Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 1991"

Prediction of steel flow stresses at high temperatures and strain rates

Abstract: The flow behavior of steels during deformation in the roll gap was simulated by means of single hit compression tests performed in the temperature range 800 °C to 1200 °C. Strain rates of 0.2 to 50 s−1 were employed on selected low-carbon steels containing various combinations of niobium, boron, and copper. The stress/strain curves determined at the higher strain rates were corrected for deformation heating so that constitutive equations pertaining to idealized isothermal conditions could be formulated. When dynamic recovery is the only softening mechanism, these involve a rate equation, consisting of a hyperbolic sine law, and an evolution equation with one internal variable, the latter being the dislocation density. When dynamic recrystallization takes place, the incorporation of the fractional softening by dynamic recrystallization in the evolution equation makes it possible to predict the flow stress after the peak. These expressions can be employed in computer models for on-line gage control during hot-rolling.

Strength and ductile-phase toughening in the two-phase Nb/Nb 5 Si 3 alloys

Abstract: The effect of heat treatment on the mechanical properties of Nb-Nb5-Si3 two-phase alloys having compositions Nb-10 and 16 pct Si (compositions quoted in atomic percent) has been investigated. This includes an evaluation of the strength, ductility, and toughness of as-cast and hot-extruded product forms. The two phases are thermochemically stable up to ∼1670 °C, exhibit little coarsening up to 1500 °C, and are amenable to microstructural variations, which include changes in morphology and size. The measured mechanical properties and fractographic analysis indicate that in the extruded condition, the terminal Nb phase can provide significant toughening of the intermetallic Nb5Si3 matrix by plastic-stretching, interface-debonding, and crack-bridging mechanisms. It has been further shown that in these alloys, a high level of strength is retained up to 1400 °C.

An Assessment of the Fe-C-Si System

Abstract: Precise knowledge of the Fe-C-Si system has been the subject of numerous previous studies because of the importance of this system for steelmaking processes and cast iron foundry. How-ever, the most recent articles on this subject still reveal uncertainties or insufficient information. The purpose of this work was to assess this system and to give it a precise description, but one which was simple enough for practical applications. An evaluation of the parameters describing the thermodynamic properties of the phases involved in the binary Fe-Si system has been achieved, mainly based on values of the silicon activity in the body-centered cubic (bcc) phase and on knowledge of the phase diagram. The A2-B2 ordering reaction of the bcc phase has been included. It has been shown that this reaction is of paramount importance for the description of the thermodynamic properties of this phase and its field of stability. This preliminary work and previous similar studies of the Fe-C and Si-C systems were used as a base for the evaluation of the ternary stable and metastable Fe-C-Si systems. Optimization of the parameters describing the properties of the phases involved in both of these systems was achieved using experimental thermodynamic data and the critical assessment of the phase diagrams.

Mechanical Properties and Retained Austenite in Intercritically Heat-Treated Bainite-Transformed Steel and Their Variation with Si and Mn Additions

Abstract: Processing peculiarities and functions of alloying elements, such as Si and Mn, were studied for improving formability of steel sheets with mixed microstructures. Annealing a sheet steel with 0.2 pct C in the intercritical range produced very fine particles of retained austenite which were moderately stabilized due to C enrichment by subsequent holding in the bainite transformation range. Its strength-ductility balance is greatly superior to that of other dual-phase steels due to transformation-induced plasticity (TRIP). The holding time in the bainite transformation range varies with temperature, depending on the activation energy of C diffusion in austenite, and shifts to longer times with an increase of Si or Mn additions. The optimum cooling rate from the intercritical region is reduced with an increase of Mn content but is not influenced by Si content. Additional Mn makes the retained austenite content larger, although uniform elongation remains the same. In this case, the product of tensile strength and total elongation is increased due to an increase in the tensile strength. Contrary to Mn, Si does not affect retained austenite content but improves the uniform elongation by increasing its stability.

