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Volume 27A, No. 8, August 1996 This Month Featuring: Symposium on In Situ Reactions for Synthesis of Composites, Ceramics, and Intermetallics--Part II; Alloy Phases; Transformations; Transport Phenomena; Mechanical Behavior; Physical Chemistry; Surface Treatment; Solidification; Materials Processing; and Composite Materials. View August 1996 Table of Contents.
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) indicated that the finely dispersed ductile Ni phase was effective in dissipating the energy of cracks propagating in the ceramic matrix.
The Use of an Electric Field as a Processing Parameter in the Combustion Synthesis of Ceramics and Composites
Z.A. MUNIR
The imposition of an electric field is shown to activate self-propagating combustion reactions and thus makes possible the synthesis of a variety of ceramic and composite phases. Experimental observations and modeling studies indicated that activation is accomplished by the localized effect of the current. The relationship between wave propagation and the direction of the applied field was investigated. The synthesis of composites by field-activated combustion synthesis (FACS) was demonstrated. It was shown that the imposition of a field during the combustion synthesis of MoSi2 results in a decrease in the product particle size. The results suggest that the field can be used as a processing parameter in self-propagating combustion synthesis.
Solid-State Diffusion-Based Processing Kinetics for Uniaxial Self-Propagating High-Temperature Synthesis of MoSi2
ROBERT W. BARTLETT and KEITH A. PRISBREY
An analysis is presented for self-propagating, high-temperature synthesis (SHS) of MoSi2 from a mixture of silicon and molybdenum powders compacted into a semi-infinite, uniaxial bar and ignited at one end. The kinetics of reacting molybdenum grains are controlled by solid-state diffusion through the interposing product shell of MoSi2 that surrounds each shrinking molybdenum grain. The previously determined temperature-dependent microkinetics of this solid particle reaction are coupled with one-dimensional (1-D) heat transfer and storage to describe the time-dependent macrokinetics of the synthesis reaction sequencing through a perfectly insulated bar. The resulting equations are solved numerically to provide computed results of temperature and conversion as a function of time and distance. Preignition time, propagation velocity, and thickness of the reaction front are also determined. Results depend primarily on the initial temperature at the end of the bar, which affects preignition time, and the molybdenum grain radius. The perfectly insulated model was relaxed by limiting the maximum temperature to arbitrary values corresponding with lateral heat dissipation, and these results compare favorably with experimentally measured propagation velocities and maximum temperatures during SHS of MoSi2. The model presented, with MoSi2 as the prototype, is expected to be applicable to the SHS of many other refractory materials.
On the Role of Magnesium and Silicon in the Formation of Alumina from Aluminum Alloys by Means of DIMOX Processing
LIU YANG, DEGUI ZHU, CHANGQING XU, JUN ZHANG, and JIAN ZHANG
This article deals with the reaction mechanisms of the DIMOX (Directed Melt Oxided) processing of aluminum alloys. An orthogonalized experimental procedure was introduced to stipulate the effects of the reaction temperature, reaction time, and additional metallic elements, magnesium and silicon, on the oxidation process of aluminum alloys. Emphasis is placed on the distribution of magnesium and silicon in the products so that the behaviors of these two crucial elements for the formation of alumina from directed oxidation of aluminum alloys could be revealed. Alterative methods, including optical and scanning electron microscopy (SEM), electron probing, and wave spectrum analysis were applied to specify the microstructure characters of the products and locate the position of both magnesium and silicon in the reaction products. Judged by the weight gain after reaction, the results indicated that the temperature is the most influential factor in controlling the oxidation kinetics. Silicon is more effective than magnesium in accelerating the process, although magnesium is indispensable for the process to take place. While judged by the morphology of the reaction products, an excessive amount of silicon is harmful to the DIMOX process in that the final products consist of a large amount of porosity. Both magnesium and silicon are rather concentrated in specific regions than homogeneously distributed in the whole products. The contents of magnesium and silicon in the surface region are not as high as expected, with most of the magnesium being concentrated in the region directly neighboring the bulky metals and most of the Si in the residual bulky metals, although the contents of these two elements in the surface region are a little higher than the regions next to the surface. These characteristics, combined with other investigations, suggest that the decisive role of the slight amount of magnesium and silicon in the nucleation and growth of Al2O3 could be explained by the proposed circulated reaction.
