Grain refinement and superplastic behaviour of a modified 6061 aluminium alloy

Abstract

The superplastic properties and microstructure evolution of a 0.15%Zr and 0.7%Cu modified 6061 aluminium alloy were examined in tension at temperatures ranging from 475 to 600°C and strain rates ranging from 7 × 10^-6 to 2.8 × 10^-2 s^-1. The refined microstructure with an average grain size of about 11 μm was produced in thin sheets by a commercially viable thermomechanical process. It was shown that the modified 6061 alloy exhibits a moderate superplastic elongation of 580% in the entirely solid state at 570°C and ? = 2.8 × 10^-4 s^-1. Superior superplastic properties (elongation to failure of 1300% with a corresponding strain rate sensitivity coefficient m of about 0.65) were found at the same strain rate and a temperature of 590°C, which is higher than the incipient melting point of the 6061 alloy (~575°C). The microstructural evolution during superplastic deformation of the 6061 alloy has been studied quantitatively. The presence of a slight amount of liquid phase greatly promotes the superplastic properties of the 6061 alloy, reducing the cavitation level.     

High strain rate superplasticity of ultrafined Ti–10V–2Fe–3Al compact prepared by sintering and isothermal forging

Abstract

High strain rate superplasticity was obtained for powder Ti–10V–2Fe–3Al (Ti-1023) alloy prepared by powder sintering and isothermal forging technology. The selected powder was cold isostatic pressed, sintered and isothermal forged to prepare this powder alloy. Tensile testing was conducted at optimum superplastic temperaure of 1023 K with different initial strain rate, and the elongation to failure, the flow stress and the microstructure were analysed. The experiment results exhibited that the microstructure of this powder alloy is extraordinary uniform and fine, resulted in considerable enhancement of optimum initial strain rate increased from 3·3×10^?4 s^?1 of conventional cast and wrought Ti-1023 alloy to 3·3×10^?3 s^?1 of this powder alloy. The elongation to failure increased first and then decreased with initial strain rate from 3·3×10^?4 to 3·3×10^?2 s^?1. The strain rate sensitivity m is about 0·46 near initial strain rate of 3·3×10^?3 s^?1, larger than conventional cast and wrought Ti-1023 alloy. Microstructure observations showed that dynamic recrystallisation and grain growth were present during superplastic deforming.     

Superplasticity of single-phase Ni–48Al intermetallics with large grains

Superplasticity was found in single-phase Ni–48Al alloy with initial grain size of 200 μm under an initial strain rate of 1.25×10⁻⁴ to 2×10⁻³ s⁻¹ at temperatures ranging from 1025 to 1100 °C. The maximum elongation of 188.2% was obtained under an initial strain rate of 1.125×10⁻³ s⁻¹ at 1100 °C. Optical metallography (OM) showed that the grains were refined during superplastic deformation from initial 200 to less than 20 μm. Transmission electron microcopy (TEM) observation showed that an unstable subgrain boundary network formed during superplastic deformation. The subgrain boundaries were transformed into low- and high-angle grain boundaries by absorbing gliding dislocations. The large-grained superplastic phenomenon could be explained by continuously dynamic recovery and recrystallization (CDRR).     

The superplasticity and microstructure evolution of coarse-grained Ti40 alloy

The superplastic deformation characteristics of coarse-grained Ti40 alloy have been studied in the temperature and strain rate range of 760–880°C and 5?×?10^?4 to 1?×?10^?2?s^?1, respectively. The alloy exhibited good superplasticity in all test conditions except at 760°C and strain rate higher than 5?×?10^?3?s^?1, with the maximum elongation of 436% at 840°C, 1?×?10^?3?s^?1. The activation energy value was found to be close to the self-diffusion activation energy of Ti40 alloy, suggesting that the rate controlling mechanism was lattice diffusion. The coarse grain was elongated and refined which can be attributed to the occurrence of dynamic recovery and continuous dynamic recrystallisation. These processes were promoted by the subgrain formation and evolution, resulting in the good superplasticity of Ti40 alloy with coarse grains.     

