Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 1996"

Numerical modeling of cellular/dendritic array growth: spacing and structure predictions

Abstract: A numerical model of cellular and dendritic growth has been developed that can predict cellular and dendritic spacings, undercoolings, and the transition between structures. Fully self-consistent solutions are produced for axisymmetric interface shapes. An important feature of the model is that the spacing selection mechanism has been treated. A small, stable range of spacings is predicted for both cells and dendrites, and these agree well with experiment at both low and high velocities. By suitable nondimensionalization, relatively simple analytic expressions can be used to fit the numerical results. These expressions provide an insight into the cellular and dendritic growth processes and are useful for comparing theory with experiment.

The balance of mechanical and environmental properties of a multielement niobium-niobium silicide-based in situ composite

Abstract: This article describes room-temperature and high-temperature mechanical properties, as well as oxidation behavior, of a niobium-niobium silicide basedin situ composite directionally solidified from a Nb-Ti-Hf-Cr-Al-Si alloy. Room-temperature fracture toughness, high-temperature tensile strength (up to 1200 °C), and tensile creep rupture (1100 °C) data are described. The composite shows an excellent balance of high- and low-temperature mechanical properties with promising environmental resistance at temperatures above 1000 °C. The composite microstructures and phase chemistries are also described. Samples were prepared using directional solidification in order to generate an aligned composite of a Nb-based solid solution with Nb3Si- and Nb5Si3-type silicides. The high-temperature mechanical properties and oxidation behavior are also compared with the most recent Ni-based superalloys. This composite represents an excellent basis for the development of advanced Nb-based intermetallic matrix composites that offer improved properties over Ni-based superalloys at temperatures in excess of 1000 °C.

Rafting in superalloys

Abstract: The phenomenon of rafting in superalloys is described, with particular reference to modern superalloys with a high volume fraction of the particulate γ’ phase. It is shown that in the elastic regime, the thermodynamic driving force for rafting is proportional to the applied stress, to the difference between the lattice parameters of the γ matrix and the γ’ particles, and to the difference of their elastic constants. A qualitative argument gives the sign of this driving force, which agrees with that determined by Pineau for a single isolated particle. Drawing on the work of Pollock and Argon and of Socrate and Parks, it is shown that after a plastic strain of the sample of order 2 × 10-4, the driving force is proportional to the product of the applied stress and the lattice misfit, in agreement with the results of the calculations of Socrate and Parks. The rate of rafting is controlled by the diffusion of alloying elements. Here, the tendency of large atoms to move from regions of high hydrostatic pressure to those of low may outweigh the influence of concentration gradients. The deformation of the sample directly produced by rafting is small, of order 4.5 × 10-4. The rafted structure is resistant to creep under low stresses at high temperatures. Under most experimental conditions at relatively high stresses, rafting accelerates creep; this effect may be less pronounced at the small strains acceptable under operational conditions.

Equiaxed dendritic solidification with convection: Part I. Multiscale/multiphase modeling

Abstract: Equiaxed dendritic solidification in the presence of melt convection and solid-phase transport is investigated in a series of three articles. In part I, a multiphase model is developed to predict com-position and structure evolution in an alloy solidifying with an equiaxed morphology. The model accounts for the transport phenomena occurring on the macroscopic (system) scale, as well as the grain nucleation and growth mechanisms taking place over various microscopic length scales. The present model generalizes a previous multiscale/multiphase model by including liquid melt convec-tion and solid-phase transport. The macroscopic transport equations for the solid and the interdendritic and extradendritic liquid phases are derived using the volume averaging technique and closed by supplementary relations to describe the interfacial transfer terms. In part II, a numerical application of the model to equiaxed dendritic solidification of an Al-Cu alloy in a rectangular cavity is dem-onstrated. Limited experimental validation of the model using a NH4C1-H2O transparent model alloy is provided in part III.

