Microtexture evolution during annealing and superplastic deformation of Al-5 pct Ca-5 pct Zn

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.     

Deformation Characteristics and Recrystallization Response of a 9310 Steel Alloy

The flow behavior and recrystallization response of a 9310 steel alloy deformed in the ferrite temperature range were studied in this work. Samples were compressed under various conditions of strain (0.6, 0.8 and multi-axial), strain rate (10^?4 seconds^?1 to 10^?1 seconds^?1) and temperature [811 K to 1033 K (538 °C to 760 °C)] using a Gleeble thermo-mechanical simulator. Deformation was characterized by both qualitative and quantitative means, using standard microscopy, electron backscatter diffraction (EBSD) analysis and flow stress modeling. The results indicate that deformation is primarily accommodated through dynamic recovery in sub-grain formation. EBSD analysis shows a continuous increase in sub-grain boundary misorientation with increasing strain, ultimately producing recrystallized grains from the sub-grains at high strains. This suggests that a sub-grain rotation recrystallization mechanism predominates in this temperature range. Analyses of the results reveal a decreasing mean dynamically recrystallized grain size with increasing Zener-Hollomon parameter, and an increasing recrystallized fraction with increasing strain.     

A Study on Microstructural Evolution and Dynamic Recrystallization During Isothermal Deformation of a Ti-Modified Austenitic Stainless Steel

Dynamic recrystallization (DRX) behavior in hot deformed (by uniaxial compression in a thermomechanical simulator in the temperatures range 1173 K to 1373 K [900 °C to 1100 °C]) Ti-modified austenitic stainless steel was studied using electron back scatter diffraction. Grain orientation spread with a “cut off” of 1 deg was a suitable criterion to partition dynamically recrystallized grains from the deformed matrix. The extent of DRX increased with strain and temperature, and a completely DRX microstructure with a fine grain size ~4 μm (considering twins as grain boundaries) was obtained in the sample deformed to a strain of 0.8 at 1373 K (1100 °C). The nucleation of new DRX grains occurred by the bulging of the parent grain boundary. The DRX grains were twinned, and a linear relationship was observed between the area fraction of DRX grains and the number fraction of Σ3 boundaries. The deviation from the ideal misorientation of Σ3 boundaries decreased with an increase in the fraction of Σ3 boundaries (as well as the area fraction of DRX) signifying that most Σ3 boundaries are newly nucleated during DRX. The generation of these Σ3 boundaries could account for the formation of annealing twins during DRX. The role of Σ3 twin boundaries on DRX is discussed.     

Plastic-flow and microstructure evolution during hot deformation of a gamma titanium aluminide alloy

The hot workability of a near gamma titanium aluminide alloy, Ti-49.5Al-2.5Nb-1.1Mn, was assessed in both the cast and the wrought conditions through a series of tension tests conducted over a wide range of strain rates (10⁻⁴ to 10⁰ s⁻¹) and temperatures (850 °C to 1377 °C). Tensile flow curves for both materials exhibited sharp peaks at low strain levels followed by pronounced necking and flow localization at high strain levels. A phenomenological analysis of the strain rate and temperature dependence of the peak stress data yielded an average value of the strain rate sensitivity equal to 0.21 and an apparent activation energy of ∼411 kJ/mol. At low strain rates, the tensile ductility displayed a maximum at ∼ 1050 °C to 1150 °C, whereas at high strain rates, a sharp transition from a brittle behavior at low temperatures to a ductile behavior at high temperatures was noticed. Dynamic recrystallization of the gamma phase was the major softening mechanism controlling the growth and coalescence of cavities and wedge cracks in specimens deformed at strain rates of 10⁻⁴ to 10⁻² s⁻¹ and temperatures varying from 950 °C to 1250 °C. The dynamically recrystallized grain size followed a power-law relationship with the Zener-Hollomon parameter. Deformation at temperatures higher than 1270 °C led to the formation of randomly oriented alpha laths within the gamma grains at low strain levels followed by their reorientation and evolution into fibrous structures containing γ + α phases, resulting in excellent ductility even at high strain rates.     

