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Volume 29A, No. 2, February 1998 This Month Featuring: Transformations; Transport Phenomena; Mechanical Behavior; Welding & Joining; Surface Treatment; Electronic, Magnetic, & Optical Material; Solidification; Materials Processing; Composite Materials. View February 1998 Contents.
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Stress-State Effects on the Stress-Induced Martensitic Transformation of Carburized 4320 Steels
I. KARAMAN, HUSEYIN SEHITOGLU, H.J. MAIER, and M. BALZER
The effect of different stress states on the stress-induced martensitic transformation of retained austenite was investigated in carburized 4320 steels with an initial retained austenite content of 15 pct. Experiments were conducted utilizing a specialized pressure rig and comparison between stress-strain behaviors of specimens with different austenitization and tempering histories was performed under these stress states. Experimental results indicated considerable asymmetry between tension and compression, with triaxial stress states resulting in the highest strength levels for the untempered material. Fine carbide precipitates due to low-temperature tempering increased the strength and ductility of the specimens and also changed the austenite-to-martensite transformation behavior. Numerical simulations of stress-strain behaviors under different stress states were obtained, with an existing micromechanical self-consistent framework utilizing the crystallographic theory of austenite/martensite transformation and the minimum complementary free-energy principle. The model was modified for carburized steels upon microstructural investigation and predicted the same trends in effective stress--effective strain behavior as observed experimentally.
Communication: Grain Boundary Precipitation Reactions in a Wrought Fe-8Al-5Ni-2C Alloy Prepared by the Conventional Ingot Process
C.H. HWANG and T.F. LIU
-NbC1-x shows a decrease with increasing metalloid concentration, whereas the diffusivity of nitrogen in -NbN1-x is nearly independent of the nonmetal concentration. The concentration dependence of the carbon diffusion coefficients in -NbC1-x is a result of a lower activation energy of carbon diffusion in the substoichiometric -NbC1-x than in the -NbC. On the contrary, the activation energy of nitrogen in -NbN1-x does not change with the nitrogen concentration. This behavior could be explained by the different occupancies of metal sublattices, which remain constant in -NbC1-x but decrease with increasing nonmetal concentration in -NbN1-x.
-Iron and Two Interstitial-Free Steels
-iron were deformed in torsion over the temperature range of 600°C to 840°C, and the textures produced were measured using conventional X-ray techniques. The conditions were chosen so that dynamic recrystallization (DRX) would take place in the ferrite and static recrystallization would be avoided during cooling after deformation. The DRX textures differ from those observed at room temperature and are dominated by the D1 (
)[111], D2 (
)[111], and E2 (
)[111] components. The D2 becomes increasingly important as the strain is increased, which leads to weakening of the D1 and disappearance of the E2 at large strains. Texture simulations were carried out using a DRX model based on sequential deformation, nucleation, and growth steps. The types of oriented nucleation and selective growth required to reproduce the experimentally observed textures are discussed. The simulations indicate that the low-energy nucleation mechanism plays a dominant role in the formation of bcc DRX textures. The results are also interpreted in terms of the continuous (in situ) and discontinuous mechanisms of dynamic recrystallization.
Effect of Prestrain on Aging and Bake Hardening of Cold-Rolled, Continuously Annealed Steel Sheets
WOO CHANG JEONG
To understand the bake hardening (BH) behavior in an actual automotive part, 40 tensile specimens were machined from the actual press-formed outer-door panel of a compact car and both bake and work hardenability distribution data were determined. Strain applied by actual press forming was estimated from the work hardening data. Finally, the effects of prestraining mode and amount on ambient aging and BH response were also investigated. The BH widely ranged from 10 to 54 MPa and the work hardenability was between 17 and 82 MPa, depending on panel location. Bake hardening in the outer-door panel decreased as the work hardening increased, indicating that the BH steel must be applied to the shallow drawn parts in order to maximize the BH effect in dent resistance. In order to establish the effects of prestrain and ambient aging time on the age and subsequent BH, the specimens were prestrained and aged at ambient temperature for various time intervals, and then baked at 170°C for 20 minutes. In the as-temper-rolled and press-formed condition, the steels were extremely resistant to ambient aging. However, it was found that a 0.3 pct tensile prestrain was sufficient to initiate ambient aging within 1 day, and the effect was accelerated with greater prestrain. With 8 days of ambient aging, all prestrained steels exhibited 20 to 25 MPa of age hardening. Irrespective of prestrain amount in the range of 0.5 to 5.0 pct, the BH decreased as the aging time increased.
