METALLURGICAL AND MATERIALS TRANSACTIONS A
	ABSTRACTS Volume 27A, No. 6, June 1996 This Month Featuring: The 1995 Distinguished Lecture in Materials and Society ASM International; Symposium on Atomisitic Mechanisms of Nucleation and Growth in Solids; Mechanical Behavior; Environment and Composite Materials. View June 1996 Table of Contents.

THE 1995 DISTINGUISHED LECTURE IN MATERIALS AND SOCIETY ASM INTERNATIONAL

SYMPOSIUM ON ATOMISITIC MECHANISMS OF NUCLEATION AND GROWTH IN SOLIDS

Theory of Nucleation with Cluster Loss and Injection: Application to Plastic Deformation and Irradiation
KENNETH C. RUSSELL
Conventional nucleation theory considers the growth and decay of clusters only by thermally activated single atom addition or subtraction. Many environments of interest, including plastic deformation and energetic particle irradiation, induce numerous dynamic processes to create a host of defects which may produce a new nucleation regime. Creation and movement of dislocations and point defects and passage of thermal and stress waves could readily destroy or remove atoms from subcritical clusters, which typically contain at most only a few tens of atoms. Displacement cascades could destroy subcritical precipitates or vacancy clusters. It is also possible that clusters are injected into the system, as with vacancy-rich displacement cascades which may act as precursors for dislocation loop or void nuclei. This article modifies the nucleation equation to account for such cluster loss and injection and solves the resulting equation analytically for the steady state. The effects of cluster loss are described by several dimensionless groupings of kinetic and thermodynamic parameters. Cluster loss is found significant only if the probability of destruction is comparable to or greater than the probability of growth by single atom capture. The effects of cluster loss are predicted to be greatest for phases which form with large critical nuclei or complex unit cells. Athermal single atom loss, as by radiation resolution in ion mixing, appreciably reduces the nucleation rate only when the rate of loss approaches or exceeds the rate of solute atom capture.

A Study on Coherency Strain and Precipitate Morphology via Discrete Atom Methods
JONG K. LEE
Morphological evolution of coherent precipitates is studied by means of a discrete atom method under a plane strain condition with a purely dilatational misfit. The method is predicated upon Hookean atomic interactions and Monte Carlo diffusion and makes no assumption of a specific precipitate shape. Precipitates having elastic constants different from those of the matrix phase are treated in both isotropic and anisotropic elastic systems. Shape evolution is examined under the condition of a constant precipitate size and an isotropic interfacial energy. The results show that in general, an elastically soft precipitate tends to have an equilibrium morphology of low symmetry such as a plate, whereas a hard particle tends to take up a shape of high symmetry such as a circle. Morphological evolution proceeds through dynamic activities of coherency-induced interfacial waves whose wavelength depends upon the difference in elastic constants, precipitate geometry, anisotropy, and diffusion temperature. Coherency-induced interfacial waves seem to be responsible for the protrusions often observed along elastically hard directions in ' particles of Ni-base superalloys and also to be a source for fresh ledges for growth via the ledge mechanism. For a highly nonequilibrium precipitate, first splitting followed by coalescence appears to be a common feature in achieving its equilibrium morphology.

Effect of Uniaxial Stress on Coarsening of Precipitate Clusters
WERNER HORT and WILLIAM C. JOHNSON
The influence of moderate applied uniaxial stresses ( app/C₄₄ 10^-3) on the coarsening behavior of misfitting coherent precipitates in binary alloys has been studied. Three-dimensional (3-D) computer simulations of the coarsening have been performed for elastically homogeneous systems with tetragonal misfit strain and elastically heterogeneous systems with dilatational misfit strain. Precipitate shapes are restricted to spheres. Results depend on the sign of the misfit strain, the sign of the applied field, and the character of the elastic heterogeneity: precipitates softer than the matrix phase with positive (negative) misfit strain align along the direction of the applied stress for compressive (tensile) fields and arrange in planes perpendicular to it for tensile (compressive) fields. Precipitates harder than the matrix behave in the opposite way.

