Role of Ti on Phase Evolution,Oxidation and Nitridation of Co–30Ni–10Al–8Cr–5Mo–2Nb–(0, 2 & 4) Ti Cobalt Base Superalloys at Elevated Temperature

Titanium is an important alloying addition to γ/γ′ cobalt-based superalloys that enhances the high temperature microstructural stability and make the alloys lighter. In this work, we probe the role of Ti composition on the phase stability and oxidation behavior of Co–30Ni–10Al–8Cr–5Mo–2Nb superalloys. With Ti addition, the γ′-solvus temperature is enhanced and the γ′-precipitate shape changes from spherical to rounded cuboids. Addition of 4 at. pct Ti to the alloy promotes topologically-close-packed (TCP) phase formation that are rich in Co, Cr, and Mo. During oxidation at 900 °C, Ti was found to facilitate the early formation of passivating oxide layers (spinel CoCr₂O₄/CoAl₂O₄) on the exposed surfaces, however, it was not effective in reducing the oxidation-induced mass gain. Microstructural analysis reveals that Ti delays the Al₂O₃ layer formation eventually leading to faster oxidation kinetics. Additionally, we also found formation of (Ti,Nb)N in the γ′ denuded zones near the alloy-oxide interface.

     

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.     

Kinetics of F.C.C. → B.C.C. heterogeneous martensitic nucleation—I. The critical driving force for athermal nucleation

Employing available experimental data for athermal f.c.c. → b.c.c. martensitic transformation in binary, ternary and multicomponent Fe-base alloys, a model is developed and tested for the critical driving force at the M_s temperature. Incorporating the theory of solid solution hardening, we describe the composition dependence of the athermal frictional work for martensitic interface motion governing the kinetics of barrierless heterogeneous nucleation. The available data suggests that the composition dependence of the athermal frictional work is of the same form as that for slip deformation. We have evaluated the athermal strengths of 14 alloying elements Al, C, Co, Cr, Cu, Mn, Mo, N, Nb, Ni, Si, Ti, V and W from the experimental data. Except for Al, Ni and Co, the athermal strengths of the common substitutional alloying elements are similar in magnitude, while the interstitial solutes C and N exert a stronger influence. Previously proposed superposition laws are used to account for the presence of multiple solutes having different athermal strengths. With an improved assessment of the magnetic parameters of alloy systems, the model predicts M_s temperatures within ±40 K for M_s > 300 K where thermal contributions to the frictional work can be neglected.     

Microstructure of a complex Nb–Si-based alloy and its behavior during high-temperature oxidation

A in-situ composite Nb–Si–Ti–Hf–Cr–Mo–Al composite material alloyed with yttrium and zirconium is studied. The evolution of the structure–phase state of the alloy during oxidation under dynamic and isothermal conditions is considered on samples prepared by vacuum remelting and directional solidification. The phase composition and the microstructure of the alloy are examined by the methods of physico-chemical analysis, and the distribution of alloying elements in initial samples and the products of oxidation is estimated. Thermogravimetric experiments are performed on powders and compacted samples during continuous (in the range 25–1400°C) and isothermal (at 900 and 1100°C) heating in air. The directional solidification of an Nb–Si–Ti–Al–Hf–Cr–Mo–Zr–Y is found to cause the formation of an ultradispersed eutectic consisting of α-Nb_ss and γ-Nb₅Si_3ss cells. The as-cast sample prepared by vacuum remelting has a dendritic structure and contains Nb₃Si apart from these phases. Oxidation leads to the formation of a double oxide layer and an inner oxidation zone, which retain the two-phase microstructure and the ratio of alloying elements that are characteristic of the initial alloy. Diffusion redistribution is only detected for molybdenum. The cyclicity of heating at the initial stage of oxidation weakly influences the oxidation resistance of the alloy.     

