Solid-state Interdiffusion Research Articles

Effects of Ion Irradiation on the Crystal-Glass Transition by Solid-State Interdiffusion*

Article Effects of Ion Irradiation on the Crystal-Glass Transition by Solid-State Interdiffusion* was published on January 1, 1988 in the journal Zeitschrift für Physikalische Chemie (volume 157, issue 1).

Solid-state interdiffusion reactions in Ni/Ti and Ni/Zr multilayered thin films

We have performed a comparative transmission electron microscopy study of solid-state interdiffusion reactions in multilayered Ni/Zr and Ni/Ti thin films. The Ni-Zr reaction product was amorphous while the Ni-Ti reaction product was a simple intermetallic compound. Because thermodynamic and chemical properties of these two alloy systems are similar, we suggest kinetic origins for this difference in reaction product.

The use of thermodynamic models in the prediction of the glass-forming range of binary alloys

Analytical expressions describing the various thermodynamic phases in binary systems can be derived by the CALPHAD approach, whereby the parameters in these expressions are obtained from fits to measured phase equilibrium data. Extrapolations of these models can be used to predict metastable equilibria. Different thermodynamic models proposed for the Ni–Ti and the Cu–Ti systems are used to calculate the T0 curves that are relevant to the prediction of the glass-forming range in rapidly cooled molten alloys. Although all the models give acceptable fits for the equilibrium range for which they were derived, significant differences are found for the metastable regime. The problems that need to be addressed in order to improve the extrapolations into the metastable regime are discussed. Simple thermodynamic considerations predict that the glass-forming range for rapidly quenched alloys exceeds that for amorphous alloys synthesized by isothermal solid-state interdiffusion reactions.

Study of the mechanism of amorphization by mechanical alloying

Amorphous Ni x Nb x−1 powders were prepared by mechanical alloying for 0.2 ⩽ x ⩽ 0.8. The structural refinement during milling was studied by optical and electron microscopic examinations. The data are in good agreement with Benjamin's model. Transmission electron micrographs give the first direct evidence that the mechanism of amorphization by mechanical alloying is governed by a solid-state interdiffusion reaction.

Formation of iscosahedral AlMn and AlRu by solid state processes

We have used ion beam mixing and solid state interdiffusion of interdigitated Al/Mn or Al/Ru layers to form icosahedral surface alloys in the solid state. Higher temperatures are required to form the icosahedral phase in AlRu than in AlMn, both with mixing (300 °C vs. 150 °C) and by interdiffusion (400 °C vs. 300 °C). Annealing to higher temperatures produces a transformation to more stable crystalline phases. Formation by these techniques indicates that these icosahedral phases nucleate and grow in the solid state in preference to crystalline phases of lower free energy.

The formation of amorphous interlayers by solid-state interdiffusion between Pd-Er and Si

Interest has been maintained in the way silicides form between a deposited metal and silicon because the relationship between the contact potential and boundary reaction remains obscure. This is partially because the cleanliness as well as the vicinality of the silicon surface can change the temperature dependence and homogeneity of the reaction products and thus the presence of localised strains in the silicon. Chemical surface analysis and electrical measurements thus need careful correlation with microstructural examination and it was with this in mind, at the suggestion of K.N. Tu, that we followed up the work of his group on the behaviour of a rare earth-noble metal alloy pair on silicon (Pd-Er/Si) with a microscopical study. The more interesting results are summarised below.

Formation of amorphous alloys by the mechanical alloying of crystalline powders of pure metals and powders of intermetallics

Amorphous powders of Ni32Ti68 and of Ni45Nb55 were synthesized by mechanical alloying (MA) starting from either a mixture of pure metal powders (in the appropriate molar ratio) or from powders of the crystalline intermetallics NiTi2 and Ni45Nb55, respectively. For both alloys, the peak temperature increase (above the average processing temperature) in the powder particles trapped between colliding balls is estimated at 38 K. Thus, the amorphization is attributed to a process other than the formation of local melts followed by the rapid solidification of these melts into the amorphous phase. The amorphization by MA starting from a mixture of pure crystalline powders is attributed to a solid state interdiffusion reaction, the kinetics of which is controlled by the excess point and lattice defects generated by plastic deformation. The amorphization by MA starting from powders of crystalline intermetallics is attributed to the accumulation of point and lattice defects which raise the free energy of the faulted intermetallic above that of the amorphous alloy.

Icosahedral-phase formation by solid-state interdiffusion.

We have formed the icosahedral phase in ${\mathrm{Al}}_{86}$${\mathrm{Mn}}_{14}$, ${\mathrm{Al}}_{82}$${\mathrm{Mn}}_{18}$, and ${\mathrm{Al}}_{75.5}$${\mathrm{Mn}}_{20}$${\mathrm{Si}}_{4.5}$ alloys by the interdiffusion of elemental layers in the solid state. Annealing at 250-425\ifmmode^\circ\else\textdegree\fi{}C produced the phase in times as short as 1 min with grains up to \ensuremath{\sim} 14 nm in diameter. Continued annealing transformed the alloy layers into crystalline phases. These results demonstrate that the icosahedral phase nucleates in the solid state in preference to crystalline phases of lower free energy, even though the initial material does not have liquidlike icosahedral short-range order.

