Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 1993"

A Multiphase Solute Diffusion Model for Dendritic Alloy Solidification

Abstract: A solute diffusion model, aimed at predicting microstructure formation in metal castings, is proposed for dendritic solidification of alloys. The model accounts for the different length scales existing in a dendritic structure. This is accomplished by utilizing a multiphase approach, in which not only the various physical phases but also phases associated with different length scales are considered separately. The macroscopic conservation equations are derived for each phase using the volume averaging technique, with constitutive relations developed for the interfacial transfer terms. It is shown that the multiphase model can rigorously incorporate the growth of dendrite tips and coarsening of dendrite arms. In addition, the distinction of different length scales enables the inclusion of realistic descriptions of the dendrite topology and relations to key metallurgical parameters. Another novel aspect of the model is that a single set of conservation equations for solute diffusion is developed for both equiaxed and columnar dendritic solidification. Finally, illustrative calculations for equiaxed, columnar, and mixed columnar-equiaxed solidification are carried out to provide quantitative comparisons with previous studies, and a variety of fundamental phenomena such as recalescence, dendrite tip undercooling, and columnar-to-equiaxed transition (CET) are predicted.

Study of the mechanism of grain refinement of aluminum after additions of Ti- and B-containing master alloys

Abstract: A highly sensitive thermal analysis technique has been used to study the mechanisms of grain refinement in high-purity aluminum. Additions of Al-Ti-B master alloys were made both below and above the peritectic concentration in reference to the Al-rich corner of the binary Al-Ti phase diagram (0.15 pct Ti in solution). The experiments were conducted at various times after the addition of grain refiner. From the results, except for formation of TiB2, no effect of boron on the Al-rich portion of the binary Al-Ti phase diagram can be observed. With hypoperitectic additions of Al-Ti-B master alloys, TiB2 particles are the most frequent nucleant for aluminum grains. Also, when Al-5Ti-lB additions are made, nucleation frequently occurs above the equilibrium liquidus temperature. From a thermodynamic point of view, this phenomenon can occur only if regions of the melt (which contain bondes and nucleate new grains) have a higher Ti concentration than is present in the bulk of the liquid. A mechanism has been proposed to account for this observation. When hyperperitectic additions of grain refiner were made, a metastable formation of Al solid was often observed to occur at 2 to 5 deg above the equilibrium peritectic temperature. Other researchers have made this observation and proposed that a metastable aluminide phase was formed, even though no X-ray evidence of this phase was found. The experiments reported here show that the metastable nucleation occurs on boride particles when cooling from high temperature, which allow high (metastable) quantities of dissolved Ti to be retained in portions of the melt.

Effects of heat treatment and reinforcement size

Abstract: The effects of heat-treatment, matrix microstructure, and reinforcement size on the evolution of damage, in the form of SiCp cracking, during uniaxial tension testing of an aluminum-alloy based composite have been determined. A powder metallurgy Al-Zn-Mg-Cu alloy reinforced with 15 vol pct of either 5 or 13 μm average size SiCp was heat treated to solution annealed (SA), underaged (UA), and overaged (OA) conditions. The SA treatment exhibited lower yield strength and higher ductility for both particulate sizes compared to the UA and OA conditions. The evolution of damage, in the form of SiCp fracture, was monitored quantitatively using metallography and changes in modulus on sequentially strained specimens. It is shown that the evolution of SiCp fracture is very dependent on particulate size, matrix aging condition, and the details of the matrix-reinforcement interfacial regions. SiCp fracture was exhibited by the UA and OA treatment over a range of strains, while a preference for failure near the SiCp/matrix interfaces and in the matrix was exhibited in the OA material. While thepercentage of cracked SiCp at each global strain typically was equal or somewhat lower in the material reinforced with 5 μm average size SiCp, theabsolute number of cracked SiCp was always higher at each global stress and strain in the material containing 5 μm average size SiCp, for each heat treatment. Damage(e.g., voids) in the matrix and near the SiCp/matrix interfaces was additionally observed, although its extent was highly matrix and particle-size dependent. It was always observed that increases in stress (and strain) produced a larger amount of fractured SiCp. However, neither a global stress-based nor a global strain-based model was sufficient in converging the amount of SiCp fractured for all heat treatments and particle sizes tested.

