Superplastic behavior and microstructure evolution in a commercial Al-Mg-Sc alloy subjected to intense plastic straining

A commercial Al-6 pct Mg-0.3 pct Sc-0.3 pct Mn alloy subjected to equal-channel angular extrusion (ECAE) at 325 °C to a total strain of about 16 resulted in an average grain size of about 1 μm. Superplastic properties and microstructural evolution of the alloy were studied in tension at strain rates ranging from 1.4 × 10⁻⁵ to 1.4 s⁻¹ in the temperature interval 250 °C to 500 °C. It was shown that this alloy exhibited superior superplastic properties in the wide temperature range 250 °C to 500 °C at strain rates higher than 10⁻² s⁻¹. The highest elongation to failure of 2000 pct was attained at a temperature of 450 °C and an initial strain rate of 5.6 × 10⁻² s⁻¹ with the corresponding strain rate sensitivity coefficient of 0.46. An increase in temperature from 250 °C to 500 °C resulted in a shift of the optimal strain rate for superplasticity, at which highest ductility appeared, to higher strain rates. Superior superplastic properties of the commercial Al-Mg-Sc alloy are attributed to high stability of ultrafine grain structure under static annealing and superplastic deformation at T ≤ 450 °C. Two different fracture mechanisms were revealed. At temperatures higher than 300 °C or strain rates less than 10⁻¹ s⁻¹, failure took place in a brittle manner almost without necking, and cavitation played a major role in the failure. In contrast, at low temperatures or high strain rates, fracture occurred in a ductile manner by localized necking. The results suggest that the development of ultrafine-grained structure in the commercial Al-Mg-Sc alloy enables superplastic deformation at high strain rates and low temperatures, making the process of superplastic forming commercially attractive for the fabrication of high-volume components.     

Low-temperature superplastic behavior of a submicrometer-grained 5083 Al alloy fabricated by severe plastic deformation

A submicrometer-grained structure was introduced in a commercial 5083 Al alloy by imposing an effective strain of ∼8 through equal channel angular pressing. In order to examine the low-temperature superplastic behavior, the as-equal channel angular pressed (as-ECAP) samples were tensile tested in the strain rate range of 10⁻⁵ to 10⁻² s⁻¹ at temperatures of 498 to 548 K corresponding to 0.58 to 0.65 T _m, where T _m is the incipient melting point. The mechanical data of the alloy at 498 and 548 K exhibited a sigmoidal behavior in a double logarithmic plot of the maximum true stress vs true strain rate. The strain rate sensitivity was 0.1 to 0.2 in the low- and high-strain rate regions and 0.4 in the intermediate-strain rate region, indicating the potential for superplasticity. At 523 K, instead of the sigmoidal behavior, a strain rate sensitivity of 0.4 was maintained to low strain rates. A maximum elongation of 315 pct was obtained at 548 K and 5×10⁻⁴ s⁻¹. The activation energy for deformation in the intermediate-strain rate region was estimated as 63 kJ/mol. Low-temperature superplasticity of the ultrafine grained 5083 Al alloy was attributed to grain boundary sliding that is rate-controlled by grain boundary diffusion, with a low activation energy associated with nonequilibrium grain boundaries. Cavity stringers parallel to the tensile axis were developed during deformation, and the failure occurred in a quasi-brittle manner with moderately diffusive necking.     

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.     

Thermal gradients,strain rate,and ductility in sheet steel tensile specimens

The effect on ductility of strain rate and thermal gradients arising from deformation is examined in tensile specimens of 1008 AK steel. The total elongatione _tot is taken as the measure of ductility, since it reflects changes in the strain hardeningn and strain-rate sensitivitym. Tensile specimens are pulled to failure in 23 °C air, at initial strain rates from 10⁻³ to 10⁻¹ s⁻¹, with thermocouples recording temperature along the 50.8 mm gauge section. The maximum temperature is ∼110 °C just prior to failure at the highest rate. Thee _tot, however, remains fairly constant with rate at ∼40 pct. When thermal gradients are prevented by immersing the specimens in circulating water at 23 °C,e _tot, increases with rate to a maximum of ∼54 pct at 10⁻¹ s⁻¹. Direct measurements of isothermal values ofm at 23, 60, and 90 °C show thatm increases with rate.e _tot, therefore, would be expected to increase with rate. Since under nonisothermal conditionse _tot does not change, it appears thatm and thermal gradients are competing influences on ductility at higher rates. Enhanced ductility in stampings should be possible by suppressing gradients, either by controlling die temperature or by heat transfer properties of a lubricant.     

