METALLURGICAL AND MATERIALS TRANSACTIONS A
ABSTRACTS
Volume 28A, No. 2, February 1997

This Month Featuring: Alloy Phases; Transformations; Transport Phenomena; Mechanical Behavior; Environment; Electronic, Magnetic, & Optical Meterial; Solidification; Materials Processing; and Composite Materials. View February 1997 Contents.

ALLOY PHASES

On the Effect of Plastic Deformation on the Coarsening of - Phase Precipitation in an Al-Cu Alloy
E. CERRI, E. EVANGELISTA, N. RYUM
In the present investigation, the focus is on dynamic coarsening of the equilibrium phase in an Al-4wt pct Cu alloy. For this purpose, specimens containing a uniform particle distribution have been produced and deformed in compression at two different temperatures (200°C and 250°C) and strain rates in the ranges of 200°C to 250°C and 10-5 to 10-2 s-1, respectively. The particle size distribution measurements performed in a scanning microscope in back-scattered mode demonstrated a double peak behavior depending on temperature: at the lowest test temperature, the dynamic coarsening is enhanced at the highest strain rate, while at 250°C, the coarsening seems to be affected by crushing of small and medium size particles during straining.

The Ce-Mg-Y System
H. FLANDORFER, M. GIOVANNINI, A. SACCONE, P. ROGL and R. FERRO
On the basis of a critical assessment, the Ce-Mg-Y system has been investigated, employing X-ray diffraction (XRD), optical microscopy, and microprobe analyses. Phase relations have been determined in the complete isothermal section at 500°C, revealing the existence of two novel ternary compounds: Ce1-xYxMg5-y (0.39 x 0.84; 0 y 0.60) with the GdMg5, or the defect Sm11Cd45-type and the cubic Laves phase Ce1-xYxMg2 (x 0.67). The binary system Ce-Y has been reinvestigated, and a constitutional diagram, temperature vs concentration, has been proposed. In contrast to earlier versions, the so-called phase with the Sm-type has been shown to be metastable.

TRANSFORMATIONS

An Experimental and Theoretical Study of Heat-Affected Zone Austenite Reformation in Three Duplex Stainless Steels
STAFFAN HERTZMAN, PAULO J. FERREIRA, and BENGT BROLUND
Three duplex grades, one molybdenum-free, one 22Cr type, and one super duplex grade, have been subjected to weld simulation treatments, and the resulting microstructures have been quantified by automatic image analysis techniques. Substantial differences between the duplex grades were observed with an increased ability to reform austenite with increased alloying content. A theoretical model has been applied, based upon the paraequilibrium concept elaborated by Hillert, and the paraequilibrium compositions of individual phases were calculated as a function of temperature using the THERMOCALC database. A model based on Cahns theory of grain boundary nucleated reactions has also been utilized to calculate the kinetics of the reaction. By using this model, the grain size effects could be included in the treatment. The results of the calculations were compared with experimental data, and the experimental results were reproduced using the same parameter set for the three materials, with the exception of the diffusion coefficient values which had to be adjusted. This adjustment has in a later study been verified experimentally. The results validate the model used and the physical relevance of using the paraequilibrium model. The appropriateness of a paraequilibrium approach is also supported by experimental evidence from weld metal compositions. It is shown that the nitrogen content of the alloys plays an important role, and a higher nitrogen content results in more efficient austenite reformation. This implies that the alloy nitrogen compositions should lie close to the upper specification limits for these materials and nitrogen losses should be avoided on welding since the material properties, both mechanical and corrosive, are strongly related to the austenite-ferrite phase ratio.

On the Rule of Additivity in Phase Transformation Kinetics
MARK LUSK and HERNG-JENG JOU
It is commonly held that a sufficient condition for the rule of additivity to be valid is that the transformation rate depend only on temperature and volume fraction. This is not true in general.

