Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 1999"

A New Hot-Tearing Criterion

Abstract: A new criterion for the appearance of hot tears in metallic alloys is proposed. Based upon a mass balance performed over the liquid and solid phases, it accounts for the tensile deformation of the solid skeleton perpendicular to the growing dendrites and for the induced interdendritic liquid feeding. This model introduces a critical deformation rate ( $$\dot \varepsilon _{p,\max } $$ ) beyond which cavitation, i.e., nucleation of a first void, occurs. As should be expected, this critical value is an increasing function of the thermal gradient and permeability and a decreasing function of the viscosity. The shrinkage contribution, which is also included in the model, is shown to be of the same order of magnitude as that associated with the tensile deformation of the solid skeleton. A hot-cracking sensitivity (HCS) index is then defined as $$\dot \varepsilon _{_{p,\max } }^{ - 1} $$ . When applied to a variable-concentration aluminum-copper alloy, this HCS criterion can reproduce the typical “Λ curves” previously deduced by Clyne and Davies on a phenomenological basis. The calculated values are in fairly good agreement with those obtained experimentally by Spittle and Cushway for a non-grain-refined alloy. A comparison of this criterion to hot cracks observed in ring-mold solidification tests indicates cavitation depression of a few kilo Pascal and tensile stresses in the coherent mushy zone of a few mega Pascal. These values are discussed in terms of those obtained by other means (coherency measurement, microporosity observation, and simulation). Even though this HCS criterion is based only upon the appearance of a first void and not on its propagation, it sets up for the first time a physically sound basis for the study of hot-crack formation.

Microstructural evolution of 6063 aluminum during friction-stir welding

Abstract: The microstructural distribution associated with a hardness profile in a friction-stir-welded, age-hardenable 6063 aluminum alloy has been characterized by transmission electron microscopy (TEM) and orientation imaging microscopy (OIM). The friction-stir process produces a softened region in the 6063 Al weld. Frictional heating and plastic flow during friction-stir welding create fine recrystallized grains in the weld zone and recovered grains in the thermomechanically affected zone. The hardness profile depends greatly on the precipitate distribution and only slightly on the grain size. The softened region is characterized by dissolution and growth of the precipitates during the welding. Simulated weld thermal cycles with different peak temperatures have shown that the precipitates are dissolved at temperatures higher than 675 K and that the density of the strengthening precipitate was reduced by thermal cycles lower than 675 K. A comparison between the thermal cycles and isothermal aging has suggested precipitation sequences in the softened region during friction-stir welding.

Grain refinement of aluminum alloys: Part I. the nucleant and solute paradigms—a review of the literature

Abstract: Despite the extensive literature on grain refinement, there is not a consensus on the mechanism of grain refinement in aluminum alloys. Recently, there has been a shift in understanding of the grain-refinement paradigm from purely being concerned with the nucleation event, called here the “nucleant paradigm,” to also being concerned with the effect of solute elements, or, the “solute paradigm,” on the final grain structure. This article is divided into two parts. In Part I, the literature underpinning both paradigms is explained, and the validity of the paradigm shift toward the solute paradigm as a more complete understanding of grain refinement is presented. Part II experimentally confirms the validity of the solute paradigm and details a mechanism which explains the need for both effective nucleants and a solute of a good segregating power in order to obtain grain refinement.

Indentation power-law creep of high-purity indium

Abstract: Using a variety of depth-sensing indentation techniques, the creep response of high-purity indium, from room temperature to 75 °C, was measured. The dependence of the hardness on the variables of indentation strain rate (stress exponent for creep (n)) and temperature (apparent activation energy for creep (Q)) and the existence of a steady-state behavior in an indentation test with a Berkovich indenter were investigated. It was shown for the first time that the indentation strain rate (-este-/h) could be held constant during an experiment using a Berkovich indenter, by maintaining the loading rate divided by the load (-este-/P) constant. The apparent activation energy for indentation creep was found to be 78 kJ/mol, in accord with the activation energy for self-diffusion in the material. Finally, by performing -este-/P change experiments, it was shown that a steady-state path independent of hardness could be reached in an indentation test with a geometrically similar indenter.

