High Temperature Superplasticity Research Articles

Room Temperature Strengthening and High-Temperature Superplasticity of Mg-Li-Al-Sr-Y Alloy Fabricated by Asymmetric Rolling and Friction Stir Processing

Magnesium-lithium alloy is the lightest alloy to date. To explore its room temperature strength and high-temperature ductility, a plate of a new fine-grained Mg-9.13Li-3.74Al-0.31Sr-0.11Y alloy was fabricated by asymmetric rolling, and the rolled plate was subjected to friction stir processing (FSP). The microstructure and mechanical properties at room and elevated temperatures were investigated by optical microscopy, X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and tensile tester. Grain refinement with an average grain size in the α-Mg phase of 1.65 μm and an average grain size in the β-Li phase of 4.24 μm was achieved in the water-cooled FSP alloy. For room temperature behavior, the ultimate tensile strength of 208 ± 4 MPa, yield strength of 193 ± 2 MPa, and elongation of 48.2% were obtained in the water-cooled FSP alloy. XRD and EDS analyses revealed that the present alloy consists of α-Mg and β-Li phases, Al2Y, Al4Sr, MgLi2Al, and AlLi intermetallic compounds. For high-temperature behavior, the maximum superplasticity or ductility of 416% was demonstrated in this fine-grained alloy with an average grain size of 10 μm at 573 K and 1.67 × 10-3 s-1. A power-law constitutive equation was established. The stress exponent was 2.29 (≈2) (strain rate sensitivity 0.44), and the deformation activation energy was 162.02 kJ/mol. This evidence confirmed that the dominant deformation mechanism at elevated temperatures is grain boundary and interphase boundary sliding controlled by lattice diffusion.

Superplastic deformation behavior of Mg alloys: A-review

The magnesium alloys are the next-generation lightweight metal materials; however, the production of the complex shape components of thermo-mechanical processed magnesium alloys is very limited in comparison to their die-casted counterparts. Both high strain rate and low-temperature processing are required to save time and cost for the project. Uniaxial tensile testing has been vastly used for determining the plasticity of magnesium alloys at elevated temperatures. Therefore, this review article sheds light on superplastic behaviors under the uniaxial tensile loading of different magnesium alloys. This review also emphasizes low and high-temperature superplasticity, high strain rate superplasticity, deformation mechanism, thermal stability, texture evaluation, and fracture mechanism of extruded/rolled and severe plastic deformed magnesium alloys. At the end of the review, some technical issues have been addressed which can be beneficial for further enhancement in the superplasticity of magnesium alloys.

Disclination dipoles are the Holy Grail for high temperature superplasticity in ceramics

A model for high-temperature plasticity of polycrystals controlled by disclination dipoles is proposed that predict a parabolic dependence of the strain rate versus the applied stress. The presence of a precise stationary disclination density explains the origin of plasticity without microstructural invariance, commonly known as superplasticity. The disclination mechanism is universal, although other processes, such as dislocation glide, are superposed to this one in many systems such as metals or metallic alloys. While, in ceramics it is likely to be the only operative mechanism. Activation of disclination dipoles is a necessary condition for plasticity and sufficient one for superplastic yielding.

High temperature superplasticity and its deformation mechanism of AA6063/SiCp

Superplastic forming is primarily used for thin-walled and complex shape structures. This process is much successful in industries mainly for the fabrication of intricate components used for aerospace and automobile industries. In this paper, superplastic forming behavior and microstructural evolution in Al/5%SiCp composites were studied at different temperatures through the hot tensile test. In a hot tensile test, a maximum elongation (i.e., 227%) was obtained at the 580 °C temperature condition. When the temperature was more than 580 °C, the elongation was reduced to less than 100%. Similarly, when the temperature is adjusted to a value lesser than 550 °C, the reduction in the elongation is seen which is less than 100%. Here, the microstructure study of the test samples is studied through the optical microscope. From the microstructure analysis, the grain refinement is taken place by dynamic re-crystallization and the intergranular deformation with grain boundary sliding is observed. It is understood that, the dynamic re-crystallization and grain boundary sliding increases the percentage of elongation and on the other hand the ductility reduces due to the presence of excessive liquid phase at the grain boundaries.