Particulate fracture during deformation of a spray formed metal-matrix composite

Abstract: The mechanisms of deformation and failure in a 2618 Al alloy reinforced with 15 vol pct SiC particilates were studied and compared with those of the unreinforced alloy, processed by spray forming as well. Tensile and fracture toughness tests were carried out on naturally aged and peak-aged specimens. The broken specimens were sliced through the middle, and the geometric features of fractured and intact particulates were measured. The experimental observations led to the conclusion that failure took place by the progressive fracture of the particulates until a critical volume fraction was reached. An influence of the particulate size and aspect ratio on the probability of fracture was found, the large and elongated particulates being more prone to fail, and the fracture stress in the particulates seemed to obey the Weibull statistics. The dif- ferences in ductility found between the naturally aged and peak-aged composites were explained in terms of the number of broken particulates as a function of the applied strain. Numerical simulations of the deformation process indicated that the stresses acting on the particulates are higher in the peak-aged material, precipitating the specimen failure. Moreover, the compressive residual stresses induced on the SiC during water quenching delayed the onset of particulate breakage in the naturally aged material.

Effect of deformation parameters on the

Abstract: The recrystallization behavior of three Nb-bearing high-strength low-alloy (HSLA) steels was investigated during multipass deformation under continuous cooling conditions. The niobium concentrations of these steels varied from 0.05 to 0.09 wt pct. The specimens were tested on a computerized torsion machine using a simulation schedule of 17 passes. Deformation tem-peratures of 1180 °C to 700 °C were employed, together with pass strains of 0.1 to 0.7, strain rates of 0.2 to 10 s-1, and interpass times of 5 to 200 seconds. By means of mean flow stressvs 1000/T diagrams, the effect of the deformation conditions on the no-recrystallization tem-perature (T nr ), the temperature at which recrystallization is no longer complete, was determined. It decreases with increasing strain and also decreases slightly with increasing strain rate. There is aT r minimum at times of about 12∼15 seconds, and both increases and decreases from this value raise this characteristic temperature. When the interpass times are short, solute atoms control the rate of recrystallization, the extent of which decreases as the time is decreased. When the interpass times are long, precipitation takes place and retards recrystallization, so that the extent of softening decreases.

Effect of changing strain paths on

Abstract: The effect of changing strain paths on forming limits of aluminum alloy 2008-T4 has been investigated by determining forming limit diagrams (FLDs) after prestraining. Sheets were pre- strained to several levels in uniaxial, biaxial, and plane-strain tension parallel and perpendicular to the prior rolling direction (RD). Abrupt changes in the strain path can be used to increase the forming limits. Prestraining in biaxial tension generally lowers the forming limits for the entire FLD, whereas prestraining in uniaxial tension raises the limits on the right-hand side of the FLD without much effect on the left-hand side. Prestraining in plane-strain tension raises both sides away from the minimum. Finally, it was found that after prestraining, the amount of the additional plane-strain deformation possible depends on the effective strain during prestrain.

Microstructural evolution in nickel during rolling from intermediate to large strains

Abstract: High-purity nickel (99. 99 pct) with a grain size of 80 to 100 µm was deformed by cold-rolling from 37 to 98 pct reductions (von Mises effective strains ofevm = 0. 5 to 4. 5). The deformation microstructures and texture at five strain levels were observed and characterized using transmission electron microscopy (TEM) and neutron diffraction. The microstructures evolved within a framework common to medium and high stacking fault energy fee polycrystals. This framework consists of structural subdivision by higher angle boundaries (geometrically necessary boundaries) at one volume scale and at a smaller volume scale by lower angle cell boundaries (incidental boundaries) for all strain levels. We have characterized the dislocation boundaries, including dense dislocation walls (DDWs), microbands (MBs), and lamellar boundaries (LBs) in terms of crystallographic and macroscopic orientations, morphology, and frequency of occurrence. The microstructural evolution is discussed with special emphasis on factors that contribute to the transition from structures characteristic of small and medium strain microstructures to those characteristic of large strain microstructures.

Phase chemistry and precipitation reactions in maraging steels: Part I. Introduction and study of Co–containing C–300 steel

Abstract: This article introduces a series of studies of phase transformations in maraging steels. Atom-probe field-ion microscopy (APFIM) was the main research technique employed. Hardness measurements, transmission electron microscopy (TEM), and thermochemical calculations were also used. The composition and morphology of precipitates in the commercial-grade C-300 steel were compared for different aging times at 510 °C to investigate the aging sequence. Both Ni3Ti and Fe7Mo6 were found to contribute to age hardening. The decomposition starts with the formation of small Mo-enriched Ni3Ti particles at very short aging times. The Fe7Mo6 phase forms at a later stage of aging. The matrix concentrations of both Ti and Mo were measured and were found to be low after standard aging conditions. The observation of the Fe7Mo6 μ phase is supported by thermochemical calculations. Austenite reversion has been found at the aging temperature, and its composition approaches the predicted equilibrium composition after 8 hours of aging.

