Effects of temperature and strain rate on tensile properties and activation energy for dynamic strain aging in alloy 625

Alloy 625 ammonia cracker tubes were service exposed for 60,000 hours at 873 K. These were then subjected to a solution-annealing treatment at 1473 K for 0.5 hours. The effects of temperature and strain rate on the tensile properties of the solution-annealed alloy were examined in the temperature range of 300 to 1023 K, employing the strain rates in the range of 3×10⁻⁵ s⁻¹ to 3×10⁻³ s⁻¹. At intermediate temperatures (523 to 923 K), various manifestations of dynamic strain aging (DSA) such as serrated flow, peaks, and plateaus in the variations of yield strength (YS) and ultimate tensile strength (UTS) and work-hardening rate with temperature were observed. The activation energy for serrated flow (Q) was determined by employing various methodologies for T<823 K, where a normal Portevien-Le Chatelier effect (PLE) was observed. The value of Q was found to be independent of the method employed. The average Q value of 98 kJ/mol was found to be in agreement with that for Mo migration in a Ni matrix. At elevated temperatures (T≥823 K), type-C serrations and an inverse PLE was noticed. The decrease in uniform elongation beyond 873 K for 3×10⁻⁵ s⁻¹ and 3×10⁻³ s⁻¹ and beyond 923 K for 3×10⁻⁴ s⁻¹ strain rates seen in this alloy has been ascribed to reduction in ductility due to precipitation of carbides and δ phase on the grain boundaries.     

Strength and plastic flow in “in situ” TiC reinforced aluminum composites

The deformation behavior of TiC particulate-reinforced aluminum composites (Al-TiC _p ) was investigated in this work using pure aluminum as the reference matrix material. Uniaxial compression tests were carried out at 293 and 623 K and at two strain rates (3.7×10⁻⁴ and 3.7×10⁻³ s⁻¹). Yield strengths of up to 127 MPa were found in composites containing 10 vol pct TiC particulates, which were almost 4 times the yield strength of pure Al. In addition, at 623 K, relatively small reductions in yield strength were found, suggesting that this property was rather insensitive to temperature for the temperatures investigated in this work. Nevertheless, at 623 K, increasing the rate of straining from 3.7×10⁻⁴ s⁻¹ to 3.7×10⁻³ s⁻¹ lowered the yield strength, particularly in 10 vol pct TiC _p -Al composites. Two stages of work hardening were identified in pure Al and a 10 vol pct TiC _p composite during plastic flow through the modified version of the Hollomon equation (σ = Kɛ ⁿ ± Δ). In particular, the work-hardening exponents found in pure Al shifted from high to low values as the extent of plastic strain was increased while the opposite was true for the 10 vol pct TiC _p composite. Finally, at 623 K, dynamic recovery mechanisms became dominant at plastic strain levels >0.2 in 10 vol pct TiC _p -Al composites, with the effect being minor at room temperature.     

Fabrication of bulk ultrafine-grained materials through intense plastic straining

Ultrafine grain sizes were introduced into samples of an Al-3 pct Mg solid solution alloy and a cast Al-Mg-Li-Zr alloy using the process of equal-channel angular (ECA) pressing. The Al-3 pct Mg alloy exhibited a grain size of ∼0.23 μm after pressing at room temperature to a strain of ∼4, but there was significant grain growth when the pressed material was heated to temperatures above ∼450 K. The Al-Mg-Li-Zr alloy exhibited a grain size of ∼1.2 μm, and the microstructure was heterogeneous after pressing to a strain of ∼4 at 673 K and homogeneous after pressing to a strain of ∼8 at 673 K with an additional strain of ∼4 at 473 K. The heterogeneous material exhibited superplastic-like flow, but the homogeneous material exhibited high-strain-rate superplasticity with an elongation of >1000 pct at 623 K at a strain rate of 10⁻² s⁻¹. It is concluded that a homogeneous microstructure is required, and therefore a high pressing strain, in order to attain high-strain-rate superplasticity (HSR SP) in ultrafine-grained materials. This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees.     