Molecular Dynamics Simulation of Martensitic Transformations in NiAl
Y. SHAO, P.C. CLAPP, and J.A. RIFKIN
Both thermally induced and stress-induced coherent nucleation and growth of an L1₀ martensitic phase have been examined and analyzed at the atomic level in molecular dynamics (MD) computer simulations of an ordered B2 NiAl lattice array using embedded atom method (EAM) interatomic potentials. Both heterogeneous and homogeneous nucleation are observed, the latter requiring an applied stress. The heterogeneous process occurs at ledge corners on stepped free surfaces and can be analyzed in terms of localized soft modes. The homogeneous nucleation can be understood as resulting from a strain spinodal instability which produces a morphology reminiscent of chemical spinodal decomposition. Self-accommodating martensite variants appear very early in the growth process, and all interfaces remain coherent with no detectable presence of dislocations in these early stages.

Computer Simulation of Ledge Migration under Elastic Interaction
MASATO ENOMOTO and JOHN P. HIRTH
The diffusional growth of a phase by the motion of disconnections (ledges which contain transformation or misfit dislocations) was studied by a finite difference computer model. The elastic stress of these dislocations is considered to alter the (local equilibrium) solute concentration at the riser of ledges and cause a complex diffusion field interaction among ledges as they migrate. In some cases, however, the ledges forming a train can migrate all at the same speed in the presence of elastic interaction. The condition under which ledges overcome the elastic barrier and form a multiple-height ledge was determined. The model was applied to the migration of ledges/Shockley partial dislocations at '-plate interfaces in Al-Ag alloys.

Bainite in the Light of Rapid Continuous Cooling Information
ANNIKA BORGENSTAM and MATS HILLERT
Rapid continuous cooling of pure iron can produce three different transformations yielding acicular structures: Widmanstätten , lath martensite, and lenticular martensite. The information on their extensions into binary systems with carbon, nickel, and chromium has been reviewed, and admittedly rough methods have been used for estimating growth rates in order to examine the role of diffusion. The effect of alloying elements on their plateau temperatures and growth rates indicates that Widmanstätten in Fe-C alloys grows under conditions close to local equilibrium for carbon, and it is suggested that the same should hold for edgewise growth of bainite. In Fe-Ni alloys, there are indications that Widmanstätten grows under a considerable solute drag, an effect which may also occur for bainite. In Fe-Cr alloys, the solute drag effect seems to be weaker but may increase with the carbon content.

Characterization of a Massive Transformation by Microstructural Analysis
R.A. VANDERMEER and B.B. RATH
Quantitative metallography was employed to investigate experimentally the nucleation behavior and growth rates in a quenched Ag + 50 at. pct Cd alloy undergoing an isothermal massive transformation at 373 K (100°C). The experimental characterization of the massively transforming microstructure as a function of time was accomplished by the stereological measurement of the microstructural variables, V_v, the volume fraction transformed, S_v, the parent/daughter interfacial area density, and _max, the largest intercept-free nodule length of the newly forming daughter phase. These data were compared to statistically derived geometrical models of the microstructure formulated on various nucleation and growth premises and expressed in terms of these same variables. A model was found which matched the data in all respects, and this allowed certain nucleation characteristics, the interface migration rates, and the nodule shapes of the daughter phase to be deduced.

