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Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 1998"


Journal ArticleDOI
TL;DR: Friction stir welding (FSW) was used to weld 7075 T651 aluminum, an alloy considered essentially unweldable by fusion processes as discussed by the authors, which exposed the alloy to a short time, high-temperature spike, while introducing extensive localized deformation.
Abstract: Friction stir welding (FSW), a new welding technique invented at TWI, was used to weld 7075 T651 aluminum, an alloy considered essentially unweldable by fusion processes. This weld process exposed the alloy to a short time, high-temperature spike, while introducing extensive localized deformation. Studies were performed on these solid-state welds to determine mechanical properties both in the longitudinal direction, i.e., within the weld nugget, and, more conventionally, transverse to the weld direction. Because of the unique weld procedure, a fully recrystallized fine grain weld nugget was developed. In addition, proximate to the nugget, both a thermomechanically affected zone (TMAZ) and heat affected zone (HAZ) were created. During welding, temperatures remained below the melting point and, as such, no cast or resolidification microstructure was developed. However, within the weld nugget, a banded microstructure that influences room-temperature fracture behavior was created. In the as-welded condition, weld nugget strength decreased, while ductility remained high. A low-temperature aging treatment failed to fully restore T651 strength and significantly reduced tensile ductility. Samples tested transverse to the weld direction failed in the HAZ, where coarsened precipitates caused localized softening. Subsequent low-temperature aging further reduced average strain to failure without affecting strength. Although reductions in strength and ductility were observed, in comparison to other weld processes, FSW offers considerable potential for welding 7075 T651 aluminum.

864 citations


Journal ArticleDOI
TL;DR: In this paper, the mechanical properties of two Zr-base bulk amorphous alloys (BAA) were studied by both tensile and compressive tests at room temperature in various test environments and the results suggest that moisture-induced hydrogen embrittlement in BAAs may be masked by catastrophic fracture following shear bands.
Abstract: The mechanical properties of two Zr-base bulk amorphous alloys (BAA), Zr-10Al-30Cu-5Ni (BAA-10) and Zr-10Al-5Ti-17.9Cu-14.6Ni (BAA-11), were studied by both tensile and compressive tests at room temperature in various test environments. The BAA ingots up to 7 mm in diameter were successfully produced by both arc melting and drop casting and induction melting and injection casting. The BAA specimens deformed mainly elastically, followed by catastrophic failure along shear bands. Examination of the fracture region revealed ductile fracture features resulting from a substantial increase in temperature, which was attributable to the conversion of the stored elastic strain energy to heat. Surprisingly, “liquid droplets” located at major shear-band cracks adjacent to the fracture section were observed, indicating the occurrence of local melting during fracture. The angle orientation of shear bands, shear-band cracks, and fracture surfaces relative to the stress axis is quite different for BAA specimens tested in tension and compression. This suggests that both shear stress and normal stress may play a role in developing shear bands during plastic deformation. The tensile properties of BAAs were found to be insensitive to the test environment at room temperature. However, the reaction of BAAs with distilled water and heavy water was detected by laser desorption mass spectrometry (LDMS). These results suggest that moisture-induced hydrogen embrittlement in BAAs may be masked by catastrophic fracture following shear bands.

486 citations


Journal ArticleDOI
TL;DR: In this article, the shearing associated with equal-channel angular (ECA) pressing was examined using optical microscopy and it was demonstrated that there is good agreement between the experimental results and the predictions of the models.
Abstract: 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.

254 citations


Journal ArticleDOI
TL;DR: In this article, the equal-channel angular (ECA) pressing of Al-1 pct Mg and Al-3 pct mg solid-solution alloys with pure Al was compared.
Abstract: Experiments were undertaken to compare the equal-channel angular (ECA) pressing of Al-1 pct Mg and Al-3 pct Mg solid-solution alloys with pure Al. The results reveal both similarities and differences between these three materials. Bands of subgrains are formed in all three materials in a single passage through the die, and these subgrains subsequently evolve, on further pressings through the die, into an array of grains with high-angle boundaries. However, the addition of magnesium to an aluminum matrix decreases the rate of recovery and this leads, with an increasing Mg content, both to an increase in the number of pressings required to establish a homogeneous microstructure and to a decrease in the ultimate equiaxed equilibrium grain size. It is concluded that alloys exhibiting low rates of recovery should be especially attractive candidate materials for establishing ultrafine structures through grain refinement using the ECA pressing technique.

