METALLURGICAL AND MATERIALS TRANSACTIONS A
	ABSTRACTS Volume 27A, No. 9, September 1996 This Month Featuring: The 1996 Institute of Metals and Mehl Award Lecture; Alloy Phases; Transformations; Transport Phenomena; Mechanical Behavior; Physical Chemistry; Environment; Welding & Joining; Electronic, Magnetic, & Optical Material; Solidification; Materials Processing; Composite Materials. View September 1996 Table of Contents.

THE 1996 INSTITUTE OF METALS AND MEHL AWARD LECTURE

ALLOY PHASES

Li activities for liquid and solid alloys at the (Mg), (Li), and (Mg)+(Li) two-phase region of the Li-Mg system were determined. Liquid alloys were examined at temperatures from 638 to 889 K at various Li concentrations. The (Mg) solid solutions were investigated in two series: at constant temperatures between 773 and 876 K, with varying Li content, and at fixed Li concentrations, equal to 0.125 and 0.160 molar fractions, at different temperatures between 772 and 849 K. At the two-phase region, (Mg)+(Li), emf measurements were performed in the temperature range 773 to 838 K, with fixed Li concentrations equal to 0.20, 0.25, and 0.275 molar fractions. For (Li) solid alloys, experiments were done at temperatures 773 to 849 K for several constant Li concentrations, between 0.30 to 0.45 molar fractions, respectively. Studies on solid alloys enabled us also to determine the boundaries (Li)/[(Mg)+(Li)] and (Mg)/[(Mg)+(Li)] at temperatures 773 to 831K. The resulting thermodynamic and phase boundary data of this study were used with other selected references for a critical assessment of the Li-Mg system. The Lukas BINGSS optimization program and BINFKT for the calculation of the thermodynamic functions and of the phase diagram were used. The calculated equilibrium phase diagram at temperatures below 750 K indicates a slightly lower solid solubility of Mg in (Li) in comparison with results from thermal analysis and the recently published Saunders evaluation.

Experimental Study of the Phase Equilibria in the Fe-Mn-Al System
XING JUN LIU, SHI MING HAO, LE YING XU, YANG FENG GUO, and HUI CHEN
The isothermal sections with lower Al content at 800°C, 900°C, 1200°C, and 1300°C and two vertical sections of 4 wt pct Al and 8 wt pct Al in the Fe-Mn-Al system were determined by means of the diffusion couple technique, metallographic method, and differential thermal analysis (DTA). It is shown that the results of isothermal and vertical sections coincide with each other, and the two-phase ( + ) region and three-phase ( + + ) region trend toward the Fe-Mn side and Mn corner with the rise of temperature, respectively. The experimental results are in agreement with those calculated by the present authors.

Retardation of Intermetallic Phase Formation in Experimental Superferritic Stainless Steels
S. NANA and M.B. CORTIE
While superferritic stainless steels containing 29 pct chromium possess excellent resistance to corrosion, they may, under certain conditions, be embrittled by the precipitation of intermetallic phases. The extent to which the precipitation reactions can be retarded by alloying additions of aluminum and copper has been evaluated. It was found that additions of aluminum to an Fe-29 pct Cr-4 pct Mo-1.5 pct Ni base alloy suppress the precipitation of the undesirable sigma and chi intermetallic phases, but additions of up to 3 pct aluminum promote 475°C embrittlement. Additions of copper slightly reduce the precipitation of sigma and chi phases under most conditions but enhance 475°C embrittlement. The resistance to corrosion in 10 pct H₂SO₄ and 10 pct FeCl₃ was assessed. All the aluminum-containing alloys performed significantly better in H₂SO₄ than the base alloy; however, large additions of aluminum had a deleterious effect on the pitting resistance in FeCl₃. Additions of copper improved the resistance to FeCl₃ and lowered the rate of corrosion in the H₂SO₄ solution used.