Surface hardening of nickel alloys by means of plasma nitriding

Surface hardening of Ni alloys by plasma nitriding was investigated by using tentative Ni binary alloys contained nitride forming elements such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, Mn, Fe, Al, or Si at the nitriding temperature from 673 to 1073 K. Surface hardness was different depending on the types of alloying elements, their contents, and their nitriding temperatures. Higher hardness than HV500 was obtained in Ti, V, Nb, and Cr containing alloys at 823 to 873 K, but other alloys showed lower surface hardness than HV400. The elements Ti, V, Nb, and Cr were the effective alloying elements for the surface hardening of nitrided Ni alloys. From transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis, the nitrided layer was composed of fine precipitate particles in the matrix of the nitrided layer. At the lower nitriding temperature, these particles were metastable fine particles or Ginier-Preston (GP) zone having coherency with the matrix, and these fine particles induced large microstrain in the matrix. However, at the higher nitriding temperature, equilibrium nitride particles were precipitated and coherency with the matrix was decreased. Therefore, the hardening of Ni alloys by plasma nitriding was due to the microstrain induced in the nitrided layer by the precipitation of metastable particles or GP.     

Toughening of MoSi2 with a ductile (niobium) reinforcement

The effect of interfacial reactions and Y₂O₃ coatings on toughening of MoSi₂ by ductile phase Nb reinforcements has been investigated. In the absence of coating the interfacial reaction layer exhibits parabolic growth with Mo₅Si₃, (Mo, Nb)₅Si₃, (Nb, Mo)₅Si₃ and Nb₅Si₃ phases forming. In precracked laminates subjected to tensile loads the ductile phase deformation is partially constrained, with debonding occurring within the interfacial reaction zone. Dense Y₂O₃ coating inhibits interdiffusion and results in more extensive debonding. In either case, significant toughening is expected with measured work of rupture values χ ≈ 5.7 to 6.3. Bulk composite MoSi₂ reinforced with 20 vol.% Nb particles subjected to a chevron-notched three point flexure test had a work of rupture almost five times larger than the unreinforced MoSi₂ matrix.     

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.     

Alloying behavior of Co3Ti

The alloying (substitution) behavior of Ll₂-type Co₃Ti({fx1433-01}) compound was investigated at an isothermal section of 1323 K by the observation of the direction of solubility lobe of Ll₂ phase. The solubility lobes of additions of V, Ta, Cr, W, and Al indicated that they substitute for Ti site, those of Ni and Cu for Co site, and that of Fe for both sites. However, the preferable substitution natures for additions of Zr, Hf, Nb, Mn, and Si, and of Mo and Ge were not determined because of the small solubility limit, and because of no preferable solubility lobe, respectively. The substitution behavior and solubility limit obtained in the ternary Co₃Ti compound were evaluated with the thermo-dynamic concept. The Research Institute for Iron, Steel and Other Metals The Research Institute for Iron, Steel and Other Metals     

Morphological stability of γ/α interface formed by carburization in Fe-C-X alloys

Effect of alloying elements on the morphological stability of austenite/ferrite interface formed by carburization of Fe-X alloys at 850 °C and 800 °C was investigated. Planar interfaces were found when the alloying elements added were from among the following: Ti, V, Nb, Ta, Cr, Mo, W, Co, and Cu. Nonplanar interfaces with Widmanstätten-like structures and/or an isolated phase were observed when the alloying elements were from the following group: P, Al, Sb, Ni, Mn, Si, and Ge. The degree of supersaturation of C in the α phase adjacent to the γ phase front was analyzed using the concept of local equilibrium. It was confirmed that there was indeed a close correlation between the morphological stability and the degree of C supersaturation, which in itself depended on whether the alloying element added was an α or γ stabilizer and how strongly it bonded with C in the ferrite phase.     

