Application of hot workability studies to extrusion processing: Part II. Microstructural development and extrusion of Al,Al-Mg,AND Al-Mg-Mn Alloys

In uniform hot working tests, the flow curves strain harden to a steady state plateau that depends on temperature and strain rate. The underlying mechanism is dynamic recovery which rises with strain to balance the work hardening, resulting in subgrains that are larger and more perfect as T rises and ge falls. Subgrains are observed in the extrudate following the pattern established in hot workability tests. Moreover, their formation in the upset billet and throughout the deformation zone has been examined as a means to delineate its thermal and mechanical development. Substructure in the extrudate substantially improves its mechanical properties. With increasing Mg solute, substructure recovery is reduced so that the strength and the tendency for recrystallization are increased. Moreover, in alloys above 5% Mg, most notably containing large Mn-rich constituent particles, dynamic recrystallization nuclei form in the most intensely strained regions of the deformation zone and also speed up static recrystallization in the extrudate.     

The mechanical nature of stress-corrosion cracking in Al-Zn-Mg alloys: II. electrochemical-mechanical model

Slow crack growth during SCC of 7075 aluminum has been shown to comprise both an electrochemical and a mechanical component. These findings prompted a review of several possible mechanical models, and seven possible controlling thermally-activated processes. Since no existing interpretation could satisfy all of the observations, an empirical model was developed. The conclusion is that slight modification of many existing proposed mechanisms could explain the general features of SCC but that any theoretical model must contain some aspect of the mechanical rupture process. Formerly with the Lawrence Radiation Laboratory, Berkeley Formerly with the Lawrence Radiation Laboratory Berkeley     

热挤压和旋锻粉末冶金纯钛的组织和力学性能

采用粉末冶金技术结合热挤压和旋锻工艺制备纯钛棒,利用万能试验机、维氏显微硬度仪、金相显微镜、高精度多功能密度计等设备测试纯钛棒的屈服强度、维氏硬度、显微组织和相对密度,研究了纯钛棒的制备工艺及其微观组织结构对材料力学性能的影响。研究表明,利用粉末冶金技术结合热挤压和旋锻工艺制备的纯钛棒屈服强度是880 MPa,均匀延伸率是4.06%,在拉伸变形过程中发生韧性断裂。纯钛棒显微组织为等轴状的细晶粒组织,平均晶粒尺寸约1μm,组织分布均匀,无明显裂纹和缺陷,有较高的相对密度。     

Modeling the microstructural changes during hot tandem rolling of AA5XXX aluminum alloys: Part III. Overall model development and validation

Part III of this article presents the overall mathematical development for the microstructural and textural evolution during industrial hot tandem rolling of AA5182 and AA5052 alloys and validation of the mathematical model, by comparison to both industrial data and information from the literature. The model consists of a plasticity module to simulate the temperature and deformation in the roll bite and an interstand module to characterize the changing microstructure, texture, and temperature in the strip between the rolling stands. The plasticity module was developed using a commercial finite-element package, DEFORM, a two-dimensional transient Lagrangian model which couples the thermal and deformation phenomena and is able to predict the temperature, strain rate, and strain distribution in the strip at any position in the roll bite. The interstand module incorporates semiempirical equations, developed in this study, which quantify the microstructural (percent recrystallization and recrystallized grain size) and textural changes in the strip between the rolling stands. The interstand model also includes a temperature module to predict the through-thickness temperature distribution in the strip based on one-dimensional heat conduction. Validation of the model against industrial data indicated that it gave reasonable predictions for the temperature, grain size, and volume fraction of some of the deformation texture components after recrystallization was completed. However, the model overestimated the mill loads in the last stands for both the AA5182 and AA5052 alloys and underestimated the amount of cube ({100}〈uvw〉) and S ({123}〈634〉) texture in the recrystallized strip.     

Modeling of gas-liquid reactions in ladle metallurgy: Part I. Physical modeling

A physical model of a ladle degassing operation was developed to simulate the reactions at rising bubbles and at the free surface. Carbon dioxide desorption from a sodium hydroxide solution was used to simulate the liquid-phase diffusion-controlled decarburization of liquid steel. It was found that under reduced pressure, the reactions were faster than attributable to solely the increase in volumetric flow rate. It was possible to separate the reactions with the bubbles from the free surface reactions; 20 to 40 pct of the reactions occurred at the free surface, depending on injection conditions. The free surface desorption rate depended on the gas flow rate and the number of injectors. The mass transfer coefficients to the bubbles were in reasonable agreement with previous work. Plume bending was observed when small bubbles were influenced by the bulk liquid flow patterns.     