Calculations of forming limit

Abstract: The method of calculating the shape of forming limit diagrams (FLDs) using a high-exponent yield criterion with the Marciniak and Kuczynski (M-K) analysis has been extended to include the effects of changing the strain paths and applied to aluminum alloy 2008 T4. Calculations incorporating abrupt path changes agreed with the general trends found experimentally. If the first stage of strain is under biaxial tension, the subsequent FLD shifts to the right and down with respect to the original FLD, whereas it shifts to the left and up when the first stage of strain is in uniaxial tension. Calculations introducing gradual strain-path changes, characteristic of stretching over a hemispherical dome, predict that the minimum of the FLD shifts to the right.

Transmission electron microscopy examination of hardening and toughening phenomena in Aermet 100

Abstract: The effect of tempering on the microstructure and mechanical properties of ultrahigh strength Aermet 100 steel was examined. In the as-quenched condition, the steel contained a dispersion of relatively fine, undissolved, (CrTiFeMo)C and (CrFeMo)23C6 carbides in a martensitic matrix. Upon tempering at 427 °C, the martensite decomposed to form a high density of cementite particles concomitant with a significant drop in toughness. Tempering at 454 °C resulted in peak strength (yield strength ∼ 1756 MPa) due to the precipitation of coherent zones of fine carbides. The peak in toughness (170 MPa√m), attained at a tempering temperature of 482 °C, was attributed to both the absence of cementite and the formation of reverted, stable austenite. Tempering at higher temperatures resulted in loss of both strength and toughness, which was suggested to be the result of precipitate coarsening and formation of unstable austenite, respectively. The details of the electron microscopy studies and mechanism of strengthening and toughening are discussed in light of the current understanding of this subject.

A theoretical evaluation of crack closure

Abstract: Premature crack closure has been considered an important factor affecting the applied driving force under cyclic load. Among several factors that induce crack closure, plasticity and oxidation or corrosion have been recognized as the most significant. An analytical estimation of both is made using dislocation theory. The analysis indicates that (a) plasticity originating from crack tip does not induce crack closure and (b) closure arising from asperity ridges due to oxides, corrosion products or surface roughness is small and insignificant unless crack is completely packed with asperities.

Toughening mechanisms in titanium aluminides

Abstract: The relevant toughening mechanisms in two-phase titanium aluminides are reviewed in order to elucidate microstructure/fracture toughness relationships. Both intrinsic and extrinsic toughening mechanisms are present in Ti3Al- and TiAl-base alloys. The former affects the initiation toughness(i.e., KIC value) at the onset of crack extension, while the latter leads to crack growth toughness by instigating a resistance-curve behavior. Intrinsic toughening arises from matrix slip and ductile-phase blunting. In contrast, extrinsic toughening originates from crack deflection, ductile-phase bridging, shear ligament toughening, microcrack shielding, twin toughening, and the growing crack singularity. The influence of microstructure on toughening mechanisms in two-phase Ti3Al- and TiAl-base alloys is discussed, with particular emphasis on the need to control the microstructure in order to achieve the desired mechanical properties.

Stress distribution in particulate-reinforced metal-matrix composites subjected to external load

Abstract: Finite element calculations were carried out to study the stress distribution in particulate SiC-reinforced Al metal-matrix composites (MMCs) subjected to external load. The results showed that, in addition to the effect of the aspect ratio of the reinforcement, particle distribution and grouping behavior-clustering, resulting from the casting process in the present materials, also contribute considerably to the nonuniform stress distribution in the material. The triaxial stress state around a particle or inside a particle cluster may change the von Mises stress and result in plastic deformation features, such as strain localization. Furthermore, the relative orientation of the particle distribution to the external load was found to be important to the stress distribution. As the applied stress increases, the state of triaxial stress inside a cluster appears to promote the early particle cracking, interface debonding, and void formation in the ductile matrix. Experimental observations of slip band distribution and other features in compression tests are consistent with the calculated results.