Formation of Structural Intermetallics by Reactive Metal Penetration of Ti and Ni Oxides and Aluminates
WILLIAM G. FAHRENHOLTZ, KEVIN G. EWSUK, RONALD E. LOEHMAN, and ANTONI P. TOMSIA
Alumina-aluminum composites can be prepared by reactive metal penetration (RMP) of mullite by aluminum. The process is driven by a strong negative free energy for the reaction (8+x) Al+3Al6Si2O13 
13Al2O3+6Si+xAl. Thermodynamic calculations reveal that titanium oxide, aluminum titanate, nickel oxide, and nickel aluminate all have a negative free energy of reaction with aluminum from 298 to 1800K, indicating that it may be possible to form alumina-intermetallic composites by reactions of the type (2+x) Al+(3/y) MOy Al2O3+AlxM3/y. Experiments revealed that aluminum reacts with titanium oxide, nickel oxide, and nickel aluminate, but not aluminum titanate, at 1673K. Reaction with the stoichiometric amount of aluminum (x = 0) leads to the formation of alumina and either titanium or nickel. In some cases, reactions with excess aluminum (x > 0) produce intermetallic compounds such as TiAl3 and NiAl.
Interdiffusion in the MgO-Al2O3 Spinel With or Without Some Dopants
P. ZHANG, T. DEBROY, and S. SEETHARAMAN
With a view to seek an improved understanding of the DIMOX process, interdiffusion of polycrystalline MgO and Al2O3 in the temperature range 1473 to 1873K was studied by diffusion couple experiments. The interdiffusivities in the spinel layer were calculated as functions of composition and temperature. The spinel portion of the phase diagram in the system MgO-Al2O3 was determined from carefully measured compositions at the phase boundaries, and the low temperature spinel region of the phase diagram was confirmed from the present results. For Zn2+ as dopant in alumina, the growth rate of spinel thickness seems to increase when compared with that of the diffusion couples without dopant. The samples containing Si4+ as dopant reveal the formation of a glass phase, and the effect of Si4+ on the diffusion process appears to be negligible.
Reactive Atomization of Silicon to Form In Situ Oxide Sintering Aids
Y. WU, X. ZENG, E.J. LAVERNIA, and J.M. SCHOENUNG
The present investigation demonstrated the feasibility of using reactive atomization to produce Si powder with in situ oxide sintering aids. With further process optimization, this powder may be an alternative starting material to the conventional, mechanically blended, Si-plus-oxide powder used to produce commercial sintered reaction bonded silicon nitride (SRBSN). In the reactive atomization approach, yttrium and aluminum additives were introduced into silicon metal during induction melting. Reactive atomization was accomplished using a N2-5 pct O2 mixture as the atomization gas. During atomization, oxygen in the atomization gas reacted with Y and Al in the Si melt to produce Y2O3 and Al2O3, which act as in situ sintering aids. The reactive atomized powder demonstrated a Gaussian distribution with a mean diameter of 36 µm. The powder fines (<38 µm) were used to produce cold isostatically pressed compacts that were subsequently reaction bonded and sintered. The results demonstrate that 
-Si3N4 formed during reaction bonding and sintering. The density of the SRBSN was 77 pct of theoretical. Transmission electron microscopy (TEM) studies indicated the presence of a glassy phase on the grain boundaries, which is typical in SRBSN and indicative of the presence of the in situ sintering aids. A kinetic model was used to study the influence of processing parameters, such as droplet temperature and oxygen partial pressure, on the kinetics of oxide formation during reactive atomization. The results suggest that the volume fraction of oxides increases with increasing droplet temperature and oxygen partial pressure in the atomization gas mixture.