Exafs Studies of Grain Boundary Diffusion and Segregation

ABSTRACTThe dominant pathway for thin film interdiffusion and interface reactions is often via grain boundaries. We have made EXAFS measurements of grain boundary constituents for two systems: interdiffusion in Ag-Au bilay-ers and solid state reaction of Nb with a copper-tin bronze to form Nb3Sn. The Ag-Au results indicate that Au in saturated Ag grain boundaries has an environment similar to a dilute Au in Ag solution with reduced coordination. For the Nb-bronze reaction, the results for the Cu environment indicate distinct changes in the grain boundary environment when small amounts of Ti, Hf, Zr, and Ta are added to the starting Nb. Both results demonstrate the ability of EXAFS to probe grain boundary environments, and to provide important structural information in understanding solid state diffusion and interdiffusion in thin film systems.

The synthesis of amorphous NiTi alloy powders by mechanical alloying

Crystalline powders of Ni and Ti were mechanically alloyed by high-energy ball milling in an inert atmosphere at T < 240 K. The alloyed Ni x Ti 1− x powders were characterized by scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry. For 0.28 < x < 0.72, the powder is amorphous. This composition range is in good agreement with that deduced from a free energy diagram for the crystalline terminal solutions and the amorphous phase. Outside this regime, the powder is a two-phase mixture of an amorphous phase and the crystalline terminal solution of the major element. After ball milling, the solubility limit of Ti in FCC Ni is approximately 28%, which is significantly larger than that in annealed Ni. Atomic pair distribution functions, G( r), were calculated from the X-ray patterns for x = 0.32, 0.4, 0.6, and 0.7. For x = 0.4, the G( r) of the mechanically alloyed powder is almost identical with that of rapidly quenched amorphous Ni 40Ti 60. For Ni 33Ti 67, the crystallization temperature, the crystallization enthalpy, and the apparent activation energy for crystallization are 712 K, 0.74 kcal/g at., and 65.8 kcal/g at., respectively. These values are within 10% of those measured by Buschow for rapidly quenched Ni 32Ti 68. The amorphization by mechanical alloying is attributed to mechanical mixing and to a solid state interdiffusion reaction taking at or near clean boundaries between polycrystalline Ni and Ti.

Influence of the electrode boundary motion on the determination of the interdiffusion coefficient by electrochemical techniques

A mathematical analysis is developed to take into account the motion of the electrolyte/electrode boundary during electrolysis at constant current or at constant potential. The change in volume of the electrode is deduced from the partial molal volume of the electroactive component, which is supposed to be constant during the experiment. The applicability of this treatment is discussed. It is shown that, after a potential step, the decrease of the current density is a linear function of t − 1 2 , as for the Cottrell law, but the proportionality factor is changed. During the passage of a constant current the surface concentration remains nearly a linear function of t 1 2 , as for the Sand law when the concentration of the electroactive component is small. In general, very large departures from the standard equations appear at high concentrations of electroactive component. Applications are given for measuring solid state interdiffusion coefficients from electrochemical techniques. This study can be used to determine the kinetics of electrolyte diffusion coating and surface alloying.

Investigations on the structural and hydrogenation characteristics of LaNi 5, HoNi 5, GdNi 5, SmNi 5, MmNi 5, and CFMmNi 4.5Al 0.5 thin films

This communication deals with the investigations made on synthesis, structural characteristics, hydrogen absorption behaviour and electrical resistivity variation with hydrogenation in thin films of rare earth metal pentanickelides— RNi 5 where R = La, Sm, Gd, Ho and Mm (Mischmetal) and a related phase cerium-free MmNi 4.5Al 0.5. The synthesis of the above mentioned intermetallics was accomplished by suitable solid-state interdiffusion and homogenization. The as-synthesized flux were characterized by X-ray diffraction. The thin films prepared by vacuum thermal vapour deposition were characterized by transmission electron microscopy in various modes. These films were amorphous and transformed to crystalline form on annealing. The hydrogen absorption behaviour (both in bulk and thin films) was studied by employing electrolytic method. It was found that amorphous forms absorb more hydrogen than their crystalline counterparts. The variation of electrical resistivity with hydrogen absorption in thin-film form of various intermetallics was also studied. Plausible explanations for the above hydrogenation characteristics have been outlined.

A study of amorphous alloys of Au with group III A elements (Y and La) formed by a solid-state diffusion reaction

Amorphous alloys Au XM 1−x, with x ≅ 0.5, where M is La or Y, have been formed through solid-state interdiffusion reaction of pure polycrystalline Au, La, and Y thin films (200 to 600 Å thickness) at temperatures of 50–130 C. For x ≳ 0.7 the end product of the reaction is a two phase material containing polycrystalline Au and amorphous AuM, while for x ≲ 0.3 the end product is amorphous AuM plus polycrystalline M. The attainment of these products is explained in terms of the common tangent rule applied to the Gibbs free energy diagram of the amorphous alloy and the crystalline compounds observed.

