Superplastic Properties Research Articles

Factors influencing superplasticity in the Ti-6Al-4V alloy processed by high-pressure torsion

Two different initial microstructures, martensitic and lamellar, were developed in a Ti-6Al-4V alloy to examine their effect on the high temperature mechanical properties and superplasticity after high-pressure torsion (HPT). Significant grain refinement was achieved in both conditions with grain sizes after HPT processing of ~30 and ~40 nm, respectively. The nanocrystalline alloy in both conditions was subjected to mechanical testing at 923–1073 K using strain rates in the range from 10−3 to 10−1 s−1. The martensitic and lamellar alloys exhibited excellent ductility at these high temperatures including superplastic elongations at 973 K with maximum elongations of 815% and 690%, respectively. The fcc phase was stable at elevated temperatures in the martensitic alloy and the results suggests the fcc phase may contribute to the superior superplastic properties of the martensitic alloy.

Effect of a minor titanium addition on the superplastic properties of a CoCrFeNiMn high-entropy alloy processed by high-pressure torsion

A CoCrFeNiMn high-entropy alloy (HEA) with an addition of 2 at% Ti was processed by high-pressure torsion to produce a grain size of ~30 nm and then tested in tension at elevated temperatures from 873 to 1073 K using strain rates from 1.0 × 10−3 to 1.0 × 10−1 s−1. The alloy exhibited excellent ductility at these elevated temperatures including superplastic elongations with a maximum elongation of 830% at a temperature of 973 K. It is shown that the Ti addition contributes to the formation of precipitates and, combined with the sluggish diffusion in the HEA, grain growth is inhibited to provide a reasonable stability in the fine-grained structure at elevated temperatures. By comparison with the conventional CoCrFeNiMn HEA, the results demonstrate that the addition of a minor amount of Ti produces a smaller grain size, a higher volume fraction of precipitates and a significant improvement in the superplastic properties.

Application of G. Lame’s and J. Gielis’ formulas for description of shells superplastic forming

The paper shows that the contour of a sheet blank at all stages of superplastic forming can be described by application of the universal equations known as a "superformula" and "superellipse". The work provides information on the ranges of values of the coefficients entering these equations and shows that their magnitudes make it possible to additionally determine the level of superplastic properties of the workpiece metal, the shape of the shell being manufactured, the stage of superplastic forming, and the presence of additional operations to regulate the flow of the deformable metal. The paper has the results of approximation by the proposed equations of shell contours that manufactured by superplastic forming by different methods.

Achieving superior superplastic properties in fine grained intermetallic alloys based on γ-TiAl + α2‑Ti3Al

Achieving superior superplastic properties in fine grained intermetallic alloys based on γ-TiAl + α2‑Ti3Al

SUPERPLASTIC DEFORMATION OF TWO PHASE MgLiAl ALLOY AFTER TCAP PRESSING

Magnesium based alloy containing 9 wt. % Li, 1,5 wt. % Al, composed of  (hcp + bcc) phases was cast under argon atmosphere and extruded at 350oC as bars of cross section 12x12 mm. Twist Channel Angular Pressing TCAP deformation was applied at a temperature of 160oC using BC rotation of samples in the following passes. TCAP tool consisted of helical part in horizontal area of the channel with an angle of lead γ = 30° to simulate back pressure and thus to increase the extrusion force. The initial grain size of the hexagonal  phase estimated at 30 m decreased in the following passes down to 6 m and that of bcc  phase decreased after TCAP from initial 12 m, down to 5 m. TEM studies after TCAP passes showed higher dislocation density in the  region than in the  phase. Crystallographic relationship (001)  || (110)  indicates parallel positioning of slip planes of both phases due to activation of slip. Electron diffraction indicates increase of grain misorientation with a number of TCAP passes. Stress/strain curves measured at temperature of 200oC show superplastic forming after 1st and 3rd TCAP passes with better superplastic properties with increasing number of passes. Values of strain rate sensitivity coefficient m were calculated at 0.31 after 1 TCAP pass using flow stress values from step strain increase curves. The sample after 3 TCAP passes showed increase of m value up to 0.47 for the strain rate range 10-5 – 5 x 10-4 s-1. Obtained values are slightly lower than those reported in the literature, however increase of a number of TCAP passes has a positive effect on superplastic properties due to fine grains and increase of their misorientation.

