Diffusion in the titanium-nickel system: II. calculations of chemical and intrinsic diffusion coefficients

Diffusion coefficients in the Ti-Ni system have been calculated by the aid of equations given by Sauer and Freise, and Wagner. Values for the TiNi (50 at. pct Ni) phase were found to be:D ^u (cm²/s) = 0.0020 exp - 142,000/R for the temperature range between 650 and 940°C. The heat of activation, expressed in J/mol, has an accuracy of ±6000. For the β-Ti(Ni) phase containing 6 at. pct Ni the temperature dependence of the diffusion coefficient is expressed by:D ^u (cm²/s) = 0.0688 exp - 141,000/RT. The uncertainty in the energy of activation is ±12000 J/mol. No clear variation of the diffusion coefficient with concentration could be detected. It was found that Ni is by far the fastest moving component in β-Ti(Ni), Ti₂Ni and TiNi (at least in the composition range between 50 and 53 at. pct Ni). Values ofD _Ni/D _Ti have been calculated with an equation derived by van Loo. The significance of the calculated values is critically examined. By means of a practical example it is shown that the calculated ratio of the intrinsic diffusion coefficients can be extremely sensitive to slight variations in the position of the marker interface.Diffusion coefficients in the Ti-Ni system have been calculated by the aid of equations given by Sauer and Freise, and Wagner. Values for the TiNi (50 at. pct Ni) phase were found to be:D ^u (cm²/s) = 0.0020 exp - 142,000/R for the temperature range between 650 and 940°C. The heat of activation, expressed in J/mol, has an accuracy of ±6000. For the β-Ti(Ni) phase containing 6 at. pct Ni the temperature dependence of the diffusion coefficient is expressed by:D ^u (cm²/s) = 0.0688 exp - 141,000/RT. The uncertainty in the energy of activation is ±12000 J/mol. No clear variation of the diffusion coefficient with concentration could be detected. It was found that Ni is by far the fastest moving component in β-Ti(Ni), Ti₂Ni and TiNi (at least in the composition range between 50 and 53 at. pct Ni). Values ofD _Ni/D _Ti have been calculated with an equation derived by van Loo. The significance of the calculated values is critically examined. By means of a practical example it is shown that the calculated ratio of the intrinsic diffusion coefficients can be extremely sensitive to slight variations in the position of the marker interface. This paper is based on a Thesis submitted by G. F. BASTIN in fulfillment of requirements for the degree of Doctor in Technological Sciences.     

Synthesis of nanodispersed phases during rapid solidification and crystallization of glasses in Ti75Ni25 alloys

The formation of the metallic glass and crystalline phases and related microstructures and the decomposition behavior of rapidly solidified Ti₇₅Ni₂₅ alloys obtained under different processing conditions have been investigated in detail. The competition between glass transition and nucleation of β-Ti during rapid solidification leads to the possibility of synthesizing the nanocomposites of β-Ti and glass. Additionally, it is shown that the presence of a small amount of Si also promotes simultaneous nucleation of fine Ti₂Ni intermetallic compound. Thermodynamic calculation of the metastable phase diagram indicates the presence of a metastable eutectic reaction between α-Ti and Ti₂Ni. Evidence of this reaction at lower cooling rates has been presented. On heating, the glass decomposes through this reaction. Finally, on the basis of understanding of the microstructural evolution during decomposition, a new approach has been adopted to synthesize a nanodispersed composite of α-Ti in the crystalline Ti₂Ni matrix with a narrow size distribution by controlling the devitrification heat treatment of the metallic glass.     