Manganese metallurgy review. Part III: Manganese control in zinc and copper electrolytes

Manganese is often associated with zinc and copper minerals, and can build up in the processing circuits. Part III of the review outlines the current practice and new developments to get a better understanding of manganese behaviour and control in electrowinning of zinc and copper, and identifies suitable methods and processes to control manganese.In zinc electrowinning, the presence of small amounts of manganese (1–5 g/L) can minimise the corrosion rate of the anodes and reduce the contamination of the cathodic zinc with lead, but excess manganese results in significant decreases in the current efficiency. The neutralized zinc feed solution that contains little acid is considered to be the best place to implement manganese control. Various methods and processes for manganese control in zinc electrowinning have been developed. Oxidative precipitation and solvent extraction are the most important methods. For the neutralized zinc solution at pH 5, oxidative precipitation using a strong oxidant such as Caro's acid and SO₂/O₂ can selectively precipitate manganese as insoluble MnO₂ or Mn(OOH), leaving other impurities, e.g., Mg, Cl⁻, F⁻, etc. in the circuit. Solvent extraction of zinc using D2EHPA (di-2-ethylhexyl phosphoric acid) can selectively recover zinc from the solution and leave other impurities including manganese in the raffinate.In copper solvent and electrowinning circuits, the problem of manganese is mainly associated with the decrease in the current efficiency and degradation of the solvent caused by the higher valent manganese species generated on the anode. The prevention or minimisation of Mn(II) oxidation during the electrowinning is critical. This can be achieved by adding ferrous ions or sulfur dioxide to control the cell potential.     

The mechanical nature of stress-corrosion cracking in Al-Zn-Mg alloys: I. Evaluation of the ductile rupture contribution

A detailed study of rapid stress-corrosion-cracking (SCC) in a 7075 aluminum alloy has allowed separation of the mechanical and chemical contributions. This was accomplished by combining scanning electron microscopy, stress-wave emission and crack growth rate observations as a function of test temperature. These established an activation energy of 11.2 kcal/mol, a stress-intensity squared dependence of crack growth, and a range of 20 to 80 pct dimpled rupture on the fracture surfaces. Thus a two-step crack growth mechanism is proposed combining a thermally activated electrochemical process and a discontinuous mechanical jumping process. Formerly with the Lawrence Radiation Laboratory, Berkeley Formerly with the Lawrence Radiation Laboratory, Berkeley This research was performed at the Inorganic Materials Research Division, Lawrence Berkeley Laboratory, and Department of Materials Science and Engineering, College of Engineering, University of California, Berkeley, California 94720.     

Powder metallurgy T15 tool steel: Part I. Characterization of powder and hot isostatically pressed material

The microstructure and constitution of T15 tool steel processed from gas-atomized powder have been characterized. From the atomized powder, four particle size ranges (≤840, 250 to 840, 44 to 100, and ≤44 Μm) were consolidated to full density by hot isostatic pressing (“hipping”) at 1130 ‡C or 1195 ‡C. Both atomized powder and consolidated material were examined by means of optical and electron microscopy, X-ray diffraction, chemical analysis, and micro-hardness. A segregated structure exists in the gas-atomized powder, independent of particle size; MC and M₂C carbides are present, primarily at cell boundaries. The matrix of the powders is a mix of martensite and retained austenite. Weight fraction and overall composition of the carbides are insensitive to particle size, but the proportion of MC carbides increases with decreasing particle size. After consolidation, MC, M₆C, and M₂₃C₆ carbides are present in a ferrite matrix. The carbide size distribution is skewed to larger carbide sizes at the higher consolidation temperature, independent of the prior particle size fraction, but there is no significant change in carbide volume fraction. For a given consolidation temperature, the size distribution of the MC and M₆C carbides is broader for the coarser particle size fractions.     