Microstructural Aspects of Grain Boundary Bulge in a Dynamically Recrystallized Mg-Al-Zn Alloy

Microstructural features of grain boundary bulging have been studied in a dynamically recrystallized (DRXed) Mg-Al-Zn alloy. Unidirectional compression was used to deform the specimens to different strains at 473 K (200 °C). Microstructural characterization of the deformed specimens was performed by using both scanning electron microscopy and transmission electron microscopy (TEM). From the present results, it is suggested that in AZ31 Mg alloy, the DRXed grain is developed from grain boundary bulging. After a grain boundary segment starts to bulge, a bridging dislocation wall forms and anchors the bulged grain boundary. During further deformation, the misorientation of this bridging wall gradually increases, then transforms into a grain boundary, and a DRXed grain forms. Electron backscattered diffraction was used to study the orientation relationship between bulges/DRXed grains and the parent grains, and no special orientation relationship was found between them.     

Evolution of Microstructure and Texture During Hot Compression of a Ni-Fe-Cr Superalloy

Superalloys are being employed in more extreme conditions requiring higher strength, which requires producers to forge products to finer grain sizes with less grain size variability. To assess grain size, crystallographic texture, and substructure as a function of forging conditions, frictionless uniaxial compression testing characteristic of hot working was performed on INCOLOY 945 (Special Metals Corporation, Huntington, WV), which is a newly developed hybrid of alloys 718 and 925, over a range of temperatures and strain rates. The microstructure and texture were investigated comprehensively using light optical microscopy, electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI), and transmission electron microscopy (TEM) to provide detailed insight into microstructure evolution mechanisms. Dynamic recrystallization, nucleated by grain/twin boundary bulging with occasional subgrain rotation, was found to be a dominant mechanism for grain refinement in INCOLOY 945. At higher strain rates, static recrystallization occurred by grain boundary migration. During deformation, duplex slip along {111} planes occurred until a stable 〈110〉 fiber compression texture was established. Recrystallization textures were mostly random but shifted toward the compression texture with subsequent deformation. An exception occurred at 1423 K (1150 °C) and 0.001 seconds⁻¹, the condition with the largest fraction of recrystallized grains, where a 〈100〉 fiber texture developed, which may be indicative of preferential growth of specific grain orientations.     

Thermomechanical response of a powder metallurgy Ti−6Al−4V alloy modified with 2.9 Pct Boron

The thermomechanical response of Ti−6A1−4V modified with 2.9 pct B produced by a blended powder metallurgy route was studied with isothermal constant strain-rate hot compression tests in the temperature range 850°C to 1200°C and strain rate range 10⁻³ to 10s⁻¹. Detailed analyses of the flow stress data were conducted to identify various microstructural deformation and damage mechanisms during hot working by applying available materials modeling techniques. In the α+β phase field, cavitation at the matrix/TiB interfaces and TiB particle fracture occurs at low strain rates (<10⁻¹ s⁻¹), while adiabatic shear banding also occurs at high strain rates. At low strain rates, the β phase deforms superplastically due to the stabilization of a fine grain size by the TiB particles. Grain boundary and matrix/TiB interface sliding with simultaneous diffusional accommodation are observed to contribute to the β superplasticity. The deformation behavior at high strain rates in the β-phase field is similar to that of the α+β phase field, with microstructural manifestations of extensive cavitation at the matrix/TiB interfaces and TiB particle fracture.     

Dynamic recrystallization of ferrite in a low-carbon steel

Plane strain compression tests were performed on a low-carbon steel from 550 °C to 700 °C (ferritephase range) at strain rates of 10 to 5 × 10⁻⁴ s⁻¹, and the deformation microstructure evolution was investigated by means of scanning electron microscopy, transmission electron microscopy (TEM), and electron backscattered diffraction (EBSD). The results indicate that under the present deformation conditions, dynamic recrystallization of ferrite can occur in the low-carbon steel and lead to grain refinement. With increasing Zener-Hollomon parameter Z, the mechanism of this process changes from discontinuous dynamic recrystallization to continuous dynamic recrystallization; the turning point is approximately at Z=1 × 10¹⁶ s⁻¹. The increase of parameter Z leads to the decrease of recrystallized grain size of ferrite under steady state of deformation, and can lead to the formation of ultrafine microstructures with average grain size of about 2 μm.     