Constitutive behavior of tantalum and tantalum-tungsten alloys

Abstract: The effects of strain rate, temperature, and tungsten alloying on the yield stress and the strainhardening behavior of tantalum were investigated The yield and flow stresses of unalloyed Ta and tantalum-tungsten alloys were found to exhibit very high rate sensitivities, while the hardening rates in Ta and Ta-W alloys were found to be insensitive to strain rate and temperature at lower temperatures or at higher strain rates This behavior is consistent with the observation that overcoming the intrinsic Peierls stress is shown to be the rate-controlling mechanism in these materials at low temperatures The dependence of yield stress on temperature and strain rate was found to decrease, while the strain-hardening rate increased with tungsten alloying content The mechanical threshold stress (MTS) model was adopted to model the stress-strain behavior of unalloyed Ta and the Ta-W alloys Parameters for the constitutive relations for Ta and the Ta-W alloys were derived for the MTS model, the Johnson—Cook (JC), and the Zerilli-Armstrong (ZA) models The results of this study substantiate the applicability of these models for describing the high strain-rate deformation of Ta and Ta-W alloys The JC and ZA models, however, due to their use of a power strain-hardening law, were found to yield constitutive relations for Ta and Ta-W alloys that are strongly dependent on the range of strains for which the models were optimized

Abnormal growth of faceted (WC) grains in a (Co) liquid matrix

Abstract: If the grains dispersed in a liquid matrix are spherical, their surface atomic structure is expected to be rough (diffuse), and their coarsening has been observed to be controlled by diffusion in the matrix. They do not, furthermore, undergo abnormal growth. On the other hand, in some compound material systems, the grains in liquid matrices are faceted and often show abnormal coarsening behavior. Their faceted surface planes are expected to be singular (atomically flat) and therefore grow by a defect-assisted process and two-dimensional (2-D) nucleation. Contrary to the usual coarsening the-ories, their growth velocity is not linearly dependent on the driving force arising from the grain size difference. If the growth of the faceted grains occurs by 2-D nucleation, the rate is expected to increase abruptly at a critical supersaturation, as has been observed in crystal growth in melts and solutions. It is proposed that this growth mechanism leads to the abnormal grain coarsening. The 2-D nucleation theory predicts that there is a threshold initial grain size for the abnormal grain growth (AGG), and the propensity for AGG will increase with the heat-treatment temperature. The AGG behavior will also vary with the defects in the grains. These predictions are qualitatively confirmed in the sintered WC-Co alloy prepared from fine (0.85-Μm) and coarse (5.48-Μm) WC powders and their mixtures. The observed dependence of the AGG behavior on the sintering temperature and the milling of the WC powder is also qualitatively consistent with the predicted behavior.

Ostwald ripening in ternary alloys

Abstract: A theory of coarsening in an isothermal, ternary alloy is developed in an effort to understand the effects of a third chemical component on the ripening behavior of a two-phase system. The analysis is valid for a general, nonideal, nondilute solution, but is limited to extremely small volume fractions of the coarsening phase and neglects off-diagonal terms in the diffusion matrix. The Gibbs-Thompson equation in a ternary system undergoing coarsening reveals that the concentrations at the particle/matrix interface are dependent on the far-field supersaturations as well as on the particle radius. In addition, the capillary length depends on the diffusivities of the two components. An asymptotic analysis shows that the exponents of the temporal power laws for the average particle radius, number of particles per unit volume, and the matrix supersaturations are the same as that found in the binary limit; however, the amplitudes of the power laws are modified. We find that the trajectory of the matrix supersaturation must lie along a tie-line, but the trajectory of the particle composition does not. An expression for the effect of dilute ternary additions to the coarsening rate of a binary alloy is also given.

The breakdown of single-crystal solidification in high refractory nickel-base alloys

Abstract: The breakdown of single-crystal solidification has been studied over a wide range of solidification conditions in ten superalloys with large variations in Re, Ta, and W content. Over the range of experimental conditions investigated, grain defect formation was sensitive to local thermaland solutal conditions. For a fixed alloy composition and withdrawal rate, the transition from single-crystal to equiaxed solidification did not occur abruptly. Instead, as thermal gradients were decreased in a series of experiments, isolated, highly misoriented columnar grains with the same composition as that of the base alloy developed in the presence of positive (stabilizing) thermal gradients with increasing frequency until the advance of the single-crystal front was completely blocked. The onset of columnar grain formation occurred when the primary dendrite arm spacing exceeded a critical value, corresponding to a morphological transition in the dendritic array. The onset of “freckling” was observed at the same primary dendrite arm spacing where misoriented columnar grains began to appear. In experiments with varying levels of refractory alloy content, there was also a strong correlation between the onset of grain formation and freckle formation. These observations strongly suggest that in high refractory content superalloys, the breakdown of single-crystal solidification and the formation of misoriented grains as well as freckle-type defects are sensitively dependent on thermosolutal convection processes.

