Mechanical behavior of a cryomilled near-nanostructured Al-Mg-Sc alloy

In the present study, the mechanical behavior of a cryomilled Al-7.5 pct Mg-0.3 pct Sc alloy was investigated at temperatures in the range of 298 to 648 K. The grain size of the as-extruded alloy was determined to be approximately 200 nm by transmission electron microscope (TEM) and X-ray diffraction (XRD) analysis. The data indicate that as a result of cryomilling, a supersaturated solid solution with high thermal stability was formed in the Al-Mg-Sc alloy. The high strength at room temperature was primarily attributed to three types of strengthening: grain size effect, solid solution hardening, and Orowan strengthening. The elevated temperature mechanical behavior of the Al-Mg-Sc alloy exhibits the following: (a) a strain-rate sensitivity, m, of less than 0.2; and (b) an activation energy, Q, that increases from 139 to 193 kJ/mol with increasing applied stress. An analysis of the experimental data at elevated temperatures shows that despite the fine-grained structure of the alloy, the deformation characteristics are not consistent with those arising from a superplastic deformation process that incorporates a threshold stress. On the other hand, the analysis suggests that the deformation characteristics agree with those associated with the transition in the creep behavior of Al-based solid solution alloys from that for the intermediate-stress region, where m=0.33 and Q=Q _D (Q _D is the activation energy for self-diffusion in Al), to that of the high-stress region, where m<0.2 and Q>Q _D .     

On the measurements of superplasticity in an Al-Cu alloy

The usual method of measuring the strain rate sensitive ‘m’ values of superplastic materials through differential cross-head speed is found to result in improperly definedm values;m is found to depend strongly on the strain to which the material is subjected, especially at low strains. In this connection, the shape of the log stress-log strain rate curve is examined for the Al-33 wt pct Cu eutectic alloy. The inherent grain growth of the very fine grains which occurs during deformation, and the strain dependence ofm at low strains, are shown to be the causes of the familiarS shape of the log stress-log strain rate curves for the Al-Cu alloy. At high strains (15 to 20 pct and higher) where the stress is no longer importantly strain sensitive, the log stress-log strain rate curve is a straight line of slope near 0.5. The elongation at fracture also does not go through a maximum but continues to increase slowly to the lowest strain rate examined: 10^-7 per s.     

On the measurements of superplasticity in an Al-Cu alloy

The usual method of measuring the strain rate sensitive ‘m’ values of superplastic materials through differential cross-head speed is found to result in improperly definedm values;m is found to depend strongly on the strain to which the material is subjected, especially at low strains. In this connection, the shape of the log stress-log strain rate curve is examined for the Al-33 wt pct Cu eutectic alloy. The inherent grain growth of the very fine grains which occurs during deformation, and the strain dependence ofm at low strains, are shown to be the causes of the familiarS shape of the log stress-log strain rate curves for the Al-Cu alloy. At high strains (15 to 20 pct and higher) where the stress is no longer importantly strain sensitive, the log stress-log strain rate curve is a straight line of slope near 0.5. The elongation at fracture also does not go through a maximum but continues to increase slowly to the lowest strain rate examined: 10^-7 per s. Formerly Research Assistant, Department of Metallurgy and Materials Science, MIT.     

Deformation mechanisms during low-and high-temperature superplasticity in 5083 Al-Mg alloy

The controlling deformation mechanisms and grain boundary sliding behavior during low-, medium-, and high-temperature superplasticity (LTSP, MTSP, and HTSP) in fine-grained 5083 Al-Mg base alloys are systematically examined as a function of strain. Grain boundary sliding was observed to proceed at temperatures as low as 200 °C. With increasing LTSP straining from the initial (ε<0.5) to later stages (ε>1.0), the strain rate sensitivity m, plastic anisotropy factor R, high-angle grain boundary fraction, grain size exponent p, and grain boundary sliding contribution all increased. During the initial LTSP stage, there was little grain size dependence and the primary deformation mechanisms were solute drag creep plus minor power-law creep. At later stages, grain size dependence increased and grain boundary sliding gradually controlled the deformation. During MTSP and HTSP, solute drag creep and grain boundary sliding were the dominant deformation mechanisms.     

