Showing papers on "Equiaxed crystals published in 2002"

The use of severe plastic deformation for microstructural control

Abstract: The microstructure of a metal may be very significantly changed by subjecting the material to severe plastic deformation (SPD) through procedures such as equal-channel angular pressing (ECAP) and high-pressure torsion. These procedures lead to a substantial refinement in the grain size so that the grains are reduced to, typically, the submicrometer or even the nanometer range. This paper describes the application of SPD to pure aluminum and aluminum-based alloys, with emphasis on the factors influencing the development of homogeneous microstructures of equiaxed grains separated by high-angle grain boundaries. Materials subjected to ECAP are capable of exhibiting exceptional mechanical properties including superplastic ductilities at very rapid strain rates. Examples of this behavior are presented and results are described showing the potential for using this approach in superplastic forming applications at high strain rates.

Solidification thermal parameters affecting the columnar-to-equiaxed transition

Abstract: Experiments were conducted to analyze the columnar-to-equiaxed transition (CET) during the upward unsteady-state directional solidification of Al-Cu and Sn-Pb alloys, under different conditions of superheat and heat-transfer efficiencies at the metal/mold interface. A combined theoretical and experimental approach is developed to quantitatively determine the solidification thermal parameters: transient heat-transfer coefficients, tip growth rates, thermal gradients, and cooling rates. The observed results do not give support to CET criteria based individually either on tip growth rate or temperature gradients ahead of the liquidus isotherm. Rather, the analysis has indicated that a more convenient criterion should encompass both thermal parameters through the tip cooling rate. The columnar growth is expected to prevail throughout the casting for a tip cooling rate higher than a critical value, which depends only on the alloy system and was observed to be about 0.2 K/s for Al-Cu alloys and 0.01 K/s for Sn-Pb alloys in the present investigation.

Elevated-temperature deformation at forming rates of 10 −2 to 10 2 s −1

Abstract: In the hot working at constant strain rate ( $$\dot \varepsilon $$ ) of Al and α Fe alloys at 0.5 to 0.9 T M (absolute melting temperature), steady-state deformation is achieved in similarity to creep, which is usually at constant stress. After an initial strain-hardening transient, the flow stress becomes constant in association with a substructure which remainsequiaxed and constant in the spacing of sub-boundaries and of dislocations in both walls and subgrains. All these spacings become larger at higher temperature (T) and lower $$\dot \varepsilon $$ values as well as with lower stress, being fully consistent with the relationships established in creep. Because hot working can proceed to a much higher true strain in torsion (∼100) and compression (∼2) as well as in extrusion (∼20) and rolling (∼5), it is possible to confirm that grains continue to elongate while the subgrains within them remain equiaxed and constant in size. When the thickness of grains reaches about 2 subgrain diameters (d s), the grain boundaries with serrations (∼d s) begin to impinge and the grains pinch off, becoming somewhat indistinguishable from the subgrains; this has been called geometric dynamic recrystallization (DRX). In polycrystals as at 20 °C, deformation bands form and rotate during hot working according to the Taylor theory, developing textures very similar to those in cold working. In metals of lower dynamic recoverability such as Cu, Ni, and γ Fe, new grains nucleate and grow (discontinuous DRX), leading to a steady state related to frequently renewed equiaxed grains, containing an equiaxed substructure that develops to a constant character and defines the flow stress.

Modeling of Globular Equiaxed Solidification with a Two-Phase Approach

Abstract: A two-phase volume averaging model for globular equiaxed solidification is presented. Treating both liquid and solid (disperse grains) as separated but highly coupled interpenetrating continua, we have solved the conservation equations for mass, momentum, species mass fraction, and enthalpy for both phases. We also consider the conservation of grain density. Exchange or source terms take into account interactions between the melt and the solid, such as mass transfer (solidification and melting), friction and drag, solute redistribution, release of latent heat, and nucleation. An ingot casting with a near globular equiaxed solidification alloy (Al-4 wt pct Cu) is simulated. Results including grain evolution, melt convection, sedimentation, solute transport, and macrosegregation formation are obtained. The mechanisms producing these results are discussed in detail.

