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Volume 29A, No. 11, November 1998 This Month Featuring: Alloy Phases; Transformations; Transport Phenomena; Mechanical Behavior; Welding & Joining; Solidification; Materials Processing; Composite Materials. View November 1998 Contents.
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= 5 Boundaries in B2 Compounds
5(310) and 5(210) symmetrical tilt grain boundaries. The effects of both boundary stoichiometry and bulk simulation block stoichiometry on grain boundary energetics were investigated in detail. The structures obtained for the three alloys are very similar. Defect energies were calculated for boundaries contained in both stoichiometric and off-stoichiometric bulks. The surface energies for these B2 aluminides were also calculated so that trends concerning the cohesive energy of the boundaries could be studied. The implications of the increasing ordering energy, stoichiometry, and multiplicity of possible structures for grain boundary brittleness are discussed.
Nanometer-Scale, Fully Lamellar Microstructure in an Aged TiAl-Based Alloy
Y.Q. SUN
This article reports on the optical microscopy, transmission electron microscopy, and hardness experiments conducted on a Ti-45Al-2Cr-Nb alloy that was aged between 400°C and 800°C. The aging treatment was conducted on specimens that were quenched to retain the high-temperature 
/2 structure. The aging treatment produced an ultrafine, nanometer-scale, fully lamellar microstructure, ranging from 8 nm in thickness at 600°C to 17 nm at 700°C. The fine lamellar microstructure produced a large strengthening effect, raising the hardness to 540 on the Vickers scale. The lamellar structure nucleates preferentially at grain boundaries and grows into the grain interior.
Grain Growth and Carbide Precipitation in Superalloy, UDIMET 520
S. XU, J.I. DICKSON, and A.K. KOUL
The results of an experimental study on the grain coarsening behavior, M23C6 carbide precipitation, and secondary MC carbide precipitation kinetics in UDIMET 520 are presented. Primary MC carbides and M(C, N) carbonitrides strongly influence the grain growth, with their dissolution near 1190°C and 1250°C, respectively, resulting in two distinct grain coarsening temperatures (GCTs). M23C6 carbides precipitate in the alloy over a wide range of temperatures varying between 600°C and 1050°C. A discrete M23C6 grain boundary carbide morphology is observed at aging temperatures below 850°C. Secondary MC carbides formed at temperatures ranging between 1100°C and 1177°C, in specimens in which primary MC dissolution had been obtained at solution treatment temperatures of 1190°C to 1250°C. A schematic time-temperature-transformation (TTT) diagram for understanding the microstructure and precipitation inter-relationships in UDIMET 520 alloy is also presented.
Transformation Relaxation and Aging in a CuZnAl Shape-Memory Alloy Studied by Modulated Differential Scanning Calorimetry
Z.G. WEI
The reverse martensitic transformation and aging processes in a polycrystalline Cu-23.52 at. pct Zn-9.65 at. pct Al shape-memory alloy have been studied using the recently developed modulated differential scanning calorimetry (MDSC) technique, and some new findings are obtained. By separating the nonreversing heat flow from the reversing heat flow, MDSC can better characterize the thermodynamic, kinetic, and hysteretic features of thermoelastic martensitic transformations. Two kinds of exothermal relaxation peaks have been identified and separated from the endothermal reverse martensitic transformation: one is associated with the movement of twin interfaces or martensite-parent interfaces, and another is due to the atomic reordering in the parent phase via a vacancy mechanism. The martensite aging processes have been examined, and two stages of the aging process been distinguished: the first stage of aging is characterized by the stabilization of martensite, as manifested in the increase in the reversing enthalpy of the reverse martensitic transformation and in the transformation temperatures, and the second stage is, in fact, the decomposition of the martensite on prolonged aging, accompanied by a decrease in the transformation enthalpy. The results suggest that the mechanisms of the relaxation in the martensite and in the parent phase may be quite different.