Communication: The Dependence of Complex Alloyed Steel Properties on Quenching and Tempering Conditions
BERTOLD B. VINOKUR, ALEXANDER VINOKUR, and VICTORIYA E. SHETSSEL

TRANSFORMATIONS

Effect of Alloying Elements on Martensitic Transformation in the Binary NiAl() Phase Alloys

Effects of Low-Temperature Aging on the Microstructure and Soft Magnetic Properties of Rapidly Quenched Fe-Si-B Alloys
T. NAOHARA
The rapidly quenched Fe_88-X Si₁₂B_X alloys with a B content ranging from 4 to 20 at. pct were subjected to prolonged aging at temperatures below 573K and their microstructure and soft magnetic properties were examined. The alloys containing more than 8 at. pct B had an amorphous phase in the as-quenched state. The low-temperature aging caused crystallization and concurrent change in the soft magnetic properties only in the amorphous alloys with 8 and 10 at. pct B; however, no such age-induced behavior was determined in the amorphous alloys containing more than 12 at. pct B. These results were reasonably interpreted as being associated with changes in the amorphous structure which occurs in the 10 to 12 at. pct B range.

Annealing and Aging of Interstitial C in -Fe, As Measured by Internal Friction
WILSON PASCHETO and G.P. JOHARI
The diffusion of interstitial C in -Fe has been studied by the dynamic mechanical relaxation method, and the isothermal aging kinetics of the removal of interstitial C by precipitation from its saturated state has been measured at several temperatures. The height of the Snoek peak is found to decrease according to the relation Q-1_max (T,t) = Q-1_max(T,t )+Q-1_max(T) exp {-t / [k₀ exp (E/RT)]}, where t is the aging time, k₀ a time constant, and E the activation energy for the precipitation of C atoms. The anelastic relaxation time is independent of the amount of C in -Fe and varies according to =3.45 X 10^-15 exp [80.0 (kJ/mol)/RT ]. The diffusion coefficient varies according to D = 6.61X10^-3 exp [-(80.0/RT) cm² s^-1], over the temperature range 303 to 357K. The effect of chemical composition on k₀ has been determined and a procedure for determining the constant of proportionality between Q-1_max and the amount of dissolved carbon in steel is described. A concept of accumulated equivalent aging time is introduced and theoretically justified on the bases of time-temperature superposition of aging effects. Calculations and experiments show that a substantial decrease occurs in the C content of steel when it is heated to high temperatures and that this effect alters the shape of the Snoek peak.

Ostwald Ripening of Solid-Liquid Pb-Sn Dispersions
I. SEYHAN, L. RATKE, W. BENDER, and P.W. VOORHEES
An experimental study on the influence of a finite volume fraction of solid on Ostwald ripening of Pb-Sn dispersions is presented. Dispersions of lead particles in a near-eutectic Pb-Sn liquid were stabilized against gravitationally induced sedimentation by the use of Lorentz forces. The Lorentz forces were induced by an electrical current within the samples and a magnetic field perpendicular to it. Dispersions from 0.3 to 0.71 vol fraction solid Pb could thus be kept free of any large-scale sedimentation for times of minutes to 15 hours. The results show that there is a definitive effect of the volume fraction on the rate of coarsening and that the experimentally measured kinetics are in close agreement with theoretical predictions. However, it is also clear that Lorentz forces accelerate the coarsening process whenever a solid skeleton of the dispersoids is present. It is unclear if this acceleration is also present in the experiments with a low volume fraction of solid.

Precipitation Behaviors in Al-Cu-Mg and 2024 Aluminum Alloys
HAN-CHENG SHIH, NEW-JIN HO, and J.C. HUANG
The precipitation behaviors and aging reactions of the pseudobinary Al-Cu-Mg alloy and the commercial 2024 alloy under unstretched and stretched conditions have been investigated in this study by means of conductivity and hardness measurements, differential scanning calorimetry, and transmission electron microscopy (TEM). The morphologies and growth modes of various defects and transition phases as well as the interaction among them were widely discussed. In particular, an electron diffraction ring pattern was found to correspond to the axial growth of the GPB2 zone. This suggested that the atom groups constructing this cylindrical zone are statistically, uniformly arranged in the adjacent {100}_Al planes and the GPB2 zone is only a partially ordered version of the GPB zone in [001]_Al directions. Moreover, few GPB2 zones can survive long time overaging due to the Gibbs-Thomson effect. As for the S' precipitates, they preferentially nucleate on dislocations. During subsequent growth, they can further coalesce into two morphologies (corrugated sheets and wide plates) for the unstretched specimens. However, for the stretched specimens, this coalescing process does not occur until a long time of overaging due to more introduced dislocations. Therefore, the rate of Ostward ripening decreases and the peak hardness becomes flattened. Finally, based on the present analyses, the aging sequence of the two alloys studied could be revised with respect to previous investigations and their isothermal aging reactions can be subdivided into five main stages. These stages correspond to (1) GPB zone precipitation, (2) fast in situ precipitation of GPB2 zones from GPB zones and their subsequent growth, (3) fast nucleation and accelerating growth of the S' phase, (4) decelerating growth of the S' phase, and (5) Ostward ripening of the S' and S phases, respectively.