Phase chemistry and precipitation reactions in maraging steels: Part IV. Discussion and conclusions

Abstract: This article summarizes our studies of phase chemistry and precipitation reactions in a variety of maraging steels. The roles of different phases and alloying elements are investigated by comparing the behavior of different steels. The phases considered are Ni3Ti, Fe7Mo6 μ phase, Fe2Mo Laves phase, ω phase, Ti6Si7Ni16 G phase, “Z phase,” austenite, and α matrix. The alloying elements discussed are Ti, AI, Mo, Si, Mn, Ni, Cr, and Co. By comparing the aging behavior of both commercial steels and model alloys, a major role of Co is confirmed to be the lowering of the matrix solubility of Mo. Of the two main hardening elements in maraging steels (namely, Ti and Mo), Ti is much more active than Mo in the very early stage of precipitation. The main Mo-rich precipitate found in this work was Fe7Mo6μ phase instead of Laves phase. The precipitation of Mo is modified by the presence of Ti. ω phase appears only in Ti-free alloys, especially when aged at a low temperature. The quantity of Ni-containing precipitates and the presence of Cr in the steels change the austenite reversion behavior. Other phases, such as G phase and “Z phase,” contribute to age hardening in different types of maraging alloys.

Phase equilibria effects on the enhanced liquid phase sintering of tungsten-copper

Abstract: The sintering behavior and mechanical properties of W-Cu are improved by the addition of elements that have solubility for W,e.g., Co, Ni, Fe, and Pd. The degree of enhancement with small concentrations of additive is dependent on specific phase diagram features, and the ranking of effectiveness does not follow the trend observed for the activated solid-state sintering of W. These observations are explained through a combination of liquid phase sintering and activated sintering theories that considers the combined W, Cu, and activator phase equilibria effects. In small concentrations, Ni and Pd have little effect on densification because they go into solution with Cu, resulting in only a slight increase in the solubility of W in the liquid phase. In this case, the sintered density, strength, and hardness increase with increasing additive concentration due to enhanced densification through solution-reprecipitation. Cobalt and Fe are the most ef-fective activators due to their limited solubility in Cu and the formation of a stable intermetallic phase with W at the sintering temperature. This promotes the formation of a high-diffusivity interboundary layer which enhances solid-state sintering of the tungsten grains at temperatures at which a liquid phase is present. With Co and Fe additions, the sintered density, strength, and hardness peak with activator concentrations of 0.35 to 0.5 wt pct. An evaluation of models for activated solid-state sintering and liquid phase sintering indicates a substantial solid-state contribution to densification when a high-diffusivity interboundary layer is present and the sol-ubility of W in the liquid phase is small.

A model for the graphite formation in

Abstract: Part I of this investigation deals with the inoculation mechanisms in ductile cast iron, with particular emphasis on the theoretical aspects of heterogeneous nucleation of graphite at inclusions. It is shown that the majority of the inclusions in ductile cast iron are primary or secondary products of the magnesium treatment(e.g., MgS, CaS, MgOSiO{ni2}, and 2MgO-SiO2). After inoculation with (X,Al)-containing ferrosilicon (X denotes Ca, Sr, or Ba), hexagonal silicate phases of the XO-SiO2 or the XO-Al2O3-2SiO2 type form at the surface of the oxide inclusions, probably through an exchange reaction with MgO. The presence of these phases, will enhance the nucleation potency of the inclusions with respect to graphite. In particular, the (001) basal planes of the crystals are favorable sites for graphite nucleation, since these facets allow for the development of coherent/semicoherent low-energy interfaces between the substrate and the nucleus. In contrast, the fading of inoculation can be explained by a general coarsening of the inclusion population with time, which reduces the total number of catalyst particles for graphite in the melt. A theoretical analysis of the reaction kinetics gives results which are in close agreement with experimental observations.

Modeling of solute redistribution in the mushy zone during solidification of aluminum-copper alloys

Abstract: A mathematical model has been established to predict the formation of macrosegregation for a unidirectional solidification of aluminum-copper alloys cooled from the bottom. The model, based on the continuum formulation, allows the calculation of transient distributions of temperature, velocity, and species in the solidifying alloy caused by thermosolutal convection and shrinkage-induced fluid flow. Positive segregation in the casting near the bottom (inverse segregation) is found, which is accompanied by a moving negative-segregated mushy zone. The effects of shrinkage-induced fluid flow and solute diffusion on the formation of macrosegregation are examined. It is found that the redistribution of solute in the solidifying alloy is caused by the flow of solute-rich liquid in the mushy zone due to solidification shrinkage. A higher heat-extraction rate at the bottom increases the solidification rate, decreasing the size of the mushy zone, reducing the flow of solute-rich liquid in the mushy zone and, as a result, lessening the severity of inverse segregation. Comparisons between the theoretical predictions from the present study and previous modeling results and available experimental data are made, and good agreements are obtained.