Interdiffusion in the Carbides of the Nb-C System
J. WOODFORD and Y.A. CHANG
Interdiffusion coefficients in Nb2C and NbC1-x were measured using bulk diffusion couples in the temperature range from 1400°C to 1700°C. Marker experiments were used to show that carbon is the only component undergoing significant diffusion in both carbides. Carbon concentrations were measured by difference using electron probe microanalysis, and interdiffusion coefficients were taken from Boltzmann-Matano analyses of the resulting concentration profiles. This analysis clearly showed that, in NbC1-x, interdiffusion coefficient varies with carbon concentration, and is expressed by
(NbC1-x) = 3.84 x 10-9 exp (23.24x) exp 
m2/swhere x is the site fraction of vacancies on the carbon sublattice. The interdiffusion coefficient in Nb2C is given by
(Nb2C) = 2.04 ± 0.57 x 10-4 exp 
m2/sParabolic layer growth coefficients were estimated from the Nb|C diffusion couples as well. They are given by
m2/s
m2/s
The value of in NbC1-x was found to be consistent with literature values for the tracer diffusivity of C in NbC1-x via the thermodynamic factor, which was determined in two ways.
Interface-Controlled Fatigue Cracking of SCS-6/Ti-22Al-23Nb ``Orthorhombic'' Titanium Aluminide Composite
Y.C. HER, P.C. WANG, and J.-M. YANG
The effect of aging at elevated temperature on interfacial stability and fatigue behavior of a SCS-6/Ti-22Al-23Nb ``orthorhombic'' (O) titanium aluminide composite is investigated. The composite was heat treated in vacuum at 900°C for up to 250 hours to change the microstructural characteristics. The stability of the matrix alloy and interfacial reaction zone after extended thermal exposure was analyzed. The effect of interface on fatigue behavior, including stiffness degradation, evolution of fatigue damage, and crack growth rates, was characterized. Finally, a modified shear-lag model was used to predict the saturated matrix crack spacing in the composite under fatigue loading. The results demonstrate that aging at elevated temperature affects the stability of the interfacial reaction zone, which, in turn, degrades the fatigue properties of the composite. However, fatigue crack will not develop from the ruptured interfacial reaction layer until the thickness of the reaction zone or the maximum applied stress exceeds a critical value.
Sliding Wear Behavior of Some Al-Si Alloys: Role of Shape and Size of Si Particles and Test Conditions
B.K. PRASAD, K. VENKATESWARLU, O.P. MODI, A.K. JHA, S. DAS, R. DASGUPTA, and A.H. YEGNESWARAN
In this investigation, effects of the shape and size of silicon particles have been studied on the sliding wear response of two Al-Si alloys, namely, LM13 and LM29. The LM13 alloy comprised 11.70 pct Si, 1.02 pct Cu, 1.50 pct Ni, 1.08 pct Mg, 0.70 pct Fe, 0.80 pct Mn, and remainder Al. The LM29 alloy contained 23.25 pct Si, 0.80 pct Cu, 1.10 pct Ni, 1.21 pct Mg, 0.71 pct Fe, 0.61 pct Mn, and remainder Al. Wear tests were conducted under the conditions of varying sliding speed and applied pressure. The alloys were also characterized for their microstructural features and mechanical properties. The presence of primary silicon particles in the alloy led to a higher hardness but lower tensile properties. Further, refinement in the size of the primary particles improved the mechanical properties of the alloy system. The wear behavior of the alloys was influenced by the presence of primary Si particles and was a function of their size. Samples with refined but identical microconstituents (e.g., pressure cast vs gravity cast LM29 in terms of the size of primary Si particles and dendritic arm spacing) exhibited better wear characteristics. Their overall effect was further controlled by the test conditions. It was observed that test conditions leading to the generation of an optimal degree of frictional heating offer the best wear resistance. This was attributed to the reduced microcracking tendency of the alloy system otherwise introduced by the Si particles. The reduced microcracking tendency in turn allows the Si phase to carry load more effectively and impart better thermal stability to the alloy system. This caused improved wear resistance under the circumstances. Further, the primary Si particles improved the wear resistance of the alloy system (e.g., gravity-cast LM29 vs gravity-cast LM13) under high operating temperature conditions. Additional thermal stability and protection offered to the matrix by the primary Si phase, under the conditions of reduced microcracking tendency, were the reasons for the improved wear characteristics of the alloy system. Conversely, a reverse effect was produced at low operating temperatures in view of the predominating microcracking tendency. The study suggests that shape, size, microcracking tendency, and thermal stability of different microconstituents greatly control the mechanical and tribological properties of these alloys. The extent of effective load transfer between the phases plays an important role in this regard. Further, the overall effect of these factors is significantly governed by the test conditions.