Communication: The Effect of Substrate Constraint on the Martensitic Transformation of Ni-Ti This Films
S.A. MATHEWS, MANFRED WUTTIG, and QUANMIN SU

TRANSPORT PHENOMENA

Average Effective Interdiffusion Coefficients and the Matano Plane Composition
M.A. DAYANANDA
From an integration of the interdiffusion flux of a component over distance in the diffusion zone of an isothermal solid-solid diffusion couple, average effective interdiffusion coefficients can be evaluated over selected ranges of composition. An analysis is developed to inter-relate, in general, the average effective interdiffusion coefficients calculated for the composition ranges on either side of the Matano plane to the composition at the Matano plane. Characteristic depths for concentration profiles on either side of the Matano plane are employed to characterize the concentration profiles and their deviations from error function solutions. The analysis is applied to the concentration profiles of a binary Fe-C as well as a ternary Cu-Ni-Zn single-phase diffusion couple. From a knowledge of concentration profiles on one side of the Matano plane, profiles on the other side can be generated from an error function model based on the analysis.

MECHANICAL BEHAVIOR

The Fracture Toughness of Niobium-Based, In Situ Composites
KWAI S. CHAN
The fracture resistance of Nb-Cr-Ti alloys or in situ composites of three different compositions, Cr₂Nb, and a Nb-10Si in situ composite was studied at ambient temperature. The crack-tip deformation and fracture behaviors were characterized using near-tip measurement techniques and fractographic analyses. The relevant fracture and toughening mechanisms were identified and related to the microstructure. Despite fracture by a combination of cleavage and slip band decohesion, the Nb solid-solution alloy exhibited a resistance-curve behavior with a relatively high toughness and local ductility. The source of toughness was modeled and explained in terms of a cracking process that involved alternate slip band decohesion and cleavage. The in situ composites, on the other hand, exhibited cleavage fracture but considerably lower toughness with little or no resistance-curve behaviors. The difference in the fracture behavior appears to arise from two factors: (1) the presence of a high constraint in the Nb solid-solution matrix in the in situ composites, and (2) the lack of plastic flow associated with cleavage of the constrained Nb solid-solution matrix.

A Model Study of Cavity Growth in Superplasticity Using Single Premachined Holes
ATUL H. CHOKSHI and TERENCE G. LANGDON
Experiments were conducted on a superplastic copper alloy to investigate the growth of single holes machined in the gage length prior to testing. Specimens were deformed in tension in the three regions of flow associated with superplastic materials. Within each flow region, three distinct stages of hole growth were identified. Initially, in stage 1, the hole simultaneously increases in length along the tensile axis but decreases in the dimension measured perpendicular to the tensile axis ("transverse contraction"). Subsequently, in stage 2, the hole grows both along and perpendicular to the tensile axis ("transverse growth"). Finally, in stage 3, a crack nucleates on either side of the hole and propagates to cause failure ("crack propagation"). It is shown that the transitions between the different stages of growth is dependent upon the initiation and development of macroscopic necking adjacent to the hole.

