METALLURGICAL AND MATERIALS TRANSACTIONS A
ABSTRACTS
Volume 29A, No. 3, March 1998

This Month Featuring: The 1997 Institute of Metals Lecture, Alloy Phases, Transformations, Transport Phenomena, Mechanical Behavior, Welding & Joining, Surface Treatment, Solidification, Materials Processing, Composite Materials. View March 1998 Table of Contents.

THE 1997 INSTITUTE OF METALS LECTURE

The Role of Nanosized Particles: A frontier in Modern Materials Science, from Nanoelectronics to Environmental Problems
M. JOSÉ-YACAMÁN
Small metal and semiconductor particles with a size of a few nanometers are one of the important systems in modern materials science. Nanoparticles have found applications in many fields, ranging from catalysis to magnetic storage. In the present work, we discuss some of the methods to characterize the structure of nanoparticles using electron microscopy. We also discuss some of the exciting novel applications of nanoparticles in nanoelectronics and nanophotonics. Finally, we show that nanoparticles play an important role in producing atmospheric pollutants.

ALLOY PHASES

Characterizations of Pore and Constituent Particle Populations in 7050-T7451 Aluminum Plate Alloys
JIMIN ZHANG, MAREK A. PRZYSTUPA, and ANNETTA J. LUÉVANO
Although qualitative relationships between fatigue lives and the sizes of the microstructural features, such as pores and particles, are well known, the quantitative models are lacking because of the unavailability of the required detailed microstructural data. The purpose of this work was to obtain such data for the high porosity (HP) and reduced porosity (RP) variants of the aluminum 7050- T7451 thick-plate alloys. Both alloys had similar tensile and fracture properties, but the reduced porosity variant showed superior fatigue performance attributed to the smaller sizes of the fatigue crack initiating particles and pores. Those size differences, as well as the differences in the through- thickness size gradients, have been characterized in this work. The sizes, shapes, and orientations of particles and pores were analyzed first on the plane sections and then converted to the true three- dimensional (3-D) characteristics using the moment method. In the conversions, the particle and pore shapes have been assumed as triaxial ellipsoids and their size distributions as lognormal. The spatial distributions were quantified using the nearest neighbor spacing method. Results confirmed that the reduced porosity alloy had smaller particles and pores than the high porosity variant. The size distributions in the former were also more confined. In both alloys, the largest particles and pores were at the plate centers and the smallest at the surface. Their spatial distributions could be categorized as random with clusters.

TRANSFORMATIONS

Computer Simulation and Experimental Investigation of the Spinodal Decomposition in the Ti-Cr Binary Alloy System
A.M. MEBED and T. MIYAZAKI
The spinodal decomposition of the Ti-Cr binary alloys system is still questionable, because there are only rare experimental and thermodynamics data; moreover, simulation results for a real alloy system have not been found. Transmission electron microscopy (TEM) and quantitative computer simulations based on Khachaturyan's diffusion equation have been employed to study the microstructural evolution occurring in the metastable Ti-Cr alloys. Our study results reveal that the metastable Ti-Cr undergoes a phase separation reaction titanium rich (1) + chromium rich (2) through a spinodal decomposition. The coherent two-phase fields show extremely fine platelike precipitates distributed homogenously through the bcc matrix, which are parallel to the ;fb100;rb plane. Those precipitates are highly elastic induced from the first step of the phase separation. There is good agreement between the observed microstructure and the simulation results. The mode of formation of phase is also discussed.

Communication: On the Characteristics of M2C Carbides in the Peak Hardening Regime of AerMet 100 Steel
RAGHAVAN AYER AND P. MACHMEIER

TRANSPORT PHENOMENA

A Diffusion Solution for the Melting/Dissolution of a Solid at Constant Temperature and Its Use for Measuring the Diffusion Coefficient in Liquids
X. WAN, Q. HAN, and J.D. HUNT
A general solution to the diffusion equation is obtained for the composition in two phases when one phase dissolves and the other grows at constant temperature. The relevance of the derivation to melting is discussed. The exact solution is used to examine the condition when diffusion may be considered to occur in only one phase. Experiments have been carried out using the analysis to obtain solute diffusion coefficients in liquid succinonitrile-water alloys. The method relies on an interface displacement measurement and does not require the composition to be measured as a function of position as is usually necessary with a diffusion couple.