Compaction and Characterization of Mechanically Alloyed Nanocrystalline Titanium Aluminides
C. SURYANARAYANA, G.E. KORTH, and F.H. FROES
Blended elemental (BE) Ti-24 at. pct Al-11 at. pct Nb (Ti-24-11) and Ti-55 at. pct Al (Ti-55) powders and prealloyed (PA) Ti-24-11 powders were mechanically alloyed in a SPEX mill or an attritor. After SPEX milling for 10 hours, the BE Ti-24-11 powder contained the B2/ bcc phase, while the BE Ti-55 powder showed the presence of an amorphous phase. The PA Ti-24-11 powder containing the B2 phase showed a decrease of crystal size on milling. These powders were consolidated by hot isostatic pressing (``hipping''), Ceracon process, and dynamic methods. On compaction, the B2/hbcc phase in the Ti-24-11 sample transformed to a mixture of the B2 and orthorhombic (``O'') phases, while the amorphous phase in the Ti-55 powder crystallized to a mixture of the -TiAl and 2-Ti3Al phases. The finest grain size in compacted material was obtained in the dynamically consolidated powder, and the grain size in the hot isostatic pressed (``hipped'') powder became larger with the increasing hipping temperature.

TRANSPORT PHENOMENA

Computer Simulation of Multicomponent Diffusion in Joints of Dissimilar Steels
THOMAS HELANDER and JOHN ÅGREN
Diffusion in a typical joint of a stainless steel and a low-alloy steel has been simulated by means of the DICTRA software in a temperature range between 600°C and 1200°C. Calculated carbon concentration profiles are shown. A comparison with experimental data is made for a simulation at 650°C. The simulations also predict how the amount of the various phases varies with the distance from the joint. The amount of carbon transported across the joint is evaluated as a function of temperature.

MECHANICAL BEHAVIOR

Role of Matrix Microstructure on Room-Temperature Tensile Properties and Fiber-Strength Utilization of an Orthorhombic Ti-Alloy-Based Composite
C.J. BOEHLERT, B.S. MAJUMDAR, S. KRISHNAMURTHY, and D.B. MIRACLE
Microstructure-property understanding obtained for a nominally Ti-25Al-17Nb (at. pct) monolithic sheet alloy was used to heat treat a unidirectional four-ply SCS-6/Ti-25Al-17Nb metal-matrix composite (MMC) and a fiberless ``neat'' material of the same alloy for enhancing mechanical properties. The unreinforced alloy and [0]4 composite recorded significant improvements in ductility and strength, which were related to the microstructural condition. Modeling of the tensile strength based on fiber fracture statistics helped in understanding how improved matrix microstructure provided more efficient utilization of fiber strength. In comparison to the [0]4 MMC, improvement of the [90]4 response was negligible, which was related to an 2 stabilized zone around the fiber. A Nb coating on the fiber was used to modify the local microstructure, and it produced a modest improvement in strength and ductility in the transverse direction. Structure-property relations of the matrix under different heat-treatment conditions are described in terms of deformation and failure mechanisms of the constituent phases; 2 (ordered hexagonal close-packed), B2 (ordered body-centered cubic), and O (ordered orthorhombic based on Ti2AlNb).

High-Temperature Tensile Deformation and Thermal Cracking of Ferritic Spheroidal Graphite Cast Iron
C.P. CHENG, S.M. CHEN, T.S. LUI, and L.H. CHEN
Ferritic spheroidal graphite (SG) cast irons of different silicon contents were used to study the tensile behavior in the temperature range of 500°C to near Ac1. The thermal-cracking behavior under cyclic heating to various temperatures from 650°C to 850°C was also explored. According to the tensile data, the temperature dependence of the flow stress is concave upward, and that of the elongation is concave downward with drastic descent after reaching the maximum. The temperature range of ascending stress and descending elongation is above Ac1, in which the eutectic cell-wall region transforms to austenite. Intergranular fracture with serious ductility loss can take place at 500°C, if the silicon content is too high (3.9 wt pct in this test). This brittle phenomenon can be eliminated through microstructure refining. As to the thermal-cycling test, it indicates that the thermal cracking occurs through intergranular fracture. Whereas the susceptibility to thermal cracking increases with increasing silicon content, it can be reduced by refining the microstructure. Unlike the cast irons heated above Ac1 with phase transformation, the heating temperature of about 750°C leads to the most severe thermal cracking. In addition, the specimens heated in air have lower thermal-cracking resistance than those heated in a neutral salt bath.