Cu-Bearing High-Strength Low-Alloy Steels: The Influence of Microstructure on the Initiation and Growth of Small Fatigue Cracks
D.L. DAVIDSON, K.S. CHAN, and R.C. McCLUNG
The fatigue characteristics of a Cu-bearing high-strength low-alloy (HSLA) steel were investigated in air, relative humidity 50 pct, as a function of microstructure, which was altered by heat treatments and welding. Small fatigue cracks (30-µm long) were naturally initiated from smooth specimens and grown past the transition length (200 µm), where they exhibited the characteristics of large fatigue cracks. The number of cycles to crack initiation depended on stress magnitude but not on microstructure, although the site of initiation was microstructurally dependent. Small cracks in all microstructures grew at K values below the large crack threshold. The as-received (polygonal ferrite) microstructure and one of the lath microstructures that resulted from heat treatment exhibited the same growth rate correlation as large cracks in the linear (Paris) region, and could be considered as an extension of the large crack growth region down to the point of initiation. Small cracks grew at rates faster than expected through one of the heat-treated and the weld microstructures; therefore, the number of cycles required for growth from initiation to the transition to large crack growth decreased about threefold, which is a potentially important factor in predicting lifetimes of structures made from this steel.

High Cycle Fatigue Behavior of Gas-Carburized Medium Carbon Cr-Mo Steel
HYUNG-JUN KIM and YOUNG-GAK KWEON
High cycle fatigue properties of gas-carburized 4140 steel were assessed to compare with those of 8620 steel which is widely used as a carburizing steel. Fatigue limit was evaluated associated with microstructure, case depth, and distribution of retained austenite and compressive residual stress near the surface. Test results indicated that the reheat quenching method of 4140 and 8620 steels produced a reduction in grain size, retained austenite level, and compressive residual stress at the surface and an increase in fatigue limit. The fatigue limit of direct-quenched 4140 steel shows substantially lower value than that of direct-quenched 8620 steel due to larger grain size of direct-quenched 4140 steel. However, the fatigue limit of reheat-quenched 4140 steel is greatly improved and is comparable to the reheat-quenched 8620 steel. This is attributed to the larger reduction ratio in grain size and deeper case depth of reheat-quenched 4140 steel as compared to direct-quenched and reheat-quenched 8620 steels.

Analysis of Damping in Particle-Reinforced Superplastic Zinc Composites
JUNSHENG LU and DAVID C. VAN AKEN
The damping behavior of superplastic zinc (SPZ) particulate composites with up to 42.5 vol pct spherical TiC particles (3 µm in diameter) was studied in the 25°C to 330°C temperature range using a low frequency torsion pendulum. The observed damping at room temperature was modeled as a combination of a diffusion-controlled dislocation relaxation and a grain boundary relaxation. Addition of TiC produced a lower dislocation damping contribution at room temperature, but this loss was offset by an increased contribution from the grain boundary relaxation. An increase in the elastic modulus was also observed for the composite. The validity of a theoretical model for predicting changes in the grain boundary relaxation peak temperature resulting from the introduction of large nondeforming particles was tested. This study demonstrates that grain sliding in SPZ alloys occurs by cooperative sliding of grain clusters containing three to five grains. The activation energy for this process was found to be 111 kJ/mole (1.15 eV), which is in agreement with previously published values for grain sliding in SPZ. A second internal friction peak at a temperature just below the eutectoid transformation temperature was also observed and this peak was associated with recrystallization.

Microstructures Relevant to Brittle Fracture Initiation at the Heat-Affected Zone of Weldment of a Low Carbon Steel
KENJI OHYA, JONGSEOP KIM, KEN'ICHI YOKOYAMA, and MICHIHIKO NAGUMO
Charpy toughness of the heat-affected zone (HAZ) of weldment of a low carbon steel has been investigated by means of an instrumented Charpy test and fractographic analysis. Microstructures were varied with thermal cycles simulating double-pass welding. The ductile-brittle transition temperature is the most deteriorated at an intermediate second-cycle heating temperature. The origin of the difference in the transition temperatures has been analyzed to exist in the brittle fracture initiation stage. Fractographic examination correlating with microstructural features has revealed that the brittle fracture initiation site is associated with the intersection of bainitic ferrite areas with different orientations rather than the martensite-austenite constituents. The role of the constraint of plastic deformation on the brittle fracture initiation is discussed.

