Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 1985"

Analysis of stress-strain, fracture, and ductility behavior of aluminum matrix composites containing discontinuous silicon carbide reinforcement

Abstract: Mechanical properties and stress-strain behavior were evaluated for several types of commercially fabricated aluminum matrix composites, containing up to 40 vol pct discontinuous SiC whisker, nodule, or particulate reinforcement. The elastic modulus of the composites was found to be isotropic to be independent of type of reinforcement, and to be controlled solely by the volume percentage of SiC reinforcement present. The yield/tensile strengths and ductility were controlled primarily by the matrix alloy and temper condition. Type and orientation of reinforcement had some effect on the strengths of composites, but only for those in which the whisker reinforcement was highly oriented. Ductility decreased with increasing reinforcement content; however, the fracture strains observed were higher than those reported in the literature for this type of composite. This increase in fracture strain was probably attributable to cleaner matrix powder, better mixing, and increased mechanical working during fabrication. Comparison of properties with conventional aluminum and titanium structural alloys showed that the properties of these low-cost, lightweight composites demonstrated very good potential for application to aerospace structures.

A two-sublattice model for molten solutions with different tendency for ionization

Abstract: Different models accounting for the introduction of an excess of cations or anions in an ionic melt are considered. A new model which describes the variation in the thermodynamic properties in a melt as it gradually changes its composition from purely metallic state to complete ionization is developed. The model is based upon Temkin’s expression for the configurational entropy of mixing. Neutral species and hypothetical vacancies with an induced charge are introduced on the anion sublattice. The induced charge is equal to the average valency of the species occupying the cation sublattice. When the tendency of ionization is weak, the model approaches the substitutional model. For binary systems the new model is formally identical to the associate solution model if the associates are defined in such a way that they contain one atom of the electronegative element. For higher-order systems the new model works with a lower number of composition variables and parameters than the associate solution model.

Rhenium additions to a Ni-base superalloy: Effects on microstructure

Abstract: Refractory metal alloying to superalloy single crystals has resulted in alloys possessing higher temperature capabilities than have previously been known. In this study, additions of rhenium to a modified MAR-M200* alloy were made at the expense of tungsten. A study was carried out on the γ′ precipitate growth characteristics, morphology, and lattice misfit as a function of rhenium content and aging time and temperature. Rhenium additions substantially lowered the γ′ coarsening kinetics and resulted in negative γ-γ′ misfit alloys. A qualitative model is developed which explains these results. Particle growth was shown to bevia the widely accepted diffusion controlled coarsening mechanism. Precipitates were also shown to grow by particle coalescence into irregular blocky shapes at high aging temperatures while remaining cuboidal at lower temperatures, and the necessity of using large precipitate volume fraction modifications to theoretical coarsening models is illustrated.

The microstructure of rapidly solidified Al 6 Mn

Abstract: The microstructure of rapidly solidified Al−Mn alloys containing 18 to 25.3 wt pct Mn was studied by transmission electron microscopy. One of the phases found in the microstructure exhibits icosahedral symmetry manifested in electron diffraction patterns having five-fold symmetry. A new structural concept is proposed to account for the observed electron diffraction patterns. The structure is assumed to be composed of many connected polyhedra. Although not forming a regular lattice, such structures are able to produce sharp diffraction peaks. The terminal stability and transformation of the icosahedral phase was also studied and reported.

Fatigue crack deflection and fracture surface contact: Micromechanical models

Abstract: The variation of cyclic crack propagation rates, under thecombined influence of crack kinking (deflection) and fracture surface contact (closure), is estimated from simple linear elastic analyses of tilted cracks. The predictions of the models are consistent with the experimental results of linear and kinked crack advance in high strength aluminum alloys testedin vacuo. Examples of crack deflection in various engineering alloy systems and some generalizations ofaverage deflection parameters based on microstructural and mechanical factors are discussed. The individual contributions to overall growth rates from deflection and closure processes are evaluated for different mechanical and metallurgical conditions. The significance, implications, and limitations of the models are outlined.