The Formation Mechanism(s), Morphology, and Crystallography of Ferrite Sideplates
G. SPANOS and M.G. HALL
The formation mechanism(s), morphology, and crystallography of secondary ferrite sideplates were investigated with transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscatter pattern (EBSP) analysis, and optical microscopy in a high-purity Fe-0.12 wt pct C-3.3 wt pct Ni alloy isothermally transformed at temperatures of 550°C, 600°C, 650°C, and 675°C. The results indicate that two different mechanisms contribute to the formation of these sideplates at austenite grain boundaries. On the first mechanism, primary sideplates form initially, followed by rapid lateral impingement along their bases, resulting in a region along the grain boundary which very early in the growth process resembles an allotriomorphic film. On the second mechanism, sympathetic nucleation of ferrite sideplates occurs atop pre-existing ferrite allotriomorphs, resulting in ferrite:ferrite grain boundaries and significant crystallographic misorientations between the sideplates and the allotriomorphs with which they are associated. These results indicate that``secondary sideplates'' and the allotriomorphs from which they evolve are not composed of mono-lithic single crystals formed by a morphological instability mechanism but are instead composed of multiple crystals formed by individual nucleation events. Previous investigations in Ti-Cr alloys and a high chromium stainless steel suggest that the findings presented here may be applicable to a number of other alloy systems as well.

Formation of Bainite in Ferrous and Nonferrous Alloys through Sympathetic Nucleation and Ledgewise Growth Mechanism
HONG-SHENG FANG, JIA-JUN WANG, ZHI-GANG YANG, C.M. LI, Y.K. ZHENG, and C.X. LI
The subunits constituting a bainitic sheaf in an Fe-C-Cr-Si alloy were discovered by scanning tunneling microscopy (STM) to consist of sub-subunits, and sub-subunits were also composed of sub-sub-subunits. Detailed investigation shows that a bainitic relief is composed of many smaller reliefs, which correspond to a different structure of bainite, i.e., subunits, sub-subunits, and sub-sub-subunits. It is determined by STM that the surface relief arising from the formation of bainite in an Fe-C-Cr alloy is tent shaped rather than an invariant plane strain (IPS) type of surface relief. Careful observation shows that the relief obtained from a sub-sub-subunit is also tent shaped. It is discovered by STM that an ₁ plate, i.e., bainite formed in Cu-Zn-Al alloys, is composed of subunits. This is also demonstrated by transmission electron microscopy (TEM). The preceding results indicate that bainitic plates in Cu-Zn-Al alloys and bainitic subunits in steels are not the smallest structural units. Based on the preceding results on the ultrafine structure and the nature of surface relief accompanying bainite, it is proposed that the bainitic structure forms through a sympathetic nucleation and ledgewise growth (SNLG) mechanism. This article shows that the SNLG mechanism can be successfully applied to interpret the complicated structure of bainite.

Ferrite Nucleation and Growth During Continuous Cooling
M. MILITZER, R. PANDI, and E.B. HAWBOLT
The austenite decomposition has been investigated in two hypoeutectoid plain carbon steels under continuous cooling conditions using a dilatometer on a Gleeble 1500 thermomechanical simulator. The experimental results were used to verify model calculations based on a fundamental approach for the dilute ternary system, Fe-C-Mn. The austenite-to-ferrite transformation start temperature can be predicted from a nucleation model for slow cooling rates and small austenite grain sizes, where ferrite nucleates at austenite grain corners. The nuclei are assumed to have an equilibrium composition and a pillbox shape in accordance with minimal interfacial energy. For higher cooling rates or larger austenite grain sizes, early growth has to be taken into account to describe the transformation start, and nucleation is also encouraged at the remaining sites of the austenite grain boundaries. In contrast to nucleation, growth of the ferrite is characterized by paraequilibrium; i.e., only carbon can redistribute, whereas the diffusion of Mn is too slow to allow full equilibrium in the ternary system. However, Mn segregation to the moving ferrite-austenite interface has to be considered. The latter, in turn, exerts a solute draglike effect on the boundary movement. Thus, growth kinetics are controlled by carbon diffusion in austenite modified by interfacial segregation of Mn. Employing a phenomenological segregation model, good agreement has been achieved with the measurements.

