Role of alloying elements in phase decomposition in alnico magnet alloys

Partition of alloying elements between the Fe-rich disordered a, and NiAl-rich ordered α₂ phases in Fe-Ni-Al base systems has been studied by the diffusion couple technique. It has been revealed that Ti, Cu, Mn, and Co are distributed to the α₂ phase rather than the α₁ phase, while Si, V, Nb, Cr, and Mo are mostly distributed to the α₁ phase. Ti remarkably narrows the miscibility gap between the α₁ and α₂ phases and shifts the summit point to the Fe-rich side. Cu promotes the diffusion rate of atoms in the α₂ phase. On the basis of these experimental results, the role of alloying elements in the spinodal decomposition of Alnico magnets has been discussed.     

Miscibility gap in Fe-Ni-Al and Fe-Ni-Al-Co systems

The phase equilibria in Fe-Ni-Al and Fe-Ni-Al-Co systems have been investigated by the diffusion couple technique, and the miscibility gap that separates α₁ (Fe-rich disordered bcc phase) from α₂ (NiAl-rich ordered bcc phase) has been determined. It has been ascertained that the three-dimensional shape of the miscibility gap is not simple helmetlike, but has a peculiar ridge at the order-disorder transition temperature. The miscibility gap is narrowed and shifted to the Fe-rich corner by the addition of Co, and Co atoms are distributed to the α₂ rather than the α₁ Furthermore, by alloying Co, the fcc γ phase is stabilized and the miscibility gap between α₁ and α₂ is hidden to some extent by the phase regions concerned with the γ phase.     

Partition of alloying elements between γ (A1), γ′ (L12), and β (B2) phases in Ni-Al base systems

Phase equilibria between γ (Al), γ′ (Ll₂), and β (B2) phases in the Ni-rich portions of the Ni-Al-X ternary systems were investigated over a temperature range of 800 °C to 1300 °C. The tie lines and phase boundaries were accurately determined by the diffusion couple technique. It was established that Co, Cu, Mn, Fe, and Cr concentrated more into the γ phase than into the γ′ phase, while Ta, Nb, Ti, V, and Si mostly partitioned to the γ′ phase. The partition coefficients for alloying elements between γ and γ′ phases varied as a function of temperature for most of the elements and also as a function of concentration for some of the elements, such as Mo, W, and V. In the equilibrium between γ′ and β phases, Mn, Fe, Co, and Cu partitioned to the β phase rather than to the γ′ phase, whereas Nb, Mo, Ta, Ti, W, V, Cu, and Si concentrated into the γ′ phase. The partition of alloying elements in the metastable equilibrium between γ and β phases, in the Ni-Al binary system, was estimated from the data on γ/γ′ and γ′/β equilibria. Based on these data, the relative stabilizing effects of alloying elements on γ, γ′, and β phases are also discussed.     

Phase chemistry and precipitation reactions in maraging steels: Part IV. Discussion and conclusions

This article summarizes our studies of phase chemistry and precipitation reactions in a variety of maraging steels. The roles of different phases and alloying elements are investigated by comparing the behavior of different steels. The phases considered are Ni₃Ti, Fe₇Mo₆ μ phase, Fe₂Mo Laves phase, ω phase, Ti₆Si₇Ni₁₆ G phase, “Z phase,” austenite, and α matrix. The alloying elements discussed are Ti, AI, Mo, Si, Mn, Ni, Cr, and Co. By comparing the aging behavior of both commercial steels and model alloys, a major role of Co is confirmed to be the lowering of the matrix solubility of Mo. Of the two main hardening elements in maraging steels (namely, Ti and Mo), Ti is much more active than Mo in the very early stage of precipitation. The main Mo-rich precipitate found in this work was Fe₇Mo₆μ phase instead of Laves phase. The precipitation of Mo is modified by the presence of Ti. ω phase appears only in Ti-free alloys, especially when aged at a low temperature. The quantity of Ni-containing precipitates and the presence of Cr in the steels change the austenite reversion behavior. Other phases, such as G phase and “Z phase,” contribute to age hardening in different types of maraging alloys. Formerly Graduate Student with the Department of Materials, Oxford University.     

