The separate roles of subgrains and forest dislocations in the isotropic hardening of type 304 stainless steel

Tests on 304 stainless steel were conducted involving first warm working in torsion, then cold working in torsion, and finally measurement of the elevated-temperature yield strength in compression. These tests permitted separation of the effects of subgrain size and forest dislocation density on the isotropic part of the flow stress. Forest dislocation strengthening appears to dominate in this material. The results are best fitted by a root-mean-square summation of strength terms representing the contributions of solutes, forest dislocations, and subgrain boundaries. The same equation successfully predicts the flow stress during elevated-temperature transient deformation (under both constant strain rate and variable strain rate) from the transient dislocation substructure. formerly Research Assistant, Department of Materials Science and Engineering, Stanford University.     

High temperature-high strain rate fracture of inconel 600

The hot fracture of Inconel 600 has been studied over the temperature range from 800° to 2000°F using a hot torsion tester that is capable of superimposing either axial tensile or compressive stresses on the torsional shearing stresses. Microscopic studies of fracture initiation have been made over the entire temperature region. From 800° to 1200°F fracture initiates at inclusions and propagates by transgranular shear. In the temperature region of minimum ductility, 1300° to 1500°F, fracture initiates at grain boundaries and propagates readily in an intergranular manner. At 1600°F and above, fracture initiates easily at grain boundaries, but because recrys-tallization intervenes crack propagation is difficult and strain to fracture is high. Microcracks initiate at the peak in the torque-twist curve. The higher the temperature the smaller is the strain at which fracture initiates. Correlations have been found between the stress state and the shearing strain at crack initiation and total fracture strain. These correlations show the strong influence of a compressive normal stress on retarding crack initiation and resisting crack propagation.     

High temperature plastic deformation of two V-Ga alloys with A15 structure

A study of high temperature plastic deformation has been undertaken on alloys of V-18 at. pct Ga and V-23 at. pct Ga. The materials were prepared by arc melting, homogenizing, and transformation annealing, resulting in polycrystalline A15 structure. Through compression testing and load-relaxation testing, plastic deformation has been studied over a strain rate range from 10^-6 to 10^-2/s and a temperature range from 1000 to 1300 °C. Flow stress decreases with increased temperature and decreased strain rate. Stress-strain rate relations may be fitted with a power law creep expression. The flow stress is influenced by the length of the 1150 °C transformation anneal; longer anneals result in a decrease in flow stresses projected from load relaxation testing. Analysis of compressive yield strength data places the activation energy for A15 V-Ga creep roughly in the 400 kJ/mol range.     

Hot deformation characteristics of INCONEL alloy MA 754 and development of a processing map

The characteristics of hot deformation of INCONEL alloy MA 754 have been studied using processing maps obtained on the basis of flow stress data generated in compression in the temperature range 700 °C to 1150 °C and strain rate range 0.001 to 100 s^-1. The map exhibited three domains. (1) A domain of dynamic recovery occurs in the temperature range 800 °C to 1075 °C and strain rate range 0.02 to 2 s^-1, with a peak efficiency of 18 pct occurring at 950 °C and 0.1 s^-1. Transmission electron microscope (TEM) micrographs revealed stable subgrain structure in this domain with the subgrain size increasing exponentially with an increase in temperature. (2) A domain exhibiting grain boundary cracking occurs at temperatures lower than 800 °C and strain rates lower than 0.01 s^-1. (3) A domain exhibiting intense grain boundary cavitation occurs at temperatures higher than 1075 °C. The material did not exhibit a dynamic recrystallization (DRX) domain, unlike other superalloys. At strain rates higher than about 1 s^-1 the material exhibits flow instabilities manifesting as kinking of the elongated grains and adiabatic shear bands. The material may be safely worked in the domain of dynamic recovery but can only be statically recrystallized.     