Effect of Test Temperature on the Dynamic Torsional Deformation Behavior of Two Aluminum-Lithium Alloys
CHANG GIL LEE, KI JONG KIM, SUNGHAK LEE, and KYUNGMOX CHO
The objective of the present study is to investigate the effect of test temperature on the dynamic torsional deformation behavior of two Al-Li alloys, i.e., 2090 and 8090 alloys. Dynamic torsional tests were conducted using a torsional Kolsky bar at room temperature and a low temperature (-196°C), and the torsionally deformed regions and the fracture surfaces of the tested specimens were examined. The dynamic properties of the two Al-Li alloys at the low temperature were improved, owing to the modification of the deformation behavior. The dynamic deformation behavior at room temperature was dominated by intergranular cracks due to planar slips and by crack propagation along the grain boundaries. At the low temperature, plastic deformation proceeded more homogeneously as planar slip was prevented. These results indicated that the overall deformation mode of both the Al-Li alloys changed from planar slip to homogeneous deformation with decreasing temperature, resulting in the improvement of cryogenic properties under dynamic torsional loading.
Microstructural Development of Adiabatic Shear Bands Formed by Ballistic Impact in a WELDALITE 049 Alloy
CHANG GIL LEE, WOO JIN PARK, SUNGHAK LEE, and KWANG SEON SHIN
The object of the present study is to investigate the microstructural development of the adiabatic shear band formed by ballistic impact in a WELDALITE 049 alloy. The microstructure of the shear band was examined by optical microscopy and transmission electron microscopy. The results indicated that the adiabatic shear band consisted of fine recrystallized grains with a high dislocation density. This microstructure was considered to be formed in an extremely short time by the combined effects of the highly localized shear deformation and the high-temperature rise that occurred within the shear band. However, no precipitates could be observed in the interior of the grains, since the temperature rise in the shear band formation process was inferred to be above 460°C and below the solidus temperature. Dynamic recrystallization was suggested as a possible mechanism to explain the microstructural development of the adiabatic shear band formed in the WELDALITE alloy.
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Microtexture Evolution during Annealing and Superplastic Deformation of Al-5 Pct Ca-5 Pct Zn
M.T. PÉREZ-PRADO, T.R. McNELLEY, O.A. RUANO, and G. GONZÁLEZ-DONCEL
The microtexture and grain boundary misorientation distributions (i.e., mesotexture) of the superplastic alloy Al-5 pct Ca-5 pct Zn have been investigated in the as-processed condition, after annealing at 520°C (for times ranging from 7 minutes to 90 hours) and after tensile straining in the transverse direction (TD). Three different superplastic straining conditions were considered:
550°C /10-2 s-1, 550°C/10-1 s-1, and 400°C/10-2 s-1. Microtexture data were obtained by means of computer-aided electron backscatter diffraction analysis methods. The retention of the deformation texture of the as-received material and the development of an increasingly bimodal grain boundary misorientation distribution following static annealing are consistent with the occurrence of recovery and continuous recrystallization. During superplastic straining, deformation texture components are also retained, but with a change in the grain boundary misorientation distribution toward random, indicating that grain switching occurs during grain boundary sliding (GBS). At the midlayer, however, a change from an initial texture component near the Cu-type texture component toward the Brass texture component, {011}<211>, was observed even as the misorientation distribution became more random. This change in texture component is associated with the occurrence of single slip during superplastic flow.
