Plastic flow phenomenology of 304L stainless steel

The isothermal plastic flow behavior of annealed 304L austenitic stainless steel in uniaxial compression and torsional modes of deformation has been established over a wide range of temperatures and strain rates. In uniaxial compression, it was found that high rates of strain hardening, which persist to large strains (?.7) at cold-working temperatures, are found only at small values of strain (?0.2) at hot-working temperatures because of the influence of dynamic softening processes. The effect of deformation heating on flow behavior, which occurs primarily at high strain rates, was most significant at cold-working temperatures. Deformation heating was observed to result in flow stress maxima and flow softening. A method of estimating high-strain rate, isothermal-flow curves in such instances was derived. Shear stress-shear strain curves derived from torsion tests exhibited dependences on temperature and strain rate similar to those observed in compression data. In contrast to the compression curves, however, the shear stress-shear strain curves showed lower rates of strain-hardening at room temperature, 400 °C, 800 °C, and (for high strain rates) 1000 °C. It was shown that the choice of definition for calculating effective stress-strain from the torsion data could not be modified to bring the two types of data into coincidence. Only a structure-sensitive explanation could be invoked to explain the difference.     

Temperature and strain rate dependence of cyclic deformation response and damage accumulation in ofhc copper and 304 stainless steel

The effects of temperature and strain rate on deformation behavior and dislocation structure were investigated for OFHC copper and type 304 stainless steel. It is shown that the cyclic stress response is inversely related to the cell size for copper cycled at different temperatures ranging from -75 to 650°C. Type 304 stainless steel underwent a change from a planar to a wavy slip character as the temperature was changed from room temperature to 760°C. At elevated temperatures, cells were observed and the size of the cells tended to increase with increase in temperature. The effects of temperature on the cyclic stress-strain parameters were investigated for copper, type 304 stainless steel and Ferrovac “E” iron. On studying the effects of temperature and strain rate on the fracture mechanisms it was found that a time dependent fracture mode was dominant at high temperature levels and low strain rates. However, at high strain rates the life was insensitive to temperature. The role of grain boundary migration on the fracture process was investigated. Grain boundary migration was found to be dependent on strain rate for copper. However, for type 304 stainless steel, the grain boundary migration was inhibited at high temperature (760°C) due to the presence of precipitates at the grain boundaries. In strain cycling of OFHC copper and type 304 stainless steel, it was found that the addition of creep-type damage to fatigue damage resulted in a total damage which was not equal to unity for failure when these different modes were imposed sequentially. The sense of the damage accumulation appeared to have no effect on this summation.     

Optimization of cold and warm workability in 304 stainless steel using instability maps

The deformation characteristics of stainless steel type AISI 304 under compression in the temperature range 20 °C to 600 °C and strain-rate range 0.001 to 100 s^-1 have been studied with a view to characterizing the flow instabilities occurring in the microstructure. At strain rates less than 5 s^-1, 304 stainless steel exhibits flow localization, whereas dynamic strain aging occurs at intermediate temperatures and below 0.5 s^-1. At room temperatures and strain rates less than 10 s^-1, martensite formation is observed. To avoid the preceding microstructural instabilities, cold and warm working should be carried out at strain rates greater than 5 s^-1. The continuum criterion, developed on the basis of the principles of maximum rate of entropy production and separability of the dissipation function, predicts accurately all the preceding instability features. S. VENUGOPAL, Scientific Officer, on leave from the Materials Development Division, Indira Gandhi Centre for Atomic Research     

Workability of commercial-purity titanium and 4340 steel during equal channel angular extrusion at cold-working temperatures

The deformation and failure of commercial-purity (CP) titanium (grade 2) and AISI 4340 steel (tempered to R _c 35) during equal channel angular extrusion were determined at temperatures between 25 °C and 325 °C and effective strain rates between 0.002 and 2.0 s⁻¹. The CP titanium alloy underwent segmented failure under all conditions except at low strain rates and high temperatures. By contrast, the 4340 steel deformed uniformly except at the highest temperature and strain rate, at which it also exhibited segmented failure. Using flow curves and fracture data from uniaxial compression and tension tests, workability analysis was conducted to establish that the failures were a result of flow localization prior to the onset of fracture. This conclusion was confirmed by metallographic examination of the failed extrusion specimens.     