Microstructure and Properties of Al2O3-Al(Si) and Al2O3-Al(Si)-Si Composites Formed by In Situ Reaction of Al with Aluminosilicate Ceramics
KEVIN G. EWSUK, S. JILL GLASS, RONALD E. LOEHMAN, ANTONI P. TOMSIA, and WILLIAM G. FAHRENHOLTZ
Al2O3-Al(Si) and Al2O3-Al(Si)-Si composites have been formed by in situ reaction of molten Al with aluminosilicate ceramics. This reactive metal penetration (RMP) process is driven by a strongly negative Gibbs energy for reaction. In the Al/mullite system, Al reduces mullite to produce 
-Al2O3 and elemental Si. With excess Al (i.e., x > 0), a composite of -Al2O3, Al(Si) alloy, and Si can be formed. Ceramic-metal composites containing up to 30 vol pct Al(Si) were prepared by reacting molten Al with dense, aluminosilicate ceramic preforms or by reactively hot pressing Al and mullite powder mixtures. Both reactive metal-forming techniques produce ceramic composite bodies consisting of a fine-grained alumina skeleton with an interpenetrating Al(Si) metal phase. The rigid alumina ceramic skeletal structure dominates composite physical properties such as the Young's modulus, hardness, and the coefficient of thermal expansion, while the interpenetrating ductile Al(Si) metal phase contributes to composite fracture toughness. Microstructural analysis of composite fracture surfaces shows evidence of ductile metal failure of Al(Si) ligaments. Al2O3-Al(Si) and Al2O3-Al(Si)-Si composites produced by in situ reaction of aluminum with mullite have improved mechanical properties and increased stiffness relative to dense mullite, and composite fracture toughness increases with increasing Al(Si) content.
Pressure-Assisted Reactive Synthesis of Titanium Aluminides from Dense 50Al-50Ti Elemental Powder Blends
E. PARANSKY, E.Y. GUTMANAS, I. GOTMAN, and M. KOCZAK
In the present research, dense 
-TiAl based intermetallic samples were fabricated by reactive synthesis of fully dense elemental 50 at. pct Al-50 at. pct Ti powder blends. Two different processing routes were attempted: thermal explosion under pressure (combustion consolidation) and reactive hot pressing. In both approaches, relatively low processing or preheating temperatures (¾900°C) were used. The entire procedure of thermal explosion under pressure could be performed in open air without noticeable oxidation damage to the final product. The application of a moderate external pressure (¾250 MPa) during synthesis was shown to be enough to accommodate the negative volume change associated with TiAl formation from the elemental components and, thereby, to ensure full density of the final product. Microstructure and phase composition of the materials obtained were characterized employing X-ray diffraction and scanning electron microscopy with energy dispersive analysis. It was found that at elevated temperatures (e.g., 900°C), the equiatomic 50Al-50Ti alloy lies beyond the homogeneity range of the -TiAl phase in the Ti-Al binary and contains, in addition to -TiAl, Al-rich Ti3Al. Mechanical properties of the materials synthesized were evaluated in compression tests at different temperatures and by microhardness measurements. Due to its very fine microstructure, the Ti-Al material synthesized via reactive hot pressing exhibited superplastic behavior at temperatures as low as 800°C.