Structure and strengthening of Al-Mg alloys during hot working

Hot deformation studies using torsion testing were conducted on high purity Al and Al-4 at. pct Mg alloy systems in the strain rate range of 0.1 to 1.0 s⁻¹ and temperatures up to 810 K (1000‡F). At all test temperatures, the flow stress of the Al-Mg alloy was higher than that of pure AL The strengthening in hot working (above 522 K (480°F)) is suggested to be due to a higher equilibrium subgrain forest dislocation density. Special quenching procedures were required to show this correlation. Conventional quenching fails to show this because structural details are lost when quenching from high temperatures. Formerly with Olin Metals Research Laboratories.     

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.     

Thermomechanical working of Astroloy

Astroloy was warm rolled at temperatures between 1450° and 1800°F, and directly aged in the temperature range from 1300° to 1600°F. Room temperature tensile strengths of 250 ksi and 0.2 pct yield strengths of 200 ksi were obtained by subjecting the alloy to a 20 pct reduction at 1550°F followed directly by an 8-hr aging treatment at 1600°F. The tensile properties of the thermomechanically treated material remained superior to those of normally heat treated astroloy up to test temperatures of 1800°F. The warm working primarily served to refine both the γ′ precipitate and the grain boundary carbides. Preliminary stress rupture tests have indicated that the resulting thermomechanically treated material is markedly inferior to the normally heat treated alloy when compared on the basis of long time properties.     

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.     

Mechanical Properties and Microstructural Evolution of Friction-Stir-Welded Thin Sheet Aluminum Alloys

Friction stir welding of thin aluminum sheets represents a potential goal for aircraft and automotive industries because of the advantages of using this new technological process. In the current work, the microstructural evolution and mechanical behavior of 6082T6-6082T6, 2024T3-2024T3, and 6082T6-2024T3 thin friction-stir-welded joints were investigated. Uniaxial tensile testing at room temperature, 443 K, 473 K, and 503 K (170 °C, 200 °C, and 230 °C) was used to determine the extent to which these ultra-thin joints can be used and deformed. The tensile stress–strain curves showed a decrease of the flow stress with increasing temperature and decreasing strain rate. The ductility of 6082T6-6082T6 joints generally improved when deformed at warm temperatures. It was almost constant for the 6082T6-2024T3 and reached the higher value in the 2024T3-2024T3 when deformed at 443 K and 473 K (170 °C and 200 °C) when compared with the room temperature value. Tensile specimens fractured in the middle of the weld zone in a ductile mode. The precipitation and growth of S’ type phases strengthens 2024T3-2024T3 joints during deformation. In the 6082T6-6082T6, β″ precipitates show some increase in size but give a lower contribution to strength. At 503 K (230 °C), recovery mechanisms (dislocation reorganization inside the deformed grains) are initiated but the temperature was not enough high to produce a homogeneous subgrain structure.     

Temperature and microstructural dependence of the deformation of a high Nb Ti-Al alloy

Compression testing of a Ti-44Al-llNb alloy was carried out at various temperatures and for different microstructures. Annealing was done at temperatures from 1000 °C to 1500 °C to produce the unrecrystallized, duplex (gamma grains plus lamellar colonies) and the fully lamellar microstructures. Samples of each of these microstructures were then tested in air at room temperature and at various temperatures from 1000 °C to 1300 °C. Results indicate that successively higher temperature anneals produce increasing grain or colony sizes from 138 jam in the unrecrystallized microstructure to 1017 ώm in the fully lamellar microstructure. A sequentially lower yield stress was produced on samples tested at increasingly higher temperatures for a given microstructure. In addition, a minimum yield stress on each yield stressvs temperature curve was recorded for the duplex microstructure with a colony size of 154ώm. One promising result was a sample of this microstructure tested at room temperature, where a yield stress of better than 800 MPa and a compressive strain at the cessation of testing of better than 14 pct were obtained.     