Plastic deformation and fracture of binary TiAl-base alloys

Abstract: The mechanical behavior of binary TiAl alloys containing 46 to 60 at. pct Al has been studied in bulk materials preparedvia rapid solidification processing. Bending and tensile tests were carried out at room temperature as a function of Al concentration. A few alloys were also tested from liquid nitrogen temperature to ∼ 1000°C. Deformation substructures were studied by analytical transmission electron microscopy and fracture modes by scanning electron microscopy (SEM). It was found that both microstructure and composition strongly affect the mechanical behavior of TiAl-base alloys. A duplex structure, which contains both primary y grains and transformedγ/α 2 lamellar grains, is more deformable than a single-phase or a fully transformed structure. The highest plasticities are observed in duplex alloys containing 48–50 at. pct Al after heat treatment in the center of theγ + α phase field. The deformation of these duplex alloys is facilitated by 1/2[110] slip and {111} twinning, but very limited superdislocation slip occurs. The twin deformation is suggested to result from a lowered stacking fault energy due to oxygen depletion or an intrinsic change in chemical bonding. Other factors, such as grain size and grain boundary chemistry and structure, are important from a fracture point of view. The results on the deformation and fracture modes as a function of test temperature are also discussed.

Ductility enhancement in NiAl (B2)-base alloys by microstructural control

Abstract: An attempt to improve ductility of NiAl (B2)-base alloys has been made by the addition of alloying elements and the control of microstructure. It has been found that a small amount of fccγ phase formed by the addition of Fe, Co, and Cr has a drastic effect not only on the hot workability but also on the tensile ductility at room temperature. The enhancement in ductility is mainly due to the modification of Β-phase grains by the coexistence ofγ phase. The effect of alloying elements on the hot forming ability is strongly related to the phase equilibria and partition behavior amongγ,γ′ (L12 structure), and Β phases in the Ni-Al-X alloy systems. The ductility-enhancement method shows promise for expanding the practical application of nickel aluminide.

Microstructural evolution of modified 9Cr-1Mo steel

Abstract: The tempering and subsequent annealing of modified 9Cr-lMo steel have been investigated to determine the influence of trace amounts of V and Nb on the sequence of precipitation processes and to identify the basis for the enhanced high-temperature strength compared to the standard 9Cr-lMo composition. Air cooling (normalizing) from 1045 °C results in the precipitation of fine (Fe, Cr)3C particles within the martensite laths. Additional carbide precipitation and changes in the dislocation structure occur during the tempering of martensite at 700 °C and 760 °C after normalizing. The precipitation of M23C6 carbides occurs preferentially at lath interfaces and dislocations. The formation of Cr2C was detected during the first hour of tempering over the range of 650 °C to 760 °C but was replaced by V4C3 within 1 hour at 760 °C. During prolonged annealing at 550 °C to 650 °C, following tempering, the lath morphology remains relatively stable; partitioning of the laths into subgrains and some carbide coarsening are evident after 400 hours of annealing at 650 °C, but the lath morphology persists. The enhanced martensite lath stability is attributed primarily to the V4C3 precipitates distributed along the lath interfaces and is suggested as the basis for the improved performance of the modified 9Cr-lMo alloy under elevated temperature tensile and creep conditions.

The effect of plastic deformation on Al2CuLi (T1) precipitation

Abstract: The enhancement ofT1 precipitation in Al-Li-Cu alloys by plastic deformation prior to aging (that is, cold work) and the subsequent increase in alloy strength is investigated. The increased understanding of the role of matrix dislocations in the nucleation and growth ofT1 plates, discussed in the previous paper,[1] permits a detailed study of the phenomenon. In this paper, the effect of different levels of plastic strain on theT1 particle distributions as a function of aging time at 190 °C is quantified, and the subsequent influence on tensile properties is thereby described. The effect of plastic deformation is shown to decrease theT1 plate length and thickness, increase the number density by almost two orders of magnitude, increase the yield strength by 100 MPa, while simultaneously reaching peak strength in 20 pct of the time required without plastic deformation.

