R.B. Schwarz

Formation of amorphous alloys by the mechanical alloying of crystalline powders of pure metals and powders of intermetallics

Amorphous powders of Ni32Ti68 and of Ni45Nb55 were synthesized by mechanical alloying (MA) starting from either a mixture of pure metal powders (in the appropriate molar ratio) or from powders of the crystalline intermetallics NiTi2 and Ni45Nb55, respectively. For both alloys, the peak temperature increase (above the average processing temperature) in the powder particles trapped between colliding balls is estimated at 38 K. Thus, the amorphization is attributed to a process other than the formation of local melts followed by the rapid solidification of these melts into the amorphous phase. The amorphization by MA starting from a mixture of pure crystalline powders is attributed to a solid state interdiffusion reaction, the kinetics of which is controlled by the excess point and lattice defects generated by plastic deformation. The amorphization by MA starting from powders of crystalline intermetallics is attributed to the accumulation of point and lattice defects which raise the free energy of the faulted intermetallic above that of the amorphous alloy.

Bulk ferromagnetic glasses prepared by flux melting and water quenching

Several ferromagnetic bulk amorphous alloys of the type Fe–(Co, Cr, Mo, Ga, Sb)–P–B–C, containing between 62 and 71 at. % Fe, have been prepared in the form of 4-mm-diam rods. The glass synthesis consists of mechanically alloying the constituents, purifying the melts in B2O3 flux inside fused silica tubes, and quenching the melts at cooling rates on the order of 100 K/s. All these glasses have a large supercooled region Tx−Tg, ranging from 35 to 61 K, within which the glass can be shaped under a relatively small applied load. It is shown that the value of Tx−Tg depends strongly on the metalloid composition. These bulk metallic glasses have very low coercivity and low hysteresis losses.

Localized heating during serrated plastic flow in bulk metallic glasses

We have studied the serrated plastic flow observed in Zr40Ti14Ni10Cu12Be24 and Pd40Ni40P20 bulk metallic glass alloys tested in uniaxial compression. Quantitative measurements with sufficient temporal resolution to record the fine-scale structure of the data are reported. These data are used to predict temperature increases in single shear bands due to local adiabatic heating caused by the work done on the sample during plastic deformation. Since the predicted temperature increases are on the order of only a few degrees Kelvin, it seems unlikely that localized heating is the primary cause of flow localization. Instead, changes in viscosity associated with increased free volume in the shear band seem more consistent with experiment. Substantial shear band heating is, however, predicted for final failure, as corroborated by evidence of melting on the fracture surface.

BULK GLASS FORMATION IN THE PD-NI-P SYSTEM

Bulk amorphous Pd–Ni–P rods with diameters ranging from 10 to 25 mm were prepared by a fluxing technique over a wide composition range. For most bulk glassy alloys studied, the difference between the glass transition temperature and the crystallization temperature, Tx−Tg, is larger than 90 K. Of all the alloy compositions examined, Pd40Ni40P20 has the highest glass formability, and 300‐g bulk amorphous cylinders, 25 mm in diameter and 50 mm in length, were easily and repeatedly formed. This size, however, is not an upper limit. The elastic properties of these bulk amorphous alloys were determined by a resonant ultrasound spectroscopy technique.

Alloys for hydrogen storage in nickel/hydrogen and nickel/metal hydride batteries

Since 1990, there has been an ongoing collaboration among the authors in the three laboratories (i) to prepare alloys of the AB5 and AB2 types, using arc-melting/annealing and mechanical alloying/annealing techniques; (ii) to examine their physiochemical characteristics (morphology, composition; (iii) to determine the hydrogen absorption/desorption behavior (pressure-composition isotherm as a function of temperature), and (iv) to evaluate their performance characteristics as hydride electrodes (charge/discharge, capacity retention, cycle life, high-rate capability). This review article presents the work carried out on representative AB5 and AB2 type modified alloys (by partial substitution of with small additives of other elements). The purpose of the modification was to optimize the thermodynamics and kinetics of the hydriding/dehydriding reactions and to enhance the stabilities of the alloys for the desired battery applications. The results of our collaboration, to date, demonstrate that: (i) alloys prepared by arc-melting/annealing and mechanical alloying/ annealing techniques exhibit similar morphology, composition and hydriding/dehydriding characteristics; (ii) alloys with the appropriate small amounts of substituent or additive elements — retain the single phase structure, improve the hydriding/dehydriding reactions for the battery applications, and enhance the stability in the battery environment — and (iii) the AB2 type alloys exhibit higher energy densities than the AB5 type alloy but the state-of-the-art, commercialized batteries are predominantly manufactured using AB5 type alloys.

