Laves Type Research Articles

Laves type intermetallic compounds as hydrogen storage materials: A review

Laves type AB 2 intermetallics belong to the most abundant group of intermetallic compounds containing over 1000 compounds. A large variety of the chemical nature of A (Mg, Ca, Ti, Zr, Rare Earth Metals) and B (V, Cr, Mn, Fe, Co, Ni, Al) metals together with the existence of the extended solid solutions formed by mixing various selected components on both A and B sites dramatically extends the list the known binary and ternary individual compounds. A vast majority of the Laves type intermetallics crystallises with C15 / FCC MgCu 2 and C14 / hexagonal MgZn 2 types of structures, both formed for a large range of ratios between the atomic radii of the A and B components outside the ideal ratio r A / r B = 1.225. Their hydrogenation performance is defined by the chemical composition and structure of the alloys and proceeds according to the following alternative / parallel mechanisms: (a) Formation of the insertion type interstitial hydrides containing up to 6–7 at. H/f.u.AB 2 ; (b) Amorphisation of the alloys on hydrogenation; (c) Disproportionation with the formation of a binary hydride of the A metal and depleted by A metal B-components based alloys/hydrides. Equilibrium pressures of hydrogen desorption from the AB 2 -type hydrides span a huge range of ten orders of magnitude and thus Laves type-based intermetallics satisfy the requirements for various applications including getters of hydrogen gas, volume- and mass-efficient hydrogen storage materials operating at ambient conditions, materials for the efficient thermally driven compression of hydrogen gas with an output pressure of several hundred bar and high capacity and high rate anode materials for the metal hydride batteries operating in a challenging temperature range - at subzero temperatures and also above 60 °C. The paper contains references to 245 publications and will guide the future work in the areas of fundamental research and also in advancing the applications of the hydrides of the Laves type intermetallics. • Laves type AB 2 intermetallics are the most abundant group of alloys. • Hydrogenation performance is defined by the chemical composition and structure. • Mechanisms of hydrogenation: interstitial hydrides / amorphization / disproportionation. • Structure-properties relationships are described. • Important applications are reviewed: H stores/MH compression/MH batteries/H getters.

Electrochemical hydrogenation, lithiation and sodiation of the GdFe2–xMx and GdMn2–xMx intermetallics

Electrochemical hydrogenation, lithiation and sodiation of the phases GdFe2–xMx and GdMn2–xMx (M=Mn, Co, Ni, Zn, and Mg) and the influence of doping components on electrochemical characteristics of electrode materials on their basis were studied using X-ray powder diffraction method, scanning electron microscopy, energy dispersive X-ray analysis, X-ray fluorescent spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. Phase analysis showed a simple correspondence between unit cell parameters of the phases and atomic radii of doping elements. Electrode materials based on GdFe2 and GdMn2 doped with 2 at.% of Co, Ni and Mg demonstrated better hydrogen sorption properties than those doped with Mn and Zn. Corrosion resistance of the doped electrodes was also better than of the binary analogues (e.g. corrosion potential of the GdFe2-based electrode was –0.162 V whereas that of GdFe1.96Ni0.04 was –0.695 V). The capacity parameters were increased in the following ranges: Zn<Mn<Mg<Co<Ni and Zn<Fe<Mg<Co<Ni for GdFe2–xMx and GdMn2–xMx, respectively. After fifty cycles of charge/discharge, we observed the changes in surface morphology and composition of the electrode samples. In the structure of studied Laves type phases with MgCu2-type structure, the most suitable sites for hydrogen atoms are tetrahedral voids 8a. During lithiation and sodiation of the phases, the atoms of the M-component of the structure are replaced by the atoms of lithium, and the atoms of gadolinium are replaced by the atoms of sodium. This difference in interaction is due to the difference in atomic sizes of the atoms. No insertion of lithium or sodium into the structural voids of the phases was observed.

