Showing papers on "Superplasticity published in 2020"

Opportunities and Issues in the Application of Titanium Alloys for Aerospace Components

Abstract: The metal titanium (Ti) and its alloys have many attributes which are attractive as structural materials, but they also have one major disadvantage, high initial cost. Nevertheless, Ti and Ti alloys are used extensively in airframes, gas turbine engines (GTE), and rocket engines (RE). The high cost is a deterrent, particularly in airframe applications, in that the other alloys it competes with are, for the most part, significantly lower cost. This is less of a concern for GTE and RE where the cost of titanium is closer to and sometimes even lower than some of the materials it competes with for these applications. In spacecraft the weight savings are so important that cost is a lesser concern. Ti and its alloys consist of five families of alloys; α-Ti, near α-alloys, α + β alloys, β-alloys, and Ti-based intermetallic compounds. The intermetallic compounds of primary interest today are those based on the compound TiAl which, at this time, are only used for engine applications because of their higher temperature capability. These TiAl-based compounds are used in a relatively low, but growing, amounts. The first production application was for low pressure turbine blades in the GE engine (GEnx) used on the Boeing 787, followed by the GE LEAP engine used on A-320neo and B-737MAX. These air foils are investment cast and machined. The next application is for the GE90X which will power the Boeing B-777X. These air foils will be made by additive manufacturing (AM). Unalloyed titanium and titanium alloys are typically melted by vacuum arc melting and re-melted either once (2X VAR) or twice (3X VAR); however a new and very different melting method (cold hearth melting) has recently become favored, mainly for high performance applications such as rotors in aircraft engines. This process resulted in higher quality ingots with a significant reduction in melt-related defects. Once melted and cast into ingots, the alloys can be processed using all the standard thermomechanical working and casting processes used for making components of other types of structural alloys. Because of their limited ductility, the TiAl-based intermetallic compounds are quite difficult to process using ordinary wrought methods. Consequently, the low-pressure turbine blades currently in service are investment cast and machined to net shape. The AM air foils will require minimal machining, which is an advantage. This paper describes some relatively recent developments as well as some issues and opportunities associated with the production and use of Ti and its alloys in aerospace components. Included are new Ti alloys, new applications of Ti alloys, and the current status of several manufacturing processes including a discussion of the promise and current reality of additive manufacturing as a potentially revolutionary method of producing Ti alloy components.

Friction Stir Processing of Magnesium Alloys: A Review

Abstract: Magnesium (Mg) alloys have been extensively used in various fields, such as aerospace, automobile, electronics, and biomedical industries, due to their high specific strength and stiffness, excellent vibration absorption, electromagnetic shielding effect, good machinability, and recyclability. Friction stir processing (FSP) is a severe plastic deformation technique, based on the principle of friction stir welding. In addition to introducing the basic principle and advantages of FSP, this paper reviews the studies of FSP in the modification of the cast structure, superplastic deformation behavior, preparation of fine-grained Mg alloys and Mg-based surface composites, and additive manufacturing. FSP not only refines, homogenizes, and densifies the microstructure, but also eliminates the cast microstructure defects, breaks up the brittle and network-like phases, and prepares fine-grained, ultrafine-, and nano-grained Mg alloys. Indeed, FSP significantly improves the comprehensive mechanical properties of the alloys and achieves low-temperature and/or high strain rate superplasticity. Furthermore, FSP can produce particle- and fiber-reinforced Mg-based surface composites. As a promising additive manufacturing technique of light metals, FSP enables the additive manufacturing of Mg alloys. Finally, we prospect the future research direction and application with friction stir processed Mg alloys.

Ultrahigh high-strain-rate superplasticity in a nanostructured high-entropy alloy

Abstract: Superplasticity describes a material’s ability to sustain large plastic deformation in the form of a tensile elongation to over 400% of its original length, but is generally observed only at a low strain rate (~10−4 s−1), which results in long processing times that are economically undesirable for mass production. Superplasticity at high strain rates in excess of 10−2 s−1, required for viable industry-scale application, has usually only been achieved in low-strength aluminium and magnesium alloys. Here, we present a superplastic elongation to 2000% of the original length at a high strain rate of 5 × 10−2 s−1 in an Al9(CoCrFeMnNi)91 (at%) high-entropy alloy nanostructured using high-pressure torsion. The high-pressure torsion induced grain refinement in the multi-phase alloy combined with limited grain growth during hot plastic deformation enables high strain rate superplasticity through grain boundary sliding accommodated by dislocation activity. Superplasticity at high strain rates is challenging to achieve in high strength materials. Here, the authors show superplastic elongation in excess of 2000% in a high entropy alloy nanostructured by high-pressure torsion.

