The Role of Refractory Metal Additions in Precipitation Processes in Superalloys

Superalloys are multi-component alloys containing many alloying additions developed for high temperature applications. Refractory metals are added to impart beneficial properties to the alloy. The role refractory metal additions in precipitation processes in superalloys are discussed in this paper. The minor variation in the alloying additions on the changes in precipitation behaviour are illustrated with the example of Alloy 718 and Alloy 625.     

Cu-Precipitation Strengthening in Ultrahigh-Strength Carburizing Steels

Ultrahigh hardness levels greater than 700 VHN can be obtained in secondary hardening carburizing steels but depend on costly Co alloying additions to maximize hardness achieved through M₂C-type carbide precipitation strengthening. This study aims to incorporate nanometer-scale bcc Cu precipitates to both provide strength as well as catalyze M₂C nucleation in the absence of or with reduced Co. Cu additions of 1.0 and 3.7?wt pct were investigated, using a series of mechanistic models coupled with thermodynamic computational tools to derive final compositions. Thirty-pound experimental heats were cast of each designed alloy, samples of which were carburized and tempered to determine their hardness response. Characterization revealed the successful incorporation of Cu alloying additions into this family of steels, demonstrating a secondary hardening response even in the absence of Co. Matrix strength levels were close to those predicted by design models; however, all four alloys demonstrated a hardness deficit of approximately 200 VHN at the carburized surface, suggesting recalibration of the M₂C precipitation strengthening model may be required in these alloys.     

Retardation of intermetallic phase formation in experimental superferritic stainless steels

While superferritic stainless steels containing 29 pct chromium possess excellent resistance to corrosion, they may, under certain conditions, be embrittled by the precipitation of intermetallic phases. The extent to which the precipitation reactions can be retarded by alloying additions of aluminum and copper has been evaluated. It was found that additions of aluminum to an Fe-29 pct Cr-4 pct Mo-1.5 pct Ni base alloy suppress the precipitation of the undesirable sigma and chi intermetallic phases, but additions of up to 3 pct aluminum promote 475 ‡C embrittlement. Additions of copper slightly reduce the precipitation of sigma and chi phases under most conditions but enhance 475 ‡C embrittlement. The resistance to corrosion in 10 pct H₂SO₄ and 10 pct FeCl₃ was assessed. All the aluminum-containing alloys performed significantly better in H₂SO₄ than the base alloy; however, large additions of aluminum had a deleterious effect on the pitting resistance in FeCl₃. Additions of copper improved the resistance to FeCl₃ and lowered the rate of corrosion in the H₂SO₄ solution used.     

The Role of Refractory Metals in the Development of Structural Intermetallic Compounds

Abstract

The role of refractory metals in high temperature applications is discussed in the context of structural intermetallic compounds. The use of refractory metals in the elemental form as well as its use as alloying additions is first discussed. A catalogue of refractory metal based structural interrnetallics is then itemised. Two examples, a niobium-based structural intermetallic compound and the role of the refractory metal W in improving the thermal stability of TiAI-based interrnetallics are then discussed.     

A melting and solidification study of alloy 625

The melting and solidification behavior of Alloy 625 has been investigated with differential thermal analysis (DTA) and electron microscopy. A two-level full-factorial set of chemistries involving the elements Nb, C, and Si was studied. DTA results revealed that all alloying additions decreased the liquidus and solidus temperatures and also increased the melting temperature range. Terminal solidification reactions were observed in the Nb-bearing alloys. Solidification microstructures in gastungsten-arc welds were characterized with transmission electron microscopy (TEM) techniques. All alloys solidified to an austenitic (γ) matrix. The Nb-bearing alloys terminated solidification by forming various combinations of γ/MC(NbC), γ/Laves, and γ/M₆C eutectic-like constituents. Carbon additions (0.035 wt pct) promoted the formation of the γ/MC(NbC) constituent at the expense of the γ/Laves constituent. Silicon (0.4 wt pct) increased the formation of the yJLaves constituent and promoted formation of the γ/M₆C carbide constituent at low levels (<0.01 wt pct) of carbon. When both Si (0.4 wt pct) and C (0.035 wt pct) were present, the γ/MC(NbC) and γ/Laves constituents were observed. Regression analysis was used to develop equations for the liquidus and solidus temperatures as functions of alloy composition. Partial derivatives of these equations taken with respect to the alloying variables (Nb, C, Si) yielded the liquidus and solidus slopes t(m _L , m _S ) for these elements in the multicomponent system. Ratios of these liquidus to solidus slopes gave estimates of the distribution coefficients (k) for these same elements in Alloy 625.     