Low-temperature superplasticity of ultra-fine-grained Ti-6Al-4V processed by equal-channel angular pressing

The low-temperature superplasticity of ultra-fine-grained (UFG) Ti-6Al-4V was established as a function of temperature and strain rate. The equiaxed-alpha grain size of the starting material was reduced from 11 to 0.3 μm (without a change in volume fraction) by imposing an effective strain of ∼4 via isothermal, equal-channel angular pressing (ECAP) at 873 K. The ultrafine microstructure so produced was relatively stable during annealing at temperatures up to 873 K. Uniaxial tension and load-relaxation tests were conducted for both the starting (coarse-grained (CG)) and UFG materials at temperatures of 873 to 973 K and strain rates of 5 × 10⁻⁵ to 10⁻² s⁻¹. The tension tests revealed that the UFG structure exhibited considerably higher elongations compared to those of the CG specimens at the same temperature and strain rate. A total elongation of 474 pct was obtained for the UFG alloy at 973 K and 10⁻⁴ s⁻¹. This fact strongly indicated that low-temperature superplasticity could be achieved using an UFG structure through an enhancement of grain-boundary sliding in addition to strain hardening. The deformation mechanisms underlying the low-temperature superplasticity of UFG Ti-6Al-4V were also elucidated by the load-relaxation tests and accompanying interpretation based on inelastic deformation theory.     

The effect of grain boundaries on the athermal stress of tantalum and tantalum-tungsten alloys

The temperature dependence of the yield stress of polycrystalline Ta, Ta-2.47 wt pct W (Ta-2.5W), and Ta-9.80 wt pct W (Ta-10W) was measured to study the effect of grain boundaries and tungsten concentration on athermal strength components. Compression tests were performed over a temperature range from 77 to 1223 K at strain rates of 10⁻⁴ and 10⁻¹ s⁻¹. The test results show that the yield stress of Ta becomes independent of temperature above about 400 K, indicating an “athermal” regime. In contrast, the temperature dependence of yield stress was still significant for Ta-10W up to the maximum test temperature. An analysis of the test data using single-crystal data in conjunction with Taylor factors was performed to assess the effect of grain boundaries on the athermal component of flow stress at 600 K. The results indicated that the long-range athermal stress at the yield point due to grain boundaries is approximately 13 to 41 MPa for the study materials and decreases with an increase in tungsten concentration. These results are discussed with regard to constitutive modeling of flow stress.     

Transition of dominant diffusion process during superplastic deformation in AZ61 magnesium alloys

The superplastic behavior of the AZ61 magnesium alloy sheet, processed by one-step hot extrusion and possessing medium grain sizes of ∼12 μm, has been investigated over the temperature range of 523 to 673 K. The highest superplastic elongation of 920 pct was obtained at 623 K and a deformation rate of 1×10⁻⁴ s⁻¹. In the lower and higher strain rate regimes, with apparent m values of ∼0.45 and ∼0.25, respectively, grain-boundary sliding (GBS) and dislocation creep appeared to dominate the deformation, consistent with the scanning electron microscopy (SEM) examination. The SEM examination also revealed that individual GBS started to operate from the very initial deformation stage in the strain rate range with m∼0.45, which was attributed to the relatively high fraction (88 pct) of high-angle boundaries. The analyses of the superplastic data over 523 to 673 K and 5×10⁻⁵ to 1×10⁻³ s⁻¹ revealed a true stress exponent of ∼2, and the activation energy was close to that for grain-boundary and lattice diffusion of magnesium at 523 to 573 K and 573 to 673 K, respectively. The transition temperature of activation energy is ∼573 K, which is attributed to the change in the dominant diffusion process from grain-boundary diffusion to lattice diffusion. It is demonstrated that the effective diffusion coefficient is a valid parameter to characterize the superplastic behavior and the dominant diffusion process.     

Evaluation of activation energy for dynamic strain aging process in 316L(N)/316(N) SS weld joints

Type 316 L(N) Stainless Steel (SS) is being currently used as a structural material for various components of Prototype Fast Breeder Reactor (PFBR). The possibility of using 316 L(N) electrodes for fabrication of 316 L(N) welding joints is being critically examined. This paper discusses about the evaluation of activation energy for Dynamic Strain Aging (DSA) process in 316L(N)/316(N) SS Weld Joints. The Gas Tungsten Arc Welding (GTAW) process was used for the root pass and Gas Metal Arc Welding (GMAW) process was used for the remaining passes. Tensile tests have been conducted in the wide temperature range from room temperature to 1023 K at a strain rate of 3 × 10⁻³ s⁻¹. Yield stress showed a continuous decrease with increasing temperature, with a plateau being observed between 823 and 923 K. A minima in elongation was also observed in this temperature range. These two properties being manifestations of dynamic strain aging, further tests at different strain rates (3 × 10⁻⁵ s⁻¹ to 3 × 10⁻² s⁻¹) were conducted in this temperature range. Detailed analyses of the results were carried out and the solute responsible for dynamic strain aging was identified to be substitutional chromium. Post test analysis of fracture surfaces and deformation substructures were correlated with the changes in tensile properties at different testing temperatures.     