MECHANICAL BEHAVIOR

Investigation of the Annealing Texture Evolution in Hafnium
R. BAI, C.L. BRIANT, D.C. PAINE, and J.R. BERESFORD
This article presents a study of the evolution of the annealing texture in hafnium, as measured by electron backscattering diffraction patterns (EBSPs). It was found that the annealing texture of as- received extruded rod depended on the annealing temperature. After low-temperature recrystallization, the deformation axis was parallel to [] or [] and the basal planes were approximately parallel to the deformation axis. These orientations were deduced by the position of the points in the standard stereographic triangle used to produce the inverse pole figure. As the annealing temperature was raised to 1700°C, the direction parallel to the rolling direction changed to [] and the grain size increased. It appeared that the increase in grain size occurred by a process of abnormal grain growth, and this abnormal grain growth appeared to be the cause of the change in the texture. Texture was also examined in samples that had been warm rolled to thickness reductions between 10 and 90 pct and then annealed at 1700°C. In these samples, the main feature of the texture was that the basal plane became parallel to the rolling plane as the amount of rolling increased. The maximum grain size was observed in samples that had been rolled to a reduction in thickness of 50 pct.

Stress-Induced Martensitic Phase Transformations in Polycrystalline CuZnAl Shape Memory Alloys under Different Stress States
KEN GALL, HUSEYIN SEHITOGLU, HANS J. MAIER, and KURT JACOBUS
The effect of different uniaxial and triaxial stress states on the stress-induced martensitic transformation in CuZnAl was investigated. Under uniaxial loading, it was found that the compressive stress level required to macroscopically trigger the transformation was 34 pct larger than the required tensile stress. The triaxial tests produced effective stress-strain curves with critical transformation stress levels in between the tensile and compressive results. It was found that pure hydrostatic pressure was unable to experimentally trigger a stress-induced martensitic transformation due to the large pressures required. Traditional continuum-based transformation theories, with transformation criteria and Clausius-Clapeyron equations modified to depend on the volume change during transformation, could not properly predict stress-state effects in CuZnAl. Considering a combination of hydrostatic (volume change) effects and crystallographic effects (number of transforming variants), a micromechanical model is used to estimate the dependence of the critical macroscopic transformation stress on the stress state.

Flow Localization in Sheet Specimens with Pairs of Holes
A.B. GELTMACHER, D.A. KOSS, M.G. STOUT, and P. MATIC
The deformation localization behavior of sheet specimens containing geometric perturbations in the form of pairs of through-thickness holes is examined. Both experiments and computational modeling are performed in either uniaxial or equal-biaxial tension in order to examine the effect of applied loading path on the far-field strain needed to initiate localized necking in the ligament between the hole pairs. The models also examine the influence of hole spacing and matrix strain hardening on ligament localization. The far-field strain needed to cause the localization of the ligament is shown to increase as the biaxiality of the loading path increases, the hole spacing increases, and the strain- hardening exponent increases. The present study also indicates that the onset of localized necking can be predicted by employing the Hill criterion, if the local strain states within the ligament are taken into account.

Relationship between Fracture Toughness and Crack Extension Resistance Curves (R Curves) for Ti-6Al-4V Alloys
TAKAO HORIYA and TERUO KISHI
The effects of microstructure, impurity content, and testing temperature on the fracture toughness (as measured by the crack tip opening displacement (CTOD)) and microcrack extension resistance curves (R curves) of Ti-6Al-4V alloys were examined. At 0°C, microstructure is the most influential factor in the toughness-strength relationship. Acicular microstructure specimens have a higher CTOD than specimens with equiaxed microstructures, regardless of strength (0.2 pct proof stress) and impurity content. At -196°C, impurity content becomes a controlling factor in the toughness-strength relationship. Extra-low impurity (ELI) specimens, which have a lower impurity content, show a higher CTOD, irrespective of microstructure. Microcracks extended from the notch tip before the maximum load was reached during testing were investigated, and crack initiation (i) and extension- resistance properties were evaluated by obtaining exact R curves of the microcracks. At 0°C, specimens with different microstructures and different impurity contents have almost the same i. But acicular-microstructure specimens with a higher CTOD at a given strength show a greater crack extension resistance. At -196°C, ELI specimens, which have a higher CTOD, show a larger crack extension resistance. It is concluded that the crack extension-resistance property of the microcracks extended from the notch tip before the maximum load is a controlling factor for the fracture toughness of Ti-6Al-4V alloys.