Electron-beam welding behavior in Mg-Al-based alloys

Abstract: The electron-beam welding (EBW) behaviors of pure Mg and the AZ31, AZ61, and AZ91 Mg alloys are examined in this study, in terms of fusion-zone characteristics, grain structures, texture evolution, and joint efficiency. With increasing A1 content, the Mg-based materials were found to be more easily fusion welded. The AZ91 alloy could be welded using a beam power of 2200 W and a weld speed of 16 mm/s, resulting in a weld depth of 29 mm with a fusion-zone aspect ratio of 8.2. The grains inside the fusion zone were nearly equiaxed in shape and ∼10 µm in size, due to the rapid cooling rate. Extended partial melting zones were observed in alloys with high solute contents, such as AZ61 and AZ91. The postweld tensile strength of the Mg alloys could recover back to ∼80 to 110 pct of the original strength. The texture in the fusion zone was traced by X-ray diffraction (XRD) and electron-backscattered diffraction (EBSD). The grain orientations inside the rapidly solidified electron-beam-welded fusion zones are still rather diversely distributed. The α 1-, α 2-, and α 3-axes of some grains tend to align at 90 or 30 deg with respect to welding direction, and the c-axis tends to align along the plate normal direction. The influence from surface tension on the weld top-surface appearance and weld depth was not pronounced for the current four Mg materials. Instead, differences in the solidus temperatures and thermal conductivity should be the primary factors.

The production of ultrafine ferrite in low-carbon steel by strain-induced transformation

Abstract: An investigation into the production of ultrafine (1 µm) equiaxed ferrite (UFF) grains in low-carbon steel was made using laboratory rolling, compression dilatometry, and hot torsion techniques It was found that the hot rolling of thin strip, with a combination of high shear strain and high undercooling, provided the conditions most suitable for the formation of this type of microstructure Although high strains could be applied in compression and torsion experiments, large volume fractions of UFF were not observed in those samples, possibly due to the lower level of undercooling achieved It is thought that ferrite refinement was due to a strain-induced transformation process, and that ferrite grains nucleated on parallel and linear deformation bands that traversed austenite grains These bands formed during the deformation process, and the undercooling provided by the contact between the strip and the work rolls was sufficient to drive the transformation to homogeneous UFF grains

A three-phase model of hydrogen pore formation during the equiaxed dendritic solidification of aluminum-silicon alloys

Abstract: A computational model for the prediction of porosity due to dissolved hydrogen in binary aluminum-silicon alloys has been developed. The model combines the cellular automata technique for the simulation of the growth of the solid phase, the finite-difference technique for the simulation of diffusion of the dissolved species, and a quasi-equilibrium model for the growth of individual bubbles. The growth of the solid and gas phases is initiated by a stochastic nucleation model, depending upon the undercooling (for the solid) or the supersaturation ratio (for the gas). The results agree favorably with experiments. The low supersaturation values needed to simulate the experimental results are consistent with a nucleation mechanism of gas pockets entrained within the melt.

Combined effect and its mechanism of Al-3wt.%Ti-4wt.%B and Al-10wt.%Sr master alloy on microstructures of Al-Si-Cu alloy

Abstract: The combined effect of Al–3wt.%Ti–4wt.%B and Al–10wt.%Sr on Al–Si–Cu alloy has been investigated. Equiaxed dendritic grain of 100–120 μm was obtained with holding time of at least 240 min through addition of more than 0.020% Ti and 0.026% B, though the laminar morphology of eutectic Si kept unaltered. Eutectic Si was completely modified for more than 240 min through addition of Al–10wt.%Sr which also had some benefit on the refinement of primary α-Al. The combined addition of both Al–3wt.%Ti–4wt.%B and Al–10wt.%Sr led to shorter working time of modification and slightly coarser grain size. The above results are mainly due to the formation of SrB6 through the reaction between Al–10wt.%Sr and Al–3wt.%Ti–4wt.%B. It was found that satisfactory grain refinement and modification could be attained by suitable addition of both Al–3wt.%Ti–4wt.%B and Al–10wt.%Sr in spite of the formation of SrB6.