Recrystallization of austenite after deformation at high temperatures and strain rates—Analysis and modeling

Abstract: Double-hit compression tests were performed on low-carbon steels containing various combinations of niobium, boron, and copper over a wide range of temperatures and strain rates pertinent to hot rolling. The kinetics of static recrystallization are characterized in terms of the mean flow stresses, which lead to more accurate results than alternative methods. The fractional softening defined by the mean flow stress method was first corrected for adiabatic heating using a simple procedure. Appropriate expressions are given for the recrystallization kinetics as a function of predeformation, temperature, and strain rate. Particular attention is paid to the effect of preloading strain rate on recrystallization kinetics. It is shown that there is a one order of magnitude increase in softening rate when the strain rate is increased by two orders of magnitude. Thus, Simple extrapolations of laboratory data determined at conventional strain rates to high-speed mill conditions are likely to be inaccurate.

The effects of Cr additions to binary TiAl-base alloys

Abstract: The effects of Cr additions to y-base alloys have been investigated, using bulk materials consolidated from rapid solidification-processed ribbons. The composition ranges studied were 0 to 4 at. pet Cr and 44 to 54 at. pet Al. It was found that Cr additions do not affect the deformation behavior of single-phase γ alloys. However, they significantly enhance the plasticity of Al-lean duplex alloys which contain grains of single-phase γ and grains of lamellar γ/α2. Other Cr effects on microstructure, phase stability, site occupancy, and deformation sub-structures were characterized and correlated to the observed mechanical behavior. It was concluded that the ductilization effect of Cr in duplex alloys is partially due to the tendency of Cr to occupy Al lattice sites. Ductilization is also partially due to the ability of Cr to modify the Al partitioning and, therefore, the thermal stability of transformed α2 laths.

Microstructure and local brittle zone phenomena in high-strength low-alloy steel welds

Abstract: This study is concerned with a correlation between the microstructure and the local brittle zone (LBZ) phenomena in high-strength low-alloy (HSLA) steel welds The influence of the LBZ on toughness was investigated by means of simulated heat-affected zone (HAZ) tests as well as welded joint tests Micromechanical processes involved in microvoid and cleavage microcrack formation were also identified using notched round tensile tests and subsequent scanning electron microscopy (SEM) analyses The LBZ in the HAZ of a mUltipass welded joint is the intercritically reheated coarse-grained HAZ whose properties are strongly influenced by metallurgical factors such as an effective grain size and high-carbon martensitic islands: The experimental results indicated that Charpy energy was found to decrease monotonically with increasing the amount of martensitic islands, confirming that the martensitic island is the major microstructural factor controlling the HAZ toughness In addition, microvoids and microcracks were found to initiate at the interface between the martensitic island and the ferrite matrix, thereby causing the reduction in toughness These findings suggest that the LBZ phenomena in the coarse-grained HAZ can be explained by the morphology and the amount of martensitic islands

Effect of reinforcement on the aging response of cast 6061 Al-Al2O3 particulate composites

Abstract: The effect of 10 and 15 vol pct alumina particulate addition on the age hardening behavior of cast 6061 Al-matrix composites was studied using microhardness, electrical resistivity, differential scanning calorimetry, and transmission electron microscopy (TEM). It was found that the kinetics of precipitation in the matrix alloy are significantly accelerated due to the presence of reinforcements. This acceleration is attributable to the decrease in incubation time required for nucleation and the increase in solute diffusivity and hence precipitate growth rate resulting from the increase in the matrix dislocation density due to coefficient of thermal expansion (CTE) mismatch between the matrix and the reinforcements. The relative amounts of the various phases were also observed to be affected by reinforcement addition. Increasing reinforcement content decreased the volume fractions of the Β’ and Β precipitates while increasing the volume fraction of the GP-I zones. The volume fraction of silicon clusters (which are the precursors to GP zones in 6061 Al) formed during postsolution treatment aging was found to decrease with increasing reinforcement addition. The above effects have been discussed with respect to the associated mechanisms, and plausible explanations have been offered.