Superhard B–C–N materials synthesized in nanostructured bulks

We report here the high-pressure synthesis of well-sintered millimeter-sized bulks of superhard BC 2 N and BC 4 N materials in the form of a nanocrystalline composite with diamond-like amorphous carbon grain boundaries. The nanostructured superhard B–C–N material bulks were synthesized under high P–T conditions from amorphous phases of the ball-milled molar mixtures. The synthetic B–C–N samples were characterized by synchrotron x-ray diffraction, high-resolution transmission electron microscope, electron energy-loss spectra, and indentation hardness measurements. These new high-pressure phases of B–C–N compound have extreme hardnesses, second only to diamond. Comparative studies of the high P – T synthetic products of BC 2 N, BC 4 N, and segregated phases of diamond + c BN composite confirm the existence of the single B–C–N ternary phases.

Synthesis of molybdenum disilicide by mechanical alloying

Abstract Considerable interest and effort are being directed towards developing molybdenum disilicide (MoSi2) alloys with low oxygen content. During alloy synthesis, oxygen combines with Si to form glassy SiO2 precipitates at the MoSi2 grain boundaries, resulting in a degradation of its mechanical properties. We have used mechanical alloying, a high-energy ball-milling process, to synthesize alloy powders of MoSi2, MoSi2-27 mol.% MoSi3, MoSi2-50 mol.% Mo5Si3 and MoSi2-50 mol.% WSi2 starting from elemental powders. The processing of the powders, as well as the loading of the powders in graphite dies, was performed under high-purity argon inside a glovebox. The finer grain and particle size of the mechanically alloyed powders enabled us to hot-press them at 1500 °C, which is 300 °C lower than the temperature currently used for hot-pressing commercial powders. We have been successful in reducing the oxygen content in our alloys to about 310 ppm by weight, as measured by nuclear (d,p) reactions. We report the formation of metastable phases in the mechanically alloyed powders and their characterization by X-ray diffraction and differential thermal analysis. We also report the characterization of the hot-pressed alloys by optical and transmission electron microscopy, and the measurement of high-temperature mechanical properties.

Calorimetry study of the synthesis of amorphous Ni-Ti alloys by mechanical alloying

Abstract We synthesized amorphous Ni33Ti67 alloy powder by ball milling (a) a mixture of elemental nickel and titanium powders and (b) powders of the crystalline intermetallic NiTi2. We characterized the reaction products as a function of ball-milling time by differential scanning calorimetry and X-ray diffraction. The measurements suggest that in process (a) the amorphous alloy forms by a solid state interdiffusion reaction at the clean Ni Ti interfaces generated by the mechanical attrition. In process (b), the crystalline alloy powder stores energy in the form of chemical disorder and lattice and point defects. The crystal-to-amorphous transformation occurs when the stored energy reaches a critical value. The achievement of the critical stored energy competes with the dynamic recovery of the lattice.

Bulk ferromagnetic glasses in the Fe–Ni–P–B System

Abstract The ferromagnetic metallic glass Fe40Ni40P14B6, available only as 30–50 μm thick ribbons, has been extensively studied over the last three decades. We used a flux-melting and water-quenching technique to prepare bulk glassy Fe40Ni40P14B6 alloys in the form of 2-mm diameter spheres and 1-mm diameter rods. The Curie temperature for the bulk glasses is higher than the average value of Curie temperatures reported for the rapidly quenched ribbons. The glass-transition temperature and the crystallization temperature of the bulk glasses are lower and higher, respectively, than the average values reported for rapidly quenched ribbons, making the supercooled-liquid region as wide as 42 K. The bulk glasses crystallize by a homogeneous nucleation followed by a growth at a constant rate. The nucleation rate in the bulk glasses is four orders of magnitude lower than in the rapidly quenched ribbons, suggesting that the previous thickness limitation was due to impurities in the melt (heterogeneous nucleation).

Elastic contants of single crystal {gamma}--TiAl

The six independent second-order elastic stiffness coefficients of a Ti{sub 44}Al{sub 56} single crystal ({ital L}1{sub 0} structure) have been measured at room temperature for the first time using a resonant ultrasonic spectroscopy (RUS) technique. These data were used to calculate the orientation dependence of Young`s modulus and the shear modulus. The Young`s modulus is found to reach a maximum near a [111] direction, close to the normal to the most densely packed planes. The elastic moduli and the Poisson`s ratio for polycrystalline materials, calculated by the averaging scheme proposed by Hill, are in good agreement with experimental data and theoretical calculations.