Austenite Decomposition During Continuous Cooling of an HSLA-80 Plate Steel
S.W. THOMPSON, D.J. COLVIN, and G. KRAUSS
Decomposition of fine-grained austenite (10-µm grain size) during continuous cooling of an HSLA-80 plate steel (containing 0.05C, 0.50Mn, 1.12Cu, 0.88Ni, 0.71Cr, and 0.20Mo) was evaluated by dilatometric measurements, light microscopy, scanning electron microscopy, transmission electron microscopy, and microhardness testing. Between 750°C and 600°C, austenite transforms primarily to polygonal ferrite over a wide range of cooling rates, and Widmanstätten ferrite sideplates frequently evolve from these crystals. Carbon-enriched islands of austenite transform to a complex mixture of granular ferrite, acicular ferrite, and martensite (all with some degree of retained austenite) at cooling rates greater than approximately 5°C/s. Granular and acicular ferrite form at temperatures slightly below those at which polygonal and Widmanstätten ferrite form. At cooling rates less than approximately 5°C/s, regions of carbon-enriched austenite transform to a complex mixture of upper bainite, lower bainite, and martensite (plus retained austenite) at temperatures which are over 100°C lower than those at which polygonal and Widmanstätten ferrite form. Interphase precipitates of copper form only in association with polygonal and Widmanstätten ferrite. Kinetic and microstructural differences between Widmanstätten ferrite, acicular ferrite, and bainite (both upper and lower) suggest different origins and/or mechanisms of formation for these morphologically similar austenite transformation products.

Copper Precipitation During Continuous Cooling and Isothermal Aging of A710-Type Steels
S.W. THOMPSON and G. KRAUSS
Precipitation in copper-containing A710 (also referred to as HSLA-80) and modified-A710 steels was investigated by transmission electron microscopy. Isothermal aging of as-quenched specimens at 675°C produced -copper precipitates located primarily at -iron matrix dislocations. The precipitates exhibited multiple variants of an orientation relationship (OR) consistent with that reported by Kurdjumov and Sachs, fine fault formation, and associated streaking in electron diffraction patterns. For reaustenitized and continuously cooled specimens, the primary precipitation event was associated with interphase precipitation of copper at ferrite/austenite interfaces. Interphase precipitates frequently displayed ORs other than that reported by Kurdjumov and Sachs, although a unique crystallographic variant was observed within any one region of interphase precipitation, faults were observed infrequently, and streaking was not observed in diffraction patterns. At high temperatures during cooling, precipitate-free ferrite formed, whereas at lower temperatures, nucleation of copper precipitates occurred at ferrite/austenite interfaces for crystals of polygonal ferrite and Widmanstätten ferrite. This latter feature precludes the formation of Widmanstätten ferrite via a displacive mechanism. Interphase precipitation was not observed for granular ferrite or acicular ferrite. Less-common precipitation events during continuous cooling included the formation of AIN and CuS.

Effect of Bainite Transformation and Retained Austenite on Mechanical Properties of Austempered Spheroidal Graphite Cast Steel
TOSHIO TAKAHASHI, TOSHIHIKO ABE, and SHUJI TADA
Austempered ductile iron (ADI) has excellent mechanical properties, but its Young's modulus is low. Austempered spheroidal graphite cast steel (AGS) has been developed in order to obtain a new material with superior mechanical properties to ADI. Its carbon content (approximately 1.0 pct) is almost one-third that of a standard ADI; thus, the volume of graphite is also less. Young's modulus of AGS is 195 to 200 GPa and is comparable to that of steel. Austempered spheroidal graphite cast steel has an approximately 200 MPa higher tensile strength than ADI and twice the Charpy absorbed energy of ADI. The impact properties and the elongation are enhanced with increasing volume fraction of carbon-enriched retained austenite. At the austempering temperature of 650 K, the volume fraction of austenite is approximately 40 pct for 120 minutes in the 2.4 pct Si alloy, although it decreases rapidly in the 1.4 pct Si alloy. The X-ray diffraction analysis shows that appropriate quantity of silicon retards the decomposition of the carbon-enriched retained austenite. For austempering at 570 K, the amount of the carbon-enriched austenite decreases and the ferrite is supersaturated with carbon, resulting in high tensile strength but low toughness.