Thermoelastic martensite and shape memory effect in ductile Cu-Al-Mn alloys

Abstract: Ductile shape memory (SM) alloys of the Cu-AI-Mn system have been developed by controlling the degree of order in the β phase. Additions of Mn to the binary Cu-Al alloy stabilize the β phase and widen the single-phase region to lower temperature and lower Al contents. It is shown that Cu-Al-Mn alloys with low Al contents have either the disordered A2 structure or the ordered L21 structure with a lower degree of order and that they exhibit excellent ductility. The disordered A2 phase martensitically transforms to the disordered Al phase with a high density of twins. The martensite phase formed from the ordered L21 phase has the 18R structure. The SM effect accompanies both the A2 → Al and L21 → 18R martensitic transformations. These alloys exhibit 15 pct strain to failure, 60 to 90 pct rolling reduction without cracking, and 80 to 90 pct recovery from bend test in the martensitic condition. Experimental results on the microstructure, crystal structure, mechanical properties, and shape memory behavior in the ductile Cu-AI-Mn alloys are presented and discussed.

Solidification of an alloy 625 weld overlay

Abstract: The solidification behavior (microsegregation, secondary phase formation, and solidification temperature range) of an Alloy 625 weld overlay deposited on 2.25Cr-1Mo steel by gas metal arc welding was investigated by light and electron optical microscopy, electron microprobe, and differential thermal analysis techniques. The overlay deposit was found to terminate solidification at {approx}1,216 C by a {gamma}/Laves eutectic-type reaction. The Laves phase was highly enriched in Nb, Mo, and Si. The solidification reaction and microsegregation potential of major alloying elements in the overlay deposit are compared to other Nb-bearing Ni base alloys and found to be very similar to those for Alloy 718. Solidification cracks observed in the overlay were attributed to the wide solidification temperature range ({approx}170 C) and formation of interdendritic ({gamma} + Laves) constituent. Reasonable agreement is obtained between the calculated and measured volume percent ({gamma} + Laves) constituent with the Scheil equation by treating the overlay system as a simple {gamma}-Nb binary and using an experimentally determined k{sub Nb} value from electron microprobe data.

Forming of tailor-welded blanks

Abstract: Beginning in 1992, tailor-welded blanks (TWBs) were used in the United States automotive industry to consolidate parts, reduce tolerances, save weight, and increase stiffness. This business is expanding rapidly; more than $500 million of annual TWB sales are expected by 1997. Welds in steel are generally stronger than the base material, such that weld failure by preferential localization is not a critical issue. However, the forming characteristics of TWBs must be understood in order to design and produce high-quality parts with reasonable production and tooling costs. Three formability issues were addressed in this study: the intrinsic ductility and relative formability of three weld types (CO2 and Nd:YAG laser welds and mash-seam welds with and without mechanical postweld processing); the value and correspondence of mechanical tests to each other and to press performance; and the prediction of the forming behavior using the finite element method (FEM). Two failure modes for TWBs were identified. While the local ductility of welds can differ greatly, little difference in press formability was measured among the weld types. More important than weld ductility are the changed deformation patterns which depend on the differential strength but depend little on local weld prop-erties. Finite element method (FEM) simulations of dome tests and scale fender-forming operations show good agreement with measurements, as long as boundary conditions are known accurately. The importance of weld-line displacement is discussed and several simulations are compared with ex-periments.