Microstructure and ordering of L12 titanium trialuminides

The present article reports and discusses the results of the microstructural characterization of various modifications of Ll₂ trialuminides containing various titanium contents, including the first ever report on their degree of ordering. The Ll₂ trialuminide alloys Al₃Ti + X, where X = Cu, Fe, Cr, and Mn were studied. The as-cast structure contains a very low level of porosity, and the amount of second phase depends on the particular alloy. After homogenization, the second phase is reduced in almost all the alloys to the level less than 0.5 pct, except for the Mn-high Ti alloy in which it remains at about 20 pct and its composition is 67.9 ± 0.6 at. pct Al, 2.2 ± 0.6 at. pct Mn, and 29.9 ± 0.3 at. pct Ti. In almost all the alloys, porosity after homogenization increases about twofold, except in the Al₃Ti + Cr alloy in which it remains at almost the as-cast level. Limited transmission electron microscopic observations have revealed the existence of very fine (≈10 nm) unidentified precipitates in the homogenized Al₃Ti + Cu alloy. The homogenized Al₃Ti + Cr and Mn alloys have greater lattice parameters than the Al₃Ti + Fe and Cu alloys. It is also found that the long-range order parameterS of the ho- mogenized Ll₂ Al₃Ti + X alloys dramatically decreases with increasing titanium content.     

Ti5Si3 whisker in-situ reinforced TiAl alloys

The effects of Fe, Cr, V, and Nb on the microstructures, tensile properties at 20 °C and 900 °C, and creep resistance at 800 °C of Ti₅Si₃ whisker-reinforced Ti₅₂Al₄₈-3Si2M alloys were investigated. The addition of Fe, Cr, Nb, and V modifies not only the morphologies but also the distribution of Ti₅Si₃ whiskers. A microstructure with a uniform and homogeneous distribution of Ti₅Si₃ whiskers was obtained in a Ti₅₂Al₄₈-3Si2Cr2V alloy by conventional ingot metallurgy. The Ti₅₂Al₄₈-3Si2Cr2V alloy has the best room-temperature tensile strength, relatively good ductility, an attractive tensile property at 900 °C, and good creep resistance at 800 °C. The improvement of properties results from not only the homogeneous distribution of Ti₅Si₃ whiskers but also from the higher fracture strength of the Ti₅Si₃ whisker and the interface property. The solubility of V in the Ti₅Si₃ phase is higher than that of Fe, Cr, and Nb. The element V is very effective in strengthening the Ti₅Si₃ whiskers. Different failure modes were found in the Ti₅Si₃ whisker-reinforced TiAl alloys at room temperature. Cleavage fracture dominates the failure of Ti₅Si₃ whiskers and γ phase in V-free alloys, whereas crack deflection and branching at the Ti₅Si₃-whisker/γ-matrix interface, subsequently followed by interface debonding and whisker bridging, were observed in Ti₅₂Al₄₈-3Si2V and Ti₅₂Al₄₈-3Si2Cr2V alloys. In addition, twinning and dislocation slip in Ti₅Si₃ whisker-reinforced TiAl alloys were investigated.     