Microstructures and properties of investment castings of γ-titanium aluminide

Abstract: γ-TiAl castings have been produced by induction skull melting (ISM), and their microstructures, hardness and tensile properties have been studied in the as-cast, hot isostatically pressed (HIPped) and HIPped+heat treated conditions. The equiaxed rosette grains found in the cast structure suggest that the alloy was cast in a partially solid condition. HIPping and heat treatment for 24 h at 1300 °C resulted in a duplex structure and decreased the hardness and tensile properties compared with the as-HIPped condition. The hardness and tensile properties also decreased with increasing bar diameter. The main fracture modes in duplex structures were transgranular (TG) in equiaxed γ-grains and translamellar (TL) in lamellar grains. The presence of an α 2 /γ lamellar structure can improve strength and ductility.

Room temperature deformation behavior of Zn-22 mass%Al alloy with nanocrystalline structure : Superplasticity and its applications

Abstract: The deformation behavior near room temperature in Zn-22 mass%Al alloy including nanocrystalline structure produced with Thermo Mechanical Controlling Process (TMCP) technology has been characterized over a wide range of strain rates from 10 -6 to 10 -1 s -1 at temperatures from 273 to 473 K. The microstructure of TMCP produced Zn-22 mass%Al alloy had both a random distribution of equiaxed Al-rich and Zn-rich phases with grain size of 1.3 μm and many nanocrystalline Zn particles in Al-rich phases. Since the flow stress in the deformation near room temperature was much larger than that in superplastic deformation and a maximum m value is only 0.3 (n = 3) at low strain rates below 10 -5 s -1 , the pure superplastic behavior may not be observed near room temperature. However it is noted that the large elongation of ∼ 200% was observed at 10 -5 s -1 . From microstructural observations of the specimens tested in the condition with the m value of 0.3 near room temperature, furthermore, it is considered that grain boundary sliding (GBS) is the dominant deformation process, and the specimen may be fractured by cavitation as well as the conventional superplastic materials. Therefore, it seems that the various factors contribute to the deformation flow at room temperature.

Room Temperature Deformation Behavior of Zn-22 mass%Al Alloy with Nanocrystalline Structure

Abstract: peratures from 273 to 473 K. The microstructure of TMCP produced Zn-22 mass%Al alloy had both a random distribution of equiaxed Al-rich and Zn-rich phases with grain size of 1.3 µm and many nanocrystalline Zn particles in Al-rich phases. Since the flow stress in the deformation near room temperature was much larger than that in superplastic deformation and a maximum m value is only 0.3 (n = 3) at low strain rates below 10 −5 s −1 , the pure superplastic behavior may not be observed near room temperature. However it is noted that the large elongation of ∼ 200% was observed at 10 −5 s −1 . From microstructural observations of the specimens tested in the condition with the m value of 0.3 near room temperature, furthermore, it is considered that grain boundary sliding (GBS) is the dominant deformation process, and the specimen may be fractured by cavitation as well as the conventional superplastic materials. Therefore, it seems that the various factors contribute to the deformation flow at room temperature.

Thermal stability of hard TiN/SiNx multilayer coatings with an equiaxed microstructure

Abstract: Hard TiN/SiN x multilayer coatings with an equiaxed microstructure were prepared using a dual-cathode unbalanced reactive-magnetron sputtering system. These multilayer coatings were then annealed at 1000 °C for 1 h in vacuum. These TiN/SiN x multilayer coatings before and after annealing were characterized at room temperature and compared in terms of microstructure and mechanical properties. X-Ray diffraction and cross-sectional transmission electron microscopy studies showed that the layer structure of these coatings is preserved after annealing at 1000 °C when the SiN x layer thickness is 0.8 nm or greater. The high hardness of these multilayer coatings is either maintained or improved after annealing. Therefore, with proper control of the SiN x thickness, TiN/SiN x multilayer coatings demonstrate excellent thermal stability and are potential candidates for high-temperature tribological applications.

