The microstructure of discontinuously precipitated lamellae in an austenitic Fe-42.4 Wt Pct Ni- 4.15 Wt Pct AI-0.45 Wt Pct C alloy

Phase decomposition of austenitic Fe-42.4 wt pct Ni-4.15 wt pct Al-0.45 wt pct C on aging at 823 K was investigated by means of electron microscopy, selected area diffraction and microdiffraction, and microprobe chemical analysis. During annealing, κ phase of L′l₂ structure fully coherent with the matrix formedvia the nucleation and growth mechanism. The matrix phase and cubic κ carbide were later gradually encroached upon by discontinuously precipitated lamellar phases. The duplex fine lamellae are composed of alternately arranged carbon-depleted Ll₂ phase and cementite. Between the two constituents in the lamellae, the Pitsch orientation relationship is fulfilled, and at the same time, the matrix phase of the grain which the discontinuously precipitated lamellar colony has left behind maintains the crystallographic cube-to-cube correspondence with the product Ll₂ phase.     

The effect of titanium on creep strength in 2.25 Pct Cr-1 Pct Mo steels

The effect of a low level of titanium on the microstructure and creep properties of 2.25 pct Cr-1 pct Mo steels has been examined as a function of carbon content and austenitizing temperature. The addition of 0.04 wt pct titanium resulted in a dramatic increase in creep strength at 565 °C, and this was found to be associated with the presence in the microstructure of very small (50 to 100 Å) titanium-bearing precipitates based upon both TiC and Mo₂C. The variation of the minimum creep rate with carbon content and austenitizing treatment was explained in terms of the solubility of TiC in austenite. The titanium-bearing carbides have an important effect on microstructural stability and on the maintenance of creep strength, but it is also apparent that solid solution strengthening by molybdenum can make a significant contribution to creep strength at low carbon levels (0.02 wt pct).     

Ferrite and carbide morphologies in and below the “bay region” of an Fe-0.19 Pct C-2.30 Pct Mo alloy

Optical and transmission electron microscopy were used to study ferrite and carbide morphology from temperatures just above to well below the bay temperature in an Fe-0.19 pct C-2.30 pct Mo alloy. Suppression of idmanstätten ferrite morphologies at the bay and their reappearance below, initially in highly degenerate form, previously reported by Boswellet al. in Fe-C-2 pct Mo alloys, were confirmed. These effects were again explained in terms of a solute drag-like effect. Just above the bay, the usual fibrous and interphase boundary Mo₂C structures appear. However, fibrous carbides disappear immediately below the bay whereas interphase boundary carbides continue to form until approximately 60 K below the bay. In a narrow temperature range below the bay, large, widely spaced Mo₂C laths are also observed. Many laths are associated with terraces and risers of ferrite superledges; some, however, nucleate on dislocations within ferrite. Nucleation of Mo₂C on such dislocations occurs throughout the temperature range investigated (898 to 748 K). At the lower temperatures studied, (Fe, Mo)₃C appears as plates approximately 10³ longer than those of Mo₂C also formed at these temperatures. Explanations for these observations on carbides are offered in terms of interactions between ferrite and carbide precipitation and the concepts of ledgewise diffusional growth Orientation relationships between ferrite and carbides in several types of microstructure are reported, but the equivocal nature of the information which these data supply about transformation mechanism is emphasized.     

