An evaluation of the flow behavior during high strain rate superplasticity in an Al-Mg-Sc alloy

An Al-3 pct Mg-0.2 pct Sc alloy was fabricated by casting and subjected to equal-channel angular pressing to reduce the grain size to ∼0.2 μm. Very high tensile elongations were achieved in this alloy at temperatures over the range from 573 to 723 K, with elongations up to >2000 pct at temperatures of 673 and 723 K and strain rates at and above 10⁻² s⁻¹. By contrast, samples of the same alloy subjected to cold rolling (CR) yielded elongations to failure of <400 pct at 673 K. An analysis of the experimental data for the equal-channel angular (ECA)-pressed samples shows consistency with conventional superplasticity including an activation energy for superplastic flow which is within the range anticipated for grain boundary diffusion in pure Al and interdiffusion in Al-Mg solid solution alloys.     

An evaluation of the flow behavior during high strain rate superplasticity in an Al−Mg−Sc alloy

An Al-3 pct Mg-0.2 pct Sc alloy was fabricated by casting and subjected to equal-channel angular pressing to reduce the grain size to ∼0.2 μm. Very high tensile elongations were achieved in this alloy at temperatures over the range from 573 to 723 K, with elongations up to >2000 pct at temperatures of 673 and 723 K and strain rates at and above 10⁻² s⁻¹. By contrast, samples of the same alloy subjected to cold rolling (CR) yielded elongations to failure of <400 pct at 673 K. An analysis of the experimental data for the equal-channel angular (ECA)—pressed samples shows consistency with conventional superplasticity including an activation energy for superplastic flow which is within the range anticipated for grain boundary diffusion in pure Al and interdiffusion in Al−Mg solid solution alloys. MINORU NEMOTO, formerly Professor, Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University.     

Achieving enhanced ductility in a dilute magnesium alloy through severe plastic deformation

Experiments were conducted to evaluate the utility of a new processing procedure developed for Mg-based alloys in which samples are subjected to a two-step processing route of extrusion followed by equal-channel angular pressing (designated as EX-ECAP). The experiments were conducted using a Mg-0.6 wt pct Zr alloy and, for comparison purposes, samples of pure Mg. It is shown that the potential for successfully using ECAP increases in both materials when adopting the EX-ECAP procedure. For the Mg-Zr alloy, the use of EX-ECAP produces a grain size of ∼1.4 μm when the pressing is undertaken at 573 K. By contrast, using EX-ECAP with pure Mg at 573 K produces a grain size of ∼26 μm. Tensile testing of the Mg-Zr alloy at 523 and 573 K after processing by EX-ECAP revealed the occurrence of significantly enhanced ductilities with maximum elongations of ∼300 to 400 pct.     

Grain refinement and superplasticity in a magnesium alloy processed by equal-channel angular pressing

The extrusion/equal channel angular pressing (EX-ECAP) processing procedure, in which magnesium-based alloys are subjected to extrusion followed by ECAP, was applied to a Mg-7.5 pct Al-0.2 pct Zr alloy prepared by casting. Microstructural inspection showed the EX-ECAP process was effective in reducing the grain size from ∼21 μm after extrusion to an as-pressed grain size of ∼0.8 μm. It is shown through static annealing that these ultrafine grains are reasonably stable up to 473 K, but grain growth occurs at higher temperatures. Tensile specimens were cut from the billets prepared by EX-ECAP and testing showed these specimens exhibited superplasticity at relatively low temperatures with maximum elongations up to >700 pct. By processing through EX-ECAP to a higher imposed strain and thereby increasing the area fraction of high-angle boundaries, it is demonstrated that there is a potential for achieving high-strain-rate superplasticity. This article is based on a presentation made at the Symposium entitled “Phase Transformations and Deformation in Magnesium Alloys,” which occurred during the Spring TMS meeting, March 14–18, 2004, in Charlotte, NC, under the auspices of the ASM-MSCTS Phase Transformations Committee.     