Micromechanisms of brittle fracture

Abstract: Mechanical processes operating in materials on the scale of the microstructure have come to be called “micromechanisms.” The fundamental science and the micromechanisms of brittle fracture are reviewed here, with particular emphasis on cleavage and intergranular fracture. Extant micromechanisms for these fracture types are evaluated. The role of solutes, particularly in intergranular fracture, is also discussed in terms of the fundamentals of brittle fracture.

Atomic mechanisms of diffusional nucleation and growth and comparisons with their counterparts in shear transformations

Abstract: An integrated overview is presented of a viewpoint on the present understanding of nucleation and growth mechanisms in both diffusional and shear (martensitic) transformations. Special emphasis is placed on the roles played by the anisotropy of interphase boundary structure and energy and also upon elastic shear strain energy in both types of transformation. Even though diffusional nucleation is based on random statistical fluctuations, use of the time reversal principle shows that interfacial energy anisotropy leads to accurately reproducible orientation relationships and hence to partially or fully coherent boundaries, even when nucleation at a grain boundary requires an irrational orientation relationship to obtain. Since the fully coherent boundary areas separating most linear misfit compensating defects are wholly immobile during diffusional growth because of the improbability of moving substitutional atoms even temporarily into interstitial sites under conditions normally encountered, partially and fully coherent interphase boundaries should be immovable without the intervention of growth ledges. These ledges, however, must be heavily kinked and usually irregular in both spacing and path if they, too, are not to be similarly trapped. On the other hand, the large shear strain energy usually associated with martensite requires that its formation be initiated through a process which avoids the activation barrier associated with nucleation, perhaps by the Olson-Cohen matrix dislocation rearrangement mechanism. During growth, certain ledges on martensite plates serve as transformation dislocations and perform the crystal structure change (Bain strain). However, the terraces between these ledges in martensite (unlike those present during diffusional growth) are also mobile during non-fcc/hcp transformations; glissile dislocations on these terraces perform the lattice invariant deformation. Growth ledges operative during both diffusional and shear growth probably migrate by means of kink mechanisms. However, diffusional kinks appear to be nonconservative and sessile (and therefore resist immediate transmission of elastic shear strain energy), whereas those associated with martensitic growth must be conservative and glissile (and fully transmit such strain energy). The broad faces of both diffusionally and martensitically formed plates contain an invariant line, as emphasized by Dahmen and Weatherly. However, in the diffusional case, minimization of growth ledge formation kinetics seems to be the main role thereby played, whereas in martensitic growth, the main purpose of such an interface is to minimize elastic shear strain energy. The latter minimization requires that martensite forms as plates (or perhaps as laths) enclosed by a pair of invariant line-containing interfaces. During diffusional transformations, on the other hand, other interfaces at which growth ledge formation kinetics are not too much faster than those at the invariant line interface can also comprise a significant portion of the interfacial area, thereby leading to the formation of other, quite different morphologies, such as intragranular idiomorphs and grain boundary allotriomorphs. Critical problems remaining unsolved in diffusional transformations include calculation of critical nucleus shapes when the crystal structures of the two phases are significantly different, highly accurate calculation of the energies of the interphase boundaries thus formed, and direct observation of atomic scale kinks on the risers of growth ledges by means of a yet-to-be-invented three-dimensional (3-D) atomic-resolution form of transmission electron microscopy. Experimental identification and characterization of transformation dislocations and experimental testing of “nucleation” mechanisms are now of special importance in fundamental studies of martensitic transformations.