Crystallography of grain boundary α precipitates in a β titanium alloy

Abstract: The crystallography of α(hcp) precipitates formed on the β(bcc) matrix grain boundaries has been studied with transmission electron microscopy (TEM) in a Ti-15V-3Cr-3Sn-3Al alloy. The α precipitates have a near-Burgers orientation relationship with respect to at least one of the adjacent β grains. Among the possible 12 variants in this orientation relationship, the variant that [11•20]α is parallel to the 〈111〉β closest to the grain boundary plane tends to be preferred by the α precipitates. Additionally, further variant selections are made so as to minimize the deviation of orientation relationship with respect to the “opposite“ β grain from the Burgers one. Such rules in variant selection often result in the formation of precipitates with a single variant at a planar grain boundary. Prior small deformation of β matrix changes the variant of α precipitates at the deformed portion of grain boundary. It is considered that the stress field of dislocations in the slip bands intersecting with the boundary strongly affects the variants of α precipitates. Discussion of these results is based upon a classical nucleation theory.

Porosity formation in AI-9 Wt Pct Si-3 Wt Pct Cu alloy systems: Metallographic observations

Abstract: The formation of porosity in Al-9 wt Pct Si-3 wt Pct Cu-X alloys was studied as a function of (1) the hydrogen content of the melt; (2) the melt treatment additives, namely, modifier (Sr), grain refiner (TiB2), and primary silicon refiner (P); (3) alloying elements for precipitation hardening such as Mg and Zn; (4) intermetallics (α-iron, β-iron, sludge, and Al2Cu); and (5) solidification conditions (so-lidification time and solidus velocity). The results were statistically analyzed, based on the quanti-tative image analysis data of the porosity observed in samples obtained from a set of 72 solidification experiments. Metallographic aspects of pore size and pore morphology related to the preceding parameters and the possible mechanisms of porosity formation are highlighted in this article. The results show that a melt hydrogen content of 0.1 mL/100 g Al has the same effect on percentage porosity as that obtained with an addition of 185 ppm strontium to the melt. Grain refiner particles, phosphorus, and magnesium reduce percentage porosity, although in different magnitudes. A Mg-Sr or Mg-GR combination further reduces the percentage porosity observed in the casting. Theβ needles of the Al5FeSi intermetallic phase are very active as pore nucleation sites. All intermetallics,viz. β needles, α-Chinese script phase, Al2Cu phase, and sludge restrict pore growth and expansion. In-creasing the local solidification time or the solidus velocity increases the pore parameters. Pore growth in the two cases is attributed, respectively, to a diffusion-controlled growth process and to the formation of hot spots.

Morphological instabilities of lamellar eutectics

Abstract: We present the results of a numerical study based on the boundary integral technique of interfacial pattern formation in directional solidification of thin-film lamellar eutectics at low velocity. Microstructure selection maps that identify the stability domains of various steady-state and nonsteady-state growth morphologies in the spacing-composition (λ –C 0) plane are constructed for the transparent organic alloy CBr4-C2Cl6 and for a model eutectic alloy with two solid phases of identical physical properties. In CBr4-C2Cl6, the basic set of instabilities that limit steady-state growth is richer than expected. It consists of three primary instabilities, two of which are oscillatory, which bound the domain of the commonly observed axisymmetric lamellar morphology, and two secondary oscillatory instabilities, which bound the domain of the nonaxisymmetric (tilted) lamellar morphology. The latter is predicted to occur over a hypereutectic range of composition which coincides well with experiment. Moreover, the steady tilt bifurcation lies between but does not directly bound either of these two domains, which are consequentlydisjoint. Four stable oscillatory microstructures, at least three of which have been seen experimentally, are predicted to occur in unstable regimes. In the model alloy, the structure is qualitatively similar, except that a stable domain of tilted steady-state growth is not found, in agreement with previous random-walk simulations. Furthermore, the composition range of stability of the axisymmetric morphology decreases sharply with increasing spacing away from minimum undercooling but extends further off-eutectic than predicted by the competitive growth criterion. In addition, oscillations with a wavelength equal to two λ lead to lamella termination at a small distance above the onset of instability. The implications of these two features for the eutectic to dendrite transition are examined with the conclusion that in the absence of heterogeneous nucleation, this transition should be histeritic at small velocity and temperature gradient.

