Texture Modification During Extrusion of Some Mg Alloys

The effects of extrusion ratio and alloying addition on the microstructure of Mg-0.2 wt pct Ce alloys are investigated by electron backscatter diffraction. The results show that in this alloy, texture randomization does not occur at high or low extrusion ratios but at a ratio of 25:1 at 400 °C. When extruded at the same temperature and extrusion ratio, Ca addition to Mg results in a weak nonbasal texture. In contrast, Mg-Al and Mg-3 wt pct Al-0.2 wt pct Ce alloys do not exhibit texture modification in single-pass extrusion. In the Mg-Al-Ce alloy, Ce and Al form Al₁₁Ce₃ particles, leaving little Ce solute in the matrix. The texture modifications in Mg-Ce or Mg-Ca alloys are related to the nature of the solid solution and consistent with dynamic strain aging during extrusion.     

The Serrated Flow Behavior of Mg-Gd(-Mn-Sc) Alloys

The tensile deformation behavior of extruded samples of Mg-0.8 pct Gd and Mg-0.8 pct Gd-0.5 pct Mn-0.45 pct Sc (at. pct) alloys has been studied. Both alloys exhibit serrated flow when they are tensile tested at temperatures ranging from 150 °C to 300 °C and at strain rates of 1.67 × 10⁻⁴ s⁻¹ to 1.67 × 10⁻² s⁻¹, and this serrated flow behavior is significantly affected by postextrusion heat treatments. Combined with observations made by transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP), the serrated flow is attributed to dynamic interactions between solute atoms and gliding dislocations. It is suggested that Gd atoms in the solid solution matrix of magnesium are mainly responsible for the serrations in the two alloys. The additions of Mn and Sc to the Mg-Gd alloy strengthen the dynamic solute-dislocation interactions and lead to a lower critical strain and larger stress drops of the serrated flow in the Mg-Gd-Mn-Sc alloy.     

The influence of Al3Zr dispersoids on the recrystallization of hot-deformed AA 7010 alloys

The recrystallization of AA 7010 alloys (Al-6 pct Zn-2.4 pct Mg-1.6 pct Cu-Zr) during solution treatment is investigated as a function of Zr content after deformation under conditions simulating hot rolling. The respective roles of the volume fraction and size of Al₃Zr dispersoids are characterized by additions of 0.05 to 0.12 pct Zr and suitable heat treatments. Plane strain compression (channeldie) tests at temperatures of 320 °C and 440 °C were conducted to strains of 1, and the samples subsequently solution-annealed at 470 °C. Recrystallization during this anneal was characterized by optical and electron microscopy and X-ray diffraction. The fraction recrystallized decreases with increasing Zr content, higher deformation temperature, and finer particle size. An original model based on the concept of the recrystallizable volume fraction is presented to predict the degree of recrystallization in materials characterized by spatially heterogeneous microstructures.     

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     

Microstructure and Impression Creep Characteristics of Cast Mg-5Sn-xBi Magnesium Alloys

The microstructure and creep behavior of a cast Mg-5Sn alloy with 1, 2, and 3 wt pct Bi additions were studied by impression tests in the temperature range 423 K to 523 K (150 °C to 250 °C) under punching stresses in the range 125 to 475 MPa for dwell times up to 3600 seconds. The alloy containing 3 wt pct Bi showed the lowest creep rates and, thus, the highest creep resistance among all materials tested. This is attributed to the favorable formation of the more thermally stable Mg₃Bi₂ intermetallic compound, the reduction in the volume fraction of the less stable Mg₂Sn phase, and the dissolution of Bi in the remaining Mg₂Sn particles. These particles strengthen both the matrix and grain boundaries during creep deformation of the investigated system. The creep behavior of the Mg-5Sn alloy can be divided into the low- and high-stress regimes, with the respective average stress exponents of 5.5 and 10.5 and activation energies of 98.3 and 163.5 kJ mol⁻¹. This is in contrast to the creep behavior of the Bi-containing alloys, which can be expressed by a single linear relationship over the whole stress and temperature ranges studied, yielding stress exponents in the range 7 to 8 and activation energies of 101.0 to 107.0 kJ mol⁻¹. Based on the obtained stress exponents and activation energies, it is proposed that the dominant creep mechanism in Mg-5Sn is pipe-diffusion controlled dislocation viscous glide the low-stress regime and dislocation climb creep with back stress in the high-stress regime. For the Mg-5Sn-xBi alloys, however, the controlling creep mechanism is dislocation climb with an additional particle strengthening effect, which is characterized by the higher stress exponent of 7 to 8.     