An Overview of the Thermomechanical Processing of α/β Titanium Alloys: Current Status and Future Research Opportunities

Abstract: Current understanding of the principles underlying the thermomechanical processing (TMP) of α/β titanium alloys is reviewed. Attention is focused on the formulation of constitutive descriptions for plastic flow under hot-working conditions, the evolution of microstructure, the occurrence of defects, and novel/emerging TMP techniques. With regard to constitutive behavior, descriptions of the plastic flow of the individual phases and two-phase alloys per se are summarized. The important influence of phase morphology, size, and volume fraction on plastic flow is emphasized. Mechanisms which underlie microstructure evolution include beta recrystallization (in the high-temperature β field), the development of dislocation substructure and its effect on dynamic and static spheroidization of colony microstructures (in the two-phase field), static and dynamic coarsening of primary α, and the development of deformation and transformation textures. In the area of defects, the effect of TMP variables and starting microstructure on the formation of cavities, the persistence of microtexture, and the development of undesirably-coarse β grain structures are described. The current status of relatively new processing techniques for α/β titanium alloys such as low-temperature superplastic forming and solid-state joining (via linear friction or friction-stir methods) are also briefly reviewed. Last, R&D which could help to resolve deficiencies in the current knowledge base for TMP of α/β titanium alloys are summarized for each of the areas.

The study of flow behavior and governing mechanisms of a titanium alloy during superplastic forming

Abstract: TA15 (Ti–6Al–2Zr–1Mo–1V) is a near-α titanium alloy and has wide applications in the aerospace industry because of its high strength to mass ratio, good weldability, and superior creep resistance at high temperatures up to 550 °C, compared to other titanium alloys. This study investigates the flow behavior and microstructural evolution as functions of temperatures and strain rates during deformations under the superplastic conditions at 880 °C/0.01s−1, 900 °C/0.01s−1, 880 °C/0.001s−1, and 920 °C/0.0005s−1. Results showed that this alloy exhibit excellent superplastic behavior for all selected temperatures and strain rates. The maximum tensile elongation of 1450% is achieved at 880 °C with a strain rate of 0.001s−1. Flow softening is observed under deformation conditions of 880 °C/0.01s−1 and 900 °C/0.01s−1, while strain hardening is observed at deformation conditions of 880 °C/0.001s−1 and 920 °C/0.0005s−1. These complex flow behaviors are rationalized by characterizing the underlying microstructures on the interrupted tensile samples using electron backscatter diffraction (EBSD) and backscattered electrons (BSE). The geometrically necessary dislocations (GNDs) density, which is caused by lattice rotation and misorientations and plays a vital role in the plastic constitutive behaviors, was for the first time, systematically revealed. Together with other key microstructures, i.e. grain sizes, texture, phase fractions, the results show that the dominant deformation mode changes at initial, intermediate, and final stages of the deformation. The probable deformation mechanisms, such as grain boundary sliding (GBS) under different deformation conditions, are discussed in terms of grain morphology, GNDs, and texture evolution. Also, it is observed that the β-phase transformation is accelerated during deformation and contributes to the enhancement of superplasticity.

Martensite transformation induced superplasticity and strengthening in single crystalline CoNiCrFeMn high entropy alloy nanowires: A molecular dynamics study

Abstract: In this work, we report simultaneous superplasticity and strengthening in single crystalline face-centered cubic (FCC) CoNiCrFeMn high entropy alloy (HEA) nanowires through synergistic martensitic phase transformation and micro-twin nucleation as revealed by molecular dynamics simulations. Furthermore, in contrast to the irreversible martensite transformation that has been previously reported in bulk HEAs under high pressure, the martensitic transformation in HEA nanowires is found to be reversible upon reverse loading. Shape memory effects can thus be enabled in HEA nanowires, although such effects are found to be mitigated for some orientations due to stacking fault crossing. Those mechanisms are dramatically different from deformation twinning dominated superplasticity in conventional FCC metal and intermetallic nanowires. While deformation twinning is still observed in FCC HEA nanowires, three pathways are found and in particular. All the novel mechanical behaviors in FCC CoNiCrFeMn HEA nanowires reported in this study can be explained by their unique negative stacking fault and martensite energies, which may shed some light on understanding the mechanical behavior of general HEAs under more complicated loading conditions.

