James Hanna

A new high-strength spinodal alloy

(Received 15 January 2005; accepted 2 February 2005) Preliminary investigations of a new high-strength alloy of composition Fe 30 Ni 20 Mn 25 Al 25 (at.%) are described in this paper. The as-cast alloy consisted of a periodic two-phase microstructure of interconnected, ∼50-nm-wide rods with fully coherent {100} interfaces, strongly suggestive of formation by a B2 to [(B 2+ body-centered cubic (bcc)] spinodal decomposition. The (Ni,Al)-rich B2 and (Fe,Mn)-rich bcc phases differed in lattice parameter by <0.5%. Hardness and yield strength of the as-cast alloy were found to be approximately 500 VPN and 1500 MPa, respectively, and increased by more than 50% after annealing at 550 °C for several days. (Fe,Mn)-rich precipitates with a –Mn structure were observed in the annealed material. The quaternary metallic Fe–Ni–Mn–Al system is of interest for its rich assortment of possible phase transformations and, hence, wide array of potential microstructures and properties. As part of an investigation into Fe– Ni–Mn–Al alloys, we cast several alloys of nominal composition Fe50�xNixMn25Al25, where x 0, 10, 20, 25, 30, 40, or 50. When x 0 or 50, a single-phase L21 structure occurs. Several other alloys in the series are two-phase, with high hardness values (HV 400–550) and what appear to be spinodal microstructures. This paper outlines our preliminary investigations of one of these spinodal alloys, Fe30Ni20Mn25Al25, which displays remarkable strength. Castings of approximately 50 g and nominal composition Fe30Ni20Mn25Al25 were produced by arc melting 99.97% Fe, 99.95% Ni, 99.9% Mn, and 99.9% Al pieces under an argon atmosphere. Subsequent measurements on a Cameca SX50 electron probe microanalyzer showed that final compositions were all within 0.4 at.% of the nominal value. Some variation in the scale of as-cast microstructures was present between castings, although properties and behavior were similar. Mechanically polished disks, 3 mm in diameter and ∼200 m thick, were electropolished in a solution of 30% nitric acid in methanol at a potential of 16V at �20 °C using a Struers Tenupol 5, and subsequently cascade rinsed in ethanol and

Surfactant-induced migration of a spherical drop in Stokes flow

In Stokes flows, symmetry considerations dictate that a neutrally buoyant spherical particle will not migrate laterally with respect to the local flow direction. We show that a loss of symmetry due to flow-induced surfactant redistribution leads to cross-stream drift of a spherical drop in Poiseuille flow. We derive analytical expressions for the migration velocity in the limit of small nonuniformities in the surfactant distribution, corresponding to weak-flow conditions or a high-viscosity drop. The analysis predicts migration toward the flow centerline.

Microstructure of a Spinodal Fe-Ni-Mn-Al Alloy

The crystal structures, chemical compositions, and morphology of the phases in a new quaternary alloy, Fe30Ni20Mn25Al25, were investigated using transmission electron microscopy (TEM), electron and x-ray diffraction, and x-ray energy dispersive spectroscopy (EDS). This alloy is of interest for its very high strength and hardness, and as an example of a class of quaternary transition metal alloys in which both spinodal decomposition behavior and ordering reactions are found. The alloy is also somewhat magnetic.