Fluid flow and weld penetration in stationary arc welds

Abstract: Weld pool fluid flow can affect the penetration of the resultant weld significantly. In this work, the computer simulation of weld pool fluid flow and its effect on weld penetration was carried out. Steady-state, 2-dimensional heat and fluid flow in stationary arc welds were computed, with three driving forces for fluid flow being considered: the buoyancy force, the electromagnetic force, and the surface tension gradient at the weld pool surface. The computer model developed agreed well with available analytical solutions and was consistent with weld convection phenomena experimentally observed by previous investigators and the authors. The relative importance of the influence of the three driving forces on fluid flow and weld penetration was evaluated, and the role of surface active agents was discussed. The effects of the thermal expansion coefficient of the liquid metal, the current density distribution in the workpiece, and the surface tension temperature coefficient of the liquid metal on weld pool fluid flow were demonstrated. Meanwhile, a new approach to free boundary problems involving simultaneous heat and fluid flow was developed, and the effort of computation was reduced significantly.

Time-dependent deformation of metals

Abstract: The basic characteristics of timedependent deformation of metals are described in terms of dislocation properties. At high temperatures, diffusion controlled climb of edge dislocations is the rate limiting process, whereas at low temperatures, other forms of recovery involving cross-slip of screw dislocations operate. A composite model of plastic flow is used to describe the coupling between these recovery processes. The model is patterned after the persistent slip band structures observed in cyclically deformed fcc single crystals. Screw dislocations are allowed to move in the cell interiors and to deposit edge dislocations into the adjoining walls. Cross-slip and climb lead to dislocation rearrangement and annihilation in the two regions. These processes are coupled not only through the dislocation microstructure, but also through the mechanics of the composite structure. The model is used to describe various deformation properties of metals, including stage II, stage III, and stage IV strain hardening and saturation of the flow stress. The coupling of cross-slip and climb controlled recovery processes leads to gradual transitions in strain hardening and gives a natural account of the transition from low temperature deformation to high temperature creep. The model also leads to polarized dislocation structures, internal stresses, and anelastic creep properties.

On macroscopic and microscopic analyses for crack initiation and crack growth toughness in ductile alloys

Abstract: Relationships between crack initiation and crack growth toughness are reviewed by examining the crack tip fields and microscopic (local) and macroscopic (continuum) fracture criteria for the onset and continued quasi-static extension of cracks in ductile materials. By comparison of the micromechanisms of crack initiationvia transgranular cleavage and crack initiation and subsequent growthvia microvoid coalescence, expressions are shown for the fracture toughness of materials in terms of microstructural parameters, including those deduced from fractographic measurements. In particular the distinction between the deformation fields directly ahead of stationary and nonstationary cracks are explored and used to explain why microstructure may have a more significant role in influencing the toughness of slowly growing, as opposed to initiating, cracks. Utilizing the exact asymptotic crack tip deformation fields recently presented by Rice and his co-workers for the nonstationary plane strain Mode I crack and evoking various microscopic failure criteria for such stable crack growth, a relationship between the tearing modulusTR and the nondimensionalized crack initiation fracture toughnessJIc is described and shown to yield a good fit to experimental toughness data for a wide range of steels.

Kinetics of solution hardening

Abstract: The flow stress of solution hardened single crystals and polycrystals is analyzed with respect to its dependence on temperature and strain rate. An evaluation of literature data, especially at low temperatures and low concentrations in fcc alloys, reveals that the interaction between dislocations and discrete, atomic-sized obstacles (or fixed clusters of them) cannot be responsible for solution hardening. A “trough” model is favored in which the effect of the solutes is postulated to be equivalent to a continuous locking of the dislocations along their entire length, during every waiting period. The macroscopic features of this model are similar to Suzuki’s chemical-hardening model. It can also explain the strong interaction of solution hardening and strain hardening at elevated temperatures, as well as basic features of dynamic strain-aging, in particular its strain dependence.