Role of solid-state skeletal sintering during processing of Mo-Cu composites

Mo-Cu composites with Mo contents up to 85 wt pct can be processed by either infiltration of a presintered Mo skeleton with liquid Cu or by liquid-phase sintering of mixed Mo and Cu powders. For both cases, the effects of particle size, sintering temperature, and sintering time on densification and microstructural evolution are compared. The effects of transition metal additions on the densification of Mo-Cu are also investigated. The liquid-phase sintering densification rate of Mo-Cu is much slower than in traditional liquid-phase sintering and is similar to the solid-state densification rate of elemental Mo. Furthermore, the poor densification behavior and absence of slumping for compositions up to 50 vol pct Cu indicate that the high dihedral angle of the Mo-Cu system stabilizes the formation of a rigid Mo skeleton during liquid-phase sintering. Results from a computer simulation that takes into account mass transport via both solid-state and liquid-phase mechanisms show that the solubility of Mo in Cu is sufficient for rapid densification, but confirm that the sintering behavior of Mo-20 vol pct Cu is best described by solid-state skeletal sintering. In this case, the liquid phase promotes microstructural coarsening by solution reprecipitation but contributes little to densification because of the rigid Mo skeleton.     

Intermetallic phases formed during DC-casting of an Al−0.25 Wt Pct Fe−0.13 Wt Pct Si alloy

The Al−Fe and Al−Fe−Si particles formed during DC-casting of an Al-0.25 wt pct Fe-0.13 wt pct Si alloy have been examined. The particles were analyzed by transmission electron microscopy (TEM) and energy dispersive spectroscopy of X-rays (EDS). Crystal faults were studied by high resolution electron microscopy (HREM). Samples for electron microscopy were taken at various positions in the ingot,i.e., with different local cooling rates during solidification. At a cooling rate of 6 to 8 K/s the dominating phases were bcc α-AlFeSi and bct Al _m Fe. The space group of bcc α-AlFeSi was verified to be Im3. Superstructure reflections from Al _m Fe were caused by faults on {110}-planes. At a cooling rate of 1 K/s the dominating phases were monoclinic Al₃Fe and the incommensurate structure Al _x Fe. In Al₃Fe, stacking faults on {001} were frequently observed. The structure of Al _x Fe is probably related to Al₆Fe. Some amounts of other phases were detected. For EDS-analysis, extracted particles mounted on holey carbon films were examined. Extracted particles were obtained by dissolving aluminum samples in butanol. Accurate compositions of various Al−Fe−Si phases were determined by EDS-analysis of extracted crystals.     

3D Quantitative Characterization of Rapidly Solidified Al-36 Wt Pct Ni

The rapid solidification of a peritectic alloy is studied. Various 2D and 3D characterization techniques were effectively utilized to investigate the effect of cooling rate on both the phase fractions and the shrinkage porosity. Particles of Al-36 wt pct Ni were produced using a drop tube impulse system. Neutron diffraction and Rietveld analysis were used to quantify the phases formed during solidification. The microstructure of the produced particles was analyzed using SEM and X-ray microtomography. It was found that increasing cooling rate resulted in decreasing the Al₃Ni₂ to Al₃Ni ratio. Also, quantitative analysis of the microtomography images revealed that the volume percent of porosity increased with increasing particle size. The distribution of porosity was found to be significantly different in small and large particles. It was concluded that the extensive growth of Al₃Ni₂ at lower cooling rates followed by the peritectic reaction made the feeding of the shrinkages more difficult, and as a result, the volume percent of porosity increased. Other findings showed that high cooling rate during solidification would result in the formation of a quasicrystalline phase, known as D-phase, and suppression of the primary Al₃Ni₂. Also, investigation of the 3D structure of the solidified particles revealed that large particles of Al-36 wt pct Ni contain multiple nucleation sites, while smaller particles contain only one single nucleation site.     

Effect of LiF as Sintering Agent on the Densification and Phase Formation in Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-4 Wt Pct Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> Ceramic Compound