An analysis of cavity growth during superplasticity

Cavity growth at high temperatures may be controlled by vacancy diffusion, giving cavities which are approximately spherical and randomly distributed, or by power-law creep, giving cavities which are elongated and aligned in the direction of the tensile stress. In general, diffusion growth is favored at low total strains, and there is a transition to power-law growth at a critical cavity radius,r _c. The value ofr _c increases with decreasing strain-rate, so that there is also a transition from predominanly power-law growth at high stress levels to predominantly diffusion growth at low stress levels. Both types of cavities have been observed in superplastic materials, but the diffusion growth rate may be enhanced if the cavity intersects a number of grain boundaries. The analysis is in good agreement with experimental results reported for three diffent superplastic materials. DAVID A. MILLER, formerly Research Associate, Department of Materials Science, University of Southern California.     

An investigation by interactive electron backscatter pattern analysis of processing and superplasticity in an aluminum-magnesium alloy

Tensile specimens from an Al-lOMg-0.lZr alloy, thermomechanically processed (TMP) according to two different schedules and deformed superplastically at 300 °C, were examined using electron backscatter pattern (EBSP) imaging in the scanning electron microscope (SEM) with computer-aided analysis. The TMP schedules differed only in the duration of the interpass anneal (IPA) time between successive rolling passes. Microtexture, grain misorientation angle distribution, and grain boundary character were analyzed for both undeformed grip and deformed gage sections of these tensile specimens. Microtexture analysis revealed the same texture components, primarily brass and S, in the undeformed grip sections of both TMP materials, while analysis of deformed gage sections revealed only a decrease in texture with no new components evident. Material processed with a prolonged IPA time exhibited highly superplastic response and contained a high proportion of boundaries of 5 to 15 deg misorientation. Material processed with a short IPA time exhibited lower superplastic elongations and contained higher-order twin boundaries, suggesting that the twin chain mechanism is active during TMP. The observed difference in tensile behavior appears to be the result of extended recovery during TMP of the more ductile material evidenced by the development of a greater proportion of boundaries of 5 to 15 deg misorientation.     

Characterization and modeling of the high temperature flow behavior of aluminum alloy 2024

Isothermal flow curves were determined for aluminum alloy 2024-0 at temperatures of 145 to 482 °C and at constant true-strain rates of 10^-3 to 12.5 s^-1 using compression tests of cylindrical specimens. The average pressure was corrected for friction and for deformation heating to determine the flow stress. At 250 °C and above, the isothermal flow curves usually exhibited a peak followed by flow softening. At 145 °C the flow curves exhibited strain hardening. For 250 °C≦ T<= 482 °C, 10^-3 s^-1 ≦ ≦ 12.5 s^-1, and ε ≦ 0.6 the flow behavior was represented by the constitutive equation σ =K (T, ε) where logK andm are simple functions of temperature and strain. The as-deformed microstructures generally supported the idea that flow softening in Al 2024-0 is caused by dynamic recovery. At the higher temperatures and strain rates, however, fine recrystallized grains were observed in local areas near second phase particles and at as-annealed grain boundaries. At 482 °C, there was evidence of re-dissolution of the CuMgAl₂ precipitate. Formerly Visiting Associate Professor, Wright State University, Dayton, OH 45435 Formerly a Mechanical Systems Engineering Student at Wright State University Formerly a Materials Engineering Student at Wright State University Formerly Director, Metallurgy Program, National Science Foundation, Washington, DC     

Superplastic behavior and microstructure evolution in a commercial Al-Mg-Sc alloy subjected to intense plastic straining