Recovery Phenomenon During Annealing of an As-Rapidly Solidified Al Alloy

It has been well documented that recovery occurring in metals/alloys produced via solid-state quenching involves only annihilation of supersaturated vacancies. Interestingly, in the present study, we observed completely different mechanisms underlying recovery during annealing of an Al-Zn-Mg-Cu (7075 Al) alloy processed via liquid-state quenching, i.e., rapid solidification (specifically melt spinning herein). The as-melt-spun alloy consists of refined grains containing tangled dislocations inside the grains. Following annealing at 393 K (120 °C) for 24 hours, refined grain structure was still retained and grain sizes essentially remained unchanged, but subgrains separated by dense dislocation walls were generated at grain interiors, with a much lower density of dislocations at subgrain interiors than that in the as-melt-spun 7075 Al alloy and dislocation arrays inside some subgrains. The microstructural evolution suggests the absence of recrystallization and the occurrence of recovery primarily via the annihilation and rearrangement of dislocations and the formation of subgrains. Based on the stored energy in dislocations in, and the annealing temperature of, the as-melt-spun 7075 Al alloy, the recovery phenomenon was analyzed and discussed in detail.

     

Age hardening and the potential for superplasticity in a fine-grained Al-Mg-Li-Zr alloy

Experiments were conducted to determine the age-hardening characteristics and the mechanical properties of an Al-5.5 pct Mg-2.2 pct Li-0.12 pct Zr alloy processed by equal-channel angular (ECA) pressing to give a very fine grain size of ∼1.2 μm. The results show that peak aging occurs more rapidly when the grain size is very fine, and this effect is interpreted in terms of the higher volume of precipitate-free zones in the fine-grained material. Mechanical testing demonstrates that the ECA-pressed material exhibits high strength and good ductility at room temperature compared to conventional Al alloys containing Li. Elongations of up to ∼550 pct may be achieved at an elevated temperature of 603 K in the ECA-pressed condition, thereby confirming that, in this condition, the alloy may be a suitable candidate material for use in superplastic forming operations.     

Influence of pressing speed on microstructural development in equal-channel angular pressing

The influence of pressing speed in equal-channel angular (ECA) pressing was investigated using samples of pure Al and an Al-1 pct Mg alloy and a range of pressing speeds from ∼10⁻² to ∼10 mm s⁻¹. The results show that the speed of pressing has no significant influence on the equilibrium grain size, at least over the range used in these experiments. Thus, the equilibrium grain sizes were ∼1.2 μm for pure Al and ∼0.5 μm for the Al-1 pct Mg alloy for all pressing conditions. However, it is shown that the nature of the microstructure is dependent on the pressing speed, because recovery occurs more easily at the slower speeds, so that the microstructure is then more equilibrated. There is also indirect evidence for the advent of frictional effects when the cross-sectional dimensions of the samples are at or below ∼5 mm.     

Developing Superplastic Ductilities in Ultrafine-Grained Metals

Conventional superplasticity is generally achieved in metals having grain sizes in the range of ∼2 to 5 μm, but processing by equal-channel angular pressing (ECAP) provides the opportunity of introducing exceptional grain refinement and producing materials with ultrafine grain sizes in the submicrometer range. These materials have the potential for exhibiting excellent superplastic properties when tested in tension at elevated temperatures and examples are presented for representative aluminum and magnesium alloys. When these ultrafine-grained materials deform in superplasticity, internal cavities develop as in conventional superplastic alloys. An example is presented for an aluminum-based alloy, and it is shown that the cavity growth processes are also similar to those in conventional alloys. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: From Basics to Application”, which occurred September 25–27, 2006, in Kloster Irsee, Germany.     

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.     

Effect of Annealing on Thermal Stability, Precipitate Evolution, and Mechanical Properties of Cryorolled Al 7075 Alloy

The effect of annealing on microstructural stability, precipitate evolution, and mechanical properties of cryorolled (CR) Al 7075 alloy was investigated in the present work employing hardness measurements, tensile test, X-ray diffraction (XRD), differential scanning calorimetry (DSC), electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM). The solution-treated bulk Al 7075 alloy was subjected to cryorolling to produce fine grain structures and, subsequently, annealing treatment to investigate its thermal stability. The recrystallization of CR Al 7075 alloys started at an annealing temperature of 423 K (150 °C) and completed at an annealing temperature of 523 K (250 °C). The CR Al 7075 alloys with ultrafine-grained microstructure are thermally stable up to 623 K (350 °C). Within the range of 523 K to 623 K (250 °C to 350 °C), the size of small η phase particles and AlZr₃ dispersoids lies within 300 nm. These small precipitate particles pin the grain boundaries due to the Zener pinning effect, which suppresses grain growth. The hardness and tensile strength of the CR Al 7075 alloys was reduced during the annealing treatment from 423 K to 523 K (150 °C to 250 °C) and subsequently it remains constant.     