Influence of grain size and stacking-fault energy on deformation twinning in fcc metals

Abstract: This article investigates the microstructural variables influencing the stress required to produce deformation twins in polycrystalline fcc metals. Classical studies on fcc single crystals have concluded that the deformation-twinning stress has a parabolic dependence on the stacking-fault energy (SFE) of the metal. In this article, new data are presented, indicating that the SFE has only an indirect effect on the twinning stress. The results show that the dislocation density and the homogeneous slip length are the most relevant microstructural variables that directly influence the twinning stress in the polycrystal. A new criterion for the initiation of deformation twinning in polycrystalline fcc metals at low homologous temperatures has been proposed as (σ tw −σ 0)/G=C(d/b)A, where σ tw is the deformation twinning stress, σ 0 is the initial yield strength, G is the shear modulus, d is the average homogeneous slip length, b is the magnitude of the Burger’s vector, and C and A are constants determined to have values of 0.0004 and −0.89, respectively. The role of the SFE was observed to be critical in building the necessary dislocation density while maintaining relatively large homogeneous slip lengths.

Influence of martensite content and morphology on tensile and impact properties of high-martensite dual-phase steels

Abstract: A series of dual-phase (DP) steels containing finely dispersed martensite with different volume fractions of martensite (Vm) were produced by intermediate quenching of a boron- and vanadium-containing microalloyed steel The volume fraction of martensite was varied from 03 to 08 by changing the intercritical annealing temperature The tensile and impact properties of these steels were studied and compared to those of step-quenched steels, which showed banded microstructures The experimental results show that DP steels with finely dispersed microstructures have excellent mechanical properties, including high impact toughness values, with an optimum in properties obtained at ∼055 Vm A further increase in Vm was found to decrease the yield and tensile strengths as well as the impact properties It was shown that models developed on the basis of a rule of mixtures are inadequate in capturing the tensile properties of DP steels with Vm>055 Jaoul-Crussard analyses of the work-hardening behavior of the high-martensite volume fraction DP steels show three distinct stages of plastic deformation

Grain refinement of aluminum alloys: Part II. Confirmation of, and a mechanism for, the solute paradigm

Abstract: In Part I of this article, the literature underpinning both the nucleant and solute paradigms was explained, and the validity of the paradigm shift toward the solute paradigm, as a more complete understanding of grain refinement, was presented. In this Part II, experimental work is presented which confirms the validity of the solute paradigm. TiB2 particle additions were found to refine the columnar zone of pure aluminum; however, an equiaxed structure was only observed when a small amount of titanium was added as solute. The potency of nucleant particles was confirmed by thermal analysis, which showed that additions of TiB2 to pure aluminium removed the nucleation undercooling. Upon the addition of more TiB2 particles and titanium as solute, the grain size continued to decrease until an apparent minimum grain size was achieved, past which little further refinement occurs. That the segregating ability of solute elements in general is essential for grain refinement, and not only that of titanium in particular, was confirmed by comparison of the Al-2Si and Al-0.05Ti systems. Finally, a mechanism of grain refinement is presented that incorporates both nucleant particles and solute segregation as essential for effective grain refinement. The solute is required to form a constitutionally undercooled zone in front of the growing solid/liquid interface to facilitate further nucleation on the substrates present. The potency of the nucleants dictates the probability of nucleation occurring for a given degree of constitutional undercooling.

A three-dimensional cellular automation-finite element model for the prediction of solidification grain structures

Abstract: A three-dimensional (3-D) model for the prediction of dendritic grain structures formed during solidification is presented. This model is built on the basis of a 3-D cellular automaton (CA) algorithm. The simulation domain is subdivided into a regular lattice of cubic cells. Using physically based rules for the simulation of nucleation and growth phenomena, a state index associated with each cell is switched from zero (liquid state) to a positive value (mushy and solid state) as solidification proceeds. Because these physical phenomena are related to the temperature field, the cell grid is superimposed to a coarser finite element (FE) mesh used for the solution of the heat flow equation. Two coupling modes between the microscopic CA and macroscopic FE calculations have been designed. In a so-called “weak” coupling mode, the temperature of each cell is simply interpolated from the temperature of the FE nodes using a unique solidification path at the macroscopic scale. In a “full” coupling mode, the enthalpy field is also interpolated from the FE nodes to the CA cells and a fraction of solid increment is computed for each mushy cell using a truncated Scheil microsegregation model. These fractions of solid increments are then fed back to the FE nodes in order to update the new temperature field, thus accounting for a more realistic release of the latent heat (i.e., the solidification path is no longer unique). Special dynamic allocation techniques have been designed in order to minimize the computation costs and memory size associated with a very large number of cells (typically 107 to 108). The potentiality of the CAFE model is demonstrated through the predictions of typical grain structures formed during the investment casting and continuous casting processes.