Serrated Flow and Enhanced Ductility in Coarse-Grained Al-Mg Alloys

Conventionally, superplasticity requires the presence of a fine-grained microstructure to enable grain-boundary sliding to take place during deformation. However, coarse-grained materials have also been shown to exhibit higher than normal amounts of ductility, provided they possess a high-enough strain rate sensitivity. In this work, coarse-grained Al-3 pct Mg, Al-5 pct Mg, and AA 5056 alloys were tested for enhanced ductility. The dependence of flow stress on temperature was found to display some unusual characteristics; these were interpreted as resulting from the occurrence of dynamic strain aging (DSA). In these materials, a local peak in elongation coincided with the presence of an unusual peak in rate sensitivity. This region of higher than normal rate sensitivity was coupled with the usual region of negative rate sensitivity found in DSA-prone materials, such as the Al-Mg alloys. A maximum ductility of 170 pct was recorded at 723 K (450 °C) and a strain rate of 5 × 10⁻² seconds⁻¹ was found in the vicinity of the rate sensitivity peak. This was found to increase to nearly 300 pct when the gage length was shortened. These peaks in elongation occurred below the maximum test temperatures.     

The effect of alloying additions on the superplastic properties of Ti-6 pct Al-4 pct V

The superplastic deformation properties of Ti-6 pct Al-4 pct V and modified alloys containing 1/4 pct, 1/2 pct, 1 pct, and 2 pct of either cobalt or nickel have been investigated in the temperature range 950 to 750 °C. The results show that both cobalt and nickel modified alloys have reduced flow stresses, in comparison with Ti-6 pct Al-4 pct V, the reductions being particularly marked at the lower temperatures and lower strain rates. The results are shown to be consistent with an isostress model for the deformation of (α + β) two-phase alloys in which the varying β volume fractions and differing diffusivities of titanium, cobalt, or nickel in the β phase are taken into account.     

Investigation of the Effects of Ni, Fe, and Mn on the Formation of Complex Intermetallic Compounds in Al-Si-Cu-Mg-Ni Alloys

The aim of this work is to partially substitute Fe and Mn for Ni in the 3HA piston alloy and to study the consequences through microstructural evaluation and the thermal analysis technique. Three types of near-eutectic alloys containing (2.6 wt pct Ni-0.2 wt pct Fe-0.1 wt pct Mn), (1.8 wt pct Ni-0.75 wt pct Fe-0.3 wt pct Mn), and (1 wt pct Ni-1.15 wt pct Fe-0.6 wt pct Mn) were produced, and their solidification was studied at the cooling rate of 0.9 K/s (°C/s) using the computer-aided thermal analysis technique. Optical microscopy and scanning electron microscopy were used to study the microstructure of the samples, and energy dispersive X-ray (EDX) analysis was used to identify the composition of the phases. Also, the quantity of the phases was measured using the image analysis technique. The results show that Ni mainly participates as Al₃Ni, Al₉FeNi, and Al₃CuNi phases in the high Ni-containing alloy (2.6 wt pct Ni). In addition, substitution of Ni by Fe and Mn makes Al₉FeNi the only Ni-rich phase, and Al₁₂(Fe,Mn)₃Si₂ appears as an important Fe-rich intermetallic compound in the alloys with the higher Fe and Mn contents.     