Factors influencing the equilibrium grain size in equal-channel angular pressing: Role of Mg additions to aluminum

Experiments were undertaken to compare the equal-channel angular (ECA) pressing of Al-1 pct Mg and Al-3 pct Mg solid-solution alloys with pure Al. The results reveal both similarities and differences between these three materials. Bands of subgrains are formed in all three materials in a single passage through the die, and these subgrains subsequently evolve, on further pressings through the die, into an array of grains with high-angle boundaries. However, the addition of magnesium to an aluminum matrix decreases the rate of recovery and this leads, with an increasing Mg content, both to an increase in the number of pressings required to establish a homogeneous microstructure and to a decrease in the ultimate equiaxed equilibrium grain size. It is concluded that alloys exhibiting low rates of recovery should be especially attractive candidate materials for establishing ultrafine structures through grain refinement using the ECA pressing technique.     

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.     

Effect of B on the microstructure and mechanical properties of mechanically milled TiAl alloys

The present study is concerned with γ-(Ti₅₂Al₄₈)_100−x B _x (x=0, 0.5, 2, 5) alloys produced by mechanical milling/vacuum hot pressing (VHPing) using melt-extracted powders. Microstructure of the as-vacuum hot pressed (VHPed) alloys exhibits a duplex equiaxed microstructure of α₂ and γ with a mean grain size of 200 nm. Besides α₂ and γ phases, binary and 0.5 pct B alloys contain Ti₂AlN and Al₂O₃ phases located along the grain boundaries and show appreciable coarsening in grain and dispersoid sizes during annealing treatment at 1300 °C for 5 hours. On the other hand, 2 pct B and 5 pct B alloys contain fine boride particles within the γ grains and show minimal coarsening during annealing. Room-temperature compressing tests of the as-VHPed alloys show low ductility, but very high yield strength >2100 MPa. After annealing treatment, mechanically milled alloys show much higher yield strength than conventional powder metallurgy and ingot metallurgy processed alloys, with equivalent ductility to ingot metallurgy processed alloys. The 5 pct B alloy with the smallest grain size shows higher yield strength than binary alloy up to the test temperature of 700 °C. At 850 °C, 5 pct B alloy shows much lower strength than the binary alloy, indicating that the deformation of fine 5 pct B alloy is dominated by the grain boundary sliding mechanism. 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.     

Mechanical Properties and Microstructural Characteristics of Friction Welded Dissimilar Joints of Aluminium Alloys

Joining of similar and dissimilar combinations of aluminium alloys 2024 and 6061 were performed using friction welding technique. Microstructure, hardness and tensile properties of the joints were characterized. Microstructure of the alloy were found to change significantly across the joint such as fully deformed, partially deformed and undeformed regions due to deformation, frictional heat and alloy characteristics. Extensive fine grain size was observed in the fully deformed region and volume fraction of finer grains was higher in the alloy 2024 as compared to alloy 6061. Hardness was lower in the weld interface region of the similar joints of AA2024 and AA6061. The lower hardness in the dissimilar metal joint was observed in the heat affected zone of alloy 6061. The tensile strengths of the similar joints were 80 and 85% of respective base metal of alloys 2024 and 6061. The strength of the dissimilar metal joint was observed to be similar to the base metal strength of 6061 alloy. Tensile fracture occurred in the region of joints where lower hardness was observed. The maximum elongation were obtained in dissimilar joints of alloys and characterized by scanning electron microscope. It revealed deep dimple patterns unlike what was observed in similar joints.     