Hot deformation mechanisms in a powder metallurgy nickel-base superalloy IN 625

The hot working behavior of the nickel-base superalloy IN 625 produced by hot extrusion of a powder metallurgy (P/M) compact has been studied by compression testing in the temperature range 900 °C to 1200 °C and true strain rate range 0.001 to 100 s⁻¹. At strain rates less than about 0.1 s⁻¹, the stress-strain curves exhibited near steady-state behavior, while at higher strain rates, the flow stress reached a peak before flow softening occurred. The processing maps developed on the basis of the temperature and strain rate and strain dependence of the flow stress exhibited three domains. (1) The first domain occurs at lower strain rates (<0.01 s⁻¹) and temperatures higher than about 1050 °C. The peak efficiency and the temperature at which it occurs have increased with strain. The microstructure of the specimen deformed in this domain exhibited extensive wedge cracking. (2) The second domain occurs in the intermediate range of strain rates (0.01 to 0.1 s⁻¹) and temperatures lower than 1050 °C, and in this domain, microstructural observations indicated dynamic recrystallization (DRX) of γ containing δ precipitates and carbide particles resulting in a fine-grained structure. (3) The third domain occurs at higher strain rates (> 10 s⁻¹) and tempe ratures above 1050 °C, with a peak efficiency of about 42 pct occurring at 1150 °C and 100 s⁻¹. Microstructural observations in this domain revealed features such as irregular grain boundaries and grain interiors nearly free from annealing twins, which are typical of DRX of homogeneous γ phase. The instability map revealed that flow instability occurs at strain rates above 1 s⁻¹ and temperatures below 1050 °C, and this is manifested as intense adiabatic shear bands. These results suggest that bulk metal working of this material may be carried out in the high strain rate domain where DRX of homogeneous γ occurs. On the other hand, for achieving a fine-grained product, finishing operations may be done in the intermediate strain rate domain. The wedge cracking domain and the regime of instability must be totally avoided for achieving defectfree products.     

High-temperature deformation behavior of coarse- and fine-grained MoSi<Subscript>2</Subscript> with different silica contents

The elevated-temperature deformation behavior of polycrystalline molybdenum disilicide (MoSi₂), in the range of 1000 °C to 1350 °C at the strain rates of 10⁻³, 5×10⁻⁴, or 10⁻⁴ s⁻¹, has been studied. The yield strength, post-yield flow behavior comprising strain hardening and serrations, as well as some of the deformation microstructures of reaction-hot-pressed (RHP) MoSi₂ samples, processed by hot pressing an elemental Mo + Si powder mixture and having a grain size of 5 μm and oxygen content of 0.06 wt pct, have been compared with those of samples prepared by hot pressing of commercial-grade Starck MoSi₂ powder, with a grain size of 27 μm and oxygen content of 0.89 wt pct. While the fine-grained RHP MoSi₂ samples have shown higher yield strength at relatively lower temperatures and higher strain rates, the coarse-grained Starck MoSi₂ has a higher yield at decreasing strain rates and higher temperatures. The work-hardening or softening characteristics are dependent on grain size, temperature, and strain rate. Enhanced dislocation activity and dynamic recovery, accomplished by arrangement of dislocations in low-angle boundaries, characterize the deformation behavior of fine-grained RHP MoSi₂ at a temperature of 1200 °C and above and are responsible for increased uniform plastic strain with increasing temperature. The silica content appears to be less effective in degrading the high-temperature yield strength if the grain size is coarse, but leads to plastic-flow localization and strain softening in Starck MoSi₂. Serrated plastic flow has also been observed in a large number of samples, mostly when deformed at specific combinations of strain rates and temperatures.     

Quasi-Static and High-Strain-Rate Experimental Microstructural Investigation of a High-Strength Aluminum Alloy

The damage mechanisms associated with the deformation and failure of high-strength Al 2139-T8 were investigated by optical and scanning electron microscopy at strain rates ranging from the quasi-static, 10⁻³ s⁻¹, to the high strain rate, at approximately 2350 s⁻¹. Deformation was more uniform at the lowest strain rate of 10⁻³ s⁻¹, where nanocracking nucleated at coarse inclusions and clustered dispersed particles. Deformation was more localized as the strain-rate increased, with microvoid nucleation, resulting from particle-matrix interfacial decohesion and particle cracking, observed at higher strain rates. Dispersed particles and coarse inclusions were observed on the fracture surfaces of the deformed samples subjected to high rates of strain. Coarse inclusions were associated with inclusion cracking and shear deformation. The dispersed particles were associated with dimpled rupture, and the orientation of the particles determined the amount of plasticity prior to failure of the alloy.     