Microstructure and Mechanical Behavior of Cr-Cr2Hf In Situ Intermetallic Composites
K.S. RAVICHANDRAN, D.B. MIRACLE, and M.G. MENDIRATTA
A detailed investigation of the effects of microstructural changes on the mechanical behavior of two in situ intermetallic composites with Cr and Cr₂Hf phases in the Cr-Hf system was performed. The nominal compositions (at. pct) of the alloys were Cr-5.6Hf (hypoeutectic) and Cr-13Hf (eutectic). The study included evaluations of strength, ductility, and fracture toughness as a function of temperature and creep behavior. Two microstructures in each alloy were obtained by heat treatments at 1250°C (fine microstructure) and 1500°C (coarse microstructure). A decrease in elastic strength (stress at the onset of inelastic response in the load-deflection curve) with the coarsening of the microstructures was noted for both alloys below 1000°C. The Cr-13Hf alloy retained strength to a higher test temperature, relative to Cr-5.6Hf alloy, under both microstructural conditions. The alloys showed no evidence of ductility at room temperature. However, in the coarse microstructure of the Cr-5.6Hf alloy, the primary Cr exhibited ductility at and above 200°C; ductility in primary Cr could be seen only at and above 1000°C for the fine microstructure. In other words, the temperature at which ductility was first observed decreased from about 1000°C to about 200°C due to high-temperature heat treatment in this alloy. Both microstructures of Cr-5.6Hf alloy showed a significant increase in fracture toughness with increasing test temperature. However, the increases in fracture toughness with temperature for the Cr-13Hf alloy microstructures were relatively small. Both alloys showed about four orders of magnitude reduction in steady-state creep rates relative to pure Cr at 1200°C. The results are analyzed in the light of deformation characteristics and fracture micromechanisms. The effects of microstructural factors, such as the size and continuity of phases, solubility levels of Hf as well as interstitial elements in Cr, on the observed mechanical behavior are discussed.

High-Temperature Deformation Processing of Ti-24Al-20Nb
P.K. SAGAR, D. BANERJEE, K. MURALEEDHARAN, and Y.V.R.K. PRASAD
Power dissipation maps have been generated in the temperature range of 900°C to 1150°C and strain rate range of 10^-3 to 10 s^-1 for a cast aluminide alloy Ti-24Al-20Nb using dynamic material model. The results define two distinct regimes of temperature and strain rate in which efficiency of power dissipation is maximum. The first region, centered around 975°C/0.1 s^-1, is shown to correspond to dynamic recrystallization of the ₂ phase and the second, centered around 1150°C/0.001 s^-1, corresponds to dynamic recovery and superplastic deformation of the phase. Thermal activation analysis using the power law creep equation yielded apparent activation energies of 854 and 627 kJ/mol for the first and second regimes, respectively. Reanalyzing the data by alternate methods yielded activation energies in the range of 170 to 220 kJ/mol and 220 to 270 kJ/mol for the first and second regimes, respectively. Cross slip was shown to constitute the activation barrier in both cases. Two distinct regimes of processing instability=mone at high strain rates and the other at the low strain rates in the lower temperature regions=mhave been identified, within which shear bands are formed.

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

Atmospheric Stress Corrosion Cracking of a Superplastic 7475 Aluminum Alloy
T.C. TSAI and T.H. CHUANG
The influence of different heat treatments upon the atmospheric stress corrosion cracking (SCC) of fine-grained 7475 Al-alloy plates has been investigated. The small size of the matrix precipitates and grain-boundary precipitates (GBPs) was found to be the main cause of atmospheric SCC susceptibility. Increasing the size of the matrix precipitates and GBPs by increasing the degree of aging could improve the atmospheric SCC resistance. The size of the matrix precipitates was the major factor affecting the atmospheric SCC resistance when GBPs were larger than a critical size that could nucleate hydrogen bubbles. However, if the size of the GBPs was smaller than this critical size, the improvement of atmospheric SCC resistance due to grain refinement, resulting from a more homogeneous slip mode, could not be obtained because hydrogen embrittlement became serious. By measuring the electrical conductivity, the influence of matrix precipitates, but not that of GBPs, on SCC susceptibility could be obtained. Retrogression and reaging (RRA) treatment could effectively improve the atmospheric SCC resistance of T6 temper because RRA temper could produce larger sizes of both the matrix precipitates and GBPs than could T6 tempered condition.