Bainitic Chromium-Tungsten Steels with 3 Pct Chromium
R.L. KLUEH, D.J. ALEXANDER, and P.J. MAZIASZ
Previous work on 3Cr-1.5MoV (nominally Fe-3Cr-2.5Mo-0.25V-0.1C), 2.25Cr-2W (Fe-2.25Cr-2W- 0.1C), and 2.25Cr-2WV (Fe-2.25Cr-2W-0.25V-0.1C) steels indicated that the impact toughness of these steels depended on the microstructure of the bainite formed during continuous cooling from the austenitization temperature. Microstructures formed during continuous cooling can differ from classical upper and lower bainite formed during isothermal transformation. Two types of nonclassical microstructures were observed depending on the cooling rate: carbide-free acicular bainite at rapid cooling rates and granular bainite at slower cooling rates. The Charpy impact toughness of the acicular ferrite was considerably better than for the granular bainite. It was postulated that alloying to improve the hardenability of the steel would promote the formation of acicular bainite, just as increasing the cooling rate does. To test this, chromium and tungsten were added to the 2.25Cr-2W and 2.25Cr-2WV steel compositions to increase their hardenability. Charpy testing indicated that the new 3Cr-W and 3Cr-WV steels had improved impact toughness, as demonstrated by lower ductile- brittle transition temperatures and higher upper-shelf energies. This improvement occurred with less tempering than was necessary to achieve similar toughness for the 2.25Cr steels and for high-chromium (9 to 12 pct Cr) Cr-W and Cr-Mo steels.

A Critical Assessment of the Mechanistic Aspects in HAYNES 188 during Low-Cycle Fatigue in the Range 25°C to 1000°C
K. BHANU SANKARA RAO, M.G. CASTELLI, G.P. ALLEN, and J.R. ELLIS
The low-cycle fatigue (LCF) behavior of a wrought cobalt-base superalloy, Haynes 188, has been investigated over a range of temperatures between 25°C and 1000°C employing a triangular waveform and a constant strain amplitude of ±0.4 pct. Correlations between macroscopic cyclic deformation and fatigue life with the various microstructural phenomena were enabled through scanning electron microscopy (SEM) and transmission electron microscopy (TEM), detailing the crack initiation and propagation modes, deformation substructure, and carbide precipitation. Cyclic stress response varied as a complex function of temperature. Dynamic strain aging (DSA) was found to occur over a wide temperature range between 300°C and 750°C. In the DSA domain, the alloy exhibited marked cyclic hardening with a pronounced maximum at 650°C. Dynamic strain aging has been documented through the occurrence of serrated yielding, inverse temperature dependence of maximum cyclic stress, and cyclic inelastic strain developed at half of the fatigue life. Additionally, the alloy also displayed a negative strain rate sensitivity of cyclic stress in the DSA regime. These macroscopic features in the DSA domain were accompanied by the substructure comprised of coplanar distribution of dislocations associated with the formation of pileups, stacking faults, and very high dislocation density. Toward the end of the DSA domain, dislocation pinning by M23C6 precipitates occurred predominantly. The deformation behavior below and above the DSA domain has also been investigated in detail. The temperature dependence of LCF life showed a maximum at 300°C. The drastic reduction in life between 300°C and 850°C has been ascribed primarily to the deleterious effects of DSA on crack initiation and propagation, while the lower life at temperatures less than 200°C has been attributed to the combined influence of low ductility and larger cyclic response stress.