Heterogeneous Nucleation of ' on Dislocations in a Dilute Aluminum-Lithium Alloy
Z.M. WANG and G.J. SHIFLET
The nucleation of ' on dislocations with small undercooling in binary aluminum-lithium alloys has been examined. The study of related microstructures was performed using transmission electron microscopy (TEM), which demonstrates that ' preferentially nucleates on dislocations with a strong edge character and locates at the side where the stress field is compressive without destroying the dislocation core structure. This qualitatively justifies the theoretical prediction by Larch;aae on coherent heterogeneous nucleation on edge dislocations. Following the evaluation of the volume free energy change for the binary system by the ideal solution model and the mean-field model by Khachaturyan, the nucleation barrier and the nucleation rate were calculated and compared with experimentally determined data based on Larché's model. Specifically, the back-calculated interfacial energies from the experimentally determined nucleation rate data are in good agreement with the interfacial energy temperature dependence predicted by the related interfacial energy model. The effects of misfit strain, volume diffusion, interfacial energy, and nucleation sites are discussed.

Transition between Internal and External Nitridation of Ni-Ti Alloys
G.C. SAVVA, G.C. WEATHERLY, and J.S. KIRKALDY
A series of Ni-Ti alloys ranging in composition from 0.1 to 5 wt pct Ti were annealed in nitrogen gas or a nitrogen/argon gas mixture between 800°C and 1020°C. The evolution of surface and subscale structures, along with the diffusion profile of Ti in Ni, were investigated using scanning electron microscopy and energy dispersive X-ray analysis (EDX), respectively. A strong extrusion of Ni to accommodate the excess volume of internal TiN precipitation was observed between 0.5 and 1.0 wt pct Ti at 1020°C, where a continuous superficial layer of stoichiometric TiN begins to form. A finite difference computational algorithm was developed based upon a ternary model of simultaneous diffusion and precipitation, which generates the concentration profile of Ti in Ni and the particle distribution of TiN and subsumes a transition from internal to external nitridation. Because there is a dearth of independent thermodynamic and kinetic data on this system, we were forced to use parameters established by a selected minimal set of our own experiments to predict outcomes for the main body of experimental work, thereby obtaining satisfactory closure between theory and experiment.

A High-Resolution Transmission Electron Microscopy Study of Interfaces between the , B2, and ₂ Phases in a Ti-Al-Mo Alloy
S. DAS, J.M. HOWE, and J.H. PEREPEZKO
Conventional and high-resolution transmission electron microscopy (HRTEM) were used to examine the interfacial structures in a Ti-50Al-5Mo (at. pct) alloy which was processed to produce combinations of , B2, and ₂ phases in a single sample. A small amount of a fourth phase labeled was also found in the microstructure. It may be the phase Ti₂AlN but confirmation requires analysis of the N content in the phase. In this alloy, the orientation relationship between the and B2 phases is {111} || {110}_B2 and < ] || <111>_B2 with a coherent habit-plane interface parallel to {474). The orientation relationship between the B2 and ₂ (and also the ) phases is {110}_B2 || (0001)_2/ and <111>_B2 || <>_2/ with a coherent interface parallel to the close-packed planes and along other orientations. The orientation relationship between the ₂ (and also the ) and phases is (0001)_2/ || {111} and <>_2/ || < ] . The ₂ phase has a coherent interface parallel to the close-packed planes, while the phase appears to adopt the {474) interface plane of the phase, similar to the B2 phase. In some cases, the interface configuration between the and B2 phases appears to be altered by the presence of ₂ phase, resulting in a semicoherent interface. The phase labeled in this study has the same orientation relationship with the and B2 phases as ₂ but consists of an ABABAC.... stacking of close-packed basal planes. The {474) habit plane interface between the and B2 phases is analyzed by several different theories of interfacial structure, and microstructural evolution in this system is also discussed.