Grain Boundary Processes in Strengthening, Weakening and Superplasticity

Abstract: Grain boundaries provide strength to materials at low temperatures by impeding slip transfer and they weaken materials at high temperatures by intergranular creep processes such as grain boundary sliding (GBS) and diffusion creep. At very fine nanocrystalline grain sizes of 0.3 deform with a large GBS contribution to total strain of approximate to 40-80%. There are a limited number of studies showing superplasticity in fine-grained high-entropy alloys, involving GBS accommodated by dislocations.

Recent Development of Superplasticity in Aluminum Alloys: A Review

Abstract: Aluminum alloys can be used in the fabrication of intricate geometry and curved parts for a wide range of uses in aerospace and automotive sectors, where high stiffness and low weight are necessitated. This paper outlines a review of various research investigations on the superplastic behavior of aluminum alloys that have taken place mainly over the past two decades. The influencing factors on aluminum alloys superplasticity, such as initial grain size, deformation temperature, strain rate, microstructure refinement techniques, and addition of trace elements in aluminum alloys, are analyzed here. Since grain boundary sliding is one of the dominant features of aluminum alloys superplasticity, its deformation mechanism and the corresponding value of activation energy are included as a part of discussion. Dislocation motion, diffusion in grains, and near-grain boundary regions being major features of superplasticity, are discussed as important issues. Moreover, the paper also discusses the corresponding values of grain size exponent, stress exponent, solute drag creep and power law creep. Constitutive equations, which are essential for commercial applications and play a vital role in predicting and analyzing the superplastic behavior, are also reviewed here.

Pronounced low-temperature superplasticity of friction stir processed Mg–9Li–1Zn alloy

Abstract: Ultrafine Mg–9Li–1Zn alloy with average grain sizes of ~0.61 μm (α phase) and ~0.96 μm (β phase) was obtained by friction stir processing. A pronounced superplasticity from 369% to 1104% was obtained at 473 K over a wide range of strain rates from 10-1/s to 10-4/s.

Experimental study of the superplastic deformation mechanisms of high-strength aluminum-based alloy

Abstract: The role of different deformation mechanisms and the contributing factors behind them needs to be clearly defined to develop materials exhibiting high strain rate superplasticity. It is still not fully understood to what extent the deformation conditions and microstructural parameters affect the mechanisms of superplastic deformation. A novel Al–3.9Zn–4.1Mg–0.8Cu–2.8Ni–0.25Zr alloy is employed to determine what effects the testing conditions, elemental and phase composition and evolution of grain and sub-grain structure make towards deformation mechanisms. To analyze the deformation behavior and microstructure evolution, uniaxial tensile tests are performed following two deformation regimes with the different constant strain rates and temperatures: (1) 2 × 10-3 s-1, 480°С and (2) 2 × 10-2 s-1, 440°С. The elongation to failure exceeds 650% and the strain rate sensitivity coefficient m is near 0.5 at both deformation regimes. Electron microscopy and focused ion beam techniques are employed to analyze mechanisms associated with grain boundary sliding and intragranular strain by using the samples in a stable flow. Surface grids indicated that superplastic flow is accompanied by the formation of striated regions on the surface, whereas grain boundary sliding involves grain neighbor switching and grain rotation. Intragranular deformation with increased dislocation density and dynamic recrystallization are also observed. Weaker intergranular and intense intragranular deformation are revealed at high strain rate deformation regime. Solute effect and the presence of two types of secondary phases, nanoprecipitates of L12-Al3Zr and micron-sized eutectic Al3Ni, are discussed in comparison to conventional AA7475 alloy in different aspects of grain growth, dynamic recrystallization and strain-rate sensitivity.