Effect of alloying elements on martensitic transformation in the binary NiAl(β) phase alloys

The characteristics of the B2(β) to L1₀(β′) martensitic transformation in NiAl base alloys containing a small amount of third elements have been investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM). It is found that in addition to the normal Ll₀ (3R) martensite, the 7R martensite is also present in the ternary alloys containing Ti, Mo, Ag, Ta, or Zr. While the addition of third elements X (X: Ti, V, Cr, Mn, Fe, Zr, Nb, Mo, Ta, W, and Si) to the binary Ni₆₄Al₃₆ alloy stabilizes the parentβ phase, thereby lowering the M_s temperature, addition of third elements such as Co, Cu, or Ag destabilizes theβ phase, increasing the M_s temperature. The occurrence of the 7R martensite structure is attributed to solid solution hardening arising from the difference in atomic size between Ni and Al and the third elements added. The variation in M_s temperature with third element additions is primarily ascribed to the difference in lattice stabilities of the bcc and fcc phases of the alloying elements.     

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.     

Ternary diffusion coefficients—Theoretical treatment and application to theIn-situ-reinforced TiB2/Al composite

In the present work, a new model is developed to predict ternary diffusion coefficients in liquid state based on hard sphere theory and the Miedema model, and the diffusion coefficients are theoretically determined starting from the quantities of the pure component. Using this model, the diffusion coefficient of Ti in the TiB₂/Al composite is evaluated and the influence of alloying element additions on the diffusion coefficient of Ti is discussed. The results indicate that additions of Si can promote the diffusion of Ti. Meanwhile, other elements including Cu, Fe, Mg, V, Ni, La, and Zr can hinder the diffusion of Ti. It is furthermore concluded that not only the phase diagram but also considerations concerning the kinetics are required to produce the desirable phases.     

The solidification behavior of 8090 Al-Li alloy

In this work, the solidification and segregation behaviors of 8090 Al-Li alloy have been investigated with differential thermal analysis (DTA) and the metallographic-electron microprobe method. The results show that 8090 Al-Li alloy has a much more complex solidification path than Al-Li binary alloy due to the addition of many alloying elements and the presence of impure elements. Solidification begins at about 635 °C with the reaction of L → α-Al + L′, and this reaction goes on to termination. The alloying element Cu and impure elements Fe and Si have a strong segregation tendency. During solidification, Cu segregates to the interdendrite and finally forms α-Al + T₂ eutectic. As a result, the solidification temperature range is greatly extended. Iron and Si form the insoluble constituents Al₇Cu₂Fe, AlLiSi,etc., although their concentrations in the alloy are quite low. With the increase of Fe content, there is a eutectic reaction of α-Al/Al₃Fe at about 595 °C. The formation of insoluble constituents is influenced by both concentrations of impure elements in the alloy and the cooling rate.     

Ductility enhancement in NiAl (B2)-base alloys by microstructural control

An attempt to improve ductility of NiAl (B2)-base alloys has been made by the addition of alloying elements and the control of microstructure. It has been found that a small amount of fccγ phase formed by the addition of Fe, Co, and Cr has a drastic effect not only on the hot workability but also on the tensile ductility at room temperature. The enhancement in ductility is mainly due to the modification of Β-phase grains by the coexistence ofγ phase. The effect of alloying elements on the hot forming ability is strongly related to the phase equilibria and partition behavior amongγ,γ′ (L1₂ structure), and Β phases in the Ni-Al-X alloy systems. The ductility-enhancement method shows promise for expanding the practical application of nickel aluminide.     