Controlled Drawing to Produce Desirable Hardness and Microstructural Gradients in Alloy 302 Wire
 M.P. RIENDEAU, M.C. MATAYA, and D.K. MATLOCK
The production of a macroscopically duplex microstructure in stainless steel alloy 302 wire, fine grains on the wire surface and coarse grains at the wire interior, was investigated by systematically varying the drawing angle from 8 to 32 deg and the reduction from 1 to 15 pct. The measured hardness gradient was correlated to the microstructure after heat treating at 1000°C for 0.5 hours. It was determined that the wire surface must exceed a hardness level of 207 KHN for recrystallization to a fine grain size, while the wire core must be hardened to a level between 166 and 207 KHN for grain growth. The deformation zone geometry parameter () for wire drawing, which is conventionally employed to give a relative measure of the strain distribution in a wire workpiece as a function of die angle and reduction, was utilized in the design of the experimental drawing schedules. The magnitude of measured hardness gradients and the corresponding calculated value of were found to vary similarly with die angle but differently with reduction. At constant total reduction, multiple- and single-step drawing schedules produced equivalent hardness gradients, even though the calculated values for indicated that the former would give a steeper gradient. Wires with two widely differing grain size gradients, coarse and fine vs. fine and coarse at the wire surface and center, were headed. The wire with fine grains on the surface had the higher resistance to surface cracking.

Deformation, Fracture, and Mechanical Properties of Low-Temperature-Tempered Martensite in SAE 43xx Steels
MARIO SAEGLITZ and GEORGE KRAUSS
Uniaxial tensile tests were performed on 4330, 4340, and 4350 steels in the as-quenched (AQ) condition and after quenching and tempering at 150°C, 175°C, and 200°C for times of 10 minutes, 1 hour, and 10 hours, respectively. Strength parameters decreased and ductility parameters increased continuously with increasing tempering. Mechanical properties are presented as a function of tempering conditions and steel carbon content, and hardness and ultimate strength changes are given as a function of Hollomon-Jaffe tempering parameters. All tempered specimens, except for some lightly tempered 4350 specimens, deformed plastically through necking instability and failed by ductile fracture. The stresses required for the ductile fracture, estimated from an analysis of the interfacial stresses at particles in the neck at fracture, showed no systematic variation with carbon content or tempering conditions despite significant variations in deformation and strain hardening. The AQ specimens of the 4340 and 4350 steels, and some of the lightly tempered 4350 steels, failed by brittle mechanisms. The deformation and fracture of the low-temperature-tempered 43xx steels are discussed in terms of the changes in fine structure, namely, the formation of transition carbides and a rearranged dislocation substructure that evolve from an AQ martensitic substructure consisting of dislocations with and without carbon atom segregation.

Effects of Test Temperature, Grain Size, and Alloy Additions on the Cleavage Fracture Stress of Polycrystalline Niobium
A.V. SAMANT and J.J. LEWANDOWSKI
The current work investigates the effects of test temperature (77 to 131 K), grain size (63 to 165 µm), and solid solution alloying additions of zirconium on the cleavage fracture stress (F) of polycrystalline niobium. Extensive fracture surface analyses of fractured notched-bend specimens revealed the location of the apparent cleavage fracture nucleation sites, while comparisons have been made to peak stress locations using existing finite element models. The effects of such microstructural changes on the magnitude of the cleavage fracture stress are rationalized via comparisons to models for cleavage fracture.