Influence of interface microstructure on the strength of the transition joint between Ti-6Al-4V and stainless steel

Solid-state diffusion bonding of Ti-6Al-4V and type 304 SS was investigated in the temperature range of 750 °C to 950 °C, under a uniaxial load for 5.4 ks in vacuum. The diffusion bonds were characterized using light and scanning electron microscopy. The scanning electron microscopic images in backscattered mode show the existence of different reaction layers in the diffusion zone. The composition of these layers was determined by energy-dispersive X-ray spectroscopy (EDS) to contain the α-Fe, χ, λ, FeTi, β-Ti, and Fe₂Ti₄O phases. The presence of these intermetallics was confirmed by X-ray diffraction. The bond strength was evaluated, and the maximum tensile strength of ∼342 MPa and the maximum shear strength of ∼237 MPa were obtained for the diffusion couple processed at 800 °C due to the finer width of the brittle intermetallic layers. With a rise in joining temperature, the bond strength drops owing to an increase in the width of the reaction layers. The activation energy and growth constant were calculated in the temperature range of 750 °C to 950 °C for the reaction products. The χ phase showed the fastest growth rate. A fracture-surface observation in a scanning electron microscope (SEM) using EDS demonstrates that failure takes place mainly through the β-Ti phase for the diffusion couples processed in the aforementioned temperature range.     

Growth of semicoherent TiFe nanoparticles in β-Ti matrix in the Ti73Fe27 rapidly quenched ribbon

A thin Ti₇₃Fe₂₇ ribbon was prepared by rapid quenching from the melt. The as-quenched ribbon was in a metastable condition with a small amount of nanoparticles of TiFe, of a size of 138±31 nm, embedded in the β-Ti matrix. The β-Ti matrix was supersaturated with Fe, and the fraction of the matrix was higher than that in the equilibrium state. High-resolution imaging of the interface of the TiFe/β-Ti showed that a periodic array of dislocations were present in the interface to accommodate the lattice mismatch. The spacing between the dislocations in the as-quenched specimen was 5.0±0.5 nm. When the ribbon was heated to 700 °C, growth of the TiFe nanoparticles to a size of 228±37 nm took place in the β-Ti matrix. The amount of β-Ti was reduced, as well as the Fe content in β-Ti. The interface between the TiFe and β-Ti remained semicoherent, except that the spacing between the interfacial dislocations was reduced to 3.5±0.6 nm. The persistence of the semicoherent interface was ascribed to the same crystal structure and close lattice parameters shared by TiFe and β-Ti. The growth kinetics of the TiFe nanoparticles during heating was examined based on the modified theory of isothermal heating. It can be considered to be controlled by the diffusion of Fe atoms in the β-Ti matrix to the TiFe phase. Prolonged heating of the ribbon below the eutectoid temperature led to partial transformation of β-Ti to α-Ti.     

Precipitation processes in near-equiatomic TiNi shape memory alloys

Metallographic studies have been made of precipitation processes in Ti-50 pct Ni and Ti-52 pct Ni (at. pct) shape memory alloys. The eutectoid and peritectoid reactions previously reported for near-equiatomic and Ni-rich TiNi alloys were not observed for either composition. In the Ti-52Ni alloy, diffusional transformations take place, similar to those in supersaturated alloys. The precipitation sequence can be written asβ ₀ → Ti₁₁Ni₁₄ → Ti₂Ni₃ → TiNi₃. The solidus line of the TiNi phase in the Ti-52Ni alloy lies at 812 ± 22 °C. Morphological characteristics of the various precipitate phases are described in detail. M. NISHIDA, formerly with the University of Illinois     

Interdiffusion in Ni-Rich,Ni-Cr-Al alloys at 1100 and 1200 °C: Part I. Diffusion paths and microstructures

Interdiffusion in Ni-rich, Ni-Cr-Al diffusion couples was studied after annealing at 1100 and 1200 °C. Recession of γ′ (Ni₃Al structure), β (NiAl structure), or α (bcc) phases was also measured. Aluminum and chromium concentration profiles were measured in the γ (fcc) phase for most of the diffusion couples. The amount and location of Kirkendall porosity suggests that Al diffuses more rapidly than Cr which diffuses more rapidly than Ni in the γ phase of Ni-Cr-Al alloys. The location of maxima and minima in the concentration profiles of several of the diffusion couples indicates that both cross-term diffusion coefficients for Cr and Al are positive and that D_CrAl has a greater effect on the diffusion of Cr than does D_A1Cr on the diffusion of Al. The γ/γ + β phase boundary has also been determined for 1200 °C through the use of numerous γ/γ+ β diffusion couples.     