An Investigation of Toughening in NiAl Composites Reinforced with Yttria--Partially Stabilized Zirconia Particles
P. RAMASUNDARAM, F. YE, R.R. BOWMAN, and W.O. SOBOYEJO
The results of an investigation of toughening mechanisms in NiAl composites reinforced with yttria--partially stabilized zirconia polycrystals are presented. Different yttria stabilization levels in the zirconia, between 0 and 6 mole pct are employed. It is shown that substantial improvements in fracture toughness are obtained in all the composites reinforced with partially stabilized zirconia particles. The phase contents and microstructures of the composite systems are characterized by X-ray diffraction and transmission electron microscopy (TEM) techniques. Crack tip deformation was also studied using crack tip TEM analysis, and laser Raman spectroscopy was used to estimate the size of the transformation zone. The results show that transformation toughening is significant only in the 2 mole pct yttria--stabilized zirconia composite. Toughening is also shown to occur via slip phenomena within the NiAl grains in the near-tip regions of the composites reinforced with 2, 4, or 6 mole pct yttria--stabilized zirconia particles.
Cyclic Deformation Behavior of High-Purity Titanium Single Crystals: Part I. Orientation Dependence of Stress-Strain Response
X. TAN, H. GU, C. LAIRD, and N.D.H. MUNROE
Randomly oriented single crystals of high-purity titanium were prepared by strain annealing and were subjected to multiple-step fatigue testing under strain-controlled conditions, in order to determine their cyclic stress-strain curves (CSSCs). These were found to fall into three groups, depending on orientation and the extent of slip and twinning. For those crystals oriented for single prismatic slip, a plateau was observed in the CSSCs, persistent slip bands (PSBs) occurred, and the plateau stress was 38 MPa. In a second group, oriented for prismatic slip but for which cross-slip and twinning was favored, the plateau was suppressed and the flow stresses were higher. In a third group, connected with orientations on the borders of the unit triangle, extensive hardening occurred, the CSSCs were steep, and there were multiple cases of slip and twinning. The results are interpreted in terms of maps in the stereographic projection recording the Schmid factors for the various deformation modes.
Cyclic Deformation Behavior of High-Purity Titanium Single Crystals: Part II. Microstructure and Mechanism
X. TAN, H. GUO, H. GU, C. LAIRD, and N.D.H. MUNROE
Strain-controlled cyclic tests have been conducted on high-purity titanium single crystals with different orientations. The fatigue mechanisms of the titanium crystals were studied by means of a scanning electron microscope (SEM) and a transmission electron microscope (TEM). It was found that single slip lines, wavy slip lines, double slip lines, twins, and associated slip lines occurred in differently oriented single crystals. A new type of fractographic morphology, parallel traces, was observed. Dislocation patterns and cyclic twins, as well as the mechanical response, were analyzed. The dependence of the deformation mechanisms on the orientations of the single crystals is discussed.
Grain-Shape Parameters for High-Temperature Creep Resistance in Powder Metallurgy Tungsten Fine Wires
K. TANOUE
Three grain-shape parameters, f1, f2, and f3, are defined to clarify the morphological effect of grains on the high-temperature creep resistance under the condition that no grain boundary cavitation occurs. The parameter f1 is characteristic of complicated grain shapes, suggesting that it can be a measure of the interlocking grain structure. The parameter f2 is an important parameter when torsional stresses are imposed on coiled wires, and f3 is characteristic of the short-range roughness of grain boundaries only when f1 is not greatly changed. The minimum creep rate decreases as the grain aspect ratio, R, increases for R < 30, and the creep rate increases as R increases for R > 30. The parameter f1, as well as Raj and Ashby's model, gives a satisfactory explanation for the former behavior. On the other hand, it is proven that their model must be modified using f3 to explain the latter behavior because of the highly elongated grain configuration that is associated with R > 30.