Modeling the microstructural changes during hot tandem rolling of AA5XXX aluminum alloys: Part II. Textural evolution

In Part II of this article, the experimental work undertaken to measure the effect of deformation parameters (temperature, strain, and strain rate) on the texture formation during hot deformation and the evolution during subsequent recrystallization is described. In addition, the isothermal kinetics of development of individual texture components were also determined. A neutron diffractometer was used to measure the texture in the as-hot-deformed aluminum samples, and the samples were then heat treated in a 400 °C salt bath for various lengths of time, with the texture being remeasured at various stages in the recrystallization process. Using data from the experimental program, the texture evolution during recrystallization was modeled by applying a modified form of the Avrami equation. Results indicated that, of the deformation parameters studied, textural development was most sensitive to the deformation temperature for both alloys. In addition, modeling results revealed that the Cu component ({112} 〈111〉) was the first to recrystallize, typically followed by the S ({123} 〈634〉) and Bs ({110} 〈112〉) components. This is in agreement with earlier work which indicated that the Bs component was the hardest to recrystallize, possibly because it is able to deform on very few slip systems and, hence, the dislocation interaction may be low.     

Modeling the microstructural changes during hot tandem rolling of AA5XXX aluminum alloys: Part I. Microstructural evolution

A comprehensive mathematical model of the hot tandem rolling process for aluminum alloys has been developed. Reflecting the complex thermomechanical and microstructural changes effected in the alloys during rolling, the model incorporated heat flow, plastic deformation, kinetics of static recrystallization, final recrystallized grain size, and texture evolution. The results of this microstructural engineering study, combining computer modeling, laboratory tests, and industrial measurements, are presented in three parts. In this Part I, laboratory measurements of static recrystallization kinetics and final recrystallized grain size are described for AA5182 and AA5052 aluminum alloys and expressed quantitatively by semiempirical equations. In Part II, laboratory measurements of the texture evolution during static recrystallization are described for each of the alloys and expressed mathematically using a modified form of the Avrami equation. Finally, Part III of this article describes the development of an overall mathematical model for an industrial aluminum hot tandem rolling process which incorporates the microstructure and texture equations developed and the model validation using industrial data. The laboratory measurements for the microstructural evolution were carried out using industrially rolled material and a state-of-the-art plane strain compression tester at Alcan International. Each sample was given a single deformation and heat treated in a salt bath at 400 °C for various lengths of time to effect different levels of recrystallization in the samples. The range of hot-working conditions used for the laboratory study was chosen to represent conditions typically seen in industrial aluminum hot tandem rolling processes, i.e., deformation temperatures of 350 °C to 500 °C, strain rates of 0.5 to 100 seconds and total strains of 0.5 to 2.0. The semiempirical equations developed indicated that both the recrystallization kinetics and the final recrystallized grain size were dependent on the deformation history of the material i.e., total strain and Zener-Hollomon parameter (Z), where and time at the recrystallization temperature.     

Modeling of mechanical alloying: Part III. Applications of computational programs

The computational modeling programs described in part II of this series are used in two ways. One is to compare program predictions to previous experimental data, thereby testing to some extent the utility of the programs. At this stage of their development, program “predictions” with respect to processing time, microstructural scale, and similar parameters are accurate to within a factor of 2 or so. Even so, the predictions offer support of the model developed in part I of this series and provide a vehicle for both model and process refinements. In addition to “testing” the model and the program in these manners, the effect of uncertainty in input material properties on program predictions is explored. Formerly Graduate Student, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903 Formerly Professor, Department of Materials Science and Engineering, University of Virginia     

Phase chemistry and precipitation reactions in maraging steels: Part III. Model alloys

This article describes studies of phase transformations during aging in a variety of model maraging steels. Atom-probe field-ion microscopy (APFIM) was the main research technique employed. Thermochemical calculation was also used during the course of the work. The composition and morphology of precipitates were compared in several maraging systems aged at different temperatures for different times to investigate the aging sequence. The APFIM results are compared with studies by other workers using different experimental techniques. In Fe-Ni(-Co)-Mo model alloys, ω phase and Fe₇Mo₆ μ phase have been found to contribute to age hardening at different stages of aging; no evidence was found for the existence of Mo-rich clusters in the as-quenched Fe-Ni-Co-Mo alloy. In a high-Si Cr-containing steel, Ti₆Si₇Ni₁₆ G phase and Ni₃Ti have been found to contribute to age hardening; reverted austenite was found after aging for 5 hours at 520 °C. In a Mn-containing steel, Fe₂Mo Laves phase and a structurally uncertain phase with a composition of Fe₄₅Mn₃₂Co₅Mo₁₉ have been found to contribute to age hardening. Formerly Graduate Student with the Department of Materials, Oxford University.     