252 citations


Journal ArticleDOI
TL;DR: In this article, thin foil specimens of 7075-T6 and 2024-T3 aluminum alloys were immersed in aerated 0.5M NaCl solution and then examined by transmission electron microscopy (TEM).
Abstract: To better understand particle-induced pitting corrosion in aluminum alloys, thin foil specimens of 7075-T6 and 2024-T3 aluminum alloys, with identified constituent particles, were immersed in aerated 0.5M NaCl solution and then examined by transmission electron microscopy (TEM). The results clearly showed matrix dissolution around the iron- and manganese-containing particles (such as Al23CuFe4), as well as the Al2Cu particles. While Al2CuMg particles tended to dissolve relative to the matrix, limited local dissolution of the matrix was also observed around these particles. These results are consistent with scanning electron microscopy (SEM) observations of pitting corrosion and are discussed in terms of the electrochemical characteristics of the particles and the matrix.

229 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of SiC volume fraction and particle size on the fatigue behavior of 2080 Al was investigated, and it was found that increasing volume fraction resulted in an increase in fatigue resistance.
Abstract: The effect of SiC volume fraction and particle size on the fatigue behavior of 2080 Al was investigated. Matrix microstructure in the composite and the unreinforced alloy was held relatively constant by the introduction of a deformation stage prior to aging. It was found that increasing volume fraction and decreasing particle size resulted in an increase in fatigue resistance. Mechanisms responsible for this behavior are described in terms of load transfer from the matrix to the high stiffness reinforcement, increasing obstacles for dislocation motion in the form of S’ precipitates, and the decrease in strain localization with decreasing reinforcement interparticle spacing as a result of reduced particle size. Microplasticity was also observed in the composite, in the form of stress-strain hysteresis loops, and is related to stress concentrations at the poles of the reinforcement. Finally, intermetallic inclusions in the matrix acted as fatigue crack initiation sites. The effect of inclusion size and location on fatigue life of the composites is discussed.

219 citations


Journal ArticleDOI
TL;DR: In this paper, the authors reviewed the recent results on the development of ferromagnetic bulk amorphous alloys prepared by casting processes and concluded that the hard magnetic properties are obtained only for the bulk Amorphous Alloy.
Abstract: This article reviews our recent results on the development of ferromagnetic bulk amorphous alloys prepared by casting processes. The multicomponent Fe-(Al,Ga)-(P,C,B,Si) alloys are amorphized in the bulk form with diameters up to 2 mm, and the temperature interval of the supercooled liquid region before crystallization is in the range of 50 to 67 K. These bulk amorphous alloys exhibit good soft magnetic properties, i.e., high B s of 1.1 to 1.2 T, low H o of 2 to 6 A/m, and high μ e of about 7000 at 1 kHz. The Nd-Fe-Al and Pr-Fe-Al bulk amorphous alloys are also produced in the diameter range of up to 12 mm by the copper mold casting process and exhibit rather good hard magnetic properties, i.e., B r of about 0.1 T, high H o of 300 to 400 kA/m, and rather high (JH)max of 13 to 20 kJ/m3. The crystallization causes the disappearance of the hard magnetic properties. Furthermore, the melt-spun Nd-Fe-Al and Pr-Fe-Al alloy ribbons exhibit soft-type magnetic properties. Consequently, the hard magnetic properties are concluded to be obtained only for the bulk amorphous alloys. The bulk Nd- and Pr-Fe-Al amorphous alloys have an extremely high T x/Tm of about 0.90 and a small ΔT m(=T m−T x) of less than 100 K and, hence, their large glass-forming ability is due to the steep increase in viscosity in the supercooled liquid state. The high T x/Tm enables the development of a fully relaxed, clustered amorphous structure including Nd-Nd and Nd-Fe atomic pairs. It is, therefore, presumed that the hard magnetic properties are due to the development of Nd-Nd and Nd-Fe atomic pairs with large random magnetic anisotropy. The Nd- and Pr-based bulk amorphous alloys can be regarded as a new type of clustered amorphous material, and the control of the clustered amorphous structure is expected to enable the appearance of novel functional properties which cannot be obtained for an ordinary amorphous structure.