High strain rate superplasticity of hot extruded and hot rolled AA 6013/20 vol.-%SiCp composite

Abstract

High strain rate superplasticity(HSRS)of an AA 6013/20SiC_pcomposite, produced by powder metallurgy and then hot extruded or hotrolled, was evaluated by means of tensile tests carried out over a range of initial strain rates from 1 × 102 to 3.8 × 10^-1 s^-1 and temperatures from 520 to 590 ° C. A maximum elongation to failure of 370% was achieved in a hot rolled composite deformed at 1 × 10^-1 s^-1 and 560 ° C. Substantially lower elongations were achieved in hot extruded composites, with a maximumof200% at1 × 10^-2 s ^-1 and 580 ° C. The lower elongations in the hot extruded composite could be related to the large quantity of intermetallic compounds, shown by TEM analyses, which probably hinder large superplastic elongations. In both hot extruded and hot rolled composite, the flow stress was strongly dependenton temperature and strain rate; a steady state flow stress region was observed in the specimen that exhibited the maximum elongation to failure. The strain rate sensitivity index m reached a maximum ofabout 0.4 for the hot rolled composite, and about 0.35 for the hot extruded composite. Analyses of the fracture surfaces of hot rolled composite deformed at the maximum elongation, were characterised by the presence of many filaments or 'whiskers', which are generally considered as evidence of a liquid phase present at grain boundaries or interfaces.     

Hot deformation behaviour of Cu-1.5Ti (wt-%) alloy

Abstract

A Cu-1.5Ti (wt-%) alloy was subjected to hot compression tests at temperatures ranging from 750 to 900°C and strain rates from 10⁰ s^-1 to 10^-3 s^-1. Flow softening was found to occur at all temperatures and strain rates studied. Deformation at 750°C and a relatively high strain rate (100 s^-1) resulted in grain refinement of the alloy with a grain size of ~25 μm. Room temperature hardness decreased with increasing deformation temperature, i.e. 145 HV10 after deforming at 750°C and 90 HV10 at 900°C. The higher values of hardness observed after deformation at 750°C are attributed to the fine grain size. A maximum value of 0.21 obtained for the strain rate sensitivity index m is not indicative of superplasticity in this alloy. Activation energy Q for the hot deformation process at 1173 K and strain rate 10^-3 s^-1 was determined to be 76 kJ mol^-1.     

Superplasticity of Fe3Al(Cr)

Abstract

The superplasticity of an Fe₃Al based intermetallic alloy with 3 at.-% chromium has been investigated in the strain rate range 10^-5-10^-2 s^-1 at test temperatures between 700 and 900°C. The composition of the iron aluminide was Fe–28Al–3Cr (at.-%) with additions of titanium and carbon. After thermomechanical processing the material possessed a coarse grained microstructure with an average grain size of 55 ± 10 μm. Strain rate exponents of 0·33≤m≤0.42 were recorded at strain rates of approximately 10^-5-10^-3 s^-1 in the temperature range 750-900°C. Superplastic elongations of 350% and more were achieved. From thermal activation analysis of superplastic flow, an activation energy of 185 ± 10 kJ mol^-1 was derived. This value is comparable to activation energies of superplastic flow in Fe₃Al(Ti) alloys. However, in unalloyed Fe₃Al the activation energy is higher, ~ 263 kJ mol^-1. Optical microscopy showed grain refinement to ~ 30 ± 5 μm in size in superplastically strained tensile specimens. Transmission electron microscopy gave evidence of the formation of subgrains of 0·3–0·5 μm in size. Superplasticity in this iron aluminide is mainly attributed to viscous dislocation glide, controlled by solute drag in the transformed B2 lattice at the deformation temperatures. During superplastic deformation, subgrain formation and grain refinement in the gauge length were revealed. From this it is concluded that dynamic recrystallisation makes an important contribution to the deformation mechanism of superplastic flow in this material.     

Hot deformation mechanisms in metastable beta titanium alloy Ti–10V–2Fe–3Al

Abstract

The mechanisms of hot deformation in the β titanium alloy Ti–10V–2Fe–3Al have been characterised in the temperature range 650–850°C and strain rate range 0·001–100 s^-1 using constant true strain rate isothermal compression tests. The β transus for this alloy is ～790°C, below which the alloy has a fine grained duplex +β structure. At temperatures lower than the β transus and lower strain rates, the alloy exhibits steady state flow behaviour while at higher strain rates, either continuous flow softening or oscillations are observed at lower or higher temperatures, respectively. The processing maps reveal three different domains. First, in the temperature range 650–750°C and at strain rates lower than 0·01 s^-1, the material exhibits fine grained superplasticity marked by abnormal elongation, with a peak at ～700°C. Under conditions within this domain, the stress–strain curves are of the steady state type. The apparent activation energy estimated in the domain of fine grained superplasticity is ～225 kJ mol^-1, which suggests that dynamic recovery in the β phase is the mechanism by which the stress concentration at the triple junctions is accommodated. Second, at temperatures higher than 800°C and strain rates lower than ～0.1 s^-1, the alloy exhibits large grained superplasticity, with the highest elongation occurring at 850°C and 0.001 s^-1; the value of this is about one-half of that recorded at 700°C. The microstructure of the specimen deformed under conditions in this domain shows stable subgrain structures within large β grains. Third, at strain rates higher than 10 s^-1 and temperatures lower than 700°C, cracking occurs in the regions of adiabatic shear bands. Also, at strain rates above 3 s^-1 and temperatures above 700°C, the material exhibits flow localisation.     

