Madan G. Mendiratta

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.

Advanced intermetallic alloys—beyond gamma titanium aluminides

Recent studies have shown that Nb-base refractory intermetallic materials may be viable for service at temperatures that compete with the nickel-based superalloys in structural applications. While advanced intermetallics in monolithic form have limited prospects for providing the required balance of properties for use at high operating temperatures, two-phase or multiphase intermetallic systems composed of a ductile, Nb-base refractory phase in equilibrium with one or more silicide intermetallics show promise for further development as structural materials. Exploratory efforts on these systems have indicated that damage tolerance, creep resistance and oxidation tolerance may be obtained and controlled simultaneously. In turn, these findings have led to alloy development strategies and approaches which are currently under investigation and are expected to lead to focused research on a small set of alloys. This manuscript presents an overview of recent results on selected Nb-base intermetallic systems with particular emphasis on phase equilibria and mechanical behavior.

Microstructures and Mechanical Behavior of Nb-Ti Base Beta + Silicide Alloys

Studies on Nb/Nb 5 Si 3 based in-situ composites have demonstrated an acceptable balance of low-temperature damage tolerance and high-temperature strength/creep resistance. However, catastrophic oxidation and embrittlement of these materials limit their usefulness in structural applications. Alloying studies were initiated at Wright Laboratory with the aim of obtaining incremental improvement in the overall oxidation response of the Nb/Nb 5 Si 3 system, while seeking microstructurally similar systems. The results showed that reduced metal recession rates and oxygen embrittlement can be obtained by Ti and Al additions to Nb-Si base alloys. This paper focuses on the effect of Ti and Al alloying additions to Nb-Si base alloys on phase equilibria, microstructures, temperature dependence of strength, low-temperature toughness, and environmental resistance.

DO3-B2-α phase relations in Fe-Al-Ti alloys

A transmission electron microscopic investigation has been conducted to determine the phase relations, transformation temperatures, and microstructures in Fe-Al-5 at. pct Ti alloys with Al content varying from 18 to 25 at. pct. As compared to the binary Fe-Al alloys, the effects of Ti addition are (i) a significant expansion of the (α + DO3) phase field, (ii) increased transition temperatures, and (iii) a tendency of the DO3 thermal anti-phase domain boundaries to exhibit anisotropy with a preference to lie on {100} planes.

Tensile flow and fracture behavior of DO3 Fe-25 at. pct Al and Fe-31 at. pct al alloys

The tensile properties, fracture modes, and deformation mechanisms of two DO3 alloys, Fe-25 and Fe-31 at. pct Al, have been investigated as a function of temperature up to 600°C. The first alloy was produced by powder metallurgy and hot-extrusion, the second by casting and hot-extrusion. At room temperature extensive plastic deformation occurs in these intermetallics, exhibiting an elongation to fracture of 8 pct and 5.6 pct, respectively. In the Fe-25Al alloy the deformation process consisted of motion and extensive cross-slip of ordinary dislocations and associated formation of antiphase-boundary (APB) bands, while in the Fe-31 Al alloy, plasticity occurred by the motion of superlattice dislocations which eventually dissociated to form APB bands. At room temperature both alloys exhibited transgranular cleavage fracture modes. The variation of tensile properties and fracture modes with temperature is presented.

Microstructures and mechanical behavior of NiAl-Mo and NiAl-Mo-Ti two-phase alloys

The phase relationship in the NiAI-Mo system is characterized by a eutectic equilibrium between binary NiAl and the terminal (Mo) solid solution, thereby offering the potential for development of ductile-phase-toughened composites. A study was conducted to evaluate the effect of varying volume fraction of the (Mo) phase on the microstructure, bend strength, and ambient temperature fracture behavior of selected NiAI-Mo two-phase alloys. Above room temperature, the NiAI-Mo alloys showed an increase in bend strength compared to monolithic NiAl, with reasonable strength retention up to ≈800 °C. The results demonstrated moderate improvements in toughness in the NiAI-Mo alloys in comparison to monolithic NiAl. A further enhancement in toughness was realized through hot working. Fractography studies showed evidence for substantial decohesion between the (Mo) phase and the NiAl matrix, thereby suggesting the presence of a weak interface. This weak interface between the (Mo) phase and the NiAl matrix, in conjunction with modulus mismatch stresses, causes the crack to deflect from the (Mo) rein-forcement and propagate preferentially along the (Mo)/NiAl interface. These attributes limit the potential for significant ductile-phase toughening in the NiAI-Mo system. An addition of 0.2 at. pct Ti resulted in a marked improvement in the room-temperature fracture toughness of NiAI-Mo. Fractography observations show some evidence for (Mo)/NiAl interface strengthening with the Ti addition.