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     

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     

Influence of Temperature and Strain Rate on Tensile Deformation and Fracture Behavior of P92 Ferritic Steel

Tensile tests were performed at strain rates ranging from 3.16 × 10^?5 to 1.26 × 10^?3 s^?1 over a temperature range of 300 K to 923 K (27 °C to 650 °C) to examine the effects of temperature and strain rate on tensile deformation and fracture behavior of P92 ferritic steel. The variations of flow stress/strength values, work hardening rate, and tensile ductility with respect to temperature exhibited distinct three temperature regimes. The fracture mode remained transgranular. The steel exhibited serrated flow, an important manifestation of dynamic strain aging, along with anomalous variations in tensile properties in terms of peaks in flow stress/strength and work hardening rate, negative strain rate sensitivity, and ductility minima at intermediate temperatures. At high temperatures, the rapid decrease in flow stress/strength values and work hardening rate, and increase in ductility with increase in temperature and decrease in strain rate, indicated the dominance of dynamic recovery.     

Influence of Strain Rate and Temperature on Tensile Deformation and Fracture Behavior of Type 316L(N) Austenitic Stainless Steel

Tensile tests were performed at strain rates ranging from 3.16 × 10^?5 to 3.16 × 10^?3 s^?1 over the temperatures ranging from 300 K to 1123 K (27 °C to 850 °C) to examine the effects of temperature and strain rate on tensile deformation and fracture behavior of nitrogen-alloyed low carbon grade type 316L(N) austenitic stainless steel. The variations of flow stress/strength values, work hardening rate, and tensile ductility with respect to temperature exhibited distinct three temperature regimes. The steel exhibited distinct low- and high-temperature serrated flow regimes and anomalous variations in terms of plateaus/peaks in flow stress/strength values and work hardening rate, negative strain rate sensitivity, and ductility minima at intermediate temperatures. The fracture mode remained transgranular. At high temperatures, the dominance of dynamic recovery is reflected in the rapid decrease in flow stress/strength values, work hardening rate, and increase in ductility with the increasing temperature and the decreasing strain rate.     

Microhardness and compressive mechanical behavior of L12 titanium trialuminides

Vickers microhardness and compressive mechanical properties of Ll₂ Al₃Ti + X intermetallics, where X = Cr, Mn, Fe, and Cu, were studied. Compressive tests were carried out at the temperature range from room temperature to 900 °C. At room temperature, both Vickers microhardness and yield strength (0.2 pct offset) decrease with increasing the long-range order parameterS. Essentially the same behavior of yield strength is observed at all compressive test temperatures up to 900 °C. In general, apparent compressive ductility (permanent deformation) increases with increasing test temperature, although with different rates, for most of the alloys, except Ll₂ Al₃Ti + Mn (high Ti), which is characterized by the lowest long-range order parameterS. Compressive ductility increases with increasing long-range order at all temperatures, but the highest rate of increase occurs at high temperatures above approximately 600 °C and the rate of increase at room temperature is minimal. At low temperatures, deformation-induced microcracks were formed in large numbers in all of the alloys studied, even in the matrix free of preexisting flaws. These microcracks can propagate catastrophically under tensile deformation conditions at a stress level barely approaching 0.2 pct offset prior to the onset of gross plastic flow, leading to a premature fracture. At high temperatures, the initiation of deformation-induced microcracks is inhibited. The mechanical behavior of L1₂ titanium trialuminides is discussed within a general framework of ductile-to-brittle transition.     

Effect of Purity Levels on the High-Temperature Deformation Characteristics of Severely Deformed Titanium

In the present investigation, high-temperature compression tests were conducted at strain rates of 0.001 to 0.1 s^?1 and at temperatures of 873 K to 1173 K (600 °C to 900 °C) in order to study the hot deformation characteristics and dynamic softening mechanisms of two different grades of commercial purity titanium after severe plastic deformation. It was observed that the effects of deformation rate and temperature are significant on obtained flow stress curves of both grades. Higher compressive strength exhibited by grade 2 titanium at relatively lower deformation temperatures was attributed to the grain boundary characteristics in relation with its lower processing temperature. However, severely deformed grade 4 titanium demonstrated higher compressive strength at relatively higher deformation temperatures (above 800 °C) due to suppressed grain growth via oxygen segregation limiting grain boundary motion. Constitutive equations were established to model the flow behavior, and the validity of the predictions was demonstrated with decent agreement accompanied by average error levels less than 5 pct for all the deformation conditions.     