Microstructural study of the interface in laser-clad Ni-Al bronze on Al alloy AA333 and its relation to cracking

The interface toughness between a laser clad and the substrate determines whether the cladding is useful for engineering application. The objective of this investigation is to correlate the interface properties of laser-clad Ni-AI bronze on Al alloy AA333 with the microstructure and crystal structure of the interface. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDX) are used to examine the interface. In a good clad track, the interface is an irregular curved zone with a varying width (occasionally keyholing structure) from 30 to 150 μm. A compositional transition from the Cu-rich clad (83 wt pct Cu) to the Al-rich substrate (3.2 wt pct Cu) occurs across this interface. Three phases in the interface are identified in TEM: Al solid solution, θ phase, and γ₁ phase, as described in the Cu-Al binary phase diagram. In a good clad track, the θ and γ₁ phases are distributed in the Al solid solution. In a clad track with cracks, the interface structure spreads to a much larger scale from 300 μm to the whole clad region. Large areas of θ and γ₁ phases are observed. The mechanism of cracking at the interface is related to the formation of a twophase region of θ and γ₁ phases. To understand the microstructure, a nonequilibrium quasibinary Cu-Al phase diagram is proposed and compared with the equilibrium binary Cu-Al phase diagram. It is found that the occurrence of many phases such as η₁η₂, ζ₁, ζ₂, ε₁, ε₂, γ₀, β₀, and β, as described in the equilibrium binary Cu-Al phase diagram, is suppressed by either the cladding process or by the alloying elements. The three identified phases (Al solid solution, θ phase, and γ₁, phase) showed significant extension of solubility. Formerly Visiting Research Associate, Department of Mechanical and Industrial Engineering, Center for Laser Aided Material Processing, University of Illinois at Urbana-Champaign, Urbana, IL 61801     

Development of TiB<Subscript>2</Subscript> reinforced in-situ Ti aluminide matrix composite through reaction synthesis

TiB₂ reinforced in-situ titanium aluminide matrix composite was made through reaction synthesis process using high purity elemental powders of Ti, Al, Cr, Nb and B. XRD of the synthesized block showed presence of mainly Al₃Ti and TiB₂ phases. To obtain γ Ti aluminide based matrix, the material was homogenized in two phase region (α₂+γ). Presence of γ phase matrix alongwith α₂ was confirmed through XRD, SEM and TEM. Uniform distribution of TiB₂ phase was confirmed through elemental mapping and by analyzing specimens of different locations. Differential scanning calorimetry of powder mixture showed presence of endothermic peak for Al melting and exothermic peak of Ti aluminide and TiB₂ formation.     

Melting of secondary-phase particles in Al-Mg-Si alloys

The melting of secondary-phase particles—or, more precisely, the melting of such particles together with the surrounding matrix—in two ternary Al-Mg-Si alloys has been studied. In the quasi-binary Al-Mg₂Si alloy, one melting reaction is found. In the alloy with an Si content in excess of that necessary to form Mg₂Si, three different melting reactions are observed. At upquenching temperatures above the eutectic temperature, the reaction rates are very high, and it is assumed that they are controlled by diffusion of the alloying elements in the liquid. Melting is also observed after prolonged annealing at temperatures below the eutectic temperature in these alloys, which is explained by the different diffusion rates of Mg and Si. The rate of the melting reaction is in this case assumed to be controlled by diffusion of the alloying elements in the solid α-Al phase. It is shown that calculation of the particle/matrix interface composition, which determines when melting is possible, cannot be made solely on the basis of the phase diagram, but must also include the rate of diffusion of Mg and Si. The melting temperatures observed differ somewhat from the accepted eutectic temperatures for these alloys. On prolonged annealing, the liquid droplets formed dissolve into the surrounding matrix and their chemical composition is found to change during dissolution. The resulting eutectic structure after quenching of a droplet is explained by the phase diagram and the different diffusion rates of Mg and Si as well as by the nucleation conditions of the constituents involved.     