Solidification of undercooled Fe-Cr-Ni alloys: Part III. Phase selection in chill casting

In Parts I and II of this series of articles, it was shown that a range of levitation-melted Fe-Cr-Ni alloys, both hypoeutectic and hypereutectic, all solidified with the hypereutectic phase (bcc) as their primary phase, except for the hypoeutectic alloys at low undercoolings. In this article, the effect of heat extraction on phase formation is studied by chill casting the undercooled alloys before nucleation. Two of the previously studied alloys are examined; one hypoeutectic and the other hypereutectic. Chill substrates employed were copper, stainless steel, alumina, zirconia, and a liquid gallium-indium bath. Contrary to the case of levitation melting and solidification, it is found that the dominant primary phase to solidify in both alloys, independent of chill substrate, is the hypoeutectic phase (fcc). It is concluded that chilling the undercooled melt results in nearly concurrent nucleation of bcc and fcc. Two different mechanisms are considered as possible explanations of the subsequent fcc phase selection during growth. These are termed “growth velocity” and “phase stability” mechanisms. Evidence favors a phase stability mechanism, in which the bcc phase massively transforms to fcc early in solidification so that fcc then grows without competition. It is suggested that this mechanism may also explain structures observed in welds and other rapid solidification processes.     

Primary dendrite spacing: Part II. Experimental studies of Pb-Pd and Pb-Au alloys

Directional solidification experiments have been extended to Pb-Pd and Pb-Au alloys. The primary spacing, λ, has been determined over a wide range of temperature gradients,G, and solidification velocities,V. The dependence of the spacing on bothG andV is found to undergo a transition as the values ofV andG are lowered. The transition withG is not correlated with the dendritic to cellular transformation. However, the transition withV appears to correlate with duplex microstructures which are observed in the dendritic to cellular transition region.     

Precipitate effects on the mechanical behavior of aluminum copper alloys: Part II. Modeling

This work focuses on a new hardening formulation accounting for precipitate-induced anisotropy in a binary aluminum-copper precipitation-hardened alloy. Different precipitates were developed upon aging at 190°C and 260°C, and corresponding work hardening characteristics were predicted for single and polycrystals. The use of single crystals facilitated the demonstration of the effect of precipitates on the flow anisotropy behavior. Pure aluminum was also studied to highlight the change in deformation mechanisms due to the introduction of precipitates in the matrix. The influence of precipitate-induced anisotropy on single-crystal flow behavior was clearly established, again relating to the precipitate character. Simulations are presented for several single-crystal orientations and polycrystals, and they display good agreement with experiments. The work demonstrates that precipitate-induced anisotropy can dominate over the crystal anisotropy effects in some cases. T. FOGLESONG formerly with the Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, IL 61801     

Precipitate effects on the mechanical behavior of aluminum copper alloys: Part II. Modeling

This work focuses on a new hardening formulation accounting for precipitate-induced anisotropy in a binary aluminum-copper precipitation-hardened alloy. Different precipitates were developed upon aging at 190 °C and 260 °C, and corresponding work hardening characteristics were predicted for single and polycrystals. The use of single crystals facilitated the demonstration of the effect of precipitates on the flow anisotropy behavior. Pure aluminum was also studied to highlight the change in deformation mechanisms due to the introduction of precipitates in the matrix. The influence of precipitate-induced anisotropy on single-crystal flow behavior was clearly established, again relating to the precipitate character. Simulations are presented for several single-crystal orientations and polycrystals, and they display good agreement with experiments. The work demonstrates that precipitate-induced anisotropy can dominate over the crystal anisotropy effects in some cases.     

