Deformation behavior of bimodal nanostructured 5083 Al alloys

Cryomilled 5083 Al alloys blended with volume fractions of 15, 30, and 50 pct unmilled 5083 Al were produced by consolidation of a mixture of cryomilled 5083 Al and unmilled 5083 Al powders. A bimodal grain size was achieved in the as-extruded alloys in which nanostructured regions had a grain size of 200 nm and coarse-grained regions had a grain size of 1 μm. Compression loading in the longitudinal direction resulted in elastic-perfectly plastic deformation behavior. An enhanced tensile elongation associated with the occurrence of a Lüders band was observed in the bimodal alloys. As the volume fraction of coarse grains was increased, tensile ductility increased and strength decreased. Enhanced tensile ductility was attributed to the occurrence of crack bridging as well as delamination between nanostructured and coarse-grained regions during plastic deformation.     

Microstructural evolution and superplastic deformation behavior of fine grain 5083Al

The microstructural evolution during superplastic deformation of a fine grain Al-4.7 pct Mg alloy (5083Al) has been studied quantitatively. Starting from an average grain size of 7 μm, grain growth was monitored in this alloy both under static annealing and with concurrent superplastic deformation at a high test temperature of 550°C. Grain size was averaged from measurements taken in longitudinal, transverse, and thickness directions and was found to grow faster during concurrent superplastic deformation than for static annealing. A grain growth law based on an additive nature between time-based and strain-based growth behavior was used to quantify the dynamics of concurrent grain growth. The extent of void formation during deformation was quantified as the area fraction of voids on L-S planes. This void fraction, referred to as the cavity area percent, was recorded at several levels of strain for specimens deformed at two different strain rates. A constitutive equation incorporating this grain growth data into the stress-strain rate data, determined during the early part of deformation, was generated and utilized to model the superplastic tensile behavior. This model was used in an effort to predict the stress-strain curves in uniaxial tension under constant and variable strain rate conditions. Particular attention was paid to the effects of a rapid prestrain rate on the overall superplastic response and hardening characteristics of this alloy.     

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     

Superplastic deformation behavior in commercial and high purity Zn- 22 Pct Al

The superplastic deformation behavior of two grades of the Zn-22 pct Al alloy was studied in detail under identical conditions of grain size, temperature, and stress. While these two grades were prepared by the same procedure, they have different impurity levels; grade 1 is of commercial purity (180 ppm of impurities) and grade 2 is of very high purity (6 ppm of impurities). The experimental results on the commercial grade show that the relationship between the applied stress and steady-state strain rate is sigmoidal and is manifested by the presence of three deformation regions; in region I (low-stress region) the stress exponent,n, is 3.8 and the activation energy for creep,Q, is higher than that for grain boundary diffusion,Q _gb; in region II (intermediate-stress region)n = 2.5 andQ ≃ Q _gb; and in region III (high-stress region)n again increases. The results on the high purity grade, when compared with those on the commercial grade, reveal a significant difference: the high purity grade, unlike the commercial grade, does not exhibit region I at low stresses. The difference in creep behavior between the two grades of Zn-22 pct Al at low stresses leads to the implication that the origin of region I during superplastic flow is related to the presence of impurity atoms. It is suggested, on the basis of consideration of various impurity-controlled deformation processes, that the creep behavior in region I is most likely a consequence of the existence of a threshold stress which is caused by impurity atom segregation at boundaries. Formerly Graduate Student, Department of Mechanical Engineering, University of California, Irvine     

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.     

Microstructure and mechanical behavior of spray-deposited High-Li Al-Li alloys

High-Li alloys, with the composition Al-3.8Li-XCu-1.0Mg-0.4Ge-0.2Zr, were synthesized using a spray deposition technique (wt. pct, X=0∼1.5). The microstructure of the spray-deposited Al-Li alloys consisted of equiaxed grains with an average grain size in the range from 20 to 50 μm. The grain-boundary phases were fine and discrete. The spray-deposited and thermomechanically processed materials were isothermally heat treated at 150 °C and 170 °C to investigate the age-hardening kinetics. It was noted that the spray-deposited Al-3.8Li-XCu-1.0Mg-0.4Ge-0.2Zr alloys exhibited relatively sluggish aging behavior. The peak-aged condition was achieved at 170 °C in the range from 20 to 90 hours. It was noted that Cu increases the hardness of alloys during aging. Moreover, the influence of Cu on age-hardening kinetics is marginal. The mechanical properties of the spray-deposited and extruded Al-Li alloys were studied in the underaged, peak-aged, and overaged conditions. For example, the peak-aged yield strength, tensile strength, and ductility of Al-3.8Li-1.0Cu-1.0Mg-0.4Ge-0.2Zr are 455 MPa, 601 MPa, and 3.1 pct, respectively. Moreover, an increase in the Cu content of the alloy led to improvements in strength, with only slight changes in ductility, for Cu contents up to 1.0 wt pct. Beyond this range, an increase in Cu content led to decreases in both strength and ductility.     

