| METALLURGICAL AND MATERIALS TRANSACTIONS A | |
|---|---|
|
|
Volume 29A, No. 9, September 1998 This Month Featuring: Symposium on Mechanical Behavior of Bulk Nanocrystalline Solids; Transformations; Mechanical Behavior; Solidification; Materials Processing; Composite Materials. View September 1998 Contents.
|
|
Microstructural Characteristics of Ultrafine-Grained Aluminum Produced Using Equal-Channel Angular Pressing
YOSHINORI IWAHASHI, MINORU FURUKAWA, ZENJI HORITA, MINORU NEMOTO, and TERENCE G. LANGDON
The shearing associated with equal-channel angular (ECA) pressing was examined using optical microscopy. Samples of pure Al with a large grain size were subjected to ECA pressing to different strains and then examined on three orthogonal planes. Samples were pressed without any rotation or with rotations of either 90 or 180 deg between each consecutive pressing. The experimental observations are compared with models which predict the shearing characteristics associated with ECA pressing under different conditions. It is demonstrated that there is good agreement, in terms of both the grain elongation and the shearing within individual grains, between the experimental results and the predictions of the models.
Microstructures and Properties of Nanocomposites Obtained through SPTS Consolidation of Powders
I.V. ALEXANDROV, Y.T. ZHU, T.C. LOWE, R.K. ISLAMGALIEV, and R.Z. VALIEV
The microstructures and properties of copper- and aluminum-based nanocomposites processed through severe plastic torsional straining (SPTS) consolidation of metallic micrometer powders and ceramic nanopowders were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), microhardness and electrical resistivity measurements, and mechanical tests. It was shown that the SPTS consolidation of powders is an effective technique for fabricating metal-ceramic nanocomposites with a high density, ultrafine grain size, and high strength. Copper samples processed under a high pressure of 6 GPa exhibited high failure strength and strain as well as unusual strain hardening. Superplastic-like behavior was found in Al-Al2O3 nanocomposite samples.
Mechanical Behavior of a Bulk Nanostructured Iron Alloy
J.E. CARSLEY, A. FISHER, W.W. MILLIGAN, and E.C. AIFANTIS
Bulk, fully dense materials were prepared from Fe-10Cu with grain diameters between 45 nm and 1.7 µm. The materials were prepared by ball milling of powders in a glove box, followed by hot isostatic pressing (hipping) or powder forging. Larger grain sizes were obtained by thermal treatment of the consolidated powders. The bulk materials were relatively clean, with oxygen levels below 1500 wpm and other contaminants less than 0.1 at. pct. The mechanical behavior of these materials was unique. At temperatures from 77 to 470 K, the first and only mechanism of plastic deformation was intense shear banding, which was accompanied by a perfectly plastic stress-strain response (absence of strain hardening). There was a large tension-compression asymmetry in the strength, and the shear bands did not occur on the plane of maximum shear stress or the plane of zero extension. This behavior, while unusual for metals, has been observed in amorphous polymers and metallic glasses. On the other hand, the fine-grained Fe-10Cu materials behaved like coarse-grained iron in some respects, particularly by obeying the Hall-Petch equation with constants reasonably close to those of pure iron and by exhibiting low-temperature mechanical behavior which was very similar to that of steels. Transmission electron microscopy (TEM) studies found highly elongated grains within shear bands, indicating that shear banding occurred by a dislocation-based mechanism, at least at grain sizes above 100 nm. Similarities and differences between the fine-grained Fe-10Cu and metals, polymers, metallic glasses, radiation-damaged metals, and quench-damaged metals are discussed.