Superplastic Property of the Ti–6Al–4V Alloy with Ultrafine‐Grained Heterogeneous Microstructure

Ti–6Al–4V alloy having a heterogeneous microstructure composed of ultrafine‐equiaxed‐α‐grains and fine‐lamellar‐α‐grains is investigated for microstructural changes during superplastic deformation at temperature of 700 °C. The Ti–6Al–4V alloy having an optimum fraction of fine‐lamellar‐α‐grains exhibits an excellent superplastic property and the highest elongation of 583% (tested at 700 °C 10−3 s−1). This is mainly due to the optimized activation of grain‐boundary‐sliding and additional accommodation mechanism associated with frequent occurrences of dynamic recrystallization and β precipitation at boundaries during deformation of the heterogeneous starting microstructure. The present result suggests the possibility that optimizing the starting microstructure so as to have an optimum heterogeneous‐microstructure serves as an additional stress accommodation mechanism and leads to a large superplastic elongation.

Deformation mechanisms for superplastic behaviors in a dual-phase high specific strength steel with ultrafine grains

The superplastic behaviors of a high specific strength steel (HSSS) with dual-phase microstructure and ultrafine grains have been investigated under a temperature range of 873–973K and at a wide strain rate range of 10−4–10−1/s. The ultrafine grained HSSS exhibits excellent superplastic properties. The microstructure observations at interrupted strains for tests under temperature of 973K and at strain rate of 10−3/s have provided evidences of different mechanisms for two stages. At the first stage (strain range from 0% to 400%), the superplastic flow is attributed to the diffusional transformation from fcc austenite phase to intermetallic compound B2 phase coupled with grain boundary sliding. While intragranular dislocation activities should be the dominant mechanism for the second stage (strain range from 400% to 629%) due to the increased realistic strain rate by diffusive necking. The grain sizes of both phases are observed to be relatively stable and remain always sub-micron level during the high temperature tensile deformation, facilitating the superplastic flow.

Determination of superplastic properties from the results of technological experiments

The problem to determine experimentally the values of material parameters for two material models of superplastic flow, σ=Kξm and σ = K′ξm′εn, from the results of technological trials is considered. With this in view, a special computational procedure is developed to minimize the deviation of the theoretically predicted forming times from experimental data recorded during constant pressure forming trials of a sheet into a circular die. As compared with similar procedures known in the literature the methods suggested enable one to obtain a unique set of material parameters by using the whole set of available experimental data. The validity of the procedures suggested is confirmed by means of comparing the results obtained with corresponding finite element solutions. The accuracy of modeling of the experimentally measured values of the forming time is found to be better than 5% for all cases considered.

Thermal stability and superplastic behaviour of an Al-Mg-Sc alloy processed by ECAP and HPT at different temperatures

An Al-3%Mg-0.2%Sc alloy was processed by ECAP and HPT at different temperatures. Afterwards, samples subjected to 10 turns of HPT at 300 and 450 K, 8 passes of ECAP at 300 K and 10 passes of ECAP at 600 K were annealed for 1 hour at 523 K and their mechanical properties and microstructure were examined using microhardness measurements and EBSD analysis. In addition, tensile specimens with similar dimensions were machined from the HPT and ECAP-processed materials and further tensile tested at 523 K. The results demonstrate that the Al alloy processed by HPT at 450 K exhibits higher microhardness values (~138 Hv) and a smaller average grain size (~0.28 µm) after annealing at 523 K among all SPD processing conditions. Accordingly, the material subjected to HPT at an elevated temperature displays superior superplastic properties such that an elongation of ~1020 % was attained after testing at 523 K at 1.0 × 10−3 s−1. Furthermore, detailed EBSD analysis revealed a significant fraction of low-angle grain boundaries (LAGBs > 33 %) in the ECAP-processed material after annealing which may be responsible for the inferior superplastic behaviour by comparison with the HPT-processed samples (LAGBs < 13 %) tested at 523 K.