Dense CoAl-Based Alloys with Improved Ductility: Solid-State Synthesis and Microstructure Control
L. FARBER, E.Y. GUTMANAS, I. GOTMAN, and M.J. KOCZAK
Dense single- and multiphase B2 CoAl-based intermetallics were synthesized from fine elemental Co-Al blends by a completely solid-state processing route at temperatures as low as 800°C. To ensure full density of the final product, a moderately high external pressure (¾500 MPa) was applied during the solid-state reactive synthesis. Microstructure and mechanical properties of the materials obtained were investigated employing X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, and microhardness and compression testing. To improve the room-temperature ductility, an attempt was made to control the microstructure of hyperstoichiometric CoAl alloys via solution treatment with subsequent aging. At the early stages of aging, fine Widmanstauatten precipitates of Co were formed; the coarsening of Co dispersions during long or higher temperature anneals resulted in the pronounced compressive ductility of the overaged samples. In addition to general precipitation, cellular precipitation was observed in CoAl alloys with a high Co content. The relatively coarse equilibrium cellular precipitates were found to rapidly overgrow the fine metastable Widmanstauatten Co plates, causing rapid overaging of these alloys. The lattice parameter of hyperstoichiometric CoAl was found to decrease linearly with increasing Co content.
-Zr, Zr2(Ni,Fe), and Zr(Fe,Cr,Ni)2+x. Fe3Zr-type and Zr3Fe-type phases were not observed in the type 304SS-Zr alloys. The changes in alloy microstructure with zirconium content have been correlated to the Fe-Zr binary phase diagram.
"-Fe16N2; the passage by thickening to semicoherent precipitates of "-Fe16N2; the dissolution of "-Fe16N2 with the concomitant formation of '-Fe4N; and the decomposition of retained austenite by tempering. The three first stages correspond to activation energies of 95, 126, and 94 kJ/mole, respectively, consistent with the nitrogen diffusion for the first and third stages and the dislocation pipe diffusion of iron for the second.
The Role of Coincident Site Lattice Boundaries During Selective Growth in Interstitial-Free Steels
PETER GANGLI, LEO KESTENS, and JOHN J. JONAS
The development of textures in interstitial-free (IF) steels as a result of annealing after cold rolling is described with the help of a combined nucleation and growth model. Nucleation is simulated by assuming that high stored energy nucleation occurs preferentially in high Taylor factor regions in the 75 to 85 pct cold reduced materials. Growth of the nuclei then takes place by means of 
<110> type as well as by 7 <111> type coincident site lattice (CSL) transformations. Of the six symmetrically equivalent <110> transformation axes, only the ones near the maximum shear stress poles are assumed to operate. The effects of the migration of individual 9, 11, 17c, 19a, 33a, and 33c <110> boundaries are analyzed. Their relative mobilities and contributions to the final texture are deduced by matching the simulated and experimental preferred orientations using a ``least-squares'' method. On the basis of experimental results for two steels, the various boundary types are observed to have the following mobility ratios: 33a:12, 19a:4, 9:1, 33c:1, and 17c:2.
Thermoelastic Martensite and Shape Memory Effect in Ductile Cu-Al-Mn Alloys
R. KAINUMA, S. TAKAHASHI, and K. ISHIDA
Ductile shape memory (SM) alloys of the Cu-Al-Mn system have been developed by
controlling the degree of order in the phase. Additions of Mn to the binary
Cu-Al alloy stabilize the phase and widen the single-phase region to lower
temperature and lower Al contents. It is shown that Cu-Al-Mn alloys with low Al
contents have either the disordered A2 structure or the ordered L21 structure
with a lower degree of order and that they exhibit excellent ductility. The
disordered A2 phase martensitically transforms to the disordered A1 phase with
a high density of twins. The martensite phase formed from the ordered L21
phase has the 18R structure. The SM effect accompanies both the A2 A1 and
L21 18R martensitic transformations. These alloys exhibit 15 pct strain
to failure, 60 to 90 pct rolling reduction without cracking, and 80 to 90 pct
recovery from bend test in the martensitic condition. Experimental results on
the microstructure, crystal structure, mechanical properties, and shape memory
behavior in the ductile Cu-Al-Mn alloys are presented and discussed.