A Study on the Subgrain Superplasticity of Extruded Al-Al3Ni Eutectic Alloy
J.Y. UAN, L.H. CHEN, and T.S. LUI
A directionally solidified Al-Al3Ni eutectic alloy was extruded to obtain micron-size subgrains with [111] fiber texture. The extrusion temperature was varied to have different distributions of the Al3Ni eutectic particles. Choosing the fiber axis as the loading axis, the tensile test results at 500°C indicate that the elongation is concave downward and strain-rate dependent. Reducing the number of intragranular particles increases the maximum elongation as well as the strain rate of maximum elongation. With the particles residing only intergranularly in the as-extruded state, the maximum elongation, which occurs under the initial strain rate of 6.3 x 10-3s-1, is about 300 pct. This subgrain superplasticity is associated with low strain-rate sensitivity but high resistance against strain softening. The fiber texture is always retained, and the microstructure reveals slip of long parallel dislocations. If intragranular particles are also present in the as-extruded state, the occurrence of dislocation tangling and dynamic recovery will give rise to early onset of strain softening and inferior ductility.

Communication: Orientation Dependence of Microfracture Behavior in a Dual-Phase High-Strength Low-Alloy Steel
DONGWOO SUH, DONGIL KWON, SUNGHAK LEE, and NACK J. KIM

ENVIRONMENT

A Mechanics-Based Approach to Cyclic Oxidation
KWAI S. CHAN
The failure process of cyclic oxidation, which involves both the formation and spallation of oxides, has been treated using a mechanics approach, in which oxidation is described via a parabolic growth law, while spallation is treated in terms of a power law derived from a fracture mechanics analysis. The spallation model is formulated on the basis that shear cracks are induced by thermal stresses during the cooldown period of a thermal cycle. Some of the shear cracks develop wing tip cleavage cracks, whose propagation and linkage with other shear cracks lead to the formation of oxide fragments that separate from the oxidizing surface during cooldown. Using a mass balance, quantitative relations are obtained between the process driving force, which is the thermal stress, and the response parameters such as the weight of oxide spalled, weight gain, and weight loss of the oxide-forming element. Applications of the proposed model for predicting the cyclic oxidation behavior of metal substrates and coating materials are demonstrated by comparing model calculations against experimental data as well as against other models in the literature.

ELECTRONIC, MAGNETIC, & OPTICAL MATERIAL

Development of a Pinned Wall Sensor Using Cobalt-Rich, Near-Zero Magnetostrictive Amorphous Alloys
C.K. KIM and R.C. O'HANDLEY
A novel type of a harmonic sensor that is based on the concept of the domain wall pinning is described. Two-step annealing of the cobalt-rich amorphous precursors gives rise to the unique signature, which is attributable to the hysteresis loop with a step change in flux. The first annealing under a magnetic field causes a field-induced uniaxial anisotropy in M-H loop whose origin is believed to be due to structural inhomogeneities produced by internal segregation of oxygen atoms on cobalt {111} lattice planes. Subsequent annealing without magnetic field gives rise to the pinned wall stepped hysteresis characteristic, which is indicative of the useful markers. During annealing, it is observed that the surface and bulk of the amorphous material undergo substantial oxidation and crystallization. Consequently, microstructural features affect tag performance significantly. Those features, including (1) crystal structure of the cobalt layer (fcc or hcp), (2) oxygen faulting in cobalt layer, (3) surface oxide, and (4) magnetic state of the cobalt layer, are discussed. In addition, a surface wall pinning model is suggested since we are convinced that wall pinning is most effectively achieved at the interface between the amorphous and its semihard cobalt layer. It is also observed that there is a good correlation between domain wall pinning field and thickness of crystalline Co layer.

SOLIDIFICATION

A Thermodynamic Description for the Ternary Al-Mg-Cu System
S.-L. CHEN, Y. ZUO, H. LIANG, and Y.A. CHANG
A thermodynamic description for the ternary Al-Mg-Cu system was obtained based on the descriptions of its three constituent binaries and ternary phase equilibrium and thermodynamic data available in the literature. All binary intermetallic phases are assumed to have negligible ternary solubility. There are five ternary intermetallic phases; three of them are taken to be line compounds and two are assumed to be semistoichiometric phases. In view of the extensive experimental data available in the Al-rich corner, particularly for the Al-rich ternary eutectic, the objective of this study was to develop a thermodynamic description that allows us to quantitatively calculate the phase equilibria in the Al-rich corner. The calculated phase equilibria in the Mg-rich corner may not be as good as those in the Al-rich corner, and those in the Cu-rich corner should be topologically correct and serve as a guide for materials researchers to carry out key experiments in the future to establish the phase equilibria in that portion of the system. Nevertheless, in the absence of additional data, the calculated phase equilibria in the Cu-rich corner can be used with caution.