Precipitation behaviors in Al-Cu-Mg and 2024 aluminum alloys

Abstract: The precipitation behaviors and aging reactions of the pseudobinary Al-Cu-Mg alloy and the commercial 2024 alloy under unstretched and stretched conditions have been investigated in this study by means of conductivity and hardness measurements, differential scanning calorimetry, and transmission electron microscopy (TEM). The morphologies and growth modes of various defects and transition phases as well as the interaction among them were widely discussed. In particular, an electron diffraction ring pattern was found to correspond to the axial growth of the GPB2 zone. This suggested that the atom groups constructing this cylindrical zone are statistically, uniformly arranged in the adjacent {100}A1 planes and the GPB2 zone is only a partially ordered version of the GPB zone in Al, directions. Moreover, few GPB2 zones can survive long time overaging due to the Gibbs-Thomson effect. As for the S' precipitates, they preferentially nucleate on dislocations. During subsequent growth, they can further coalesce into two morphologies (corrugated sheets and wide plates) for the unstretched specimens. However, for the stretched specimens, this coalescing process does not occur until a long time of overaging due to more introduced dislocations. Therefore, the rate of Ostward ripening decreases and the peak hardness becomes flattened. Finally, based on the present analyses, the aging sequence of the two alloys studied could be revised with respect to previous investigations and their isothermal aging reactions can be subdivided into five main stages. These stages correspond to (1) GPB zone precipitation, (2) fast in situ precipitation of GPB2 zones from GPB zones and their subsequent growth, (3) fast nucleation and accelerating growth of the S' phase, (4) decelerating growth of the S' phase, and (5) Ostward ripening of the S' and S phases, respectively.

Microstructural aspects of the dissolution and melting of Al2Cu phase in Al-Si alloys during solution heat treatment

Abstract: The dissolution and melting of Al2Cu phase in solution heat-treated samples of unmodified Al-Si 319.2 alloy solidified at ≈10 °C were studied using optical microscopy, image analysis, electron probe microanalysis (EPMA), and differential scanning calorimetry (DSC). The solution heat treat-ment was carried out in the temperature range 480 °C to 545 °C for solution times of up to 24 hours. Of the two forms of Al2Cu found to exist,i.e., blocky and eutectic-like, the latter type is more pronounced in the unmodified alloy (at ≈10 °C) and was observed either as separate eutectic pockets or precipitated on preexisting Si particles, β-iron phase needles, or the blocky Al2Cu phase. Dissolution of the (Al + Al2Cu) eutectic takes place at temperatures close to 480 °C through frag-mentation of the phase and its dissolution into the surrounding Al matrix. The dissolution is seen to accelerate with increasing solution temperature (505 °C to 515 °C). The ultimate tensile strength (UTS) and fracture elongation (EL) show a linear increase when plotted against the amount of dissolved copper in the matrix, whereas the yield strength (YS) is not affected by the dissolution of the Al2Cu phase. Melting of the copper phase is observed at 540 °C solution temperature; the molten copper-phase particles transform to a shiny, structureless phase upon quenching. Coarsening of the copper eutectic can occur prior to melting and give rise to massive eutectic regions of (Al + Al2Cu). Unlike the eutectic, fragments of the blocky Al2Cu phase are still observed in the matrix, even after 24 hours at 540 °C.

Electron microscope study of Al-Fe-Si intermetallics in 6201 aluminum alloy

Abstract: The Al-Fe-Si intermetallics present in a commercial cast 6201 electrical conductor alloy have been studied using high resolution electron microscopy. The β-Al5FeSi phase is highly faceted and contains multiple (001) growth twins parallel to the growth direction. The α-Al8Fe2Si phase which forms in a Chinese script morphology has a nonfaceted interface with the aluminum matrix and exhibits no growth twinning. Formation of the β phase is believed to occurvia a peritectic decomposition of α-Al8Fe2Si at 612 °C. Observations made by transmission electron microscopy (TEM) support this hypothesis. When 30 ppm strontium is added to this alloy, the α phase is stabilized and very little β-Al5FeSi appears in the microstructure. A silicon-rich layer is found around the α-phase particles. It is proposed that strontium adsorbs to the α-phase interface, and in so doing, the diffusion of silicon into the α phase, necessary for its transformation to β, is prevented.