Crystal Plasticity Forming Limit Diagram Analysis of Rolled Aluminum Sheets
P.D. WU, K.W. NEALE, E. VAN DER GIESSEN, M. JAIN, A. MAKINDE, and S.R. MacEWEN
Numerical simulations of forming limit diagrams (FLDs) are performed based on a rate-sensitive polycrystal plasticity model together with the Marciniak--Kuczynski (M--K) approach. Sheet necking is initiated from an initial imperfection in terms of a narrow band. The deformations inside and outside the band are assumed to be homogeneous, and conditions of compatibility and equilibrium are enforced across the band interfaces. Thus, the polycrystal model needs only to be applied to two polycrystalline aggregates, one inside and one outside the band. Each grain is modeled as an fcc crystal with 12 distinct slip systems. The response of an aggregate comprised of many grains is based on an elastic-viscoplastic Taylor-type polycrystal model. With this formulation, the effects of initial imperfection intensity and orientation, initial distribution of grain orientations, crystal elasticity, strain-rate sensitivity, single slip hardening, and latent hardening on the FLD can be assessed. The predicted FLDs are compared with experimental data for the following rolled aluminum alloy sheets:
AA5754-0-A, AA5754-0-B, AA6111-T4-A, AA6111-T4-C, and AA6111-T4-D.
Ir-Base Refractory Superalloys for Ultra-High Temperatures
Y. YAMABE-MITARAI, Y. RO, T. MARUKO, and H. HARADA
The microstructure and compression strengths of Ir-15 at. pct X (X ;eq Ti, Ta, Nb, Hf, Zr, or V) binary alloys at temperatures between room temperature and 1800°C were investigated to evaluate the potential of these alloys for ultra-high-temperature use. The fcc and L12 two-phase structures of these alloys were examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The strengths of the Ir-Ta, -Nb, -Hf, and -Zr alloys were above 800 MPa at temperatures up to 1200°C and about 200 MPa at 1800°C. The strengths of these alloys under 1000°C are equivalent to or higher than those of the commercially used Ni-base superalloys, MAR-M247 and CMSX-10. The Nb concentration dependence of strength was investigated using a series of Ir-Nb alloys with Nb concentrations from 0 to 25 at. pct. It was found that the Ir-base alloys were strengthened by L12 precipitation hardening. The potential of the Ir-base alloys for ultra-high temperature use is discussed.
Brittle Fracture Initiation Associated with the Strain Localization in a Heat-Affected Zone of a Low Carbon Steel
KEN'ICHI YOKOYAMA and MICHIHIKO NAGUMO
Brittle fracture initiation in the ductile-brittle fracture transition region in the heat-affected zone (HAZ) of weldments of a low carbon steel has been investigated. Consistent with the previous results from blunt notch Charpy tests, brittle fracture initiation was observed in the case of J-integral tests to take place at the intersection of small bainitic ferrite grains of different orientations within a mixed area of bainitic ferrite and quasipolygonal ferrite in proximity to the boundary between a coarse bainitic ferrite. Partial load drop during loading, pop-in phenomena, in fracture mechanics tests in the low-temperature region is caused by essentially the same mechanism as for unstable brittle fracture initiation. Inhomogeneous microstructure in the HAZ gives rise to intense strain localizations in the mixed area of bainitic ferrite and quasipolygonal ferrite due to the constraint of plastic deformation therein and may produce accumulated defects that form an incipient crack for the brittle fracture. Partial load drop proceeds in association with repetitive initiations of brittle facets and their ductile linking. The strong temperature dependence of the magnitude of partial load drop is likely to show that the temperature dependence of the brittle fracture initiation is controlled by the first initiation of a brittle facet and the ductile linking with the following induced facets. Existence of coarse bainitic ferrite grains is a prerequisite for the extension of an incipient crack.
Microstructure and Creep Behavior of an Orthorhombic Ti-25Al-17Nb-1Mo Alloy
J.W. ZHANG, C.S. LEE, D.X. ZOU, S.Q. LI, J.K.L. LAI
Microstructural evolution during three heat-treatment schedules and the terminal microstructures in an orthorhombic alloy of Ti-25Al-17Nb-1Mo were observed and analyzed with optical microscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The creep behavior of the alloy with three different microstructures (a coarse-lath, fine-lath, and fine equiaxed microstructure) was studied over a temperature range of 600°C to 750°C and over a stress range of 150 to 400 MPa in air. The steady-state creep rates, apparent stress exponents, and apparent creep activation energies of the various samples have been determined. The results show that creep behaviors in the alloy are strongly influenced by microstructure. The effect on creep by some of the microstructural features, such as the multivariants within the coarse laths and the interfaces of the laths and the equiaxed grains, is also discussed.