Hydrides of Laves type Ti–Zr alloys with enhanced H storage capacity as advanced metal hydride battery anodes

The present work was focused on the studies of the effect of variation of stoichiometric composition of Ti–Zr based AB2±x Laves phase alloys by changing the ratio between A (Ti + Zr) and B (Mn + V + Fe + Ni) components belonging to both hypo-stoichiometric (AB1.90, AB1.95) and over-stoichiometric (AB2.08) alloys further to the stoichiometric AB2.0 composition to optimize their hydrogen storage behaviours and performances as the alloy anodes of nickel metal hydride batteries.AB2-xLa0.03 Laves type alloys (A = Ti0.15Zr0.85; B = Mn0.64–0.69V0.11–0.119Fe0.11–0.119Ni1.097–1.184; x = 0, 0.05 and 0.1) were arc melted and then homogenized by annealing.The studies involved probing of the phase-structural composition by X-Ray diffraction (XRD), together with studies of the microstructural state, hydrogen absorption–desorption and thermodynamic characteristics of gas–solid reactions and electrochemical charge-discharge performance, further to the impedance spectroscopy characterization. The alloys were probed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and XRD.These studies concluded that the alloys contained the main C15 FCC Laves type AB2 intermetallic co-existing with a secondary C14 hexagonal Laves phase and a small amount of LaNi intermetallic. The gaseous H storage capacity and electrochemical performances were found to be closely dependent on the stoichiometric compositions of the alloys. High discharge capacities approaching 500 mAh/g were achieved for the AB1.95 alloy. At 500 mA/g current density, the discharge capacity was maintained as high as 80%, with very low capacity retention after 500 cycles. The alloy which had the highest capacity retention after 500 cycles was found to be AB2.0 alloy. Furthermore, AB2.0 alloy showed an excellent cyclic stability together with a high hydrogen diffusion coefficient. Studies of hydrogen diffusion coefficients showed that annealing enhanced the H diffusion rates allowing advanced performance at high discharge current densities. This has been related to a higher content of C15 Laves compound which allows to reach a higher H mobility in contrast with higher storage capacity observed for the C14 Laves type polymorph.

Studies of Zr-based C15 type metal hydride battery anode alloys prepared by rapid solidification

The objective of the present work was to study an interrelation between the conditions of the rapid solidification casting and electrochemical properties of a C15 Laves-type Zr-based metal hydride battery anode alloy. A 7-component overstoichiometric with B/A>2 alloy (Ni + Mn + V + Fe/Zr + Ti + La = 2.078) was Zr- and Ni-rich. Melt spinning was performed using as cast mother ingot at different rotation speeds of 5, 16.5, and 33 Hz with a 20-cm diameter Cu cooling wheel. The phase composition and morphology of the ribbons were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) along with X-ray energy dispersive spectroscopy (EDS). Partial Mn vaporization took place during the melt spinning and became more pronounced with the increase in the quenching rate at higher wheel rotation speeds. The SEM study showed a successful refinement and a more homogeneous distribution of La in the melt-spun alloys. However, the alloys demonstrated a more difficult activation performance as compared to the as-cast sample which was associated with introduction of Zr into the composition of the secondary La-and Ni-rich phase when quenching the melt. Presence of Zr in the secondary phase results in inferior rate of its hydrogenation. The formed during the rapid solidification (La,Zr) (Ni,Mn) secondary phase also contained a significant amount of oxygen additionally worsening the activation performance. The grain size gradually decreased from 3.5 μm in the as-cast alloy to approximately 250 nm for the melt spun sample obtained at the highest cooling rate. The two-phase alloys contained a mixture of two Laves type intermetallics, C15 and C14 ones. The abundance of the secondary C14 phase decreased with increase of the cooling rate as compared to the as-cast sample. The electrochemical performance was significantly affected by the compositional and microstructural changes associated with the variation in the cooling rate. The highest electrochemical discharge capacity of 414 mAh/g was obtained for the 16.5 Hz melt spun rate because of an optimized composition of the Laves phase. Furthermore, the diffusion coefficient of hydrogen in this alloy was the highest in a full state of charge, which indicates that faster hydrogen diffusion and shorter hydrogen diffusion paths were achieved via an application of the rapid solidification process. An optimization of the rapid solidification conditions is required to improve an activation behavior of the studied Laves types metal hydride anode alloys.

