T.D. Shen

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.

THE STRUCTURE AND PROPERTY CHARACTERISTICS OF AMORPHOUS NANOCRYSTALLINE SILICON PRODUCED BY BALL-MILLING

The structural transformation of polycrystalline Si induced by high energy ball milling has been studied. The structure and property characteristics of the milled powder have been investigated by x-ray diffraction, scanning electron microscopy, high-resolution electron microscopy, differential scanning calorimetry, Raman scattering, and infrared absorption spectroscopy. Two phase amorphous and nanocrystalline components contain some defects such as dislocations, twins, and stacking faults which are typical of defects existing in conventional coarse-grained polycrystalline materials. The volume fraction of amorphous Si is about 15% while the average size of nanocrystalline grains is about 8 nm. Amorphous elemental Si without combined oxygen can be obtained by ball milling. The distribution of amorphous Si and the size of nanocrystalline Si crystallites is not homogeneous in the milled powder. The amorphous Si formed is concentrated near the surface of milled particles while the grain size of nanocrystalline Si ranges from 3 to 20 nm. Structurally, the amorphous silicon component prepared by ball milling is similar to that obtained by ion implantation or chemical vapor deposition. The amorphous Si formed exhibits a crystallization temperature of about 660-degrees-C at a heating rate of 40 K/min and crystallization activation energy of about 268 kJ/mol. Two possible amorphization mechanisms, i.e., pressure-induced amorphization and crystallite-refinement-induced amorphization, are proposed for the amorphization of Si induced by ball milling.

ON THE ELASTIC MODULI OF NANOCRYSTALLINE FE, CU, NI, AND CU-NI ALLOYS PREPARED BY MECHANICAL MILLING/ALLOYING

The Young`s moduli of nanocrystalline, Fe, Cu, Ni, and Cu--Ni alloys prepared by mechanical milling/alloying have been measured by the nanoindentation technique. The results indicate that the Young`s moduli of nanocrystalline Cu, Ni, and Cu--Ni alloys with a grain size ranging from 17 to 26 nm are similar to those of the corresponding polycrystals. The dependence of Young`s modulus of nanocrystalline Fe on grain size corresponds well to a theoretical prediction, which suggests that the change in the Young and shear moduli of nanocrystalline materials, free of porosity, with a grain size larger than about 4 nm, should be very limited ({lt}10%). It is likely that reported large decreases in the Young and shear moduli of nanocrystalline materials prepared by gas-condensation/vacuum consolidation result from a relatively large volume fraction of pores. {copyright} 1995 {ital Materials} {ital Research} {ital Society}.

Boron suboxide: As hard as cubic boron nitride

The Vickers hardness of boron suboxide single crystals was measured using a diamond indentation method. Under a loading force of 0.98 N, our test gave an average Vickers hardness of 45 GPa. The average fracture toughness was measured as 4.5 MPa m1/2. We also measured the hardness of the cubic boron nitride and sapphire single crystals for comparison. The average measured hardness for boron suboxide was found to be very close to that of cubic boron nitride under the same loading force. Our results suggest that the boron suboxide could be a new superhard material for industrial applications, surpassed in hardness only by diamond and cubic boron nitride.

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.

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).

Magnetocaloric effect in bulk amorphous Pd40Ni22.5Fe17.5P20 alloy

We have studied the temperature (T) and field (H) dependence of the magnetocaloric effect in a bulk amorphous Pd40Ni22.5Fe17.5P20 alloy. With decreasing T, and depending on H, the alloy exhibits superparamagnetic, field-induced ferromagnetic, and spin-glass behavior. The temperature-dependence of the magnetic entropy change, ΔS=S(H)−S(H=0), exhibits extreme values: a minimum at the superparamagnetic-to-ferromagnetic transition and a maximum at the ferromagnetic-to-spin-glass transition. At 80 K, and for H=50 kOe, the entropy change is −0.029 kB per Fe atom in the alloy. This value compares favorably with that measured in other crystalline and amorphous Fe-based alloys, but it is lower than that measured in some rare-earth elements.

Structural disorder and phase transformation in graphite produced by ball milling

Nanocrystalline graphite with a crystallite size of about 2 nm is formed after 8 h of ball milling. Further milling produces a mixture of nanocrystalline and amorphous phases. The nanocrystalline graphite is relatively stable against heating when compared with some nanocrystalline metals.

Formation, solid solution hardening and softening of nanocrystalline solid solutions prepared by mechanical attrition

Abstract Nanocrystalline Ti 100 − x Cu x ( x = 0–8), Nb 100 − x Cu x ( x = 0–20), Ni 100 − x Cu x ( x = 0–50), Cr 100 − x Cu x ( x = 0–20), Fe 100 − x Cu x ( x = 0–15). Cu 100 − x Ni x ( x = 0–50), Cu 100 − x Fe x ( x = 0–50) and Cu 100 − x Co x ( x = 0–50) solid solutions have been prepared by mechanical attrition of elemental powder mixtures. X-ray diffraction, differential scanning calorimetry, and microhardness measurements are utilized to investigate the structural evolution and the thermal and mechanical properties of the solid solutions. The strength of the nanocrystalline solid solution depends on both solid solution hardening and grain boundary hardening while the latter makes the major contribution to the total strength. The increase or decrease in hardness is dependent on the combined effects of solid solution hardening and either increased or decreased hardness resulting from changes in grain boundary hardening.

Bulk amorphous Pd–Ni–Fe–P alloys: Preparation and characterization

Bulk amorphous alloys of Pd{sub x}Ni{sub y}Fe{sub 80{minus}x{minus}y}P{sub 20} (25{le}x{le}60, 20{le}y{le}55, x+y{ge}60) were prepared by a flux-melting and water-quenching method. Seven-mm diameter glassy rods of Pd{sub 40}Ni{sub 40{minus}x}Fe{sub x}P{sub 20} (0{le}x{le}20) were studied in greater detail. For these alloys, the difference between the crystallization and glass transition temperatures ranges from 102 K for x=0 to 53 K for x=20. In this composition range, the reduced glass transition temperature, T{sub rg}, ranges from 0.66 to 0.57. The change in density upon crystallization ranges from 0.24{plus_minus}0.04{percent} for x=0 to 1.33{plus_minus}0.24{percent} for x=10. The partial molar volume of Fe in amorphous Pd{sub 40}Ni{sub 40{minus}x}Fe{sub x}P{sub 20} alloys is significantly larger than the molar volume of (metastable) fcc Fe. This, as well as a comparison with the molar volumes of crystalline compounds, suggests chemically selective Fe{endash}Pd bonding in these glasses. {copyright} {ital 1999 Materials Research Society.}