Metastable Alloy Formation Research Articles

Formation of metastable phases during spark plasma sintering of Fe@Au core–shell particles

ABSTRACT The present work reports the formation of Fe-Au metastable alloys by galvanic replacement, pressureless spark plasma sintering (SPS) and acid treatment. The reaction of partial galvanic replacement between micrometer-sized iron particles and HAuCl4 solution allowed synthesising core–shell Fe@Au particles. When the Fe@Au particles were treated in HCl solution, iron was selectively dissolved and hollow Au particles were formed. Pressureless SPS of the Fe@Au core–shell particles led to the formation of a metastable intermetallic phase, FeAu3, and a supersaturated solid solution, Au(Fe). The latter was resistant to HCl solution; at the same time, the FeAu3 phase decomposed during HCl treatment to form a porous gold structure. This work shows that a combination of galvanic replacement, SPS and acid treatment can be used for the preparation of metastable Fe-Au alloys with different phase compositions and particle morphologies.

Contact Resistance Improvement for Advanced Logic by Integration of Epi, Implant and Anneal Innovations

ABSTRACTAs advanced CMOS scaling with FinFETs continues beyond the 10/7nm nodes, contact resistance (Rc) remains a dominant component affecting device performance. The FinFET Source/Drain (S/D) contact area has become smaller with fin pitch scaling, resulting in drastically increased Rc. To achieve higher drive currents and fully realize the performance gain from FinFET architectural changes, it is critical to continue to reduce contact resistivity (ρc) < 1.0x10-9 Ω.cm2 for both NMOS and PMOS. In this paper, we review the recent trends for ρc reduction for advanced CMOS devices and discuss approaches that have demonstrated reduction in ρc, such as in-situ heavily doped epitaxial films for S/D, advanced ion implantation and laser anneals. The implant techniques include pre-amorphization implants (PAI), dopant boosting implants, cryogenic (-100°C) implants for damage engineering and plasma doping (PLAD) for conformal doping of high aspect ratio (HAR) contacts. With such high levels of doping from epi and implants, advanced laser anneals are key for epitaxial regrowth and formation of metastable alloys for dopant supersaturation or segregation in top layers. Millisecond laser anneal (MSA) improves dopant activation and nanosecond laser anneal (NLA) permits superactivation, and both have become key enablers for ρc reduction. This paper also reviews two alternative contact approaches: dual silicide scheme and wrap-around contact (WAC), as potential pathways to further reduce Rc for advanced CMOS nodes.

Effect of Nb addition on Fe-7Cr-Nb and Fe-15Cr-Nb metastable alloy formation and their thermal stability

Thermal stability of any structural nanomaterial is extremely important for its prospective high temperature applications. Here, we have investigated the influence of Nb content (up to 1at%) on the thermal stability of mechanically alloyed nanocrystalline Fe-Cr alloys for its possible nuclear first wall reactor application. The alloys were prepared by mechanical milling for 25 h in a high purity argon atmosphere (purity <10 ppm O2) at room temperature. The milling was performed in hardened steel grinding media using SPEX 8000M high energy mill and the powder to ball mass ratio was kept at 1:10. Evolution of disordered Fe-Cr-Nb solid solutions has been analyzed through estimating free energy (Gibbs) change as per Toop’s model, variation of precise lattice parameter and TEM-SAED study. The Fe-15%Cr-Nb alloys showed effective thermal stability due to the addition of only 0.25%Nb. The crystallite size of the stabilize alloy was ∼58 nm after annealing at 1000 °C. TEM grain size analysis also corroborated the same. The corresponding microhardness value was found to be quite high (7.8 GPa) which also correlated the stabilizing effect. The grain boundary segregation by oversize Nb atoms along with solid solution hardening by Cr is identified to play the pivotal role for such attractive thermal stability.

Development of a Novel Ni-Fe-Cr-B-Si Interlayer Material for Transient Liquid Phase Bonding of Inconel 718

A Ni-Cr-Fe-Si-B based interlayer material was developed by mechanical alloying (MA) process in a high-energy planetary ball mill. Equiaxed alloy powders of size 12 µm was obtained after milling for 50 hours. X-ray diffraction analysis of the milled powder revealed that milling of elemental powders initially resulted in microcrystalline alloy powder having face centered cubic structure, which on subsequent milling resulted in nano-crystallice alloy powder with a crystallite size of 3.2 nm. XRD analysis also reveals formation of metastable eutectic alloys resulting in lowering of the melting point of the interlayer material to 1025 °C. IN 718 superalloy samples were joined at 1050°C using the developed interlayer. A homogeneous joint was formed by the newly developed interlayer material. Three different zones were observed at the bond (i) isothermally solidified zone, (ii) diffusion affected zone and (iii) unaffected base metal. In the diffusion-affected zone, boron was present at the grain boundaries of Ni γ matrix in bulky metal borides form. The diffusion of boron from interlayer material into the base material was mechanism of isothermal solidification and bond formation in transient liquid phase bonding of IN 718.

