Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.

Mechanical Alloying And Milling

A comprehensive review is presented of the extractive metallurgy of rare earths. The topics covered are: world rare earth resources and production; ore processing and separation of individual rare earths; reduction, refining, and ultrapurification of rare earth elements; methods for rare earth materials analysis; and a selection of the numerous rare earth applications. World rare earth reserves are abundant and would last for well beyond the next century. However, all of the 16 naturally occurring rare earth elements are not equally distributed in the ore minerals. This, compounded with the problems specific to the isolation and recovery of each of the rare earths, sets the stage for an unequal rare earth availability. The close chemical similarity of rare earths looses its importance when divergent physical properties determine the processes for rare earth element reduction and refining. The rare earth metals, alloys, and compounds have been as pure as could be determined. Finally, the commercial...

Extractive Metallurgy of Rare Earths

A review is given of the physical properties, composition and crystal structure of intermetallic compounds formed between rare-earth elements and non-magnetic metals, with emphasis on the magnetic properties. Included are the properties of compounds in which the non-rare-earth component is a 4d or 5d transition element. Special consideration is given to the properties of pseudo-binary compounds. Results of magnetisation measurements, neutron diffraction and neutron scattering are discussed together with results derived from NMR, ESR and Mossbauer effect spectroscopy. An evaluation is given of the relevance of the experimental results with respect to different types of exchange interactions in this class of intermetallics. Special consideration is also given to the influence of crystal field effects on the magnetic properties and, furthermore, to the occurrence of intermediate valences in several of the compounds of Ce, Sm, Eu and Yb.

https://iopscience.iop.org/article/10.1088/0034-4885/42/8/003/pdf

Intermetallic compounds of rare earths and non-magnetic metals

Planetary ball mills are well known and used for particle size reduction on laboratory and pilot scales for decades while during the last few years the application of planetary ball mills has extended to mechanochemical approaches. Processes inside planetary ball mills are complex and strongly depend on the processed material and synthesis and, thus, the optimum milling conditions have to be assessed for each individual system. The present review focuses on the insight into several parameters like properties of grinding balls, the filling ratio or revolution speed. It gives examples of the aspects of grinding and illustrates some general guidelines to follow for modelling processes in planetary ball mills in terms of refinement, synthesis' yield and contamination from wear. The amount of energy transferred from the milling tools to the powder is significant and hardly measurable for processes in planetary ball mills. Thus numerical simulations based on a discrete-element-method are used to describe the energy transfer to give an adequate description of the process by correlation with experiments. The simulations illustrate the effect of the geometry of planetary ball mills on the energy entry. In addition the imaging of motion patterns inside a planetary ball mill from simulations and video recordings is shown.

Process engineering with planetary ball mills

We discuss recent research on phase transitions in cerium under pressure, including new experiments on the transition, which indicate that it is a hidden structural phase transition. We also discuss some models of relevance to the field, both theoretical and computational.

The puzzle of the $ \gamma\to\alpha$ and other phase transitions in cerium

The common methods presently used to prepare samples of the heavy rare-earth metals for lattice parameter determinations were shown to give high results due to contamination by interstitial impurities, especially hydrogen. Two methods to produce fine-grained homogeneous samples in the form of wires were established. Both methods have the advantage of not requiring further heat treatment after preparation of the surface to be X-rayed. The lattice parameters of Gd, Tb, Dy, Ho, Er, Tm, Lu, Y and Sc were determined on well-characterized samples. The density, mole atomic volume, atomic radius for CN12, and c a ratios were calculated based on the lattice parameters determined. The effect of hydrogen on the lattice parameters of these metals was determined.

The effect of impurities, particularly hydrogen, on the lattice parameters of the “ABAB” rare earth metals

The solubility of hydrogen in Gd, Er, Tm, Lu and Y was determined from 25 to 850°C when the metal was in equilibrium withRH2−x
 (x varies between 0.1 and 0.2 depending on the rare earth metal). The room temperature solubilities determined by the lattice parametric method were found to be <0.1, 3.6, 7.7, 20.6 and 19.0 at, pct H in Gd, Er, Tm, Lu and Y, respectively. The change in unit cell volume for each atomic percent hydrogen added was nearly the same for all metals. The solubility of hydrogen increases more rapidly with temperature in those metals with low solubility at room temperature. Thus the solubility of hydrogen at 850°C is nearly the same in all five of the metals studied, that is, 35.0, 36.2, 36.0, 36.0 and 37.3 at. pct H in Gd, Er, Tm, Lu and Y, respectively. The equilibrium pressure of H2 in these studies was the equilibrium pressure of hydrogen in contact withRH2−x
 at the temperature concerned. A change in slope was observed in the solubility curves of the Gd-, Er-, Tm- and Lu-H systems. The logC) (at. pct H inR) was plottedvs 1/T for each system. Straight lines were obtained at temperatures above and below the changes in slope of the solubility curves. A calculation of the approximate ΔH of solution ofRH2−x
 in the metal sfrom the slope of the lines gave 4.35, 1.88, 1.28, 0.61 and 0.55 kcal/mole for Gd, Er, Tm, Lu and Y, respecitively in the low temperature portion. The change in slope which occurs at some point between 350°C and 650°C, depending on the metal, indicates a lower heat of solution ofRH2−x
 in these metals at the higher temperatures. In Lu there appears to be yet another change in slope in the neighborhood of 250°C.