The electrochemical performance of melt-spun C14-Laves type Ti[sbnd]Zr-based alloy

The main objective of the present work is to study the effect of rapid solidification on the electrochemical performance of Zr-based Laves type alloy with a nominal composition Ti12Zr21.5V10Cr7.5Mn8.1Co8Ni32.2Al0.4Sn0.3. The samples were prepared from the as-cast arc melted buttons by melt spinning at different copper wheel rotation speeds of 5, 16.5, 33, and 100 Hz, which are equivalent to linear speeds of 6.3, 21, 41, and 62.8 m s−1 respectively using a cooling wheel with a diameter of 20 cm. The phase composition and morphology of the ribbons were analyzed by X-Ray diffraction (XRD) and scanning electron microscopy (SEM). The microstructural changes of the ribbons induced by the variations in the wheel rotation speed were found to be closely related to the electrochemical performances. High discharge capacities exceeding 400 mAh∙g−1 were achieved for the melt spun samples during the measurements at low current densities. Furthermore, melt spun casting performed at the highest wheel rotation speed of 100 Hz resulted in the best rate performance of the alloy. As this alloy has the smallest crystallite size, this resulted in the shortest H atoms diffusion distances, and thus increased the efficient H diffusion rate and improved the electrochemical performance.

Ternary gallides RE 4Rh9Ga5, RE 5Rh12Ga7 and RE 7Rh18Ga11 (RE=Y, La–Nd, Sm, Gd, Tb) – intergrowth structures with MgCu2 and CaCu5 related slabs

Abstract Fourteen ternary gallides RE 4Rh9Ga5, RE 5Rh12Ga7 and RE 7Rh18Ga11 (RE=Y, La–Nd, Sm, Gd, Tb) were synthesized from the elements by arc-melting, followed by different annealing sequences either in muffle or induction furnaces. The samples were characterized through Guinier powder patterns and the crystal structures of Ce4Rh9Ga5, Ce5Rh12Ga7, Ce7Rh18Ga11, Nd5Rh10.44(4)Ga8.56(4), Nd4Rh9Ga5 and Gd4Rh9Ga5 were refined from single crystal X-ray diffractometer data. The new gallides are the n=2, 3 and 5 members of the RE 2+n Rh3+3n Ga1+2n structure series in the Parthé intergrowth concept. The slabs of these intergrowth structures derive from the cubic Laves phase MgCu2 (Mg2Ni3Si as ternary variant) and CaCu5 (CeCo3B2 as ternary variant). Only the Nd5Rh10.44(4)Ga8.56(4) crystal shows Rh/Ga mixing within the Laves type slabs. Magnetic susceptibility measurements reveal Pauli paramagnetism for Y4Rh9Ga5 and Curie-Weiss paramagnetism for Gd4Rh9Ga5 and Tb4Rh9Ga5. Low-temperature data show ferromagnetic ordering at T C=78.1 (Gd4Rh9Ga5) and 55.8 K (Tb4Rh9Ga5).