Different amounts of LiF were added to an Al₂O₃-4 pct Nb₂O₅ basic ceramic, as sintering agent. Improved new ceramics were obtained with LiF concentrations varying from 0.25 to 1.50 wt pct and three sintering temperatures of 1573 K, 1623 K, and 1673 K (1300 °C, 1350 °C, and 1400 °C). The addition of 0.5 wt pct LiF yielded the highest densification, 94 pct of the theoretical density, in association with a sintering temperature of 1673 K (1400 °C). Based on X-ray diffraction (XRD), this improvement was due not only to the presence of transformed phases, more precisely Nb₃O₇F, but also to the absence of LiAl₅O₈. The preferential interaction of LiF with Nb₂O₅, instead of Al₂O₃, contributed to increase the alumina sintering ability by liquid phase formation. Scanning electron microscopy (SEM) results revealed well-connected grains and isolated pores, whereas the chemical composition analysis by energy dispersive energy (EDX) indicated a preferential interaction of fluorine with niobium, in agreement with the results of XRD. It was also observed from thermal analysis that the polyethylene glycol binder burnout temperature increased for all LiF concentrations. This may be related to the formation of hydrogen bridge bonds.     

Flow of interdendritic liquid and permeability in pb-20 Wt Pct Sn alloys

Directionally solidified Pb-20 wt pct Sn alloys of uniform microstructures were produced with various primary and secondary dendrite arm spacings. Permeabilities of these alloys were investigated with approximately 0.19 and 0.29 volume fraction liquid, for flow parallel to the direction of primary dendrite arms. The permeabilities of the samples with approximately 0.19 volume fraction liquid were also obtained for flow normal to the primary dendrite arms. It was found that for flow parallel to the primary dendrite arms, permeability varied with d ₁ ² and g _L ² (d₁ is the primary arm spacing and g_L is the volume fraction of liquid). There appears to be no relation between permeability for this parallel flow and the secondary dendrite arm spacing. For flow perpendicular to the primary dendrite arms, permeability is approximately 0.06 to 0.20 that for parallel flow, and in this case the permeability appears to be strongly dependent upon the secondary dendrite arm spacing.     

An examination of the interparticle contact area during sintering of W-0.3 Wt Pct Co

As a powder compact sinters, its microstructure evolves. One way to quantify the scale of the microstructure is to consider the interparticle contact area. This study examines two known models for calculating the interparticle contact area: the classic two-sphere model and the Voronoi cell model. Both models have particular assumptions about the microstructure that make them not applicable for treating densification to near full density with concurrent grain growth. The classic two-sphere model assumes a regular packing of particles and a perfectly spherical particle geometry and neglects an increasing particle coordination number with sintering. The Voronoi cell model assumes that the scale of the microstructure remains constant; i.e., as long as the compact is densifying, grain growth does not occur. We propose a modified Voronoi cell that accounts for an increasing grain size, making it applicable to a general case where grain growth occurs during sintering. The three models are compared to the interparticle contact area data, obtained by stereology techniques, for W-0.3 wt pct Co sintered from green state to near full density. The original Voronoi cell model fits the data only at low temperatures, before the onset of grain growth. Below approximately 90 pct relative density, the two-sphere model with an assumed coordination number of six (coordination number in a green compact) and the modified Voronoi cell model provide a good fit to the data. At higher densities, both models overestimate the interparticle contact area.     

Microstructure and yield strength of UDIMET 720LI alloyed with Co-16.9 Wt Pct Ti

We proposed a new method for developing Ni-base turbine disc alloy for application at temperatures above 700 °C by mixing a Ni-base superalloy U720LI with a two-phase alloy Co-16.9 wt pct Ti in various contents. The microstructure and phase stability of the alloys were analyzed using an optical microscope, a scanning electron microscope, energy-dispersive spectroscopy, and an X-ray diffractometer. The yield strength was studied by compression tests at temperatures ranging from 25 °C to 1200 °C. The results show that all the alloys had a dendritic structure. Ni₃Ti (η) phase was formed in the interdendritic region in the alloys with the addition of Co-16.9 wt pct Ti, and its volume fraction increased with the increase in the addition of Co-16.9 wt pct Ti. The results of exposure at 750 °C show that the addition of Co-16.9 wt pct Ti to U720LI had a great effect on suppressing the formation of σ phase due to the reduced Cr content in the γ matrix. Compared to U720LI, the alloys with the addition of Co-16.9 wt pct Ti possessed higher yield strength. The solid-solution strengthening of γ and γ′ and higher volume fraction of γ′ were assumed to cause this strength increase.     