Effect of B on the Microstructure and Mechanical Properties of Mechanically Milled TiAl Alloys
SANG H. KIM, H.H. CHUNG, SUNG G. PYO, S.J. HWANG, and NACK J. KIM
The present study is concerned with 
-(Ti52Al48)100-xBx (x = 0, 0.5, 2, 5) alloys produced by mechanical milling/vacuum hot pressing (VHPing) using melt-extracted powders. Microstructure of the as-vacuum hot pressed (VHPed) alloys exhibits a duplex equiaxed microstructure of 
2 and with a mean grain size of 200 nm. Besides 2 and phases, binary and 0.5 pct B alloys contain Ti2AlN and Al2O3 phases located along the grain boundaries and show appreciable coarsening in grain and dispersoid sizes during annealing treatment at 1300 °C for 5 hours. On the other hand, 2 pct B and 5 pct B alloys contain fine boride particles within the grains and show minimal coarsening during annealing. Room-temperature compressing tests of the as-VHPed alloys show low ductility, but very high yield strength > 2100 MPa. After annealing treatment, mechanically milled alloys show much higher yield strength than conventional powder metallurgy and ingot metallurgy processed alloys, with equivalent ductility to ingot metallurgy processed alloys. The 5 pct B alloy with the smallest grain size shows higher yield strength than binary alloy up to the test temperature of 700 °C. At 850 °C, 5 pct B alloy shows much lower strength than the binary alloy, indicating that the deformation of fine 5 pct B alloy is dominated by the grain boundary sliding mechanism.
Mechanical Properties, Ductility, and Grain Size of Nanocrystalline Iron Produced by Mechanical Attrition
T.R. MALOW and C.C. KOCH
The main goal of this investigation is to determine the influence of grain size on the mechanical properties and, specifically, the intrinsic ductility of nanocrystalline (nc) Fe. Ball-milled nc Fe was consolidated into compacts of near theoretical density by uniaxial warm pressing. Compaction parameters and annealing treatments resulted in a range of grain sizes for subsequent mechanical testing. The miniaturized disk bend test, hardness, and the automated ball indentation (ABI) method were used to test nanocrystal (nc) iron in compression and tension. The deformation and fracture morphologies of the tested samples were characterized by light and scanning electron microscopy. The hardness, as a function of the grain size, was described with a Hall-Petch slope, which was smaller than that in coarse-grained Fe. In tension, the material failed in a macroscopically brittle manner, while local ductility in very concentrated shear bands was observed. The compressive characteristics of the nc Fe were similar to those of a perfectly plastic material. The results are discussed in the context of the mechanical behavior of coarse-grained polycrystalline metals and alloys.
Static Recrystallization Kinetics with Homogeneous and Heterogeneous Nucleation Using a Cellular Automata Model
R.L. GOETZ and V. SEETHARAMAN
The kinetics of homogeneous and heterogeneous static recrystallization in a single-phase material were analyzed using two-dimensional (2-D) and three-dimensional (3-D) cellular automata (CA). A CA model was developed, which was then validated using the theory based on relationships developed by Johnson and Mehl, Avrami, and Kolmogorov (JMAK) for homogeneous site-saturated and constant-rate nucleation. The model was then modified for heterogeneous nucleation at grain boundaries, with either a fixed number of nuclei or a constant rate of nucleation. The fraction of boundary sites nucleated, for the case of fixed nucleation, varied from 0.006 to 0.28, resulting in Avrami exponents (k) ranging from 1.8 to 1.1 (site saturation). Site saturation with fixed nucleation produced a lamellar microstructure. The parameters of q and m, from Vandermeer's microstructural path method, were calculated and compared with theoretical values. Constant-rate nucleation at grain boundaries between newly recrystallized grains and the unrecrystallized matrix resulted in k values of 
1. Simulated microstructures revealed that with a low nucleation rate, recrystallized grains formed in clusters, while a high nucleation rate resulted in a necklace microstructure, with kinetics similar to those observed in dynamic recrystallization (k = 1.4).