The kinetics of composite particle formation during mechanical alloying

Abstract: The kinetics of composite particle formation during attritor milling of insoluble binary elemental powders have been examined The effects of processing conditions (ie, mill power, temperature, and charge ratio) on these kinetics were studied Particle size distributions and fractions of elemental and composite particles were determined as functions of milling time and processing conditions This allowed the deduction of phenomenological rate constants describing the propensity for fracture and welding during processing For the mill-operating conditions investigated, the number of particles in the mill generally decreased with milling time, indicating a greater tendency for particle welding than fracture Moreover, a bimodal size distribution is often obtained as a result of preferential welding Copper and chromium 'alloy' primarily by encapsulation of Cr particles within Cu This form of alloying also occurs in Cu-Nb alloys processed at low mill power and/or for short milling times For other conditions, however, Cu-Nb alloys develop a lamellar morphology characteristic of mechanically alloyed two-phase ductile metals Increasing mill power or charge (ball-to-powder weight) ratio (CR) increases the rate of composite particle formation

Three-dimensional probabilistic simulation of solidification grain structures: Application to superalloy precision castings

Abstract: A two-dimensional (2-D) probabilistic model, previously developed for the prediction of microstructure formation in solidification processes, is applied to thin section superalloy precision castings. Based upon an assumption of uniform temperature across the section of the plate, the model takes into account the heterogeneous nucleation which might occur at the mold wall and in the bulk of the liquid. The location and crystallographic orientation of newly nucleated grains are chosen randomly among a large number of sites and equiprobable orientation classes, respectively. The growth of the dendritic grains is modeled by using a cellular automaton technique and by considering the growth kinetics of the dendrite tips. The computed 2-D grain structures are compared with micrographie cross sections of specimens of various thicknesses. It is shown that the 2-D approach is able to predict the transition from columnar to equiaxed grains. However, in a transverse section, the grain morphology within the columnar zone differs from that of the experimental micrographs. For this reason, a three-dimensional (3-D) extension of this model is proposed, in which the modeling of the grain growth is simplified. It assumes that each dendritic grain is an octaedron whose half-diagonals, corresponding to the crystallographic orientations of the grain, are simply given by the integral, from the time of nucleation to that of observation, of the velocity of the dendrite tips. All the liquid cells falling within a given octaedron solidify with the same crystallographic orientation of the parent nucleus. It is shown that the grain structures computed with this 3-D model are much closer to the experimental micrographie cross sections.

The dendritic growth of γ' precipitates and grain

Abstract: The growth pattern of γ precipitates in the grains and at the grain boundaries has been investigated in a Ni-24Co-4Al-4Ti-5Cr-5Mo (weight percent) alloy of very small lattice misfit between the precipitate and the matrix phases under varying heat-treatment conditions. When aged at temperatures lower than the solvus temperature (T s = 1150 °C) by more than 30 °C after direct cooling from the solution-treatment temperature, the nucleation density is high. In this condition, the supersaturation is quickly removed because of the overlapping diffusion fields and the precipitates undergo Ostwald ripening from the early stage. The precipitates then have an equilibrium shape of spheres in the grains and truncated spheres at nearly straight grain boundaries. The precipitates at the grain boundaries are coherent with one of the grains, and their number density is not much larger than that in the grains, apparently because of a large contact angle (about 150 deg) with the grain boundary. Quenching the alloy after the solution treatment and aging at any temperature also produce high precipitate number density and equilibrium shapes. When aged at temperatures just belowT s (above 1140 °C), the nucleation density is low, the precipitates grow dendritically in the grains, and the grain boundaries become serrated. The observed dendritic growth characteristics do not quantitatively agree with the predictions of Mullins and Sekerka theory, but the discrepancy may be due to the uncertainties in both the observed and calculated quantities. By deeply etching the matrix, it is shown that the grain boundary serration is produced by the precipitates growing preferentially in the direction of the incoherent boundary because of the rapid solute diffusion along the grain boundary. The dendritic growth and grain boundary serration can be obtained also by slowly cooling through the temperature range just belowT s.

A model for the graphite formation in ductile

Abstract: In the present investigation, mathematical models have been developed to quantify the extent of carbon diffusion occurring in ductile cast iron during cooling from the eutectic temperature. Computer calculations show that small variations in the cooling conditions may significantly alter the number density and size distribution of graphite nodules in the iron matrix, in agreement with experimental observations. This makes it difficult to compare microstructure data from various section size materials without allowing for differences in the kinetic strength of the thermal cycles with respect to carbon diffusion.