MgCo2-D2 and MgCoNi-D2 systems synthesized at high pressures and interaction mechanism during the HDDR processing

MgCo2 and MgNiCo crystallize with hexagonal Laves type intermetallic structures of the C14 type and do not form hydrides at ambient hydrogen pressures. However, applying high hydrogen pressures in the GPa range forces the hydrogen absorption and leads to the formation of multi-phase compositions, which contain approximately 2.5 atoms H per formula unit of MgCo2 or MgNiCo and remain thermally stable under normal conditions.The hydrogenation of MgCo2 resulted in its decomposition to a ternary Mg2CoD5 deuteride and metallic cobalt. Phase-structural transformations accompanying the vacuum desorption of deuterium in the temperature range of 27–500°C were studied using in situ neutron powder diffraction. The investigation showed a complete recovery of the initial MgCo2 intermetallic via a Hydrogenation-Disproportionation-Desorption-Recombination process. At 300°C, the Mg2CoD5 deuteride first decomposed to elementary Mg and hexagonal Co. At 400°C, a MgCo phase was formed by interaction between Mg and Co. At the highest processing temperature of 500°C, a solid-state interaction of MgCo and Co resulted in the recovery of the initial MgCo2.The interaction of MgNiCo with deuterium under the synthesis conditions of 2.8GPa and 200°C proceeded in a more complex way. A very stable ternary deuteride MgNi2D3 was leached away while Co was separated in the form of Mg2CoD5 and the remaining nickel formed a solid solution with Co with the approximate composition Ni0.7Co0.3.The thermal desorption of deuterium from MgCo2D2.5 and from MgNiCoD2.5 has been studied by Thermal Desorption Spectroscopy with deuterium released into a closed volume. The observed effects nicely correlate with changes in the phase structural composition of the hydrides formed.MgCo2 is a new example of the hydrogen storage alloy, in which a successful HDDR processing results in the reversible formation of the initial intermetallic at much lower temperatures than in the equilibrium phase diagram of the Mg-Co system.

Hydrogen in La2MgNi9D13: The Role of Magnesium

Reversible hydrogen storage capacity of the La(3-x)Mg(x)Ni(9) alloys, charged by gaseous hydrogen or by electrochemical methods, reaches its maximum at composition La(2)MgNi(9). As (La,Mg)Ni(3-3.5) alloys are the materials used in advanced metal hydride electrodes in Ni-MH batteries, this raises interest in the study of the structure-properties interrelation in the system La(2)MgNi(9)-H(2) (D(2)). In the present work, this system has been investigated by use of in situ synchrotron X-ray and neutron powder diffraction in H(2)/D(2) gas and by performing pressure-composition-temperature measurements. The saturated La(2)MgNi(9)D(13.1) hydride forms via an isotropic expansion and crystallizes with a trigonal unit cell (space group R3m (No.166); a = 5.4151(1) Å; c = 26.584(2) Å; V = 675.10(6) Å(3)). The studied hybrid structure is composed of a stacking of two layers resembling existing intermetallic compounds LaNi(5) (CaCu(5) type) and LaMgNi(4) (Laves type). These are occupied by D to form LaNi(5)D(5.2) and LaMgNi(4)D(7.9). The LaNi(5)D(5.2) slab has a typical structure observed for all reported LaNi(5)-containing hybrid structures of the AB(5) + Laves phase types. However, the Laves type slab LaMgNi(4)D(7.9) is different from the characterized individual LaMgNi(4)D(4.85) hydride. This results from the filling of a greater variety of interstitial sites in the La(2)MgNi(9)D(13)/LaMgNi(4)D(7.9), including MgNi(2), Ni(4), (La/Mg)(2)Ni(2), and (La/Mg)Ni(3), in contrast with individual LaMgNi(4)D(4.85) where only La(2)MgNi(2) and Ni(4) interstitials are occupied. Despite a random distribution of La and Mg in the structure, a local hydrogen ordering takes place with H atoms favoring occupation of two Mg-surrounded sites, triangles MgNi(2) and tetrahedra LaMgNi(2). A directional bonding between Ni, Mg, and hydrogen is observed and is manifested by a formation of the NiH(4) tetrahedra and MgH(6) octahedra, which are connected to each other by sharing H vertexes to form a spatial framework.