Effect of sulfur removal on Al2O3 scale adhesion

Abstract: The effect of removing sulfur impurity on the adhesion of Al2O3 scale to NiCrAl was investigated in four experiments. It was found that removing sulfur to concentration less than 1 ppm per weight is sufficient to produce a very significant degree of alpha-Al2O3 scale adhesion to undoped NiCrAl alloys. Results of experiments show that repeated oxidation, and polishing after each oxidation cycle, of pure NiCrAl alloy lowered sulfur content from 10 to 2 ppm by weight (presumably by removing the segregated interfacial layer after each cycle); thinner samples became adherent after fewer oxidation-polishing cycles because of more limited supply of sulfur. It was found that spalling in subsequent cyclic oxidation tests was a direct function of the initial sulfur content. The transition between the adherent and nonadherent behavior was modeled in terms of sulfur flux, sulfur content, and sulfur segregation.

The thermal and metallurgical state of steel strip during hot rolling: Part III. Microstructural evolution

Abstract: A mathematical model has been developed to compute the changes in the austenite grain size during rolling in a hot-strip mill. The heat-transfer model described in the first of this series of papers has been employed to calculate the temperature distribution through the thickness which serves as a basis for the microstructure model. Single-and double-hit compression tests have been conducted at temperatures of 900 °C, 850°C, 950 °C, and 875 °C on 0.34 and 0.05 pct carbon steels to determine the degree of recrystallization by metallographic evaluation of quenched samples and by measuring the magnitude of fractional softening. The Institut de Recherches de la Siderurgie Francaise, (IRSID) Saint Germain-en-Laye, France equation has been found to yield the best characterization of the observed recrystallization kinetics. The equations representing static recrystallization kinetics, recrystallized grain size, and grain growth kinetics have been incorporated in the model. The principle of additivity has been invoked to permit application of the isothermal recrystallization data to the nonisothermal cooling conditions. The model has been validated by comparing predicted austenite grain sizes with measurements made on samples quenched after one to four passes of rolling on the CANMET pilot mill. The austenite grain size evolution during rolling of a 0.34 pct carbon steel on Stelco’s Lake Erie Works (LEW) hot-strip mill has been computed with the aid of the model. The grain size decreased from an initial value of 180μm to 35μm in the first pass due to the high reduction of 46 pct. The changes in austenite grain size in subsequent passes were found to be small in comparison because of the lower per pass reductions. It has been shown that the equation employed to represent grain growth kinetics in the interstand region has a significant influence on the computed final grain size. Altering the rolling schedule had a negligible influence on the final grain size for a given finished gage. A 200°C increase in entry temperature to the mill resulted in a 20μm increase in final grain size, which is significant. This can be attributed to increased grain growth at the higher temperature.

Material effects in fretting wear : application to iron, titanium, and aluminum alloys

Abstract: Fretting wear tests were performed on several alloys (low alloyed and stainless steels, Ti6A14V titanium alloy, 2024 and 7075 aluminum alloys) slid against themselves in air under relatively low stresses for various displacements (±15 to ±50 μm). Friction logs, where tangential force is plotted as a function of displacement and number of cycles, were used to characterize the fretting behavior of the materials. Wear scars and cross sections were characterized by optical and scanning electron microscopy. Depending on the amplitude of displacement, sticking, partial slip, or gross slip occurs at the interface. Gross slip leads to debris formation. Metallic particles are detached from localized, very highly deformed areas whose properties and structures are different from those of the initial material. Sticking is observed on titanium and aluminum alloys tested under the smallest displacement. Samples are only deformed elastically. During partial slip, cracks can initiate and propagate in titanium and aluminum alloys. Millimeters-long cracks are observed on aluminum alloys after 106 cycles. Mechanisms for crack formation and propagation are described in terms of fatigue properties.

Microstructural development in transient liquid-phase bonding

Abstract: The applicability of conventional models of the transient liquid-phase (TLP) bonding process to the joining of nickel using ternary Ni-Si-B insert metals is considered in this article. It is suggested that diffusion of boron out of the liquid and into the solid substrate before the equilibration of the liquid and solid phases can result in the development of significant boron concentrations in the substrate. This, in turn, leads to the precipitation of boride phases in the substrate during holding at bonding temperatures below the binary nickel-boron eutectic temperature. The formation of boride phases during holding at the bonding temperature is of importance, because first, it is not predicted by the standard models of the TLP process, and second, the borides are not removed by prolonged holding at the bonding temperature and therefore may influence the in-service properties of the joint. In contrast, when bonding above the binary nickel-boron eutectic temperature, localized liquation of the substrate takes place. This liquid region resolidifies following prolonged holding and does not result in the formation of persistent boride phases. Experimental support is presented for the formation of borides during bonding, and characterization of the boride phases formed in the substrate is described.