Solid Solution Strengthening for Mg-3.0 Mass?Pct (2.71 At.?Pct)Al and Mg-0.06 Mass?Pct (0.036 At.?Pct)Ca Alloys

Tensile tests were carried out at 123 K to 373 K (–150 °C to 100 °C) on pure Mg, Mg-3.0 mass pct (2.71 at. pct) Al alloy, and Mg-0.06 mass pct (0.036 at. pct) Ca alloy. Little decrease occurred in the yield stress of the pure Mg and the Mg-Ca alloy with increasing temperature from 223 K to 373 K (–50 °C to 100 °C). For the Mg-Al alloy, however, its yield stress decreased with increasing temperature from 223 K to 373 K (–50 °C to 100 °C). Analyses based on the existing solid-solution strengthening theories, focusing on the athermal component of stress, revealed that the dominant strengthening mechanism is the shear modulus effect for the Mg-Ca alloy and the chemical interaction for the Mg-Al alloy. It is suggested that the shear modulus effect is dominant at a low concentration and the chemical interaction is dominant at a high concentration for Mg alloys.     

Thermal-Treated Pitches as Binders for TiB2/C Composite Cathodes

The effect of thermal treatment on the microstructure and properties of pitches and thermal-treated, pitch-based TiB₂/C composite cathodes were investigated. Thermal treatments were performed at 473 K, 523 K, 573 K, 623 K, and 673 K (200 °C, 250 °C, 300 °C, 350 °C, and 400 °C), respectively. The results show that the aromaticity of the treated pitches increases with an increasing thermal treatment temperature, and subsequently, the coking value and quinoline-insoluble (QI) content increase from 60.62 wt pct to 79.09 wt. pct and from 8.97 wt pct to 32.54 wt pct when the treatment temperature increases from 473 K to 623 K (200 °C to 350 °C). The volume fraction of coalesced mesophase in semicoke decreases with an increasing thermal treatment temperature, and after 673 K (400 °C) is reached, the coalesced mesophase is almost invisible. The bulk density and compressive strength of modified pitch-based cathodes increase with an increasing thermal treatment temperature from 2.24 g cm⁻³ to 2.39 g cm⁻³ and from 24.21 MPa to 54.85 MPa, whereas open porosity decreases from 34.62 pct to 27.06 pct. Both electrical resistivity and electrolysis expansion ratio first decrease and then increase with an increasing thermal treatment temperature, and the lowest values (45.63 μΩ m and 0.65 pct) are achieved at 573 K (300 °C). Compared with those of the parent pitch-based cathode, the properties of the modified pitch-based cathodes had improved significantly. The mechanisms of the improvements are discussed in the text.     

Deformation behavior of dilute SnBi(0.5 to 6 at. pct) solid solutions

Tensile and creep tests were conducted to characterize the deformation behavior of four dilute SnBi alloys: SnBi0.5 at. pct, SnBi1.5 at. pct, SnBi3 at. pct, and SnBi6 at. pct, the last two being supersaturated solid solutions at room temperature. The test temperatures were − 20 °C, 23 °C, 90 °C, and 150 °C, and the strain rates ranged from approximately 10⁻⁸ to 10⁻¹ 1/s. In the tensile tests, all the alloys showed strain-hardening behavior up to room temperature. At higher temperatures, only the higher-Bi-content alloys exhibited strain softening. The deformation behavior of the alloys can be divided into two stress regimes, and the change from the low-stress regime to the high-stress regime occurred at around 6 × 10⁻⁴<σ/E<7.5 × 10⁻⁴. The results suggest that, at the low-stress regime, the rate-controlling deformation mechanism changes from dislocation climb to viscous glide with the increasing Bi content of the alloy. At the high-stress regime, the activation energy of deformation is about equal in all the alloys (∼60 kJ/mol) and the stress exponents are high (10<n<12.5). Unlike in the other alloys, bismuth precipitated at room temperature from the solution-annealed and quenched SnBi6 at. pct alloy by the discontinuous mechanism. This strongly affects the mechanical properties and makes the alloy brittle at lower test temperatures. A comparison of the deformation behavior of the dilute SnBi alloys to that of the eutectic SnBi alloy suggests that the deformation of eutectic structure is controlled by the Sn-rich phase containing the equilibrium amount of dissolved Bi.     

