Aging behavior of Fe-30 Ni alloys containing niobium

A study has been made of the precipitation reactions in Fe-30 wt pct alloys containing up to 5 wt pct Nb. The as-quenched structures of these alloys consist, of austenite, martensite in twinned as well as in massive form, and Ni₃Nb and Fe₂Nb precipitates. On aging at 700° and 800°C the main precipitation reaction results in the formation of hexagonal Laves phase Fe₂Nb, but Ni₃Nb in both bct and orthorhombic structures also precipitates. The precipitation of Fe₂Nb is a heterogeneous process and results in a considerable increase in the hardness of the alloy.     

Design of quaternary Ir-Nb-Ni-Al refractory superalloys

We propose a method for developing new quaternary Ir-Nb-Ni-Al refractory superalloys for ultra-high-temperature uses, by mixing two types of binary alloys, Ir-20 at. pct Nb and Ni-16.8 at. pct Al, which contain fcc/L1₂ two-phase coherent structures. For alloys of various Ir-Nb/Ni-Al compositions, we analyzed the microstructure and measured the compressive strengths. Phase analysis indicated that three-phase equilibria—fcc, Ir₃Nb-L1₂, and Ni₃Al-L1₂—existed in Ir-5Nb-62.4Ni-12.6Al (at. pct) (alloy A), Ir-10Nb-41.6Ni-8.4Al (at. pct) (alloy B), and Ir-15Nb-20.8Ni-4.2Al (at. pct) (alloy C) at 1400 °C; at 1300 °C, three phase equilibria—fcc, Ir₃Nb, and Ni₃Al—existed in alloys A and C and four-phase equilibria—fcc, Ir₃Nb, Ni₃Al, and IrAl-B2—existed in alloy B. The fcc/L1₂ coherent structure was examined by using transmission electron microscopy (TEM). At a temperature of 1200 °C, the compressive strength of these quaternary alloys was between 130 and 350 MPa, which was higher than that of commercial Ni-based superalloys, such as MarM247 (50 MPa), and lower than that of Ir-based binary alloys (500 MPa). Compared to Ir-based alloys, the compressive strain of these quaternary alloys was greatly improved. The potential of the quaternary alloys for ultra-high-temperature use is also discussed.     

Structure of martensites in Fe-Ni-Ti alloys

The morphology, internal structure and crystal structure of martensites in three Fe-Ni-6 pct Ti alloys containing 10, 20, and 25 pct by weight of nickel have been studied. The 10 and 20 pct nickel alloys transform to lath martensite with a dislocated sub-structure, and are not tetragonal. The 25 pct nickel alloy forms martensite plates on transformation, with a substructure of dislocations and twins. This alloy is tetragonal in both twinned and dislocated regions with ac/a ratio of 1.017. Electron diffraction evidence for a tetragonal phase produced by the Bain strain of Ni₃Ti has also been obtained. The observations support the hypothesis that the tatragonality results from the formation of an ordered Ni₃Ti phase in austenite prior to transformation to martensite.     

Phase Structure and High-Temperature Mechanical Properties of Two-Phase Fe-25Al-xZr Alloys Compared to Three-Phase Fe-30Al-xZr Alloys

The structure and high-temperature mechanical properties of Fe-30 at. pct Al and Fe-25 at. pct Al alloys with various Zr contents are compared. The scanning electron microscope images in chemical contrast mode (R-BSE) as well as EDS, EBSD, and X-ray diffraction were used to determine the structure and phase composition. The as-cast alloys (both Fe-30Al and Fe-25Al) were observed to be two-phase DO₃/B2 + Laves phase λ ₁ (Fe,Al)₂Zr alloys with typical fine lamellar eutectic areas. During the heat treatment of the Fe-25Al alloys, their structure transformed from a DO₃/B2 matrix with fine lamellar eutectic into λ ₁ globular particles situated in a DO₃/B2 matrix. The same structure of Fe-30Al alloys decomposed into three phases: λ ₁ and τ ₁ Zr(Fe,Al)₁₂ particles in a DO₃/B2 matrix. The hardening in both groups of alloys (Fe-25Al and Fe-30Al) due to the presence of Zr-containing λ ₁ and τ ₁ phases is compared.     