The solubility of R H 2−x in Gd, Er, Tm, Lu and Y from ambient to 850°C

Lattice parameters of La, Ce (β and γ), Pr, Nd, Sm, Eu and Yb were determined on well-characterized, high-purity metals in the form of small-grained wires. The effect of sample preparation on the lattice parameters of these metals is discussed. A method to obtain crystallographically-pure polycrystalline d.h.c.p. lanthanum is presented.

The lattice parameters of La, Ce, Pr, Nd, Sm, Eu and Yb

The low-temperature heat capacity of the superconducting ${\mathrm{LaS}}_{x}$ ($1.333l~xl~1.500$) compounds with the ${\mathrm{Th}}_{3}$${\mathrm{P}}_{4}$-type structure were measured from 1.3 to 20.0 K in magnetic fields of 0, 2.50, 5.39, 7.62, and 9.98 T. The compositional variation of the electronic specific-heat parameter $\ensuremath{\gamma}$ and the Debye temperature ${\ensuremath{\Theta}}_{D}$ in $\mathrm{La}{\mathrm{S}}_{x}$ are discussed in connection with the electric properties of these alloys. The results also show that the $\mathrm{La}{\mathrm{S}}_{x}$ alloys near $x=1.333$ are strong-coupling type-II superconductors having large values of the electron-phonon coupling constant values, $\ensuremath{\lambda}=0.71$, and upper critical field values of ${H}_{c2}(0)$, e.g., 16.2 T for ${\mathrm{LaS}}_{1.366}$. The ${\mathrm{LaS}}_{1.346}$ and ${\mathrm{LaS}}_{1.355}$ alloys undergo a cubic-to-tetragonal phase transition at 85 and 55 K, respectively. The former composition has the intrinsic coherence length ${\ensuremath{\xi}}_{0}$ of 158 \AA{}, which gives a large value for the intrinsic Ginzburg-Landau parameter, ${\ensuremath{\kappa}}_{0}=11$. However, this tetragonal phase, which forms only in the narrow composition region of $1.333l~xl~1.362$, has a lower $\ensuremath{\gamma}$ value than the cubic-phase alloys with $x\ensuremath{\simeq}1.37$. Furthermore, the superconducting transition temperatures of $\mathrm{La}{\mathrm{S}}_{x}$ compounds hardly have any compositional variation for $xl~1.36$. Finally, there is a large drop in ${H}_{c2}(0)$ from 16.2 T for the cubic phase at $x$ = 1.366 to 9.7 T for the tetragonal phase at $x=1.346$. This study also clearly illustrates that lattice instability is an important factor governing the superconducting behavior of these alloys, but the best superconducting properties are found in the stable phase (bcc) just before it becomes unstable, rather than in the phase which actually undergoes the transformation.

Heat capacity in superconducting and normal-state LaS/sub x/ (1. 333< or =x< or =1. 500) compounds

The resistivity \ensuremath{\rho}(T) of ${\mathrm{CeH}}_{\mathrm{x}}$, x=2.70, 2.78, and ${\mathrm{LaH}}_{\mathrm{x}}$, x=2.70, 2.80 2.90, and 2.93, was measured in the temperature range 100\ensuremath{\le}T\ensuremath{\le}500 K, using an indirect Q-meter technique. The measured values of ${\mathrm{CeH}}_{2.78}$ are in excellent agreement with the previous direct-contact measurements of Libowitz, Pack, and Binnie. In all cases except ${\mathrm{LaH}}_{2.93}$, \ensuremath{\rho}(T) peaks at 240\ensuremath{\le}T\ensuremath{\le}270 K, in rough agreement with the onset of the slight tetragonal distortion of the fcc phase and/or the formation of an ordered hydrogen superlattice. Thus, the temperature coefficient of the resistivity \ensuremath{\alpha} is positive below the peak but negative above. In ${\mathrm{LaH}}_{2.90}$ the peak is very sharp and the behavior semiconductinglike above 250 K. In ${\mathrm{LaH}}_{2.93}$ the behavior is semiconductinglike throughout the measured range. The room-temperature resistivity of ${\mathrm{LaH}}_{\mathrm{x}}$ rises sharply with x, from around 7\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ \ensuremath{\Omega} cm at x=2.70 to around 0.5 \ensuremath{\Omega} cm at x=2.93. These results are discussed in light of previous experimental results and band-structure calculations of ${\mathrm{LaH}}_{2}$ and ${\mathrm{LaH}}_{3}$. It is suggested that the peak in \ensuremath{\rho}(T) is induced by an order-disorder transition, and the density of states at the Fermi level rapidly decreases to zero and a gap is consequently opened up as the concentration x approaches the trihydride stoichiometric limit.

B. J. Beaudry

Papers

The effect of impurities, particularly hydrogen, on the lattice parameters of the “ABAB” rare earth metals

The solubility of R H 2−x in Gd, Er, Tm, Lu and Y from ambient to 850°C

The lattice parameters of La, Ce, Pr, Nd, Sm, Eu and Yb

Heat capacity in superconducting and normal-state LaS/sub x/ (1. 333< or =x< or =1. 500) compounds

Q-factor measurements of the bulk resistivity and nonmetal-metal transitions in LaHx and CeHx (x >= 2.70).