John L. Walter

The influence of oxygen and other impurities on the crystallization of NiZr2 and related metallic glasses

The role of oxygen and other impurities on the crystallization characteristics of Ni‐Zr glasses near the composition NiZr2, as well as for FeZr2, CoZr2, and NiHf2, has been investigated. For NiZr2 glasses with 1 at. % oxygen, the first crystallization product is the metastable E93 structure with a =1.227 nm instead of the equilibrium C16 structure. A similar effect is found for samples containing ≳3 at. % B. For FeZr2, CoZr2, and NiHf2 the first crystallization product is also E93 structure, even with very small levels of oxygen (≤0.2 at. %). The formation of the E93 structure is always accompanied by an increase in the electrical resistivity, an increase which transmission electron microscopy shows is intrinsic to the phase and unrelated to crystallite size. For Ni36.5Zr63.5 and Ni42Zr58 the crystallization is also accompanied by an increase in electrical resistance and the evolution of a crystal structure similar to the E93 structure in the size of the unit cell and packing fraction but with a different...

Stability of amorphous metallic alloys

In a previous paper it was concluded that the migration of phosphorus was causing the embrittlement of amorphous Fe40Ni40P14B6 at temperatures as low as 100 °C. In this paper we compare the stability of the above alloy to amorphous Fe40Ni40B20, Fe50Ni30P14B6, and Fe50Ni30B20. The stability is evaluated from brittleness measurements after annealing at temperatures between 100 and 400 °C. The results confirm the prediction that the removal of phosphorus suppresses the embrittlement at low temperatures; temperatures above 225 °C are required to cause embrittlement in the phosphorus‐free alloys. At higher temperatures the fracture strain approaches that of the phosphorus‐containing alloy. It is concluded that the replacement of P by B inhibits the low‐activation processes involved in the embrittlement process. The changes are not related to the changes in the glass transition temperatures.

Formation and magnetic properties of Fe-B-Si amorphous alloys

The magnetic properties and crystallization temperatures of alloys in the ternary Fe-B-Si system are reported. The Curie temperature increases slightly on replacement of boron by silicon. This results in a sharp ridge of relatively constant room-temperature saturation magnetization extending from Fe 80 B 20 to Fe 82 B 12 Si 6 . The coercivity exhibits a broad minimum, both before and after stress relief annealing, in the region around Fe 81 B 15 Si 4 and extending at least to Fe 77 B 13 Si 10 . The crystallization temperature increases with increasing silicon and with decreasing iron and boron. The alloys with silicon are generally easier to prepare in the amorphous state than the binary Fe-B alloys. Thus for the highest saturation magnetization alloy combined with ease of preparation, stability, and lowest losses, the alloys between Fe 81 B 17 Si 2 and Fe 82 B 12 Si 6 are preferred.

An Auger analysis of the embrittlement of the amorphous alloy Ni40Fe40P14B6

Abstract The amorphous alloy Ni40Fe40P14B6 becomes brittle when heated to temperatures in excess of about 100 °C for 2 h. Auger analysis of the fracture surface of ribbon samples heated at 325 °C and 350 °C showed phosphorus concentrations on the fracture surface greater than twice that of the expected bulk composition. Ion milling to a depth of ∼ 60 A reduced the phosphorus concentration to near that expected for the bulk. The concentrations of Fe, Ni and B on the fracture surface showed only small changes during ion milling. The results indicate that phosphorus has become segregated during the anneal to form discrete regions of high phosphorus concentration which are less than 60 angstrom in diameter. These high phosphorus regions are the cause of embrittlement and may be the nuclei for subsequent crystallization. Auger analysis of the ribbon surface indicates that the material is inhomogeneous, being high in phosphorus near the ribbon surface.

Spark erosion: A method for producing rapidly quenched fine powders

Spark erosion is a method for producing fine powders of metals, alloys, semiconductors, and compounds. The technique involves maintaining repetitive spark discharges among chunks of material immersed in a dielectric liquid. As a result of the spark discharge there is highly localized melting or vaporization of the material. The powders are produced by the freezing of the molten droplets or the condensation and freezing of the vapor in the dielectric liquid. Since the powders are quenched in situ , they may be extremely rapidly cooled. Particles can be produced in sizes ranging from 5 nm to 75 μm. The average powder size and production rate depend on the power parameters, material used, and the dielectric liquid.