Structural Stability of Super Duplex Stainless Weld Metals and Its Dependence on Tungsten and Copper
J.-O. NILSSON, T. HUHTALA, P. JONSSON, L. KARLSSON, and A. WILSON
Three different superduplex stainless weld metals have been produced using manual metal arc welding under identical welding conditions. The concentration of the alloying elements tungsten and copper corresponded to the concentrations in commercial superduplex stainless steels (SDSS). Aging experiments in the temperature range 700°C to 1110°C showed that the formation of intermetallic phase was enhanced in tungsten-rich weld metal and also dissolved at higher temperatures compared with tungsten-poor and tungsten-free weld metals. It could be inferred from time-temperature-transformation (TTT) and continuous-cooling-transformation (CCT) diagrams produced in the present investigation that the critical cooling rate to avoid 1 wt pct of intermetallic phase was 2 times faster for tungsten-rich weld metal. Microanalysis in combination with thermodynamic calculations showed that tungsten was accommodated in X phase, thereby decreasing the free energy. Experimental evidence supports the view that the formation of intermetallic phase is enhanced in tungsten-rich weld metal, owing to easier nucleation of nonequilibrium X phase compared with 
phase. The formation of secondary austenite (2) during welding was modeled using the thermodynamic computer program Thermo-Calc. Satisfactory agreement between theory and practice was obtained. Thermo-Calc was capable of predicting observed lower concentrations of chromium and nitrogen in 2 compared with primary austenite. The volume fraction of 2 was found to be significantly higher in tungsten-rich and tungsten + copper containing weld metal. The results could be explained by a higher driving force for precipitation of 2 in these.
Modeling of Ferrite Growth in Nodular Cast Iron
MAGNUS WESSEN and INGVAR L. SVENSSON
In nodular cast iron, ferrite forms around the graphite nodules and growth proceeds until pearlite nucleates and consumes the remaining austenite. In order to simulate the structure, it is therefore necessary to have accurate models for the ferrite growth. Some investigators have proposed that the growth is completely governed by carbon diffusion through the ferrite shell. In the present work, it is shown that the ferrite growth in nodular cast iron can be divided into three different stages where the growth initially is governed by carbon diffusion in the austenite until the graphite nodule is entirely enveloped by a ferrite shell. During the second stage, it is proposed that the growth is controlled by the incorporation rate of carbon atoms on the graphite nodule. During the later stages of the transformation, the diffusion distance has increased considerably, and therefore, the diffusion of carbon through the ferrite shell will determine the growth rate.
Microstructural Development of a Gas-Atomized and Hot-Pressed Super-
2 Alloy
R. XU, Y.Y. CUI, D.M. XU, D. LI, Q.C. LI, and Z.Q. HU
A variety of heat treatments have been employed to explore the microstructure
in Ti-25Al-10Nb-3V-1Mo alloy prepared by gas atomization and hot pressing.
These treatments include quenching by oil cooling and water cooling and aging
at temperatures between 530°C and 950°C. Quenching transformations from
the -phase field include the formation of O phase in oil quenching and (disordered) + O phase in water quenching. The metastable
phase decomposes into O + ``
'', O, or 2 +
0/B2 phase when the as-quenched alloy is aged at various temperatures. By comparing the selection area diffraction patterns, it has been found that the ordered phase in the alloy studied in this article is distinct in
structure to the `` type'' (P3m1) and B82 phase which
are formed in the parent matrix of the ordered (B2,D03) phases. It has also been shown by X-ray diffraction (XRD) analyses that the lattice parameters of the as-aged O phase do not remain constant in the alloy at various temperatures.
/
interface were studied experimentally at 1000°C using the diffusion couple
technique. Coefficients for interdiffusion and tracer diffusion have been
calculated at the intersection points of two independent diffusion paths with a
common composition. A concentration dependence for the tracer diffusion
coefficients for each component was calculated and found to be consistent with
the literature data in the binary Cu-Ni and Fe-Ni systems. The calculated
vacancy flux in the couples was consistent with the experimentally observed
marker shift.