Precipitate effects on the mechanical behavior of aluminum copper alloys: Part I. Experiments

This article focuses on understanding the mechanical behavior of precipitation-hardened alloys by studying single and polycrystalline deformation behavior with various heat treatments. Aluminumcopper alloys are the focus in this work and their changing stress-strain behavior is demonstrated resulting from the different hardening mechanisms brought about by the various precipitates. Extensive transmission electron microscopy investigations facilitated the interpretation of the stress-strain behavior and the work hardening characteristics. The use of both single and polycrystals proved valuable in understanding the role of anisotropy due to crystal orientation vs precipitate-induced anisotropy. The experiments show that precipitation-induced anisotropy could offset the crystal orientation anisotropy depending on the orientation. This is clearly demonstrated with similar [111] and [123] behaviors under 190 °C and 260 °C aging temperatures. Experiments on pure aluminum crystals are also provided for comparison and understanding the crystal anisotropy in the absence of precipitates. Part I of this article will focus on experiments, and part II will describe the modeling of the effect of different metastable phases in the matrix acting as barriers to dislocation motion. FOGLESONG for-merly with the Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, IL 61801     

Precipitate effects on the mechanical behavior of aluminum copper alloys: Part I. Experiments

This article focuses on understanding the mechanical behavior of precipitation-hardened alloys by studying single and polycrystalline deformation behavior with various heat treatments. Aluminumcopper alloys are the focus in this work and their changing stress-strain behavior is demonstrated resulting from the different hardening mechanisms brought about by the various precipitates. Extensive transmission electron microscopy investigations facilitated the interpretation of the stress-strain behavior and the work hardening characteristics. The use of both single and polycrystals proved valuable in understanding the role of anisotropy due to crystal orientation vs precipitate-induced anisotropy. The experiments show that precipitation-induced anisotropy could offset the crystal orientation anisotropy depending on the orientation. This is clearly demonstrated with similar [111] and [123] behaviors under 190 °C and 260 °C aging temperatures. Experiments on pure aluminum crystals are also provided for comparison and understanding the crystal anisotropy in the absence of precipitates. Part I of this article will focus on experiments, and part II will describe the modeling of the effect of different metastable phases in the matrix acting as barriers to dislocation motion.     

INCOLOY 908, a low coefficient of expansion alloy for high-Strength cryogenic applications: Part I. Physical metallurgy

INCOLOY 908 is a low coefficient of thermal expansion (COE) iron-nickel base superalloy that was developed jointly by The Massachusetts Institute of Technology and the International Nickel Company for cryogenic service. The alloy is stable against phase transformation during prolonged thermal treatments and has a COE compatible with that of Nb₃Sn. These properties make the material ideal for use as a structural component in superconducting magnets using Nb₃Sn. The evolution of microstructure has been studied as a function of time at temperature over the temperature range of 650 °C to 900 °C for times between 50 and 200 hours. A detailed analysis of precipitated phases has been conducted using X-ray diffraction (XRD), transmission electron microscopy (TEM), and analytical scanning and scanning transmission electron microscopy (STEM) techniques. The primary strengthening phase has been found to be γ’, Ni₃(Al, Ti). INCOLOY 908 is stable against overaging, which is defined as the transformation of γ’ to η, Ni₃Ti, for times to 100 hours at temperatures up to 750 °C. Upon overaging, the strengthening phase transforms to η. A new phase,H _x, has been identified and characterized.     

Sulfides and oxides in Fe-Mn alloys: Part III. Formation of oxysulfides during freezing of steel

The previous study of the phase relations (Part I) is now extended to the solid-liquid regions of Fe-Mn-S and Fe-Mn-S-O systems. The differential thermal analysis measurements made with 1 to 5 pet Mn-Fe alloys containing sulfur and oxygen substantiated the positions of the univariant curves in pet Mn vsT plots derived from other available data and theoretical considerations. It is found that if sufficient oxygen and sulfur are present in the Fe-Mn alloys, there may be a liquid oxysulfide phase present at temperatures above 900°C, depending on the manganese content. The higher the manganese content in solution, the higher is the temperature below which no liquid phase is present,e.g. for 10 ppm, 900°C and for 10 pet Mn, ∼1200°C. The formation of oxides, sulfides, and liquid oxysulfides in the interdendritic regions during solidification, as predicted from the phase relations in the Fe-Mn-S-O system, was verified by microscopic examination using scanning electron microscopy and metallographic techniques.