Effect of deformation heating and strain rate sensitivity on flow localization during the torsion testing of 6061 aluminum

The effect of deformation heating and strain rate sensitivity on flow localization during the torsion testing of 6061 aluminum was investigated both theoretically and experimentally. From the theoretical viewpoint, a simple analysis of the torsion test was carried out based on the torque equilibrium and one-dimensional heat transfer equations. The problem formulation was discretized to enable numerical solution of the governing equations and prediction of the effect of material properties on the development of deformation and temperature gradients. The analysis was validated by conducting high strain rate experiments on the 6061 alloy at a variety of temperatures. At low temperatures, at which the flow stress and temperature changes due to deformation heating are large and the strain rate sensitivity is low, marked flow localization occurred. The analysis modeled this behavior correctly, indicating that strain concentrations can occur solely as a result of the temperature gradients set up by heat transfer during testing, i.e. in the absence of geometric or deformation defects. At the higher temperatures, at which temperature changes due to deformation heating are small and the rate sensitivity is large, the flow remained nominally uniform until fracture intervened. The numerical simulations of these tests also showed good agreement with the observations.     

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.     

Optimization of hot workability in stainless

The hot working behavior of 304L stainless steel is characterized using processing maps developed on the basis of the Dynamic Materials Model and hot compression data in the tem- perature range of 700 °C to 1200 °C and strain-rate range of 0.001 to 100 s♪-1. The material exhibits a dynamic recrystallization (DRX) domain in the temperature range of 1000 °C to 1200 °C and strain-rate range of 0.01 to 5 s_-1. Optimum hot workability occurs at 1150 °C and 0.1 s_-1, which corresponds to a peak efficiency of 33 pct in the DRX domain. Finer grain sizes are obtained at the lower end of the DRX domain (1000 °C and 0.1 s^-1). The material exhibits a dynamic recovery domain in the temperature range of 750 °C to 950 °C and at 0.001 s"1. Flow instabilities occur in the entire region above the dynamic recovery and recrystallization domains. Flow localization occurs in the regions of instability at temperatures lower than 1000 °C, and ferrite formation is responsible for the instability at higher temperatures.     

Cyclic creep of type 304 stainless steel during unbalanced tension-compression loading at elevated temperature

Samples of Type 304 stainless steel were subjected to cyclic stresses with a positive mean stress at 300 and 560°C. Very rapid net elongation was observed whenever the stress limits were such as to produce a plastic strain amplitude of the same order of magnitude as the elastic strain at the peak stress. The maximum mean strain-rate, or cyclic creep rate, for a given peak tensile stress was achieved when the mean stress was just slightly above zero. Increasing the mean stress caused the mean strain rate to de-crease. The sensitive dependence of the mean strain-rate on the plastic strain ampli-tude and inverse dependence on the mean stress indicates that remobilization of disloca-tions by the reverse strain is an important mechanism for cyclic-creep acceleration. Although rapid cyclic creep was observed at both temperatures, a measurable mean strain rate was found for a much narrower range of stress conditions at 560 than at 300°C. The strain accumulated during cyclic creep did not produce any strain hardening, but did influence the shape of the stress-strain curve in a subsequent tensile test.     

The Recrystallization Behavior of an Austenitic Stainless Steel Ingot Structure Due to Hot Deformation

The kinetics of recrystallization were determined metallographically for an ingot casting of AISI type 304 stainless steel deformed over a range of strains at temperatures of 1600 to 2250°F (860 to 1232°C) at several strain rates, and annealed at temperatures of 1900 to 2250°F (1037 to 1232°C). As with recrystallization following room temperature cold work, the time for recrystallization was reduced for increasing deformation and annealing temperature and for increasing strains. Decreasing the deformation temperature resulted in a reduction of time for recrystallization at a given strain and annealing temperature. Increasing strain rate resulted in a reduction of recrystallization time for a given deformation and annealing temperature. The dependence of recrystallization time upon strain rate and deformation temperature is related to the change in deformation stress encountered for the various deformation conditions.     

Tensile failure of austenitic iron at intermediate strain rates

The basic failure behavior of austenitic iron has been established for the temperature range 950 to 1350°C and the strain-rate range 2.8 x 10^-5 to 2.3 x 1(10^-2 s^-1. Failure in zone-refined iron is determined solely by plastic deformation, leading first to multiple necking, continuing by the exclusive growth of a single neck, and concluding by separation at a point within that neck. With the increasing impurity content of electrolytic iron, Fe-0.05 C and Fe-5.2 Mn, this failure process is interrupted at the lower temperatures by fracture at either second-phase particles or grain boundaries. The regimes of these two fracture modes have been determined as functions of strain rate, deformation temperature, and annealing temperature. Recrystallization is prevalent during the plastic deformation of austenitic iron and influences the necking process to some extent. Recrystallization is more influential as a means of stabilizing arrays of intergranular cracks, thereby allowing the cracks to undergo appreciable plastic deformation during the final stage of failure. The concept of failure diagrams is introduced as a simple means of representing the complex interposition of plastic instability, recrystallization, and fracture during the failure process.     