Effect of Pitting Corrosion in NaCl Solutions on the Statistics of Fracture of Beryllium
RAJENDRA U. VAIDYA, MARY ANN HILL, MARILYN HAWLEY, and DARRYL P. BUTT
The effect of pitting corrosion, occurring in NaCl solutions, on the bending behavior of commercial beryllium was studied. Three concentrations of NaCl solutions (0.01, 0.1, and 1 M) were used in the experiments. Bend specimens were exposed to these solutions for a total of 168 hours. Weibull statistics were used to analyze the experimental bend-strength data and were applied to the failure strength, displacement to failure, and yield strength. Samples exposed to the 1 M NaCl solution exhibited a high Weibull modulus, with a higher retained mean failure strength and displacement to failure. On the other hand, the samples exposed to the 0.01 and 0.1 M NaCl solution exhibited significantly lower mechanical property values and Weibull moduli, accompanied by a larger scatter in the failure strength and displacement to failure. The engineering yield strength of the samples did not change significantly with exposure to the NaCl solutions, although there was an increase in the scatter of the values. These effects on the mechanical properties were attributed to a decreasing propensity for the formation of localized deep pits and an increasing propensity for uniform shallow pitting with increasing NaCl concentrations. Scanning electron microscopy (SEM) was primarily used for quantifying these corrosion pitting effects. In addition, atomic-force microscopy (AFM) and potentiodynamic polarization experiments were also conducted to provide supportive evidence for the differences in the pitting behavior of the beryllium with different NaCl concentrations.
Transformation Kinetics in the Heat-Affected Zone of Commercially Pure Titanium Arc Welds 
phase transformation in the heat-affected zone (HAZ) of commercially pure titanium gas tungsten arc welds. In situ SRXRD experiments were conducted using a 180-µm-diameter X-ray beam at the Stanford Synchrotron Radiation Laboratory (SSRL) (Stanford, CA) to probe the phases present in the HAZ of a 1.9 kW weld moving at 1.1 mm/s. Results of sequential linear X-ray diffraction scans made perpendicular to the weld direction were combined to construct a phase transformation map around the liquid weld pool. This map identifies six HAZ microstructural regions between the liquid weld pool and the base metal: (1) -Ti that is undergoing annealing and recrystallization; (2) completely recrystallized -Ti; (3) partially transformed -Ti, where -Ti and -Ti coexist; (4) single-phase -Ti; (5) back-transformed -Ti; and (6) recrystallized -Ti plus back-transformed -Ti. Although the microstructure consisted predominantly of -Ti, both prior to and after the weld, the crystallographically textured starting material was altered during welding to produce different -Ti textures within the resulting HAZ. Based on the travel speed of the weld, the transformation was measured to take 1.83 seconds during heating, while the transformation was measured to take 0.91 seconds during cooling. The transformation was characterized to be dominated by long-range diffusional growth on the leading (heating) side of the weld, while the transformation was characterized to be predominantly massive on the trailing (cooling) side of the weld, with a massive growth rate on the order of 100 µm/s.
Charpy V-Notch Properties and Microstructures of Narrow Gap Ferritic Welds of a Quenched and Tempered Steel Plate
G.L.F. POWELL and G. HERFURTH
Multipass welds of quenched and tempered 50-mm-thick steel plate have been deposited by a single wire narrow gap process using both gas metal arc welding (GMAW) and submerged arc welding (SAW). Of the five welds, two reported much lower Charpy V-notch (CVN) values when tested at -20°C. The CVN toughness did not correlate with either the welding process or whether the power source was pulsed or nonpulsed. The only difference in the ferritic microstructure between the two welds of low Charpy values and the three of high values was the percentage of acicular ferrite. There was no effect of the percentage of as-deposited reheated zones intersected by the Charpy notch or the microhardness of the intercellular-dendritic regions. In all welds, austenite was the microconstituent between the ferrite laths. The percentage of acicular ferrite correlated with the presence of MnO, TiO2, 
Al2O3, or MnO. Al2O3 as the predominant crystalline compound in the oxide inclusions. In turn, the crystalline compound depended on the aluminum-to-titanium ratio in both the weld deposits and the oxide inclusions. In addition to the presence of less acicular ferrite, the two welds that showed lower Charpy values also reported more oxide inclusions greater than 1 µm in diameter. The combination of more oxide inclusions greater than 1 µm and less acicular ferrite is considered to be the explanation for the lower Charpy values.