Effect of magnesium on the crystal structure and thermodynamics of the La 3− xMg xNi 9 hydrides

The present work gives the data of systematic studies of the influence of magnesium on the crystal structure and hydrogenation behaviour of the PuNi 3 -type La 1− x Mg x Ni 3 ( x = 0–0.67) intermetallic alloys. Synchrotron X-ray diffraction studies revealed that substitution of La in LaNi 3 by Mg proceeds in an ordered way, only within the Laves type layers of the hybrid crystal structures build from the MgZn 2 - and CaCu 5 -type slabs. When completed, it leads to the formation of LaMg 2Ni 9 (2MgNi 2 + LaNi 5). Gradual increase of Mg content is accompanied by a linear decrease of the volumes of the unit cells. Interestingly, a substantial contraction takes place also for the Mg-free CaCu 5 -type slabs. Hydrogen interaction with the La 1− x Mg x Ni 3 alloys was investigated by in situ synchrotron X-ray, neutron powder diffraction and pressure–composition–temperature studies. In the whole substitution range, La 1− x Mg x Ni 3 alloys form intermetallic hydrides with H/ M ratio ranging from 0.77 to 1.16. Magnesium influences structural features of the hydrogenation process and determines various aspects of the hydrogen interaction with the La 1− x Mg x Ni 3 intermetallics causing: (a) more than 1000 times increase in equilibrium pressures of hydrogen absorption and desorption for the Mg-rich LaMg 2Ni 9 as compared to the Mg-poor La 2.3Mg 0.7Ni 9 and a substantial modification of the thermodynamics of the formation–decomposition of the hydrides; (b) an increase of the reversible hydrogen storage capacities following increase of Mg content in the La 1− x Mg x Ni 3 to ∼1.5 wt.% H for La 2MgNi 9; (c) improvement of the resistance against hydrogen-induced amorphisation and disproportionation; (d) change of the mechanism of the hydrogenation from the anisotropic to isotropic one. Thus, optimisation of the magnesium content provides different possibilities in improving properties of the studies alloys as hydrogen storage and battery electrode materials.

Magnetic and transport properties of Laves phase Dy 1− xMm xCo 2 ( x = 0–0.5) alloys

The influence of Mm substitution (Mm = mischmetal) on structural, transport and magnetic properties of (Dy 1− x Mm x )Co 2 ( x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) alloys has been investigated by means of X-ray diffraction (XRD), temperature dependent electrical resistivity ( ρ( T)), ac susceptibility ( χ( T)) and thermopower ( S( T)). XRD patterns show the formation of solid solutions crystallizing with cubic Laves (C15) type structure at room temperature. The pronounced discontinuities in the resistivity and thermopower at Curie temperature ( T C) are explained based on the suppression of the spin-fluctuation contribution. The gradual decrease in T C and sharpness of discontinuities in ρ( T) and S( T) with increasing Mm substitution has been discussed.

Composite Hypo-Hyper-d-Intermetallic and Interionic Phases as Supported Interactive Electrocatalysts

Interactive, strong interbonding and highly electron conductive nonstoichiometric titanium suboxide catalytic supports, Magneli phases (Ti(n)O(2n-1), on average Ti(4)O(7)), have been used in the electrocatalysis of hydrogen (HELR) and oxygen (OELR) electrode reactions with remarkable consequences and advanced achievements. The theory of hypo-hyper-d-interelectronic bonding of transition metal ions and atoms has been employed for selective ordered grafting and shown to stay in the core of the strong metal-support interaction (SMSI) in heterogeneous catalysis and electrocatalysis, and thereby the substantial cause for the improved synergistic activity of composite (electro)catalysts. The same fundament has been the thermodynamic basis for the thermal production of symmetric intermetallic Laves type phases of nanostructured electrocatalysts, in particular the ones with higher oxophilic properties of hypo-d-elements. Remarkably advanced in electrocatalytic activity, highly monatomically dispersed deposits of Pt upon Magneli phases are shown to be unique and highly promising electrocatalysts for the cathodic oxygen reduction (ORR). Nanostructured Au upon a thin nanocrystalline film of anatase titania has been confirmed by X-ray photoelectron spectroscopy (XPS) as a typical classical paradigm of the SMSI, and at the same time affording the basis for gold with strained d-orbitals, as the reversible hydrogen electrode. Magneli phases have been shown to be the best electrocatalytic supports with unique properties both for low temperature PEM fuel cells (LT PEM FCs) with pronounced CO tolerance and water electrolysis in membrane type hydrogen generators.