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     

Mechanical properties of new stainless steels based on the system Fe‐30Mn‐5A1‐XCr‐0.5C

New stainless steels based on the system Fe‐30Mn‐5AI‐XCr‐0.5C (Cr mass contents of ≤ 9 %) were developed and evaluated as a replacement of conventional AISI 304 steel. The alloys were produced by induction melting and thermomechanically processed to obtain a fine equiaxed microstructure. A typical thermomechanical processing for AISI 300 austenitic stainless steels was used and included forging at 1200°C, rolling at 850 °C and final recrystallization at 1050 °C. A final fully austenitic microstructure with grains of about 150 μm in size was obtained in all the steels. Tensile tests at temperatures ranging from ‐196 to 400 °C showed similar results for the various alloys tested. In accordance with the values for the elongation to fracture, this temperature range was subdivided into three regions. In the temperature range of ‐196 °C to room temperature, elongation to fracture increases with decreasing temperature. At temperatures ranging from 100 to 300 °C, elongation to fracture increases with testing temperature and serrations on the stress‐strain curve were observed. Finally, higher testing temperatures were accompanied by a decrease in ductility. Examination of the microstructures after deformation led to the conclusion that mechanical twinning was the dominant mechanism of deformation at the tested temperatures.     

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.     

Plastic anisotropy in a superplastic Al-Li-Mg-Cu alloy

The plastic anisotropy of AA8090 Al-Li-Cu-Mg alloy sheet has been evaluated by tensile testing and by deep drawing at temperatures in the range 200 °C to 525 °C. At temperatures of about 500 °C and strain rates of about 10^-3 s^-1, this material exhibits a high strain-rate sensitivity of flow stress which reduces any tendency to strain localization in stretching and allows so-called superplastic forming of the sheet. Most models of the material behavior in this regime require highly inhomogeneous deformation on the scale of the material’s grain size. The plastic anisotropy measured in the superplastic regime was similar in form, though of reduced magnitude, to that measured under conditions associated with a much smaller strain-rate sensitivity. Homogeneous slip models predict the correct form of anisotropy, and inclusion of slip-rate senitivity can reduce the magnitude of anisotropy predicted but not sufficiently to give good correlation with the experimental results unless very high values are used. The development of the preferred crystallographic orientation in deep drawing has also been examined. The predictions of homogeneous slip models correlate quite well with experimental results at low temperatures, but the situation is more complex in the superplastic regime where, although there is some evidence of texture changes as predicted, there is a general reduction in the intensity of preferred orientation with deformation. However, the results indicate that a greater contribution of homoeneous slip deformation is involved in superplastic deformation than is assumed in some models of superplasticity.     

Deformation behavior of irradiated Zr-2.5Nb pressure tube material

A study of the deformation behavior of irradiated highly textured Zr-2.5Nb pressure tube material in the temperature range of 30 °C to 300 °C was undertaken to understand better the mechanism for the deterioration of the fracture toughness with neutron irradiation. Strain localization behavior, believed to be a main contributor to reduced toughness, was observed in irradiated transverse tensile specimens at temperatures greater than 100 °C. The strain localization behavior was found to occur by the cooperative twinning of the highly textured grains of the material, resulting in a local softening of the material, where the flow then localizes. It is believed that the effect of the irradiation is to favor twinning at the expense of slip in the early stages of deformation. This effect becomes more pronounced at higher temperature, thus leading to the high-temperature strain localization behavior of the material. A limited amount of dislocation channeling was also observed; however, it is not considered to have a major role in the strain localization behavior of the material. Contrary to previous reports on irradiated zirconium alloys, static strain aging is observed in the irradiated material in the temperature range of 150 °C to 300 °C.     

Hot working of Ti-6Al-4V via equal channel angular extrusion

The deformation behavior of Ti-6Al-4V during high-temperature equal channel angular extrusion (ECAE) with or without an initial increment of upset deformation was determined for billets with either a lamellar or an equiaxed alpha preform microstructure. For conventional ECAE (i.e., deformation by simple shear alone), flow localization and fracture occurred at temperatures between 900 °C and 985 °C. In contrast, billets deformed at temperatures between 845 °C and 985 °C using an initial increment of upset deformation immediately followed by the simple shear deformation of ECAE exhibited uniform flow with no significant cracking or fracture. A simple flow-localization criterion was used to explain the influence of preupsetting on the suppression of localization in billets with the lamellar microstructure. The suppression of flow localization for the equiaxed microstructure and the elimination of edge cracking for both types of microstructures were explained in terms of heat transfer (die chill) and workpiece geometry. Further evidence of the relative importance of microstructural and thermal effects was extracted from the results of two-pass extrusions, the first with upsetting and the second without upsetting.     