镁、锰、铁元素含量对5083铝合金超塑性行为的影响

采用金相、扫描电镜等检测手段，对不同镁、锰及铁元素含量5083铝合金超塑性变形后微观组织与结构进行观察。结果表明，在相同的应变率下，镁、锰、铁含量高的合金（合金A）形成了由Al-Mg-Fe及Al-Mg-Mn金属间化合物组成的第二相粒子，这些金属间化合物为超塑性变形中空洞的形核提供了条件。     

Superplastic behavior of two-phase titanium aluminides

A two-phase Ti(57 at. pct)-Al(43 at. pct) alloy with an initial lamellar microstructure was thermomechanically processed to form an equiaxed fine-grained structure. The fine-grained (- L = 5 μm) material was superplastic in the temperature range 1000 °C to 1100 °C, exhibiting a stress exponent of about 2 with a tensile ductility of 275 pct. The rate-controlling deformation mechanism is proposed to be grain boundary sliding accommodated by slip controlled by lattice diffusion in TiAl. At room temperature, the lamellar and fine-grained materials exhibit the same compressive yield stress. The compressive strain to failure, however, for the fine-grained material was about 28 pct compared to 6 pct for the lamellar material.     

High-strain-rate superplasticity in bulk cryomilled ultra-fine-grained 5083 Al

The ductility and creep of bulk ultra-fine-grained (UFG) 5083 Al (grain size ∼440 nm) processed by gas atomization, cryomilling, and consolidation were studied in the temperature range 523 to 648 K. Also, the creep microstructure developed in the alloy was examined by means of transmission electron microscopy (TEM). The ductility as a function of strain rate exhibits a maximum that shifts to higher strain rates with increasing temperature. An analysis of the experimental data indicates that the true stress exponent is about 2, and the true activation energy is close to that anticipated for boundary diffusion in 5083 Al. These creep characteristics along with the ductility behavior of 5083 Al are a reflection of its creep behavior as a superplastic alloy and not as a solid-solution alloy. In addition, the observation of elongations of more than 300 pct at strain rates higher than 0.1 s⁻¹ is indicative of the occurrence of high-strain-rate (HSR) superplasticity. Microstructural evidence for the occurrence of HSR superplasticity includes the retention of equiaxed grains after deformation, the observation of features associated with the occurrence of grain boundary sliding, and the formation of cavity stringers. Grain size stability during the superplastic deformation of the alloy is attributed to the presence of dispersion particles that are introduced during gas spraying and cryomilling. These particles also serve as obstacles for dislocation motion, which may account for the threshold stress estimated from the creep data of the alloy.     

Superplastic behavior of two ultrahigh boron steels

The high-temperature deformation behavior of two ultrahigh boron steels containing 2.2 pct and 4.9 pct B was investigated. Both alloys were processedvia powder metallurgy involving gas atomization and hot isostatic pressing (hipping) at various temperatures. After hipping at 700 °C, the Fe-2.2 pct B alloy showed a fine microstructure consisting of l-μm grains and small elongated borides (less than 1μm) . At 1100 °C, a coarser microstructure with rounded borides was formed. This alloy was superplastic at 850 °C with stress exponents of about two and tensile elongations as high as 435 pct. The microstructure of the Fe-4.9 pct B alloy was similar to that of the Fe-2.2 pct B alloy showing, in addition, coarse borides. This alloy also showed low stress exponent values but lacked high tensile elongation (less than 65 pct), which was attributed to the presence of stress accumulation at the interface between the matrix and the large borides. A change in the activation energy value at theα-γ transformation temperature was seen in the Fe-2.2 pct B alloy. The plastic flow data were in agreement with grain boundary sliding and slip creep models. J.A. JIMéNEZ, Postdoctoral Fellow, formerly with Centro Nacional de Investigaciones Metalurgicas, C.S.I.C.     