Adiabatic shear localization in titanium and Ti-6 pct Al-4 pct V alloy

Abstract: Ballistic impact experiments were conducted on 12.5 mm thick commercial purity titanium and Ti-6 pct Al-4 pct V alloy plates using steel “stepped” projectiles with 10.5 mm diameter. The impact velocities varied between 578 m per second and 846 m per second, and a flash X-ray technique was used to determine projectile velocity and to assure the normality of impact. The microstructural damage mechanisms associated with impact (shear band formation, shock wave propagation, and dynamic fracture) were analyzed by optical, and scanning and transmission electron microscopy. Elliptical and spherical cavities were observed along the bands. Microindentation hardness differences between the bands and adjacent regions were slight for the targets; for the projectiles, the hardness in the band was significantly lower than that of surrounding regions. Observation of the fractured regions along the bands showed unique features indicating possible melting. Transmission-electron microscopy of a shear band in titanium revealed microcrystalline features (∼0.3 µm diameter) with poorly defined grain boundaries.

The influence of cobalt, tantalum, and tungsten on the microstructure of single crystal nickel-base superalloys

Abstract: The influence of composition on the microstructure of single crystal nickel-base superalloys was investigated. Co was replaced by Ni, and Ta was replaced by either Ni or W, according to a matrix of compositions based on MAR-M247. Substitution of Ni for Co caused an increase inγ′ solvus temperature, an increase inγ-γ′ lattice mismatch, and the precipitation of W-rich phases in the alloys with high refractory metal levels. Substitution of Ni for Ta caused large decreases inγ′ solvus temperature,γ′ volume fraction, andγ-γ′ lattice mismatch, whereas substitution of W for Ta resulted in smaller decreases in these features. For the alloys withγ′ particles that remained coherent, substitution of Ni for Co caused an increase inγ′ coarsening rate. The two alloys with the largest magnitude of lattice mismatch possessedγ′ particles which lost coherency during unstressed aging and exhibited anomalously low coarsening rates. Creep exposure at 1000 °C resulted in the formation ofγ′ lamellae oriented perpendicular to the applied stress axis in all alloys.

Theory of workhardening 1934-1984

Abstract: The evolution of the theory of workhardening through the past fifty years has laid a secure basis, but much research still lies ahead. A guiding principle in the prevailing, so-called meshlength theory is that glide dislocations arrange into stress-screened, low-energy structures, the most common being the cell structure, and that the flow stress is the stress needed to generate new glide dislocations. Further, it makes extensive use of the “principle of similitude”. Remnant stresses due to dislocations with just one Burgers vector orientation are very often relieved by additional dislocations with other Burgers vectors which form not in response to the applied stress but to those remnant stresses. Such dislocations are commonly misnamed “forest” dislocations. The theory closely reproduces stages II and III of the typical workhardening curve. Stage I results in single glide from sources which initially are isolated from each other so that pile-ups form, which then may interact among neighboring pile-ups of opposite sign, so as to generate mats of dipoles parallel to the active glide plane. Stage II behavior is expected as long as similitude is obeyed so that the average free dislocation path shrinks inversely proportional with the root of the dislocation density. Stage III, finally, results when the free dislocation path is constant. At low temperatures, thermal activation can make the critical difference for the release of hair-trigger poised loops. This is the cause of creep effects whose magnitude is limited to less than the elastic strain. Computer calculations indicate the presence of longer-range (i.e., cell diameter scale) stresses whose sign changes with the cell’s sense of rotation. This suggests that rectangled dislocation cells with a common rotation axis, arranged into a three-dimensional checkerboard pattern in which the sense of rotation alternates from cell to cell, should minimize stored energy. Such cell patterns are increasingly reported in the literature. The fact that the average cell diameter tends to be inversely proportional to the applied stress is also readily explained through those stresses. In retrospect, Taylor’s theory of workhardening may be recognized as a variant of the meshlength theory of stage III, in that it is based on a stress-screened network obeying the principle of similitude while the free dislocation path, given by the spacing between the mosaic block walls, remains constant.