Models of Mutual Solubility Increasing under the Pulse Loading

Since 1975 a so-called anomalous mass-transfer in metals and alloys under pulse loading is being investigated in the Institute of Metal Physics. This phenomenon remains to be a challenge to theoreticians. Besides, one more phenomenon (observed at the same systems) was discovered – formation of metastable alloys (solutions, and, sometimes, ordered phases), with solubility limits far exceeding the equilibrium values and depending on the deformation rate. Since formation of nonequilibrium phases is also typical for alloys under irradiation, it seemed natural for us to use some of concepts and models invented in the “materials under irradiation” community. We propose 3 types of models for description of metastable solid solution formation in diffusion couple under pulse loading: 1) flux balance for both components at the interface with account of non-equilibrium defects generated during pulse loading; 2) “ballistic jump” concept, which had been invented by George Martin et al. for materials under irradiation or ball-milling, combined with another Martin’s kinetic model (1994) of diffusion; 3) modification of such a concept for the case of non-equilibrium interstitial defects. Non-equilibrium phase diagrams obtained by all 3 models are built and compared with experimental data which reflect a qualitative correspondence to one another.

Atomistic Modeling of Crystal-to-Amorphous Transition and Associated Kinetics in the Ni−Nb System by Molecular Dynamics Simulations

With the aid of ab initio calculations, an n-body potential of the Ni-Nb system is constructed under the Finnis-Sinclair formalism and the constructed potential is capable of not only reproducing some static physical properties but also revealing the atomistic mechanism of crystal-to-amorphous transition and associated kinetics. With application of the constructed potential, molecular dynamics simulations using the solid solution models reveal that the physical origin of crystal-to-amorphous transition is the crystalline lattice collapsing while the solute atoms are exceeding the critical solid solubilities, which are determined to be 19 atom % Ni and 13 atom % Nb for the Nb- and Ni-based solid solutions, respectively. It follows that an intrinsic glass-forming ability of the Ni-Nb system is within 19-87 atom % Ni, which matches well with that observed in ion beam mixing/solid-state reaction experiments. Simulations using the Nb/Ni/Nb (Ni/Nb/Ni) sandwich models indicate that the amorphous layer at the interfaces grows in a layer-by-layer mode and that, upon dissolving solute atoms, the Ni lattice approaches and exceeds its critical solid solubility faster than the Nb lattice, revealing an asymmetric behavior in growth kinetics. Moreover, an energy diagram is obtained by computing the energetic sequence of the Ni(x)Nb(100)(-)(x) alloy in fcc, bcc, and amorphous structures, respectively, over the entire composition range, and the diagram could serve as a guide for predicting the metastable alloy formation in the Ni-Nb system.

Study of metastable alloy formation by thermodynamic calculation and ion beam manipulation in an equilibrium immiscible Au–W system

Based on thermodynamic calculations, a Gibbs free energy diagram was constructed to predict the possibility of metastable alloy formation in the equilibrium immiscible Au–W system characterized by a large positive heat of formation of +17kJ/mol. Accordingly, in the properly designed Au–W multilayered films, a unique super-saturated fcc solid solution was obtained by ion beam irradiation on the Au-rich side. More interestingly, on the W-rich side, an amorphous alloy was formed through a two-step structural transition. First, a low dose (2×1015 Xe+/cm2) irradiation resulted in the formation of a super-saturated bcc solid solution. Second, the bcc solid solution transformed into an amorphous state upon further irradiation up to a dose of 8×1015 Xe+/cm2. We report, in this paper, results of thermodynamic calculations and the experimental observations concerning the metastable alloy formation in the Au–W system together with a brief discussion of the structural transition mechanism.

Metastable Alloy Formation in the Immiscible Cu–Ru System by Ion Beam Manipulation

In the equilibrium-immiscible Cu–Ru system, a metastable crystalline (MC) alloy of face-centered cubic (FCC) structure was formed in the appropriately designed Cu 25 Ru 75 multilayered films upon room-temperature ion beam manipulation using 200 keV xenon ions. More interestingly, an amorphous alloy was obtained in the Cu 50 Ru 50 multilayered films at low irradiation dose and it transformed into the FCC MC alloy upon further irradiation at high dose. We report, in this Letter, experimental observations of the metastable alloy formation and briefly discuss the role of interfacial free energy in the metastable alloy formation observed in the Cu–Ru system.

