Search

Two-phase materials, such as α+β Titanium (Ti) alloys, are technologically important. A number of factors can affect deformation behavior, including the interaction stresses between phases, the crystallographic relationships between phases, and the morphology. As a result, the deformation mechanisms of two-phase alloys may be different from the individual single-phase materials. For example, twinning may not occur in a single phase material if the grain size is very small but twinning can occur in a very fine grained alloy if the second phase contributes to the interfacial stresses due to elastic interactions. Interaction stresses can result from the difference in the elastic properties of the two phases. In particular, these elastic interaction stresses can be quantified by the finite element method (FEM). In this paper recent developments regarding two-phase deformation mechanisms will be reviewed and the ramifications on mechanical behavior in regard to two-phase Ti alloys in particular and on two-phase metallic materials in general will be outlined.

We have experimentally investigated the effect of V and Zr insertion thickness on perpendicular magnetic anisotropy (PMA) in [Co/Pt multilayers]/[Zr, V]/CoFeB (Hard/Soft) composite structures. Micromagnetic simulations have been performed to study the effect of the Gilbert damping constant on the switching characteristics in a hard/soft composite structure for weak and strong interlayer exchange coupling. The V and Zr insertions are studied due to their low Gilbert damping constant with the CoFeB interface. For both insertions, the interlayer exchange coupling remained ferromagnetic in nature for all the thicknesses studied. We observed an increase in PMA with increasing V and Zr insertion thicknesses. The composite layer with V insertion provides thermal stability comparable to the previously reported Ta insertion. From micromagnetic simulations, we observed that the effect of the Gilbert damping constant on switching characteristics is largest for thinner insertions (strong ferromagnetic coupling) and the effect diminishes with increasing insertion thickness (weak ferromagnetic coupling).

Ferritic-austenitic cast duplex stainless steels (CDSS) undergo significant thermal aging embrittlement in service in light water nuclear reactors. Cast CF–3 and CF–8 duplex stainless steels have been laboratory aged for up to 17,200 h at four different temperatures (280 °C, 320 °C, 360 °C, 400 °C) in order to characterize the evolution of microstructure and mechanical properties. Standard Charpy V-Notch (CVN) impact testing and tensile testing have been conducted to measure the progression of embrittlement of the steels. Concurrently, nanoindentation measurements were performed to measure the hardening of the individual ferrite and austenite phases. Atom probe tomography (APT) analysis of the constituent phases is used to investigate the relationship between spinodal decomposition of the ferrite and the mechanical property evolution during aging. The effects of the local microstructure and property changes in the ferrite phase on the overall bulk mechanical property degradation of these steels are discussed.

The microstructure and mechanical properties in unaged and thermally aged (at 280 °C, 320 °C, 360 °C, and 400 °C to 4300 h) CF–3 and CF–8 cast duplex stainless steels (CDSS) are investigated. The unaged CF–8 steel has Cr-rich M23C6 carbides located at the δ–ferrite/γ–austenite heterophase interfaces that were not observed in the CF–3 steel and this corresponds to a difference in mechanical properties. Both unaged steels exhibit incipient spinodal decomposition into Fe-rich α–domains and Cr-rich α’–domains. During aging, spinodal decomposition progresses and the mean wavelength (MW) and mean amplitude (MA) of the compositional fluctuations increase as a function of aging temperature. Additionally, G–phase precipitates form between the spinodal decomposition domains in CF–3 at 360 °C and 400 °C and in CF–8 at 400 °C. The microstructural evolution is correlated to changes in mechanical properties.

The phase properties and deformation behavior of the δ –ferrite and γ –austenite phases of CF–3 and CF–8 cast duplex stainless steels were characterized by nanoindentation and microstructure-based finite element method (FEM) models. The elastic modulus of each phase was evaluated and the results indicate that the mean elastic modulus of the δ –ferrite phase is greater than that of the γ –austenite phase, and the mean nanoindentation hardness values of each phase are approximately the same. The elastic FEM model results illustrate that greater von Mises stresses are located within the δ –ferrite phase, while greater von Mises strains are located in the γ –austenite phase in response to elastic deformation. The elastic moduli calculated by FEM agree closely with those measured by tensile testing. The plastically deformed specimens exhibit an increase in misorientation, deformed grains, and subgrain structure formation as measured by electron backscatter diffraction (EBSD).

Ta inserts in double magnetic tunnel junctions have been shown to induce perpendicular magnetic anisotropy. We fabricated the central layers of a CoFeB/MgO based double magnetic tunnel junction with a Ta insertion layer between the free layers of the magnetic tunnel junctions. The thickness of the Ta insert and CoFeB layer were varied from 0.5 to 1.1 nm and 0.9 to 1.7 nm respectively, to find which minimum thickness of Ta will induce perpendicular anisotropy in the MTJ. FMR studies were performed to measure the interfacial anisotropy Ku,i. We demonstrated that the most effective stack minimized the CoFeB thickness while maximizing the Ta thickness. This was balanced with the need to keep both CoFeB layers from decoupling or becoming magnetically dead.

Cast duplex stainless steel piping in light water nuclear reactors experience thermal aging embrittlement during operational service. Interest in extending the operational life to 80 years requires an increased understanding of the microstructural evolution and corresponding changes in mechanical behavior. We analyze the evolution of the microstructure during thermal aging of cast CF-3 and CF-8 stainless steels using electron microscopy and atom probe tomography. The evolution of the mechanical properties is measured concurrently by mechanical methods such as tensile tests, Charpy V-notch tests, and instrumented nanoindentation. A microstructure-based finite element method model is developed and utilized in conjunction with the characterization results in order to correlate the local stress-strain effects in the microstructure with the bulk measurements. This work is supported by the DOE Nuclear Energy University Programs (NEUP), contract number DE-NE0000724.

The (111) orientation preferred L10 FePtB has been obtained by post-deposition annealing sputtered FeB/Pt multilayers on thermally oxidized silicon substrates. Block copolymer templating was employed to pattern FeBPt film. A matrix study of etch time and etch angle showed that ion-milling at 75° for 3 min yielded the highest coercivity. Reannealing after patterning process was found to improve the crystalline structure and coercivity significantly. These results suggested (111) orientation preferred L10 FePt patterned by block copolymer templating may be promising for tilted media and bit patterned media.

Comparisons of microstructural evolution and magnetic properties were made of a FeB12/Pt10/[FeB1.2/Pt1]15/Ta5 nm multilayered structure with a FeB12/Pt10/FeBPt33/Ta5 nm co-deposited structure. The Ta capping layer was used to protect the films from oxidation. Both these samples were sputtered in the same planetary deposition system onto thermally oxidized silicon substrates. They both represent layer-by-layer deposition, with the second type of deposition having atomically fine layers, more than an order of magnitude finer than the first type. The samples were annealed at a range of times, temperatures, and vacuum conditions. X-ray diffraction (XRD), transmission electron microscopy, and alternating gradient magnetometry were employed to characterize the structural and magnetic properties, respectively. Significant differences were observed between the two types of structures. A maximum coercivity of 8.9 kOe was seen for the atomically fine multilayer, about 10% more than that for the coarse multilayer. XRD analysis confirmed that both the coarse and fine multilayers were in the L10 phase after annealing. Our results indicate that the co-deposited film, which is really composed of atomically fine multilayers, is superior to the coarse multilayered FeB/Pt for the formation of L10-phase FePt.