condition, the terminal Nb phase can provide significant toughening of the intermetallic Nb5Si3 matrix by plastic-stretching, interface-debonding, and crack-bridging mechanisms. It has been further shown that in these alloys, a high level of strength is retained up to 1400 °C.

Plastic Flow and Fracture of B2 NiAl-based Intermetallic Alloys Containing a Ductile Second Phase.

The use of NiAl as a structural material has been hindered by its lack of tensile ductility or toughness at room temperature. The operative flow and fracture mechanisms in monolithic NiAl leading to these poor low temperature properties are analyzed, demonstrating the need for ductile phase toughening. Progress in ductile phase reinforced intermetallics and NiAl-based materials are reviewed and the primary mechanisms involved in the flow and fracture of ductile phase reinforced alloys are clarified by recent investigations of directionally solidified NiAl-based materials. The mechanical behavior of these model alloys (Ni-39Al and Ni-30Fe-20Al (at%)) are discussed. The prospects for developing a ductile phase toughened NiAl-based alloy and the shortcoming presently inherent in these systems are analyzed.

Ductile Phase Toughening of Brittle Intermetallics

AbstractOne method of imparting damage tolerance into intermetallic compounds is to introduce ductile particles in a fine uniform distribution. Systematic criteria are given as to the necessary requirements for such ductile phase reinforced systems. Two composite systems, Cr2Nb/Nb and Nb3Al/Nb were selected for further evaluation. Both microstructural evaluations as well as thermal stability were demonstrated in both systems. Small cracks introduced via micro-hardness indentations were measured as a function of precipitate size. In Cr2Nb/Nb the crack lengths decreased with increasing particle size which was explained through increasing effectiveness of ductile particles with minimum effective particle radii calculated at 6-10 μm. Larger particles along with poor ductile phase distributions led to lower damage tolerance in aged Nb3Al/Nb. By maintaining uniform ductile particle distributions, intermetallic compounds possessing superior damage tolerance have been demonstrated.

Ductile-Phase Toughening in Niobium-Niobium Silicide Powder Processed Composites

AbstractThe Nb-Si system offers a possibility of ductile-phase toughening of the brittle Nb5Si3 intermetallic with the terminal niobium-silicon solid solution. Powder composites have been made in which the volume fraction of the terminal Nb-Si phase is systematically varied in a matrix of Nb5Si3 in order to study the extent of toughening. The Nb-Si solid-solution phase was observed to exhibit cleavage failure under both as hot pressed and heat treated conditions, thereby limiting the toughening attained by the presence of this phase. Hot working of the composite results in a dramatic improvement in toughness because of a change in the plastic behavior and fracture mode of the terminal Nb-Si phase from brittle cleavage to a mixed mode of cleavage and ductile microvoid growth and coalescence (dimpled) fracture.

Toughening Mechanisms in Intermetallic γ-TiAl Alloys Containing Ductile Phases

AbstractThis work is aimed at developing understanding of ductile phase toughening in powder-processed intermetallic γ-TiAl alloys. The nominally ductile phases studied include Nb and Ti6Al4V, in the form of pancake shaped particles. Toughening is primarily due to the formation of crack face bridging zones. Toughness increases in the composites with a Ti6Al4V phase are limited due to particle embrittlement during processing. In this case, toughening is associated with the formation of short, low ductility/high strength bridge zones. Larger toughness increases and significant resistance curves are observed in composites containing Nb particles. In this case, long, high ductility/low strength bridge zones are observed. The behavior of the Nb composites is consistent with current understanding of constrained deformation of embedded particles and bridge zone toughening models. Additional toughening was observed in the Nb composites due to crack deflection and blunting in orientations where the crack intersects the broad pancake face.