On the Nuances in the Power Law Description and Interpretation of High Homologous Temperature Creep and Superplasticity Data

“Power law’’ representation is used to describe minimum creep rate and “steady state” superplastic deformation. In creep isothermal log stress – log strain rate relationship is linear for so long as the rate controlling mechanism remains unchanged. During optimal superplastic flow the slope of this curve changes even when there is no change in the rate controlling mechanism, i.e. the stress exponent, n, at a constant temperature and grain size is a function of strain rate. For a constant rate controlling mechanism, in both the phenomena, n decreases with increasing temperature. Grain size has no effect on creep, but its effect is significant in superplasticity. Therefore, analyzing creep and superplasticity data by treating n for any given mechanism as a constant independent of stress and temperature is questionable. In this analysis stress is normalized with respect to a reference stress, rather than the shear modulus. The microstructure dependence comes through the Buckingham Pi theorem. When contribution from microstructure terms to isothermal strain rate is constant, Laurent’s theorem helps generate a set of values for n. It is shown that the simplest solution, viz. n is independent of stress, but is a linear function of temperature, describes steady state creep. (The case n is independent of both stress and temperature follows as a special case.) The second simplest solution, viz. n is a linear function of both temperature and stress corresponds to steady state superplasticity. Using the equations, the values of n, activation energies for the rate controlling processes and strain rates at different temperatures and stresses could be estimated for both creep and superplasticity. The analysis is validated using experimental results concerning many systems. iiThis lecture is dedicated to the sacred memory of late Prof. Oleg D. Sherby.

High temperature superplasticity and deformation behavior of naturally aged Zn-Al alloys with different phase compositions

Two Zn-Al alloys, eutectoid Zn-22Al and dilute Zn-0.3Al, were processed through equal-channel angular pressing (ECAP). The ECAP-processed alloys were subjected to long-term natural aging at room temperature for 120 days in order to investigate the effect of long-term natural aging on high-temperature superplasticity and the deformation behavior of these alloys. The grain size of the Zn-22Al alloy increased from 200 nm after ECAP to 400 nm after the natural aging process. The Zn-0.3Al alloy with a grain size of 1.3 µm after ECAP demonstrated a reasonably stable microstructure with the grain size of ~ 1.7 µm after long-term natural aging. The naturally aged ultrafine-grained (UFG) Zn-22Al alloy showed an excellent superplastic elongation of about 1800% at 200 °C. Grain boundary sliding was determined as the dominant deformation mechanism in this alloy. On the contrary, a considerably low elongation to failure of 230% was recorded in the naturally aged fine-grained (FG) Zn-0.3Al alloy at 150 °C, and dislocation creep was evaluated as the main deformation mechanism for this alloy. Detailed microstructural analyses showed that the FG Zn-0.3Al alloy was exposed to grain boundary corrosion during the natural aging process due to the presence of Al precipitates at grain boundaries whereas such formation was not observed in the UFG Zn-22% Al alloy. Grain boundary sliding was hindered by grain boundary corrosion and the cavity nucleation at the corroded grain boundaries caused a premature failure leading to such a low elongation in the Zn-0.3Al alloy.

Formation of Gradient Structures in the Zone of Joining a Deformable Nickel Alloy and a Single-Crystal Intermetallic Alloy during Thermodiffusion Pressure Welding and Subsequent Heat Treatment

The possibility of formation of a high-quality solid-phase joint of an Ni3Al-based single-crystal intermetallic VKNA-25 blade alloy with a high-temperature deformable EP975 disk alloy by pressure welding is studied to create high-performance one-piece blisk unit for the next-generation aviation gas turbine engines and to decrease the unit mass. The influence of the conditions of thermodiffusion pressure welding under the hightemperature superplasticity of the disk alloy and the influence of heat treatment of welded joints on the gradient structures in the welded joint zone and the structure at the periphery of the welded samples are investigated.

Erratum to: Evolution of Dislocation Structure in a Ni3Al Single Crystal Under the Conditions of High-Temperature Superplasticity

The evolution of defective structure in a single crystal of a Ni3Al-base intermetallic with iron and chromium additives under the conditions of superplasticity (δ = 140%) is studied by transmission electron microscopy. The tensile deformation is performed a 1150°C. Planar flaws and recrystallized grains are detected in the fractured zone of the metal. The mechanisms of high-temperature superplasticity of alloys based on Ni3Al intermetallic are discussed.

高温変形・クリープ・超塑性に関する今後の展望

高温変形・クリープ・超塑性に関する今後の展望

Evolution of Dislocation Structure in a Ni3Al Single Crystal Under the Conditions of High-Temperature Superplasticity

Evolution of Dislocation Structure in a Ni3Al Single Crystal Under the Conditions of High-Temperature Superplasticity

Research of Solid Compound Formation during Thermal Deformation Effect on Intermetallic Ni<sub>3</sub>Al-Alloy with a Heat-Resistant Alloy EP975 and Influence on the Physical, Mechanical and Performance Properties

The work is aimed at processing highly efficient one-piece units of gas turbine aircraft engines discs with blades (blisks) to exclude heavy locking joints and to reduce the weight of the units while improving their reliability. Intermetallic γ ́+γ alloys based on γ ́-Ni3Al phase of the VKNA type selected as blade material and Ni-based high alloy EP975 selected for discs have similar initial melting temperatures (~1340оС and~1300оC, respectively). Solid state joints (SSJ) EP975 //VKNA-mono are obtained at the homologous temperatures of 0.86-0.91 Tm (K).It is shown that the most efficient method for producing solid joints between the Ni3Al based intermetallic alloys and the EP975-type high-temperature nickel alloys is a solid state pressure welding in the conditions of high-temperature superplasticity; this technique has been developed in the IMSP RAS (grant RFBR No 130812200).