Thermal stability in bulk cryomilled ultrafine-grained 5083 Al alloy

Thermal stability in bulk ultrafine-grained (UFG) 5083 Al that was processed by gas atomization followed by cryomilling, consolidation, and extrusion, and that exhibited an average grain size of 305 nm, was investigated in the temperature range of 473 to 673 K (0.55 to 0.79 T _m , where T _m is the melting temperature of the material) for different annealing times. Appreciable grain growth was observed at temperatures > 573 K, whereas there was limited grain growth at temperatures < 573 K even after long annealing times. The values of the grain growth exponent, n, deduced from the grain growth data were higher than the value of 2 predicted from elementary grain growth theories. The discrepancy was attributed to the operation of strong pinning forces on boundaries during the annealing treatment. An examination of the microstructure of the alloy suggests that the origin of the pinning forces is most likely related to the presence of dispersion particles, which are mostly introduced during cryomilling. Two-grain growth regimes were identified: the low-temperature region (<573 K) and the high-temperature region (>573 K). For temperatures lower than 573 K, the activation energy of 25 ± 5 kJ/mol was determined. It is suggested that this low activation energy represents the energy for the reordering of grain boundaries in the UFG material. For temperatures higher than 573 K, an activation energy of 124 ± 5 kJ/mol was measured. This value of activation energy, 124 ± 5 kJ/mol, lies between that for grain boundary diffusion and lattice diffusion in analogous aluminum polycrystalline systems. The results show that the strength and ductility of bulk UFG 5083 Al, as obtained from tensile tests, correlate well with substructural changes introduced in the alloy by the annealing treatment.     

Superplastic behavior of powdered titanium nickelide during pressing

Conclusions In the process of pressing of a titanium nickelide powder a high level of "elastic" recovery is observed, due to the superelasticity of the material. The following characteristic features of the superelastic behavior of a titanium nickelide powder undergoing densification have been established: The maximum degree of volume recovery does not attain 100%; the degree of recovery diminishes with increasing degree of densification of material, and increases with rise in temperature up to 400C, the increase being the greater the higher the compact porosity; repeated application of the same pressure during the pressing of the powder markedly decreases the compact porosity.Translated from Poroshkovaya Metallurgiya, No. 2(266), pp. 13–17, February, 1985.     

TA32钛合金板材的超塑性性能研究

研究了真空环境中TA32钛合金在950℃,初始变形速率在5.32×10-4~2.08×10-2s-1条件下的超塑性变形行为。结果表明,不同应变速率条件下,板材的流变应力曲线特征和显微组织演变呈现显著不同。在应变速率较低条件下(5.32×10-4 ~3.33×10-3s-1),拉伸真应力-应变曲线呈传统超塑变形的稳态流动特征,变形后的板材中初生α相晶粒尺寸较大;在高应变速率（8.31×10-3 s-1~2.08×10-2 s-1）条件下,拉伸真应力-应变曲线中流变应力增大到峰值后快速单调递减直到断裂,变形后的板材中初生α相发生动态再结晶,晶粒尺寸与低应变速率条件拉伸的板材相比显著细化。在950℃下,TA32钛合金板材均具有超塑性变形能力,超塑性延伸率在145%~519%之间,当应变速率为5.32×10-4s-1时,板材具有最佳的超塑性性能,拉伸延伸率可达519%。断裂区分析发现,TA32钛合金板材的超塑性断裂模式为空洞聚集-连接-长大型断裂。     

Microstructure and mechanical behavior of spray-deposited Al-Cu-Mg(-Ag-Mn) alloys

The effect of alloy composition on the microstructure and mechanical behavior of four spray-deposited Al-Cu-Mg(-Ag-Mn) alloys was investigated. Precipitation kinetics for the alloys was determined using differential scanning calorimetry (DSC) and artificial aging studies coupled with transmission electron microscopy (TEM) analysis. DSC/TEM analysis revealed that the spray-deposited alloys displayed similar precipitation behavior to that found in previously published studies on ingot alloys, with the Ag containing alloys exhibiting the presence of two peaks corresponding to precipitation of both Ω-Al₂Cu and θ′-Al₂Cu and the Ag-free alloy exhibiting only one peak for precipitation of θ′. The TEM analysis of each of the Ag-containing alloys revealed increasing amounts of Al₂₀Mn₃Cu₂ with increasing Mn. In the peak and over-aged conditions, Ag-containing alloys revealed the presence of Ω, with some precipitation of θ′ for alloys 248 and 251. Tensile tests on each of the alloys in the peak-aged and overaged (1000 hours at 160 °C) conditions were performed at both room and elevated temperatures. These tests revealed that the peak-aged alloys exhibited relatively high stability up to 160 °C, with greater reductions in strength being observed at 200 °C (especially for the high Mn, low Cu/Mg ratio (6.7) alloy 251). The greatest stability of tensile strength following extended exposure at 160 °C was exhibited by the high Cu/Mg ratio (14) alloy 248, which revealed reductions in yield strength of about 2.5 pct, with respect to the peak-aged condition, for the alloys tested at both room temperature and 160 °C.     