Effect of dendritic arm spacing on mechanical properties and corrosion resistance of Al 9 Wt Pct Si and Zn 27 Wt Pct Al alloys

It has been reported that the mechanical properties and the corrosion resistance (CR) of metallic alloys depend strongly on the solidification microstructural arrangement. The correlation of corrosion behavior and mechanical properties with microstructure parameters can be very useful for planning solidification conditions in order to achieve a desired level of final properties. The aim of the present work is to investigate the influence of heat-transfer solidification variables on the microstructural array of both Al 9 wt pct Si and Zn 27 wt pct Al alloy castings and to develop correlations between the as-cast dendritic microstructure, CR, and tensile mechanical properties. Experimental results include transient metal/mold heat-transfer coefficient (h _i), secondary dendrite arm spacing (λ₂), corrosion potential (E _Corr), corrosion rate (i _Corr), polarization resistance (R ₁), capacitances values (Z _CPE), ultimate tensile strength (UTS, σ _u ), yield strength (YS, σ _y ), and elongation. It is shown that σ _U decreases with increasing λ₂ while the CR increases with increasing λ₂, for both alloys experimentally examined. A combined plot of CR and σ _U as a function of λ₂ is proposed as a way to determine an optimum range of secondary dendrite arm spacing that provides good balance between both properties.     

Formation of the ωphase during the aging of βTi (7.1 Wt Pct Fe)—a mössbauer study

The formation of the ω -phase during the decomposition of a retained βTi (7.1 wt pct Fe) alloy was studied by means of Mössbauer spectroscopy and X-ray diffraction. The linear dependence of (I.S.)_ω on the Fe content of the alloy upon quenching was used to deriveC _ω-the Fe concentration of the ω-phase in alloys aged at 350°C.C _ωwas found to decrease with increasing aging time, from 5.91 wt pct Fe after 45 mins, to 4.5 wt pct Fe after 180 h of aging. The kinetic of the ω-phase formation is discussed qualitatively. The dependence of the (I.S.)_β and (Q.S.)_β on the iron concentration-C_β was also determined.     

Supercooling effects in Cu-10 Wt Pct Co alloys solidified at different cooling rates

Electromagnetic levitation and electron beam surface melting were employed to study the effects of supercooling and cooling rate on the solidification of Cu-10 wt pct Co alloys. Two major effects were observed in the supercooled alloys: the nucleation of a metastable copper-rich phase which contains 13 wt pct to 20 wt pct Co in samples supercooled between 105 and 150 K and liquid phase separation which occurs in samples supercooled 150 K or more. The microstructure of the electron beam melted surfaces consisted of very fine spheres which were similar to those of the sample supercooled more than 150 K but with a refined microstructure. The results indicate that a dynamic bulk supercooling of 150 K may exist in the molten pool during the solidification of electron beam melted surfaces.     

High strain rate superplasticity of a 25 Wt Pct Cr-7 Wt Pct Ni-3 Wt Pct Mo-0.14 Wt Pct N duplex stainless steel

Effects of prior thermomechanical treatments on the superplasticity of a 25 wt pct Cr-7 wt pct Ni-3 wt pct Mo-0.14 wt pct N δ/γ duplex stainless steel have been studied by means of hot tensile testing with constant crosshead speeds. The objective is to increase the strain rate suitable for superplasticity. The strain rate is found to be markedly increased by a special prior treatment,i.e., solution treatment at temperatures in the δ single-phase region with subsequent heavy cold-rolling. In hot tensile tests at 1273 K, elongations greater than 1000 and 300 pct were observed at initial strain rates (έ) of 10⁻³ to 10⁻¹ s⁻¹ and 1 x 10⁰ s⁻¹, respectively. The results for strain rates 〈10⁻¹ s⁻¹ can be explained in terms of a structural superplastic effect due to grain refinement. In the case of έ 〉 10⁻¹ s⁻¹, transformation superplastic effects due to γ-phase precipitation from the σ-ferrite matrix are also important, especially in the early stages of deformation. In the equiaxedδ/γ microduplex structures during stable superplastic deformation, there exists a mixture of two different structures,i.e., dislocated and recovered/ recrystallized δ grains with a homogeneous dispersion of dislocation-free γ particles. This result shows that dynamic recrystallization ofδ grains occurs locally and intermittently due to the dispersion of relatively hardγ particles. The apparent average grain growth rate during deformation is small compared to static grain growth, because grain refinement due to dynamic recrystallization reduces the superplasticity-enhanced grain growth.     