Elevated Temperature Compressive Properties of Zr-Modified NiAl
J. DANIEL WHITTENBERGER and R.D. NOEBE
Small Zr additions are known to substantially affect the deformation behavior and strength of polycrystalline NiAl, yet little information is currently available regarding the high-temperature properties of such alloys. Utilizing prealloyed powder technology, a series of four NiAl alloys have been produced containing from 0.05 to 0.7 at. pct Zr. The creep behavior of these alloys was characterized in compression between 1000 and 1400 K at strain rates ranging from ~0.1 to 10^-9 s^-1. All the Zr-modified alloys were significantly stronger than binary NiAl under lower temperature and faster strain-rate conditions; however, the single-phase materials (Zr ¾ 0.1 at. pct) and binary NiAl had similar strengths at high temperatures and slow strain rates. The two-phase NiAl-Ni₂AlZr alloys containing 0.3 and 0.7 at. pct Zr had nearly identical strengths. While the two-phase alloys were stronger than the single-phase materials at all test conditions, the degree of microstructural damage in the two-phase alloys due to internal oxidation during testing appeared to increase with Zr level. Balancing the poor oxidation behavior with the consistent strength advantage of the two-phase alloys, it is concluded that optimum elevated-temperature properties could be obtained in Heusler-strengthened NiAl containing between 0.1 and 0.3 at. pct Zr.

High-Temperature Behavior of Precious Metal Base Composites
I.M. WOLFF and G. SAUTHOFF
The high thermodynamic stability and exceptional mechanical properties of intermetallic compounds based on platinum group metals (PGMs) make them potentially exploitable as structural materials for elevated temperature service. In this study, the development of high-temperature strength was evaluated for a selection of eutectic composites based on Ru-Al, Ir-Al, Ru-Nb, Ir-Nb, and Ru-Al-Ni using compression tests. All the alloys tested were characterized by high strength at extreme temperatures, none more so than an Ir₈₄Nb₁₆ alloy which had a proof stress in excess of 600 MPa at 1300°C. In addition, several alloys showed significant compressional ductility at room temperature. Of particular interest is the synergy of soft constituent phases in evolving strengths greater than that anticipated by simple rules of mixture. The potential of secondary phases to advance both strength and room-temperature ductility makes these composites worthy of further investigation.

Corrosive Wear of SiC Whisker- and Particulate-Reinforced 6061 Aluminum Alloy Composites
S.Y. YU, H. ISHII, and T.H. CHUANG
Wear tests on SiC whisker- and SiC particulate-reinforced 6061-T6 aluminum matrix composites (SiCw/Al and SiCp/Al), fabricated using a high pressure infiltration method, were performed in laboratory air, ion-exchanged water and a 3 pct NaCl aqueous solution using a block-on-ring type apparatus. The effects of environment, applied load, and rotational (sliding) speed on the wear properties against a sintered alumina block were evaluated. Electrochemical measurements in ion-exchanged water and a 3 pct NaCl aqueous solution were also made under the same conditions as the wear tests. A comparison was made with the properties of the matrix aluminum alloy 6061-T6. The SiC-reinforced composites exhibited better wear resistance compared with the monolithic 6061 Al alloy even in a 3 pct NaCl aqueous solution. Increase in the wear resistance depended on the shape, size, and volume fraction of the SiC reinforcement. Good correlation was obtained between corrosion resistance and corrosion wear. The ratios of wear volume due to the corrosive effect to noncorrosive wear were 23 to 83 pct, depending on the wear conditions.

Effects of Nitrogen Implantation on Low Cycle Fatigue Behavior of Ferritic Fe-24Cr-4Al Stainless Alloy
S.M. ZHU and S.C. TJONG
The effects of nitrogen implantation on cyclic deformation response, near-surface dislocation substructure, surface slip band formation, crack initiation, and fatigue life under low cycle fatigue of the ferritic Fe-24Cr-4Al stainless alloy were investigated. Implantation was carried out at an energy of 65 keV and at a fluence of 2 X 10¹⁷ ions/cm². Nitrogen implantation resulted in a substantial cyclic hardening in the alloy. Homogeneous planar dislocation arrangements were formed in the near-surface region of implanted specimens after fatigue, while dislocation loop debris and patches were developed in the un-implanted specimens. Moreover, formation of persistent slip bands (PSBs) was greatly suppressed in the surface of the implanted specimens. Nitrogen implantation also resulted in an alteration of the crack initiation mode from the grain boundary to the surface penetration of the PSBs nucleated below the surface layer. Fatigue life improvements after nitrogen implantation could only be obtained when the PSBs were not only suppressed but also homogenized in the implanted surface layer.