Studies on hot deformation of sintered cobalt

The hot deformation behavior of sintered cobalt powder was studied. The Co powder prepared by thermal decomposition of cobalt oxalate was subsequently compacted by cold isostatic pressing (CIP) and sintered at 1300 °C under H₂ atmosphere. Cobalt rods of 95 pct theoretical density were obtained. Strain rate change tests in compression were conducted in the temperature range of 900 °C to 1300 °C by changing strain rates from 0.001 to 3.2 s⁻¹. Uninterrupted hot compression tests at constant strain rates and selective temperatures were also conducted. Microcracks as well as surface cracks were observed in the samples tested below 1200 °C. It was observed that the strain rate sensitivity (SRS) increased with increasing temperature and decreasing strain rate, with the maximum SRS of 0.3 being obtained at 1285 °C and strain rate of 10⁻³ s⁻¹. Despite the higher SRS at low strain rates, the hot workability of sintered cobalt was found to be poor. Extensive grain boundary microcracking was observed, with the density being lowered after deformation. However, the samples tested at higher strain rates showed less microcracking and an increase in density. On the basis of the results, it was concluded that ease of grain boundary sliding at lower strain rates and higher temperatures was responsible for the poor workability at these conditions.     

Modelling the microstructural evolution during hot compression of low carbon steel

Compression tests have been performed on low carbon cylindrical specimens in the temperature range of 900–1100°C in a thermomechanical simulator at a strain rate of 10 s^?1. True stress/true strain and load-displacement curves have been characterised over a strain of 0 to 0.8 at above temperatures. The specimens were helium quenched after an incremental true strain of 0.2 for microstructural study. From the experimental data, flow stress of the material at high temperatures has been determined as a function of Zener-Hollomon parameter. The flow stress equation was employed in a coupled finite element flow formulation model to compute the load for various incremental displacements. The predicted results of load-displacement and change in specimen geometry during compression showed good agreement with the measured values. The predicted rise in temperature due to deformation was of the order of 52 to 34°C in the temperature range of 900 to 1100°C at a strain rate of 10 s^?1. The prior austenite grain size has been measured in the specimen compressed up to a strain of 0.6 at 1100°C and compared with the predicted austenite grain size employing the microstructural model. Metallographic study showed an equiaxed recrystallized grains network in most of the region at the center of the specimen with average grain size of 43 μm. A coarse deformed grain structure with few recrystallized grains at the intersection boundary of austenite grains was observed at the top surface and bulge surface with an average grain size of 74 and 84 μm, respectively. The model predicted the evidence of fully dynamically recrystallised grains at the center of the specimen with a grain size of 42 μm. The predicted grain size at the top and bulge surface has been calculated as 90 and 106 μm, respectively.     

Mechanical behavior of a fine-grained duplex γ-TiAl alloy

The mechanical behavior of a fine-grained duplex γ-TiAl alloy was studied in compression at strain rates ranging from 0.001 to 2000 s⁻¹ and temperatures from −196 °C to 1200 °C. The temperature dependence of the yield and flow stresses is found to depend on the strain rate. At strain rates of 0.001 and 0.1 s⁻¹, the yield stress decreases as the temperature increases, with a plateau between 600 °C and 800 °C. At strain rates of 35 and 2000 s⁻¹, the yield stress exhibits a positive temperature dependence at temperatures above 600 °C; however, postyield flow stresses exhibit a reduced temperature dependency. The work-hardening rate decreases dramatically with temperature at low and high temperatures, with a plateau occurring at intermediate temperatures for all strain rates. The workhardening-rate plateau is seen to extend to higher temperatures as the strain rate increases. The strain-rate sensitivity at strain rates of 0.1 s⁻¹ and greater is lower than 0.1, although it increases slightly with temperature. At 0.001 s⁻¹, the strain-rate sensitivity increases dramatically at high temperatures (equal to 4.5 at 1200 °C). The anomalous (positive) temperature dependence of the yield stress at high strain rates (>1 s⁻¹) and high temperatures (>600 °C) is explained via a dislocation-jog pinning mechanism. The negative temperature dependence of the yield stress at low strain rates (<1 s⁻¹) and high temperatures (>900 °C) is thought to be due to a thermally activated dislocation-jog climb process in the grain interiors and/or deformation and recovery processes at/near grain boundaries. The decreased anomalous temperature dependence of the flow stress at high strain rates and high temperatures is ascribed to dynamic recovery promoted by adiabatic heating.     