Surface alloying of metals by nanosecond laser pulses under transparent overlays

A thorough study was made of the processes determining the formation of metastable surface alloys in a film–substrate system [using Au(110 nm)–Ni and Sn(40 nm)–Cr systems as an example] irradiated by laser pulses of nanosecond duration. The irradiation was performed in conventional conditions (in air) and, following the applied method, through a transparent overlay. Scanning/transmission electron microscopy and Rutherford backscattering spectroscopy investigations of alloys were complemented by the measurements of acoustic signals initiated by laser pulses in the substrate. It has been ascertained that the alloy formation in air is accompanied by two competitive processes: laser sputtering of the film substance and its diffusion into the molten substrate layer. As the irradiation intensity goes up, the hydrodynamical sputtering mechanism is replaced by the atomic vaporization. As a result, sufficiently high vapor pressure (up to 102 bar) is generated above the film surface, the film is pressed to the substrate, and the heat and mass transport through the film–substrate interface is realized. However, no more than 20% of the atoms (of their total amount in the film) penetrate into the substrate, and the depth of alloying is within 100 nm. The experiments on the irradiation of the same systems through transparent overlays (water or “liquid glass”) have demonstrated a marked change in the pattern of processes: alloying atoms efficiently penetrate from the film into the substrate, the depth of alloying increases to 400 nm, and defects are formed in deep layers of the substrate.

Experimental evidence for the role of supersaturated interfacial alloys on the shear elastic softening of Ni/Mo superlattices

The importance of the interfaces in governing the elastic behavior of nanometer-scale metallic superlattices has long been recognized, although their intrinsic nature and the underlying physical mechanisms are still subject to controversy. In the present study, the key role of an interfacial alloying effect on the dramatic softening (-62%) in the shear elastic constant ${C}_{44}$ of Ni/Mo multilayers is clearly evidenced. By combining x-ray diffraction and Brillouin light scattering experiments on both Ni/Mo superlattices and ${\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Mo}}_{x}$ solid solution samples, we show that the formation of metastable alloys, obtained either by cosputtering or stabilized at the interfaces, induces an elastic lattice instability. In addition, the extremely low value of ${C}_{44}$ observed for the multilayers, at small modulation periods, suggests the stabilization under epitaxial growth of a highly supersaturated interfacial alloyed layer.

Metastable alloy formation in lead implanted nickel

Metastable lead–nickel alloys were produced by room temperature random implantations of 240 keV Pb + ions into Ni (1 1 0) single crystals to a fluence of 10 16 cm −2. Rutherford backscattering (RBS)/channeling analysis supplemented with transmission electron microscopy (TEM) showed that the alloy was in form of a supersaturated substitutional solid solution with a peak Pb concentration of 2.4 at.% while a fairly small fraction of the implanted Pb was confined within nanoscale precipitates. The precipitates were found to be mostly single crystalline with size in a range from about 2 to 15 nm although bi-, tri- and even tetracrystals were occasionally observed. Post-implantation annealing at different temperatures upto 860 K leads to a strong outdiffusion of a substantial fraction of the implanted lead. Upon annealing, the lattice occupancy of the originally substitutional lead atoms was found to change to different types of lattice locations (regular and displaced substitutional, octahedral interstitial and random) which appeared to be strongly correlated with the temperature of annealing.

Electron spectroscopic evidence of metastable alloy formation of Ag and Au in Co

High-resolution x-ray photoemission spectroscopy measurements of the conduction bands of dilute noble metal/Co metastable alloy films show a narrowing of the noble-metal d bands at concentrations below 6 at. % of noble metal consistent with coordination of the noble metal atoms with Co. Such alloying contrasts strongly with Ag and Au precipitation at higher concentrations, and with conventional metallurgical experience. It is consistent, however, with recent observations of modified interlayer exchange coupling in magnetic trilayer structures doped with noble-metal atoms [U. Ebels et al., Phys. Rev. B 58, 6367 (1998)].

Interface-driven alloying and metallic glass formation in nano-multilayers in an immiscible Y–Nb system

Several new amorphous alloys were obtained in an immiscible Y–Nb system by room temperature 190 keV xenon ion mixing of Y–Nb multilayered films, which were designed to include a sufficient fraction of interfacial atoms and thus possessed a high free energy comparable to that of the amorphous state. In addition, two metastable f.c.c. phases were formed in Y-rich and Nb-rich multilayered films, respectively. The growth kinetics of the f.c.c. phases and their effect on the composition range of amorphization are also discussed. To understand the observed unusual alloying behaviors, a Gibbs free-energy diagram was constructed based on Miedema's model. The diagram included the free-energy curves of all the involved phases as well as that of the Y–Nb multilayered films including the extra interfacial free energy and thus gave a qualitative and reasonable interpretation to the formation of metastable alloys upon ion mixing. Furthermore, some multilayered films with a sufficient fraction of interfacial atoms were subjected to steady-state thermal annealing, which also resulted in the formation of the Y–Nb amorphous phases, suggesting that alloying in this immiscible system was actually driven by the interfacial free energy.