A commercial Al-6 pct Mg-0.3 pct Sc-0.3 pct Mn alloy subjected to equal-channel angular extrusion (ECAE) at 325 °C to a total strain of about 16 resulted in an average grain size of about 1 μm. Superplastic properties and microstructural evolution of the alloy were studied in tension at strain rates ranging from 1.4 × 10⁻⁵ to 1.4 s⁻¹ in the temperature interval 250 °C to 500 °C. It was shown that this alloy exhibited superior superplastic properties in the wide temperature range 250 °C to 500 °C at strain rates higher than 10⁻² s⁻¹. The highest elongation to failure of 2000 pct was attained at a temperature of 450 °C and an initial strain rate of 5.6 × 10⁻² s⁻¹ with the corresponding strain rate sensitivity coefficient of 0.46. An increase in temperature from 250 °C to 500 °C resulted in a shift of the optimal strain rate for superplasticity, at which highest ductility appeared, to higher strain rates. Superior superplastic properties of the commercial Al-Mg-Sc alloy are attributed to high stability of ultrafine grain structure under static annealing and superplastic deformation at T ≤ 450 °C. Two different fracture mechanisms were revealed. At temperatures higher than 300 °C or strain rates less than 10⁻¹ s⁻¹, failure took place in a brittle manner almost without necking, and cavitation played a major role in the failure. In contrast, at low temperatures or high strain rates, fracture occurred in a ductile manner by localized necking. The results suggest that the development of ultrafine-grained structure in the commercial Al-Mg-Sc alloy enables superplastic deformation at high strain rates and low temperatures, making the process of superplastic forming commercially attractive for the fabrication of high-volume components.     

The strain rate dependent plastic flow behavior of zirconium and its alloys

The effect of strain rate (10⁻⁵ { } 10⁻¹ min⁻¹) on the plastic flow behavior was examined by means of tension tests over a range of temperatures (RT { } 500°C). The principal material examined was Zircaloy-2, and the others were iodide zirconium and binary alloys of Zr-0.1 pct O and Zr-1.5 pct Sn by weight. In Zircaloy-2, the behavior was characterized by a sudden increase of flow strength with decreasing strain rate; concurrently the ductility decreased. Other interrupted and hold time experiments showed that the process is essentially that of a strain rate-induced strengthening phenomenon, with several features which could be identified with the strain aging process. It was further shown that the critical range of strain rates and temperatures at which the anomalous behavior took place could be correlated with the minimum in the strain rate sensitivity of flow stress. A similar but less pronounced flow behavior was observed with pure zirconium and its binary alloys, but with no marked change in the tensile ductility. From these results, the role of oxygen and tin on the early stage of plastic flow behavior was discussed in terms of dislocation-impurity interaction mechanisms. It was however concluded that these elements are not directly responsible for the ductility loss at slow strain rate in Zircaloy-2.     

An investigation of the effect of texture on the high-temperature flow behavior of an orthorhombic titanium aluminide alloy

The effect of mechanical and crystallographic texture on the flow properties of a Ti-21Al-22Nb (at. pct) sheet alloy was determined by conducting uniaxial tension and plane-strain compression tests at temperatures between 900°C and 1060°C and strain rates between 10⁻⁴ and 10⁻² s⁻². Despite the presence of noticeable initial texture, all of the mechanical properties for a given test temperatur and strain rate (i.e., peak stress, total elongation to failure, strain-rate sensitivity, and normal plastic anisotropy), were essentially identical irrespective of test direction relative to the rolling direction of the sheet. The absence of an effect of Mechanical texture on properties such as ductility was explained by the following: (1) the initially elongated second-phase particles break up during tension tests parallel to the rolling direction of the sheet, thereby producing a globular morphology similar to that noted in samples taken transverse to the rolling direction; and (2) failure was flow localization, rather than fracture, controlled. Similarly, the absence of an effect of mechanical texture on strain-rate sensitivity (m values), normal plastic anisotropy (r values), and the ratio of the plane strain to uniaxial flow stresses was rationalized on the basis of the dominance of matrix (dislocation) slip processes within the ordered beta phase (B2) as opposed to grain boundary sliding. Aggregate theory predictions supported this conclusion inasmuch as the crystallo graphic texture components determined for the B2 phase ((001) [100] and (−112) [110]) would each produce identical r values and uniaxial and plane-strain flow stresses in the rolling and transverse directions.     