Recent research progresses in Al-7075 based in-situ surface composite fabrication through friction stir processing: A review

Abstract: Friction stir processing (FSP) is a new and exciting technique being extensively explored because of its thermo-mechanical surface modification which has the capability to enhance the mechanical, microstructural and tribological properties. Various studies were described on friction stir processed aluminium alloys using several reinforcement particles containing micro and nano size grains thereby the formation of homogeneous and identical microstructure. However, this review is mainly focused on Al-7075 alloy which is a potentially superplastic alloy for making metal matrix composites. In this review, the recent research progress in FSP is discussed in detail with particular emphasis on their preparing method, characterization and various mechanical properties including present challenges and future prospects. Though, a remarkable progress has been made during the past few years, the present status in materials processing is far from ideal, especially in obtaining desire composites by using single or dual reinforcements.

Superplastic deformation behavior of fine-grained AZ80 magnesium alloy prepared by friction stir processing

Abstract: Friction stir processing (FSP) is a new severe plastic deformation technique where microstructure refinement, homogenization, and densification occur simultaneously. In this study, the microstructure and superplastic deformation behavior of AZ80 magnesium alloy prepared by FSP were investigated. FSP led to remarkable grain refinement and modification of texture for AZ80 magnesium alloy. A maximum elongation of 606% was obtained at 3 × 10−4 s−1 and 350 ℃. When superplastic deformation was conducted at 3 × 10-3 s−1 and 300 ℃, or at 1 × 10-4 s−1 and 400 ℃, the superplasticity was relatively lower, thus exhibiting a lower strain rate sensitivity. The coarsening of both grains and β-Mg17Al12 precipitates deteriorated superplastic elongation. Applied deformation promoted the premature dissolution of β precipitates at 400 ℃ during superplastic deformation. Moreover, superplastic deformation weakened and spread the basal texture. Although the activation of prismatic and pyramidal slips is beneficial for superplastic deformation, grain boundary sliding is the main superplastic deformation mechanism.

Evidence of Inverse Hall-Petch Behavior and Low Friction and Wear in High Entropy Alloys.

Abstract: We present evidence of inverse Hall-Petch behavior for a single-phase high entropy alloy (CoCrFeMnNi) in ultra-high vacuum and show that it is associated with low friction coefficients (~03) Grain size measurements by STEM validate a recently proposed dynamic amorphization model that accurately predicts grain size-dependent shear strength in the inverse Hall-Petch regime Wear rates in the initially soft (coarse grained) material were shown to be remarkably low (~10–6 mm3/N-m), the lowest for any HEA tested in an inert environment where oxidation and the formation of mixed metal-oxide films is mitigated The combined high wear resistance and low friction are linked to the formation of an ultra-nanocrystalline near-surface layer The dynamic amorphization model was also used to predict an average high angle grain boundary energy (087 J/m2) This value was used to explain cavitation-induced nanoporosity found in the highly deformed surface layer, a phenomenon that has been linked to superplasticity

Superplastic deformation mechanism of the friction stir processed fully lamellar Ti-6Al-4V alloy

Abstract: The rolled Ti-6Al-4V alloy sheets were subjected to friction stir processing (FSP) using high heat-input parameters at a tool rotation speed of 325 rpm with a traverse speed of 50 mm/min, and a fully lamellar microstructure was obtained in the stir zone (SZ). The high-temperature tensile tests were then conducted on this fully lamellar microstructure in the temperature range of 850–900 °C at the strain rates of 3 × 10-2–3 × 10-4 s-1. The superplasticity with elongations of above 400% was achieved at all the testing temperatures with the appropriate strain rates, and the maximum elongation of 553% was achieved at the temperature of 875 °C and 1 × 10-3 s-1, which was attributed to the dynamic globularization at the low strain stage and subsequent boundary sliding (BS) at the high strain stage. The main dynamic globularization mechanisms were considered as discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) accompanied with the β phase growing towards the inside of the groove along the low angle grain boundaries (LAGBs). The present spheroidized microstructure shows an excellent thermal-mechanical stability because the co-existence of the two phases of α and β could effectively inhibit the severe grain growth and facilitate the continuous operating of the BS. In addition, the β phase transferring from compressive boundaries to tensile boundaries and the strain/stress induced the α to β phase transformation can act as the additional accommodation mechanisms to relax the stress concentration and inhibit the formation of the cavities, which can facilitate the achievement of the enhanced superplasticity.