Prediction of Grain Structures in Various Solidification Processes

Abstract: Grain structure formation during solidification can be simulatedvia the use of stochastic models providing the physical mechanisms of nucleation and dendrite growth are accounted for. With this goal in mind, a physically based cellular automaton (CA) model has been coupled with finite element (FE) heat flow computations and implemented into the code3- MOS. The CA enmeshment of the solidifying domain with small square cells is first generated automatically from the FE mesh. Within each time-step, the variation of enthalpy at each node of the FE mesh is calculated using an implicit scheme and a Newton-type linearization method. After interpolation of the explicit temperature and of the enthalpy variation at the cell location, the nucleation and growth of grains are simulated using the CA algorithm. This algorithm accounts for the heterogeneous nucleation in the bulk and at the surface of the ingot, for the growth and preferential growth directions of the dendrites, and for microsegregation. The variations of volume fraction of solid at the cell location are then summed up at the FE nodes in order to find the new temperatures. This CAFE model, which allows the prediction and the visualization of grain structures during and after solidification, is applied to various solidification processes: the investment casting of turbine blades, the continuous casting of rods, and the laser remelting or welding of plates. Because the CAFE model is yet two-dimensional (2-D), the simulation results are compared in a qualitative way with experimental findings.

Mechanical deformation of dendrites by fluid flow

Abstract: It is generally accepted that liquid agitation during alloy solidification assists in crystal multiplication, as in dendrite fragmentation and the detachment of side arms in the mushy region of a casting. Even without deliberate stirring by electromagnetic or mechanical means, there is often vigorous interdendritic fluid flow promoted by natural thermosolutal convection. Interdendritic fluid flow rates in metals might be as high as 10 mm s{sup {minus}1}. It is the purpose of this article to examine whether such fluid flow can cause mechanical deformation of dendrites, sufficient to cause side arms to bend or break. Metals are so ductile at their melting points that applied forces could only be expected to cause bending, as opposed to fracture, although there are no reports of which the authors are aware of dendritic arms being mechanically bent in this way. The following estimates demonstrate why even bending is not to be expected. In this analysis, the authors shall estimate the stress at the root of a secondary dendrite arm of aluminum arising from the action of a flow of molten metal past the dendrite arm.

Increased ductility in high velocity electromagnetic ring expansion

Abstract: Thin rings have been rapidly expanded using large, transient magnetic fields to study the effect of deformation velocity on strains to failure of ductile metals. A classical electrodynamics analysis similar to one developed previously by Gourdin was employed to estimate sample velocities. Within expansion velocities studied (50 to 300 m/s), the experimental results show that ductility of Al 6061 and OFHC Cu increases monotonically with increasing velocity. In each case, sample strain at failure is almost twice as great at 300 m/s as in the static condition. Comparison to a one-dimensional rigid-viscoplastic dynamic finite element method analysis suggests that inertial effects are mainly responsible for enhanced ductility over a wide range of velocity.

The influence of niobium supersaturation in austenite on the static recrystallization behavior of low carbon microalloyed steels

Abstract: This work describes the effect of Nb supersaturation in austenite, as it applies to the strain-induced precipitation potential of Nb(CN), on the suppression of the static recrystallization of austenite during an isothermal holding period following deformation. Four low carbon steels, microalloyed with Nb, were used in this investigation. Three of the steels had variations in Nb levels at constant C and N concentrations. Two steels had different N levels at constant C and Nb concentrations. The results from the isothermal deformation experiments and the subsequent measurement of the solution behavior of Nb in austenite show that the recrystallization-stop temperature (TRXN) increases with increasing Nb supersaturation in austenite. Quantitative transmission electron microscopy analysis revealed that the volume fraction of Nb(CN) at austenite grain boundaries or subgrain boundaries was 1.5 to 2 times larger than Nb(CN) volume fractions found within the grain interiors. This high, localized volume fraction of Nb(CN) subsequently led to high values for the precipitate pinning force (FPIN). These values forFPIN were much higher than what would have been predicted from equilibrium thermodynamics describing the solution behavior of Nb in austenite.