Microstructural development in high volume fraction gamma prime Ni-base oxide-dispersion-strengthened superalloys

The mechanical alloying process has successfully combined oxide-dispersion-strengthening with conventional gamma prime precipitation hardening for advanced gas turbine materials. INCONEL* alloy MA 6000, a mechanically alloyed Ni-base superalloy, has the highest temperature capability among commercially available superalloys. Further improvement of the intermediate temperature strength has been pursued by both increasing the gamma prime content up to 80 vol pct and controlling the additions of refractory metals. The microstructural development of these new experimental alloys is reported in this paper, especially for an alloy, nominally identified as Alloy 51, having the composition Ni-9.3 pct Cr-8.5 pct Al-6.6 pct W-3.4 pct Mo-0.15 pct Zr-0.01 pct B-l.l pct Y₂O₃ (wt pct). Both the primary and the secondary recrystallized microstructures of the alloy were characterized in terms of gamma grain structure, gamma prime precipitate morphology, orientation relationships, dispersoids, carbide/nitride particles, and chemical composition of intermetallic phases. The microstructural stability of the alloy under stress rupture conditions was also investigated in terms of coarsening/coalescence of gamma prime precipitates. Correlation of the microstructural information with the high temperature properties of the alloy is also briefly discussed. Formerly with the INCO R&D Center, Suffern, NY.     

合金元素Mo对钛合金相析出行为的影响

钛合金的相变过程中伴随着复杂多变的析出相,不同组织结构的析出相导致合金具有不同的机械性能。添加合金元素可以人为地控制钛合金中相的析出行为,但由于合金元素种类众多,合金元素对钛合金析出行为影响的机理有待进一步探讨研究。综述了合金元素Mo对钛合金析出行为影响的研究现状,以析出相的形态和尺度为主要考察对象,归纳总结了在α型、α+β型以及β型钛合金中添加Mo元素对析出相作用的机理,并指出了目前研究过程中存在的不足,希望为以后的钛合金析出行为的研究提供帮助。     

Effect of Mn,Si and Mn‐Si Alloying on the Precipitation of Ultra‐fine Oxides in Fe during Solidification

The effect of Mn, Si and Mn‐Si alloying on the precipitation of oxide inclusions in Fe during solidification has been investigated. The results show that the inclusions precipitating during solidification are relatively small and distribute uniformly in Mn, SI and Mn‐Si alloyed steels. Most of the inclusions are nearly spherical. The maximum diameter of inclusions is only 3.3μm in the three types of alloyed steels. The average size of inclusions is 1.3μm, 1.2μm and 1.1μm in Mn, Si and Mn‐Si alloyed steels, respectively. The maximum amount of inclusions is 1.1×10⁵ per unit volume (mm^?3) in Mn alloyed steel and the minimum is 5.8×10⁴ per unit volume in Si alloyed steel. The volume fractions of the inclusions with Mn, Si and Mn‐Si alloying are 0.01%, 0.005% and 0.006%, respectively.     

Effect of alloying additions on secondary hardening behavior of Mo-containing steels

The effect of alloying additions on secondary hardening behavior in Fe-Mo-C steels has been investigated by means of the successive alloying additions of Cr, Co, and Ni. The Cr additions promote M₃C cementite formation. The Ni additions destabilize the cementite formation, while the Co additions retard dislocation recovery and present the necessary sites for M₂C formation which provides the secondary hardening. Professor Kwon is jointly appointed at the Center for Advanced Aerospace Materials.     

Stabilization of thermosolutal convective instabilities in Ni-based single-crystal superalloys: Carbide precipitation and rayleigh numbers

The influence of carbide precipitation on grain-defect formation during unidirectional solidification of experimental single-crystal Ni-based superalloys has been assessed over a wide range of compositions with large variations in Re, W, and Ta. In all instances, carbon additions of up to 0.15 wt pct were determined to be statistically significant with respect to stabilizing against the formation of grain defects, such as freckle chains, during solidification. Assessment of the segregation behavior of the constituent alloying additions via a Scheil-type analysis enabled estimation of critical Rayleigh numbers denoting the onset of thermosolutal convection. Precipitation of Ta-rich MC carbides near the liquidus temperature of the alloy was found to interact strongly with the mechanisms associated with freckle formation. Segregation analyses and phase-transformation temperature measurements were used to assess the corresponding Rayleigh numbers for the experimental alloys and to modify the Rayleigh criterion to account for carbide precipitation. Mechanisms pertaining to the interaction of carbides with the onset of thermosolutal convection are discussed.     