Excellent superplastic properties achieved in Mg–4Y–3RE alloy in high strain rate regime

Abstract: This study is focused on high strain rate superplasticity of commercial magnesium alloy Mg–4Y–3RE (WE43) in ultrafine-grained condition prepared by equal channel angular pressing (ECAP). Attaining microstructure with the average grain size of ~340 nm and high density of secondary phase particles provided a possibility to significantly decrease temperature during the deformation at strain rates of 10−2 s−1 and 10−1 s−1 in comparison with previous studies. Consequently, extent of negative effect of grain growth was partially suppressed and the investigated alloy did exhibit exceptionally high deformability with a maximum elongation of ~1230% at strain rate of 10−2 s−1 and at two temperatures: 350 °C and 400 °C. Increase in the strain rate to 10−1 s−1 resulted in maximum elongation of ~1000% at the temperature of 400 °C. The microstructural analysis after the deformation showed that the microstructure was fine-grained even after large deformation and plasticity-controlled growth of cavities did occur only at the temperature of 450 °C. The optimization of ECAP processing resulted in very weak texture and, therefore, the deformability should be more isotropic in comparison with strongly textured materials usually investigated in this regard.

Process Development for a Superplastic Hot Tube Gas Forming Process of Titanium (Ti-3Al-2.5V) Hollow Profiles

Abstract: Tube forming technologies based on internal forming pressures, such as hydroforming or hot tube gas forming, are state of the art to manufacture complex closed profile geometries. However, materials with excellent specific strengths and chemical properties, such as titanium alloys, are often challenging to shape due to their limited formability. In this study, the titanium alloy Ti-3Al-2.5V was processed by superplastic hot tube gas forming to manufacture a helically shaped flex tube. The forming process was investigated in terms of process simulation, forming tool technology and process window for the manufacturing of good parts. Within a simulation study, a strain rate optimized forming pressure–time curve was defined. With the newly developed tool design, forming temperatures up to 900 °C and internal forming pressures up to 7 MPa were tested. A process window to manufacture good parts without necking or wrinkling has been successfully identified. The experiment data showed good agreement with the numerical simulations. The detailed study of the process contributes to an in-depth understanding of the superplastic forming of Ti-3Al-2.5V during hot tube gas forming. Furthermore, the study shows the high potential of superplastic hot tube gas forming of titanium alloys for the manufacturing of helical flex tubes and bellows.

Inducing superplasticity in extruded pure Mg by Zr addition

Abstract: Addition of 0.7 wt% Zr to pure Mg resulted in a bimodal structure, consisting of both coarse and fine recrystallized grains with a mean grain size of 2.8 μm, and a large fraction of high angle grain boundaries after extrusion. A maximum strain rate sensitivity index of 0.42, obtained by shear punch test (SPT) at 673 K, together with activation energy of 96 kJ mol–1 indicated that grain boundary sliding controlled by grain boundary diffusion is the dominant mechanism in superplastic regime.

Analysis of the creep behavior of fine-grained AZ31 magnesium alloy

Abstract: Double-shear creep testing was used to evaluate the creep behavior of a magnesium AZ31 alloy processed by equal-channel angular pressing to produce an average grain size of ~2.7 μm. The results show that rapid creep rates are observed in the early stages of deformation due to the occurrence of grain boundary sliding in the fine-grained structure but the creep rates decrease with increasing deformation due to grain growth. The stress exponent for flow in the early stages is ~2 and the activation energy is ~92 kJ mol−1 where these values are consistent with the expectations for grain boundary sliding under superplastic conditions. Annealing the material for 24 h at 723 K before creep testing produces a significantly larger grain size of ~50 μm and this prevents grain boundary sliding and leads to an increasing stress exponent at higher stresses. Deformation mechanism maps are constructed incorporating both the present experimental results for a fine-grained magnesium alloy and results from published data for the AZ31 alloy. These maps provide a useful tool for evaluating the experimental conditions that are necessary for achieving superplastic forming operations.