低铼钼合金高温烧结过程的研究

研究了Ｍｏ３ ％ Ｒｅ 合金的垂熔和干氢高温烧结过程。结果表明，与垂熔法相比，干氢烧结的Ｍｏ３ ％ Ｒｅ 合金具有较细的晶粒组织，较高的抗拉强度、延伸率和显微硬度。     

Strength and ductility under dynamic loading in fine- grained IN905XL aluminum alloy

Three IN905XL aluminum alloys with fine grain (1 μm), intermediate grain (3 μm), and coarse grain (5 μm) have been developed by a combination of mechanical alloying (MA) and conventional extrusion in order to investigate their mechanical properties at dynamic strain rates of 1 × 10³ and 2 × 10³ s⁻¹ and a quasi-static strain rate of 10^-3 s⁻¹. Flow stresses are found to increase with decreasing grain size for all the strain rates tested. Negative strain-rate sensitivity of flow stress is observed up to 1 × 10³ s⁻¹ in both intermediate- and coarse-grained IN905XL. At the highest strain rate of 2 × 10³ s⁻¹ however, all samples showed a positive strain-rate sensitivity of strength. Total elongation at high strain rates is generally larger than that at low strain rates. Total elongation also decreases with grain size for all the strain rates. This decrease in elongation results from an initiation of microcracks at interfaces between the matrix and particles finely dispersed near grain boundary regions, introduced during MA processing; then, this initiation leads elongation of alloys to small limited values. Formerly with the Department of Mechanical Systems Engineering, University of Osaka Prefecture. This article is based on a presentation made in the symposium “Dynamic Behavior of Materials,” presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee.     

Characterization of superplastic deformation behavior of a fine grain 5083 Al alloy sheet

Superplastic deformation behavior of a fine grain 5083 Al sheet (Al-4.2 pct Mg-0.7 pct Mn, trade name FORMALL 545) has been investigated under uniaxial tension over the temperature range of 500 °C to 565 °C. Strain rate sensitivity values >0.3 were observed over a strain rate range of 3 × 10⁻⁵ s⁻¹ to 1 × 10⁻² s⁻¹, with a maximum value of 0.65 at 5 × 10⁻⁴ s⁻¹ and 565 °C. Tensile elongations at constant strain rate exceeded 400 pct; elongations in the range of 500 to 600 pct were obtained under constant crosshead speed and variable strain rates. A short but rapid prestraining step, prior to a slower superplastic strain rate, provided enhanced tensile elongation at all temperatures. Under the two-step schedule, a maximum tensile elongation of 600 pct was obtained at 550 °C, which was regarded as the optimum superplastic temperature under this condition. Dynamic and static grain growth were examined as functions of time and strain rate. It was observed that the dynamic grain growth rate was appreciably higher than the static growth rate and that the dynamic growth rate based on time was more rapid at the higher strain rate. Cavitation occurred during superplastic flow in this alloy and was a strong function of strain rate and temperature. The degree of cavitation was minimized by superimposition of a 5.5 MPa hydrostatic pressure during deformation, which produced a tensile elongation of 671 pct at 525 °C. R. VERMA, formerly Visiting Scientist, Department of Materials Science and Engineering, University of Michigan     

The role of grain size on the ductile-brittle transition of a 2.25 Pct Cr-1 Pct Mo steel

The ductile-brittle transition temperature of a 2 1/4 pct Cr-1 pct Mo steel has been meas-ured using ‘V’ notch Izod impact specimens for an unembrittled and embrittled 2 1/4 pct Cr-1 pct Mo steel with prior austenite grain sizes within the range 40 to 150 μm. The mi-crostructure of this steel was upper bainite. The variation of yield strength with grain size obeys a Hall-Petch relationship. The ductile-brittle transition temperature was found to have a pronounced grain size dependence for both unembrittled, 15 K mm^1/2, and embrittled, 19 K mm^1/2, specimens. The bainite colony size was found to vary as the prior austenite grain size. From the low temperature quasi-cleavage facet size, together with metallographic observations of crack path, it has been concluded that bainite colony size rather than prior austenite grain size is the effective grain size.     

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.     