Analysis of Void Growth During Superplastic Deformation of Commercial AL5083 Alloy

Superplastic alloys and metals possess the ability to undergo large uniform strains prior to failure. A number of materials are subject to the cavitation during superplastic deformation. Cavitation usually leads to either the undesirable post-forming characteristics or to the premature tensile failure. It is also apparent that the cavities can preexist in the form of cracks and decohered interfaces, which develop during thermo-mechanical processing necessary to produce the superplastic microstructures. Evidently, extensive cavitation imposes significant limitations on their commercial application. The material constitutive equation constants of commercial AL5083 alloy contain strength coefficient, and strain rate sensitivity index is determined by superplastic bulge forming tests for 400, 450, 500, 550 °C. By comparing the results of a deformed sample, good accordance between experimental and FEM results is observed. Using the calculated values for C and m parameters, the effect of material properties such as the cavity growth rate, strain rate sensitivity index, strain hardening exponent and number of intentionally preexisted voids on specimens voids growth subject to the tensile deformation and to the biaxial deformation has been determined numerically. The tensile tests have been simulated by the FEM software ABAQUS v6.9 using commercial aluminum 5083 alloy that presents superplastic properties at temperatures in the range 400–550 °C. The simulations are implemented at 400, 450, 500 and 550 °C. The results of the numerical prediction obtained are in good agreement with the results of the experiments done by some other authors.

Influence of heat treatment parameters on the structure and mechanical properties of a titanium VT6 alloy in the submicrocrystalline state

The effect of additional annealing on the structure and mechanical properties of the titanium VT6 alloy in the submicrocrystalline state is studied. It is shown that the annealing at 833 K for 20 min does not significantly affect the mechanically properties of the alloy at room temperature. However, this annealing severely deteriorates the superplastic properties of the alloy. Annealing at 873 K for 5 min results in a sharp decrease in the strength properties of the alloy at room temperature (by about 20%). Nevertheless, the superplastic properties of the alloy after this annealing are the highest ones among those considered in this work. It is assumed that the state of grain boundaries plays the decisive role in the development of the superplastic flow of the VT6 alloy after three-dimensional pressing and subsequent annealing.

Enhanced strength and plasticity of Mg–6Zn–0.5Zr alloy by low-temperature indirect extrusion

Abstract The microstructure and tensile properties of Mg–6Zn–0.5Zr alloy processed by low-temperature indirect extrusion utilizing artificial cooling were investigated. Remarkable grain refinement was obtained by the application of artificial cooling, resulting in an increase in the yield strength of as much as 53 MPa at room temperature, as compared to the case (245 MPa) of indirect extrusion without artificial cooling. Besides this, the fine-grained alloy was found to have superplastic properties at elevated temperature, revealing tensile elongations of 720–810% at 200 °C at strain rates of 2 × 10 −4 –1 × 10 −3 s −1 . Mechanisms responsible for the high strength and superplastic properties are discussed.

微結晶粒セラミック材料における低温高速超塑性の可能性

For attaining high-strain-rate superplasticity (HSRS) in ceramic materials, prerequisites are described on the basis of existing knowledge about high-temperature plastic deformation and intergranular cavitation mechanisms. The knowledge suggests that a combination of grain-size refinement to 100-200 nm or smaller and enhanced diffusion caused by doping may highly enhance stress-relaxation around triple junctions and may lead to HSRS at temperatures lower than 1250-1300°C in structural ceramics. From this point of view, recent studies are reviewed and discussed on ceramic superplasticity at strain rates higher than 1 × 10−2 s−1. Attention is also placed on the fabrication of materials with nanosized grains and the resultant superplastic properties.