Grain boundary sliding during high-temperature tensile deformation in superplastic Fe-6.6Mn-2.3Al steel

Abstract: In the present study, grain boundary sliding (GBS) occurring during high-temperature deformation, i.e., a critical strain and accommodation process of GBS, was investigated using Fe-6.6Mn-2.3Al (wt%) steel, which was recently reported as a superplastic steel. For this purpose, high-temperature interrupted tensile tests were conducted at 880 °C with an initial strain rate of 1 × 10−3 s−1, and the microstructures of the tensile specimens were then observed at room temperature as a function of the true strain (e) at 880 °C. Variations in certain microstructural features with e, in this case the maximum intensity of the orientation distribution function, the average grain size, the void fraction, the misorientation distribution, the aspect ratio of the grains and the kernel average misorientation, found that dislocation plasticity occurred up to e = 0.69, followed by both GBS and grain rotation. This indicates that the critical strain for GBS (eGBS) is 0.69. GBS occurred via a dislocation accommodation process, i.e., the second type of Rachinger sliding, where the original shape of the grains is maintained and subgrains do not form during GBS.

Superplastic deformation behavior of the as-extruded AZ110 magnesium alloy with La-rich Mish metal addition

Abstract: Tensile test at high temperature and electron backscatter diffraction (EBSD) techniques were combined to investigate the deformation behavior and microstructure evolution of the as-extruded AZ110 magnesium alloy with La-rich Mish Metal addition (AZ110LC for short). The stretching temperature and strain rate have great effect on the flow stress of the alloy. The flow stress rapidly reaches the peak stress and then decreases with further deformation until the fracture. The maximum elongation of the alloy is 840% under 573 K and 1.7 × 10−s. The excellent superplasticity ascribes to the refinement of microstructure and the distribution of sub-structures of the as-extruded AZ110LC alloy. Moreover, the strain rate sensitivity coefficient of the as-extruded AZ110LC alloy is 0.41 and the active energy is 87 kJ/mol under 523 K and 4.2 × 10−s indicating that the deformation mechanism of the alloy is GBS. Finally, the shapes and fraction of cavities closed to tensile fracture have a great influence on the elongation of the tensile samples at different strain rates under 573 K. The formation and growth of the cavities can effectively relax the stress concentration and can effectively coordinate and compensate the GBS mechanism in superplastic deformation of the alloy.

High Strain Rate Superplasticity in Al-Zn-Mg-Based Alloy: Microstructural Design, Deformation Behavior, and Modeling

Abstract: Increasing the strain rate at superplastic forming is a challenging technical and economic task of aluminum forming manufacturing. New aluminum sheets exhibiting high strain rate superplasticity at strain rates above 0.01 s−1 are required. This study describes the microstructure and the superplasticity properties of a new high-strength Al-Zn-Mg-based alloy processed by a simple thermomechanical treatment including hot and cold rolling. The new alloy contains Ni to form Al3Ni coarse particles and minor additions of Zr (0.19 wt.%) and Sc (0.06 wt.%) to form nanoprecipitates of the L12-Al3 (Sc,Zr) phase. The design of chemical and phase compositions of the alloy provides superplasticity with an elongation of 600–800% in a strain rate range of 0.01 to 0.6/s and residual cavitation less than 2%. A mean elongation-to-failure of 400% is observed at an extremely high constant strain rate of 1 s−1. The strain-induced evolution of the grain and dislocation structures as well as the L12 precipitates at superplastic deformation is studied. The dynamic recrystallization at superplastic deformation is confirmed. The superplastic flow behavior of the proposed alloy is modeled via a mathematical Arrhenius-type constitutive model and an artificial neural network model. Both models exhibit good predictability at low and high strain rates of superplastic deformation.

Using High-Pressure Torsion to Achieve Superplasticity in an AZ91 Magnesium Alloy

Abstract: An AZ91 magnesium alloy (Mg-9%, Al-1% Zn) was processed by high-pressure torsion (HPT) after solution-heat treatment. Tensile tests were carried out at 423, 523, and 623 K in the strain rate range of 10−5−10−1 s−1 to evaluate the occurrence of superplasticity. Results showed that HPT processing refined the grain structure in the alloy, and grain sizes smaller than 10 µm were retained up to 623 K. Superplastic elongations were observed at low strain rates at 423 K and at all strain rates at 523 K. An examination of the experiment data showed good agreement with the theoretical prediction for grain-boundary sliding, the rate-controlling mechanism for superplasticity. Elongations in the range of 300–400% were observed at 623 K, attributed to a combination of grain-boundary-sliding and dislocation-climb mechanisms.