The eutectoid region of the Zr−Ga system

The zirconium-rich portion of the Zr?Ga phase diagram was determined by the optical examination of microstructures of isothermally annealed and quenched alloys. A deviation from binary equilibrium, was observed even though careful selection of materials and techniques held impurities to a minimum and produced alloys with a purity of at least 99.9 pct. The slopes of the α-β boundaries are depressed and the range of solubility of the solid solution phases is restricted when compared to the phase diagrams of other Group IIIB elements, apparently as a result of the large difference in atomic size between zirconium and gallium. Thea ₀ andc ₀ lattice constants of cph zirconium are contracted and the axial ratio is expanded by the addition of gallium. The change inc/a at 1 at. pct was very close to the change observed in Zr-In alloys, in agreement with general dependence of these properties in zirconium alloys upon electron to atom ratio. A eutectoid reaction occurs at 860°C with β solid solution (1.8 at. pct Ga) decomposing into α solid solution (0.8 at. pct Ga) and Zr₃Ga. Cast microstructures suggest a eutectic reaction in which liquid (21.0 at. pct Ga) decomposes into β (8.0 at. pct Ga) and Zr₅Ga₃. It is proposed that intermediate phases are formed at 25.0 at. pct Ga (Zr₃Ga), 37.5 at. pct Ga (Zr₅Ga₃), and 50.0 at. pct Ga (ZrGa) although the exact composition was not determined.     

Effect of Ti/Al ratio and Cr,Nb, and Hf additions on material factors and mechanical properties in TiAl

The effect of the Ti/Al ratio and Cr, Nb, and Hf additions on material factors, such as the grain size, second phase, la tice parameters and the axial ratio, and on mechanical properties in TiAl-base alloys has been studied. The grain size was decreased by the deviation from the stoichiometric composition o the Ti-rich side and the addition of the third elements. The Cr element was contained a little more in Ti₃Al phase than in TiAl phase in two-phase Ti-rich alloys. The lattice parameters,a andc, and the axial ratio,c/a, of the binary alloys varied linearly with decreasing Al content even in the dual-phase region. The Cr addition decreased thea and c and alsoc/a. The Nb addition increased weakly thea andc andc/a. On the contrary, the Hf addition increased thea andc but decreased thec/a ratio. In the Cr added alloys, the decrease of volume of a unit cell, due to the substitution of Cr atoms for Ti and Al atoms, was larger than that expected from the difference of atom sizes. The Nb addition should decrease the volume of a unit cell, but it increased the volume. The Hf addition caused a larger increase of volume of a unit cell than that expected from the difference of atom sizes. We suggested that the Cr addition increases and the Nb and Hf additions decrease the bond strength in TiAl. The deviation from stoichiometry and the addition of third elements caused an increase of work-hardening rate. The alloys with Ti-rich composition have superior mechanical properties compared to those of alloys vith Al-rich composition. The Cr addition resulted in high solution hardening, and the Ti-47A1 3Cr (in atomic percent) alloys had the highest fracture strain of 2.7 pct in all alloys tested. The Nb addition resulted in poor ductility in both Ti- and Al-rich alloys. The Hf additions to the Ti-rich composition caused better mechanical properties than those of Al-rich alloys. Thi; trend was also similar to the Nb-added alloys. In the Hf-added alloys, the Ti-49Al-2Hf alloy has rather high ductility of about 2.15 pct. The effect of structural parameters on mechanical properties was discussed. The smaller grain size and the smaller axial ratio tended to result in larger ductility. The increase of the bond strength might improve ductility. Formerly with Sumitomo Light Metal Industries, Ltd., Technical Research Laboratories, Nagoya 455, Japan Formerly with NKK Corporation, td., Kawasaki 210, Japan Formerly Professor, Institute for Materials Research, Tohoku University     