Improvement of high strain rate and room temperature superplasticity in Zn–22Al alloy by two-step equal-channel angular pressing

Abstract The Zn–22Al alloy was subjected to equal-channel angular pressing (ECAP) to improve its high strain rate (HSR) superplasticity at room temperature (RT). A well-designed two-step ECAP process formed an ultrafine-grained (UFG) microstructure with an average grain size of 200 nm as the lowest one obtained so far after ECAP processing of this alloy. Also, agglomerate- and texture-free microstructure with UFG Al-rich α- and Zn-rich η-grains separated mostly by high-angle grain boundaries (HAGBs) was produced by this process. The maximum RT elongation was achieved to be 400% with a strain rate sensitivity of 0.30 at a very high strain rate of 5×10 −2 s −1 after the two-step ECAP process. This elongation value is the highest one obtained at RT and at all strain rates for this alloy up to now. The current results demonstrate that such an improvement in superplasticity of Zn–22Al alloy after the two-step ECAP process can enhance its applications where RT and HSR superplasticity are strongly needed.

Study of the properties and the choice of alloys for bladed disks (blisks) and a method for their joining

The choice of materials for the bladed disks (blisks) that are intended for next-generation aviation gas turbine engines is grounded. As blade materials, single crystals of light heterophase γ′ + γ VKNA-type alloys based on the γ′(Ni3Al) intermetallic compound with an ordered structure are proposed. The choice of novel deformable EP975-type nickel superalloys, which are intended for operation at 800–850°C, as the disk material is grounded. It is shown that the most effective method for forming one-piece joints of an Ni3Al-based alloy and a high-alloy EP975-type nickel superalloy is the new process of solid-phase pressure welding under conditions of high-temperature superplasticity. Solid-phase joints are formed for heterophase Ni3Al-based alloy single crystals and deformable EK61 and EP975 nickel alloys. The gradient structures in the zone of the solid-phase joints that form under the conditions of low- and high-temperature superplasticity at homologous temperatures of ∼0.6T m and 0.9T m are studied. The character and direction of the diffusion processes at the joint of an intermetallic alloy single crystal and a deformable polycrystalline alloy are determined.

Structural changes during superplastic deformation of high-strength alloy 1933 of the Al-Mg-Zn-Cu-Zr system

The strain rate-temperature conditions of manifestation of the effect of high-temperature superplasticity by samples of an 1933 alloy have been established. Features of the structural state, phase composition, and deformation relief of samples of this alloy have been investigated. It has been established that grain-boundary sliding occurs intensely along both boundaries of ultrafine grains and boundaries of coarse polygonized grains parallel to the tension axis of the sample. In near-surface grain-boundary pores and in cracks that were formed and developed at the surface of the working part of these samples in the course of superplastic deformation, fiber structures were detected. The chemical composition of fibers has been investigated. The kinetics of the occurrence of phase transformations in the 1933 alloy in the course of heating of its samples to the temperature of tests has been studied, and probable causes for partial melting of the 1933 alloy have been analyzed. Fracture mechanisms for the samples deformed superplastically under the optimum conditions have been established.

Superplasticity and machinability in a four-phase ceramic

Fine grained four-phase ceramic materials were fabricated to have a combination of high temperature superplasticity and room temperature machinability. The composite ceramics were made of 25vol.% 3Y-TZP/8YSZ–25vol.% Al2O3–25vol.% MgAl2O4–25vol.% LaPO4, using LaPO4 powders fabricated in-house. X-ray diffraction and scanning electron microscopy revealed that the grain size for the best mixed samples was in the range of 600nm, tetragonal ZrO2 transformed into monoclinic, and reactions in the sintered samples produced a new phase, magnetoplumbite (LaMgAl11O19) with lath-like grains. The formation of magnetoplumbite was facilitated by the presence of yttria and by a liquid phase generated at elevated temperatures. These four-phase ceramics had a maximum hardness of 12GPa and fracture toughness was no more than 3MPa√m. Deformation rates at 1400°C under 40MPa stress were in the superplastic range of 10−3s−1 for most compositions. These four-phase ceramics were machinable as demonstrated using conventional tungsten drill bits.