Diffusion in the nickel-rich part of the Ni−Al system at 1000° to 1300°C; Ni3Al layer growth,diffusion coefficients,and interface concentrations

The formation of the Ni₃Al layer in NiAl (55 at. pct Ni)-pure Ni diffusion couples at temperatures above 1000°C has been found to be controlled almost completely by volume diffusion. At 1000°C and below, the relatively small grain size of the Ni₃Al compound in the layers caused such a large contribution from grain boundary diffusion, that the layer growth rates at 1000°C exceeded those at 1100°C and even those at 1200°C. In Ni₃Al (75at. pct Ni)-pure Ni diffusion couples the Ni₃Al compound rapidly converted into the solid solution of aluminum in nickel. Volume-diffusion coefficients calculated by the Boltzmann-Matano method yielded heats of activation of 55, 64, and 65 kcal·mol^?1 for NiAl, Ni₃Al and the solid solution of aluminum in nickel, respectively. In addition, eleven different types of diffusion couples were prepared from various Ni?Al alloys and annealed at 1000°C. Marker interface displacements and observations of porosity in these couples yielded a more detailed picture of the Kirkendall-effect than earlier work had done. The ratio of the intrinsic diffusion coefficients at the marker interface,D _NI/D _Al, is greater than one in the nickel-rich NiAl phase. For the Ni₃Al phase no statement can be made on the basis of this work. When the marker interface is located in the nickel solid solution,D _Ni/D _Al is smaller than one. The phase boundary concentrations in these couples did not show the expected deviation from the equilibrium concentrations in two-phase alloys; this finding is discussed with regard to the free-energycomposition diagram.     

Interface microstructures in the diffusion bonding of a titanium alloy Ti 6242 to an INCONEL 625

A Ti6242 alloy has been diffusion bonded to a superalloy INCONEL 625. The microstructures of the as-processed products have been analyzed using optical metallography, scanning electron microscope (SEM), and scanning transmission electron microscope (STEM) techniques. The interdiffusion of the different elements through the interface has been determined using energy-dispersive spectroscopy (EDS) microanalysis in both a SEM and a STEM. Several regions around the original interface have been observed. Starting from the superalloy INCONEL 625, first a sigma phase (Cr₄Ni₃Mo₂), followed by several phases like NbNi₃, Ŋ/Ni₃Ti, Cr(20 pct Mo), β Cr₂Ti, NiTi, TiO, TiNi, and Ti₂Ni intermetallics, just before the Ti6242 have been identified. Because the diffusion of Ni in Ti is faster than the diffusion of Ti in the superalloy, a Kirkendall effect was produced. The sequence of formation of the different phases were in agreement with the ternary Ti-Cr-Ni diagram.     

Interface stability in the Ni-Cr-AI system: Part II. morphological stability of β-Ni5OAI vs γ-Ni40Cr diffusion couple interfaces at 1150 °C

The morphological stability of the bond interface in β-Ni50Alvs γ-Ni40Cr solid-solid diffusion couples annealed at 1150 °C has been examined as a function of annealing time. Interface breakdown results in a microstructure consisting of an irregular layer of α-Cr,β precipitates within the α layer, and lamellar areas of α + β touching the γ region. A series of experiments was designed specifically to examine the early stages of diffusion, interface motion, and intermediate phase formation. Pronounced vaporization of Al from the β-Ni50Al end-member occurs within the initial heat-up period to 1150 °C and during the early stages of bonding. The subsequent deposition of Al onto the faying surface of the γ end-member results in the conversion of the γ surface to a β region. The low mobility of Cr and the inability of the β-γ surfaces to maintain equilibrium causes the formation of an intermediate layer of α -Cr. Moreover, the newly formed β region exhibits a fine-grained structure with significant porosity; the number density of pores increases with increased solid-state diffusion. The grain boundaries and pores result in irregular migration patterns of the constituent elements, causing the formation and growth of β protrusions into the a layer. Lateral rejection of Cr from the protrusion then occurs, leading to the formation of α-Cr regions adjoining the β outgrowth. Sympathetic growth of alternate regions of α and β follows, giving rise to a lamellar microstructure that can be explained on the basis of a low mismatch between the α and β lattice parameters, compatible crystal structures, and a pronounced orientation relationship. In a separate but related experiment, the degenerate α+ β morphology observed in the normal diffusion couples was replaced by a layer of agglomerated a-Cr when the β-γ couples were annealed under applied high pressure.     