A study of superplasticity in a modified 5083 Al-Mg-Mn alloy

The superplastic (SP) properties of a modified 5083 alloy (Al-4.7Mg-1.6Mn) were evaluated by tensile tests and microstructural characterization over a range of strain rates from 0.0005 to 0.1 s⁻¹, temperatures from 500 °C to 550 °C, and initial grain sizes from 8.7 to 17 μm. The fine-grained material was found to exhibit strain-rate sensitivity values of greater than 0.5 over the strain-rate range of 0.002 to 0.1 s⁻¹, while the coarser-grained material appeared to deform as a Class I solid solution by glide-controlled dislocation creep. It was found that the mechanical properties could be adequately represented by a semiempirical constitutive equation which reflected the flow hardening due to dynamic grain growth, the change in m with strain and strain rate, and the transition between SP deformation and dislocation creep with strain rate. Microstructural examination revealed the presence of several pre-existing cavities associated with intermetallic particles. Tensile elongations of up to 525 pct were obtained at a strain rate of 10⁻³s⁻¹.     

Hot Deformation Characteristics of Functionally Graded Steels Produced by Electroslag Remelting

In this work, a hot compression test was carried out at 1173 K to 1473 K (900°C to 1200 °C), with a strain rate of 0.01 to 1 s⁻¹ up to ~50 pct height reduction on functionally graded steel (FGS) specimens comprised of ferritic, bainitic, austenitic, and martensitic layers (αβγMγ). The stress-strain curves are strongly dependent on temperature and strain rate. Compressive flow stress varied from 40 to 105 MPa depending on the applied temperature and strain rates. Variation in steady-state flow stress with temperature and strain rates was studied. The strain-rate-sensitivity exponent (m) and deformation activation energy (Q) for the αβγMγ composite under studied condition were 0.106 and 354.8 KJ mol⁻¹, respectively, which are within the values of boundary layers of ferrite (304.9 KJ mol⁻¹) and austenite (454.8 KJ mol⁻¹) layers. Given the alternative microstructure of the αβγMγ FGS, a range of deformation mechanisms from dynamic recovery to dynamic recrystallization maybe prevails, where the intensity of each mechanism depends on temperature and strain rates. In accordance with the experimental results, an empirical power-law equation was developed over the range of temperatures and strain rates investigated. The equation accurately describes temperature and strain-rate dependence of the flow stress.     

High strain rate superplasticity of a 25 Wt Pct Cr-7 Wt Pct Ni-3 Wt Pct Mo-0.14 Wt Pct N duplex stainless steel

Effects of prior thermomechanical treatments on the superplasticity of a 25 wt pct Cr-7 wt pct Ni-3 wt pct Mo-0.14 wt pct N δ/γ duplex stainless steel have been studied by means of hot tensile testing with constant crosshead speeds. The objective is to increase the strain rate suitable for superplasticity. The strain rate is found to be markedly increased by a special prior treatment,i.e., solution treatment at temperatures in the δ single-phase region with subsequent heavy cold-rolling. In hot tensile tests at 1273 K, elongations greater than 1000 and 300 pct were observed at initial strain rates (έ) of 10⁻³ to 10⁻¹ s⁻¹ and 1 x 10⁰ s⁻¹, respectively. The results for strain rates 〈10⁻¹ s⁻¹ can be explained in terms of a structural superplastic effect due to grain refinement. In the case of έ 〉 10⁻¹ s⁻¹, transformation superplastic effects due to γ-phase precipitation from the σ-ferrite matrix are also important, especially in the early stages of deformation. In the equiaxedδ/γ microduplex structures during stable superplastic deformation, there exists a mixture of two different structures,i.e., dislocated and recovered/ recrystallized δ grains with a homogeneous dispersion of dislocation-free γ particles. This result shows that dynamic recrystallization ofδ grains occurs locally and intermittently due to the dispersion of relatively hardγ particles. The apparent average grain growth rate during deformation is small compared to static grain growth, because grain refinement due to dynamic recrystallization reduces the superplasticity-enhanced grain growth.     