Microstructural characterization of warm-worked commercially pure aluminum

The deformation microstructures of commercially pure aluminum deformed by plane strain compression to 50 pct thickenss reduction at temperatures between 100 °C and 300 °C, under two strain rates, 5×10⁻² s⁻¹ and 5×10⁻⁴ s⁻¹, have been characterized by transmission electron microscopy. As the deformation temperature increases, the deformation microstructure gradually changes from a checkerboard pattern into an equiaxed subgrain structure with increasing subgrain size. The fraction of geometrically necessary boundaries (GNBs) found in warm-worked aluminum is much less than that found at room temperature. The average misorientation of dislocation boundaries appears to be independent of deformation temperature and strain rate. The constancy of the average misorientations is a combined effect of the variation of the fractions of GNBs and incidental dislocation boundaries (IDBs) and the variation of the average misorientations of GNBs and IDBs. Scaling theory can apply to both boundary misorientations and subgrain sizes that formed at different temperatures and strain rates. Subgrain size distributions for different temperatures and strain rates all resemble a lognormal distribution.     

Microstructural characterization of warm-worked commercially pure aluminum

The deformation microstructures of commercially pure aluminum deformed by plane strain compression to 50 pct thickness reduction at temperatures between 100 °C and 300 °C, under two strain rates, 5 × 10⁻² s⁻¹ and 5 × 10⁻⁴ s⁻¹, have been characterized by transmission electron microscopy. As the deformation temperature increases, the deformation microstructure gradually changes from a checkerboard pattern into an equiaxed subgrain structure with increasing subgrain size. The fraction of geometrically necessary boundaries (GNBs) found in warm-worked aluminum is much less than that found at room temperature. The average misorientation of dislocation boundaries appears to be independent of deformation temperature and strain rate. The constancy of the average misorientations is a combined effect of the variation of the fractions of GNBs and incidental dislocation boundaries (IDBs) and the variation of the average misorientations of GNBs and IDBs. Scaling theory can apply to both boundary misorientations and subgrain sizes that formed at different temperatures and strain rates. Subgrain size distributions for different temperatures and strain rates all resemble a lognormal distribution.     

Microstructural characterization of warm-worked commercially pure aluminum

The deformation microstructures of commercially pure aluminum deformed by plane strain compression to 50 pct thickenss reduction at temperatures between 100 °C and 300 °C, under two strain rates, 5×10⁻² s⁻¹ and 5×10⁻⁴ s⁻¹, have been characterized by transmission electron microscopy. As the deformation temperature increases, the deformation microstructure gradually changes from a checkerboard pattern into an equiaxed subgrain structure with increasing subgrain size. The fraction of geometrically necessary boundaries (GNBs) found in warm-worked aluminum is much less than that found at room temperature. The average misorientation of dislocation boundaries appears to be independent of deformation temperature and strain rate. The constancy of the average misorientations is a combined effect of the variation of the fractions of GNBs and incidental dislocation boundaries (IDBs) and the variation of the average misorientations of GNBs and IDBs. Scaling theory can apply to both boundary misorientations and subgrain sizes that formed at different temperatures and strain rates. Subgrain size distributions for different temperatures and strain rates all resemble a lognormal distribution.     

Mechanical behavior of a fine-grained duplex γ-TiAl alloy

The mechanical behavior of a fine-grained duplex γ-TiAl alloy was studied in compression at strain rates ranging from 0.001 to 2000 s⁻¹ and temperatures from −196°C to 1200°C. The temperature dependence of the yield and flow stresses is found to depend on the strain rate. At strain rates of 0.001 and 0.1 s⁻¹, the yield stress decreases as the temperature increases, with a plateau between 600°C and 800°C. At strain rates of 35 and 2000 s⁻¹, the yield stress exhibits a positive temperature dependence at temperatures above 600°C; however, postyield flow stresses exhibit a reduced temperature dependency. The work-hardening rate decreases dramatically with temperature at low and high temperatures, with a plateau occurring at intermediate temperatures for all strain rates. The work-hardening-rate plateau is seen to extend to higher temperatures as the strain rate increases. The strain-rate sensitivity at strain rates of 0.1 s⁻¹ and greater is lower than 0.1, although it increases slightly with temperature. At 0.001 s⁻¹, the strain-rate sensitivity increases dramatically at high temperatures (equal to 4.5 at 1200°C). The anomalous (positive) temperature dependence of the yield stress at high strain rates (>1 s⁻¹) and high temperatures (>600°C) is explained via a dislocation-jog pinning mechanism. The negative temperature dependence of the yield stress at low strain rates (<1 s⁻¹) and high temperatures (>900°C) is though to be due to a thermally activated dislocation-jog climb process in the grain interiors and/or deformation and recovery processes at/near grain boundaries. The decreased anomalous temperature dependence of the flow stress at high strain rates and high temperatures is ascribed to dynamic recovery promoted by adiabatic heating. Z. JIN, formerly Technical Staff Member, Materials Science and Technology Division, Los Alamos National Laboratory     