Thickness of grain-boundary liquid phase and its effect on the mechanism of superplastic deformation

For a series of matrix aluminum alloys, an estimation of the thickness of the grain-boundary liquid phase under the optimum conditions of the manifestation of high-temperature superplasticity has been performed. It has been established that the contribution of grain-boundary sliding to the total deformation under these conditions is determining. It has been shown that the greater values of the thickness of the grain-boundary liquid phase correspond to greater values of the parameter m of the strain-rate sensitivity of the flow stress. The obtained dependences of the thickness of the grain-boundary liquid phase on m indicate that with increasing amount of the liquid phase the grain-boundary sliding progressively acquires features of viscous flow.

Microstructure and Mechanical Properties of a 1.6C (pct) Ultra-Fine Grained Ultra-High Carbon Steel

With massive trials, spheroidized by austeniting at 810°C and cooling by 1°C/min, a 1.6C (pct) Ultra-high Carbon Steel shows a microstructure of uniformly distributed fine carbides in the ultra-fine ferrite matrix. The grain size of ferrite matrix and spheroidized carbides are about 5um and 0.1～2um, respectively. Further investigation by TEM shows that much dislocation together with twins is obtained for the UHCS, and generally finer grains have higher dislocation density. The spheroidized steel exhibits high tensile strength of 910 MPa and high yielding strength of 653 MPa at room temperature, together with excellent elongation of 18.3%, which shows the UHCS can entirely satisfy certain grades of engineering materials and means the steel may substitute present engineering steel considering lower cost. Furthermore, the steel owns good high-temperature superplasticity, the elongation of 216% obtained at 800°C under a strain rate of 2.5×10-4. Initial analysis suggests that the superplastic deformation mechanics of the steel is grain boundary sliding and grain rotating (GBSR), coordinated by migration of dislocation.

Phase transformations and structural changes in the course of the high-temperature superplastic deformation of aluminum alloys

Phase and structural transformations, which develop under conditions characteristic of high-temperature superplasticity in matrix multicomponent aluminum alloys, have been investigated. It has been shown that the appearance of a metastable liquid phase is in principle possible in the structure of such alloys in the process of the establishment of the equilibrium state during their heating to the temperature of tests and in the course of superplastic deformation. The characteristic properties of the structural changes that occur in the course of the superplastic deformation of such alloys have been studied. The basic conditions that lead to the origin and the development of fibrous species in the course of the high-temperature superplastic deformation have been determined. The determining role of magnesium in the process of origin and development of such fibrous formations is shown.

Microstructural Refinement and Mechanical Property Improvements of Ti-5Al-1Sn-1V-1Zr-0.76Mo Alloy by Equal Channel Angular Extrusion Processing

Severe plastic deformation (SPD) by equal channel angular extrusion (ECAE) was investigated as a way to produce ultra fine grain (UFG) microstructure in Ti-5Al-1Sn-1V-1Zr-0.76Mo. DEFORM 3D Finite Element Model (FEM) was used to predict loads and strain distribution associated with ECAE which were validated experimentally The effects of resulting ultra fine grain microstructure on room temperature mechanical properties and high temperature super plasticity were investigated. ECAE processing: resulted in the conversion of coarse Widmanstatten structure to fine equiaxed microstructure with an average grain size of 4 μm. The alloy with fine grained equiaxed microstructure has 20- 25% higher yield strength, 15% higher ultimate tensile strength, and 50-100% higher % elongation to fracture and % reduction in area compared to the as received alloy containing coarse Widmanstatten microstructure. The ECAE processed alloy exhibits attractive superplastic formability charactersistics at 900-950oC with m value >0.4, flow stresses < 20 MPa, and superplastic elongations > 200%

Dynamic formation of zircon during high temperature deformation of zirconia–silica composites with alumina additions

A three phase ceramic composite of 8 mol% Y2O3 stabilized ZrO2 (YSZ), SiO2, and Al2O3 was evaluated for potential high temperature superplasticity. The amorphous SiO2 content was 5 wt.%, and increasing additions of Al2O3 were made. The effect of varying the Y2O3 stabilizer concentration in ZrO2 was also studied. Samples sintered at 1200 °C contained only YSZ, Al2O3, and amorphous SiO2, but ZrSiO4 formed in the samples above 1300 °C. Mullite (3Al2O3 · 2SiO2) was not detected in any samples. Specimens of 1 wt.% Al2O3–YSZ/SiO2 had an anomalously high deformation rate of ∼2 × 10−4 s−1 at 1200 °C when compared to YSZ/SiO2 without Al2O3 (∼4 × 10-5 s−1). Higher amounts of Al2O3 additions decreased the strain rate. Extensive deformation of Al2O3 doped YSZ/SiO2 at 1200 °C induced the formation of ZrSiO4 due to enhanced reaction rates. This distributed, yet locally interconnected, zircon phase rapidly eroded the strain rate after ∼60% deformation.