Michael Nastasi

Stability of amorphous Cu/Ta and Cu/W alloys

Although both the Cu/W and Cu/Ta alloy systems are known to be immiscible, the response of coevaporated amorphous films of a‐CuxW1−x (x=0.45–0.72) and a‐CuxTa1−x (x=0.10–0.55) alloys to thermal annealing and ion irradiation is very different. Alloys of a‐CuxTa1−x crystallize between 600 and 800 °C, with the crystallization temperature increasing with Ta concentration, while a‐CuxW1−x alloys crystallize below 200 °C. Likewise in Xe‐ and Ne‐ion irradiation studies, a‐Cu/Ta can still be observed after ∼13 displacements per atom (DPA), whereas a‐Cu/W alloys completely transform to solid solutions after ∼0.08 DPA. The results of this study, in combination with previous investigations of amorphous alloys of immiscible systems, show that the crystallization temperature Tc is not predicted equally well for systems with a positive or negative heat of formation. It appears that the kinetic model of crystallization only applies if the transformation proceeds through long‐range diffusion. Amorphous alloys which have ...

Initial phase formation and dissociation in the thin‐film Ni/Al system

We have investigated the interactions of Al/Ni, Al/NiAl/Ni, Al/Ni3Al, and NiAl3/Ni thin films between 300 and 425 °C. The films were prepared by sequential evaporation and coevaporation, then vacuum annealed and analyzed by Rutherford backscattering and x‐ray diffraction. The reaction always started at the interface in contact with the most Al‐rich film and resulted in formation and growth of NiAl3, unless that was the most Al‐rich layer. Subsequently the reaction proceeded to Ni2Al3. We conclude that the kinetics rather than the thermodynamics is responsible for determining the growth of these phases.

Phase formation by ion beam mixing in Ni/Al, Pd/Al, and Pt/Al bilayers

Ion beam and thermal reactions in thin‐film bilayers of Ni/Al, Pd/Al, and Pt/Al have been investigated using electron diffraction and Rutherford backscattering (RBS). Ion mixing was performed with 600‐keV Xe ions at room temperature with doses ranging between 0.25 to 12×1015 ions/cm2. The compositions of the ion induced crystalline phases, NiAl and PdAl, differ both from the overall atomic compositions of the ion mixed layers (as determined by RBS) and from the compositions of the layers obtained by thermal annealing. In Pt/Al bilayers ion mixing forms an amorphous region with atomic composition Pt40Al60 while thermal annealing shows the formation of crystalline Pt2Al3. We believe that actual crystal compound formation during mixing is dependent on the high quench rate in the collision cascade region.

The quasicrystalline transformation in the AlCr system

Amorphous Al/sub 80/Cr/sub 20/ films were made by coevaporation and by room temperature ion irradiation of the coevaporated films. The amorphous phase was transformed into the quasicrystalline state through two routes: thermal and ion beam assisted anneal. The intensity of the quasicrystalline electron diffraction pattern increases continuously within the annealing temperature range from 547 to 607 /sup 0/C. The starting state of the films (as-deposited or ion-irradiated codeposited) had no effect on the thermal transformation to the quasicrystalline state. Ion irradiation of the amorphous phase at 200 /sup 0/C produces a more complete set of icosahedral diffraction lines. Icosahedral AlCr has the same reciprocal lattice spacings as icosahedral AlMn.

Phase transformation of Ni2Al3 to NiAl. I. Ion irradiation induced

An ion‐irradiation‐induced phase transformation from Ni2Al3 to NiAl has been investigated by transmission electron diffraction. Compound samples of Ni2Al3 were irradiated at liquid nitrogen, room temperature, and 100 °C with 300‐keV Xe and 60‐keV Ne ions. Transformation to NiAl was observed in all cases. Detailed examination of the closely related equilibrium NiAl and Ni2Al3 structures permits explanation of this transformation based on the structural similarity of the two phases and using a model built around radiation disordering of vacancies.

Failure temperature of amorphous Cu‐Ta alloys as diffusion barriers in Al‐Si contacts

Self‐supporting films of amorphous Cu20Ta80 alloys have a crystallization temperature as high as 800 °C, yet they fail as diffusion barriers in aluminum‐silicon contacts at much lower temperature because of compound formation. On Si(100) the a‐Cu20Ta80 films react above the temperature at which TaSi2 forms (650 °C). In contact with aluminum, Al3Ta starts to form at 500 °C, which is also the failure temperature of a‐Cu20T80 as diffusion barrier between Al and Si(100). The reaction characteristics of the barrier constituents with the surrounding elements as well as the crystallization temperature determine the thermal stability of amorphous alloy diffusion barriers.

Stability of metallic CsCl‐structured alloys under ion irradiation

The ion irradiation stability of the CsCl phases CoAl, IrAl, SiRu, and AuZn has been investigated. Thin‐film (∼500‐A‐thick) transmission electron microscopy samples were irradiated with Xe to a maximum dose of 4×1015 ions/cm2 at room temperature. Irradiation‐induced transformations in the compound samples were examined by transmission electron diffraction. The alloys CoAl, IrAl, and AuZn remained crystalline while SiRu became amorphous. Analysis of our findings and other previously reported experimental results indicate that CsCl compound stability under ion irradiation is strongly correlated with atomic mobilities.

Effect of ion beam mixing on wear and friction of Fe-Ti multilayered structure on AISI 304 stainless steel

Dry sliding properties of ion beam mixed multilayered Fe‐Ti structures deposited on AISI 304 stainless steel were studied over the composition range 0–100% Ti using samples with a linearly varying composition of constituents. Our measurements revealed that a ductile surface was formed with good sliding properties over the wide composition range by ion beam mixing of deposited layers. These beneficial effects were not obtained on as‐deposited samples or by ion bombardment of either a substrate or pure iron layer deposited on a substrate. Thus, it is concluded that both the ion beam processing and composition as well as the resulting microstructure are essential for improved tribology.

Stability and formation of NiAl 3 under ion irradiation

Compound samples of NiAl 3 as well as Ni 23 Al 77 multilayered samples have been irradiated by either Xe or Ne ions to doses of 2 × 10 15 Xe ions/cm 2 and 1.3 × 10 6 Ne ions/cm 2 at temperatures of 100 K, 300 K, and 373 K. In the case of compound irradiation, NiAl 3 stability appears to be determined by regrowth kinetics and increased with lighter irradiating ion mass and higher irradiation temperature. The formation of NiAl 3 by ion mixing Ni/Al multilayers was not affected by irradiating ion mass and appears to be limited by nucleation.

Mobility of Ni versus Zr in an amorphous Ni-Zr alloy

A method for determining the dominant diffusing constituent in a binary amorphous alloy is demonstrated with the Ni‐Zr system. Nickel is the dominant moving species in an amorphous Ni‐Zr alloy near the a‐Ni50Zr50 composition at 250 °C. In comparison to the Ni, the Zr has a negligible mobility under similar thermodynamic driving forces. Rutherford backscattering spectrometry is used to measure the change in the concentration of a thin film of Ni between thick amorphous layers. Based upon the change in the Ni profile, the diffusivity of Ni in an amorphous Ni‐Zr alloy is between 1×10−17 and 1×10−16 cm2/s at 250 °C.