The effect of tempering temperature on near- threshold fatigue crack behavior in quenched and tempered 4140 steel

Fatigue crack growth in compact tension samples of high purity 4140 steel quenched and tempered to various strength levels was investigated. Tempering temperatures of 200, 400, 550, and 700 °C produced yield strengths from 1600 to 875 MPa, respectively. Crack propagation and crack closure were monitored inK-decreasing tests performed underR = 0.05 loading conditions in laboratory air. Results indicated that as the yield strength increased the crack growth rate increased at a given ΔK and ΔK_th decreased. Threshold values varied from 2.8 MPa m^1/2 (200 °C temper) to 9.5 MPa m_1/2 (700 °C temper). Cracks in the 200 °C tempered samples grew by an intergranular mechanism following prior austenite grain boundaries probably caused by hydrogen embrittlement or tempered martensite embrittlement. Tempering above 200 °C produced transgranular fatigue crack growth. The level of crack closure increased with tempering temperature and with crack propagation in a given tempered condition. Crack closure was caused by a combination of plasticity-induced and oxide-induced mechanisms. The use of an effective stress intensity range based on crack closure consolidated the fatigue crack growth curves and the threshold values for all tempering temperatures except 200 °C. Formerly Graduate Research Assistant, Department of Materials Science and Engineering, Stanford University, Stanford, CA. Formerly Professor, Department of Materials Science and Engineering, Stanford University, Stanford, CA.     

High-Temperature Tensile Behaviors of Base Metal and Electron Beam-Welded Joints of Ni-20Cr-9Mo-4Nb Superalloy

Electron beam welding of Ni-20Cr-9Mo-4Nb alloy sheets was carried out, and high-temperature tensile behaviors of base metal and weldments were studied. Tensile properties were evaluated at ambient temperature, at elevated temperatures of 625 °C to 1025 °C, and at strain rates of 0.1 to 0.001 s^?1. Microstructure of the weld consisted of columnar dendritic structure and revealed epitaxial mode of solidification. Weld efficiency of ~?90 pct in terms of strength (UTS) was observed at ambient temperature and up to an elevated temperature of 850 °C. Reduction in strength continued with further increase of test temperature (up to 1025 °C); however, a significant improvement in pct elongation is found up to 775 °C, which was sustained even at higher test temperatures. The tensile behaviors of base metal and weldments were similar at the elevated temperatures at the respective strain rates. Strain hardening exponent ‘n’ of the base metal and weldment was ~?0.519. Activation energy ‘Q’ of base metal and EB weldments were 420 to 535 kJ mol^?1 determined through isothermal tensile tests and 625 to 662 kJ mol^?1 through jump-temperature tensile tests. Strain rate sensitivity ‘m’ was low (<?0.119) for the base metal and (<?0.164) for the weldment. The δ phase was revealed in specimens annealed at 700 °C, whereas, twins and fully recrystallized grains were observed in specimens annealed at 1025 °C. Low-angle misorientation and strain localization in the welds and the HAZ during tensile testing at higher temperature and strain rates indicates subgrain formation and recrystallization. Higher elongation in the weldment (at Test temperature >?775 °C) is attributed to the presence of recrystallized grains. Up to 700 °C, the deformation is through slip, where strain hardening is predominant and effect of strain rate is minimal. Between 775 °C to 850 °C, strain hardening is counterbalanced by flow softening, where cavitation limits the deformation (predominantly at lower strain rate). Above 925 °C, flow softening is predominant resulting in a significant reduction in strength. Presence of precipitates/accumulated strain at high strain rate results in high strength, but when the precipitates were coarsened at lower strain rates or precipitates were dissolved at a higher temperature, the result was a reduction in strength. Further, the accumulated strain assisted in recrystallization, which also resulted in a reduction in strength.