Densification of Al 5083 Mechanically Alloyed Powder by Equal Channel Angular Pressing

Equal channel angular pressing (ECAP), one of the most important methods in SPD, is used for the consolidation of mechanically alloyed Al 5083 powder. This paper mainly focuses on the densification of Al 5083 mechanically alloyed powder by ECAP with and without application of back pressure up to three passes with four different routes at room temperature. Aluminum can is used to encapsulate the powder. The particle size, crystallite size, microstructure and density were evaluated by scanning electron microscope and X-ray diffraction peak profile analysis. The crystallite size was measured by Williamson Hall analysis. Density and hardness were increased with increasing number of passes and upon sintering after ECAP. Good densification as well as good powder bonding was observed after three passes of ECAP.     

Superplastic behavior of the Sn-Pb eutectic in the as-worked state

Superplastic behavior of the Sn-Pb eutectic was studied in the as-worked state by mechanically working the cast material to varying degrees. The flow behavior was explored in the temperature range of 298 to 443 K and metallographic observations of longitudinal and transverse sections were made. The microstructure was inhomogeneous for low degrees of working, while at high reductions the grains were nonequiaxed. There was strain softening or hardening up to some strain depending on the test conditions. The grains became more equiaxed and coarser in size with increasing deformation. Due to these changes in microstructure and the nonunique stress-strain rate relation, the as-worked material is not suitable from the viewpoint of assessing the mechanisms of superplastic flow based on steady state. Strain dependency of flow stress was also observed in specimens specially processed to obtain equiaxed grains in the as-worked state. formerly a graduate student at Indian Institute of Technology, Kanpur, India.     

Analysis and Simulation of Parallel Tubular Channel Angular Pressing of Al 5083 Tube

Severe plastic deformation (SPD) is divided to a number of branches. One of the most important branches of SPD of tube materials is parallel tubular channel angular pressing (PTCAP). The PTCAP process is more complicated than another SPD process for metal tubes. The PTCAP process includes two half cycles for obtaining the desired product. There are some factors, which affect the PTCAP process. For instance, deformation ratio, clamping force, channel angles, and curvature angles. In this paper, the PTCAP process for aluminum alloy (Al 5083) was simulated through the finite element ABAQUS software in order to investigate the effect of die parameters. Thus, the influences of deformation ratio, channel angles, and curvature angles on the PTCAP process were examined numerically. In addition, to verify the finite element method (FEM) results, the PTCAP process was done experimentally. To some extent, good conformity was observed between the FEM calculations and experimental results.     

Superplastic flow and cavitation in Zn-22 pct Al doped with Cu

The sigmoidal relationship between stress and steady-state strain rate that has been reported for micrograin superplastic alloys is characterized by the presence of three regions: region I at low stresses, region II (the superplastic region) at intermediate stresses, and region III at high stresses. Recent results on the superplastic Zn-22 pct Al eutectoid have shown that the characteristics of region I are influenced by the impurity level of the alloy, and that neither region I nor significant cavitation is observed when such a level is reduced to about 6 ppm. These observations are in agreement with the suggestion that the origin of region I is related to strong impurity segregation at boundaries. The present investigation was conducted to study the effect of Cu, as a selected impurity, on superplastic deformation and cavitation in Zn-22 pct Al. The results show that Zn-22 pct Al-0.13 pct Cu exhibits two primary characteristics: region I is absent and cavitation is not extensive. These characteristics, which are essentially similar to those reported previously for high-purity Zn-22 pct Al but are different from those documented for a grade of the alloy containing a comparable atomic concentration of Fe, suggest that Cu has little or no tendency to segregate at boundaries. Indirect evidence in support of this suggestion is inferred from studying the effect of impurities on former α boundaries that form in the microstructure of Zn-22 pct Al as a result of solution treatment above the eutectoid temperature. Although further studies are needed to provide direct evidence for the absence of Cu segregation at boundaries, the present results clearly indicate that superplastic flow and cavitation at low stresses are controlled not only by the impurity level, but also by its type.