E. Rozhkova

Synthesis of Ni-based bulk amorphous alloys by warm extrusion of amorphous powders

Abstract A high strength Ni-based bulk amorphous alloy is synthesized by warm extrusion of gas atomized amorphous powder. The Ni59Zr20Ti16Si2Sn3 amorphous powder obtained by a high pressure Ar gas atomization method has a wide super-cooled liquid region of 63 K. Warm extrusion of the amorphous powders in the super-cooled liquid state successfully yielded a fully consolidated bulk amorphous Ni59Zr20Ti16Si2Sn3 alloy. The processing conditions for extrusion are obtained from the time–temperature-transformation curve for the onset of crystallization of the amorphous powder. Lower extrusion ratio of 5 is preferred for the retention of the single amorphous phase during extrusion at 848 K. The extruded bulk amorphous Ni59Zr20Ti16Si2Sn3 alloy exhibits a high strength level (∼2.0 GPa) similar to that of an as-cast bulk amorphous Ni59Zr20Ti16Si2Sn3 alloy (∼2.2 GPa). The mechanical behavior of the extruded alloy under the compressive condition shows no anisotropy in the longitudinal and transverse directions to the extrusion direction.

Oxygen-stabilized glass formation in Zr80Pt20 melt-spun ribbons

The as-quenched structure of Zr80Pt20 melt-spun ribbons containing measured oxygen contents ranging from 184 to 4737 ppm mass was studied. Ribbons containing less than 500 ppm mass oxygen are fully crystallized and consist predominantly of a metastable ordered β-Zr phase with significant solution of Pt. Increasing oxygen content to 1053 and 1547 ppm mass produces a transition to fully amorphous and to mixed amorphous and quasicrystalline structures, respectively. Samples containing 4737 ppm mass consist of quasicrystalline and crystalline phases in an amorphous matrix. The results from this study suggest a critical level of oxygen is required to promote glass formation in Zr80Pt20 melt-spun ribbons produced at a specific quench rate.

Consolidation of gas atomized Cu47Ti34Zr11Ni8 amorphous powders

Abstract The dense, random atomic configuration of bulk metallic glasses enhances their stability against crystallization in the supercooled liquid state. This stability provides the opportunity to deform the material by plastic processing methods such as warm extrusion. Gas atomized Cu 47 Ti 34 Zr 11 Ni 8 powders were consolidated at various temperatures above their glass transition temperature (688 K) at extrusion ratios of 5, 9 and 13. At extrusion temperatures approaching T x (743 K), powders consolidated at an extrusion ratio of 5 were predominantly amorphous while at higher ratios the powders were completely devitrified, yet fully consolidated. Gradients in the degree of devitrification were observed along the length (i.e., top to bottom) of all extruded materials.

Devitrification studies of Zr–Pd and Zr–Pd–Cu metallic glasses

Abstract High temperature X-ray diffraction (HTXRD) is used to investigate the devitrification pathway for two related Zr-based metallic glasses, Zr 70 Pd 30 and Zr 70 Pd 20 Cu 10 . Both alloys have similar as-quenched structures and initially devitrify to form a meta-stable quasicrystalline phase. The HTXRD data for the Zr 70 Pd 30 alloy shows the coexistence of the quasicrystalline and the Zr 2 Pd (I4/mmm) crystalline phases over a range of 25 K. Conversely, the Zr 70 Pd 20 Cu 10 alloy shows an additional transformation of the quasicrystalline phase to a meta-stable Zr 2 (PdCu) intermetallic (Zr 2 Si type structure, I4/mcm) that polymorphically transforms to the Zr 2 Ni type structure (I4/mmm) over a very narrow temperature range.

Synthesis route-dependent formation of quasicrystals in Zr70Pd30 and Zr70Pd20Cu10 amorphous alloys

The devitrification of mechanically milled amorphous Zr70Pd30 and Zr70Pd20Cu10 powders occurs via a single-step, first-order transformation to a stable Zr2Pd tetragonal structure. This is in sharp contrast to the devitrification of the same amorphous alloys prepared by melt spinning, in which a primary metastable quasicrystalline phase forms. Since the mechanical milling process does not involve direct liquid phase formation of an amorphous structure, it is inferred that the short-range order in the solid state derived amorphous powder is different from that in the melt spun ribbon.

Formation of quasicrystals in Zr–Pd–(Cu) melt spun ribbons and mechanically milled powders

Abstract Amorphous Zr70Pd30 and Zr70Pd20Cu10 alloys were prepared by mechanical milling and melt spinnng to compare their devitrification behaviors. The devitrification of mechanically milled Zr70Pd30 and Zr70Pd20Cu10 powders occurs via a single-step, first-order transformation to a stable Zr2Pd tetragonal structure. This is in sharp contrast to the devitrification of the same amorphous alloys prepared by melt spinning, in which a primary meta-stable quasicrystalline phase forms. Since the mechanical milling process does not involve direct liquid phase formation of an amorphous structure, it is inferred that the short-range order in the solid state derived amorphous powder is different from that in the melt spun ribbon. During mechanical milling of an amorphous melt spun ribbon, crystallization of the quasicrystalline phase appears to precede disordering into an amorphous structure having an different short range order. Deformation of an amorphous melt spun ribbon by repetitive rolling at ambient temperature crystallizes the meta-stable quasicrystalline phase.

Coincident lattice sites between cubic β-Zr(Pt) and an isochemical icosahedral phase in rapidly solidified Zr80Pt20 alloys

Epitaxial growth of a bcc hyperstoichiometric β-Zr(Pt) (Im3m) superstructure on an icosahedral phase has been studied in rapidly solidified Zr80Pt20 alloys. Icosahedral twofold axes coincide with 〈110〉, 〈111〉, 〈112〉 and 〈113〉 axes in β-Zr(Pt). The β-Zr(Pt) has a close crystallographic match and nearly identical stoichiometry to the i phase, but, it is not an approximant. Both the β-Zr(Pt) and the i phase are distorted.

Structural Relationships between QC and Meta-stable Crystalline Phases in Melt Spun Zr 80 Pt 20 Ribbons

Two competing meta-stable crystalline phases, a bcc hyperstoichiometric β-Zr(Pt) (Im3m) superstructure and a non-stoichiometric fcc e-Zr 6 Pt 3 O (Fd3m) phase, have been observed to coexist with a quasicrystalline (i) phase, respectively, in as-spun Zr 80 Pt 20 ribbons with low oxygen content (184ppm mass O) and high oxygen content (4737 ppm mass O). Transmission electron microscopy (TEM) results show that the β-Zr(Pt) superstructure and i phase have a well defined orientational relationship, good crystallographic match and nearly identical stoichiometry. Icosahedral two-fold axes coincide with , , and axes in β-Zr(Pt); {110} β-Zr planes register with icosahedral {211111} fivefold or {221001} twofold planes. The observed orientational relationship and the space group (Im3m) preclude β-Zr(Pt) as a cubic approximant to the i phase. Both β-Zr(Pt) and the i phase are distorted; β-Zr(Pt) maintains a basic β-Zr Bravais lattice with an aperiodic superlattice. Morphologies suggest that the i phase forms first, followed by an easy nucleation of the β-Zr(Pt) on the surfaces of the i phase. Also, a similar orientational relationship and lattice match between e-Zr 6 Pt 3 O and i phase has been revealed by TEM.