Dislocations, kink bands, and room-temperature plasticity of Ti 3 SiC 2

Abstract: Transmission electron microscopy (TEM) of aligned, macrograined samples of Ti3SiC2, deformed at room temperature, shows that the deformed microstructure is characterized by a high density of perfect basal-plane dislocations with a Burgers vector of 1/3〈112 0〉. The dislocations are overwhelmingly arranged either in arrays, wherein the dislocations exist on identical slip planes, or in dislocations walls, wherein the same dislocations form a low-angle grain boundary normal to the basal planes. The arrays propagate across entire grains and are responsible for deformation by shear. The walls form as a result of the formation of kink bands. A dislocation-based model, that builds on earlier ideas proposed for kink-band formation in hexagonal metallic single crystals, is presented, which explains most of the microstructural features. The basic elements of the model are shear deformation by dislocation arrays, cavitation, creation of dislocation walls and kink boundaries, buckling, and delamination. The delaminations are not random, but successively bisect the delaminating sections. The delaminations and associated damage are contained by the kink boundaries. This containment of damage is believed to play a major role in endowing Ti3SiC2 and, by extension, related ternary carbides and nitrides with their damage-tolerant properties.

Room-temperature ductile carbides

Abstract: Large-grained, oriented, polycrystalline samples of Ti3SiC2 loaded in compression at room temperature deform plastically. When the basal planes are oriented in such a way that allows for slip, deformation occurs by the formation of shear bands. The minimum critical resolved shear stress at room temperature is ≈36 MPa. When the slip planes are parallel to the applied load—a situation where ordinary glide is impossible—deformation occurs by a combination of delamination of, and kink band formation in, individual grains, as well as by shear band formation. It is this unique multiplicity of deformation modes that allows the material to deform plastically in any arbitrary orientation.

Part I. The microstructural evolution in Ti-Al-Nb O+Bcc orthorhombic alloys

Abstract: Phase transformations and the resulting microstructural evolution of near-Ti2AlNb and Ti-12Al-38Nb O+bcc orthorhombic alloys were investigated. For the near-Ti2AlNb alloys, the processing temperatures were below the bcc transus, while, for Ti-12Al-38Nb, the processing temperature was supertransus. Phase evolution studies showed that these alloys contain several constituent phases, namely, bcc, O, and α 2; when present, the latter was in small quantities compared to the other phases. The transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray investigations of samples that were solutionized and water quenched were used to estimate the phase fields, and a pseudobinary diagram based on Ti=50 at. pct was modified. The aging-transformation behavior was studied in detail. For solutionizing temperatures between 875 °C and the bcc transus, the phase composition and volume fraction of the near-Ti2AlNb alloys adjusted through relative size changes of the equiaxed B2, O, and α 2 grains. The aging behavior followed three distinct transformation modes, dependent on the solutionizing and aging temperatures. Widmanstatten formation was observed when a new phase evolved from a parent phase. Thus, Widmanstatten O phase precipitated within the B2 phase for supertransus fully B2 microstructures, as well as for substransus α 2+B2 microstructures. Similarly, Widmanstatten B2 phase can form from a fully O microstructure, a transformation that has not been observed before. In the case of equiaxed O+B2 solutionized and water-quenched microstructures, Widmanstatten O-phase formation occurred only below 875 °C. For the subtransus-solutionized and water-quenched microstructures, a second aging transformation mode, cellular precipitation, was dominant below 750 °C. This involved formation of coarse and lenticular O phase that grew into the prior B2 grains from the grain boundaries. A third transformation mode involved composition-invariant transformation, where the fully B2 supertransus-solutionized and water-quenched microstructure transformed to a fully O microstructure at 650 °C. This microstructure reprecipitated B2 phase out of the O phase with continued aging time. For Ti-12Al-38Nb, Widmanstatten O precipitation remained the only transformation mode. It is shown that subtransus processing offers flexibility in controlling microstructures through postprocessing heat treatments.