Structural superplasticity in a fine-grained eutectic intermetallic NiAl−Cr alloy

A rapidly solidified and thermomechanically processed fine-grained eutectic NiAl−Cr alloy of the composition Ni₃₃Al₃₃Cr₃₄ (at, pct) exhibits structural superplasticity in the temperature regime from 900°C to 1000°C at strain rates ranging from 10⁻⁵ to 10⁻³ s⁻¹. The material consists of a B2-ordered intermetallic NiAl(Cr) solid solution matrix containing a fine dispersion of bcc chromium. A high strain-rate-sensitivity exponent of m=0.55 was achieved in strain-rate-change tests at strain rates of about 10⁻⁴ s⁻¹. Maximum uniform elongations up to 350 pct engineering strain were recorded in superplastic strain to failure tests. Activation energy analysis of superplastic flow was performed in order to establish the diffusion-controlled dislocation accommodation process of grain boundary sliding. An activation energy of Q _c=288±15 kJ/mole was determined. This value is comparable with the activation energy of 290 kJ/mole for lattice diffusion of nickel and for ⁶³Ni tracer selfdiffusion in B2-ordered NiAl. The principal deformation mechanism of superplastic flow in this material is grain-boundary sliding accommodated by dislocation climb controlled by lattice diffusion, which is typical for class II solid-solution alloys. Failure in superplastically strained tensile samples of the fine-grained eutectic alloy occurred by cavitation formations along NiAl‖‖Cr interfaces.     

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.     

Effect of strain rate on the flow stress of three liquid phase sintered tungsten alloys

Stress/strain tests were carried out in compression on three liquid phase sintered tungsten alloys, with tungsten contents of 90, 95, and 97.4 wt pct, in the strain rate range 10⁻³ s⁻¹ to 10³ s⁻¹. Each alloy shows a gradual increase of flow stress with strain rate, and evidence of work softening is observed when the strain rate is of the order of 2 s⁻¹ or greater. The work softening effect is shown to result from a temperature rise due to the plastic deformation and partly masks the strain rate effect at strains greater than 0.1. The 97.4 pct tungsten alloy also shows variable behavior due to cracking associated with the presence of a brittle phase at the tungsten particle/matrix interface.     

Dynamic-coarsening behavior of an α/β titanium alloy

The dynamic-coarsening behavior of Ti-6Al-4V with an equiaxed α microstructure was established via isothermal hot-compression testing of cylindrical samples cut from an ultra-fine-grain-size (UFG) billet. Compression experiments were conducted at 900 and 955 °C, strain rates between 10⁻⁴ and 1 s⁻¹, and imposed true strains between 0 and 1.4. Following deformation, quantitative metallography revealed marked coarsening of the primary α particles at low strain rates (10⁻⁴ and 10⁻³ s⁻¹). The dynamic-coarsening rate followed rⁿ vs time kinetics, in which n was between 2 and 3, or behavior between those of bulk-diffusion and interface-reaction controlled. An examination of the temperature and strain-rate dependence of theoretical coarsening rates, however, strongly suggested that bulk diffusion (with n=3) was more important. The dynamic-coarsening behavior was also interpreted in the context of the observed plastic-flow behavior. At low strain rates, high values of the strain-rate sensitivity (m>0.5) and the overall shape of log stress-log strain rate plots indicated that the majority of the imposed strain was accommodated by grain-boundary sliding (gbs) and only a small amount via dislocation glide/climb processes. In addition, an analysis of the flow hardening that accompanied dynamic coarsening indicated that the flow stress varied approximately linearly with the α particle size, thus providing support for models based on gbs accommodation by dislocation activity in grain-mantle regions.     

High temperature deformation of a commercial aluminum alloy

A study of high temperature deformation of a commercial aluminum alloy has been undertaken through tensile tests at strain rates ranging from 5.6×10⁻⁵ s⁻¹ to 5.6×10⁻² s⁻¹ and load relaxation testing in the temperature range 473 to 873 K. Experiments have established that maximum ductility is reached at about 623 K and at maximum strain rates. Maximum fracture ductility corresponds to minimum uniform elongation. The deformation and fracture mechanisms operating in the temperature range 473 to 573 K seem to differ from those between 623 K and 823 K; different strain rate sensitivities are also observed. Dynamic recovery is the dominant softening mechanism in high temperature plastic deformation—that is, a thermally activated process whose kinetics can be suitably described by an empirical power relation.     