Stress-corrosion cracking in equiaxed 7075

The environmental response of commercially produced high-strength Al alloys, such as 7075, depends strongly on the anisotropy of the grain structure. Minimum resistance to both stresscorrosion cracking (SCC) and hydrogen embrittlement is observed in the short transverse direction of the “pancake” grain structure in commercially produced alloys. It has not been established, however, exactly how the morphology of the grain structure mediates the SCC response or the SCC mechanism. Therefore, stress-corrosion testing of a high-purity 7075 Al alloy (low in Fe, Si, and Cr), having equiaxed grains, under tension (mode I) and torsion (mode III) loading in a solution of IN A1C1₃ has been performed. The SCC results in the two loading modes, including fractography, appeared to suggest that the predominant processes of SCC were hydrogen embrittlement in mode I and anodic dissolution in mode III, in agreement with prior work on a commercially produced 7075 alloy, but that severe corrosion during longer tests renders those results unsuitable for threshold determination in this very aggressive testing environment. Formerly with Carnegie Mellon University Formerly with Carnegie Mellon University     

Characterization and mechanical properties of ultrahigh boron steels produced by powder metallurgy

The present work is part of an investigation into the use of rapid solidification and powder metallurgy techniques to obtain iron-boron alloys with good mechanical properties. Two Fe-B binary alloys and two ultrahigh boron tool steels were gas atomized and consolidated by hot isostatic pressing (HIP) at temperatures ranging from 700 °C to 1100 °C to have a fine microstructure. Optimum properties were achieved for the binary alloys at low consolidation temperatures, since the solidification mi-crostructure from the original powders is eliminated and, at the same time, fine microstructures and low porosity are obtained in the alloys. At high temperatures and low strain rates, three of the four alloys exhibited low stress exponents, but only the Fe-2.2 pct B alloy showed tensile elongations higher than 100 pct. At low temperatures, only the Fe-2.2 pct B alloy deformed plastically. This alloy showed values of tensile elongation and ultimate tensile strength that were strongly dependent on testing and consolidation temperatures. J.A. JIMéNEZ, Postdoctoral Fellow, formerly with Centra Nacional de Investigaciones Metalúrgicas, C.S.I.C     

仿美7075高强度铝合金

介绍了国产仿美7075合金薄板,厚板,棒、管、型材,线材,锻件,模锻件样品的初步分析结果。仿美7075有三种合金成份:即等效美国的低纯7075,高纯7075,高纯7175。并制订了-T6、-T73、-T76状态的热处理制度。仿美7075的力学性能已达到美国7075技术标准。且低、高温力学性能稳定,缺口敏感性、周期强度、断裂韧性等主要综合性能较好。     

Early Stages of Recrystallization in Equal-Channel Angular Pressing (ECAP)-Deformed AA3104 Alloy Investigated Using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) Orientation Mappings

Early stages of recrystallization in alloys containing complex structure of second phase particles are of considerable practical interest. They were observed for the AA3104 alloy in which large particles occur against the background of randomly distributed dispersoids. The samples were deformed by equal channel angular pressing and then slightly annealed to obtain the state of partial recrystallization. The highly deformed alloy contained a structure of flat grains with the spacing between high-angle grain boundaries ranging from 100 nm to 1 ??m. On annealing, the structure coarsened and got transformed into nearly equiaxed grains by both discontinuous and continuous recrystallization. The nucleation of new grains was observed in statically recrystallized bulk samples using scanning electron microscopy, and during in-situ recrystallization in a transmission electron microscope. Special attention was paid to the nucleation of new grains in areas close to large second phase particles, where a relatively high stored energy was expected to stimulate nucleation. A particular role in the rise of nuclei is attributed to migration of low angle boundaries. During recrystallization at 623 K (350?°C), in most of the observed cases, the growth of grains occurred by coalescence of neighbouring cells and by migration of high-angle grain boundaries. These processes led to nearly equiaxed grains of similar size. Orientation mappings showed that although orientations of new grains were widely scattered, they were not completely random.     

Wetting of grain boundaries in hard-magnetic Nd-Fe-B alloys

Samples of Nd-Fe-B alloys, which have been the main hard-magnetic materials with the highest magnetic energy since the end of the 1980s, are investigated. Magnets based on them are obtained by liquid-phase sintering or spin coating. In this article, wetting of the Nd₂Fe₁₄B grains by the neodymium-enriched liquid phase is investigated. The microstructure of the Fe-12.3 at %Nd-7.6 at %B alloy quenched after annealing at T = 700?1100°C in the presence of a neodymium-enriched melt is studied. The acquired data indicate that the transition from incomplete to complete wetting of grain boundaries occurs as the temperature increases. The results are compared with the reference data for alloys of the Nd-Fe-B system obtained by liquid-phase sintering. The relation between the wetting phase transition of grain boundaries and magnetic properties is also discussed.