Crystallography of Grain Boundary Precipitates in a Titanium Alloy
T. FURUHARA, S. TAKAGI, H. WATANABE, and T. MAKI
The crystallography of (hcp) precipitates formed on the (bcc) matrix grain boundaries has been studied with transmission electron microscopy (TEM) in a Ti-15V-3Cr-3Sn-3Al alloy. The precipitates have a near-Burgers orientation relationship with respect to at least one of the adjacent grains. Among the possible 12 variants in this orientation relationship, the variant that [] is parallel to the <111> closest to the grain boundary plane tends to be preferred by the precipitates. Additionally, further variant selections are made so as to minimize the deviation of orientation relationship with respect to the ``opposite'' grain from the Burgers one. Such rules in variant selection often result in the formation of precipitates with a single variant at a planar grain boundary. Prior small deformation of matrix changes the variant of precipitates at the deformed portion of grain boundary. It is considered that the stress field of dislocations in the slip bands intersecting with the boundary strongly affects the variants of precipitates. Discussion of these results is based upon a classical nucleation theory.

Phase Transformations in Nb-Al-Ti Alloys
E.S.K. MENON, P.R. SUBRAMANIAN, and D.M. DIMIDUK
Phase relationships as well as morphological and crystallographic features in Nb-rich Nb-Al and Nb-Al-Ti alloys have been investigated. The phase boundaries involving the bcc and Nb₃Al (Al5 structure) were experimentally determined and several isothermal sections of the Nb-rich corner of the Nb-Al-Ti phase diagram established. The present findings show that (a) the solubility of Al in Nb is considerably less than that reported previously, (b) the high-temperature bcc phase undergoes an ordering transformation to the B2 structure, and (c) the phase also forms in these alloys. The sequence of decomposition of the high-temperature bcc phase during isothermal decomposition in the bcc + Nb₃Al phase field has been systematically studied in these alloys. A wide variety of morphological features were found to be associated with the Nb₃Al precipitates that formed in the bcc/B2 matrix during isothermal heat treatments. The lengthening kinetics of the plate-shaped Nb₃Al precipitates were also studied.

The Mechanism of Formation of a Fine Duplex Microstructure in Ti-48Al-2Mn-2Nb Alloys
R.V. RAMANUJAN and P.J. MAZIASZ
The mechanism of formation of the fine duplex microstructure resulting from the transformation in water-quenched Ti-48Al-2Mn-2Nb alloys was studied using transmission and analytical electron microscopy. As-cast Ti-48Al-2Mn-2Nb alloys were heat treated in the phase field and water quenched to room temperature. The resulting microstructure (referred to as a fine duplex microstructure) consisted of equiaxed grains and abutting lath colonies. Both the colonies and the grains were composed of the phase, twinned laths, and ₂ laths. It was found that the transformation from to in the fine duplex microstructure took place through long range diffusional processes, and competitive growth between the equiaxed and lath morphology occurred. Nucleation of the phase from the matrix can occur through nucleation on stacking faults, followed by growth through the sympathetic nucleation and growth of new laths on a substrate lath. The observed misorientations and the interfacial structures between the laths were found to be consistent with such a mechanism. Competition between such nucleation and growth mechanisms for the equiaxed and lath morphologies of leads to the formation of lath colonies (of and ₂) interspersed with equiaxed grains in these alloys.

Precipitation in Lead-Calcium Alloys Containing Tin
H. TSUBAKINO, M. TAGAMI, S. IOKU, and A. YAMAMOTO
The effect of Sn addition on the precipitation in Pb-Ca binary alloys has been studied by means of metallographic observations, hardness and resistivity measurements, and transmission electron microscopy (TEM) observations. With increasing Sn content, the cell advance of discontinuous precipitation is retarded and continuous precipitation occurs preferentially in higher Sn content alloys. The amount of Sn necessary for the retardation is higher for higher Ca contents. The retardation effect is attributed to the segregation of Sn at advancing cell boundaries. Precipitates in ternary alloys are L1₂-type ordered (Pb, Sn)₃Ca.