An Investigation by Interactive Electron Backscatter Pattern Analysis of Processing and Superplasticity in an Aluminum-Magnesium Alloy
T.R. McNELLEY and M.E. McMAHON
Tensile specimens from an Al-10Mg-0.1Zr alloy, thermomechanically processed
(TMP) according to two different schedules and deformed superplastically at 300
°C, were examined using electron backscatter pattern (EBSP) imaging in the
scanning electron microscope (SEM) with computer-aided analysis. The TMP
schedules differed only in the duration of the interpass anneal (IPA) time
between successive rolling passes. Microtexture, grain misorientation angle
distribution, and grain boundary character were analyzed for both undeformed
grip and deformed gage sections of these tensile specimens. Microtexture
analysis revealed the same texture components, primarily brass and S, in the
undeformed grip sections of both TMP materials, while analysis of deformed gage
sections revealed only a decrease in texture with no new components evident.
Material processed with a prolonged IPA time exhibited highly superplastic
response and contained a high proportion of boundaries of 5 to 15 deg
misorientation. Material processed with a short IPA time exhibited lower
superplastic elongations and contained higher-order twin boundaries, suggesting
that the twin chain mechanism is active during TMP. The observed difference in
tensile behavior appears to be the result of extended recovery during TMP of
the more ductile material evidenced by the development of a greater proportion
of boundaries of 5 to 15 deg misorientation.
Fatigue and Fracture Behavior of Nb Fiber-Reinforced MoSi2 Composites
W.O. SOBOYEJO, F. YE, and D.S. SCHWARTZ
Fatigue and fracture mechanisms in Nb fiber-reinforced MoSi2 composites are
elucidated in this article. The effects of fiber diameter on fracture and
crack-tip shielding mechanisms are discussed after a review of micromechanical
models which are applied to the prediction of residual stress levels,
toughening, and microcracking phenomena. Toughening is shown to occur by a
combination of crack bridging and crack-tip blunting under monotonic and cyclic
loading. However, the observed failure mechanisms are different under monotonic
and cyclic loading. Composites with smaller (250-µm) fiber diameters are
shown to have better fatigue resistance and lower fracture toughness than
composites with larger (750-µm) fiber diameters. The occurrence of slower
fatigue crack growth rates in the composites reinforced with smaller diameter
Nb fibers is rationalized by assessing the combined effects of fiber spacing
and interfacial crack growth on the average crack growth rates within the
composites.
Deformation Behavior of an Al-3.37 Wt Pct Li Alloy
A. THAKUR, B.P. KASHYAP, and M.K. MALIK
Al-3.37 wt pct Li alloy was deformed by differential strain rate and constant initial strain rate test techniques to investigate deformation and failure behavior over the strain rate range of 10-5 to 10-2 s-1 and the temperature range of 22°C to 580°C. Flow stress first increases then decreases with an increase in test temperature, whereas ductility shows a sigmoidal relationship with the test temperature. The maximum ductility of about 80 pct is obtained at intermediate strain rate and 550°C. Failure is noted to occur by cavity interlinkage and crack formation. Strain rate sensitivity (m) and activation energy (Q) for deformation are determined to be 0.04 to 0.13 and 96.2 to 157.4 kJ/mol, respectively. Toward lower test temperatures, both the m and Q are found to have lower values. Deformation at high temperature is suggested to be controlled by dislocation climb. However, under non-steady-state conditions due to cavitation, m and Q both vary with strain.
-(Mo,Ti)Al and 
-(Mo,Ti)3Al. The - boundary is incoherent, whereas the TiAl- and -Mo boundaries are semicoherent. The reaction zone grows with increasing heat-treatment time in a parabolic form. The incorporated Mo exhibits lower hardness than the TiAl matrix, implying that ductilizing and toughening of TiAl by introducing Mo as a ductile reinforcement are possible.