The development of γ-γ′ lamellar structures in a nickel-base superalloy during elevated temperature mechanical testing

Abstract: Changes in the morphology of the γ′ precipitate were examined during creep and tensile testing at temperatures between 927 and 1038 °C in [001]-oriented single crystals of a model Ni-Al-Mo-Ta superalloy. In this alloy, the γ′ particles link together to form lamellae, or rafts, which are aligned with their broad faces perpendicular to the applied tensile axis. The dimensions of the γ and γ′ phases were measured as the lamellar structure developed and were related to time and strain in an attempt to trace the changing γ-γ′ morphology. The results showed that directional coarsening of γ′ began during primary creep, and the attainment of a fully developed lamellar structure did not appear to be directly related to the onset of steady-state creep. The rate of directional coarsening during creep increased as the temperature was raised and also increased as the stress level was raised at a given testing temperature. The raft thickness remained equal to the initial γ′ size from the start of the creep test up through the onset of tertiary creep for all testing conditions. It was found that extensive rafts did not develop during the shorter testing times of the tensile tests, and that tensile testing of pre-rafted structures did not alter the morphology of the rafts. The overall behavior of the alloy was a clear indication of the stability of the finely-spaced γ-γ′ lamellar structure.

Ferrite recrystallization and austenite formation in cold-rolled intercritically annealed steel

Abstract: The recrystallization of ferrite and austenite formation during intercritical annealing were studied in a 0.08C-1.45Mn-0.21Si steel by light and transmission electron microscopy. Normalized specimens were cold rolled 25 and 50 pct and annealed between 650 °C and 760 °C. Recrystallization of the 50 pct deformed ferrite was complete within 30 seconds at 760 °C. Austenite formation initiated concurrently with the ferrite recrystallization and continued beyond complete recrystallization of the ferrite matrix. The recrystallization of the deformed ferrite and the spheroidization of the cementite in the deformed pearlite strongly influence the formation and distribution of austenite produced by intercritical annealing. Austenite forms first at the grain boundaries of unrecrystallized and elongated ferrite grains and the spheroidized cementite colonies associated with ferrite grain boundaries. Spheroidized cementite particles dispersed within recrystallized ferrite grains by deformation and annealing phenomena were the sites for later austenite formation.

Elevated temperature creep-rupture behavior of the single crystal nickel-base superalloy NASAIR 100

Abstract: The creep and rupture behavior of [001] oriented single crystals of the nickel-base superalloy NASAIR 100 was investigated at temperatures of 925 and 1000 °C. In the stress and temperature ranges studied, the steady state creep rate, time to failure, time to the onset of secondary creep, and the time to the onset of tertiary creep all exhibited power law dependencies on the applied stress. The creep rate exponents for this alloy were between seven and eight, and the modulus-corrected activation energy for creep was approximately 350 kjoule/mole, which was comparable to the measured activa-tion energy for Ostwald ripening of the γ′ precipitates. Oriented γ′ coarsening to form lamellae perpendicular to the applied stress was very prominent during creep. At 1000 °C, the formation of a continuous γ-γ′ lamellar structure was completed during the primary creep stage. Shear through the γ-γ ' interface is considered to be the rate limiting step in the deformation process. Gradual thickening of the lamellae appeared to be the cause of the onset of tertiary creep. At 925 °C, the fully developed lamellar structure was not achieved until the secondary or tertiary creep stages. At this temperature, the γ-γ′ lamellar structure did not appear to be as beneficial for creep resistance as at the higher temperature.

Deformation characteristics in Β phase Ti-Nb alloys

Abstract: Deformation modes in Β-phase Ti-Nb alloys containing from 36 to 52 wt pct Nb have been investigated as a function of composition, crystallographic orientation, or deformation temperature using both two surface trace analysis and transmission electron microscopy. 332 (113) twinning occurs in the low niobium content alloys independent of orientation and temperature. With increasing niobium content, 332 twinning or (111) slip occurs dependent on orientation and temperature. The operative twinning systems are explained by considering both the polarization of twinning shear and the Schmid factor. The 332 (113) twinning is suppressed in the high niobium content alloys and also suppressed by aging or oxygen addition in the low niobium content alloy. These characteristics have previously been observed in Β phase Ti-V and Ti-Mo alloys. The occurrence of 332 (113) twinning in Β-phase Ti alloys is related to the instability of Β phase.