Structural effects on kinetic properties for hydrogen electrode reactions and CO tolerance along Mo–Pt phase diagram

The effect of structural and surface versus bulk properties of Mo–Pt alloys and intermetallic phases taken along their phase diagram upon kinetic and electrocatalytic features for the cathodic hydrogen evolution (HER) has been investigated and displayed. All specimens along Mo–Pt phase diagram in broader reversible potential range feature Volmer–Tafel mechanism with the catalytic recombination of Tafel as the rate determining step (RDS), while further polarization plot in semi-logarithmic η versus log j system shows Volmer–Heyrowski mechanism with the electrochemical desorption of Heyrowski reaction being the RDS; the extension of the former depends on the degree of MoO 3 coverage and blocking active centers of electrode surface. XPS, UPS, XRD and work function characterization of all specimens revealed congenial volcano plots relative to the same dependence in electrocatalytic activity. As the main observation and rule, the most stable and prevailing Pt content specimens feature the best electrocatalytic and kinetic properties. Activated (MoO 3 free) MoPt 3 and MoPt 4 catalysts feature all along the Tafel plot reversible Tafel catalytic reaction as the RDS, and create properties of super-activity within a broader current density range. It has been pointed out that an intermetallic phase with prevailing Mo atomic percentage (Mo 3Pt 2) features pronounced electrocatalytic properties for the HER. XPS measurements of nanostructured intermetallic phases of the same nominal composition, revealed that oxide species of Mo form solid solution with Pt, while significant portion of Pt surface atoms become screened by two-dimensional Mo oxide clusters. Such a structural modification alters the chemisorptive properties of Pt, so that XPS measurements reveal its partial oxidation, the oxide state being the primary oxide (Pt–OH), that spillovers along with MoO 2OH, both being imposed by adsorptive dissociation of water upon titania (TiO 2). It has been inferred that such structural and surface state a priori defines the overall behavior of composite Mo–Pt electrocatalysts. BRIEFS: Brewer hypo-hyper-d-interelectronic (average) configurational, Interionic and crystal structural effects of composite Laves type intermetallic phases and alloys taken along Mo–Pt phase diagram on kinetic, Synergistic electrocatalytic and super-active properties for hydrogen electrode reactions (HELR) and CO tolerance in macro-size and nanostructured electrodes.

Synthesis of Fe 2W by low pressure laser spraying

Fe 2W is the Laves type intermetallic compound with hexagonal crystal structure and is known to be a type of hard alloy. The low-pressure laser spraying (LPLS) method was applied to synthesize Fe 2W clad layers on a mild steel substrate. Structure and atomic composition of surface clad layers were analyzed by X-ray diffraction and energy dispersive X-ray spectroscopy, respectively. When the substrate was exposed to a laser energy density of 36 W/mm 2, the tungsten concentration of clad layers was estimated to be 20 wt.% at maximum with a microhardness of 240 Hv. The microhardness remarkably increased with heat treatment at 973 K in vacuum, suggesting that the Fe 2W phase was formed with heat treatment.