Mechanisms for fracture and fatigue-crack propagation in a bulk metallic glass

Abstract: The fracture and fatigue properties of a newly developed bulk metallic glass alloy, Zr41.2Ti13.8Cu12.5 Ni10Be22.5 (at. pct), have been examined. Experimental measurements using conventional fatigue precracked compact-tension C(T) specimens (∼7-mm thick) indicated that the fully amorphous alloy has a plane-strain fracture toughness comparable to polycrystalline aluminum alloys. However, significant variability was observed and possible sources are identified. The fracture surfaces exhibited a vein morphology typical of metallic glasses, and, in some cases, evidence for local melting was observed. Attempts were made to rationalize the fracture toughness in terms of a previously developed micromechanical model based on the Taylor instability, as well as on the observation of extensive crack branching and deflection. Upon partial or complete crystallization, however, the alloy was severely embrittled, with toughnesses dropping to ∼1 MPa % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9vqpe0x% c9q8qqaqFn0dXdir-xcvk9pIe9q8qqaq-dir-f0-yqaqVe0xe9Fve9% Fve9qapdbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOaaaeaaie% aacaWFTbaaleqaaaaa!3A30! $$\sqrt m $$ . Commensurate with this drop in toughness was a marginal increase in hardness and a reduction in ductility (as measured via depthsensing indentation experiments). Under cyclic loading, crack-propagation behavior in the amorphous structure was similar to that observed in polycrystalline steel and aluminum alloys. Moreover, the crack-advance mechanism was associated with alternating blunting and resharpening of the crack tip. This was evidenced by striations on fatigue fracture surfaces. Conversely, the (unnotched) stress/life (S/N) properties were markedly different. Crack initiation and subsequent growth occurred quite readily, due to the lack of microstructural barriers that would normally provide local crack-arrest points. This resulted in a low fatigue limit of ∼4 pct of ultimate tensile strength.

Microstructural Evolution in a 17-4 PH Stainless Steel after Aging at 400°C

Abstract: The microstructure of 17-4 PH stainless steel at various stages of heat treatment, i.e., after solution heat treatment, tempering at 580 °C, and long-term aging at 400 °C, have been studied by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). The solution-treated specimen consists largely of martensite with a small fraction of δ-ferrite. No precipitates are present in the martensite phase, while spherical fcc-Cu particles are present in the δ-ferrite. After tempering for 4 hours at 580 °C, coherent Cu particles precipitate in the martensite phase. At this stage, the Cr concentration in the martensite phase is still uniform. After 5000 hours aging at 400 °C, the martensite spinodaly decomposes into Fe-rich α and Cr-enriched α′. In addition, fine particles of the G-phase (structure type D8a, space group Fm\(\bar 3\)m) enriched in Si, Ni, and Mn have been found in intimate contact with the Cu precipitates. Following spinodal decomposition of the martensite phase, G-phase precipitation occurs after long-term aging.

Surface oxide and the role of magnesium during the sintering of aluminum

Abstract: The effect of trace additions of magnesium on the sintering of aluminum and its alloys is examined. Magnesium, especially at low concentrations, has a disproportionate effect on sintering because it disrupts the passivating Al2O3 layer through the formation of a spinel phase. Magnesium penetrates the sintering compact by solid-state diffusion, and the oxide is reduced at the metal-oxide interface. This facilitates solid-state sintering, as well as wetting of the underlying metal by sintering liquids, when these are present. The optimum magnesium concentration is approximately 0.1 to 1.0 wt pct, but this is dependent on the volume of oxide and, hence, the particle size, as well as the sintering conditions. Small particle-size fractions require proportionally more magnesium than large-size fractions do.