Continuous measurement of m-parameter for analyzing plastic instability in a superplastic ultra-fine grained magnesium alloy

In this paper, the superplastic properties of ultra-fine grained AZ31 Mg alloy prepared by ECAP are investigated. Ultra-fine grained materials (UFG) often exhibit superplastic behavior, but UFG microstructure is not stable at increased temperatures. Due to recovery and grain growth the m-parameter of strain rate sensitivity may decrease during the tensile test. Methodology of continuous measurement of m-parameter during the tensile test is presented. Tensile tests were conducted at relatively low temperatures of 150–250°C. The m-parameter decreased with increasing true strain for all studied conditions from approx. 0.5–0.3. The decrease was most pronounced for the highest temperature (250°C) and was attributed to recovery and recrystallization. Three different criteria of plastic instability were evaluated. Hart's criterion significantly underestimated the stability of plastic deformation, whereas other criteria based on geometrical model of grain boundary sliding were more appropriate for the description of elongation up to fracture.

Finite element modeling of superplastic co-doped yttria-stabilized tetragonal-zirconia polycrystals

Yttria-stabilized tetragonal-zirconia polycrystals (Y-TZP) have been shown to have superplastic properties at high temperatures, opening a way for the manufacture of complex pieces for industrial applications by a variety of techniques. However, before that is possible, it is important to analyze the deformation and fracture mechanisms at a macroscopic level based on continuum theory. In this paper, an elastic-plastic material model with a theoretical large deformation is constructed to simulate the true stress-true strain relationships of superplastic ceramics. The simplified constitutive law used for the numerical simulations is based on piecewise linear connections at the turning points of different deformation stages on the experimental stress-strain curves. The finite element model (FEM) is applied to selected tensile tests on 3-mol%-Y-TZP (3Y-TZP) co-doped with germanium oxide and other oxides (titanium, magnesium, and calcium) to verify its applicability. The results show that the stress-strain characteristics and the final deformed shapes in the finite element analysis (FEA) agree well with the tensile test experiments. It can be seen that the FEM presented can simulate the mechanical behavior of superplastic co-doped 3Y-TZP ceramics and that it offers a selective numerical simulation method for advanced development of superplastic ceramics.

Superplasticity of Friction-Stir Welded Al-Mg-Sc Alloy with Ultrafine-Grained Microstructure Obtained by Warm Severe Plastic Deformation

High-strength sheets of Al-5.4Mg-0.2Sc-0.1Zr alloy were produced by equal-channel angular pressing (ECAP) to 12 passes via route BC at 300 °C (573 K) followed by isothermal rolling at 300 °C (573 K) to a total thickness reduction of 80%. The final sheets with ultra-fine grained (UFG) structure were joined by friction stir welding (FSW). The tensile samples including all of the characteristic FSW microstructural zones were machined perpendicular to welding direction. The material demonstrated excellent superplastic properties in the range of temperatures from 350 (623 K) to 450 °C (723 K) at strain rates ranging from 8.3×10-3 s-1 to 3.3×10-1 s-1. The base material was found to be prone to abnormal grain growth at the testing temperature. This led to localization of the superplastic deformation in the stir zone section of the joints and thus limited total elongation-to-failure. The relationship between superplastic ductility and microstructure and application of this technique for the fabrication of large-scale superplastic sheets are discussed.

The effect of interfaces on mechanical and superplastic properties of titanium alloys

The effect of volume fraction of the β-phase on the mechanical and superplastic properties of ultrafine-grained titanium alloys with grain size d of ~0.2 µm was investigated by transmission and scanning electron microscopy, X-ray diffraction analysis, and tensile test experiments. The ultrafine-grained structure of the materials was formed by the multi-directional pressing technique. The structure in question is shown to improve the mechanical properties by 30–50 % and to lower down to 823 K, the temperature at which superplastic flow starts as compared to coarse-grained analogs, no matter what the phase composition and concentration of the alloying elements used. The reduced temperature is attributable to the activation of diffusion-controlled grain boundary sliding in the case of nonequilibrium interfaces of materials produced by severe plastic deformation. The fraction of the β-phase and its precipitation pattern are found to have significant influence on the temperature range in which superplastic flow occurs and on the maximum elongation to failure. A near-β Ti-5Al-5Mo-5V-1Cr-1Fe alloy with a large fraction of the β-phase (>34 %) under superplastic conditions exhibits record-breaking strains (>1300 %) that do not cause fracture of the material and extremely low flow stresses. This is associated with the activation of the grain boundary sliding due to an increase in the diffusivity along the phase boundaries in a case of microduplex structure.