Modeling the Flow Curve Characteristics of 410 Martensitic Stainless Steel Under Hot Working Condition

The hot deformation behavior of AISI 410 martensitic stainless steel was investigated by conducting hot compression tests between 1173 K (900 °C) and 1423 K (1150 °C) and between strain rates of 0.001 s⁻¹ to 1 s⁻¹. The hyperbolic sine function described the relation well between flow stress at a given strain and the Zener–Hollomon parameter (Z). The variation of flow stress with deformation temperature gave the average value of apparent activation energy as 448 kJ/mol. The strain and stress corresponding to two important points associated with flow curve (i.e., peak strain and the onset of steady-state flow) were related to the Z parameter using power-law equations. A model also was proposed based on the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation to estimate the fractional softening of dynamic recrystallization at any given strain. This model can be used readily for the prediction of flow stress. The values of n and k, material constants in the JMAK equation, were determined for the studied material. The strains regarding the peak and the onset of steady-state flow were formulated in term of applied strain rate and the constants of the JMAK equation. A good agreement was found between the predicted strains and those obtained by the experimental work.     

Characterization and modeling of the high temperature flow behavior of aluminum alloy 2024

Isothermal flow curves were determined for aluminum alloy 2024-0 at temperatures of 145 to 482 °C and at constant true-strain rates of 10^-3 to 12.5 s^-1 using compression tests of cylindrical specimens. The average pressure was corrected for friction and for deformation heating to determine the flow stress. At 250 °C and above, the isothermal flow curves usually exhibited a peak followed by flow softening. At 145 °C the flow curves exhibited strain hardening. For 250 °C≦ T<= 482 °C, 10^-3 s^-1 ≦ ≦ 12.5 s^-1, and ε ≦ 0.6 the flow behavior was represented by the constitutive equation σ =K (T, ε) where logK andm are simple functions of temperature and strain. The as-deformed microstructures generally supported the idea that flow softening in Al 2024-0 is caused by dynamic recovery. At the higher temperatures and strain rates, however, fine recrystallized grains were observed in local areas near second phase particles and at as-annealed grain boundaries. At 482 °C, there was evidence of re-dissolution of the CuMgAl₂ precipitate. Formerly Visiting Associate Professor, Wright State University, Dayton, OH 45435 Formerly a Mechanical Systems Engineering Student at Wright State University Formerly a Materials Engineering Student at Wright State University Formerly Director, Metallurgy Program, National Science Foundation, Washington, DC     

Effect of friction stir processing on the kinetics of superplastic deformation in an Al-Mg-Zr alloy

The effect of friction stir processing on the superplastic behavior of extruded Al-4Mg-1Zr was examined at 350 °C to 600 °C and at initial strain rates of 1×10⁻³ to 1 s⁻¹. A combination of a fine grain size of 1.5 μm and high-angle grain boundaries in the friction stir-processed (FSP) alloy led to considerably enhanced superplastic ductility, much-reduced flow stress, and a shift to a higher optimum strain rate and lower optimum temperature. The as-extruded alloy exhibited the highest superplastic ductility of 1015 pct at 580 °C and an initial strain rate of 1×10⁻²s⁻¹, whereas a maximum elongation of 1280 pct was obtained at 525 °C and an initial strain rate of 1×10⁻¹s⁻¹ for the FSP alloy. The FSP alloy exhibited enhanced superplastic deformation kinetics compared to that predicted by the constitutive relationship for superplasticity in fine-grained aluminum alloys. A possible origin for enhanced superplastic deformation kinetics in the FSP condition is proposed.     

Atomic-Scale Study of Plastic-Yield Criterion in Nanocrystalline Cu at High Strain Rates

Large-scale molecular dynamics (MD) simulations are used to understand the macroscopic yield behavior of nanocrystalline Cu with an average grain size of 6 nm at high strain rates. The MD simulations at strain rates varying from 10⁹ s⁻¹ to 8 × 10⁹ s⁻¹ suggest an asymmetry in the flow stress values in tension and compression, with the nanocrystalline metal being stronger in compression than in tension. The tension-compression strength asymmetry is very small at 10⁹ s⁻¹, but increases with increasing strain rate. The calculated yield stresses and flow stresses under combined biaxial loading conditions (X-Y) gives a locus of points that can be described with a traditional ellipse. An asymmetry parameter is introduced that allows for the incorporation of the small tension-compression asymmetry. The biaxial yield surface (X-Y) is calculated for different values of stress in the Z direction, the superposition of which gives a full three-dimensional (3-D) yield surface. The 3-D yield surface shows a cylinder that is symmetric around the hydrostatic axis. These results suggest that a von Mises-type yield criterion can be used to understand the macroscopic deformation behavior of nanocrystalline Cu with a grain size in the inverse Hall–Petch regime at high strain rates.     

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