Mechanism of Al 2 CuLi ( T 1 ) nucleation and growth

Abstract: Conventional strain contrast transmission electron microscopy (CTEM) and high-resolution transmission electron microscopy (HRTEM) were performed to establish the nucleation and growth mechanism of Al2CuLi (T1) precipitates in an Al-Li-Cu alloy. It is shown that the growth mechanism ofT 1 precipitate plates occurs by the diffusional glide of growth ledges composed of b = 1/6〈112〉 partial dislocations on 111 matrix planes and that the growth ledges migrate by the ledge-kink mechanism, as previously suggested by Cassadaet al. 1 for this system.T 1 plate nucleation is modeled as the dissociation of a perfect b = 1/2〈110〉 matrix dislocation in the vicinity of a dislocation jog. The coordinated dissociation of the dislocation line segments on each side of the sessile jog provides the displacement necessary for the formation of a new hexagonal plate or plate ledge. Strain contrast analysis of the Burgers vector of plate edges and the edges of growth ledges indicates the stacking of partial dislocations is of mixed displacement. Formerly Graduate Student, Department of Materials Science, University of Virginia,

Fatigue behavior of a 2xxx series aluminum alloy reinforced with 15 vol pct sicp

Abstract: The fatigue behavior of a naturally aged powder metallurgy 2xxx series aluminum alloy (Alcoa MB85) and a composite made of this alloy with 15 vol pct SiCp, has been investigated. Fatigue lives were determined using load-controlled axial testing of unnotched cylindrical samples. The influence of mean stress was determined at stress ratios of −1, 0.1, and 0.7. Mean stress had a significant influence on fatigue life, and this influence was consistent with that normally observed in metals. At each stress ratio, the incorporation of SiC reinforcement led to an increase in fatigue life at low and intermediate stresses. When considered on a strain-life basis, however, the composite materials had a somewhat inferior resistance to fatigue. Fatigue cracks initiated from several different microstructural features or defect types, but fatigue life did not vary significantly with the specific initiation site. As the fatigue crack advanced away from the fatigue crack initiation site, increasing numbers of SiC particles were fractured, in agreement with crack-tip process zone models.

High-damping metals and alloys

Abstract: High-Damping Metals (HIDAMETS) are the physical metallurgist’s answer to unwanted noise and vibrations. However, the characterization of the damping properties of metals and alloys is neither simple nor straightforward. This is mainly because the damping mechanisms involved depend upon the stress-induced movement of defects in the metal in question which, in turn, implies a dependence upon the microstructure (thermomechanical history) of the sample. To properly characterize the damping performance of a HIDAMET in a well-defined structural state, one must measure the mechanical damping as a function of vibration frequency, temperature, vibration strain amplitude, and static bias load over the ranges of these variables to be encountered in the application in question. This requires the use of a variety of equipment adapted to different modes of vibration and their overtones, especially when the damping is nonlinear (amplitude-dependent). Our approach to this problem is described and illustrated by test results obtained on several HIDAMETS.

Dynamic recrystallization during hot deformation of aluminum: A study using processing maps

Abstract: The hot deformation behavior of aluminum of different purities has been studied in the temperature range of 250 °C to 600 °C and strain-rate range of 10 3 to 102 s’1. On the basis of the flow stress data, the strain-rate sensitivity (m) of the material is evaluated and used for establishing power dissipation maps following the Dynamic Materials Model. These maps depict the variation of the efficiency of power dissipation [η = 2m/(m +1)] with temperature and strain rate. A domain of dynamic recrystallization (DRX) could be identified in these maps. While the strain rate at which the efficiency peak occurred in this domain is 10-3 s−1 the DRX temperature is purity dependent and is 375 °C for 99.999 pct Al, 450 °C for 99.995 pct Al, 550 °C for 99.94 pct Al, and 600 °C for 99.5 pct Al. The maximum efficiency of power dissipation for DRX in aluminum is about 55 pct. The sigmoidal increase of grain size with temperature in the DRX domain and the decrease in the DRX temperature with increase in the purity of aluminum are very similar to that observed in static recrystallization, although DRX occurred at much higher temperatures.