Mechanical properties of 0. 40 Pct C-Ni-Cr-Mo high strength steel having a mixed structure of martensite and bainite

Abstract: A study has been systematically made of the effect of bainite on the mechanical properties of a commercial Japanese 0.40 pct C-Ni-Cr-Mo high strength steel (AISI 4340 type) having a mixed structure of martensite and bainite. Isothermal transformation of lower bainite at 593 K, which appeared in acicular form and partitioned prior austenite grains, in association with tempered marprovided provided a better combination of strength and fracture ductility, improving true notch tensile strength (TNTS) and fracture appearance transition temperature (FATT) in Charpy impact tests. This occurred regardless of the volume fraction of lower bainite present and/or the tempering conditions employed to create a difference in strength between the two phases. Upper bainite which was isothermally transformed at 673 K appeared as masses that filled prior austenite grains and had a very detrimental effect on the strength and fracture ductility of the steel. Significant damage occurred to TNTS and FATT, irrespective of the volume fraction of upper bainite present and/or the tempering conditions employed when the upper bainite was associated with tempered martensite. However, when the above two types of bainite appeared in the same size, shape, and distribution within tempered martensite approximately equalized to the strength of the bainite, a similar trend or a marked similarity was observed between the tensile properties of the mixed structures and the volume fraction of bainite. From the above results, it is assumed that the mechanical properties of high strength steels having a mixed structure of martensite and bainite are affected more strongly by the size, shape, and distribution of bainite within martensite than by the difference in strength between martensite and bainite or by the type of mixed bainite present. The remarkable effects of the size, shape, and distribution of bainite within martensite on the mechanical properties of the steel are briefly discussed in terms of the modified law of mixtures, metallographic examinations, and the analyses of stress-strain diagrams.

New eutectic alloys and their heats of transformation

Abstract: Eutectic compositions and congruently melting intermetallic compounds in binary and multicomponent systems among common elements such as Al, Ca, Cu, Mg, P, Si, and Zn may be useful for high temperature heat storage. In this work, heats of fusion of new multicomponent eutectics and intermetallic phases are reported, some of which are competitive with molten salts in heat storage density at high temperatures. The method used to determine unknown eutectic compositions combined results of differential thermal analysis, metallography, and microprobe analysis. The method allows determination of eutectic compositions in no more than three steps. The heats of fusion of the alloys were measured using commercial calorimeters, a differential thermal analyzer, and a differential scanning calorimeter.

The contiguity of liquid phase sintered microstructures

Abstract: Contiguity is a measure of the dispersed phase contact area in a composite microstructure and is particularly important to the properties of liquid phase sintered materials. For an assumed spherical geometry, the contiguity is calculated for various interfacial energies, grain size ratios, and volume fractions of solid phase. The volume fraction solid is linked to the dihedral angle to indicate conditions where grain shape accommodation is necessary. The effect of a distribution in solid phase grain sizes is to lower slightly the contiguity from the monosized grain value. Coalescence is linked to the occurrence of curved boundaries at intergrain contacts involving differing grain sizes. The results are correlated to prior observations on cemented carbides.

Mobility of martensitic interfaces

Abstract: Using a dislocation model of interfacial structure, kinetic theories of dislocation motion are adapted to predict the mobility of martensitic interfaces. Defining generalized driving forces and activation parameters, analytical models are developed which describe the kinetics of motion controlled by various types of obstacle interactions. The behaviors of martensitic interfaces and slip dislocations in identical microstructures are compared. For a lattice-invariant shear by slip, the martensitic interface behaves similarly to a collection of glide dislocations. The interface/obstacle interaction is much weaker if the martensite is internally twinned, giving a higher relative mobility.

