Recycling of rare earths: a critical review

Modeling hardness of polycrystalline materials and bulk metallic glasses

A model is presented that explains the yield stress and hardness enhancements that have been observed in superlattice thin films. The stress required for dislocations to glide across layers with different shear moduli was calculated using an expression that accounts for core effects and all interfaces in trapezoidal or sawtooth composition modulations. The predicted strength/hardness enhancement increased with increasing superlattice period Λ, before reaching a saturation value that depended on interface widths. A second mechanism, where dislocations glide within individual layers, was important at large Λ and gave a decrease in strength/hardness with increasing Λ. The combination of these two mechanisms gives a strength/hardness maximum versus Λ in good quantitative agreement with experimental results for nitride and metal superlattices. The results indicate that superlattice strength/hardness depends strongly on interface widths and the difference in shear moduli of the two components for Λ values below the maximum, and on the average shear modulus for larger Λ.

Model of superlattice yield stress and hardness enhancements

Although steel has been the workhorse of the automotive industry since the 1920s, the share by weight of steel and iron in an average light vehicle is now gradually decreasing, from 68.1 per cent in 1995 to 60.1 per cent in 2011 (refs 1, 2). This has been driven by the low strength-to-weight ratio (specific strength) of iron and steel, and the desire to improve such mechanical properties with other materials. Recently, high-aluminium low-density steels have been actively studied as a means of increasing the specific strength of an alloy by reducing its density. But with increasing aluminium content a problem is encountered: brittle intermetallic compounds can form in the resulting alloys, leading to poor ductility. Here we show that an FeAl-type brittle but hard intermetallic compound (B2) can be effectively used as a strengthening second phase in high-aluminium low-density steel, while alleviating its harmful effect on ductility by controlling its morphology and dispersion. The specific tensile strength and ductility of the developed steel improve on those of the lightest and strongest metallic materials known, titanium alloys. We found that alloying of nickel catalyses the precipitation of nanometre-sized B2 particles in the face-centred cubic matrix of high-aluminium low-density steel during heat treatment of cold-rolled sheet steel. Our results demonstrate how intermetallic compounds can be harnessed in the alloy design of lightweight steels for structural applications and others.

Brittle intermetallic compound makes ultrastrong low-density steel with large ductility

Review on hydrometallurgical recovery of rare earth metals

This research involves using molten magnesium (Mg) to remove neodymium (Nd) from NdFeB magnet scrap by diffusion. Mg was melted over pieces of NdFeB scrap and held at temperatures in the range 675–705 °C for 2–8 h. The Mg was allowed to solidify, and the castings were then sectioned and characterized using scanning electron microscopy, x-ray diffraction, and chemical analysis. Nd was found to have diffused out of the solid scrap into the molten Mg. The thickness of the diffusion layer was measured, the diffusion of Nd through the NdFeB scrap into liquid Mg was described, and the diffusion coefficient of Nd in liquid Mg was estimated.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1129&context=mse_pubs

Liquid metal extraction of Nd from NdFeB magnet scrap

Previous studies of Cu-20 vol.% Nb alloys used optical and SEM techniques to evaluate the thickness and spacing of the Nb filaments. It had been known that the filament sizes were smaller than the resolution limit of the SEM at high deformation strain but it was assumed that this problem would not significantly change the observed trends. Results of this TEM study using dark-field techniques on the same materials show that the previous SEM measurements significantly overestimated the spacing at the higher deformations. Analysis of resistivity data on the samples also confirms this result. The strength dependence on Cu channel spacing, t Cu , was found to display a large transition region, with the dependence changing from a t Cu −0.5 at low strains to a t Cu −0.38 at high strains. The significance of these results is discussed. It is also pointed out that the filament sizes observed at high strains in these materials range to below 20 nm and they exhibit strength/filament size characteristics similar to vapor deposited nanocrystalline materials.

Measurement of filament spacing in deformation processed CuNb alloys

The strength, electrical conductivity, and substructure of deformation processed Cu-15vol%X alloys have been studied where X included Nb, Ta, and Cr. One alloy of Cu-15Nb was studied in which 2% Ag was added to examine solid solution strengthening of the Cu matrix. The alloys were prepared by consumable arc melting and ingot diameters of 7.6 and 15.2 cm were examined. Deformation was carried out at room temperature by rolling, press forging, and axisymmetric modes. The results show that the strength/conductivity properties of the Nb, Ta, and Cr alloys are essentially the same and are slightly better than the Cu-20vol%Nb alloys previously measured. The Ag alloy was found to be stronger at a given deformation, but the solid solution Ag decreased the conductivity more than it increased strength so that the net effect was to reduce the strength at a given conductivity. TEM studies showed that the substructures of all the alloys were similar to each other and to previous results on Cu-20vol%Nb alloys. Deformation by both press forging and rolling are not as effective at increasing strength as is axisymmetric deformation.

Development of deformation processed copper-refractory metal composite alloys

The phase relationships and solid-solution region of the Bi 2 Sr 2 Ca 1 Cu 2 O y (2212) superconductor were investigated for compositions around the ideal 2212 stoichiometry at 865°C in one bar of oxygen. Changes in the starting composition were accommodated by changes in the types and composition of the secondary phases present, the number of intergrowths within the 2212 phase, and the solid solution composition of the 2212 phase. Compositional analysis of the superconducting phase was carried out in the transmission electron microscope (TEM) in order to separate the effects of solid-solution changes and the presence of intergrowths upon measured compositions. The actual solid-solution region determined from TEM measurements showed most of the solid solution substitution to occur between Sr and Ca. The highest superconducting transition temperatures ( T c ) were found in annealed samples for which the measured Sr+Ca content of the 2212 phase was closest to the ideal stoichiometric ratio of 3 (42.86 at.%). No correlation was found between the transition temperature and lattice parameters of the superconducting phase.

Characterization of the phase relations and solid solution range of the Bi2Sr2Ca1Cu2Oy superconductor

Measurements were made of the compositional range of the Bi 2 Sr 2 CaCu 2 O x superconductor in which the effects of intergrowths and grain-boundary phases were eliminated. At 800°C, the solid solution extends from Bi 2.06 Sr 2.09 Ca 0.83 Cu 2.01 O y to Bi 2.09 Sr 1.13 Ca 1.79 Cu 1.99 O y . Most of the cation interchange occurs between Sr and Ca while only a small variation in the Cu content was observed. In addition, substitution of Bi for Sr and/or Ca extends the range of compositions, although the reverse substitution of Sr or Ca for Bi does not seem to occur. The degree of interchange between Sr and Ca was found to decrease slowly with increasing temperatures in pure oxygen. In low oxygen partial pressures (0.1% 0 2 ), a significant portion of the Ca-rich solid solution region was found to be unstable.

L.S. Chumbley

Papers

Liquid metal extraction of Nd from NdFeB magnet scrap

Measurement of filament spacing in deformation processed CuNb alloys

Development of deformation processed copper-refractory metal composite alloys

Characterization of the phase relations and solid solution range of the Bi2Sr2Ca1Cu2Oy superconductor

Solid solution region of the Bi2Sr2CaCu2Oy superconductor