Molybdenum in Nb-C alloys

The effects of molybdenum alloying additions to niobium on the carbide phases and their precipitation behavior were investigated. The experimental alloys included Nb-0.1C, Nb-15Mo-0.1C, and Nb-30Mo-0.1C. After selected heat treatments the microstructural changes were determined by metallography and the carbide phases were extracted and identified by X-ray diffraction and chemical analysis. The results are essentially in agreement with recent phase diagram determinations. Additions of 30 wt pct Mo appears to slightly increase the solubility of carbon in niobium at temperatures around 1650°C. The solubility of molybdenum in Nb₂C is very small. Discontinuous precipitation of β-Nb₂C was found to occur in the Nb-30Mo-0.1C alloy during annealing at 1200°C. The important, overall effect of molybdenum in Nb-C alloys is to decrease the rate of niobium carbide precipitation so that appreciable carbon supersaturation can be achieved even after comparatively slow furnace cooling.     

Some Mechanical Properties of Powder-Forged Iron/Alloy Mixtures

Abstract

An investigation was made of the feasibility of producing alloy forgings (nominal composition Fe–0·5C–0·6% Mn), from a powder prepared by mixing the alloying additions with iron powder. The carbon was added as microcrystalline graphite and the manganese as elemental powder and as ferro-manganese powder. Additions of copper and ammonium chloride powders were also made for the purpose of assisting the manganese to alloy with the iron. The copper addition improved the tensile strength but lowered the ductility. The ammonium chloride had little effect except for an apparent lowering of hardness. It was found that useful mechanical properties could be obtained in forgings made from mixtures of the alloying ingredients.     

Spangle formation in galvanized sheet steel coatings

Very large grains, termed “spangles,” are produced on galvanized sheet steel coatings when lead is added to the zinc bath. The spangles have been attributed to melt undercooling prior to solidification. The present results indicate this is not the case, undercooling being less than 1 °C. The spangle diameter is shown to be dependent on the alloy addition to the bath, large spangles being obtained with Bi and Sb as well as Pb. The spangle size is related to the surface tension of the alloying addition, the size decreasing as the melt vapor surface tension of the alloying element increases. It is proposed that spangles form dendritically from a nucleus in the melt. Alloy additions with low interfacial energies and very limited solid solubility are highly concentrated ahead of the dendrite tip. This decreases the tip radius and increases the dendrite velocity, producing large grains. The basal plane orientation of the samples varies between 17 and 80 deg with respect to the steel sheet surface, which is inconsistent with basal plane dendritic growth in Zn along (1010) directions. It is proposed that solute additions to the melt and growth in a thin liquid layer can modify the dendrite growth direction, accounting for the spangle orientation. On leave from Obafemi Awolowo University, lie Ife, Oyo State, Nigeria     

The role of nickel,copper, and columbium (Niobium) in strengthening a low-carbon ferritic steel

The contributions of the alloying elements nickel, copper, and columbium are evaluated in a low-carbon precipitation hardenable ferritic steel. The influences of nickel, copper, and columbium as individual and dual elemental additions on microstructure and mechanical properties are described. These elements each contribute to the as-rolled strength by grain refinement and solid solution strengthening. Copper provides an additional strength increment of about 15,000 psi as a precipitation hardener upon 1050°F aging.     

Use of Alloying to Effect an Equiaxed Microstructure in Additive Manufacturing and Subsequent Heat Treatment of High-Strength Titanium Alloys

This paper addresses the use of alloying additions to titanium alloys for additive manufacturing (AM) with the specific objective of producing equiaxed microstructures. The additions are among those that increase freezing ranges such that significant solutal undercooling results when combined with the rapid cooling rates associated with AM, and so be effective in inducing a columnar-to-equiaxed transition (CET). Firstly, computational thermodynamics has been used to provide a simple graphical means of predicting these additions; this method has been used to explore additions of Ni and Fe to the alloy Ti–6Al–4V (Ti64). Secondly, an experimental means of determining the minimum concentration of these alloying elements required to effect the CET has been developed involving gradient builds. Thirdly, it has been found that additions of Fe to Ti64 cause the alloy to change from an α/β Ti alloy to being a metastable β-Ti alloy, whereas additions of Ni do not produce the same result. This change in type of Ti alloy results in a marked difference in the development of microstructures of these compositionally modified alloys using heat treatments. Finally, hardness measurements have been used to provide a preliminary assessment of the mechanical response of these modified alloys.