Superplasticity in an alulllinium alloy 2124/SiCp composite

Abstract

The superplastic potential of an aluminium alloy 2124/SiC_p composite, fabricated by powder metallurgy techniques, has been investigated. Instead of any special thermomechanical processing or hot extrusion, simple warm rolling has been employed to obtain a fine grained structure before superplastic testing. Constant strain rate tests were performed to characterise the superplastic behaviour of the composite. All tests were performed in air at temperatures of 743–783 K and in the strain rate range 10^-3-10^-1 S^-l. A maximum elongation of 425% was achieved at a temperature of 763 K and a strain rate of 8.3 × 10^-2 S^-1. The highest value obtained for the strain rate sensitivity index (m) was 0.41. Differential scanning calorimetry was used to ascertain the possibility of any partial melting in the vicinity of optimum superplastic temperature. These results suggested that no liquid phase existed where maximum elongation was achieved and deformation took place entirely in the solid state. Optical and electron microscopy were used to examine the materials microstructure before and after superplastic testing.     

Superplasticity in Al–33Cu eutectic alloy in as extruded condition

Abstract

Experiments were carried out to determine the superplastic properties of the Al–33Cu eutectic alloy in an as extruded condition. It is shown that the stress–strain curves do not attain a steady state condition and, except at high strain rates greater than ~10^?2 s^?1, the curves show strain hardening due to concurrent grain growth. There is a sigmoidal relationship between stress and strain rate, with a maximum strain rate sensitivity of ~ 0·5 at intermediate strain rates in region 2 and a decrease in the strain rate sensitivity to ~ 0·3 at low strain rates in region 1. The maximum elongation to failure in these experiments is ~1400% at an initial strain rate of 6·7 × 10^?5 s^?1 and there is a decrease in the elongations to failure at both lower and higher strain rates. From detailed experimental measurements of grain growth, it is demonstrated by calculation that there is a genuine region 1 at low strain rates in this alloy in the as extruded condition.

MST/911     

Effect of alloying element modification on superplastic deformation properties of Ti–4Mo–4Al–2Sn–0·5Si (IMI550)

Abstract

The superplastic deformation properties of a commercial titanium alloy, IMI550 (Ti–4Mo–4Al–2Sn–0·5Si, wt-%) are compared with a specially melted ‘modified’ alloy in which 1 wt-%Mo (slow diffusing element) is replaced with 1 wt-%Fe (fast diffusing element) on the basis that the diffusional properties of the β phase, to which these alloying elements segregate, is a significant factor in determining the overall flow properties. It is shown that the superplastic deformation properties of both alloys in the temperature range 800–850°C develop with strain as the initial heterogeneous α + β structure develops into uniform duplex equiaxed microstructures. Thereafter, the modified alloy exhibits enhanced superplastic properties (reduced flow stresses in the strain rate range 5 × 10^-5–5 × 10^-3 s^-1). The results are analysed quantitatively on the basis of the Ashby and Verrall model applied to the α and β phases combined according to the isostress model for two phase deformation which accounts for the interaction of the α and β phases.     

Tensile properties of fine-grained 7475 aluminium alloy at various temperatures and strain rates

For a superplastic 7475 alloy, tensile properties are tested under various conditions in order to investigate the effects of the tensile conditions on the mechanical properties as well as on the microstructure after working. The results obtained here suggest a possibility of high-speed forming of this alloy. Under conditions of 400°C and 8.3×10⁻²s⁻¹ strain rate, a comparatively large elongation of about 200% can be attained. This may be due to the fact that the fine subgrains arise during the deformation. Also, mechanical properties at room temperature of the specimen prestrained under this condition are found to be fairly satisfactory.     