Transformation Superplasticity of Iron and Fe/TiC Metal Matrix Composites
PETER ZWIGL and DAVID C. DUNAND
Unreinforced iron was thermally cycled around the /
phase field under an externally applied uniaxial tensile stress, resulting in strain increments which could be accumulated, upon repeated cycling, to a total strain of 450 pct without failure. In agreement with existing theory attributing transformation superplasticity to the biasing of the internal allotropic strains by the external stress, the measured strain increments were proportional to the applied stress at small stresses. However, for applied stresses higher than the nominal yield stress, strain increments increased nonlinearly with stress, as a result of strain hardening due to dissolved carbon and iron oxide dispersoids. Also, the effects of transient primary creep and ratchetting on the superplastic strain increment values were examined. Finally, partial cycling within the / phase field indicated an asymmetry in the superplastic strain behavior with respect to the temperature cycling range, which is attributed to the different strengths of ferrite and austenite. Transformation superplasticity was demonstrated in iron-matrix composites containing 10 and 20 vol pct TiC particles: strain increments proportional to the applied stress were measured, and a fracture strain of 230 pct was reached for the Fe/10TiC composite. However, the strain increments decreased with increasing TiC content, a result attributed to the slight dissolution of TiC particles within the matrix which raised the matrix yield stress by solid-solution strengthening and by reducing the transformation temperature range.
Communication: Sticking Mechanism during Hot Rolling of Two Stainless Steels
SUNGHAK LEE, DONGWOO SUH, SEUNGCHAN OH, and WON JIN
A Novel Approach for Predicting the Tensile Strength of Brazed Joints
KUNIMASA TAKESHITA and YUHJI TERAKURA
A method for predicting the tensile strength of brazed joints is presented. A finite element method (FEM) of analysis is used, taking into account the failure within the soft interlayer metal region of the joint. A modified failure criterion is derived in consideration of void growth and incorporated into the FEM analysis. The predicted tensile strength is discussed and compared with experimental results obtained from titanium joints brazed with aluminum.
The measurements are carried out along the constant compositional paths of xSb = 0.1 and 0.2 over a temperature range of 1073 to 1273 K. The measured activity data in the system exhibit positive deviations from ideality. The deviations increase progressively with the In content of the alloys for a constant compositional path of Sb. The magnitude of the aAl as a function of In is found to decrease significantly with the increase in the Sb content of the alloys. The behavior is consistent with the existence of the liquid-liquid immiscibility in the system.
Solidification and Spangle Formation of Hot-Dip-Galvanized Zinc Coatings
J. STRUTZENBERGER and J. FADERL
Solidification of hot dip coatings was studied regarding thermal conditions. Optical phenomena occurring at the liquid zinc surface were documented and the solid zinc surface was characterized in respect to optical and microscopic appearance, distribution of Pb and Al, crystal orientation, and topography. Resulting from these observations, a solidification model can be derived: zinc nucleation occurs at the steel/zinc interface. Due to thermal conditions in the slightly undercooled liquid zinc film, solidification occurs by rapid sideways dendritic expansion of the nucleated grains along the steel/zinc interface. Dendritic growth is controlled by interaction of crystal orientation of the nucleated zinc grain and thermal conditions in the undercooled layer. This leads to formation of different shaped grains with thicker and thinner sectors. The mechanism of sideways expansion continues until the entire interface is covered with dendritic zinc grains. Even though the zinc outer surface is still a liquid phase, final spangle size, as well as surface appearance and shape of the grains, is already determined at that early stage of solidification. Further growth only leads to a thickening of the solid layer; however, its relief remains almost unchanged. Thickening occurs relatively slowly due to the fact that marginal heat flow toward the surface now represents the limiting factor. Growth of the solid zinc layer results in continuous enrichment of Pb and Al in the residual liquid. Then, outer surface solidification starts as segments of single grains emerge. Distribution of the enriched residual melt in between the already solid areas depends on the relief of the solid layer. Finally, eutectic Zn-Pb reaction with precipitation of Pb particles takes place, which defines the dull appearance of these regions. Solidification for ``lead-free'' coatings is essentially the same, except that the final eutectic Zn-Pb reaction is missing. Additional investigations of dendritic secondary arm spacing indicate that Pb does not act by suppressing zinc nucleation. Pronounced dendritic growth is proposed to be favored by a change in interfacial energy. The new solidification model is applicable for a wide range of processing conditions and explains the origin of the typical spangle structure.