     

FAST DIFFUSION ALLOYING FOR POWDER FORGING USING A LIQUID PHASE

Abstract

Prealloyed powders are dominant items in the economics of powder forgings today. Present estimates predict a maximum powder consumption that is too low to bring about essential reductions in the cost of these materials. Long homogenization discourages the use of plain iron powders with admixed alloying elements, especially where induction heating is employed, and the use of cheap alloying elements such as manganese is hampered by oxidation problems. A way of overcoming these difficulties is described. If the alloying additions needed for hardenability are made in the form of a low-melting master-alloy powder, diffusion times can be very much reduced. A condition is that the molten prealloy wets the iron particles, reducing the diffusion distance to the order of one particle radius. It is also desirable that the master alloy should penetrate quickly along the grain boundaries of the iron, further reducing the diffusion distance. Low-melting alloys of manganese with copper satisfy these conditions. Forgings can be produced from plain iron powders with copper-borne manganese additions without excessive oxidation of the manganese and preforms can be sufficiently homogenized within a heating time of a few minutes to give hardenability and tensile properties similar to those of conventional quenched and tempered steels. Since only small amounts of master-alloy powder are needed, ‘solid-liquid alloyed’ plain iron powders appear to offer great flexibility in alloy composition at a cost substantially below that of conventional prealloyed powders.     

Thermodynamics and Kinetics to Alloying Addition on <Emphasis Type="Italic">In-Situ</Emphasis> AlN/Mg Composites Synthesis <Emphasis Type="Italic">via</Emphasis> Displacement Reactions in Liquid Mg Melt

Based on the Wilson equation, extended Miedema model, and hard sphere theory, new models are developed theoretically only using the quantities of the pure component and are applied to investigate the thermodynamical and kinetic effect of alloying additions on in-situ AlN formation via displacement reaction in Mg-Al alloy melt. The results show that the alloying additions such as Si, Zn, and Cu can promote the formation of AlN in Mg-Al melt both in thermodynamics and kinetics. Meanwhile, other elements, including Mn, Nd, Ce, Ni, and La, must be matched properly in order to produce the desired reinforcement AlN in liquid Mg-Al melt.     

Dynamic Precipitation of Laves Phase in FeCrAl Alloy with Zr Addition

FeCrAl alloy is one of potential candidates for accident-tolerant-fuel (ATF)-cladding materials due to its excellent oxidation and corrosion resistance at accident temperature, combining good mechanical properties at service temperature. Alloying strategy is an important way for improving comprehensive properties of FeCrAl alloy through the precipitation of fine Laves phase. Zr alloying can stabilize the Laves phase due to its lower diffusion coefficient and solubility in body-centered-cubic ferrite matrix. Herein, it is found that Zr addition changes the dynamic precipitation features of Laves phase in FeCrAl alloy during high-temperature deformation, from only one type of Fe₂M (M = Nb, Mo, Ta) Laves phase to Fe₂Zr combining Fe₂M-type Laves phase. The Fe₂Zr-type Laves phase precipitates dynamically first, and the interface precipitates between which with ferrite matrix creates more nucleation sites for subsequent precipitation of Fe₂M Laves phase. The results can be possibly applied for alloy design and microstructure tailoring in series of FeCrAl alloys used for ATF cladding in the near future.     

SINTERED IRON-BASE ALLOYS WITH HIGH STRENGTH USING SELECTED NITRIDES AND SILICIDES

Abstract

The techniques used in alloying in iron powder metallurgy have been extended by employing special compounds. The introduction of the alloying elements in this form and the decomposition of selected nitrides and silicides are described. Elements that oxidize readily at high temperatures (e.g. Cr, Si) can be added in a relatively pure and homogeneous state. These elements stabilize the α phase and thus improve the sintering behaviour.

The paper deals mainly with the preparation of binary Fe-Cr, Fe-Si, and also ternary Fe-Cr-Si alloys obtained by ‘in situ’ decomposition of Si₃N₄, Cr₂N, and CrSi₂ in an iron matrix (WP-150).

The study covers the properties of the powders and their mixtures, the pressing and sintering conditions, the sintering behaviour in the range 1000–1300°C with varying alloying additions, for different sintering times and atmospheres. The tensile strengths observed are ～525 N/mm² at a densityof 6·7 g/cm³, with ～3% elongation at fracture. With respect to the low density and the carbon free state of the alloys, the strength values may be considered as rather high. A study of the homogenization process is being carried out.     

Lean Cr iron powders for obtaining high performance PM steel grades

Abstract

Ecological and economic demands are driving PM markets to incorporate less expensive, yet effective, alloying elements in iron based powders. To investigate their potential for this purpose recently developed sinter-hardening iron powders containing Cr and smaller amounts of Mo and Ni were industrially sintered at 1120 and at 1240°C under laboratory conditions. One set of samples, containing 0·8% graphite additions, was cooled so to obtain sinter-hardened steels. A second set of samples, containing 0·3% graphite, was cooled under normal conditions and subjected to a secondary carburising treatment. The microstructures and mechanical properties developed were compared.