Substructures and recrystallization of deformed (100)[001]-oriented crystals of high-purity silicon-iron

Abstract (100)[001]-oriented crystals of high-purity 3% silicon-iron were rolled to reductions of 10–90 per cent of thickness and annealed. The substructures of the deformed crystals, before and after annealing, were observed by transmission electron microscopy. At low reductions (10–20 per cent), the substructure consists of cells, 0.2–0.3 μ in diameter. With further reduction to 50 per cent, the cells elongate in the rolling direction and the crystal reorients to form two major texture components contained within deformation bands. The major components are related to the initial orientation of the single crystal by rotations about an axis normal to the (100) or rolling plane. With heavier reductions, the elongated cells evolve into sub-bands, 0.2–0.3 μ wide, separated by low-angle tilt boundaries. Groups of sub-bands form “transition bands” which separate the deformation bands and across which the change in orientation from one deformation band to the next is accomplished. When annealed, the low-angle boundaries become more perfect tilt boundaries. Migration of portions of the tilt boundaries across successive adjacent sub-bands leads to formation of recrystallization nuclei. The nuclei have the same orientation as the orientation of the sub-band from which they originate.

Crystallization of the amorphous alloys Fe50Ni30B20and Fe80oB20

The structure and growth characteristics of crystals growing in the amorphous alloys Fe50Ni30B20 and Fe8oB20 were studied by transmission electron microscopy and X-ray diffraction. The alloys, in the form of ribbon, were annealed for 2 h at 380° to initiate crystallization. The Fe3B crystals in both alloys have characteristic cylindrical shapes, and their structure is b.c. tetragonal witha = 8.63å andc = 4.29å. Growth occurs by formation of columnar regions which grow outward from the center of the crystal. Twinning on {211} planes of the Fe3B divides the crystals into gradrants. There is a small amount of alpha-iron incorporated in the crystal which has a preferred orientation relationship with the Fe3B. From these results and the results of earlier work, it is seen that the shape and structure of the crystals depend upon the metalloids present in the alloy.

The variation of interface dislocation networks with lattice mismatch in eutectic alloys

Abstract Dislocation networks at the interface between phases were studied in directionally solidified eutectics NiAl-Cr and NiAl-Cr(Mo) to determine the effect of lattice parameter mismatch on the spacing and configuration of the interface dislocations. The lattice mismatch was varied by adding different amounts of Mo to the eutectic NiAl-Cr and by quenching NiAl-Cr from temperatures near the melting point to retain the high temperature solid solution and reduced lattice mismatch. The dislocation networks were examined by transmission electron microscopy and the lattice mismatch calculated from the spacing of the dislocations. Lattice parameter mismatch was also measured by X-ray techniques at room temperature and at elevated temperatures to determine the effects of thermal expansion and solubility on the mismatch. In the case of the slowly cooled NiAl-Cr and NiAl-Cr(Mo) alloys, the mismatch calculated from the spacing of the dislocations at room temperature agreed with the measured mismatch at room temperature. In the heated and quenched NiAl-Cr eutectic, the reduced mismatch at elevated temperature was reflected by rotated networks and increased dislocation spacing (compared with the slowly cooled eutectics) at room temperature. When mismatch decreases in the solid, the interface dislocations move from the interface network into the NiAl matrix. When mismatch increases in the solid, stresses deform the matrix and the glide dislocations move from the matrix into the interface to decrease the spacing of the dislocations in the interface networks.

The effects of addition on the crystallization temperatures of amorphous Fe82−xMxB18 alloys

Abstract Nickel, cobalt, chromium, vanadium, platinum, molybdenum, gold, niobium, silicon and dysprosium were added singly to the base amorphous alloy Fe 82− x M x B 18 . Amorphous ribbons were prepared from alloys containing up to 6 wt.% (12 at.% for nickel) of the addition element. The crystallization temperatures were measured by differential scanning calorimetry and were related to the difference between the radius of the addition atom and the radius of the iron atom. There was found to be a direct correlation between the atomic radius (determined from the atomic volume per atom of the crystalline form of the element) and the increase in crystallization temperature. The effect is related to the viscosity of the alloy. That is, the larger the addition atom, the greater is the increase in viscosity and, therefore, the higher will be the crystallization temperature.

STABILITY OF THE DIRECTIONALLY SOLIDIFIED EUTECTICS NiAl--Cr AND NiAl--Mo.

The eutectics NiAl-Cr with cylindrical chromium fibers and NiAl-Mo with faceted molybdenum fibers were heated at 1400°C to determine the stability of the composite structure and to compare the stability of the nonfaceted fibers with that of the faceted fibers in the NiAl matrix. Fiber size and size distribution and number of fibers per unit area were measured as a function of time at temperature. The number of fibers in the NiAl-Cr eutectic decreased continuously reaching half the initial value in about 30 h at temperature. Spheroidization of the fibers occurred and was complete in 160 h. In the NiAl-Mo eutectic, the number of fibers per unit area remained constant to 150 h and the fiber size was constant to 331 h at 1400°C. In NiAl-Cr, the cylindrical chromium fibers first formed pinchedoff segments at random diameter variations along the length of the fibers. The segments gradually shortened and thickened and finally spheroidized. The faceted molybdenum fibers remained stable because the Mo-NiAl interface is constrained to lie in the facet plane which inhibits the formation of faults leading to pinching off of the fibers.