Binder deformation in WC-(Co, Ni) cemented carbide composites

Abstract: The microstructural responses to monotonic and cyclic compressive loading of three WC-(Co,Ni) alloys have been characterized and measured by high voltage transmission electron microscopy and neutron diffraction. A base alloy comprising WC-17 wt pct Co was prepared and evaluated, along with two alloys in which the binder composition was altered by replacing 15 pct and 30 pct of the total cobalt by nickel. Results are presented for strains of 0, 0.75, and 5.0 pct, and for two fatigue stress levels, both at 0.5 million cycles. Predominant binder-deformation mechanisms were observed to shift with increasing Ni content from the fcc-hcp martensitic transformation to slip plus twinning over the composition range studied. In the base alloy, 44.5 vol pct of the binder had transformed at the highest strain level, while only 11.4 pct transformation occurred at the same strain in the 30 pct Ni-binder alloy. The shift in binder plasticity mechanisms and the corresponding changes in composite stressstrain behavior have been discussed with respect to several theories on the role of binder deformation in cermet mechanical response.

Plastic flow in dispersion hardened materials

Abstract: In this review consideration is given to the sequence of processes which can occur in dispersion strengthened materials during plastic deformation. Consideration is given to the role of elastic back stresses and their influence on the magnitude of the Bauschinger effect and on dimensional stability. The formation and stability of the plastic zones around second phase particles is described and the competitive processes of void formation and diffusional relaxation are considered. Attention is given to the description of the microstructural features of dispersion hardened materials in terms of the local volume fraction of clustered particles rather than the average size and volume fraction. Evidence is presented to show that a number of important properties such as fracture behavior and recrystallization need to be described in terms of local volume fractions. In the final section of the review emphasis is placed on the relationship between basic concepts of dispersion hardening and the development of new processing methods capable of producing synthetic microstructures with specific properties.

Microanalytical study of the heterogeneous phases in commercial Al-Zn-Mg-Cu alloys

Abstract: X-ray microanalysis and Convergent Beam Diffraction (CBD) studies were conducted on the second phase constituent and dispersoid particles in 7075 and 7475 aluminum alloys. Partial substitution of alloying elements was found to occur in all the second phase particles causing small deviations from the stoichiometric compositions reported for the binary and ternary compounds. The coarse constituent phases were identified to be Al7Cu2Fe, (Al,Cu)6(Fe,Cu), Mg2Si, α-Al12Fe3Si, amorphous silicon oxide, and a modified Al6Fe compound in decreasing order of abundance. The dispersoid particles were Al18Mg3Cr2 compound, and they formed in both triangular and spherical morphologies. Their compositions were found to vary slightly with the aging treatment. The crystal structure of the dispersoid phase consisted of a disordered form of a cubic structure (Fd3m) reported for the Al18Mg3Cr2 compound. The uniqueness of CBD analysis in the crystal structure determination is emphasized.

Grain boundary fracture of L12 type intermetallic compound Ni3Ai

Abstract: AES analysis of intergranular fracture surfaces of Ni3Al showed that grain boundaries are free from any detectable amount of impurity segregation. From this finding it was suggested that grain boundary brittleness in Ni3Al is not due to the segregation of harmful elements. X-ray diffraction and SEM observation of the fracture surfaces detected a plastically strained layer of which thickness is compa-rable to grain size.

The effect of hydrogen as a temporary alloying element on the microstructure and tensile properties of Ti-6Al-4V

Abstract: Ti-6Al-4V alloy, to which 0.6 wt pct to 1.0 wt pct (22 to 33 at. pct) hydrogen has been added, can undergo a phase transformation which produces unique, fine microstructures. Specimens of the alloy were heated to 870°C, transformed at temperatures between 540°C and 700°C, and the microstructures were determined as a function of hydrogen content and transformation temperature. Microstructures and tensile properties of sheet specimens were determined after such transformation followed by dehydrogenation at temperatures between 650°C and 760°C. The highest yield strength (1130 MPa) and good ductility (9 pct El) were associated with a fine equiaxed microstructure obtained in material charged with approximately 1.0 wt pct hydrogen, transformed at 565°C and dehydrogenated at 675°C. Lower strengths and ductilities were associated with acicular microstructures produced by transformation at higher temperatures or coarser structures producted at higher dehydrogenation temperatures.