The precipitation of Ni3Nb phases in a Ni−Fe−Cr−Nb alloy

The age hardening of a Ni?Fe?Cr?Nb alloy containing 4.85 wt pct Nb has been studied using transmission electron microscopy. The major hardening phase in this alloy isγ*, DO₂₂-ordered Ni₃Nb, which precipitates as a fine dispersion of square plates. It is shown that nucleation ofγ* plates may be dependent upon matrix excess vacancy concentration, but nucleation ofγ* plates is also observed at dislocations and extrinsic stacking faults. Theγ* phase is metastable with respect to the orthorhombic Ni₃Nb phase, β, which precipitates by either a cellular or an intragranular reaction, depending upon the aging temperature. It is proposed that the intragranular nucleation of β laths proceeds by the growth of stacking faults from withinγ* plates; theγ* plates subsequently dissolve in favor of the β laths. Room temperature deformation of theγ* dispersion is shown to produce faults within theγ* plates; possible dislocation reactions occurring during this deformation are discussed.     

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.     

Phase stability and yield stress of Ni-base superalloys containing high Co and Ti

Ni-base superalloys containing high Co (>20 wt pct) and Ti (>5.5 wt pct) were designed in order to study the effects of Co16.9 wt pct Ti addition on phase stability and mechanical property. These new alloys, though they contained high Ti, mainly consisted of γ and γ′ phases. Ni₃Ti (η) phase was observed along the grain boundaries in some of the alloys. The formation of η phase was mainly related to the Ti/Al ratio, Ti content, and alloy composition. Tensile and compression tests showed that these new alloys exhibited higher yield stress than that of the baseline alloy, TMW-1(U720LI). The possible strengthening mechanisms were discussed in terms of solid-solution and precipitation strengthening effects by the Co16.9 wt pct Ti additions. Preliminary results show promising trends for the development of new superalloys for turbine disc applications.     

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.     

Ir-Nb-Si ternary refractory superalloys with a three-phase Fcc/L12/silicide structure for high-temperature applications: Phase and microstructural evolution

To find a new phase with the potential to improve the high-temperature strength of Ir-based superalloys, the novel idea of introducing silicides into the Ir-Nb binary was implemented. Hypoeutectic Ir-10Nb, eutectic Ir-16Nb, and hypereutectic Ir-25Nb alloys were used as bases, and 5 mol pct Si was added through the removal of Ir. XRD (XRD), scanning electron microscopy (SEM), and electron-probe microanalysis (EPMA) revealed the formation of a three-phase fcc/L1₂/silicide microstructure in the Ir-Nb-Si ternary after Si addition. The type of silicide formed was dependent on heat-treated temperatures and Nb content. After heat treatment at 1750 °C and 1600 °C, a tie-triangle composed of fcc/L1₂/silicide (Ir₂Si) appeared in the Ir-10Nb-5Si and Ir-16Nb-5Si alloys; in the Ir-25Nb-5Si alloy, an L1₂ and silicide (Ir,Nb)₂Si tie-line was observed. In the as-cast and 1300 °C heat-treated samples, the Ir-10Nb-5Si microstructure changed to a two-phase fcc/silicide structure, while the Ir-16Nb-5Si alloy maintained a three-phase fcc/L1₂/silicide structure. The Ir-25Nb-5Si alloy, however, had the same phases as that at 1600 °C. Silicides typically continuously or discontinuously distribute along the interdendritic regions or grain boundaries of the fcc or the L1₂ phase. With the addition of Si, it was found that both the eutectic point and solid solubility of Nb in Ir would shift toward Ir.     

The effect of metallic elements on the crystallization behavior of amorphous Fe-Si-B alloys

The crystallization behavior of amorphous Fe_84-X Si₆B₁₀M_X (M=Nb, Zr, V, or Cu) alloys was examined using differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) with the aim of clarifying the effect of additional M elements. The compositional dependence of the first crystallization temperatureT _x1 increased in the order of Zr > Nb > V; however, the addition of 1 at. pct Cu caused a decrease inT _x1. Such an effect of the M elements on the thermal stability of an amorphous phase was interpreted in terms of the difference in the atomic size. These alloys were composed of a mixed structure ofα-Fe and amorphous phases after aging for 3.6 ks in the first exothermic temperature range. The addition of more than 3 at. pct Nb or Zr significantly affected the morphology and grain size of theα-Fe phase. However, their particles possessed dendritic morphology with a grain size of 0.1 to 0.3 μm, when the Nb or Zr content was less than 2 at. pct. Further addition of these elements brought about the formation of sphericalα-Fe particles. The average grain size, for instance, was as small as 20 nm in the aged alloy containing 6 at. pct Nb, which shows that a remarkable grain refinement occurs with increasing Nb content.     