Solidification of hypereutectic Al-38 Wt Pct Cu alloy in microgravity and in unit gravity

Solidification in microgravity aboard the space shuttle Endeavour resulted in a dramatic change in the morphology of the primary Al₂Cu phase compared to ground-based solidification in unit gravity. An Al-38 wt pct Cu ingot directionally solidified at a rate of 0.015 mm/s with a temperature gradient of 1.69 K/mm exhibited large, well-formed dendrites of primary Al₂Cu phase. Ingots solidified under similar conditions in unit gravity contained primary Al₂Cu phase with smooth, faceted surfaces. The primary Al₂Cu phase spacing in the microgravity ingot was much greater than that in the unit gravity ingot, 670 μm compared to 171 μm. It is suggested that thermosolutal mixing in the unit gravity ingot reduces the buildup of an Al-rich layer at the solid/liquid interface, which increases the stability of the interface resulting in smooth, faceted particles of Al₂Cu phase. It is also suggested that the large difference in primary phase spacings is due mostly to the difference in morphology rather than changes in parameters that might influence dendrite ripening mechanisms. The presence or absence of gravity had no effect on the interlamellar spacing of the inter-Al₂Cu phase eutectic. The ingot solidified in microgravity exhibited almost no longitudinal macrosegregation, in agreement with the theory of inverse segregation in the absence of thermosolutal convection. The ingot solidified in unit gravity exhibited considerable longitudinal macrosegregation, with the chilled end having about 6 wt pct more Cu than the average composition. It is not clear whether the segregation results from thermosolutal convection during solidification or from sedimentation during melting.     

Microstructural development during liquid phase sintering of Fe and Fe–Mo alloys containing elemental boron additions

Abstract

The sintering behaviour of Fe and Fe–Mo prealloyed powder compacts containing from 0·5 to 3·5 wt-%Mo and fixed boron additions has been studied with special emphasis on the microstructural development, the formation of the liquid phase and the liquid phase sintering mechanisms involved during the densification process. The basic phenomena involving the formation of a liquid phase and the temperature at which the liquid is generated is strongly influenced by the Mo/B ratio in the initial powder mixture. The effect produced by Mo and its concentration, both, on the final microstructure and on the behaviour of boron prior to, during and after the formation of the liquid phase, was studied under both the optical and the scanning electron microscope. For this purpose interrupted sintering experiments followed by water quenching from specific temperatures and times within the sintering cycle have been carried out. The study shows that the formation of a liquid phase is preceded by noticeable enhancement of solid state sintering at intermediate temperatures. This is accompanied by boron diffusion into the metallic particles, generating inter- and intragranular precipitates in amounts dependent on the Mo concentration. At a later stage boron is found to be preferentially located at the boundaries as the formation of a continuous Fe/Mo/B liquid phase with excellent wetting characteristics proceeds thus producing densification by pore filling and shape accommodation. Final densities up to 7·82 g cm^?3 were obtained for these alloys.     