Precipitation sequence in friction stir weld of 6063 aluminum during aging

Abstract: The precipitation sequence in friction stir weld of 6063 aluminum during postweld aging, associated with Vickers hardness profiles, has been examined by transmission electron microscopy. Friction stir welding produces a softened region in the weld, which is characterized by dissolution and growth of the precipitates. The precipitate-dissolved region contains a minimum hardness region in the aswelded condition. In the precipitate-dissolved region, postweld aging markedly increases the density of strengthening precipitates and leads to a large increase in hardness. On the other hand, aging forms few new precipitates in the precipitate-coarsened region, which shows a slight increase in hardness. The postweld aging at 443 K for 43.2 ks (12 hours) gives greater hardness in the overall weld than in the as-received base material and shifts the minimum hardness from the as-welded minimum hardness region to the precipitate-coarsened region. These hardness changes are consistent with the subsequent precipitation behavior during postweld aging. The postweld solution heat treatment (SHT) and aging achieve a high density of strengthening precipitates and bring a high hardness homogeneously in the overall weld.

Atom probe and transmission electron microscopy investigations of heavily drawn pearlitic steel wire

Abstract: Transmission electron microscopy (TEM) and atom probe field ion microscopy (APFIM) observations of pearlitic steel wire show that drawing to a true strain of 4.22 causes fragmentation of cementite lamellae into nanoscale grains. The drawing strain amorphizes some portions of the cementite lamellae in regions where the interlamellar spacing is very small, but most of the cementite lamellae are polycrystalline with nanoscale grains. The carbon concentration in the ferrite is inhomogeneous and varies from 0.2 to 3 at. pct; the carbon concentration in nanocrystalline cementite is less than 18 at. pct, significantly lower than that in stoichiometric Fe3C. Silicon is segregated to ferrite/cementite boundaries, but, in regions with a small interlamellar spacing, the silicon concentration is uniform across the lamellae. Annealing at 200 °C for 1 hour does not cause noticeable changes in the microstructure. Annealing at 400 °C or above for 1 hour causes spherodization of the cementite lamellae, and the carbon concentrations in ferrite and in cementite return to the predeformation values.

Effect of initial microstructure on plastic flow and dynamic globularization during hot working of Ti-6Al-4V

Abstract: Plastic flow behavior and globularization kinetics during subtransus hot working were determined for Ti-6Al-4V with three different transformed beta microstructures. These conditions consisted of fine lamellar colonies, a mixture of coarse colonies and acicular alpha, and acicular alpha. Isothermal hot compression tests were performed on cylindrical samples at subtransus temperatures and strain rates typical of ingot breakdown (i.e., T∼815 °C to 955 °C, \(\bar \varepsilon \)∼0.1 s−1). For all three material conditions, true stress-true strain curves exhibited a peak stress followed by noticeable flow softening; the values of peak stress and flow softening rate showed little dependence on starting microstructure. On the other hand, the kinetics of dynamic globularization varied noticeably with microstructure. By and large, the globularization rate under a given set of deformation conditions was most rapid for the fine acicular microstructure and least rapid for the mixed coarse-colony/acicular structure. At temperatures close to the beta transus, however, the difference in globularization rates for the three microstructures was less, an effect attributed to the rapid (continuous) coarsening of the laths in the acicular microstructure during preheating prior to hot working. The absence of a correlation between the globularization kinetics and the observed flow softening at low strains suggested platelet/lath bending and kinking as the primary deformation mechanism that controls the shape of the flow curves.

The effect of Mg on the microstructure and mechanical behavior of Al-Si-Mg casting alloys

Abstract: The microstructure and tensile behavior of two Al-7 pct Si-Mg casting alloys, with magnesium contents of 0.4 and 0.7 pct, have been studied. Different microstructures were produced by varying the solidification rate and by modification with strontium. An extraction technique was used to determine the maximum size of the eutectic silicon flakes and particles. The eutectic Si particles in the unmodified alloys and, to a lesser extent, in the Sr-modified alloys are larger in the alloys with higher Mg content. Large Fe-rich π-phase (Al9FeMg3Si5) particles are formed in the 0.7 pct Mg alloys together with some smaller β-phase (Al5FeSi) plates; in contrast, only β-phase plates are observed in the 0.4 pct Mg alloys. The yield stress increases with the Mg content, although, at 0.7 pct Mg, it is less than expected, possibly because some of the Mg is lost to π-phase intermetallics. The tensile ductility is less in the higher Mg alloys, especially in the Sr-modified alloys, compared with the lower Mg alloys. The loss of ductility of the unmodified alloy seems to be caused by the larger Si particles, while the presence of large π-phase intermetallic particles accounts for the loss in ductility of the Sr-modified alloy.