The anisotropic mechanical properties of a Ti matrix composite reinforced with SiC fibers

Abstract: The anisotropic mechanical properties of a Ti alloy composite reinforced with SiC fibers have been investigated and rationalized using analytical models. The appropriate material model for this composite involves the following features: an interface that debonds and slides, a flaw insensitive ductile matrix, and high-strength elastic fibers subject to residual compressive stress caused by thermal expansion mismatch. This, model is broadly consistent with the longitudinal, transverse, and shear properties of the composite.

Evolution of boride morphologies in TiAl-B alloys

Abstract: The solidification of γ-TiAl alloys with relatively low (<2 at. pct) additions of boron is discussed. Binary Ti-Al alloys containing 49 to 52 at. pct Al form primary α-(Ti) dendrites from the melt, which are subsequently surrounded by γ segregate as the system goes through the peritectic reactionL + α →γ. Alloys between 45 and 49 at. pct Al go through a double peritectic cascade, forming primary β-(Ti) surrounded by α-(Ti) and eventually by γ in the interdendritic spaces. Boron additions to these binary alloys do not change the basic solidifi-cation sequence of the matrix but introduce the refractory compound TiB2 in a variety of mor-phologies. The boride develops as highly convoluted flakes in the leaner alloys, but needles, plates, and equiaxed particles gradually appear as the B content increases above ∼1 at. pct. Increasing the solidification rate initially promotes the formation of flakes over plates/needles and ultimately gives way to very fine equiaxed TiB2 particles in the interdendritic spaces of the metallic matrix. Furthermore, the primary phase selection in the 49 to 52 at. pct Al range changes from α-(Ti) to β-(Ti) at supercoolings of the order of 200 K. The different boride morphologies are fully characterized, and their evolution is rationalized in terms of differences in their nucleation and growth behavior and their relationship to the solidification of the inter-metallic matrix.

Eutectic Growth under Rapid Solidification Conditions

Abstract: The model for rapid solidification of eutectics developed recently by the authors is extended: (1) the effect of the localization of the diffusion field which leads to a complex series solution is described by an approximation which makes the prediction of eutectic microstructures a simple matter and (2) the loss of local equilibrium at the solid-liquid interface is taken into account through a growth rate-dependent effective liquidus slope and solute distribution coefficient, leading to a depression of the eutectic temperature and a substantial increase in the solute content of the eutectic phases at growth rates larger than some centimeters per second. The importance of high growth rate effect is demonstrated for a linearized phase diagram.

Eutectic spacing selection in lead-based alloy systems

Abstract: Directional solidification studies have been carried out in Pb-Au, Pb-Pd, Pd-Cd, and Pb-Sn systems to characterize the variation in eutectic spacing with velocity. In the Pb-Cd, Pb-Sn, and Pb-Pd systems, statistical distributions of spacings at each velocity were determined and a significant spread was observed. The smallest and the largest spacings, along with the average spacing, have been characterized. A broad distribution curve has been observed for low velocities which becomes sharper as the velocities increase. A comparison with the theoretical model shows that the average spacings are consistently larger than the spacings predicted by the minimum undercooling criterion, whereas the smallest observed spacing corresponds to the theoretical extremum value. Dynamical studies have been carried out to examine the spacing selection process at a given velocity by initially starting with a higher or a lower average spacing. The final spacing distribution at a given velocity was found to be the same irrespective of the initial starting condition which establishes that a definite distribution of spacings exists in eutectic systems without any hysteresis effect and that dynamical effects are not responsible for the existence of a band of stable spacing.