Unconstrained and constrained tensile flow and fracture behavior of an Nb-1.24 At. Pct Si alloy

The unnotched and notched tensile behavior of the β-phase constituent (Nb with Si in solid solution) of the (Nb)/Nb5Si3 composite has been investigated at room temperature and -196 °C. At room temperature, the unnotched tensile behavior comprises significant strengthening due to Si, low strain-rate sensitivity, low strain hardening, extensive ductility, and ductile microvoid coalescence fracture, even at strain rates as high as 1.1 s−1. At −196 °C, the unnotched alloy exhibited much higher strength, good ductility, and cleavage fracture. At room temperature, the notched specimens exhibited cleavagelike fracture with significant plasticity, and at −-196 °C, they exhibited cleavagelike fracture with much lower plasticity at the notch. A finite-element analysis (FEA) of stress and strain fields in the vicinity of the notch root, together with un-notched tensile behavior, indicates that plasticity plays an important role in nucleating cracks, while the high-axial tensile stress component governs crack propagation. These results are used to rationalize the observed toughening and fracture behavior of a (Nb)/Nb5Si3 composite.

Notch fracture inγ-titanium aluminides

The notch fracture behavior of twoγ-titanium aluminide alloys, having duplex and fully lamellar microstructures, has been investigated as a function of notch geometry and test temperature. The unnotched tensile properties and notch fracture loads are used to perform finite element analysis (FEA) to determine triaxial tensile stresses and effective plastic strains in the vicinity of notch roots. These results, together with fractographic examinations of notch failures, indicate that a crack nucleates in the triaxial tensile field when the effective von Mises stress just exceeds the uniaxial tensile yield stress. The high tensile stress component then propagates the nucleated microcrack to failure with local stress intensity reaching the toughness of the material. Thus, both plasticity and high tensile stress are required to cause notch failure.

A Review of Recent Developments in Iron Aluminides

This paper presents a review of recent research conducted at the Air Force Wright Aeronautical Laboratories (AFWAL)/Materials Laboratory to develop iron-aluminides as elevated temperature structural materials. The research consisted of investigations on the microstructure, tensile behavior, deformation, and fracture mechanisms of the DO 3 and B2 ironaluminides. Binary Fe-Al alloys with a wide range of Al contents, solidsolution ternary alloys and precipitation- and dispersion-strengthened two-phase alloys have been investigated. It is shown that iron-aluminides have a potential to be structural materials at least up to 650°C.

Anisotropic analysis of dislocation line energies and possible dislocation core dissociations in MoSi2

Abstract The line energy factors of(110)[001], [331], [11],[11];(100)[010];(103)[331], [010]; and (011)[100], [111] dislocations as well as the elastic interaction forces between two 1/4<331] and two 1/4<111] partials, in MoSi2, are calculated using anisotropic elasticity theory. The line energy factors are found to be relatively large (170–250 GPa) and isotropic, whereas the non-radial interaction forces are found to be a small fraction of the radial forces. The atomic configurations around planar faults on the {110), {013) and {116) planes are analysed using the embedded atom method technique. Results of such calculations are used to energetically rank the possible core dissociations of 1/2<331], 1/2<111], <110], and <100] dislocations on the {110), {013) amd {116) planes in MoSi2. Collectively, these results suggest that the core structures of 1/2<331], 1/2<111], <110] and <100] dislocations are expected to be complicated and non-planar, similar to dislocation cores in b.c.c derivative B2 structures. T...