Alloying mechanism of beta stabilizers in a TiAl alloy

The effects of beta stabilizers such as Fe, Cr, V, and Nb on the microstructures and phase constituents of Ti₅₂Al₄₈-xM (x=0, 1.0, 2.0, 4.0, or 6.0 at. pct and M=Fe, Cr, V, and Nb) alloys were studied. The dependence of the tensile properties and creep resistance of TiAl on the alloying elements, especially the formation of B2 phase, was investigated. Fe is the strongest B2 stabilizer, Cr is second, V is an intermediate stabilizer, and Nb is the weakest stabilizer. The composition partitioning of Fe, Cr, V, and Nb in the γ phase is affected by the formation of B2 phase. The peaks of the tensile strengths and creep rupture life of Ti₅₂Al₄₈-xM generally occur at the maximum solid solution of these elements in the γ phase, which is just before the formation of B2 phase. Ti₅₂Al₄₈-0.5Fe shows an attractive elongation of 2.5 pct at room temperature, and the Ti₅₂Al₄₈-1V, Ti₅₂Al₄₈-Cr, and Ti₅₂Al₄₈-2Nb alloys have about 1.1 to 1.3 pct elongation at room temperature. The increase of tensile strengths and creep resistance with increasing Fe, Cr, V, and Nb contents is chiefly attributed to the solid-solution strengthening of these elements in the γ phase. The appearance of B2 phase deteriorates the creep resistance, room-temperature strengths, and ductility. With respect to the maximum solid-solution strengthening, an empirical equation of the Cr equivalent [Cr] is suggested as follows: [Cr]=Cr+Mn+3/5V+3/8Nb+3/2 (W+Mo)+3Fe=1.5 to 3.0. The solid-solution strengthening mechanism of Fe, Cr, V, and Nb at room temperature arises from the increase of the Ti 3s and Al 2s binding energies in Ti-Ti and Al-Al bonds, and the retention of the strength and creep resistance at elevated temperatures in Ti₅₂Al₄₈-xM is mainly attributed to the increase of the Ti 3s and Al 2s binding energies in Ti-Al bonds in γ phase. The decrease of the Ti 3p and Al 2p binding energies in Ti-Ti, Ti-Al, and Al-Al bonds benefits the ductility of TiAl.     

Electrochemical Behavior of Bulk Amorphous Steel Fe55M2Cr12Mo10B6C13Y2（M=Ni, Cu, Nb）

 Fe55Ni2Cr12Mo10B6C13Y2,Fe55Cu2Cr12Mo10B6C13Y2 and Fe55Nb2Cr12Mo10B6C13Y2 alloys with diameter of 4mm were produced by copper mold casting. The role of alloying additions (Ni, Cu or Nb) on corrosion resistance of Fe55Nb2Cr12Mo10B6C13Y2, Fe55Ni2Cr12Mo10B6C13Y2 and Fe55Cu2Cr12Mo10B6C13Y2 alloys were studied by polarization curves and electrochemical impedance spectroscopy (EIS). The results show that Fe55Ni2Cr12Mo10B6C13Y2 and Fe55Cu2Cr12Mo10B6C13Y2 alloys can be cast into bulk metallic glasses. Fe55Ni2Cr12Mo10B6C13Y2 and     

Effect of Ti Addition and Microstructural Evolution on Toughening and Strengthening Behavior of as Cast or Annealed Nb–Si–Mo Based Hypoeutectic and Hypereutectic Alloys

Room temperature fracture toughness along with compressive deformation behavior at both room and high temperatures (900 °C, 1000 °C and 1100 °C) has been evaluated for ternary or quaternary hypoeutectic (Nb–12Si–5Mo and Nb–12Si–5Mo–20Ti) and hypereutectic (Nb–19Si–5Mo and Nb–19Si–5Mo–20Ti) Nb-silicide based intermetallic alloys to examine the effects of composition, microstructure, and annealing (100 hours at 1500 °C). On Ti-addition and annealing, the fracture toughness has increased by up to ~ 75 and ~ 63 pct, respectively with ~ 14 MPa√m being recorded for the annealed Nb–12Si–5Mo–20Ti alloy. Toughening is ascribed to formation of non-lamellar eutectic with coarse Nb_ss, which contributes to crack path tortuosity by bridging, arrest, branching and deflection of cracks. The room temperature compressive strengths are found as ~ 2200 to 2400 MPa for as-cast alloys, and ~ 1700 to 2000 MPa after annealing with the strength reduction being higher for the hypoeutectic compositions due to larger Nb_ss content. Further, the compressive ductility has varied from 5.7 to 6.5 pct. The fracture surfaces obtained from room temperature compression tests have revealed evidence of brittle failure with cleavage facets and river patterns in Nb_ss along with its decohesion at non-lamellar eutectic. The compressive yield stress decreases with increase in test temperature, with the hypoeutectic alloys exhibiting higher strength retention indicating the predominant role of solid solution strengthening of Nb_ss. The flow curves obtained from high temperature compression tests show initial work hardening, followed by a steady state regime indicating dynamic recovery involving the formation of low angle grain boundaries in the Nb_ss, as confirmed by electron backscattered diffraction of the annealed Nb–12Si–5Mo alloy compression tested at 1100 °C.