A Study on Fractography in the Low-Temperature Brittle Fracture of an 18Cr-18Mn-0.7N Austenitic Steel
SHI CHENG LIU, T. HASHIDA, H. TAKAHASHI, H. KUWANO, and Y. HAMAGUCHI
The fracture mode and crack propagation behavior of brittle fracture at 77 and 4 K in an 18Cr- 18Mn-0.7N austenitic stainless steel were investigated using optical and scanning electron microscopy. The fracture path was examined by observing the side surface in a partially ruptured specimen. The relationship of the fracture facets to the microstructures was established by observing the fracture surface and the adjacent side surface simultaneously. Three kinds of fracture facets were identified at either temperature. The first is a smoothly curved intergranular fracture facet with characteristic parallel lines on it. The second is a fairly planar facet formed by parting along an annealing twin boundary, a real {111} plane. There are three sets of parallel lines on the facet and the lines in different sets intersect at 60 deg. The third is a lamellar transgranular fracture facet with sets of parallel steps on it. Fracture propagated by the formation of microcracks on a grain boundary, annealing twin boundary, and coalescence of these cracks. The observation suggests that the ease of crack initiation and propagation along the grain boundary and the annealing twin boundary may be the main reason for the low-temperature brittleness of this steel. A mechanism for grain boundary cracking, including annealing twin boundary parting, has been discussed based on the stress concentration induced by impinging planar deformation structures on the grain boundaries.

The Critical Resolved Shear Stress of a Superalloy as a Combination of Those of Its Matrix and ' Precipitates
ASTRID NITZ and ECKHARD NEMBACH
The critical resolved shear stress (CRSS), 0, of peak-aged single crystals of the '-hardened commercial nickel-base superalloy NIMONIC 105 has been measured as a function of temperature T and orientation [hkl] of the compression axis. The same measurements have been carried out for the two constituent phases of NIMONIC 105: for the single-phase matrix and for the single-phase L12- long-range ordered ' phase. The value of T ranged from 283 to 1150 K, and the following orientations were studied: [001], [011], [], and []. The specimens were compression tested. The 0 values of NIMONIC 105 and of the ' phase are anisotropic; the anisotropy of NIMONIC 105 is similar to that of the ' phase, but less pronounced. The 0 of the phase is isotropic. The 0 values of the and of the ' phase vary with T, whereas the 0 of NIMONIC 105 is nearly independent of T for 400 K T 1000 K. A model is presented that relates the function 0 ([h, k, l], T) of NIMONIC 105 to the analogous functions of its constituent and ' phase.

Deformation Structure and Subsurface Fatigue Crack Generation in Austenitic Steels at Low Temperature
OSAMU UMEZAWA and KOTOBU NAGAI
In order to progress in the understanding of fatigue crack generation for high-strength alloys, the subsurface fatigue crack initiation sites were characterized and the deformation structure was investigated for the solution-treated 24Cr-15Ni-4Mn-0.3N and 32Mn-7Cr-0.1N austenitic steels. High- cycle fatigue tests of those steels were carried out at 4, 77, and 293 K. Subsurface crack initiation was detected in the lower-peak stress and/or in the longer-life range at the three temperatures. The subsurface crack initiation sites were intergranularly formed. The localized deformation and/or strain concentration by dislocation arrays of the (111)-<110> system assisted intergranular cracking due to incompatibility at grain boundaries. Dislocation movements were restricted to their slip planes. Even at the lower stress level, dislocations had generated in more than one slip system and piled up to a grain boundary. The peak cyclic stress was lowered with the increasing size of the subsurface crack initiation site. The dependence of the subsurface crack size on the peak cyclic stress was discussed.

Communication: Effect of Temperature on Silicon Particle Damage in A356 Alloy
ARUN M. GOKHALE, MANISH D. DIGHE, AND MARK HORSTEMEYER

WELDING & JOINING

A Study of the Structure of Dissimilar Submerged Arc Welds
N.A. McPHERSON, T.N. BAKER, and D.W. MILLAR
Butt welds have been produced using the submerged arc welding (SAW) process to join a carbon steel plate of normal shipbuilding grade to an austenitic stainless steel 316LN plate. Variables used in this work were the position of the consumable wire in the weld preparation and the ferrite number of the consumable wire. Abnormally high hardnesses were measured in some regions of the welds. These were related to the central position of the consumable wire in the weld preparation. Undesirably low ferrite numbers were related to the ferrite number of the consumable wire and also to the central position of the consumable wire in the weld preparation. The position of the consumable wire in the weld preparation controlled the relative dilution from the parent plates, and when the dilution from the 316LN steel was increased by off-setting the wire to that side of the weld preparation, the high hardness regions were no longer found. Similar optical microstructures were found to have significantly different hardnesses, which were related to the dilution effects, which were also related to the wire position within the weld preparation. Martensite was observed by transmission electron microscopy (TEM) in some of the regions of high hardness, and in other areas, the presence of extensive precipitation of M23C6 was seen. The presence of M23C6 was due to the effects of high heat input or reheating of areas by subsequent passes. The determination of the ferrite number in dissimilar welds was problematic. Feritscope determinations appeared to be influenced by magnetic effects from adjacent areas of the carbon steel. Image analysis was found to be less reliable than the Feritscope for the determination of the ferrite numbers in dissimilar welds.