206 citations


Journal ArticleDOI
TL;DR: Equal-Channel angular (ECA) pressing is rapidly becoming an established procedure for inducing microstructural refinement in polycrystalline materials through the process of intense plastic straining as discussed by the authors, where a sample is pressed through a die having two channels, equal in cross section, which intersect at an angle of F with simple shear occurring as the sample moves through the angle subtended at the intersection of the two channels.
Abstract: Equal-channel angular (ECA) pressing is rapidly becoming an established procedure for inducing microstructural refinement in polycrystalline materials through the process of intense plastic straining. To date, there have been numerous reports of the use of the ECA pressing procedure to produce grain refinement down to the submicrometer, or occasionally the nanometer, level in a range of metallic materials[1–10] and in some intermetallic compounds.[11,12] In ECA pressing, a sample is pressed through a die having two channels, equal in cross section, which intersect at an angle of F, with simple shear occurring as the sample moves through the angle subtended at the intersection of the two channels.[13,14] Since the cross section of the sample remains unchanged on passage through the die, the procedure is readily amenable to repetitive pressings of the same sample in order to introduce high total strains. Pressings are generally conducted using dies having F . 90 deg, and under these conditions the equivalent strain introduced on each passage through the die is approximately equal to 1.[15] In practice, experiments have demonstrated that the microstructural characteristics introduced by ECA pressing, including the structural homogeneity, the average shapes of the individual grains, and the misorientation angles of the boundaries between adjacent grains, are all dependent upon experimental conditions such as the number of passages through the die,[5,9,10,16] the shearing directions in each separate passage as manifested by any rotation of the sample between separate pressings,[5,9,16] and the temperature at which the straining is conducted.[10] When repetitive pressings are undertaken, it is a standard procedure to identify three different routes, designated A, B, and C, in which the pressings are conducted without any rotation of the sample, with rotation about the longitudinal axis by 90 deg between each pressing and with rotation by 180 deg between each pressing, respectively. Two recent reports have compared directly the efficiency of these different processing routes for establishing a homogeneous structure consisting of an array of equiaxed grains. In experiments by Ferrasse et al.[9] on Cu and Al alloys, it was concluded that processing by route C produces a more ho-

198 citations


Journal ArticleDOI
TL;DR: In this article, the Hall-Petch equation was used to model the behavior of fine-grained Fe-10Cu powders with grain diameters between 45 nm and 1.7 µm.
Abstract: 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.

178 citations


Journal ArticleDOI
TL;DR: In this paper, the microstructure and compression strengths of Ir-15 at. pct X (X=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.
Abstract: The microstructure and compression strengths of Ir-15 at. pct X (X=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.

169 citations


Journal ArticleDOI
TL;DR: In this article, the authors highlight significant improvements in high-temperature intergranular degradation susceptibility and weldability arising from increasing the relative proportion of crystallographically "special" low-Σ CSL grain boundaries in the microstructure.
Abstract: The principal limitation of today’s Ni- and Fe-based superalloys continues to be their susceptibility to intergranular degradation arising from creep, hot corrosion, and fatigue. Many precipitation-strengthened superalloys are also difficult to weld, owing to the formation of heat-affected zone (HAZ) cracks during postweld heat treatments (PWHTs). The present work highlights significant improvements in high-temperature intergranular degradation susceptibility and weldability arising from increasing the relative proportion of crystallographically “special” low-Σ CSL grain boundaries in the microstructure. Susceptibility to intergranular degradation phenomena is reduced by between 30 and 90 pct and is accompanied by decreases in the extent and length of PWHT cracking of up to 50-fold, with virtually no compromise in mechanical (tensile) properties upon which the functionality of these specialty materials depends. Collectively, the data presented suggest that “engineering” the crystallographic structure of grain boundaries offers the possibility to extend superalloy lifetimes and reliability, while minimizing the need for specialized welding techniques which can negatively impact manufacturing costs and throughput.