The cracking mechanism of silicon particles in an A357 aluminum alloy

Abstract: The cracking of Si particles in an A357 Al alloy has been investigated over a spectrum of stress and strain by varying aging strength and applying different tensile strains. The variation of the fraction of broken Si particles with stress, strain, and cleavage plane orientation has been obtained. The features of cracking reveal that cracking of Si particles is a very localized event. A dislocation pileup mechanism is the most probable one among all crack-initiation theories for explaining the behavior. Based on this mechanism, further deduction has been made to obtain the relationship between the fraction of broken particles and metallurgical factors. The present data, along with Gurlandrss and that of Lowet al., have been found to verify this relationship for the effect of stress, strain, and cleavage plane orientation.

The quench sensitivity of cast Al-7 wt pct Si-0.4 wt pct Mg alloy

Abstract: The effect of quenching condition on the mechanical properties of an A356 (Al-7 wt pct Si-0.4 wt pct Mg) casting alloy has been studied using a combination of mechanical testing, differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). As the quench rate decreases from 250 °C/s to 0.5 °C/s, the ultimate tensile strength (UTS) and yield strength decrease by approximately 27 and 33 pct, respectively. The ductility also decreases with decreasing quench rate. It appears that with the peak-aged condition, both the UTS and yield strength are a logarithmic function of the quench rate,i.e., UTS orσ y =A logR +B. The termA is a measure of quench sensitivity. For both UTS and yield strength of the peak-aged A356 alloy,A is approximately 32 to 33 MPa/log (°C/s). The peak-aged A356 alloy is more quench sensitive than the aluminum alloy 6063. For 6063,A is approximately 10 MPa/log (°C/s). The higher quench sensitivity of A356 is probably due to the high level of excess Si. A lower quench rate results in a lower level of solute supersaturation in the α-Al matrix and a decreased amount of excess Si in the matrix after quenching. Both of these mechanisms play important roles in causing the decrease in the strength of the peak-aged A356 with decreasing the quench rate.

Effect of stress state on the stress-induced martensitic transformation in polycrystalline Ni-Ti alloy

Abstract: The effect of stress state on the character and extent of the stress-induced martensitic transformation in polycrystalline Ni-Ti shape memory alloy has been investigated. Utilizing unique experimental equipment, uniaxial and triaxial stress states have been imposed on Ni-Ti specimens and the pseudoelastic transformation strains have been monitored. Comparisons between tests of differing stress states have been performed using effective stress and effective strain quantities; a strain offset method has been utilized to determine the effective stress required for transformation under a given stress state. Results of the tests under different stress states indicate that (1) despite the negative volumetric strain associated with the austenite-to-martensite transformation in Ni-Ti, effective stress for the onset of transformation decreases with increasing hydrostatic stress; (2) effective stressvs effective strain behavior differs greatly under different applied stress states; and (3) austenite in Ni-Ti is fully stable under large values of compressive hydrostatic stress.

Equiaxed dendritic solidification with convection: Part II. Numerical simulations for an Al-4 Wt pct Cu alloy

Abstract: The multiphase model developed in part I for equiaxed dendritic solidification with melt convection and solid-phase transport is applied to numerically predict structural and compositional development in an Al-4 wt pct Cu alloy solidifying in a rectangular cavity. A numerical technique combining a fully implicit control-volume-based finite difference method with a multiple time-step scheme is developed for accurate and efficient simulations of both micro- and macroscale phenomena. Quantitative results for the dendritic microstructure evolution in the presence of melt convection and solid movement are obtained. The remarkable effects of the solid-liquid multiphase flow pattern on macrosegregation as well as the grain size distribution are illustrated.

Control of iron nitride layers growth kinetics in the binary Fe-N system

Abstract: This study is within the framework of a research program dedicated to defining the optimal conditions for the nitriding of iron and steels at atmospheric pressure by using various mixtures, NH3-N2-H2 and NH3-Ar. After studying the mechanisms of phase formation and mass transfer at the gas-solid interface, a mathematical model is developed in order to predict the nitrogen transfer rate in the solid, the nitride layer growth rate, and the nitrogen concentration profiles. In order to validate the model and to show its possibilities, it is compared with thermogravimetric experiments, analyses, and metallurgical observations (X-ray diffraction, optical microscopy, and electron microprobe anal-ysis). The results obtained allow us to demonstrate the sound correlation between the experimental results and the theoretical predictions. By applying the model to the iron-nitrogen binary system, when the e/γ/α configuration referred to the Fe-N phase diagram is formed, we have experimentally determined the effective diffusion coefficient of nitrogen in the e phase. The latter is constant for a composition of the e nitride between 8 and 9.5 wt pct nitrogen. All the results obtained show that it is possible, by means of dynamic gas flow regulation, to eliminate the incubation period and to control the thickness, composition, and structure of the compound layer at the beginning of the treatment.