Mechanical properties of Ir-Nb alloys containing Ni and Al

Two alloys made by adding 5 or 10 at. pct, respectively, of Ni-18.9 at. pct Al to an Ir-15 at. pct Nb alloy were investigated. The microstructure and compressive strength at temperatures between room temperature and 1800 °C were investigated to evaluate the potential of these alloys for ultra-high-temperature use. Their microstructural evolution indicated that the two alloys formed fcc and L1₂-Ir₃Nb two-phase structures. The fcc and L1₂ two-phase structures were examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The 0.2 pct flow stresses were above 1000 MPa at temperatures up to 1200 °C, about 150 MPa at 1500 °C, and over 100 MPa at 1800 °C. The strength of the quaternary Ir-base alloys at 1200 °C was even higher than that of Ir-base binary and ternary alloys. And the strength of quaternary Ir-Nb-Ni-Al was equivalent to that of the Ir-15 at. pct Nb binary alloy at 1800 °C. The compressive ductility of quaternary (around 20 pct) was improved drastically compared with that of the Ir-base binary alloy (lower than 10 pct) and the ternary Ir-base alloys (about 11 pct). An excellent balance of high-temperature strength and ductility was obtained in the alloy with 10 at. pct Ni-18.9 at. pct Al. The effect of Ni and Al on the strength of the Ir-Nb binary alloy is discussed.     

Solidification of Aluminum-Copper B206 Alloys with Iron and Silicon Additions

Solidification of B206 aluminum alloys with additions of iron and silicon was studied to investigate their combined effect on the formation and precipitation of intermetallics, particularly Fe-rich phases. Iron is precipitated mainly by either β(CuFe) or α(MnFe) phases, or both depending of the iron and silicon content, as well as the cooling rate. It was found that in alloys having up to 0.3 wt pct Fe, the precipitation of β(CuFe) phase can be largely suppressed if the ratio Si/Fe is close to 1 and the cooling rate is moderately high. The low mobility of the large facets of the β(CuFe) platelets is likely the cause limiting the amount of this phase, especially when the iron atoms have the possibility to be captured by another phase, in this case, the α(MnFe) phase.     

Deformation behavior of a Ni3Al(B,Zr) alloy during cold rolling: Part I. changes in order and structure

A hypostoichiometric Ni₃Al(B,Zr) alloy was homogenized and cold rolled by amounts ranging from 25 to 73 pct. The alloy consisted of two phases—a partially ordered γ′ phase (L1₂) and a Ni-rich fcc solid solution (γ). On deforming the alloy by rolling at room temperature, the order parameter showed a gradual change. In fact, between 35 and 45 pct deformation, the order characteristic of the L1₂ structure changed into that of a DO₂₂ structure. The possibility of transition from L1₂ to DO₂₂ structure is also corroborated from strain parameter, microhardness, and detailed x-ray diffraction (XRD) measurements. This structural transformation is accompanied by a change in the deformation mode (from slip to twinning), as is evident from the relevant microstructures.     

Electron diffraction study of a noncrystalline Zr−Ni phase

Metastable noncrystalline phases were retained by rapid quenching Zr?Ni alloys from the liquid state over the composition range 25 to 70 at. pct Ni. The X-ray and electron diffraction patterns of the metastable phases showed broad diffraction maxima indicating absence of long range crystalline order in them. The pair function of the metastable noncrystalline phase in a splat cooled Zr_0.7Ni_0.3 alloy was determined from the electron diffraction data. The various interatomic distances corresponding to the positions of the peaks in the pair function curve were determined. The nearest-neighbors coordination number was computed from the area under the first peak. The radius of the first near neighbor shell is 2.95 Å and the ratio of the second to the first interatomic distance is equal to 1.41 which is significantly lower than that observed in most metallic liquids. The local atomic arrangement in the non-crystalline Zr_0.7Ni_0.3 alloy was shown to be based on simple fcc coordination.     