Influence of pressing speed on microstructural development in equal-channel angular pressing

Abstract: The influence of pressing speed in equal-channel angular (ECA) pressing was investigated using samples of pure Al and an Al-1 pct Mg alloy and a range of pressing speeds from ∼10−2 to ∼10 mm s−1. The results show that the speed of pressing has no significant influence on the equilibrium grain size, at least over the range used in these experiments. Thus, the equilibrium grain sizes were ∼1.2 µm for pure Al and ∼0.5 µm for the Al-1 pct Mg alloy for all pressing conditions. However, it is shown that the nature of the microstructure is dependent on the pressing speed, because recovery occurs more easily at the slower speeds, so that the microstructure is then more equilibrated. There is also indirect evidence for the advent of frictional effects when the cross-sectional dimensions of the samples are at or below ∼5 mm.

The debonding and fracture of Si particles during the fatigue of a cast Al-Si alloy

Abstract: Constant-amplitude high-cycle fatigue tests (σmax=133 MPa, σmax/σy=0.55, and R=0.1) were conducted on cylindrical samples machined from a cast A356-T6 aluminum plate: The fracture surface of the sample with the smallest fatigue-crack nucleating defect was examined using a scanning electron microscope (SEM). For low crack-tip driving forces (fatigue-crack growth rates of da/dN 1 × 10−6 m/cycle), silicon particles ahead of the crack tip were fractured, and the crack subsequently propagated through the weakest distribution of prefractured particles in the Al-Si eutectic. Only small rounded silicon particles were observed to debond while the fatigue crack grew at high rates. Using fracture-surface markings and fracture mechanics, a macroscopic measure of the maximum critical driving force between particle debonding vs fracture during fatigue-crack growth was calculated to be approximately K max tr ≈6.0 MPa √m for the present cast A356 alloy.

The effect of solidification rate on the growth of small fatigue cracks in a cast 319-type aluminum alloy

Abstract: A study was conducted to investigate the effect of solidification rate on the growth behavior of small fatigue cracks in a 319-type aluminum alloy, a common Al-Si-Cu alloy used in automotive castings. Fatigue specimens were taken from cast material that underwent a hot isostatic pressing (HIP) process in order to eliminate shrinkage pores and to facilitate the observation of surface-initiated cracks by replication. Naturally initiated surface cracks ranging in length from 17 µm to 2 mm were measured using a replication technique. Growth rates of the small cracks were calculated as a function of the elastic stress-intensity-factor range (ΔK). Long-crack growth-rate data (10 mm≤length≤25 mm) were obtained from compact-tension (CT) specimens, and comparison to the small-crack data indicates the existence of a significant small-crack effect in this alloy. The solidification rate is shown to have a significant influence on small-crack growth behavior, with faster solidification rates resulting in slower growth rates at equivalent ΔK levels. A stress-level effect is also observed for both solidification rates, with faster growth rates occurring at higher applied-stress amplitudes at a given ΔK. A crack-growth relation proposed by Nisitani and others is modified to give reasonable correlation of small-crack growth data to different solidification rates and stress levels.

Simulation of Convection and Macrosegregation in a Large Steel Ingot

Abstract: Melt convection and macrosegregation in casting of a large steel ingot are numerically simulated. The simulation is based on a previously developed model for multicomponent steel solidification with melt convection and involves the solution of fully coupled conservation equations for the transport phenomena in the liquid, mush, and solid. Heat transfer in the mold and insulation materials, as well as the formation of a shrinkage cavity at the top, is taken into account. The numerical results show the evolution of the temperature, melt velocity, and species concentration fields during solidification. The predicted variation of the macrosegregation of carbon and sulfur along the vertical centerline is compared with measurements from an industrial steel ingot that was sectioned and analyzed. Although generally good agreement is obtained, the neglect of sedimentation of free equiaxed grains prevents the prediction of the zone of negative macrosegregation observed in the lower part of the ingot. It is also shown that the inclusion of the shrinkage cavity at the top and the variation of the final solidification temperature due to macrosegregation is important in obtaining good agreement between the predictions and measurements.