Wear-Resistant Coatings Produced by Shock-Wave Compaction of Powders
A.A. KIISKI, P.R. RUUSKANEN, and J.B. RUBIN
Wear-resistant metal-matrix composite coatings with a thickness of 1.5 mm were fabricated on low-alloy steel substrates by explosively generated shock waves. Starting materials were equivolume mixtures of WC or Cr3C2 powder mixed with either titanium or cobalt powder as a binder phase. Three different planar geometries were used, with powder layer thicknesses varying from 1 to 3 mm. Microstructural examination showed that fully dense, crack-free coatings could be produced with a uniform distribution of the carbides within the metallic binder phase. Shear strengths in excess of 40 MPa were measured for coatings composed of equivolume powder mixtures of (Cr3C2 + Ti) and (WC + Ti). The weight loss of a coating produced from an equivolume (WC + Co) powder mixture measured from a two-body abrasive wear test was significantly lower than that measured for a wear-resistant tool steel used as a reference material.
Modeling of Microsegregation in Macrosegregation Computations
H. COMBEAU, J.-M. DREZET, A. MO, and M. RAPPAZ
A general framework for the calculation of micro-macrosegregation during solidification of metallic alloys is presented. In particular, the problems of back diffusion in the primary solid phase, of eutectic precipitation at the end of solidification, and of remelting are being addressed for an open system, i.e., for a small-volume element whose overall solute content is not necessarily constant. Assuming that the variations of enthalpy and of solute content are known from the solution of the macroscopic continuity equations, a model is derived which allows for the calculation of the local solidification path (i.e., cooling curve, volume fraction of solid, and concentrations in the liquid and solid phases). This general framework encompasses four microsegregation models for the diffusion in the solid phase: (1) an approximate solution based upon an internal variable approach; (2) a modification of this based upon a power-law approximation of the solute profile; (3) an approach which approximates the solute profile in the primary phase by a cubic function; and (4) a numerical solution of the diffusion equation based upon a coordinate transformation. These methods are described and compared for several situations, including solidification/remelting of a closed/open volume element whose enthalpy and solute content histories are known functions of time. It is shown that the solidification path calculated with method 2 is more accurate than using method 1, and that 2 is a very good approximation in macrosegregation calculations. Furthermore, it is shown that method 3 is almost identical to that obtained with a numerical solution of the diffusion equation (method 4). Although the presented results pertain to a simple binary alloy, the framework is general and can be extended to multicomponent systems.
Directional Solidification of White Cast Iron
J.S. PARK and J.D. VERHOEVEN
Several studies of the ledeburite eutectic (Fe-Fe3C), in pure Fe-C alloys have shown that it has a lamellar morphology under plane front growth conditions. The structure of ledeburite in white cast irons, Fe-C-Si, consists of a rod morphology. It is generally not possible to produce plane front growth of Fe-C-Si eutectic alloys in the Fe-Fe3C form, because at the slow growth rates required for plane front growth, the Fe3C phase is replaced by graphite. By using small additions of Te, the growth of graphite was suppressed, and the plane front growth of the ledeburite eutectic in Fe-C-Si alloys was carried out with Si levels up to 1 wt pct. It was found that the growth morphology became a faceted rod morphology at 1 wt pct Si, but in contrast to the usual rod morphology of white cast irons, the rod phase was Fe3C rather than iron. It was shown that the usual rod morphology only forms at the sides of the two-phase cellular or dendritic growth fronts in Fe-C-Si alloys. Possible reasons for the inability of plane front directional solidification to produce the usual rod morphology in Fe-C-Si alloys are discussed. Also, data are presented on the spacing of the lamellar eutectic in pure Fe-C ledeburite, which indicates that this system does not follow the usual 
2V = constant relation of regular eutectics.