Superplastic behavior of thermomechanically treated P/M 7091 aluminum alloy

The superplastic behavior of thermomechanically treated P/M 7091 aluminum alloy was assessed in the temperature range of 573 to 773 K. The thermomechanical treatment (TMT) comprised of three steps of solution treatment, overaging, and warm rolling. There are large η-phase (MgZn₂) precipitate particles of average size of 1.30 μm in the overaged condition. The warm-rolled alloy undergoes continuous recrystallization at the test temperatures of 573 and 623 K, exhibiting a maximum tensile elongation of 450 pct at 573 K and a strain rate of 8 × 10⁻⁵ s⁻¹. The precipitate particles play a major role in the process of continuous recrystallization. For a given volume fraction of precipitate particles and constant amount of warm rolling (in the course of TMT), an optimum precipitate particle size is expected to maximize the rate of continuous recrystallization and render the finest recrystallized grain size. The warm-rolled alloy undergoes static recrystallization at temperatures above 673 K. The grain growth accompanying the deformation at these test temperatures limits the tensile ductility to a lower value. Irrespective of the test temperature and strain rate, the specimens undergo extensive cavitation when deformed at elevated temperatures.     

Nanocrystalline iron sintering behavior and microstructural development

Nanocrystalline (20 nm) iron powder was closed-die sintered in a hydrogen atmosphere at a stress of 10.1 MPa and at temperatures between 670 and 1270 K. The maximum densification rate was approximately 6 × 10⁻⁴ s⁻¹. Density greater than 90 pct was obtained at sintering temperatures greater than 990 K. Densification was marked microstructurally by local gradients which appeared after initial cold compaction. Oxygen content in the starting powder was high but was effectively a monolayer of surface adsorbed oxygen. Despite the reducing sintering atmosphere, oxide was present in dense specimens as a fine dispersion of order 0.1 to 1μm. The extent of oxide formation can be controlled by closed-die sintering to a stable structure of interconnected porosity followed by open-die resintering in the reducing atmosphere. Final grain size in material sintered 1 hour at 1080 K was generally less than 200 nm, although scattered coarsening to approximately 5 μm was observed.     

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.     

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.     

The kinetics of atomic diffusion to stacking faults and the related special problems in cold-worked alpha brasses

The diffusion kinetics of solute atoms to the stacking fault layers were investigated by the Laplace transformation method in binary homogeneous alloys. The room temperature annealing kinetics of stacking faults in cold-worked alpha brasses, which were studied by an X-ray diffraction technique, were analyzed in terms of the present theoretical findings. The chemical diffusivities in cold-worked (filings) Cu−10Zn and Cu−22.7Zn alpha brasses were determined as equal to 1.7×10⁻²¹ and 1.5×10⁻¹⁹ cm²/s at room temperature (300 K), respectively. Finally, the concentration of athermal monovacancies was estimated in plastically deformed Cu−10Zn (the particle size 20 μm) and Cu−22.7Zn (the particle size 75 μm) alloys and was found to be about 2.0×10¹⁹ and 3.0×10¹⁹ cm⁻³, respectively.     

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.     

Grain coarsening in Ti-5Al-2.5Sn

Grain coarsening in a Ti-5 Al-2.5 Sn titanium alloy, deformed in tension to 13 pct uniform elongation and then heated to 1144 K (1600°F) for one h, was investigated. The influence of deformation temperature (77 to 598 K), grain size (10.7, 11.8, and 22.5 μm), and strain rate (2.67 × 10^-2, 6.67 × 10^-4, 2.67 × 10^-5 s^-1) was also studied. Critical elongation and work input values for maximum grain coarsening varied with deformation temperature. The critical elongation value increased from 9 to 12 pct as the temperature decreased from 598 to 367 K and decreased from 12 to 9 pct as temperature decreased from 367 to 77 K. The critical work energy input increased linearly with decreasing temperature.     

W–Re/石墨复合材料的制备、微观组织及性能

以W–Re片和石墨块为原料,Mo/Zr混合粉为焊料,在1850 ℃和30 MPa的条件下通过真空热压烧结制备了W–Re/石墨复合材料,对复合材料的微观组织、剪切断口及室温剪切性能进行了分析。研究结果表明,在热压烧结复合材料的Mo/Zr焊料层中发生了明显的分层现象,靠近石墨区域的物相组成是ZrC和Mo₂Zr,靠近W–Re区域的物相组成是Mo₂Zr和锆基固溶体（Zrss）。复合材料的断裂主要发生在焊料与石墨界面上,焊料并未明显渗透到石墨中,未能形成钉扎作用,试样的室温剪切强度约5 MPa。