Synthesis and cyclic oxidation behavior of a (Ni, Pt) Al coating on a desulfurized Ni-base superalloy

Abstract: The influences of sulfur impurities and Pt incorporation on the scale adhesion behavior of aluminide coatings were studied and compared. Low-sulfur NiAl coatings were prepared on a desulfurized, yttrium-free, single-crystal Ni-based superalloy by a modified version of a conventional aluminizing procedure based on chemical vapor deposition. The sulfur level in the resulting NiAl coatings was measured to be less than ∼0.5 ppmw by glow-discharge mass spectroscopy. Platinum-modified aluminide coatings were synthesized by first electroplating a thin layer of Pt (∼7 µm) on the superalloy, followed by the same low-sulfur aluminizing procedure. The measured sulfur content in the (Ni, Pt)Al coating was substantially higher than that of the low-sulfur NiAl coating due to contamination during the Pt electroplating process. A very adherent α-Al2O3 scale formed on the grain surfaces of the low-sulfur NiAl coating during cyclic oxidation testing at 1150 °C, but scale spallation eventually occurred over many of the NiAl grain boundaries. In contrast, despite the higher level of sulfur in the (Ni Pt)Al coating, a very adherent scale was formed over both the coating grain surfaces and grain boundaries during thermal cycling. These results suggest that Pt additions can mitigate the detrimental influence of sulfur on scale adhesion.

Characterization of the W2C phase formed during the high velocity oxygen fuel spraying of a WC + 12 pct Co powder

Abstract: A variety of experimental techniques have been used to study a WC-12 pct Co powder and the coatings produced by high velocity oxygen fuel (HVOF) spraying of the powder onto a steel substrate. Many of the structural characteristics of the powder were also found in the coating. However, when the metallic matrix of the powder was melted during thermal spraying, the carbides were partially dissolved and a very heterogeneous liquid phase was produced in which the W/C ratio varied from about 1 to 4. These variations have been linked with oxidation of the liquid phase during spraying. The factors influencing the formation of W2C in the coating have been identified as (1) an in situ transformation of WC into W2C maintaining the original WC faceted morphology and (2) the precipitation of W2C from the W-rich liquid phase matrix as the coating cools. A cobalt containing carbide of the M6C-M12C type has also precipitated from the liquid phase when the W/C and W/Co ratios were high.

Mechanism of spallation in platinum aluminide/electron beam physical vapor-deposited thermal barrier coatings

Abstract: The spallation failure of a commercial thermal barrier coating (TBC), consisting of a single-crystal RENE N5 superalloy, a platinum aluminide (Pt-Al) bond coat, and an electron beam-deposited 7 wt pct yttria-stabilized zirconia ceramic layer (7YSZ), was studied following cyclic furnace testing. In the uncycled state and prior to deposition of the ceramic, the Pt-Al bond-coat surface contains a cellular network of ridges corresponding to the underlying bond-coat grain-boundary structure. With thermal cycling, the ridges and associated grain boundaries are the sites of preferential oxidation and cracking, which results in the formation of cavities that are partially filled with oxide. Using a fluorescent penetrant dye in conjunction with a direct-pull test, it is shown that, when specimens are cycled to about 80 pct of life, these grain-boundary regions show extensive debonding. The roles of oxidation and cyclic stress in localized grain boundary region spallation are discussed. The additional factors leading to large-scale TBC spallation are described.

Thermodynamic assessment of the Al-Fe-Si system

Abstract: The phase equilibria and thermodynamic properties of the ternary Al-Fe-Si system were analyzed and a complete thermodynamic description of the ternary system was obtained. The thermodynamic descriptions of the three binary systems were taken from the literature. Most of the binary intermetallic phases, except Al13Fe4(ϑ), FeSi2-L, and FeSi2-H, were assumed to have no ternary solubility. Based on the experimental data, seven stable ternary intermetallic phases were considered in the system. Two of them were treated as semistoichiometric compounds with homogeneity ranges for Al and Si, and the others were treated as stoichiometric compounds. Reasonable agreement was obtained between calculation results and experimental information in the ternary system.