Effects of Liquidus and Solidus Curvature in Solidification Modeling of Binary Systems with Constant Partition Ratio
M. RETTENMAYR and T. KRAFT
A microsegregation model is used to investigate the effect of approximating liquidus and solidus lines in binary phase diagrams by straight lines during solidification modeling. Even if repartitioning of solute can be described by a constant partition coefficient, the curvature of the phase boundary lines exerts an influence on results of microsegregation calculations. Deviations of liquidus and solidus lines from linearity have a distinct influence on microstructural parameters predicted for a wide range of cooling conditions, owing to the effect of pronounced changes of the solidified fraction at a given temperature. Results obtained with simplified phase diagrams should therefore be considered with care.

Nucleation and Phase Selection in Undercooled Fe-Cr-Ni Melts: Part I. Theoretical Analysis of Nucleation Behavior
T. VOLKMANN, W. LÖSER, and D.M. HERLACH
The selection of the primary solidifying phase in undercooled stainless steel melts is theoretically analyzed in terms of nucleation theory. Nucleation phenomena are considered using different models for the solid-liquid interface energy. The classical nucleation theory for sharp interfaces and an improved modification, the diffuse interface theory, are applied. The influence of deviations of the nucleus composition from the overall alloy composition is also revealed. A preferred nucleation of the metastable bcc phase in fcc equilibrium solidification-type alloys is predicted. The critical undercooling of metastable crystallization as a function of alloy composition is calculated for an isoplethal section at 69 at. pct Fe of Fe69Cr31-xNix alloys. The results are summarized in a phase selection diagram predicting the primary solidification mode as a function of undercooling and melt composition.

Nucleation and Phase Selection in Undercooled Fe-Cr-Ni Melts: Part II. Containerless Solidification Experiments
T. VOLKMANN, W. LÖSER, and D.M. HERLACH
The solidification behavior of undercooled Fe-Cr-Ni melts of different compositions is investigated with respect to the competitive formation of -bcc (ferrite) and -fcc phase (austenite). Containerless solidification experiments, electromagnetic levitation melting and drop tube experiments of atomized particles, show that (bcc) solidification is preferred in the highly undercooled melt even at compositions where is metastable. Time-resolved detection of the recalescence events during crystallization at different undercooling levels enable the determination of a critical undercooling for the transition to metastable bcc phase solidification in equilibrium fcc-type alloys. Measurements of the growth velocities of stable and metastable phases, as functions of melt undercooling prior to solidification, reveal that phase selection is controlled by nucleation. Phase selection diagrams for solidification processes as functions of alloy composition and melt undercooling are derived from two types of experiments: X-ray phase analysis of quenched samples and in situ observations of the recalescence events of undercooled melts. The experimental results fit well with the theoretical predictions of the metastable phase diagram and the improved nucleation theory presented in an earlier article. In particular, the tendency of metastable phase formation in a wide composition range is confirmed.