Eutectoid Decomposition in Ag-Ga
J.K. CHEN, C.W. SPENCER, M.E. EKSTRAND, G. CHEN, and W.T. REYNOLDS, JR.
The kinetics and mechanisms of eutectoid decomposition in alloys near Ag-15.3 wt pct Ga (Ag-21.8 at. pct Ga) were investigated. Isothermal decomposition of the parent phase exhibits C-shaped time-temperature-transformation (TTT) curves below the eutectoid temperature. Pearlite forms from the eutectoid temperature (380°C) to at least 85°C below this temperature. Two distinct types of pearlite form: a coarse lamellar structure at higher reaction temperatures and a fine lamellar structure with characteristically faceted boundaries. The fine pearlite forms in competition with coarse pearlite below 315°C, and the interlamellar spacings of the two pearlites differ by an order of magnitude. The constituents of coarse pearlite are the equilibrium and ' phases with fcc and hP9 crystal structures, respectively. Fine pearlite is composed of and a metastable phase, '', with a crystal structure identified as 9R (structure type hR3). Both types of pearlite undergo coarsening by a discontinuous, or cellular, reaction.

MECHANICAL BEHAVIOR

[^{-1 (1/2 + )} d^{-n(1/2 + )} T^-1 exp (-Q/kT)]^2/3

is proposed, where Q is the activation energy associated with substitutional diffusion. The proposed model fits well with the experimental data of an Al-3.7 wt pct Mg alloy.

Temperature-Dependent Deformation of Polydomain Phases in an In-22.5 At. Pct Tl Shape Memory Alloy
HARSH DEEP CHOPRA, ALEXANDER L. ROYTBURD, and MANFRED WUTTIG
Mechanically synthesized, reproducible polytwins of an In-22.5 at. pct Tl shape memory alloy were studied quantitatively for their deformation behavior due to bending stresses, in the ferroelastic and rubberlike temperature regime, with in situ video recordings of the evolving mesostructure. The experimental ratios of the twin modulus, E_T, to the elastic modulus, E_el, were two orders of magnitude higher than the theoretically calculated values. Further investigations to reconcile this discrepancy revealed a fine substructure within each domain of the polytwin. The response coefficient of this substructure, rather than the elastic modulus, was found to be the controlling deformation parameter for the polytwin. Theoretically predicted results were modified, which established a close agreement with the experimental results.

ENVIRONMENT

Influence of Long-Term Annealing on Tensile Properties and Fracture of Near- Titanium Alloy Ti-6Al-2.75Sn-4Zr-0.4Mo-0.45Si
CHRISTOPH LEYENS, MANFRED PETERS, DIRK WEINEM, and WOLFGANG A. KAYSSER
Long-term exposure at 600°C of the near-alpha titanium alloy Ti-6Al-2.75Sn-4Zr-0.4Mo-0.45Si (TIMETAL 1100) influences tensile properties and fracture morphology due to microstructural changes by aging as well as environmentally induced effects. The influences of aging and oxygen penetration on the subsurface of the alloy were evaluated individually. Metallographic investigations, microhardness measurements, and Auger electron spectroscopy (AES) were performed to examine the oxygen-affected zone. Whereas ultimate tensile stress (UTS) and 0.2 pct yield stress (YS) are mainly influenced by aging at 600°C up to 1000 hours in bimodal and lamellar microstructures, ductility sensitivity depends on oxygen embrittlement of the subsurface zone. Hereafter, little strain cracks are initiated which rapidly penetrate into the unaffected material. Fracture morphology which was investigated by scanning electron microscopy changes little during exposure. Ductile fracture exhibiting honeycomb morphology was observed on bimodal microstructure and interlamellar fracture with little ductile portion on lamellar microstructure. The oxygen-penetrated zone fails by brittle fracture.

COMPOSITE MATERIALS