Processing map for hot working of alpha-zirconium

Abstract: The hot deformation characteristics of alpha-zirconium in the temperature range of 650 °C to 850 °C and in the strain-rate range of 10-3 to 102 s-1 are studied with the help of a power dissipation map developed on the basis of the Dynamic Materials Model.[7,8,9] The processing map describes the variation of the efficiency of power dissipation (η =2m/m + 1) calculated on the basis of the strain-rate sensitivity parameter (m), which partitions power dissipation between thermal and microstructural means. The processing map reveals a domain of dynamic recrystallization in the range of 730 °C to 850 °C and 10−2 to 1−1 with its peak efficiency of 40 pct at 800 °C and 0.1 s-1 which may be considered as optimum hot-working parameters. The characteristics of dynamic recrystallization are similar to those of static recrystallization regarding the sigmoidal variation of grain size (or hardness) with temperature, although the dynamic recrystallization temperature is much higher. When deformed at 650 °C and 10-3 s-1 texture-induced dynamic recovery occurred, while at strain rates higher than 1 s-1, alpha-zirconium exhibits microstructural instabilities in the form of localized shear bands which are to be avoided in processing.

Analysis of an aluminum single crystal with unstable initial orientation (001) [110] in channel die compression

Abstract: An aluminum single crystal with an unstable initial orientation (001) [110] was compressed in a channel die device at room temperature. The crystal split into regions which rotated in opposite directions about an axis in the zero strain direction. At large reductions, these regions approached the stable copper texture components. These experiments were analyzed using a simplified analytical model and detailed finite element models. The predicted rotation of the crystal lattice with strain and the predicted stress-strain response are in good agreement with the experiments. The analyses show that, for this particular crystal orientation, the lattice rotation rate is very sensitive to the constraints on the crystal, and the lattice rotation is relatively insensitive to the constitutive model.

Recrystallization and superplasticity at 300 C in an aluminum-magnesium alloy

Abstract: Variations in thermomechanical processing (TMP) which regulate the microstructural characteristics and superplastic response of an Al-lOMg-0.1Zr alloy at 300 °C were evaluated. Mechanical property data revealed that the superplastic ductility can be enhanced by simultaneously increasing the total rolling strain, the reduction per pass, and the duration of reheating intervals between passes during isothermal rolling. Texture and microscopy data were consistent with the development of a refined microstructure by recovery-dominated processes, i.e., continuous recrystallization, during the processing. The mechanisms by which a refined substructure can be progressively converted into a fine-grained structure during repeated cycles of deformation and annealing are addressed. A qualitative description of the complex sequence of developments leading to a microstructure better suited to support superplastic response is presented.

The influence of applied stress, crack length, and stress intensity factor on crack closure

Abstract: Experimental and analytical evidence for the influence of applied stress, crack length, and stress intensity factor on crack closure is critically compared and evaluated. Fatigue crack opening behaviors are broadly catalogued into three classes. Class I comprises “near-threshold” behavior, where crack closure levels increase with decreasing stress intensity factor. In class II, the “stable” regime, the crack opening level is independent of the stress intensity factor and crack length but is influenced by the applied stress. Class III is characterized by the loss of elastic constraint accompanying extensive yielding at the crack tip or in the remaining ligament, especially with further crack growth. Here, the crack opening level decreases with increasing crack length until little or no closure occurs. These three different classes of closure behavior are extensively illustrated with both experimental data and the results of numerical closure simulations, particularly original finite element (FE) analyses. No single relationship between crack opening levels and the fundamental fatigue parameters is found to hold universally, due to the wide range of mechanisms which cause or influence closure.

Anomalous combustion effects during mechanical alloying

Abstract: Combustion in solid-state displacement reactions during mechanical alloying has been investigated. It was found that mechanical alloying decreases the critical reaction temperature at which unstable combustion reactions propagate. Some reactions exhibited an unusual “interrupted combustion” effect, whereby reactant powders which had been milled for subcombustion times and left to age at room temperature combusted on the immediate resumption of milling. For one reaction, the reduction of ZnO by Ti, combustion was only observed in aged powders. Interrupted combustion is associated with the diffusion of reactant species during aging and with a decrease in the temperature required for combustion.