( + NbC) and L ( + Laves). The Ni-base alloys with a high C/Nb ratio represented the only exception to this general solidification sequence. This group of alloys terminated solidification with the L ( + NbC) reaction and did not exhibit the /Laves constituent. At similar levels of solute elements (Nb, Si, and C), the Fe-base alloys always formed more of the /Laves eutectic-type constituent than the corresponding Ni-base alloys. Silicon additions also increased the amount of the /Laves constituent that formed in the as-solidified microstructure, while C additions promoted formation of /NbC. The influence of Nb was dependent on the C content of the alloy. When the C content was low, Nb additions generally promoted formation of /Laves, while Nb additions to alloys with high C led to formation of the /NbC constituent. The results of this work are combined with quantitative analyses for developing -Nb-C pseudoternary solidification diagrams in a companion article.
Solidification of Nb-Bearing Superalloys: Part II. Pseudoternary Solidification Surfaces
J.N. DuPONT, C.V. ROBINO, A.R. MARDER, and M.R. NOTIS
Equilibrium distribution coefficients and pseudoternary solidification surfaces for experimental superalloys containing systematic variations in Fe, Nb, Si, and C were determined using quenching experiments and microstructural characterization techniques. In agreement with previous results, the distribution coefficient, k, for Nb and Si was less than unity, while the ``solvent'' elements (Fe, Ni, and Cr) exhibited little tendency for segregation (k 
1). The current data were combined with previous results to show that an interactive effect between kNb and nominal Fe content exists, where the value of kNb decreases from 0.54 to 0.25 as the Fe content is increased from 2 wt pct to 47 wt pct. This behavior is the major factor contributing to formation of relatively high amounts of eutectic-type constituents observed in Fe-rich alloys. Pseudoternary -Nb-C solidification surfaces, modeled after the liquidus projection in the Ni-Nb-C ternary system, were proposed. The Nb compositions, which partially define the diagrams, were verified by comparison of calculated amounts of eutectic-type constituents (via the Scheil equation) and those measured experimentally, and good agreement was found. The corresponding C contents needed to fully define the diagrams were estimated from knowledge of the primary solidification path and k values for Nb and C.
<110> Dendrite Growth in Aluminum Feathery Grains
S. HENRY, P. JARRY, and M. RAPPAZ
Automatic indexing of electron backscattered diffraction patterns, scanning electron microscopy, and optical microscopy observations have been carried out on aluminum-magnesium-silicon, aluminum-copper, and aluminum-silicon alloys directionally solidified or semicontinuously cast using the direct chill casting process. From these combined observations, it is shown that the feathery grains are made of <110> primary dendrite trunks (e.g., [
]) split in their centers by a coherent (111) twin plane. The average spacing of the dendrite trunks in the twin plane (about 10 to 20 µm) is typically one order of magnitude smaller than that separating successive rows of trunks (or twin planes). The [] orientation of these trunks is close to the thermal gradient direction (typically within 15 deg)--a feature probably resulting from a growth competition mechanism similar to that occurring during normal <100> columnar dendrite growth. On both sides of these trunks, secondary dendrite arms also grow along <110> directions. Their impingement creates wavy noncoherent twin boundaries between the coherent twin planes. In the twin plane, evidence is shown that <110> branching mechanisms lead to the propagation of the twinned regions, to the regular arrangement of the primary dendrite trunks along a [
] direction, and to coherent planar twin boundaries. From these observations, it is concluded that the feathery grains are probably the result of a change from a normal <100> to a <110> surface tension/attachment kinetics anisotropy growth mode. This change might be induced by the added solute elements, by the local solidification conditions (thermal gradient, growth rate, and melt convection), and possibly by the help of the twin plane itself. Convection in the melt could also play a role in the symmetrization of the <110> growth directions of the side arms. Finally, the proposed mechanisms of feathery grain growth are further supported by the observation of <110> dendrite growth morphologies in thin aluminum-zinc coatings.