Melting of secondary-phase particles in Al-Mg-Si alloys

The melting of secondary-phase particles—or, more precisely, the melting of such particles together with the surrounding matrix—in two ternary Al-Mg-Si alloys has been studied. In the quasi-binary Al-Mg₂Si alloy, one melting reaction is found. In the alloy with an Si content in excess of that necessary to form Mg₂Si, three different melting reactions are observed. At upquenching temperatures above the eutectic temperature, the reaction rates are very high, and it is assumed that they are controlled by diffusion of the alloying elements in the liquid. Melting is also observed after prolonged annealing at temperatures below the eutectic temperature in these alloys, which is explained by the different diffusion rates of Mg and Si. The rate of the melting reaction is in this case assumed to be controlled by diffusion of the alloying elements in the solid α-Al phase. It is shown that calculation of the particle/matrix interface composition, which determines when melting is possible, cannot be made solely on the basis of the phase diagram, but must also include the rate of diffusion of Mg and Si. The melting temperatures observed differ somewhat from the accepted eutectic temperatures for these alloys. On prolonged annealing, the liquid droplets formed dissolve into the surrounding matrix and their chemical composition is found to change during dissolution. The resulting eutectic structure after quenching of a droplet is explained by the phase diagram and the different diffusion rates of Mg and Si as well as by the nucleation conditions of the constituents involved.     

Synthesis of ductile titanium-titanium boride (Ti-TiB) composites with a beta-titanium matrix: The nature of TiB formation and composite properties

The study focused on the in-situ synthesis of titanium (Ti)-titanium boride (TiB) composites with β phase in the matrix by reaction sintering of TiB₂ with Ti and alloying element powders. The goal was to examine the nature of TiB whisker formation in three different kinds of powder mixtures: (1) β-Ti alloy powders and TiB₂; (2) α-Ti powder, a master alloy (Fe-Mo) powder containing the β-stabilizing elements, and TiB₂; and (3) α-Ti powder, a β-stabilizing elemental powder (Mo or Nb), and TiB₂. The effects of powder packing and the relative locations of powder particles on the morphological changes in TiB whisker formation and their growth were studied at processing temperatures ranging from 1100°C to 1300°C. The morphology, size, and distribution of whiskers were found to be influenced by the powder-packing conditions. A large particle-size ratio in bimodally packed mixtures led to the formation of a TiB monolithic layer around β grains. With a relatively finer starting powder, smaller size ratio, and trimodal packing arrangement, the TiB whiskers were found to be distributed more homogeneously in the matrix. The study also used the X-ray direct comparison method and the structure factor for the β phase to determine the volume fraction of TiB phase from X-ray data. Tensile tests and fractographic investigations were carried out on selected composites. The evolution of the composite microstructure, the influence of powder-packing variables, and the morphology and growth of TiB whiskers and their effect on mechanical properties are discussed.     

Thermodynamic effect of alloying addition on <Emphasis Type="Italic">in-situ</Emphasis> reinforced TiB<Subscript>2</Subscript>/Al composites

From the viewpoint of thermodynamics, using the Wilson equation and an extended Miedema model, the effect of the alloying element on the stability of the precipitated phases during the fabrication of in-situ reinforced TiB₂/Al composites was evaluated. The result shows that additions of alloying elements, such as Mg, Cu, Zr, Ni, Fe, V, and La, can promote the formation of Al₃Ti and TiB₂ phases. Particularly, Zr has the most pronounced effect among these alloying elements. In addition, alloying elements can hinder the formation of AlB₂ to a small extent. The calculation results also show that it is easier for magnesium to react with the salts to form TiB₂ than aluminum during the fabrication of in-situ reinforced TiB₂/Al using the flux-assisted synthesis (FAS) technology.     

Precipitation-strengthened austenitic Fe−Mn−Ti alloys

The precipitation of intermetallic compounds in the Fe−20Mn−2Ti and Fe−28Mn−2Ti alloy systems has been investigated over the temperature range 700 to 900°C by hardness measurements, optical and scanning electron microscopy, and X-ray diffraction. In both systems only the equilibrium Laves phase was observed. The precipitate was identified as C14(MgZn₂) type hexagonal Laves phase with a chemical composition close to Fe₂ (Ti, Mn). In an as-annealed sample precipitation occurred in a heterogeneous manner, predominantly along grain boundaries. The effect of a cold deformation between the solution annealing and aging processes was also investigated. In addition to a high density of dislocations, martensitic phases were induced by deformation: a γ→∈ transformation occurred in the Fe−28Mn−2Ti alloy while a γ→α′ transformation was predominant in the Fe−20Mn−2Ti alloy. Subsequent aging was conducted at temperatures above theA _f . A large number of very fine precipitates formed randomly in the matrix after a short aging period. This cold work plus aging treatment resulted in an increase in yield strength. The enhancement of mechanical properties is due to the randomly distributed precipitates combined with the high defect density and fine substructure.     