     

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.     

Atom-probe microanalysis of a nickel-base superalloy

A FIM atom-probe has been used to investigate the phase compositions in a Nb-Mo bearing nickel based superalloy. The composition of γ′ precipitates in fully heat-treated conditions was found to vary with their mean sizes. The matrix analyses revealed the presence of fine secondary precipitates (30 to 100 Å) which occupy 10 pct of the overall volume of the material. The high spatial resolution of the atom-probe allowed the γ-γ′ interface characterization. Composition profiles show that the transition between the phases occurs within one interplanar spacing. Finally, a long range order study of the ordered γ′ phase has been performed. The analysis of the L1₂ type (Ni, X)₃ (Al, Y) precipitates, made on an atomic plane-by-plane basis, shows how alloying elements substitute for Ni and Al in the γ′ sublattice. The observed results, expressed in terms of occupancy probabilities for both types of sites, indicate that Ti, Nb, and Mo preferentially occupy Al sites while Cr and Co substitute for Ni.     

Kinetic analysis of the combustion synthesis of molybdenum and titanium silicides

The temperature profiles associated with the passage of self-propagating combustion waves during the synthesis of MoSi₂ and Ti₅Si₃ were determined. From these profiles, kinetic analyses of the combustion synthesis process for these two silicides were made. The synthesis is associated with high heating rates: 1.3 × 104 and 4.9 × 104 K·s₋₁ for MoSi₂ and Ti₅Si₃, respectively. The width of the combustion zone was determined as 1.3 and 1.8 mm for the silicides of Mo and Ti, respectively. The degree of conversion, η, and its spatial distribution and the conversion rate, ∂η/∂t, were determined. However, because of the inherent characteristics of wave propagation in MoSi₂, only in the case of Ti₅Si₃ could the activation energy be calculated. An average value of 190 kJ μ mol₋1 was determined for titanium suicide.     

Stability and Heat Resistance of Silicide Coatings on Refractory Metals. Part 3. Stability of Silicide Coatings on Niobium Heated in Air at 1500-1800°C

The kinetics of phase redistribution in the (Mo, W)Si₂ Nb system at 1500-1800°C was investigated. The kinetic parameters for growth of the lower silicides (Mo, W, Nb) ₅Si₃ + Nb₅Si₃ and decrease in the layer thickness of the higher silicide (Mo, W)Si₂ as function of the oxidation temperature were determined. It was established that the stability of the multiphase and multicomponent system was more than twice that of the system MoSi₂ Nb, and 15-18 times that of MoSi₂ Mo.     

Preparation of Mo–Si–B nanocomposite powders by mechanical alloying and heat treating

Abstract

In this study, an attempt has been made to synthesis Mo–Si–B nanocomposite alloys using a combination of mechanical alloying (MA) and heat treatment in various primary elemental compositions. For this purpose, Mo–14Si–10B, Mo–57Si–10B and Mo–47Si–23B (at.‐%) elemental powders were separately milled using an attritor mill. Mechanically alloyed (MAed) powders were annealed in an atmosphere controlled furnace under constant temperature for 10?h. Metallurgical characteristics of MAed and/or annealed powders were evaluated by atomic absorption spectrometry, SEM, TEM and X‐ray diffraction. The results did not show any formation of related intermetallics after MA. However, MoSi₂, Mo₅Si₃, Mo₅SiB₂, MoB and Mo were successfully formed, when the MAed Mo–57Si–10B powders were subjected to annealing at a high temperature.