Finite Element Modeling of Distortion during Liquid Phase Sintering
RAMNATH GANESAN, ANTHONY GRIFFO, and RANDALL M. GERMAN
Liquid phase sintering (LPS) is a common technique to consolidate materials that are difficult to process by fusion techniques, such as tungsten heavy alloys. One of the major processing difficulties associated with liquid phase sintered alloys is component distortion and loss of component shape. In LPS, this distortion is the result of viscous flow driven by curvature effects and gravity. A finite element model is developed for viscous flow of the semisolid sintering structure using Stokes equations. This model considers solid volume fraction and effective viscosity of the solid-liquid mixture. The simulation predictions are compared to distortion results for microgravity and ground-based sintering experiments, and they show good agreement. The model results indicate that the effective semisolid viscosity is significantly greater than the liquid metal viscosity. Hence, future work needs to quantitatively examine the factors controlling viscosity and the benefits from such high viscosities in liquid phase sintered systems.
Evolution of Texture and Microstructure in a Thermomechanically Processed Al-Li-Cu-Mg Alloy
A.K. SINGH, G.G. SAHA, A.A. GOKHALE, and R.K. RAY
The development of texture and microstructure in a thermomechanically processed quaternary Al-Li-Cu-Mg alloy has been investigated. Textures on both the surface (1/8 thickness (T)) and midthickness levels of specimens were measured using the conventional pole figure as well as the orientation distribution function (ODF) method. Microstructural characterization was carried out with the help of optical microscopy and transmission electron microscopy (TEM). The processing schedule involved hot cross rolling, followed by several stages of cold rolling (CR) with intermediate solution treatments (STs). A marked through-thickness texture inhomogeneity developed in the processed sheets during the course of thermo mechanical treatment (TMT). In general, the texture produced at the midthickness level was 2 to 3 times sharper than the texture at the surface. The alloy, after hot cross rolling, showed nearly equally strong Bs {110} <112> and Bs/S {168} <211> components at midthickness. After three cycles of cold rolling (CR) and solution treatments (STs), the overall texture intensity came down by a factor of nearly 2. The final processed sheet material showed a moderately strong Bs and a predominant S {123} <634> component at midthickness. Solution treatments did not produce much change in the texture of the cold-rolled materials. Microstructural evidence indicated extensive recovery and, at best, partial recrystallization of the deformed structure. No significant effect of second-phase particles in texture development was noticed.
Failure of SiC Particulate-Reinforced Metal Matrix Composites Induced by Laser Thermal Shock
Y.C. ZHOU, Z.P. DUAN, and Q.B. YANG
Thermal failure of SiC particulate-reinforced 6061 aluminum alloy composites induced by both laser thermal shock and mechanical load has been investigated. The specimens with a single-edge notch were mechanically polished to 0.25 mm in thickness. The notched-tip region of the specimen is subjected to laser beam rapid heating. In the test, a pulsed Nd:glass laser beam is used with duration 1.0 ms or 250 µs, intensity 15 or 70 kW/cm2, and spot size 5.0 mm in diameter. Threshold intensity was tested and fracture behavior was studied. The crack-tip process zone development and the microcrack formation were macroscopically and microscopically observed. It was found that in these materials, the initial crack occurred in the notched-tip region, wherein the initial crack was induced by either void nucleation, growth, and subsequent coalescence of the matrix materials or separation of the SiC particulate-matrix interface. It was further found that the process of the crack propagation occurred by the fracture of the SiC particulates.
Communication: Microstructural Characterization of the Matrix in the SiC Fiber-Reinforced Ti-15-3 Composite
S.Q. GUO and Y. KAGAWA
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