Effect of β/B2 phase on cavitation behavior during superplastic deformation of TiAl alloys

Fine-grained Ti-(42.5–43.5)Al-8Nb-0.2W-0.2B-(0–0.1)Y (in at.%) alloys with two different phase constituents (i.e., (α2+γ) alloy and (β/B2+γ) alloy) have been prepared by means of thermo-mechanical processing. Tensile tests were performed at 1000 °C under an initial strain-rate of 10−4 s−1 to evaluate the superplastic properties, and it was found that both alloys exhibited impressive superplasticity so that the elongations reached ∼400%. Texture measurements preliminarily revealed that intragranular deformation and the subsequent dynamic recrystallization was the predominant superplastic deformation mechanism in (β/B2+γ) alloy, while grain boundary sliding was responsible for the large elongation of (α2+γ) alloy. Cavities caused by superplastic deformation have been quantitatively evaluated by using SEM and X-ray tomography to reveal the effect of β/B2 phase on cavitation behavior. It was found that in (α2+γ) alloy the cavities preferentially nucleated in the triple-junctions of interphase boundaries, whereas most of the small cavities in (β/B2+γ) alloy were located along the β/γ interfaces. Quantitative analysis revealed that the alloys had comparable cavity number density after deformation, but the (β/B2+γ) alloy was characterized by larger cavity sizes, and thus by higher cavitation volume fraction and coalescence parameter in comparison with that of (α2+γ) alloy. Moreover, the large cavities in (β/B2+γ) alloy preferentially elongated along the direction parallel to the tensile axis, whereas the large cavities were more equiaxed in (α2+γ) alloy. The different cavity nucleation–growth–coalescence behavior of the two alloys was clarified based on the divergence in superplastic deformation mechanisms. Besides, theoretical evaluations were performed and revealed that the cavity growth rate of γ-TiAl was much higher than ordinary Al alloys under similar deformation conditions. This was believed to be the main reason for the relatively lower superplastic elongations of TiAl-based intermetallics.

Superplastic Behavior of an Al-Cu-Mg-Mn-Ag Alloy

The superplastic properties and microstructural evolution of a commercial Al-4.4%Cu-0.5%Mg-0.4%Mn-0.5%Ag-0.1%Ti alloy were examined under tension at temperatures ranging from 450 to 520°C and strain rates ranging from 6.9x10-5 to 6.9x10-2s-1. The refined microstructure with an average grain size of about 11m was produced in thin sheets by a commercially viable thermomechanical process. The highest elongation to failure of 540% was attained at a temperature of 500°C and an initial strain rate of 6.9x10-4 s-1 with the corresponding strain rate sensitivity coefficient of 0.55. The microstructural evolution during superplastic deformation of the aluminum alloy has been studied quantitatively. Processing at temperatures above 475°C and strain rate below 1.4x10-3s-1 resulted in fracturing almost without necking with cavitation playing a major role in the failure. In contrast, at low temperatures and/or high strain rates, fracture occurred in a ductile manner by localized necking. The relationship between superplastic ductility and microstructural evolution is analyzed.

Superplasticity of high-strength Al-based alloys produced by thermomechanical treatment

The superplastic properties of two high strength aluminium alloys are investigated. The first alloy contains fine, coherent Al3(Zr,Sc) particles, and the second alloy contains both fine, coherent Al3Zr particles and coarse, eutectic Al3Ni particles. Due to its fine, stable grain structure, the alloy containing both Al3Zr and Al3Ni particles exhibits significantly better superplastic properties. The superplastic behaviour in tension under both constant and initial strain rates was compared, and the effective activation energy of superplastic deformation was calculated. The alloy containing both fine and coarse particles exhibit 1100%–1200% elongation at strain rates of 1 × 10−2 s−1–1 × 10−1 s−1 and temperature of 440 °C.