Communication: Determination of the Solidification Curve of RENE N4 Superalloy
SINN-WENN CHEN and SHIH-CHANG JENG

MATERIALS PROCESSING

On the Precipitation of TiC in Liquid Iron by Reactions between Different Phases
ZHENSHENG LIU and HASSE FREDRIKSSON
In order to study the in situ formation of carbide-reinforced metal matrix composites and the inoculation reactions of some low alloy steels, titanium carbide precipitation in liquid iron was investigated. TiC particles were formed directly in liquid iron solution through reactions between different phases. The high-temperature reactions studied in the present work are reactions between (a) solid graphite and liquid Fe-Ti alloys, (b) Fe-Ti and Fe-C liquid alloys, and (c) pure solid titanium and liquid Fe-C alloys. Small samples for these reactions have been processed using a mirror furnace facility in a precisely controlled manner. Samples of Fe-Ti alloys reacted with graphite were also processed by a high frequency induction furnace and other facilities. The precipitation behavior of titanium carbide in various reactions and the distribution of carbide particles in iron melts were investigated. Different growth morphologies of the carbide were observed under various conditions. The effects of convection on the reactions were observed and discussed, and by processing the samples with various orientations relative to gravity, various patterns of carbide distribution were obtained.

Undulation of W/Matrix Interface by Resintering of Cyclically Heat-Treated W-Ni-Fe Heavy Alloys
HEUNG-SUB SONG, JOON-WOONG NOH, WOON-HYUNG BAEK, SUK-JOONG L. KANG, and BYUNG-SUN CHUN
When liquid-phase-sintered W-Ni-Fe alloys were cyclically heat treated at 1100°C and resintered at 1485°C, undulation of W/matrix interface resulted. The irregularity of the interface increased with the number of heat-treatment cycles. The residual thermal stress of W grains measured by X-ray diffraction increased with the number of heat-treatment cycles and exceeded the yield stress of W single crystal in certain crystallographic directions. A calculation by the finite element method (FEM) also showed nonuniform distribution of thermal stress on W grains. Local yielding of W grains is believed to occur during the cyclic heat treatment. The observed undulation of W/matrix interface appears therefore to result from preferential dissolution of material from regions with higher strain energy and precipitation of material at regions with lower strain energy at the resintering temperature. The undulation disappeared with the grain growth during prolonged resintering.

COMPOSITE MATERIALS

Tribological Properties of Aluminum Alloy Matrix TiB2 Composite Prepared by In Situ Processing
C.A. CARACOSTAS, W.A. CHIOU, M.E. FINE, and H.S. CHENG
An investigation of the wear behavior, in lubricated sliding and rolling of in situ prepared TiB2 particle-reinforced 2024 T4 Al alloy matrix composites against 52100 steel and hardened pearlitic nodular cast iron, respectively, was undertaken. In sliding contact, the 10 vol pct 0.3-µm TiB2-metal matrix composite (MMC) showed slightly less wear than the 10 vol pct 1.3-µm TiB2-MMC. Transmission electron microscopy of cross sections, taken normal to the wear track and parallel to the sliding direction, revealed that the TiB2 particles on the wear track were polished and particle pullout was largely absent. This was attributed to the strong interfacial bonding between the Al-alloy matrix and the TiB2 reinforcing phase. The TiB2 particles on the wear track inhibited spalling. Subsurface damage of the MMC did not occur. The wear of the steel mating surfaces worn against the TiB2- MMCs was minor and caused by the cutting action of the TiB2 particles that resided on the MMC wear track. In rolling contact, the 0.3-µm-size TiB2-MMC showed 5 times higher weight loss than the 1.3-µm TiB2-MMC for the same content of reinforcement, but the weight loss of the cast iron mating surface was less for the former. For the smaller particle size, the wear of 5 and 10 vol pct TiB2-MMCs was the same. A high density of surface cracks was present on the wear track of the 0.3-µm TiB2-MMC but not on the 1.3-µm MMC. The significance of strong particle/matrix interfacial bonding and particle size effect on the wear behavior of ceramic particulate-reinforced MMCs in lubricated sliding and rolling wear is discussed.
Direct questions about this or any other Metallurgical and Materials Transactions page to mettrans@andrew.cmu.edu.

Search TMS Document Center Tables of Contents Subscriptions Met. and Mat. Trans. TMS OnLine