Keith E. Knipling

Criteria for developing castable, creep-resistant aluminum-based alloys - A review

Abstract We describe four criteria for the selection of alloying elements capable of producing castable, precipitation-strengthened Al alloys with high-temperature stability and strength: these alloying elements must (i) be capable of forming a suitable strengthening phase, (ii) show low solid solubility in Al, (iii) low diffusivity in Al, and (iv) retain the ability for the alloy to be conventionally solidified. With regard to criterion (i), we consider those systems forming Al3M trialuminide compounds with a cubic L12 crystal structure, which are chemically and structurally analogous to Ni3Al in the Ni-based superalloys. Eight elements, clustered in the same region of the periodic table, fulfill criterion (i): the first Group 3 transition metal (Sc), the three Group 4 transition metals (Ti, Zr, Hf) and the four latest lanthanide elements (Er, Tm, Yb, Lu). Based on a review of the existing literature, these elements are assessed in terms of criteria (ii) and (iii), which satisfy the need for a dispersion in Al with slow coarsening kinetics, and criterion (iv), which is discussed based on the binary phase diagrams.

Influence of Co and Ni addition on the magnetocaloric effect in Fe88−2xCoxNixZr7B4Cu1 soft magnetic amorphous alloys

We have studied the magnetocaloric effect in a series of Fe88−2xCoxNixZr7B4Cu1 alloys. The partial substitution of Fe by Co and Ni leads to a monotonic increase in the Curie temperature (TC) of the alloys from 287 K for x=0 to 626 K for x=11. The maximum magnetic entropy change (ΔSMpk) at an applied field of 1.5 T, shows a value of 1.98 J K−1 kg−1 for x=8.25. The refrigerant capacity (RC) has maximum values near 166 J kg−1 (for x=0 and 2.75). These values place the present series of alloys among the best magnetic refrigerant materials, with an RC ∼40% larger than Gd5Si2Ge1.9Fe0.1 and ∼15% larger than Fe-based amorphous alloys.

Optimization of the refrigerant capacity in multiphase magnetocaloric materials

The refrigerant capacity (RC) of magnetocaloric materials can be enhanced using multiphase materials or composites, which expand the temperature range over which a significant magnetic entropy change can be obtained. Numerical simulations show that by controlling the parameters of the composite (the fraction of the different phases and their Curie temperatures) improvements of RC of ∼83% are possible. The maximum applied field plays a crucial, nonmonotonic, role in the optimization. As a proof of concept, it is shown that the combination of two Fe88−2xCoxNixZr7B4Cu1 alloys produces an enhancement in RC of ∼37%, making it ∼92% larger than that of Gd5Si2Ge1.9Fe0.1.

Fe-based nanocrystalline soft magnetic alloys for high-temperature applications

We report on improved high-temperature soft magnetic properties in Fe88−2xCoxNixZr7B4Cu1 nanocrystalline alloys. Substituting 5.5 at. % Co and Ni for Fe enhances the magnetization by 5% at ambient temperature and by 30% at 650 °C. The Curie temperature of the residual amorphous phase is also raised significantly (from 67 °C for x=0 to 298 °C for x=5.5), resulting in low coercivities (<30 A m−1) for Fe77Co5.5Ni5.5Zr7B4Cu1 over the temperature range 50–500 °C. The higher magnetization and Curie temperature as compared with other Fe-based alloys, and smaller Co content as compared with (Fe,Co)-based alloys, make this alloy attractive as an affordable high-temperature soft magnetic material.

Material Flow in Friction Stir Welds

Friction stir welding generates periodic features within the weld. These “onion ring” features are associated with variations in both texture and the orientation of that texture along the length of the weld. Analysis of an AA2195 friction stir weld reveals the presence of periodic oscillations between the dominant B and

Atom probe tomographic studies of precipitation in Al-0.1Zr-0.1Ti (at.%) alloys.

\overline{\text{B}}

Three-dimensional composition reconstruction of InAlAsSb lattice-matched to InP for top cell implementation

components of the ideal shear texture, suggesting a periodic reversal in the predominant shear orientation during welding that is inconsistent with current understandings of the friction stir welding process. Microstructural features present in the weld and machine force variations during welding indicate that these textures may arise from the oscillation of an off-centered tool. Such a tool oscillation can generate a periodic extrusion of material around the tool, giving rise to the observed flow features, machine force variations, and reversals of the local shear texture orientations. A new model of material flow during friction stir welding is proposed to explain the observed features.

Mechanisms of Particle Coarsening and Phase Transformation in Oxide Dispersion Strengthened Steels During Friction Stir Welding

The critical exponents of the alloy have been determined with the Kouvel–Fisher method to predict the field dependence of the magnetic entropy change ΔSM. The nonlinear fit of ΔSM(H) to a power law provides a field exponent in perfect agreement with the predictions of the relevant scaling laws using the obtained critical exponent values. It is shown that possible discrepancies between these two methods for determining the field dependence of ΔSM might arise due to a poor resolution in the temperature of the experiments.