PHYSICAL CHEMISTRY

ENVIRONMENT

The Influence of Aqueous Environments on Low K and High K Fatigue Crack Propagation Behavior in Low Carbon Structural Steels

Initiation of Stress Corrosion Cracking for Pipeline Steels in a Carbonate-Bicarbonate Solution
Z.F. WANG and A. ATRENS
The linearly increasing stress test (LIST) was used to study the stress corrosion cracking (SCC) behavior of a range of pipeline steels in carbonate-bicarbonate solution under stress rate control at different applied potentials. Stress corrosion cracking, at potentials below -800 mV(SCE), was attributed to hydrogen embrittlement. Stress corrosion cracking, in the potential range from about -700 to -500 mV(SCE), was attributed to an anodic dissolution mechanism. In the anodic potential region, the SCC initiation stress was larger than the yield stress and was associated with significant plastic deformation at the cracking site. The relative SCC initiation resistance decreased with increasing yield strength. In the cathodic potential region, the SCC initiation stress was smaller than the yield stress of steel; it was approximately equal to the stress at 0.1 pct strain (_{0.1 pct}) for all the steels. The original surface was more susceptible to SCC initiation than the polished surface.

WELDING & JOINING

ELECTRONIC, MAGNETIC, & OPTICAL MATERIAL

SOLIDIFICATION

Determination of the Solidification Curves of Commercial Aluminum Alloys
SINN-WEN CHEN and SHIH-CHANG JENG
The understanding of solidification curves is crucial for the control of many solidification processes. The solidification curves of seven commercial aluminum alloys (1050, 1070, 1100, 3003, 3004, 5052, and 7075) were determined by using a differential thermal analysis (DTA) coupled with a mathematical modeling method, which has advantages over the conventional quenching method or the solidification modeling method. With the proposed technique in this study, the whole spectrum of the solidification curve can be determined with high reliability, without requiring the knowledge of a number of thermophysical parameters, which are usually difficult to obtain.

Orientation Dependence of Primary Dendrite Spacing
CH.-A. GANDIN, M. ESHELMAN, and R. TRIVEDI
The orientation dependence of the primary dendrite spacing is examined through the solidification of a succinonitrile-3.61 wt pct acetone alloy with a Bridgman-type device. Primary dendrite spacing has been studied as a function of the primary dendrite trunk orientation with respect to the thermal gradient direction under different growth rate conditions. The observations show that the orientation dependence of the primary dendrite spacing can be related to the formation of new primary dendrites at divergent grain boundaries via the branching mechanism. A branching-based model is developed to study the interaction between secondary and tertiary dendrite arms. Based on this model, a simple analytical relationship is proposed to account for the orientation dependence of the primary dendrite spacing.

Transitions between Type A Flake, Type D Flake, and Coral Graphite Eutectic Structures in Cast Irons
J.S. PARK and J.D. VERHOEVEN
Directional solidification experiments were used to measure the transition velocities between the type A and coral eutectic structures in high-purity cast irons and between the type A and type D eutectic structures in S and Te doped cast irons. Introduction of O into the gas atmosphere was found to have little effect on the A D transition velocities in S doped alloys, but it produced a strong reduction in the A coral transition velocities in high-purity irons. Transmission electron microscopy revealed interesting variations in the defect structures of the graphite in the flake irons vs the type of flake (A or D) and the type of doping element. Scanning Auger microscopy demonstrated that both S and Te segregate to the iron/graphite interface. In the S doped alloys, type A flakes are generally covered with a monolayer of S with patches of O in the form of iron oxide having a thickness on the order of 2 nm. A series of experiments, including examination of fracture surfaces at the quenched solid/liquid growth front, have shown that S segregates to the iron/graphite interfaces from the liquid at the growth front, but O forms at these interfaces during the cooldown. These results are discussed in relation to current models of eutectic growth in cast irons.