Flow stress and ductility of duplex stainless steel during high-temperature torsion deformation

Hot torsion tests were performed on a duplex stainless steel (DSS)-type EN1.4462 steel. The temperature was varied in the range from 950 °C to 1200 °C, while the strain rate was varied from 0.01/s to 2/s. The mean flow stress (MFS) was fitted to the hyperbolic sine function proposed by Sellars and Tegart. An activation energy for plastic deformation of Q _HW = 425 kJ/mol was obtained. The high value was explained by the fact that, in addition to the softening of ferrite (α) and austenite (γ), there was a decrease in the volume fraction of the high-strength austenite with increasing temperature. For higher values of the Zener-Holomon parameter (Z), the MFS showed a linear dependence on ln (Z), which was related to the gradual disappearance of a yield-point-elongation-like effect. This yield-point-elongation-like effect was characterized by a nonstrengthening plateau during the initial stages of plastic deformation. The strain to rupture and the dynamic softening were both found to decrease for higher values of ln (Z). Therefore, the ductility was directly related to the amount of dynamic softening, i.e., dynamic recrystallization (DRX) of the austenitic phase. At higher strain rates, significant dynamic softening was only observed for temperatures above 1100 °C. The strain-rate sensitivity (m) was found to vary from 0.13 at 950 °C to 0.22 at 1200 °C.     

Temperature-dependent void-sheet fracture in Al-Cu-Mg-Ag-Zr

Previous research showed that tensile fracture strain increases as temperature increases for AA2519 with Mg and Ag additions, because the void-sheet coalescence stage of microvoid fracture is retarded. The present work characterizes intravoid-strain localization (ISL) between primary voids at large constituents and secondary-void nucleation at small dispersoids, two mechanisms that may govern the temperature dependence of void sheeting. Most dispersoids nucleate secondary voids in an ISL band at 25 °C, promoting further localization, while dispersoid-void nucleation at 150 °C is greatly reduced. Increased strain-rate hardening with increasing temperature does not cause this behavior. Rather, a stress relaxation model predicts that flow stress and strain hardening decrease with increasing temperature or decreasing strain rate due to a transition from dislocation accumulation to diffusional relaxation around dispersoids. This transition to softening causes a sharp increase in the model-predicted applied plastic strain necessary for dispersoid/matrix interface decohesion. This reduced secondary-void nucleation and reduced ISL at elevated temperature explain retarded void sheeting and increased fracture strain.     

High-temperature deformation and failure of an orthorhombic titanium aluminide sheet material

The high-temperature deformation and failure behavior of an orthorhombic titanium aluminide sheet alloy (fabricated by diffusion bonding of six thin foils) was established by conducting uniaxial tension and plane-strain compression tests at 980 °C and strain rates between 10⁻⁴ and 10⁻² s⁻¹. The stress-strain response was characterized by a peak stress at low strains followed by moderate flow softening. Values of the strain-rate sensitivity index (m) were between 0.10 and 0.32, and the plastic anisotropy parameter (R) was of the order of 0.6 to 1.0. Cavity nucleation and growth were observed during tensile deformation at strain rates of 10⁻³ s⁻¹ and higher. However, the combined effects of lowm, low cavity growth rateη, and flow softening were deduced to be the source of failure controlled by necking and flow localization rather than cavitation-induced fracture prior to necking.     

Tensile Properties and Fracture Behavior of ATI 718Plus Alloy at Room and Elevated Temperatures

The effect of temperature over the range of ambient to 704 °C and strain rate from 10⁻⁴ to 10⁻² s⁻¹ on the tensile properties and fracture behavior of ATI 718Plus was investigated. The results showed that with increase in temperature at a strain rate 10⁻⁴ s⁻¹, there is a small reduction in the yield strength, but a large drop in ductility at 704 °C. This reduction was accompanied by a change in fracture mode from ductile transgranular to brittle intergranular cracking. Detailed analysis of the microstructure and microchemistry of the areas around the crack using electron microscopy showed that the driving mechanism behind the failure at elevated temperatures and slow strain rates is oxygen-induced intergranular cracking, a dynamic embrittlement mechanism. In addition, the results suggest that the δ precipitates on the grain boundaries tend to oxidize and may facilitate the oxygen-induced intergranular cracking. Finally, an increase in strain rate at 704 °C caused a small increase in the yield strength and a huge increase in ductility. This increase in ductility was accompanied by a change in fracture mode from brittle-to-ductile failure. Possible mechanisms for the deformation, failure mechanisms, and strain rate dependence are discussed.