Workability of commercial-purity titanium and 4340 steel during equal channel angular extrusion at cold-working temperatures

Abstract: The deformation and failure of commercial-purity (CP) titanium (grade 2) and AISI 4340 steel (tempered to Rc35) during equal channel angular extrusion were determined at temperatures between 25 °C and 325 °C and effective strain rates between 0.002 and 2.0 s−1. The CP titanium alloy underwent segmented failure under all conditions except at low strain rates and high temperatures. By contrast, the 4340 steel deformed uniformly except at the highest temperature and strain rate, at which it also exhibited segmented failure. Using flow curves and fracture data from uniaxial compression and tension tests, workability analysis was conducted to establish that the failures were a result of flow localization prior to the onset of fracture. This conclusion was confirmed by metallographic examination of the failed extrusion specimens.

Microstructural effects on high-cycle fatigue-crack initiation in A356.2 casting alloy

Abstract: The effects of various microconstituents on crack initiation and propagation in high-cycle fatigue (HCF) were investigated in an aluminum casting alloy (A356.2). Fatigue cracking was induced in both axial and bending loading conditions at strain/stress ratios of −1, 0.1, and 0.2. The secondary dendrite arm spacing (SDAS) and porosity (maximum size and density distribution) were quantified in the directionally solidified casting alloy. Using scanning electron microscopy, we observed that cracks initiate at near-surface porosity, at oxides, and within the eutectic microconstituents, depending on the SDAS. When the SDAS is greater than ∼ 25 to 28 µm, the fatigue cracks initiate from surface and subsurface porosity. When the SDAS is less than ∼ 25 to 28 µm, the fatigue cracks initiate from the interdendritic eutectic constituents, where the silicon particles are segregated. Fatigue cracks initiated at oxide inclusions whenever they were near the surface, regardless of the SDAS. The fatigue life of a specimen whose crack initiated at a large eutectic constituent was about equal to that when the crack initiated at a pore or oxide of comparable size.

A New Analysis for the Determination of Ternary Interdiffusion Coefficients from a Single Diffusion Couple

Abstract: Concentration profiles that develop in a ternary diffusion couple during an isothermal anneal can be analyzed directly for average ternary interdiffusion coefficients. A new analysis is presented for the determination of average values for the main and cross-interdiffusion coefficients over selected regions in the diffusion zone from an integration of interdiffusion fluxes, which are calculated directly from experimental concentration profiles. The analysis is applied to selected isothermal diffusion couples investigated with α (fcc) Cu-Ni-Zn alloys at 775 °C, β (bcc) Fe-Ni-Al alloys at 1000 °C, and γ (fcc) Ni-Cr-Al alloys at 1100 °C. Average ternary interdiffusion coefficients treated as constants are calculated over composition ranges on either side of the Matano plane and examined for the diffusional interactions among the diffusing components. The ternary interdiffusion coefficients determined from the new analysis are observed to be consistent with those determined by the Boltzmann-Matano analysis at selected compositions in the diffusion zone. The ternary interdiffusion coefficients are also employed in analytical solutions based on error functions for the generation of concentration profiles for the selected diffusion couples. The generated profiles are a good representation of the experimental profiles including those exhibiting uphill diffusion or zero-flux plane (ZFP) development for the individual components. Uncertainties in the values of the interdiffusion coefficients calculated on the basis of the new analysis are found to be minimal.

Finite-element modeling of nonisothermal equal-channel angular extrusion

Abstract: Deformation during conventional (nonisothermal) hot working of metals via equal-channel angular extrusion (ECAE) was investigated using two-dimensional (2-D) and three-dimensional (3-D) finite-element modeling (FEM) analysis. The effects of material flow properties, die-workpiece heat-transfer and friction conditions, and die design on metal flow were examined. Friction and die design were shown to be the most important parameters governing the formation of dead-metal zones during extrusion. On the other hand, thermal gradients induced by die chill and deformation heating were found to exacerbate the extent of flow localization that arises due to material-flow softening alone. The FEM predictions were validated by ECAE experiments on a Ti-6Al-4V alloy with a colony alpha microstructure. Preforms exhibited minor edge cracking and mild flow localization during extrusion at 985 °C, but severe shear localization and fracture during extrusion at 900 °C. The 2-D FEM simulations predicted deformation detail, including shear localization, that was in good agreement with the experimental results, but 3-D FEM simulations were required to realistically predict die chill. A combined approach, in which thermal data were extracted from 3-D simulations and inserted into 2-D simulations, produced load-stroke and fracture predictions in general agreement with measured values.