Heterogeneous Microstructures and Microtextures in Cube-Oriented Al Crystals after Channel Die Compression
QING LIU, CLAIRE MAURICE, JULIAN DRIVER, and NIELS HANSEN
Pure aluminum crystals of cube orientation have been deformed in plane-strain compression to strains of unity using a channel die. The macrostructures and microstructures were characterized in three dimensions by a range of metallographic techniques including optical, scanning, and transmission electron microscopy. Particular attention was paid to quantifying global textures and local variations in crystal orientation by means of X-ray pole figures, automatic electron back-scattered diffraction (EBSD) and semiautomatic transmission electron microscope (TEM) Kikuchi line analysis. Cube crystals are observed to break up into macroscopic deformation bands aligned along the elongation direction and strongly disorientated by rotations mostly, but not uniquely, about the transverse direction. The bands develop deformation substructures of dislocation boundaries or, in certain cases, of intersecting dislocation boundaries which have characteristic microtexture signatures of alternating lattice rotations. The transition regions between the bands are composed of equiaxed dislocation cells which accommodate continuous orientation gradients over distances of about 20 µm. Compared to the behavior of rolled Al crystals, the macroscopic bands are observed to lie in different planes, but the microscopic subdivisions and microtextures developed in the channel die and in rolling are very similar. The origins of the macroscopic and microscopic subdivisions are discussed in terms of the local deformation modes and slip amplitudes and their relation to the behavior of rolled crystals.
High-Temperature Deformation of Commercial-Purity Aluminum
E.S. PUCHI and M.H. STAIA
The stress-strain behavior of aluminum 99.5 pct (2-9) purity deformed under hot-working conditions has been found to be satisfactorily described by combining the exponential saturation equation earlier proposed by Voce and a latter model advanced by Kocks. Voce's equation describes the strain dependence of the flow stress, whereas the temperature and strain rate dependencies of both the initial flow stress and the saturation or steady-state stress are introduced by means of Kocks' model, which leads to the definition of a different temperature-compensated strain rate parameter. The basic principles of the dynamic materials model (DMM) advanced by Gegel and co-workers has been reassessed, leading to a different proposition in relation to the calculation of both the power dissipator co-content (J) and the power dissipation efficiency (
), which makes use of the constitutive equation previously developed. Such concepts are later applied to the analysis of a typical industrial hot-rolling process conducted on commercial-purity aluminum. From the microstructural point of view, hot rolling of commercial-purity aluminum has been found to be conducted under conditions of relatively low power dissipation efficiency ( 0.20 to 0.25), which is likely to be associated with the predominance of dynamic recovery as the main dislocation rearrangement mechanism.
An Investigation of the Fracture and Fatigue Crack Growth Behavior of Forged Damage-Tolerant Niobium Aluminide Intermetallics
F. YE, C. MERCER, and W.O. SOBOYEJO
The results of a recent study of the effects of ternary alloying with Ti on the fatigue and fracture behavior of a new class of forged damage-tolerant niobium aluminide (Nb3Al-xTi) intermetallics are presented in this article. The alloys studied have the following nominal compositions: Nb-15Al-10Ti (10Ti alloy), Nb-15Al-25Ti (25Ti alloy), and Nb-15Al-40Ti (40Ti alloy). All compositions are quoted in atomic percentages unless stated otherwise. The 10Ti and 25Ti alloys exhibit fracture toughness levels between 10 and 20 MPa
at room temperature. Fracture in these alloys occurs by brittle cleavage fracture modes. In contrast, a ductile dimpled fracture mode is observed at room-temperature for the alloy containing 40 at. pct Ti. The 40Ti alloy also exhibits exceptional combinations of room-temperature strength (695 to 904 MPa), ductility (4 to 30 pct), fracture toughness (40 to 100 MPa), and fatigue crack growth resistance (comparable to Ti-6Al-4V, monolithic Nb, and inconnel 718). The implications of the results are discussed for potential structural applications of the 40Ti alloy in the intermediate-temperature (~700 °C to 750 °C) regime.