Journal ArticleDOI
TL;DR: In this paper, the reaction sequences responsible for microstructural development were found to be similar to those expected in the Ni-Nb-C ternary system and commercial superalloys of comparable composition.
Abstract: The solidification reaction sequences of experimental superalloys containing systematic variations in Fe, Nb, Si, and C were studied using differential thermal analysis (DTA) and microstructural characterization techniques. The reaction sequences responsible for microstructural development were found to be similar to those expected in the Ni-Nb-C ternary system and commercial superalloys of comparable composition. The solute-rich interdendritic liquid generally exhibited two eutectic-type reactions at the terminal stages of solidification: L → (γ+NbC) and L → (γ+Laves). The Ni-base alloys with a high C/Nb ratio represented the only exception to this general solidification sequence. This group of alloys terminated solidification with the L → (γ + NbC) reaction and did not exhibit the γ/Laves constituent. At similar levels of solute elements (Nb, Si, and C), the Fe-base alloys always formed more of the γ/Laves eutectic-type constituent than the corresponding Ni-base alloys. Silicon additions also increased the amount of the γ/Laves constituent that formed in the assolidified microstructure, while C additions promoted formation of γ/NbC. The influence of Nb was dependent on the C content of the alloy. When the C content was low, Nb additions generally promoted formation of γ/Laves, while Nb additions to alloys with high C led to formation of the γ/NbC constituent. The results of this work are combined with quantitative analyses for developing γ-Nb-C pseudoternary solidification diagrams in a companion article.

Journal ArticleDOI
TL;DR: In this paper, analytical electron microscopic studies were performed on the constituent particles in commercial 7075-T6 and 2024-T3 alloys, and the crystal structure and chemistry of the Al23CuFe4, Al2CuMg, and Al2cu phases in these alloys were identified.
Abstract: To better understand the role of constituent particles in pitting corrosion, analytical electron microscopic studies were performed on the constituent particles in commercial 7075-T6 and 2024-T3 alloys. Five phases, namely, Al23CuFe4 and amorphous SiO2 in 7075-T6 and Al2CuMg, Al2Cu, and (Fe,Mn) x Si(Al,Cu) y in 2024-T3, were identified. The crystal structure and chemistry of the Al23CuFe4, Al2CuMg, and Al2Cu phases in these alloys are in good agreement with the published data. Small deviations from their stoichiometric compositions were observed and are attributed to the influence of alloy composition on the phase chemistry. For the (Fe,Mn) x Si(Al,Cu) y (approximately, x=3 and y=11) phase, a rhombohedral structure, with lattice parameter a=b=c=1.598 nm and α=β=γ=75 deg, was identified and is believed to be a modified form of either Al8Fe2Si or Al10Mn3Si. Information from this study provided technical support for studying the electrochemical interactions between the individual particles (or phases) and the matrix. The corrosion results are reported in a companion article.

Journal ArticleDOI
TL;DR: In this article, the microstructural details of the Ni-base superalloy formed at various stages of aluminizing were analyzed and it was concluded that the coating growth in the above process takes place primarily by inward Al diffusion initially, followed by an intermediate stage when the growth involves both inward Al and outward Ni diffusion.
Abstract: This study deals with the aluminizing of a directionally cast Ni-base superalloy, namely CM-247, by a single-step process using a high-activity pack. It is observed that significant incorporation of Al into the substrate surface during aluminizing continues over a period of about 1 hour and is not restricted merely to the first few minutes, as reported in the literature. Based on the microstructural details of the coatings formed at various stages of aluminizing, it is concluded that the coating growth in the above process takes place primarily by inward Al diffusion initially, followed by an intermediate stage when the growth involves both inward Al and outward Ni diffusion. In the final stages, the outward diffusion of Ni dominates the coating formation process. The above mechanism of coating formation is different from the one that prevails in the conventional two-step high-activity coating process in that the reaction front for the formation of NiAl remains spatially stationary despite the outward diffusion of nickel during the intermediate stage. It is also shown in the present study that the content of the Al source in the pack affects the coating structure significantly. It is further demonstrated that the microstructure of the aluminide coatings depends not only on the amount of Al incorporated in the sample during aluminizing but also on the time over which the uptake of this Al takes place.