An investigation of the effect of texture on the high-temperature flow behavior of an orthorhombic titanium aluminide alloy

The effect of mechanical and crystallographic texture on the flow properties of a Ti-21Al-22Nb (at. pct) sheet alloy was determined by conducting uniaxial tension and plane-strain compression tests at temperatures between 900 °C and 1060 °C and strain rates between 10⁻⁴ and 10⁻² s⁻¹. Despite the presence of noticeable initial texture, all of the mechanical properties for a given test temperature and strain rate (i.e., peak stress, total elongation to failure, strain-rate sensitivity, and normal plastic anisotropy) were essentially identical irrespective of test direction relative to the rolling direction of the sheet. The absence of an effect of mechanical texture on properties such as ductility was explained by the following: (1) the initially elongated second-phase particles break up during tension tests parallel to the rolling direction of the sheet, thereby producing a globular morphology similar to that noted in samples taken transverse to the rolling direction; and (2) failure was flow localization, rather than fracture, controlled. Similarly, the absence of an effect of mechanical texture on strain-rate sensitivity (m values), normal plastic anisotropy (r values), and the ratio of the plane strain to uniaxial flow stresses was rationalized on the basis of the dominance of matrix (dislocation) slip processes within the ordered beta phase (B2) as opposed to grain boundary sliding. Aggregate theory predictions supported this conclusion inasmuch as the crystallographic texture components determined for the B2 phase ((001) [100] and ( 12) [110]) would each produce identical r values and uniaxial and plane-strain flow stresses in the rolling and transverse directions.     

Nanostructure in an Al-Mg-Sc alloy processed by low-energy ball milling at cryogenic temperature

Spray-atomized Al-7.5Mg-0.3Sc (in wt pct) alloy powders were mechanically milled at a low-energy level and at cryogenic temperature (cryomilling). The low-energy milling effectively generated a nanoscale microstructure of a supersaturated face-centered cubic (fcc) solid solution with an average grain size of ∼26 nm. The nanoscale microstructure was fully characterized and the associated formation mechanisms were investigated. Two distinct nanostructures were identified by transmission electron microscopy (TEM) observations. Most frequently, the structure was comprised of randomly oriented equiaxed grains, typically 10 to 30 nm in diameter. Occasionally, a lamellar structure was observed in which the lamellas were 100 to 200 nm in length and ∼24 nm wide. The morphology of the mixed nanostructures in the cryomilled samples indicated that high-angle grain boundaries (HAGBs) formed by a grain subdivision mechanism, a process similar to which occurs in heavily cold-rolled materials. The microstructural evidence suggests that the subdivision mechanism observed here governs the development of fine-grain microstructures during low-energy milling.     

应变速率对18Cr-12Mn-0.55N高氮奥氏体不锈钢塑性流变行为的影响

研究了拉伸应变速率对高氮奥氏体不锈钢18Cr-12Mn-0.55N(质量分数/%)室温力学性能和塑性流变行为的影响.结果表明,随应变速率的升高,实验钢的屈服强度R0.2增大,断后延伸率A减小,抗拉强度Rm略有降低,断面收缩率Z变化不大;在各应变速率下,实验钢的塑性流变行为均可用Ludwigson模型进行描述;随应变速率的升高,实验钢的加工硬化能力和发生屈服时第一根位错开动所需的短程作用力降低;增大应变速率促进多系滑移和交滑移,降低瞬变应变,使实验钢的塑性流变行为在更低的应变水平符合Ludwik模型.