Intermetallic phases in the iron-rich region of the Zr[sbnd]Fe phase diagram

Intermetallic phases in the Fe-rich region of the ZrFe system are studied by X-ray diffraction and optical and electron microscopy. The chemical composition of each phase has been quantitatively measured in a electron microprobe. The stable phases found in this region are ZrFe2, Zr6Fe23 and (αFe). ZrFe2 is identified as a cubic Laves type phase (C15) and the ZrFe2/ZrFe2 + Zr6Fe23 boundary composition is 73 ± 1 at.% Fe. Zr6Fe23 is a cubic phase of the Th6Mn23 type and its composition is 80.0 ± 1.5 at.% Fe. The eutectic L ⇌ Zr6Fe23 + τ-Fe transformation temperature and composition are 1325°C and 91 ± 1 at.% Fe, respectively. The solubility of Zr in τ-Fe at 1012°C is 500 ± 50 appm and 1000 ± 100 appm close to the eutectic temperature.

Crystal Structure and Magnetic Properties of (YxZr1-x)Co2.9

Crystal structure of (Y x Zr 1- x )Co 2.9 is the PuNi 3 type in x >0.3, four slabs hexagonal structure in 0.3> x ≥0.1 and the MgCu 2 type at x =0. X-ray diffraction patterns of the compounds show that the three structures are similar each other. The magnetic moment and the Curie temperature decrease with decreasing yttrium content x in x ≥0.1. ZrCo 2.9 has a larger moment and a higher Curie temperature than (Y 0.1 Zr 0.9 )Co 2.9 . The change of magnetic moment and crystal structure is analysed by the stacking model of Laves type (Y,Zr) 2 Co 4 and CaCu 5 type (Y,Zr)Co 5 slabs.

Phase composition, structure, and properties of steel R9M4K8

1. The retained austenite in quenched steel R9M4K8 has a smaller lattice constant, which increases during tempering. This is evidently due to elastic compression of the austenite lattice during the martensitic transformation and the subsequent reduction of elastic deformation of austenite during tempering resulting from stress relaxation. 2. No stable cementite is observed in quenched steel R9M4K8. It appears after tempering for 1 h at 350–600°. This may indicate a substantial change in the properties of carbides of the cementite type formed in the presence of high-carbon martensite and austenite in the elastically compressed condition. The higher strength and toughness of quenched steel R9M4K8 in comparison with other high-speed steels are evidently due to the absence of cementite. 3. Intermetallic phases of the Laves type are observed in the steel in all structural conditions. 4. Special carbides of the MC (VC) and M2C (Mo2C) types are precipitated during tempering of steel R9M4K8. Highly dispersed M6C carbides are observed after tempering at temperatures above 650°, and thus M6C is a weakening phase. 5. After the optimal heat treatment (quenching from 1220°, austenite grain size grade 10–11, triple tempering at 560° for 1 h), ensuring the maximum hardness, strength, and toughness of steel R9M4K8, the structure consists of highly alloyed (∼7.3% alloying elements) twin-dislocation martensite, excess carbides (not dissloved during quenching) of the M6C and MC types, intermetallic phase of the Laves type (∼8.7%), and dispersed special carbides of the MC and M2C types with a size of order 50 A and M3C in the form of platelets with a length of 1000–5000 A and thickness 100–700 A (∼5.0% in all).

The Atomic Moments and Hyperfine Fields in Fe2Ti and Fe2Zr

The magnetic properties of Fe2Ti and Fe2Zr, intermetallic compounds with Laves type structures, were studied using the techniques of neutron diffraction and nuclear resonance fluorescence (Mössbauer effect). Both compounds are ferromagnetic, with magnetic moments of 0.35 and 2.56 Bohr magnetons, respectively, at room temperature, and 0.92 and 3.12 Bohr magnetons, respectively, at liquid helium temperatures. A magnetic form factor for the iron atoms in Fe2Zr was determined. The nuclear resonance fluorescence experiments gave measures of the magnetic fields at the iron nuclei. At room temperature, the field for Fe2Zr was 190±10 kgauss, while that for Fe2Ti was very low, less than 5 kgauss. The relation between the atomic magnetic moments and the magnetic fields at the nuclei is discussed.