Orientation Selective Recrystallization of Nonoriented Electrical Steels
L. KESTENS, J.J. JONAS, P. VAN HOUTTE, and E. AERNOUDT
A nonoriented electrical steel that was commercially hot rolled and then given a 70 pct cold reduction on a laboratory mill was annealed at 680°C for 6 minutes. The sheet was then submitted to a second rolling reduction of 5.2 pct, followed in turn by a second annealing at 730°C for various times. The textures were measured after the first and second recrystallization treatments and analyzed using a nucleation and growth model. In the model, the nucleus orientation distribution function is first calculated by assessing the nucleation probability for each deformed matrix orientation. The nucleation texture is then transformed into the recrystallization texture by means of an appropriate growth criterion. The calculations indicate that the annealing texture of the conventionally rolled (70 pct reduction) sheet can be accounted for on the basis of random nucleation followed by selective growth. The latter is characterized by the following physical features: (a) the low mobility of low angle grain boundaries, (b) the enhanced mobility of {110} plane matching boundaries, and (c) variant selection of the {110} plane that carries the largest amount of slip during deformation. The computer simulations also show that low stored energy nucleation is favored in the lightly rolled sheet. These nuclei grow into the matrix by a selection mechanism that involves the increased mobility of 19a and 33a <110> coincident site lattice (CSL) boundaries.
Communication: Nanoscale Brass/Steel Multilayer Composites Produced by Cold Rolling
SATYAM S. SAHAY, KAKKAVERI S. RAVICHANDRAN, and J. GERALD BYRNE
The Effect of Volume Percent and Morphology of Phases on the Damping Behavior of Epoxy/Aluminum Composites
JYOTHI G. RAO and SREERAMAMURTHY ANKEM
In this investigation, the finite element method (FEM) has been employed to predict the effects of volume percent and morphology, including size, shape, and continuity of phases, on damping behavior of epoxy/Al composites. It is shown that for a given volume percent of phases, the loss factor of the composite increases with an increase in particle size. The effect of matricity was obtained by selecting a composite with 50 vol pct of each phase and arranging, in one case, aluminum as the particle phase and, in the other case, aluminum as the matrix phase. The loss factor obtained for the former was found to be much higher. This was attributed to the ability of the epoxy phase when it is in the form of matrix to damp/deform relatively freely. The normal stress distributions and two-dimensional (2-D) hydrostatic stress distributions were also predicted. In general, the stresses were found to be higher in the stiffer aluminum phase and the stress gradients were found to increase with an increase in particle size for a given volume percent of phases. The 2-D hydrostatic stresses were also found to be higher in the stiffer aluminum phase and the stress gradients were found to increase with an increase in particle size as well.
Synthesis of Nanocrystalline Titanium Carbide Alloy Powders by Mechanical Solid State Reaction
M. SHERIF EL-ESKANDARANY
A high-energy ball mill operated at room temperature has been used for preparing titanium carbide (TiC) alloy powders, starting from elemental titanium (Ti) and carbon (C) powders. X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) have been used to follow the progress of the mechanical solid state reaction of Ti and C powders. A complete single phase of fcc-Ti44C56 alloy powders is obtained after a very short milling time (20 ks). The lattice parameter (a0) of the end product of Ti44C56 was calculated to be 0.4326 nm. The presence of excess starting reactant materials (Ti and/or C atoms) in the final product of the alloy powders could not be detected. The end product of Ti44C56 alloy powders possesses homogeneous, smooth spherical shapes with an average particle diameter of less than 0.5 µm. The internal structure of the particles is marked by fine cell-like features of about 3 nm. On the basis of the results of the present study, the mechanical alloying (MA) process appears to provide a powerful tool for the fabrication of Ti44C56 alloy powders at room temperature. The mechanism of mechanical solid state reaction for formation of Ti44C56 alloy powders is discussed.
Communication: Wear Behavior of Aluminum-Based Metal Matrix Composites Reinforced with a Preform of Aluminosilicate Fiber
S.C. TJONG, H.Z. WANG, and S.Q. WU
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