SURFACE TREATMENT

Effects of Sulfur Impurity on the Scale Adhesion Behavior of a Desulfurized Ni-Based Superalloy Aluminized by Chemical Vapor Deposition
W.Y. LEE, Y. ZHANG, I.G. WRIGHT, B.A. PINT, and P.K. LIAW
The surface of a single-crystal Ni-based superalloy, which contained a bulk sulfur content of ~0.4 ppmw, was aluminized in a hot-wall chemical vapor deposition (CVD) reactor, using AlCl3 and H2 as gaseous precursors, at 1100°C. The chemical composition and microstructure of the resulting aluminide coating were characterized with particular emphasis on sulfur incorporation as an impurity during aluminizing. Depth profiling by glow-discharge mass spectroscopy (GDMS) was used as a qualitative means of assessing the level of sulfur in the coating structure. Sulfur contamination, which was initially observed at the coating surface and the substrate-coating interface, could be reduced by some minor reactor modifications. With the reduced sulfur content, scale adhesion on the surface of the aluminide grains of the coating was significantly improved during cyclic oxidation, whereas scale spallation at the coating's grain boundaries became more apparent.

SOLIDIFICATION

Solidification of a Ternary Metal Alloy: A Comparison of Experimental Measurements and Model Predictions in a Pb-Sb-Sn System
MATTHEW JOHN M. KRANE, FRANK P. INCROPERA, and DAVID R. GASKELL
Experiments were performed with a Pb-5 pct Sb-35 pct Sn system to experimentally validate simulations made with a continuum mixture model for ternary alloy solidification. Temperature histories were measured during casting, and composition profiles were found in the solidified ingot. Dendritic arm spacings were found from optical micrographs of the alloy microstructure and used to determine a constant in the Blake-Kozeny submodel for the mushy zone permeability in the liquid-solid interaction term of the momentum equations. Scaling analysis from a previous work and a large uncertainty in the permeability constant suggested that predictions of the composition are extremely sensitive to the choice of a permeability model. Three simulations were performed using the permeability constant as a parameter, and measured temperatures and compositions were compared with predictions based on different model constants. While the thermal histories had good agreement, the results near the liquidus interface, where all of the significant advection of solute occurs, suggest that the Blake-Kozeny model significantly underpredicts the resistance of the dendritic array to fluid flow.

Observations of the Columnar-to-Equiaxed Transition in Stainless Steels
WARREN J. POOLE and FRED WEINBERG
Observations are reported for the columnar-to-equiaxed transition (CET) in stainless steel bars which have been solidified slowly and progressively in a horizontal configuration. For ferritic, austenitic, and ferritic/austenitic stainless steels containing more than 0.085 wt pct carbon, CETs occur at about the same distance from the start of solidification at a given growth rate. With increasing growth rates, the transition occurs closer to the start of solidification. At low carbon levels, near 0.02 wt pct carbon, the ferritic/austenitic steel is entirely columnar, in most cases. Adding nickel to the ferritic/austenitic steel, which makes the leading phase austenitic, produces a CET with small equiaxed grains. This suggests that different particles which are effective with austenitic growth become operative as nucleants. The transition from a columnar to an equiaxed structure occurs abruptly across the diameter of the sample. There is extensive fluid flow in the bulk melt, which produces shallow temperature gradients in the melt prior to the onset of solidification. The bulk melt flow does not appear to interact significantly with the melt in the interdendritic region or the melt immediately ahead of this region. The width of the solid/liquid region in the present experiments is observed to be between 10 and 20 mm, depending on the growth velocity and the distance from the start of solidification.