Studies on hot deformation of sintered cobalt

The hot deformation behavior of sintered cobalt powder was studied. The Co powder prepared by thermal decomposition of cobalt oxalate was subsequently compacted by cold isostatic pressing (CIP) and sintered at 1300 °C under H₂ atmosphere. Cobalt rods of 95 pct theoretical density were obtained. Strain rate change tests in compression were conducted in the temperature range of 900 °C to 1300 °C by changing strain rates from 0.001 to 3.2 s⁻¹. Uninterrupted hot compression tests at constant strain rates and selective temperatures were also conducted. Microcracks as well as surface cracks were observed in the samples tested below 1200 °C. It was observed that the strain rate sensitivity (SRS) increased with increasing temperature and decreasing strain rate, with the maximum SRS of 0.3 being obtained at 1285 °C and strain rate of 10⁻³ s⁻¹. Despite the higher SRS at low strain rates, the hot workability of sintered cobalt was found to be poor. Extensive grain boundary microcracking was observed, with the density being lowered after deformation. However, the samples tested at higher strain rates showed less microcracking and an increase in density. On the basis of the results, it was concluded that ease of grain boundary sliding at lower strain rates and higher temperatures was responsible for the poor workability at these conditions.     

Modeling of Grain Size Distributions during Single Hit Deformation of a Nb-Containing Steel

A homogenized (to remove segregation effects from continuous casting) 0.046 wt pct Nb-microalloyed steel was used to examine the validity of the Dutta–Sellars equation in describing the degree of recrystallization, grain size distributions, and the influence of precipitation at this Nb level for deformation between 1248 and 1348 K (975 and 1075 °C). Using the Dutta–Sellars equations with the mode grain size overestimated the degree of recrystallization for deformation at temperatures above 1283 K (1010 °C), while modeling the behavior for individual grain size classes gave better agreement for the degree of recrystallization and also predicted the resulting grain size distributions. Within the conditions studied (reaustenitization at 1498 K (1225 °C) and single hit thermomechanical simulation with 0.3 strain, 10-second hold), there was no effect of strain-induced precipitation on the extent of recrystallization detected.     

Selective Oxidation and Reactive Wetting of 1.0 Pct Si-0.5 Pct Al and 1.5 Pct Si TRIP-Assisted Steels

Heat treatments were performed using an isothermal bainitic transformation (IBT) temperature compatible with continuous hot-dip galvanizing on two high Al–low Si transformation induced plasticity (TRIP)-assisted steels. Both steels had 0.2 wt pct C and 1.5 wt pct Mn; one had 1.5 wt pct Al and the other had 1 wt pct Al and 0.5 wt pct Si. Two different intercritical annealing (IA) temperatures were used, resulting in intercritical microstructures of 50 pct ferrite (α)-50 pct austenite (γ) and 65 pct α-35 pct γ. Using the IBT temperature of 465 °C, five IBT times were tested: 4, 30, 60, 90, and 120 seconds. Increasing the IBT time resulted in a decrease in the ultimate tensile strength (UTS) and an increase in the uniform elongation, yield strength, and yield point elongation. The uniform elongation was higher when using the 50 pct α-50 pct γ IA temperature when compared to the 65 pct α-35 pct γ IA temperature. The best combinations of strength and ductility and their corresponding heat treatments were as follows: a tensile strength of 895 MPa and uniform elongation of 0.26 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 90-second IBT time; a tensile strength of 880 MPa and uniform elongation of 0.27 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 120-second IBT time; and a tensile strength of 1009 MPa and uniform elongation of 0.22 for the 1 pct Al-0.5 pct Si TRIP steel at the 50 pct γ IA temperature and 120-second IBT time.     