Effect of Ternary Alloying Elements Addition on the Order-Disorder Transformation Temperatures of B2-Type Ordered Fe-Al-X Intermetallics

The effect of alloying element additions on B2↔A2 order-disorder phase transformation temperatures of B2-type ordered Fe_0.5(Al_1−n X _n )_0.5 intermetallics (X = Cr, Ni, Mo, Ta, Mn, Ti, and W) that readily form single-phase solid solution for X = 1 at. pct were investigated experimentally. It was shown that the type of the ternary substitutional alloying elements have a profound effect on the variation of order-disorder transition temperature of Fe_0.5(Al_1−n X _n )_0.5 alloys. Based on the magnitude of partial ordering energies of the Al-X and Fe-X atomic pairs, predicted normalized transition temperatures, ∆T/T _o , were verified experimentally. Besides the normalized transition temperature, the relative partial ordering energy (RPOE) parameter, β, was also defined to estimate the extent of variation in B2↔A2 order-disorder phase transformation temperatures upon ternary alloying additions. The RPOE parameter, β, takes into account both the effects of magnitude of partial ordering energies of Al-X and Fe-X atomic pairs and also the lattice site occupation preferences of X element atoms over B2-type ordered Fe-Al sublattices. The alloying elements, which are preferentially distributed Fe sublattice sites, β > 0, and owing to β >> 1, are more effective in increasing order-disorder transformation temperature in Fe-Al (B2) intermetallics. On the contrary, alloying elements having β < 1 tend to decrease the transition temperature slightly relative to the binary FeAl intermetallic. The experimentally determined B2↔A2 order-disorder transition temperatures are in good qualitative or semiquantitative agreement with theoretical predictions for all X ternary alloying elements. Accordingly, the present experimental results confirm the validity of the theoretical model and calculations proposed in our previous study on the B2↔A2 order-disorder transition temperatures of single-phase Fe_0.5(Al_1−n X _n )_0.5 intermetallics.     

Reaction diffusion and phase equilibria in the V-N system

The formation of phase bands in in situ diffusion couples of the V-N system was studied by the reaction of vanadium sheet with pure nitrogen within the temperature range 1100 °C to 1700 °C and the nitrogen pressure range 2 to 24 bar. Under these conditions, phase bands of β-V₂N and δ-VN_1−x develop. The morphology of the β-V₂N/α-V(N) interface depends on the saturation state of the α-V(N) core. If the nitrogen content in α-V(N) is high, the interface has a jagged appearance, whereas at low nitrogen contents of the α-V(N) phase, the interface is planar. Electron probe microanalysis (EPMA) was used to measure the diffusion profiles within the couples. The homogeneity regions of the nitride phases were established and the phase diagram accordingly corrected. From the growth rates of the phase bands, the mean composition-independent nitrogen diffusivities in β-V₂N and δ-VN_1−x were derived. These diffusivities follow an Arrhenius equation with activation energies of 2.92 (β-V₂N) and 2.93 eV (δ-VN_1−x ). By using δ-VN_1−x as a starting material and a low nitrogen pressure during annealing, it could be shown that the direction of nitrogen diffusion can be reversed, i.e., β-V₂N is formed on the surface of the couple as a result of out-diffusion of nitrogen.     

On the equilibrium silicide in beta Ti-V alloys containing Si

The addition of small amounts of silicon (usually less than R ~1 at. pct) to strengthen α(hcp)-β(bcc) and martensitic Ti alloys is well established. The equilibrium suicide formed in these alloys has been identified as hexagonal Ti₅Si₃ (Refs. 1, 2) (or (Ti,Zr)₅Si₃ phase in alloys containing Zr), although there is also a report of a tetragonal Ti₃Si phase. The use of Si to age harden β Ti alloys, specifically Ti-V-Si alloys, has also been reported. While the precipitation sequence in these alloys involves an identifiable hexagonal