Journal ArticleDOI
TL;DR: In this article, the authors show how a ferrite-bainite-martensite microstructure containing retained austenite can improve the mechanical properties of a cold-rolled low-carbon, low-silicon steel.
Abstract: Dual-phase and transformation-induced plasticity (TRIP)-assisted multiphase steels are related families of high-strength formable steels exhibiting excellent mechanical characteristics. This study shows how a ferrite-bainite-martensite microstructure containing retained austenite can improve the mechanical properties of a cold-rolled low-carbon, low-silicon steel. Such a multiphased microstructure is obtained by a heat treatment involving intercritical annealing followed by a bainite transformation tempering. Depending on the heat-treatment parameters, the samples present a variety of microstructures. Due to the presence of retained austenite, some samples exhibit a TRIP effect not anticipated with such a low silicon content. A composite strengthening effect also results from the simultaneous presence of a ductile ferrite matrix with bainite and martensite as hard second phases. A true stress at maximum load of 800 MPa and a true uniform strain of 0.18 can be obtained by forming a ferrite-bainite-martensite microstructure containing up to 10 pct of retained austenite. These properties correspond to a favorable evolution of work hardening during plastic deformation.

Journal ArticleDOI
TL;DR: In this article, the pseudoternary γ-Nb-C solidification surfaces, modeled after the liquidus projection in the Ni-C ternary system, were proposed.
Abstract: Equilibrium distribution coefficients and pseudoternary solidification surfaces for experimental superalloys containing systematic variations in Fe, Nb, Si, and C were determined using quenching experiments and microstructural characterization techniques. In agreement with previous results, the distribution coefficient, k, for Nb and Si was less than unity, while the “solvent” elements (Fe, Ni, and Cr) exhibited little tendency for segregation (k ≈ 1). The current data were combined with previous results to show that an interactive effect between kNb and nominal Fe content exists, where the value of kNb decreases from 0.54 to 0.25 as the Fe content is increased from ≈2 wt pct to ≈47 wt pct. This behavior is the major factor contributing to formation of relatively high amounts of eutectic-type constituents observed in Fe-rich alloys. Pseudoternary γ-Nb-C solidification surfaces, modeled after the liquidus projection in the Ni-Nb-C ternary system, were proposed. The Nb compositions, which partially define the diagrams, were verified by comparison of calculated amounts of eutectic-type constituents (via the Scheil equation) and those measured experimentally, and good agreement was found. The corresponding C contents needed to fully define the diagrams were estimated from knowledge of the primary solidification path and k values for Nb and C.

Journal ArticleDOI
TL;DR: In this article, the fracture path of plane-strain fracture-toughness specimens of 7050 alloy (a typical alloy of the 7XXX series) is quantitatively characterized as a function of degree of recrystallization, specimen orientation, and aging condition.
Abstract: The fracture toughness of Al-Zn-Mg-Cu-based 7XXX aluminum alloys decreases with an increase in the extent of recrystallization. In this contribution, the fracture path of plane-strain fracture-toughness specimens of 7050 alloy (a typical alloy of the 7XXX series) is quantitatively characterized as a function of degree of recrystallization, specimen orientation, and aging condition. The fracture path is quantitatively correlated to fracture toughness, and the bulk microstructural attributes estimated via stereological analysis. In the companion article, these quantitative data are used to develop and verify a multiple-fracture micromechanism-based model that relates the fracture toughness to a number of microstructural parameters of the partially recrystallized alloy plate.

Journal ArticleDOI
TL;DR: In this article, micro-macrosegregation calculations have been performed for a rectangular cavity containing either a Pb-48 wt pct Sn alloy or a Sn-5 wtpct Pb alloy, and the numerical results calculated with a finite volume method and a finite element method were compared with experimental results obtained by Hebditch and Hunt.
Abstract: Micro-macrosegregation calculations have been performed for a rectangular cavity containing either a Pb-48 wt pct Sn alloy or a Sn-5 wt pct Pb alloy. The numerical results calculated with a finite volume method (FVM) and a finite element method (FEM) are compared with experimental results previously obtained by Hebditch and Hunt. The two methods are based on the same average conservation equations governing heat and mass transfer and the same assumptions: lever rule, equal and constant density of the solid and liquid phases (except in the buoyancy term), permeability of the mushy zone given by the Carman-Kozeny relation, and no transport of the solid phase. Although the same parameters are used in both calculations, small differences are observed as a result of the different formulations. In particular, the instabilities appearing in the mushy zone (channels) of the Sn-5 wt pct Pb alloy are more pronounced with the FVM formulation as compared with FEM, whereas the opposite trend is observed for the Pb-48 wt pct Sn alloy. Nevertheless, the final segregation maps at the end of solidification compare fairly well with the experimental findings.