Effect of the lamellar grain size on plastic flow behavior and microstructure evolution during hot working of a gamma titanium aluminide alloy

Abstract: The kinetics of dynamic spheroidization of the lamellar microstructure and the associated flow-softening behavior during isothermal, constant-strain-rate deformation of a gamma titanium aluminide alloy were investigated, with special emphasis on the role of the prior-alpha grain/colony size. For this purpose, fully lamellar microstructures with prior-alpha grain sizes between 80 and 900 µm were developed in a Ti-45.5Al-2Nb-2Cr alloy using a special forging and heat-treatment schedule. Isothermal hot compression tests were conducted at 1093 °C and strain rates of 0.001, 0.1, and 1.0 s−1 on specimens with different grain sizes. The flow curves from these tests showed a very strong dependence of peak flow stress and flow-softening rate on grain size; both parameters increased with alpha grain/colony size. Microstructures of the upset test specimens revealed the presence of fine, equiaxed grains of γ + α 2 + β phases resulting from the dynamic spheroidization process that initiated at and proceeded inward from the prior-alpha grain/colony boundaries. The grain interiors displayed evidence of microkinking of the lamellae. The frequency and severity of kinking increased with strain, but were also strongly dependent on the local orientation of lamellae with respect to the compression axis. The kinetics of dynamic spheroidization were found to increase as the strain rate decreased for a given alpha grain size and to decrease with increasing alpha grain size at a given strain rate. The breakdown of the lamellar structure during hot deformation occurred through a combination of events, including shear localization along grain/colony boundaries, microbuckling of the lamellae, and the formation of equiaxed particles of γ + β 2 + α 2 on grain/colony boundaries and in zones of localized high deformation within the microbuckled regions.

Determination of Upper Limit Temperature of Strain-induced Transformation of Low Carbon Steels

Abstract: Two quenching methods were applied in the experiments carried out on Gleeble 1 500 thermomechanical simulator: (1) spurting water; (2) making the samples drop into water, and the cooling rate distributed un-uniformly in the samples quenched by the former and uniformly by the latter. The critical cooling rate of preventing ferrite precipitation was determined, and the microstructure observation position in which the microstructure could be frozen to room temperature in the samples quenched by spurting water was also determined, and using this method the A d3 was determined to be about 830°C for tested materials. Using second quenching method, single-pass deformation experiments at different temperatures were carried out, and the microstructure analysis indicates that, as the deformation temperature decreased, the ferrite morphology changed from Widmanstatten and allotriomorphs to equiaxed at about 800°C. Combining the dilation-temperature curves measured during cooling process after deformation at different temperatures, the A d3 was determined to be also about 830°C for tested materials.

Dislocation glide and dynamic recrystallization in LiF single crystals

Abstract: Single crystals of LiF with 〈111〉 orientation have been dynamically recrystallized during steady state of plastic deformation at 673 K. It is shown that localization of single dislocation glide plays an important role in the initiation of continuous dynamic recrystallization. Bands of elongated subgrains of rectangular shape are formed in areas of localized deformation at small strains. Upon subsequent deformation uniform multiple slip occurs and leads to the formation of equiaxed subgrains between bands of elongated subgrains. Further plastic deformation leads to a continuous increase in misorientation of subgrain boundaries and their eventual conversion into high angle boundaries. The mechanisms of the formation of recrystallized grains with different grain boundary configurations and the relationship between grain boundary configuration and character of dislocation slip are discussed.

Microstructure and mechanical properties of superplastically deformed silicon nitride–silicon oxynitride in situ composites

Abstract: Silicon nitride–silicon oxynitride in situ composites were fabricated by plane-strain-compressing dense silicon nitrides, starting from 93 wt.% ultrafine β-Si3N4 and 7 wt.% cordierite, at 1600 °C under a constant load of 40 MPa and subsequent annealing at 1750 °C for 30 min. The resulting composites featured a microstructure of elongated Si2N2O grains (∼0.64 μm in diameter and ∼5.5 in aspect ratio) dispersed in a fine-grained β-Si3N4 matrix (∼ 0.30μm in diameter and ∼3.5 in aspect ratio), with the amount of Si2N2O, which had relatively strong textures, being strain-dependent. The mechanical properties were found to be improved due to the development of elongated Si2N2O grains, the texture formation, and the coarsening of β-Si3N4. Fracture toughness, however, was still low (∼5.2 MPa m1/2) for these composites in comparison to self-reinforced silicon nitrides, resulted from the strong Si2N2O-matrix interfacial bond and nearly equiaxed β-Si3N4 with a small grain size. Anticipated property anisotropies were clearly observed as a result of the textured microstructure.