The development of some dual-phase steel structures from different starting microstructures

Abstract: The development of dual-phase structures with different morphologies has been studied in detail by intercritical annealing specimens of a steel containing 0.11 pct C and 1.6 pct Mn with different microstructures before annealing. The kinetics of formation of the two-phase structure at the annealing temperature and the redistribution of substitutional solute elements were measured in specimens quenched from the intercritical annealing temperature. The structure before annealing was either banded ferrite-pearlite, homogenized ferrite-pearlite, lath martensite, spheroidal cementite dispersed in ferrite, or austenite. No measurable partitioning of silicon or molybdenum, present in the steel in small concentrations, was found. However, close to equilibrium partitioning of manganese occurred on annealing specimens with either a ferrite-pearlite or a lath martensite structure, but during the separation of ferrite from austenite in step-quenched specimens there was no partitioning. Surprisingly, measurements of manganese concentrations using an electron beam of 1 nm diameter at intervals of 5 nm or less revealed the presence of narrow spikes in the concentration profile at many ferriteaustenite interfaces in specimens with a ferrite-pearlite or martensite starting structure as well as in those step-quenched from austenite. In some instances, a minimum in the concentration profile was found in ferrite, adjacent to a maximum at an interface. Thus, adsorption of manganese at ferriteaustenite interfaces produces concentrations in excess of the concentrations indicated by the equilibrium diagram. The probable diffusion processes controlling the kinetics of transformation in the different microstructures are identified.

Grain structure and solidification cracking in oscillated arc welds of 5052 aluminum alloy

Abstract: The effect of arc oscillation on grain structure and solidification cracking in GTA welds of 5052 aluminum alloy was investigated using a four-pole magnetic arc oscillator and a modified fish-bone crack test. Two different mechanisms of crack reduction were identified: one in the low frequency range of arc oscillation and the other in the high frequency range. The former was the alteration of the orientation of columnar grains, while the latter was grain refining. Neither mechanism was operative in the intermediate frequency range and solidification cracking was severe, especially when the amplitude of arc oscillation was small. Alteration of grain orientation was obtained in welds made with transverse and circular arc oscillations, but not longitudinal arc oscillation. Grain refining, on the other hand, was achieved in welds made with all three types of arc oscillation patterns. The differences between the response of alloy 5052 to arc oscillation and that of alloy 2014 observed previously were discussed.

Effect of dihedral angle on the morphology of grains in a matrix phase

Abstract: Minimum interface energy configurations of a uniformly intermixed grain-matrix aggregate are determined for various dihedral angles and matrix contents by numerical analysis of a model which consists of a rhombic dodecahedron grain in contact with matrix at its curved surfaces along truncated edges and corners. For dihedral angles, Φ, greater than 90 deg, the total interface energy,E, increases monotonically with the matrix volume fraction,Vm. For Φ= 0 deg,E decreases withVm until the grains become spherical atVm = 26 pct. For 0 deg Φ ≤ 75 deg,E vs Vm curves show the minima which represent the stable configurations to be obtained whenVm can be freely varied. For Φ ≤ 60 deg, the matrix is always continuous along the grain edges. For Φ 75 deg, the matrix becomes separated at the grain corners below certain critical values ofVm. The contiguity decreases monotonically withVm. The slope ofE vs Vm curve is shown to be an effective pressure on the specimen surface, which represents the driving force for changing the grain configuration with a corresponding change ofVm while keeping the grain volume constant. The implications of these results on solid state sintering, liquid phase sintering, and the penetration of liquid into liquid phase sintered alloys are discussed. Finally, the results of a previous analysis by Beere are shown to disagree with the present work for systems with low dihedral angles apparently because of inaccuracy in his calculation.