Ternary dissolution kinetics in the Fe-Ni-P system

The dissolution of (FeNi)₃P in the ternary Fe-Ni-P system has been studied by optical and electron microprobe techniques. Precipitates of (FeNi)₃P, initially in equilibrium with their ternary matrix (a at 750°C, y at 875°C), were examined after being partially dissolved by heating at 975°C. In addition, diffusion couples with starting compositions similar to the equilibrated ternary alloys were examined after also being heat treated at 975°C. Phosphide, (FeNi)₃P, dissolution in the α or γ phase is diffusion controlled at 975°C. The ternary dissolution paths observed in each of the diffusion couples are unique and the same as those observed in the comparable alloys. The dissolution rate of (FeNi)₃P is controlled by the diffusion rate of P in the α or y phases. The Ni interface compositions in (FeNi)₃P and α or γ and the dissolution path through the ternary are determined by the rate of dissolution and the major Ni ternary diffusion coefficients. It is possible to calculate both the dissolution path and rate for (FeNi)₃P by using the binary dissolution equations in combination with the Fe-Ni-P diagram and the major (ternary) diffusion coefficients. In addition, numerical solutions can be correctly calculated for diffusion controlled dissolution where impingement of overlapping gradients occurs. Formerly Graduate Assistant, Department of Metallurgy and Materials Science, Lehigh University, Bethlehem, Pennsylvania     

Morphological stability of α-β phase interfaces in the Cu−Zn−Ni system at 775°C

An experimental investigation into the morphological stability of α-β phase interfaces in the Cu−Zn−Ni system at 775°C has been undertaken. As a preliminary to this study, it was necessary to also determine certain significant diffusion and equilibrium parameters for the composition range of interest (35 to 50 wt pct Zn and 0 to 10 wt pct Ni). Using two-phase infinite diffusion couples, all with the same α terminal composition but with various β terminal compositions, the transition from a stable to an unstable planar α-β interface was indexed. The time evolution of unstable phase interfaces was also examined. The experimentally observed transition is in good agreement, with the transition which is predicted on the basis of linear perturbation theory.     

TiNi Shape Memory Alloys with High Sintered Densities and Well-Defined Martensitic Transformation Behavior

This study demonstrates that a high density and a high transformation heat can be attained for PM TiNi. With the use of fine elemental powders, a composition of Ti₅₁Ni₄₉, two-step heating, and persistent liquid-phase sintering at 1553 K (1280 °C), a 95.3 pct sintered density was attained for compacts with a green density of 66 pct. A transformation heat, ΔH, of 31.9 J/g was also achieved, which is much higher than reported previously for sintered TiNi and is approaching the highest ΔH reported to date, 35 J/g, for wrought TiNi with low C, O, and N contents. The main reason for having these properties in powder metal TiNi with higher amounts of C, O, and N is that the extra Ti, that over the equiatomic portion in the Ti-rich Ti₅₁Ni₄₉, forms Ti₂Ni compound, which traps most of the C, O, and N. This results in low interstitial contents and a high Ti/Ni ratio of 50.5/49.5 in the TiNi matrix. The tensile strength, elongation, and shape recovery rate after five training cycles were 638 MPa, 14.6, and 99.1 pct, respectively, despite the presence of Ti₂Ni compounds at grain boundaries.     

