Matthew F. Besser

Synthesis of Cu47Ti34Zr11Ni8 Bulk Metallic Glass By Warm Extrusion of Gas Atomized Powders

Cu 4 7 Ti 3 4 Zr 1 1 Ni 8 amorphous gas atomized powders were consolidated by warm extrusion. After consolidation near 723 K using an extrusion ratio of 5, the material retains between 88% and 98% of the amorphous structure found in the gas atomized powder. The onsets of the glass transition and crystallization temperatures of this extruded material are observed respectively at slightly higher and lower temperatures than those of the starting powders. These temperature shifts are attributed to a composition change in the remaining amorphous phase during partial devitrification throughout the extrusion process. Powders extruded at the same temperature, but using higher extrusion ratios of 9 and 13, exhibit substantial devitrification during the consolidation process yet still deform homogeneously.

Abrasive wear behavior of Al–Cu–Fe quasicrystalline composite coatings

Abstract Quasicrystals are a relatively new class of materials which exhibit unusual atomic structure and useful physical and chemical properties. The inherent brittleness of quasicrystals has limited their potential use to primarily surface coating applications. This study examined the effect on abrasive wear behavior of Al–Cu–Fe quasicrystalline coatings through the addition (e.g. 0–100 v/o) of a relatively ductile Fe–Al phase. Coatings were deposited by plasma arc spraying techniques. The incorporation of discrete Fe–Al particles into the quasicrystalline coating matrix improves the abrasive wear resistance. Moreover, it is observed that low-level additions of the Fe–Al phase (e.g. 1 v/o) produce the most abrasive wear-resistant coating. The wear behavior of the quasicrystalline and composite coatings is discussed in terms of wear mode and coating hardness.

Particle size effects on chemistry and structure of Al-Cu-Fe quasicrystalline coatings

Gas atomized Al63Cu25Fe12 powders of varying size fractions were plasma sprayed onto hot (~600 °C) and cool (~25 °C) substrates using Mach I and subsonic plasma gun configurations. The chemical composition and phase contents of coatings were determined. Furthermore, coatings were annealed in vacuum at 700 °C for 2 h to observe phase changes. It was found that finer particles (e.g., <25 μm) tend to vaporize Al during spraying, which shifts the coating composition away from the quasicrystalline (ψ) single-phase region in the Al-Cu-Fe phase diagram. Coatings deposited on hot substrates were denser, richer in theψ phase, and harder than the corresponding coatings deposited onto cool substrates.

Characterization of a commercially produced Al-Cu-Fe-Cr quasicrystalline coating

A commercially produced Al–Cu–Fe–Cr thermal spray cookware coating was characterized. Gas-atomized powders with a nominal composition of Al71Cu10Fe8.5Cr10.5 were apparently used to form the coatings. The coating is very uniform and contains a quasicrystalline icosahedral phase along with a second phase that is either a quasicrystalline decagonal phase or its nearly identical crystalline approximant. The phases show evidence of structural imperfection and appear to be in a highly transitional state, which is likely caused by the rapid cooling during thermal spraying. A large fraction of unmelted particles and microcracks were observed throughout the coating.

Surface oxidation of Al-Cr-Fe alloys characterized by X-ray photoelectron spectroscopy

Abstract We present X-ray photoelectron spectroscopy (XPS) measurements of several Al–Cr–Fe samples which are mixtures of approximants of the decagonal phase. Some samples also contain a hexagonal γ-brass phase. Our purpose is to evaluate the effect of chemical composition, particularly Cr content, on the response of the surface to oxidation. Under mild conditions only aluminium oxidizes, but under extreme conditions (water immersion at room temperature, or oxygen exposure at high temperatures), chromium oxidizes as well. XPS data also provide a measure of the oxide thickness. Cr has no discernible effect on oxide thickness when the oxidizing environment is the gas phase, but provides significant protection against water immersion, where high concentrations of Cr reduce the thickness by as much as 40%. These results for the Al–Cr–Fe samples are compared with results for approximants and quasicrystals in other systems.

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.

Surface oxidation of Al-Cu-Fe alloys: A comparison of quasicrystalline and crystalline phases

Abstract We have used X-ray photoelectron spectroscopy and Auger electron spectroscopy to examine the characteristics of oxide surfaces on a family of Al-Cu-Fe alloys. The alloys studied are compositionally similar but structurally different: two are crystalline and one is quasicrystalline. The samples all are formed by consolidation of powders, resulting in multiple grains with random surface orientations. They are oxidized to saturation in a variety of environments at room temperature. Under the conditions of our experiments, there is no detectable difference in the oxidation characteristics of the three phases. That is, there is no difference in the elemental constituents which oxidize, or in the relative extent of oxygen-induced Al segregation, or in the depth of the oxide formed. Hence, the oxidation chemistry of these alloys is determined by their Al-rich chemical composition, and not by their atomic or electronic bulk structure, under these conditions.

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.