MATERIALS PROCESSING

Communication: Perovskite Phase Lead Zirconate Titanate Thin Film Deposition on Pt/SiO2/Si Substrate at Low Temperature
CHUNG CHENG CHANG

COMPOSITE MATERIALS

Processing and Mechanical Properties of Magnesium-Lithium Composites Containing Steel Fibers
J.A. JENSEN and L.S. CHUMBLEY
Deformation-processed metal-metal composites (DMMC) of Mg-Li alloys containing steel reinforcing fibers were prepared by infiltrating a preform of steel wool with the molten matrix. The Li content was varied to control the crystal structure of the matrix; Mg-4 wt pct Li is hexagonal close packed (hcp), while Mg-12 wt pct Li is body-centered cubic (bcc). The low carbon steel used as the reinforcing fiber is essentially bcc. The hcp/bcc and bcc/bcc composites were subsequently deformed by rolling and by extrusion/swaging and mechanically tested to relate the tensile strength of the composites to true deformation strain. The hcp/bcc composites had limited formability at temperatures up to 400°C, while the bcc/bcc composites had excellent formability during sheet rolling at room temperature but limited formability during swaging at room temperature. The tensile strengths of the hcp/bcc composite rod and the bcc/bcc composite sheet and rod increased moderately with deformation, though less than predicted from rule-of-mixtures (ROM) calculations. This article presents the experimental data for these DMMC materials and comments on the possible effect of texture development in the matrix and fiber phases on the deformation characteristics of the composite material.

Al-TiC Composites In Situ-Processed by Ingot Metallurgy and Rapid Solidification Technology: Part I. Microstructural Evolution
X.C. TONG and H.S. FANG
The present work was undertaken to highlight a novel in situ process in which traditional ingot metallurgy plus rapid solidification techniques were used to produce Al-TiC composites with refined microstructures and enhanced dispersion hardening of the reinforcing phases. Microstructures of the experimental materials were comprehensively characterized by optical microscopy, electron microscopy, and X-ray diffraction. The results show that the in situ-synthesized TiC particles possess a face-centered cubic crystal structure with an atomic composition of TiC0.8 and a lattice parameter of 0.431 nm. The typical ingot metallurgy microstructures exhibit aggregates of TiC particles segregated generally at the -Al subgrain or grain boundaries and consisting of fine particles of 0.2 to 1.0 µm in size. The rapidly solidified microstructures formed under certain thermal history conditions contained a uniform, fine-scale dispersion of TiC phase particles with a size range of 40 to 80 nm in an -Al supersaturated matrix of 0.30 to 0.85 µm in grain size. These dispersed TiC particles generally have a semicoherent relationship with the -Al matrix. Based on the experimental results, a comprehensive kinetic mechanism of in situ TiC synthesis, which includes a solid-liquid interface reaction between the carbon particles and the Al melt and multiple nucleation and growth of TiC from the Al melt, was proposed. Then, the evolution of the aggregated TiC particles in a superheated melt before rapid solidification, i.e., dissolution, nucleation, and growth of the regenerated TiC dispersed particles, was analyzed. Furthermore, the behavior of rapid solidification kinetics, the nucleation of -Al on TiC-dispersed particles, and the interaction between TiC particles and the solidification front were documented experimentally and theoretically. These studies provided the theoretical criteria and an experimental basis for the optimum design of this kind of composite.

Al-TiC Composites In Situ-Processed by Ingot Metallurgy and Rapid Solidification Technology: Part II. Mechanical Behavior
X.C. TONG and H.S. FANG
In Part II of this article, the high-strength Al-Si/TiC composite and the elevated-temperature-resistant Al-Fe(-V-Si)/TiC composite, developed on the basis of the in situ Al-TiC composites, (Part I of the article), have been evaluated for their room- and elevated-temperature mechanical behavior. The microstructural characteristics of ingot metallurgy (IM) or rapid solidification (RS) Al-Si/TiC and Al-Fe(-V-Si)/TiC composites could be thought of as a combination of the related alloy matrix microstructures and the IM or RS Al/TiC composites. The IM Al/TiC and the Al-Si/TiC composites show superior strength and ductility to the relevant aluminum-based composites. The RS Al/TiC and the Al-Fe-V-Si/TiC exhibit high Young's moduli and substantial improvements in room- and elevated-temperature tensile properties compared to those of rapidly solidified alloys and conventional composites. The Young's modulus values of RS Al/TiC and Al-Fe-V-Si/TiC composites are well within Hashin-Shtrikman (H-S) limits, in keeping with the strong interfacial bonding. In the micromechanics approach, the principal strengthening mechanisms for the present dispersed, particle-hardened RS in situ Al-TiC composites would include Orowan strengthening, grain-size and substructure strengthening, and solid-solution strengthening.

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