Numerical Analysis of the Formability of an Aluminum 2024 Alloy Sheet and Its Laminates with Steel Sheets
HIROHIKO TAKUDA and NATSUO HATTA
A criterion for ductile fracture is applied to the formability prediction of an aluminum 2024 alloy sheet and its laminated composite sheets. Axisymmetric deep-drawing processes of the 2024 sheet and the laminates clad by mild steel sheets are simulated by the finite-element method. From the calculated distributions and histories of stress and strain, the fracture initiation site and the forming limit are predicted by means of the ductile fracture criterion. The predictions so obtained are compared with experimental observations. The results show that the fracture initiation in the 2024 sheet with no appearance of necking is successfully predicted by the present numerical approach. Furthermore, it is found that the formability of the 2024 sheet is improved by sandwiching it with the mild steel sheets.
'/ Precipitation in Al-10 pct Mg Alloy (solid solution) ' (metastable hex precipitate) (Al3Mg2, stable complex cubic precipitate); and (2) . Equilibrium precipitates, transformed from metastable ', were observed to possess a wide variety of orientation relationships with the matrix and were often observed to be twinned. A more restricted orientation relationship (only three variants) between and matrix was observed in direct decomposition of to , and precipitates, within these orientation relationships, were never observed to be twinned. In a predominantly binary Al-Mg system, direct precipitation of was observed to dominate. However, the presence of trace amounts of boron nitride and/or boron (or a large supply of matrix dislocations) either from cold work, or (as in case of composites) from the thermal mismatch between the SiC and Al matrix, produced both precipitation events with event 1 dominant.
Effect of SiC Volume Fraction and Particle Size on the Fatigue Resistance of a 2080 Al/SiC;yp Composite
N. CHAWLA, C. ANDRES, J.W. JONES, and J.E. ALLISON
The effect of SiC volume fraction and particle size on the fatigue behavior of 2080 Al was investigated. Matrix microstructure in the composite and the unreinforced alloy was held relatively constant by the introduction of a deformation stage prior to aging. It was found that increasing volume fraction and decreasing particle size resulted in an increase in fatigue resistance. Mechanisms responsible for this behavior are described in terms of load transfer from the matrix to the high stiffness reinforcement, increasing obstacles for dislocation motion in the form of S' precipitates, and the decrease in strain localization with decreasing reinforcement interparticle spacing as a result of reduced particle size. Microplasticity was also observed in the composite, in the form of stress-strain hysteresis loops, and is related to stress concentrations at the poles of the reinforcement. Finally, intermetallic inclusions in the matrix acted as fatigue crack initiation sites. The effect of inclusion size and location on fatigue life of the composites is discussed.
Damage Mechanisms in a Cast Ductile Iron and a Al2O3p/Al Composite
J.H. ZHU, P.K. LIAW, J.M. CORUM, J.G.R. HANSEN, and J.A. CORNIE
Mechanical behavior and damage mechanisms of an Al2O3 particulate-reinforced Al matrix composite (Al2O3p/Al) prepared by pressure infiltration are investigated and compared with those of a cast ductile iron. In addition to low cost and reduced weight, the composite has a Young's modulus comparable to the ductile iron. However, its fracture toughness is lower than that of the ductile iron. Interface debonding between the graphite and ferrite is responsible for the crack initiation behavior of the ductile iron. The crack in the ductile iron is arrested by the ductile ferrite phase surrounding the graphite, leading to high fracture toughness. For the Al2O3p/Al composite, the dominating crack initiation mode is particulate cracking. Interface debonding and zigzag cracking of particulates are additional fracture modes. The high content of Al2O3 particulates and the high thermal and elastic incompatibilities between the Al matrix and Al2O3 particulates result in brittle fracture and low fracture toughness for the composite. Possible ways to increase the fracture toughness of the Al2O3p/Al composite material are also outlined.
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