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.     

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.     

High Strength and High Ductility of Ultrafine-Grained,Interstitial-Free Steel Produced by ECAE and Annealing

Interstitial-free steel (IF steel) underwent severe plastic deformation by equal-channel angular extrusion/pressing (ECAE/P) to improve its strength, and then it was annealed to achieve a good strength-ductility balance. The coarse-grained microstructure of IF steel was refined down to the submicron level after eight-pass ECAE. The ultrafine-grained (UFG) microstructure with high dislocation density brought about substantially improved strength but limited tensile ductility. The limited ductility was attributed to the small, uniform elongation caused by early plastic instability. The annealing at temperatures below 723 K (450 °C) for 1 hour did not lead to remarkable softening, whereas annealing at temperatures up to 923 K (650 °C) resulted in complete softening depending on the development of recrystallization. Therefore, the temperature of approximately 923 K (650 °C) can be considered as a critical recrystallization temperature for UFG IF steel. The annealing at 873 K (600 °C) for different time intervals resulted in different stress–strain response. Uniform tensile elongation increased at the expense of strength with annealing time intervals. After annealing at 873 K (600 °C) for 60 minutes, the yield strength, tensile strength, uniform elongation, and total elongation were found to be 320 MPa, 485 MPa, 15.1 pct, and 33.7 pct, respectively, showing the better combination of strength and ductility compared with cold-rolled samples.     

Stability and mechanical properties of some metastable austenitic steels

The relation between austenite stability and the tensile properties, as affected by testing temperature and processing, was studied for a series of alloys of increasing compositional complexity, viz., the Fe-Ni, Fe-Ni-C, and Fe-Ni-Cr-Mn-C systems. The “stress” and “strain induced” modes of transformation to martensite differed significantly in their influence on the shape of the stress-strain curve. Under certain testing conditions, unusually low yield strengths and high work hardening rates were observed in some of these alloys. Maxima in yield strengths were observed for all austenitic alloys containing carbon that were processed at deformation temperatures between 200° and 300°C. Evidence gleaned from electron microscopy and magnetic and mechanical testing suggested that the maxima were due to the formation of carbon atmospheres on dislocations during processing. The influence of austenite stability on the mechanical properties of steels, varied by systematic changes in test temperature (22° to -196°C), composition (8 pct, 12 pct, 16 pct, and 21 pct Ni) and deformation temperature (25° to 450°C), was evaluated quantitatively. An erratum to this article is available at .     

Alumina Solubility and Diffusion Coefficient of the Dissolved Alumina Species in Low-Temperature Fluoride Electrolytes

The solubility of alumina was measured by rotating an alumina cylinder (~500 rpm) in a high-purity melt for ~3 to 6 hours, crushing and sampling the frozen melt, and determining the oxygen content in a Leco analyzer. The alumina solubilities determined were as follows: (1) 3.2 ± 0.3 wt pct in NaF-AlF₃ eutectic at 1023 K (750 °C); (2) 3.0 ± 0.3 wt pct in NaF-AlF₃-CaF₂ (5 wt pct) at 1023 K (750 °C); and (3) 5.2 ± 0.5 wt pct in a KF-AlF₃ eutectic at 1003 K (730 °C). The alumina solubility in the KF-AlF₃ eutectic was 2 wt pct more than in the sodium analogue, offering the possibility of operating a low-temperature aluminum smelting cell without the need for an alumina slurry. The diffusion coefficient of the dissolved alumina species was determined in the NaF-AlF₃ eutectic at 1023 K (750 °C) using the rotating disc method and applying the Levich equation. Through a limited range of rotation rates, the system seemed to be mass-transfer controlled, and the diffusion coefficient was estimated to be in the range 1.8 to 2.2 × 10⁻⁶ cm² s⁻¹. This value is about five times lower than the values encountered at traditional aluminum smelting temperatures (~1233 K (960 °C)) and would result in relatively low mass transfer coefficients.