On deformation-induced continuous recrystallization in a superplastic AlLiCuMgZr alloy

The microstructural changes of a warm rolled AlLi alloy occurring during static annealing and superplastic deformation at 515°C were studied by means of transmission electron microscopy. Deformation induces a continuous recrystallization with a rapid subgrain growth and a rapid increase in boundary misorientations. The higher strain rate results in a faster subgrain growth and a finer recrystallized grain size. The increasing rate of boundary misorientations and the strain at which the average misorientation reaches about 20° increase with increasing strain rate. The increase in boundary misorientations is proportional to the subgrain growth during the whole static annealing process. Deformation results in a more rapid increase in boundary misorientation with subgrain size than static annealing. Dislocation gliding plays an important role before the formation of high angle grain boundaries during superplastic deformation. The absorption of dislocations into subgrain boundaries results in a more rapid increase in boundary misorientation during deformation. Thus, the mechanism of the deformation-induced continuous recrystallization is suggested to be the generation of dislocations in grains and the absorption of gliding dislocations into subgrain boundaries.     

Deformation characteristics of isothermally forged UDIMET 720 nickel-base superalloy

The hot deformation behavior of nickel-base superalloy UDIMET 720 in solution-treated conditions, simulating the forging process of the alloy, was studied using hot compression experiments. Specimens were deformed in the temperature range of 1000 °C to 1175 °C with strain rates of 10⁻³ to 1 s⁻¹ and total strain of 0.8. Below 1100 °C, all specimens showed flow localization as shear band through the diagonal direction, with more severity at higher strain rates. A uniform deformation was observed when testing between 1100 °C and 1150 °C with dynamic recrystallization as the major flow softening mechanism above 1125 °C. Deformation above γ′ solvus temperature was accompanied with grain boundary separation. The hot working window was determined to be in the interval 1100 °C to 1150 °C. Thermomechanical behavior of the material was modeled using the power-law, the Sellars-Tegart, and an empirical equation. The flow stress values showed a nonlinear dependence of strain rate sensitivity to strain rate. The analysis indicated that the empirical method provides a better constitutive equation for process modeling of this alloy. The apparent activation energy for deformation was calculated and its variations with strain rate and temperature are discussed.     

Plastic deformation kinetics of fine-grained alumina

The plastic deformation kinetics of a commercial fine-grained alumina with ∼300 ppm MgO and grain sizes from 1.4 to 2.9 μm were determined in tension at 1475 °C to 1600 °C and strain rates from 10⁻⁵ to 10⁻³ s⁻¹, employing stress relaxation (SR) as the principal test mode. The constants in the Weertman-Dorn (W-D) equation were determined and had the following values: A=2.9±0.5×10⁹, n=2.2±0.1, p=1.9±0.1, Q=492±3 kJ/mole, and threshold stress σ ₀=0. These constants are in accord with grain boundary sliding (GBS) accommodated by dislocation glide and climb with Al³⁺ ion lattice diffusion as the rate-controlling mechanism.     

Thermomechanical response of a powder metallurgy Ti-6Al-4V alloy modified with 2.9 pct boron

The thermomechanical response of Ti-6Al-4V modified with 2.9 pct B produced by a blended powder metallurgy route was studied with isothermal constant strain-rate hot compression tests in the temperature range 850 °C to 1200 °C and strain rate range 10⁻³ to 10 s⁻¹. Detailed analyses of the flow stress data were conducted to identify various microstructural deformation and damage mechanisms during hot working by applying available materials modeling techniques. In the α + β phase field, cavitation at the matrix/TiB interfaces and TiB particle fracture occurs at low strain rates (<10⁻¹ s⁻¹), while adiabatic shear banding also occurs at high strain rates. At low strain rates, the β phase deforms superplastically due to the stabilization of a fine grain size by the TiB particles. Grain boundary and matrix/TiB interface sliding with simultaneous diffusional accommodation are observed to contribute to the β superplasticity. The deformation behavior at high strain rates in the β-phase field is similar to that of the α + β phase field, with microstructural manifestations of extensive cavitation at the matrix/TiB interfaces and TiB particle fracture.