Journal ArticleDOI
TL;DR: In this paper, a bicrystal approach using crystal plasticity theory was proposed to predict the evolution of subgrain misorientations, and these mechanically induced rotations are shown to occur at the high strain rate associated with adiabatic shear band formation.
Abstract: A mechanical subgrain rotation model is proposed to account for the recrystallized grains which have been observed to form in adiabatic shear bands in a number of materials. The model is based on a “bicrystal” approach using crystal plasticity theory to predict the evolution of subgrain misorientations. These mechanically induced rotations are shown to occur at the high strain rate associated with adiabatic shear band formation. Recrystallized grain formation is proposed to occur by the formation and mechanical rotation of subgrains during deformation, coupled with boundary refinement via diffusion during shear band cooling. This model is referred to as progressive subgrain misorientation recrystallization and appears to account for shear band microstructures in a variety of metals.

Journal ArticleDOI
TL;DR: In this article, the influence of grain size on the mechanical properties and intrinsic ductility of nanocrystalline (nc) Fe has been investigated and the results are discussed in the context of the mechanical behavior of coarse-grained polycrystalline metals and alloys.
Abstract: 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.

Journal ArticleDOI
TL;DR: In this paper, plane strain compression and torsion deformation modes have been used to analyze the microstructural evolution and the mechanical behavior of duplex stainless steel in as-cast and wrought conditions, as a function of spatial phase distribution, the nature of interface, and the relative mechanical properties of both phases.
Abstract: In the hot deformation of the duplex stainless steels, the complexity of the microstructure evolution and mechanical response is increased as compared with those of single-phase ferritic or austenitic stainless steels. In the present work, plane strain compression and torsion deformation modes have been used to analyze the microstructural evolution and the mechanical behavior of a duplex stainless steel in as-cast and wrought conditions, as a function of spatial phase distribution, the nature of interface, and the relative mechanical properties of both phases. The law of mixtures has been used to explain the different flow curves obtained when changing the phase distribution and/or the deformation mode. On deforming as-cast microstructures, the deformation partitions vary heterogeneously between both phases and some austenite areas act as hard nondeforming particles. Cracks have been observed to occur at the interface of such regions, from relatively low strains, for which the initial Kurdjumov-Sachs orientation relationship between ferrite and austenite is still present.

Journal ArticleDOI
TL;DR: In this article, the authors examined the slope of the Hall-Petch plot for FL microstructures, paying particular attention to the lamellar microstructural variables, and showed that these spacings influence the value of k ≥ 2 in the HP relationship.
Abstract: More than 5 years ago, wrought processing was first used to produce fully lamellar (FL) microstructures in TiAl alloys having grain sizes less than ≈400 µm. These alloys exhibit an improvement in overall balance of properties, especially at high temperatures. More recently, such microstructural forms led to exceptional yield strengths (500 to 1000 MPa at low temperatures) while maintaining attractive high-temperature properties. The improvements appeared to be related to an unusually high apparent sensitivity of strength to grain size. Studies reported an apparent value for the slope of the Hall-Petch (HP) plot approaching 5 MPa√m for FL gamma alloys, while that for single-phase or duplex microstructures is near unity. The present investigations examine the slope of the HP plot for FL microstructures, paying particular attention to the lamellar microstructural variables. Results show that the α 2 lamellar thickness and spacing and the γ lamellar thickness can vary over more than two orders of magnitude with typical process methods. These spacings influence the value of k y in the HP (grain size) relationship. Since they often change concomitantly with grain size in processing, they can give rise to a large scatter in the HP plot. The investigations also examine the flow behavior, glide barriers, and slip multiplicity for polysynthetically twinned (PST) crystals (the single-grain analogue of FL material), and then map this behavior into an explanation of the yield behavior of high-strength FL gamma alloys.