Formation of an Amorphous Alloy by Solid-State Reaction of the Pure Polycrystalline Metals

Single-phase ${\mathrm{Au}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}$ alloys, with $0.3\ensuremath{\lesssim}x\ensuremath{\lesssim}0.5$, have been formed through solid-state interdiffusion of pure polycrystalline Au and La thin films (100-600 \AA{}) at temperatures of 50-80\ifmmode^\circ\else\textdegree\fi{}C. The reaction is driven by the large negative heat of mixing in the amorphous alloy and occurs at low temperatures by the anomalously fast diffusion of Au in La. The composition regime giving a single-phase amorphous product is explained with use of a free-energy diagram.

On the occurrence of modulated phases in binary and ternary rare earth metal trialuminides

The investigations made on the synthesis and structural characterization of rare earth metal binary and ternary trialuminides are reported. These alloys were made by suitable homogenization using solid state interdiffusion and were tested by X-ray diffraction. Thin films from the initial agglomerate of the alloys were made by thermal vapour deposition in vacuum. These thin films were studied using transmission electron microscopy. High resolution transmission electron microscopy was also employed for the detailed structural characterization. The as-grown thin films were found to be amorphous. When the films were annealed, they underwent an amorphous-to-crystalline transformation accompanied by the formation of modulated phases.

Formation and Transformation of Amorphous Silicide Alloys

ABSTRACTAmorphous silicide films can be formed by rapid quenching using techniques of vapor deposition and ion beam mixing and also by slow heating using solid state interdiffusion and reaction. For example, amorphous TaSi2 films can be formed by sputtering or dual electron guns co-deposition. Amorphous Pt2Si3 films have been produced by mixing PtSi and Si at room temperature with an ion beam at about 100 to 300keV. Recently, an amorphous Rh-Si alloy phase has been made by slowly heating to 300°C a very thin crystalline Rh films (∼50Å) on amorphous Si. The formation and crystallization behavior of these amorphous silicide alloys has been studied by transmission electron microscopy and electrical conductivity measurement.

Metal-Joining Methods

Metal-joining methods play a prominent role in modern technologies since virtually all products are fabricated by joining processes that require that joint integrity equal or surpass component integrity. This field is extremely broad (1-6), involving many scientific and engineering disciplines, a myriad of metals and alloys, and all conceivable design criteria. Typical methods include arc welding, electron beam welding, diffusion bonding, chemical vapor deposition, adhesive bonding, soldering, and brazing. Although originally developed on an empirical basis, these methods are becoming more and more dependent on sound scientific and engineering prin­ ciples to satisfy the needs of advancing materials technology. Indeed, the joining of metals is so important that a modern society could not exist without considerable expertise in this field. Joining may be defined formally as the process of bringing two or more surfaces into intimate contact in order to establish continuity of a field across the resulting interface-electrical joining denotes establishment of continuity of an electric field across an interface, mechanical joining denotes establishment of continuity of a stress field across an interface, etc. Initially, surfaces to be joined must be prepared so that they are compatible with one another, both chemically and mechanically. Many technological approaches are associated with circumventing contact prob­ lems and may involve a protective atmosphere, insertion of an intermediate layer between surfaces, or cleansing, fluxing, melting, or coating of surfaces to be joined. Even when satisfactory joining processes are devised, subsequent performance often is limitcd by structural alterations that occur during the joining process and con­ comitant degradation of engineering properties thereafter. Typical joining problems are associated with preparation of clean surfaces, nonuniform solidification of weld materials, interfacial stress induced by periodic heating and cooling, solid-state interdiffusion, undesirable phase and structural formations, creep, corrosion, fracture, and gradual degradation of electrical and mechanical properties in the

Metal Oxycarbides as Cutting Tool Materials

The role of chemical stability of cutting tool materials in tool wear was investigated by studying the wear characteristics of titanium oxycarbides. The oxycarbides TiC0.25O0.75, TiC0.5O0.5, TiC0.6O0.4, and TiC0.75O0.25 were produced by solid state interdiffusion of TiO and TiC. Their hardnesses and lattice spacings were determined as functions of their chemical composition. The chemical interaction of these oxycarbides with steel was investigated by diffusion couple experiments. Then commercially available cemented carbide tools were coated with TiC0.5O0.5 and TiC0.75O0.25 by RF diode sputtering. The wear resistance of these coated tools was determined by cutting tests. The wear rate of these oxycarbide coated tools was comparable to that of a TiC coated tool, although the free energies of formation of oxycarbides were lower and their hardnesses comparable. It is, therefore, concluded that while the data on free energy of formation and hardness can be useful in the initial screening steps of tool materials, the kinetics of the wear process needs to be more fully understood to define the chemical stability of tool materials in a cutting environment.