) of 9.4 X 103 for the aluminum-copper system. The diffusivity of heat, thus, greatly exceeds the diffusivity of solute in this system. These results indicate that testing for the steady state during directional solidification experiments by looking for constant primary dendrite arm spacings can lead to errors, since the microsegregation profiles adjust more slowly than the spacings. It is suggested that constancy of composition also be tested for critical experiments investigating steady-state microsegregation effects.
Communication: Discussion of "Effect of Dendrite Arm Coarsening on Microsegregation"
T. KRAFT AND Y.A. CHANG
Authors' Reply
H. COMBEAU, J.-M. DREZET, A. MO, AND M. RAPPAZ
The Sweep Constant Concept in Phase Coarsening
C.S. PANDE, R.A. MASUMURA, and S.P. MARSH
An expression for sweep constant originally introduced for grain growth is derived in terms of the parameters of the size distribution. The sweep constant is extended to the phenomenon of phase coarsening. It is shown that a relationship exists between the sweep constant, coarsening rate, and volume fraction of the particles undergoing coarsening.
Transmission Electron Microscope Specimen Preparation of Zn Powders Using the Focused Ion Beam Lift-Out Technique
B.I. PRENITZER, L.A. GIANNUZZI, K. NEWMAN, S.R. BROWN, R.B. IRWIN, T.L. SHOFNER, and F.A. STEVIE
Particles of Zn powder have been studied to show that high-quality scanning electron microscope (SEM) and transmission electron microscope (TEM) specimens can be rapidly produced from a site-specific region on a chosen particle by the focused ion beam (FIB) lift-out technique. A TEM specimen approximately 20-µm long by 5-µm wide was milled to electron transparency, extracted from the bulk particle, and micromanipulated onto a carbon coated copper mesh TEM grid. Using the FIB lift-out method, we were able to prepare a site-specific TEM specimen from a difficult material in under 3 hours. The TEM analysis of the lift-out specimen revealed a large amount of thin area free from characteristic signs of damage that may be observed as a result of conventional argon ion milling. The overall microstructure of the specimen prepared by the FIB lift-out method was consistent with samples prepared by conventional metallographic methods. A grain size of ~10 to 20 µm was observed in all specimens by both TEM and SEM analysis. Light optical microscopy revealed the presence of internal voids in ~10 to 20 pct of all particles. The SEM analysis showed the voids to extend over ~70 pct of the particle volume in some cases.
Investigation of Thermomechanical Behavior of a Work Roll and of Roll Life in Hot Strip Rolling
C.G. SUN, C.S. YUN, J.S. CHUNG, and S.M. HWANG
An integrated finite element-based model is presented for the prediction of the steady-state thermomechanical behavior of the roll-strip system and of roll life in hot strip rolling. The model is comprised of basic finite-element models, which are incorporated into an iterative-solution procedure to deal with the interdependence between the thermomechanical behavior of the strip and that of the work roll, which arises from roll-strip contact, as well as with the interdependence between the thermal and mechanical behavior. Comparison is made between the predictions and the measurements to assess solution accuracy. Then, the effect of various process parameters on the detailed aspects of thermomechanical behavior of the work roll and on roll life is investigated via a series of process simulations.
A Novel Way of Amorphous Phase Formation during Mechanical Alloying of Copper and Cadmium Powders
D.L. ZHANG and T.B. MASSALSKI
Phase formation during high-energy ball milling of copper and cadmium powders has been investigated. Both X-ray diffractometry (XRD) and differential scanning calorimetry (DSC) have been used to characterize the resulting phases. An amorphous phase forms during milling in the copper-cadmium system by a reaction between the equilibrium 
phase and one of the constituent metals (copper). However, the amorphous phase cannot be produced by a direct reaction between the copper and cadmium powders, nor by milling of a powder consisting of only the phase. It appears most likely that the contribution from the generated copper/ interfaces, and the accompanying increase in the total free energy, provide the additional driving force for the amorphous phase formation in this system.
| Search | TMS Document Center | Tables of Contents | Subscriptions | Met. and Mat. Trans. | TMS OnLine |
|---|