Journal ArticleDOI
TL;DR: The divorced eutectoid transformation (DET) was shown to be faster in the presence of small spheroidal cementite particles with spacings on the order of a few microns as mentioned in this paper.
Abstract: Experiments are presented which show that the eutectoid transformation in steel can occur by two different modes for temperatures just slightly below A 1. In the normal mode, the transformation product is lamellar pearlite. The second mode occurs if the austenite contains cementite particles or nuclei with a spacing on the order of a few microns or less. In this case, the transformation product consists of spheroidal cementite particles in a ferrite matrix. This second mode is here called the divorced eutectoid transformation (DET), after recent work by Sherby and co-workers. A literature survey shows that the faster kinetics of the DET over lamellar pearlite in the presence of inhomogeneous austenite was established before 1940, but has received little attention. The inhomogeneities are generally small cementite particles. Experiments show that the DET does not occur by a shell of one phase (ferrite) forming around the other phase of the eutectoid (cementite), as is the case in divorced eutectic growth. Rather, a fairly planar austenite/ferrite front simply advances into the austenite, with no apparent effect on its shape being produced by the cementite particles. A first-order kinetic model is presented for the growth velocity as a function of undercooling below A 1 and is compared to the velocity vs undercooling for lamellar pearlite. The simple model indicates that the velocity of the divorced mode should be faster than the lamellar mode at low undercooling for cementite nuclei distributed in the austenite with spacings less than a few microns. This result is consistent with the experimental data.

Journal ArticleDOI
TL;DR: In this article, the results of the directional solidification (DS) experiments on particle engulfment and pushing by solidifying interfaces (PEP), conducted on the space shuttle Columbia during the Life and Microgravity Science (LMS) Mission, are reported.
Abstract: Results of the directional solidification (DS) experiments on particle engulfment and pushing by solidifying interfaces (PEP), conducted on the space shuttle Columbia during the Life and Microgravity Science (LMS) Mission, are reported. Two pure aluminum (99.999 pct) 9 mm cylindrical rods, loaded with about 2 vol pct 500µm-diameter zirconia particles, were melted and resolidified in the microgravity (µg) environment of the shuttle. One sample was processed at a stepwise increased solidification velocity and the other at a stepwise decreased velocity. It was found that a pushing/engulfment transition (PET) occurred in the velocity range of 0.5 to 1 µm/s. This is smaller than the ground PET velocity of 1.9 to 2.4 µm/s. This demonstrates that natural convection increases the critical velocity. A previously proposed analytical model for PEP was further developed. A major effort to identify and produce data for the surface energy of various interfaces required for calculation was undertaken. The predicted critical velocity for PET was 0.775 µm/s.

Journal ArticleDOI
TL;DR: In this article, the effects of Y, Sr, and Nd additions on the microstructure and microfracture mechanism of the four squeeze-cast magnesium alloys based on the commercial AZ91 alloy were investigated.
Abstract: This study aims to investigate the effects of Y, Sr, and Nd additions on the microstructure and microfracture mechanism of the four squeeze-cast magnesium alloys based on the commercial AZ91 alloy. Microstructural observation, in situ fracture tests, and fractographic observation were conducted on the alloys to clarify the microfracture process. Microstructural analyses indicated that grain refinement could be achieved by small additions of alloying elements, although the discontinuously precipitated Mg17Al12 phases still existed on grain boundaries. From in situ fracture observation of an AZ91-Sr alloy, it was seen that coarse needle-shaped compound particles and Mg17Al12 phases located on the grain boundary provided easy intergranular fracture sites under low stress intensity factor levels, resulting in the drop in toughness. On the other hand, the AZ91-Y and AZ91-Nd alloys showed improved fracture toughness, since deformation and fracture paths proceeded into grains rather than to grain boundaries, as the planar slip bands and twinnings actively developed inside the grains. These findings suggested, on the basis of the well-developed planar slip bands and twinnings, that the small addition of Y or Nd was very effective in improving fracture toughness.