Phase Equilibria in the Ni - Sc - Ti System in the Region TiNi - Ni - Sc1.13Ni At 900°C

Physicochemical analysis methods have been applied to TiNi - Ni - Sc_1.13Ni alloys at 900°C. It is shown that the phase interactions under these conditions are unaltered by comparison with those at subsolidus temperatures. The phase based on TiNi₃, which is the most thermally and thermodynamically stable, is in equilibrium with all the other phases in the TiNi - Ni - Sc_1.13Ni subsystem. The solubilities of the components in the phases based on Ti_{50– x} NiSc_x, TiNi₃, and the Laves phase are reduced at 900°C. The temperature of the polymorphic transformation in the phase based on ScNi₅ is raised by the dissolution of titanium in it to 930°C. An isothermal section is constructed of the phase diagram for the TiNi - Ni - Sc_1.13Ni subsystem together with two polysections.__________Translated from Poroshkovaya Metallurgiya, Nos. 1–2(441), pp. 39–46, January–February, 2005.     

Interfacial microstructure and mechanical properties of diffusion-bonded joints of titanium TC4 (Ti-6Al-4V) and Kovar (Fe-29Ni-17Co) alloys

Diffusion bonding is a near net shape forming process that can join dissimilar materials through atomic diffusion under a high pressure at a high temperature.Titanium alloy TC4(Ti-6 Al-4 V)and 4 J29 Kovar alloy(Fe-29 Ni-17 Co)were diffusely bonded by a vacuum hot-press sintering process in the temperature range of 700-850°C and bonding time of 120 min,under a pressure of 34.66 MPa.Interfacial microstructures and intermetallic compounds of the diffusion-bonded joints were characterized by optical microscopy,scanning electron microscopy,X-ray diffraction(XRD)and energy dispersive spectroscopy(EDS).The elemental diffusion across the interface was revealed by electron probe microanalysis.Mechanical properties of joints were investigated by micro Vickers hardness and tensile strength.Results of EDS and XRD indicated that(Fe,Co,Ni)-Ti,TiNi,Ti_2Ni,TiNi_2,Fe_2 Ti,Ti_(17) Mn_3 and Al_6 Ti_(19) were formed at the interface.When the bonding temperature was raised from 700 to 850°C,the voids of interface were reduced and intermetallic layers were widened.Maximum tensile strength of joints at 53.5 MPa was recorded by the sintering process at 850°C for 120 min.Fracture surface of the joint indicated brittle nature,and failure took place through interface of intermetallic compounds.Based on the mechanical properties and microstructure of the diffusion-bonded joints,diffusion mechanisms between Ti-6 Al-4 Vtitanium and Fe-29 Ni-17 Co Kovar alloys were analyzed in terms of elemental diffusion,nucleation and growth of grains,plastic deformation and formation of intermetallic compounds near the interface.     

Embrittlement of titanium alloys by long time,high temperature exposure

α stabilized titanium alloys are known to exhibit embrittlement after long-time exposures above ∼800°F. The time-temperature dependency of this embrittlement phenomenon in the Ti-6Al-2Sn-4Zr-2Mo and Ti-5Al-6Sn-2Zr-lMo-0.25Si alloys was observed using a substandard fracture mechanics test. Room temperature slow-bend tests of fatigue precracked Charpy specimens were used to monitor toughness degradation after unstressed thermal exposures in the temperature range of 800° to 1100°F for times to 5000 hr. The activation energy for the embrittlement process was found to be ∼25 to 28 kcal per g mole, which approximates that for diffusion of aluminum or tin in α-Ti. The embrittlement is attributed to the Ti₃X (X = Al, Sn) phase with the rate controlling step that of diffusion controlled growth of the Ti₃X phase domains. The embrittlement process is reversible by heat treatment at temperatures above the α + Ti₃X two phase region.     