Liquidlike sintering behavior of nanometric Fe and Cu powders: Experimental approach

Nanometric Fe and Cu powders were sintered in vacuum, He, and H₂ atmospheres after uniaxial cold pressing. The shrinkage behavior of samples was studied using three different dilatometric techniques: constant heating rate, isothermal annealing, and the Dorn method. Density greater than 90 pct was obtained at sintering temperatures of 900 °C. In nanometric powders, densification and grain coarsening occurred in a narrow temperature interval. Despite the low oxide content in the starting powders (1.5 to 4 wt pct), the reducing atmosphere plays a relevant role in the sintering process. The self-diffusion activation energies obtained for nanometric Fe were 116 and 60 kJ/mole in vacuum and H₂, and those obtained for nanometric Cu were 70 and 43 kJ/mole in He and H₂. According to the present results, the activation energies obtained from both nanometric powders in H₂ could be associated with those for self-diffusion in liquid Fe (65 kJ/mole) and Cu (41 kJ/mole).     

Densification kinetics of W-Fe-Sn pseudoalloys in liquid phase sintering

The densification kinetics in liquid phase sintering of W-Fe-Sn powder composites containing 90 wt.% of a refractory component and 10 wt.% of a low-melting component is strongly dependent on the iron content in the melt. As the iron content in composites increases, the concentration dependence of their densification shows a maximum. Samples containing more than 1.5 wt.% Fe grow intensively because of the formation of W₆Fe₇ intermetallide whose decomposition temperature is higher than that of liquid phase sintering. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 5–6 (455), pp. 22–29, 2007.     

The Sintering, Sintered Microstructure and Mechanical Properties of Ti-Fe-Si Alloys

A systematic study has been conducted of the sintering, sintered microstructure and tensile properties of a range of lower cost Ti-Fe-Si alloys, including Ti-3Fe-(0-4)Si, Ti-(3-6)Fe-0.5Si, and Ti-(3-6)Fe-1Si (in wt pct throughout). Small additions of Si (??1?pct) noticeably improve the as-sintered tensile properties of Ti-3Fe alloy, including the ductility, with fine titanium silicides (Ti₅Si₃) being dispersed in both the ?? and ?? phases. Conversely, additions of ?>1?pct Si produce coarse and/or networked Ti₅Si₃ silicides along the grain boundaries leading to predominantly intergranular fracture and, hence, poor ductility, although the tensile strength continues to increase because of the reinforcement by Ti₅Si₃. Increasing the Fe content in the Ti-xFe-0.5/1.0Si alloys above 3?pct markedly increases the average grain size and changes the morphology of the ??-phase phase to much thinner and more acicular laths. Consequently, the ductility drops to <1?pct. Si reacts exothermically with Fe to form Fe-Si compounds prior to the complete diffusion of the Fe into the Ti matrix during heating. The heat thus released in conjunction with the continuous external heat input melts the silicides leading to transient liquid formation, which improves the densification during heating. No Ti-TiFe eutectoid was observed in the as-sintered Ti-Fe-Si alloys. The optimum PM Ti-Fe-Si compositions are determined to be Ti-3Fe-(0.5-1.0)Si.     

Microstructure and mechanical properties of Ti-40 Wt Pct Ta (Ti-15 At. Pct Ta)

Of the β-isomorphous Ti-X alloy systems, Ti-Ta is one of the least studied. In the current work, the microstructure and mechanical properties of Ti-40 wt pct Ta (Ti-15 at. pct Ta) are investigated. Annealing at 810 °C produces a two-phase microstructure consisting of Ti-richa idiomorphs in a continuous Ta-rich β matrix; this suggests the β-transus temperature is higher than indicated by the most recently published phase diagram. Water quenching from 810 °C causes the β phase to partially transform to orthorhombic martensite (α), while furnace cooling yields secondarya The primary α formed isothermally remains unchanged in both cases. Subsequent aging causes transformation of the martensite to type 1a plus residual β, with a corresponding increase in strength and decrease in ductility. The maximum ductility (20 pct elongation) occurs in the water-quenched condition in which metastable β is retained. Analysis of the true stresstrue strain data suggests that transformation-induced plasticity may contribute to the enhanced ductility of the water-quenched material.