New explosive welding technique to weld

Abstract: Various aluminum alloys and stainless steel were explosively welded using a thin stainless steel intermediate plate inserted between the aluminum alloy driver and stainless steel base plates. At first, the velocity change of the driver plate with flying distance is calculated using finite- difference analysis. Since the kinetic energy lost by collision affects the amount of the fused layer generated at the interface between the aluminum alloy and stainless steel, the use of a thin stainless steel intermediate plate is effective for decreasing the energy dissipated by the collision. The interfacial zone at the welded interface is composed of a fine eutectic structure of aluminum and Fe4Al13, and the explosive welding process of this metal combination proceeds mainly by intensive deformation of the aluminum alloy. The weldable region for various aluminum alloys is decided by the change in collision velocity and kinetic energy lost by collision, and the weldable region is decreased with the increase in the strength of the aluminum alloy.

Determination of heat of transformation in a cold-rolled martensitic TiNi alloy

Abstract: We studied the heat of transformation, ΔH, in martensitic transformations in a cold-rolled equiatomic TiNi alloy with differential scanning calorimetry (DSC), X-ray diffraction (XRD), and microhardness measurements. Results of our experiment indicate that the martensite stabilization and stress-induced parent (SIP)B2 phase are introduced when the TiNi martensite is cold rolled at room temperature. The SIP formation seems to be related to the lattice softening phenomenon occurring in the martensite, while the ΔH value of the first reverse martensitic transformation decreases enormously for the cold-rolled equiatomic TiNi alloy. We are proposing possible explanations for these results: (1) the occurrence of SIP, which reduces the transformable martensite volume; (2) the release of accumulated elastic energy induced by the cold rolling; and (3) the recovery of defects induced by cold rolling and release of the heat of recovery. We also found that the retained dislocations can depress the martensitic transformation temperatures and induce the R-phase transformation after the occurrence of the first reverse martensitic transformation.

The influence of matrix microstructure and particle reinforcement on the creep behavior of 2219 aluminum

Abstract: The influence of matrix microstructure and reinforcement with 15 vol pct of TiC particles on the creep behavior of 2219 aluminum has been examined in the temperature range of 150 ‡C to 250 ‡C. At 150 ‡C, reinforcement led to an improvement in creep resistance, while at 250 ‡C, both materials exhibited essentially identical creep behavior. Precipitate spacing in the matrix exerted the predominant influence on minimum creep rate in both the unreinforced and the reinforced materials over the temperature range studied. This behavior and the high-stress dependence of minimum creep rate are explained using existing constant structure models where, in the present study, precipitate spacing is identified as the pertinent substructure dimension. A modest microstructure-independent strengthening from particle reinforcement was observed at 150 ‡C and was accurately modeled by existing continuum mechanical models. The absence of reinforcement creep strengthening at 250 ‡C can be attributed to diffusional relaxation processes at the higher temperature.

An analytical model for solute redistribution during solidification of planar, columnar, or equiaxed morphology

Abstract: Existing models for solute redistribution (microsegregation) during solidification were reviewed. There are no analytical models that take into account limited diffusion in both the liquid and the solid phases. A new analytical mathematical model for solute redistribution was developed. Diffusion in liquid and in solid was considered. This model does not require a prescribed movement of the interface. It can be used for one-dimensional (1-D) (plate), two-dimensional (cylinder), or three-dimensional (3-D) (sphere) calculations. Thus, it is possible to calculate microsegregation at the level of primary or secondary arm spacing for columnar dendrites or for equiaxed dendrites. The solution was compared with calculations based on existing models, as well as with some available experimental data for the segregation of base elements in as cast Al-4. 9 wt pct Cu, INCONEL 718, 625, and plain carbon (0. 13 wt pct C) steel.

Residual stresses and their effects on deformation

Abstract: Residual stresses induced by thermal expansion mismatch in metal-matrix composites are studied by three-dimensional (3-D) elastic-plastic finite element analyses. Typically, the stress-free state is 150 to 300 K above room temperature. The coefficient of thermal expansion of the matrix is 3 to 5 times larger than that of the ceramic inclusion, resulting in compressive stresses of order 200 MPa in the inclusions. Both compressive and tensile stresses can be found in the matrix. Since the stress may exceed the matrix yield strength near the particles, plastic flow occurs. The authors find a significant influence of this flow on the elastic and plastic properties of the composite. The calculated residual strains in TiC particles due to thermal expansion mismatch and external loads compare well with recent neutron diffraction experiments (Bourkeet al.) The present work is the first reported three-dimensional analysis of spherical inclusions in different arrays (simple cubic (sc) and face-centered cubic (fcc)) that permit a study of particle interactions.

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

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