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TL;DR: The structure of the Guinier-Preston (GP) zone in an Al-1.6 mass pct Mg2Si alloy aged at 343 and 423 K was investigated by high-resolution transmission electron microscopy (HRTEM) as mentioned in this paper.
Abstract: The structure of the Guinier-Preston (GP) zone in an Al-1.6 mass pct Mg2Si alloy aged at 343 and 423 K was investigated by high-resolution transmission electron microscopy (HRTEM). The morphology of the GP zone is that of a fine plate having the dimensions of a monolayer in thickness, 2.5 nm in width, and less than 30 nm in length. Its elongated direction is parallel to the 〈100〉 m direction. The chemical composition of the GP zone in an Al-Mg2Si alloy indicated a ratio of Mg/Si=1.0. The zones aggregate with increasing aging time and exhibit a more complicated morphology because of the transformation to the metastable phase that takes place inside the GP zones.

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TL;DR: In this paper, the evolution mechanisms of dislocation microstructures and new grains at high strains of above 4 were studied by means of multiple compression of a polycrystalline copper (99.99 pct).
Abstract: The evolution mechanisms of dislocation microstructures and new grains at high strains of above 4 were studied by means of multiple compression of a polycrystalline copper (99.99 pct). Deformation was carried out by multipass compression with changing of the loading direction in 90 deg in each pass at temperatures of 473 K to 573 K (0.35 to 0.42 T m ) under a strain rate of 10−3 s−1. The flow stresses increase to a peak followed by a work softening accompanied mainly by dynamic recrystallization (DRX) at 523 K to 573 K. In contrast, the steady-state-like flow appears at 473K accompanied with the development of fine grains at strains as high as 4.2. The relationship of flow stress to the new grain size evolved can be expressed by a power law function with a grain size exponent of about −0.35, which is different from −0.75 for high-temperature DRX at above 0.5 T m . At 473 K, misorientations of deformation-induced dislocation subboundaries increase with increasing strain, finally leading to the evolution of new grains. It is concluded that the dynamic grain formation at 473 K cannot result from DRX, but from the evolution of deformation-induced dislocation subboundaries with high misorientations and, concurrently, the operation of dynamic recovery.

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TL;DR: In this article, a solidification model for hot dip coatings was derived based on optical and microscopic properties of the liquid zinc surface, including optical properties, distribution of Pb and Al, crystal orientation, and topography.
Abstract: 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.

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TL;DR: In this article, an equal-channel angular (ECA) pressing procedure was used to obtain high-strain-rate superplasticity in ultrafine-grained materials.
Abstract: Ultrafine grain sizes were introduced into samples of an Al-3 pct Mg solid solution alloy and a cast Al-Mg-Li-Zr alloy using the process of equal-channel angular (ECA) pressing. The Al-3 pct Mg alloy exhibited a grain size of ∼0.23 µm after pressing at room temperature to a strain of ∼4, but there was significant grain growth when the pressed material was heated to temperatures above ∼450 K. The Al-Mg-Li-Zr alloy exhibited a grain size of ∼1.2 µm, and the microstructure was heterogeneous after pressing to a strain of ∼4 at 673 K and homogeneous after pressing to a strain of ∼8 at 673 K with an additional strain of ∼4 at 473 K. The heterogeneous material exhibited superplastic-like flow, but the homogeneous material exhibited high-strain-rate superplasticity with an elongation of >1000 pct at 623 K at a strain rate of 10−2 s−1. It is concluded that a homogeneous microstructure is required, and therefore a high pressing strain, in order to attain high-strain-rate superplasticity (HSR SP) in ultrafine-grained materials.

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TL;DR: In this article, a pole mechanism based on a jogged [110]/2 ordinary dislocation is proposed to explain the available experimental data on deformation twinning in γ-TiAl single crystals.
Abstract: Physical properties that are relevant to mechanical behavior of single-phase TiAl and Ti3Al and two-phase TiAl/Ti3Al alloys are summarized. By using planar-fault energies and temperature-dependent elastic constants, dislocation dissociation reactions applicable to twin formation in TiAl are analyzed, and a pole mechanism based on a jogged [110]/2 ordinary dislocation is proposed to explain the available experimental data on deformation twinning in γ-TiAl single crystals. The strong plastic anisotropy reported in TiAl polysynthetically twinned (PST) crystals is attributed in part to the localized slip along lamellar interfaces, thus lowering the yield stress for soft orientations. The experimental findings reported on cleavage habit planes of PST crystals are discussed in terms of the calculated ideal work of adhesion and possible extrinsic factors.