Structural and chemical investigation of epitaxial [formula omitted] interfaces and of metastable [formula omitted] surface alloys

The formation of the Pt/Co(10 1 ̄ 0) interface has been investigated by AES, XPS, UPS and LEED. We evidence, at room temperature, a quasi-layer-by-layer growth mode with the presence of a slight interdiffusion process at the interface. Through a rather simple kinematical approach of the LEED patterns, we discuss both the in-plane and out-of-plane parameters of the layers. At room temperature, when increasing thicknesses, the long-range order is progressively lost and the epilayer undergoes a structural transition (1×1)→C(2×2)→(1×1). The formation of metastable alloys stabilized in the near surface region has been investigated by AES, XPS, UPS, EELS and LEED for both growth at 600 K and anneals. The growth at 600 K leads to a perfect rotational epitaxy and ends up with a Pt 2Co alloy layer on top of which pure Pt layers grow. Annealing of room temperature deposited layers improves the structural quality of the layer and in some cases leads to reconstructions enhancing the layer density. In this case interdiffusion needs a 2.5 ML (monolayer) critical thickness. The composition of the alloyed layer is driven by the temperature and not by the annealing time, evidencing the metastable nature of the surface alloys.

A comparative study of metastable alloy formation by ion mixing and thermal annealing of multilayers in the immiscible Y–Zr system

Amorphous alloys were formed in an immiscible Y–Zr system by ion mixing and thermal annealing of multilayered films consisting of nine alternate metal layers. In addition, two metastable crystalline phases, i.e. a fcc Zr-rich and a fcc Y-rich phase, were also obtained. A compatible thermodynamic interpretation was proposed and illustrated by a Gibbs free energy diagram, which was constructed based on Miedema's model and included the free energy curve of the multilayered films possessing excess interfacial energy. The formation mechanism of the metastable crystalline phase is also discussed in terms of the electronic structure as well as the growth kinetics.

Solid-State Alloying in Binary Systems with a Positive Heat of Mixing

ABSTRACTThe Extended X-ray Absorption Fine Structure (EXAFS) technique has been employed to monitor the formation of metastable alloys by mechanical alloying in two positive-heat-of-mixing systems, Cu-Fe and Ag-Fe. With its unique atom selectivity and local atomic environment sensitivity, EXAFS provides clear evidence of atomic-level alloying forming bcc (Fe80Cu20) and fee (Fe50Cu50) solid solutions in the Cu-Fe system. In contrast, such alloying appears to be absent in Ag-Fe. The possible availability of a thermodynamic driving force for alloying in certain nanostructured mixtures of immiscible elements is discussed. A preliminary study was performed on the possibility of alloying and amorphization in vacuum-deposited Cu/Ta multilayer films.

Formation of metastable aluminium alloys by mechanical alloying

Formation of metastable aluminium alloys by mechanical alloying

Formation of new metastable transition metal alloys by combination of mechanical alloying and chemical leaching

The leaching of Al atoms from B2 structures of mechanically alloyed Ni and Co aluminides led to the formation of metastable B2 Ni and Co. B2 Ni is strong paramagnet not stable above 470 K. The magnetic moment of Co atom in ferromagnetic B2 phase is smaller than in fcc one. B2 Ni demonstrates high catalytic activity in hydrogenation of organic compounds.

Formation of metastable aluminium-based alloys by mechanical alloying

Report in details the preparation and procedure and some physical properties of a few Al-based metastable alloys in the amorphous or quasicrystalline phase by MA. Furthermore, their properties are compared with those of the similar composition prepared by some other conventional techniques. The following compositions were mechanically alloyed for several hours until metastable phases wre obtained, either in amorphous or in quasi-crystalline form: Al 50 Mn 20 Si 30 , Al 50 Mn 20 Ge 30 and Al 50 Mn 20 Si 20 Ge 10

Growth, Structure and Properties of Thin Films and Superlattice Structures

AbstractThis paper addresses growth mechanisms, characterization and properties of thin films grown from the vapor phase. The emphasis is placed on artificial structures such as metastable alloys and layered materials, including superlattices. For metastable alloy formation it is demonstrated that a bombardment of the growing film surface by low‐energy ions can be used to promote growth of metastable alloys even at temperatures high enough for epitaxial films to form. In the case of layered materials it is shown using a combination of X‐ray diffraction and high resolution transmission electron microscopy, that superlattices with atomically sharp interfaces can be grown not only in the case of materials with covalent bonds (e.g., semiconductors) but also in the case of materials with metallic bonds. Finally, some properties of metallic superlattices are also discussed.