Interdiffusion structures and paths for multiphase Fe-Ni-Al diffusion couples at 1000 °C

Diffusion studies were carried out in the Fe-Ni-Al system at 1000 °C using solid-solid diffusion couples assembled with β (B₂), γ (fcc) single phase, and (β + γ) two-phase alloys. The diffusion couples were encapsulated in quartz tubes under vacuum and annealed for 48 hours. The diffusion structures were examined by optical and scanning electron microscopy. For all β vs (β + γ) couples, growth of the β phase was observed as the (β + γ) two-phase region recessed with the dissolution of the γ phase. For multiphase couples assembled with two (β + γ) terminal alloys, demixing of the (β + γ) two-phase alloys occurred with the formation of single-phase β and γ layers. The development of an interphase boundary between the (β + β′) two-phase region and the γ phase is reported for the first time for a Fe-Ni-Al diffusion couple assembled with single-phase, β, and γ terminal alloys. Various diffusion structures for the couples were related to their diffusion paths constructed from concentration profiles determined by electron probe microanalysis. Interdiffusion fluxes of individual components were determined directly from the experimental concentration profiles and examined in light of diffusional interactions and the development of zero-flux planes and flux reversals. In addition, the boundaries for the miscibility gap between the ordered β and disordered β′ phases of the Fe-Ni-Al system at 1000 °C were determined with the aid of diffusion couples that developed β and β′ phases in the diffusion zone.     

Diffusion paths and structures in multiphase Cu−Ni−Zn couples at 775°C

Multiphase diffusion was investigated in the Cu−Ni−Zn system at 775°C for the development of diffusion structures involving two interfaces. Selected series of diffusion couples characterized by a common γ (cubic) terminal alloy joined to a set of α (fcc) alloys developed an intermediate β (bcc) phase with two interfaces, α/β and β/γ. The α/β interface showed transitions from planar to nonplanar and back to planar morphology, as the copper concentration of the α terminal alloy was decreased from 100 to about 30 at. pct. Planar β/γ interfaces were observed for all but two of the couples. The compositions on either side of planar α/β interfaces were consistent with those based on equilibrium tielines, while the compositions at nonplanar α/β interfaces differed from those of equilibrium. Selected series of couples assembled with γ and β alloys were also investigated for the development of interface instability at the β/γ interface. The diffusion paths of γ/β couples were consistent with those of γ/α couples.     

Interface stability in the Ni-Cr-AI system: Part I. morphological stability of β-γ diffusion couple interfaces at 1150°C

Aluminide coatings on Ni-base superalloys offer resistance to oxidation and hot corrosion at elevated temperatures. Complex depositional and subsequent diffusional interactions of the coating with the substrate result in a multiphase product consisting primarily of β-NiAl and γ′-Ni₃Al intermediate phases. An understanding of interfacial stability between the coating and the substrate is therefore necessary in order to explain the formation of such phases. The Ni-Cr-AI system serves to simplify the complex chemistry of most Ni-base superalloys. In this study, reaction diffusion and interfacial stability were investigated in solid-solid diffusion couples, consisting of a common β-Ni50Al end-member and a series of γ-pure Ni, binary Ni-Cr, and ternary Ni-Cr-Al alloys, isothermally annealed at 1150 °C for 49 hours. The morphological development of the interface was examined using optical metallography and quantitative information obtained using electron-probe microanalysis. A transition from a stable or planar to an unstable or nonplanar interface in the β-γ diffusion couples was observed with the systematic variation in Cr content of the γ end-member. Interface breakdown in the β-γ couples was explained by means of microstructural information gathered about interfaces, measured diffusion paths, and a knowledge of phase constitution relationships.     

The early stage of Ni3AI layer growth in NiAI/Ni diffusion couples

The growth kinetics and morphology of the Ni₃Al intermediate phase present in NiAI/Ni diffusion couples have been studied for short times ranging from a few minutes, or less, to a few hours at 1100 °C. Despite high heating rates (<240 seconds to reach 1100 °C), a significant portion of the Ni₃Al layer forms upon heating such that the layer growth which occurs during heating is approximately equal to that which occurs during the first hour of isothermal interdiffusion. TEM studies indicate that the irregular interfaces present at both interphase boundaries are associated with grain boundaries within the Ni₃Al phase. A qualitative model accounting for enhanced layer growth by a grain boundary contribution to diffusion within the Ni₃Al layer is described. formerly with the Department of Metallurgy and Materials Engineering, Lehigh University, Bethlehem, PA,.