Ferrite nucleation and growth during continuous cooling

Abstract: The austenite decomposition has been investigated in two hypoeutectoid plain carbon steels under continuous cooling conditions using a dilatometer on a Gleeble 1500 thermomechanical simulator. The experimental results were used to verify model calculations based on a fundamental approach for the dilute ternary system, Fe-C-Mn. The austenite-to-ferrite transformation start temperature can be predicted from a nucleation model for slow cooling rates and small austenite grain sizes, where ferrite nucleates at austenite grain corners. The nuclei are assumed to have an equilibrium composition and a pillbox shape in accordance with minimal interfacial energy. For higher cooling rates or larger austenite grain sizes, early growth has to be taken into account to describe the transformation start, and nucleation is also encouraged at the remaining sites of the austenite grain boundaries. In contrast to nucleation, growth of the ferrite is characterized by paraequilibrium;i.e., only carbon can redistribute, whereas the diffusion of Mn is too slow to allow full equilibrium in the ternary system. However, Mn segregation to the moving ferrite-austenite interface has to be considered. The latter, in turn, exerts a solute draglike effect on the boundary movement. Thus, growth kinetics are controlled by carbon diffusion in austenite modified by interfacial segregation of Mn. Employing a phenomenological segregation model, good agreement has been achieved with the measurements.

Overview of Geometric Effects on Coarsening of Mushy Zones

Abstract: An overview is presented of the thermodynamic, kinetic, statistical, and geometric factors that govern phase coarsening in dendritic mushy zones. The coarsening behavior of such systems is best quantified through the kinetics of the decay rate of the specific surface area,S v. The geometry of the complex solid-melt interfaces comprising a mushy zone is described statistically as an areal distribution of local curvature parameters. These parameters capture both the intensive and extensive thermodynamic characteristics of the mushy zone. The effects of local interface shape, negative mean, and Gaussian curvatures and the appearance of inactive lengthscales on the coarsening kinetics of dendritic structures are discussed. The combined contribution of all these geometrical effects yields global coarsening rates for ramified mushy zones that are comparable to those predicted from theory for a collection of spherical particles having the identical volume fraction of solid.

Austenite decomposition during continuous cooling of an HSLA-80 plate steel

Abstract: Decomposition of fine-grained austenite (10-µm grain size) during continuous cooling of an HSLA-80 plate steel (containing 0.05C, 0.50Mn, 1.12Cu, 0.88Ni, 0.71Cr, and 0.20Mo) was evaluated by dilatometric measurements, light microscopy, scanning electron microscopy, transmission electron microscopy, and microhardness testing. Between 750 °C and 600 °C, austenite transforms primarily to polygonal ferrite over a wide range of cooling rates, and Widmanstatten ferrite sideplates frequently evolve from these crystals. Carbon-enriched islands of austenite transform to a complex mixture of granular ferrite, acicular ferrite, and martensite (all with some degree of retained austenite) at cooling rates greater than approximately 5 °C/s. Granular and acicular ferrite form at temperatures slightly below those at which polygonal and Widmanstatten ferrite form. At cooling rates less than approximately 5 °C/s, regions of carbon-enriched austenite transform to a complex mixture of upper bainite, lower bainite, and martensite (plus retained austenite) at temperatures which are over 100 °C lower than those at which polygonal and Widmanstatten ferrite form. Interphase precipitates of copper form only in association with polygonal and Widmanstatten ferrite. Kinetic and microstruc-tural differences between Widmanstatten ferrite, acicular ferrite, and bainite (both upper and lower) suggest different origins and/or mechanisms of formation for these morphologically similar austenite transformation products.