Equiaxed Dendritic Solidification with Convection: Part I. Multiscale/Multiphase Modeling
C.Y. WANG and C. BECKERMANN
Equiaxed dendritic solidification in the presence of melt convection and solid-phase transport is investigated in a series of three articles. In part I, a multiphase model is developed to predict composition and structure evolution in an alloy solidifying with an equiaxed morphology. The model accounts for the transport phenomena occurring on the macroscopic (system) scale, as well as the grain nucleation and growth mechanisms taking place over various microscopic length scales. The present model generalizes a previous multiscale/multiphase model by including liquid melt convection and solid-phase transport. The macroscopic transport equations for the solid and the interdendritic and extradendritic liquid phases are derived using the volume averaging technique and closed by supplementary relations to describe the interfacial transfer terms. In part II, a numerical application of the model to equiaxed dendritic solidification of an Al-Cu alloy in a rectangular cavity is demonstrated. Limited experimental validation of the model using a NH₄Cl-H₂O transparent model alloy is provided in part III.

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

Equiaxed Dendritic Solidification with Convection: Part III. Comparisons with NH₄Cl-H₂O Experiments
C. BECKERMANN and C.Y. WANG
This third article on equiaxed dendritic solidification is intended to provide experimental validation of the multiphase model developed in part I. Numerical and experimental results are presented for the solidification of a NH₄Cl-70 wt pct H₂O solution inside a square cavity cooled equally from all sidewalls. The numerical simulations were performed using the numerical procedures developed in part II. The experiments were conducted to measure the temperature histories via thermocouples and to record the images of the solidification process using a shadowgraph system. Preliminary validity of the multiphase model is demonstrated by the qualitative agreement between the measurements and predictions of cooling curves as well as of the evolution of the crystal sediment bed. In addition, several important features of equiaxed dendritic solidification are identified through this combined experimental and numerical study, including the grain generation and growth behaviors in the presence of liquid flow, the sedimentation of equiaxed crystals, the formation of a crystal sediment bed, and a bottom zone of negative segregation resulting from the countercurrent solid-liquid multiphase flow. Quantitative comparisons between the numerical simulation and experiment reveal several areas for future research.

MATERIALS PROCESSING

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

COMPOSITE MATERIALS

Tensile Ductility and Fracture of Superplastic Aluminum-SiC Composites Under Thermal Cycling Conditions
GASPAR GONZALEZ-DONCEL and OLEG D. SHERBY
The tensile and fracture creep behavior of aluminum-SiC composites has been evaluated under isothermal and cyclic heating conditions. The true strain to fracture under thermal cycling conditions is shown to decrease linearly with the logarithm of the applied stress. This trend is unexpected since the strain rate sensitivity exponent, m, is high (typical of superplastic materials) and constant over the range of stresses studied. The results obtained are explained by a fracture mechanics model in which the crack size increases by a strain-induced mechanism. The results are compared with similar behavior obtained in superplastic ceramics.

Observation of Short Fatigue Crack-Growth Process in SiC-Fiber-Reinforced Ti-15-3 Alloy Composite
S.Q. GUO, Y. KAGAWA, and K. HONDA
The microscopic fatigue damage characteristics and short fatigue crack growth of an unnotched SiC(SCS-6) fiber-reinforced Ti-15-3 alloy composite were investigated in tension-tension fatigue tests (R=0.1) carried out at room temperature for applied maximum stress of 450, 670, and 880 MPa. In situ observation of the damage-evolution process was done using optical and scanning laser microscopies, which were attached in the fatigue machine. The first damage for the composite started from a cracking of the reaction layer followed by fiber fracture. The matrix cracking initiated near the broken fiber when the microhardness of the matrix just to the side of the fracture fiber reached ~6 GPa, and the number of cycles for the initiation of this cracking decreased with the increase of applied stress. The slope of the relation of surface crack growth length vs number of cycles fell into two characteristic stages; in the first stage, the rate was lower than the second stage and accelerated. The surface crack growth rate, d (2c) /dN, vs surface crack length relation also fell into two stages (stages I and II). With the increase in surface crack length, the crack-growth rate, d (2c) /dN, decreased in stage I and increased in stage II. The transition from stage I to stage II occurred due to the fracture of fibers located around the first fractured fiber. It was concluded that the fatigue crack growth resistance of the composite in the short-crack region was controlled by the fiber fracture and matrix work hardening near the fractured fiber. When the fiber fracture occurred, the surface crack growth rate was accelerated and became faster than that of the monolithic matrix.