Microstructure of Precipitation Hardenable Powder Metallurgical Ni Alloys Containing 35 to 45 pct Cr and 3.5 to 6 pct Nb

Ni-based alloys with high Cr contents are not only known for their excellent high temperature and hot corrosion resistance, but are also known for poor mechanical properties and difficult workability. Powder metallurgical (PM) manufacturing of alloys may overcome several of the shortcomings encountered in materials manufacturing involving solidification. In the present work, six PM Ni-based alloys containing 35 to 45 wt pct Cr and 3.5 to 6 wt pct Nb were produced and compacted via hot isostatic pressing. Samples were heat treated for up to 1656 hours at either 923 K or 973 K (650 °C or 700 °C), and the microstructures and mechanical properties were quantified and compared to thermodynamic calculations. For the majority of the investigated alloys, the high Cr and Nb contents caused development of primary populations of globular α-Cr and δ (Ni₃Nb). Transmission electron microscopy of selected alloys confirmed the additional presence of metastable γ″ (Ni₃Nb). A co-dependent growth morphology was found, where the preferred growth direction of γ″, the {001} planes of γ-Ni, caused precipitates of both α-Cr and δ to appear in the form of mutually perpendicular oriented disks or plates. Solution heat treatment at 1373 K (1100 °C) followed by aging at 973 K (700 °C) produced a significant strength increase for all alloys, and an aged yield strength of 990 MPa combined with an elongation of 21 pct is documented for Ni 40 wt pct Cr 3.5 wt pct Nb.     

Coherency strengthening in Ni base alloys hardened by DO22 γ′ precipitates

The strengthening phase in nickel-base Alloy 718 is a metastable DO₂₂ precipitate termed γ′ which is based upon Ni₃Nb. Since the coherent γ′ particles produce a tetragonal distortion of the matrix, the specific variants of γ′ present can be controlled by the application of stress during aging. The resultant variation in strength is appreciable. A detailed comparison with likely strengthening mechanisms indicates that hardening is primarily due to the coherency strains arising from the γ′ precipitate. Implications with regard to design of alloys strengthened by the DO₂₂ phase are considered.     

The growth and morphology of directionally solidified nickel base γ/γ′- σ superalloys

A series of Ni-Nb-Al-Cr(γ/γ′- σ) alloys in the composition ranges Nb 19.3 to 23.2 wt pct, Al 2.5 to 5.2 wt pct and Cr 0 to 7.05 wt pct have been directionally solidified under high thermal gradient (G) at both steady state and under conditions of abruptly or gradually changing growth rate(ft). The critical ratio of G andR, (g/r)*, to achieve two-phase plane frontin- situ composite growth increases as chromium and niobium (Cb) concentration deviates from the trough or surface of two-fold saturation. Interlamellar spacing of composites tend to decrease with increasing chromium content. Structures produced at steady state growth in whichG/R < (G/R)* are consistent with previous work and can be related to the location of the alloy composition with respect to the line of two-fold saturation. For alloys, which at lowG/R exhibited σ dendrites, any perturbation in growth velocity (atG/R > (G/R)*) precipitated a single phase σ (Ni₃Nb) band. For alloys which at lowG/R exhibited γ dendrites a similar effect was achieved only when growth rate was reduced abruptly by more than an order of magnitude. Interlamellar spacing of two alloys (approximately Ni-20 wt pct Nb-2.5 wt pct Al-6 wt pct Cr) was studied and for abrupt reductions in growth rate in which bands were not produced, it was observed to decay slowly to the new steady state value over distances which are inconsistent with the assumption of simple niobium diffusion control. A gradual increase in growth velocity for one of these alloys resulted in extremely slow adjustment of interlamellar spacing occurring over a period greater than one hour. An abrupt increase in growth velocity for all alloys caused immediate adjustment of interlamellar spacing to the new steady state value.