Application of high-pressure torsion to Al-6 %Cu-0.4 %Zr alloy for ultrafine-grain refinement and superplasticity

An Al-6 %Cu-0.4 %Zr alloy was processed by high-pressure torsion to produce an ultrafine-grained structure with a grain size of ~200 nm at the steady-state level where the hardness remains constant with straining. Tensile testing showed that a maximum elongation of ~530 % was attained at 673 K with an initial strain rate of 1 × 10^?3 s^?1. Evaluation of the strain-rate sensitivity and the activation energy for the deformation confirmed that grain boundary sliding through grain boundary diffusion is the rate-controlling process for the superplastic deformation.     

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.

     

Realization of high strength and high ductility for AZ61 magnesium alloy by severe warm working

Extruded Mg–6%Al–1%Zn (AZ61) alloy bar was subjected to 4-pass Equal Channel Angular Extrusion (ECAE) processing at 448–573 K. At the processing temperature of 448 K, extremely fine grains with the average grain size of 0.5 mm are formed as a result of dynamic recrystallization originated by fine Mg₁₇Al₁₂ (b) phase particles having 50–100 nm diameter dynamically-precipitated during ECAE processing. The sizes of both α matrix and β phase decrease with decreasing processing temperatures. In tensile test at room temperature under the strain rate of 1×10^—3 s^—1, tensile strength increases with decreasing ECAE processing temperatures due to fine grains, fine precipitates and residual strain hardening. Especially, highest strength of 351 MPa was achieved in the specimen ECAE-processed at 448 K. In addition to such high strength, elongation reaches 33% in that specimen. This specimen exhibits clear strain rate dependencies of both flow stress and elongation even at room temperature. As a result, higher elongation of 67% is obtained under low strain rate of 1×10^—5 s^—1.In such specimen, non-basal slip and grain boundary sliding occur in addition to basal slip. Furthermore, there are grains with no dislocations, suggesting the occurrence of dynamic recovery. The contribution of all the deformation mechanisms would cause high ductility in fine-grained AZ61 alloy specimen with high strength.     

Superplasticity in NiAl intermetallics macroalloyed with iron

In this paper, we aim to examine the superplastic behavior of an extruded Ni–28.5Al–20.4Fe (at.%) alloy, which consists of β+γ phases with an average linear intercept grain size of 30–50 μm. Its tensile properties were determined at temperatures from 1123 to 1323 K and initial strain rates from 1.04×10⁻² to 1.04×10⁻⁴ s⁻¹. A maximum elongation of 233% was obtained at 1123 K and a strain rate of 5.2×10⁻⁴ s⁻¹. Transmission electron microscope (TEM) observation found many dislocation-free grains adjacent to grains with a high-dislocation density and subgrains and subgrain boundaries, which indicate that dynamic recrystallization has occurred as an efficient accommodation mechanism. Scanning electron microscope (SEM) examination of the fracture sample after superplastic deformation reveals many voids on the fracture surface. By correlating with the results of TEM observation, it is suggested that the superplastic deformation in this alloy should be controlled by a grain boundary sliding-based mechanism accommodated by the movement of dislocation and dynamic recrystallization.     

Superplastic behavior of a powder metallurgy superalloy during isothermal compression

In this work,the flow behaviors and microstructure evolution of a powder metallurgy nickel-based superalloy during superplastic compression is investigated.Based on the strain rate sensitivity m determined by flow data,superplastic region is estimated at relatively low temperature and strain rate domains,specifically around 1000 ℃/10~(-3)s~(-1).Thereafter,the cylinder specimens are isothermally compressed at 1000 ℃/10~(-3)s~(-1) and 1025 ℃/10~(-3)s~(-1) with different strains,to exam the superplasticity and related mechanisms.The experimental results indicate that the accumulated dislocations are mainly annihilated by dynamic recovery and dynamic recrystallization(DRX),and the grain boundary sliding(GBS)contributes to the total strain during superplastic compression as well.In addition,the cavities and cracks at triple junctions or interfaces between matrix and second phase particle have not been detected,which is different from superplastic tensile deformation.     

Superplasticity and microstructure of TC21 titanium alloy during thermomechanical treatment

Abstract

As rolled TC21 titanium alloy was subjected to isothermal constant strain rate tensile tests using an electronic tensile testing machine. After tensile deformation, the alloys were subjected to double annealing. Superplastic behaviour and microstructure evolution were systematically investigated. Experimental results show that as rolled TC21 alloy exhibits good superplasticity at temperatures ranging from 870 to 930°C and strain rates ranging from 3×10^?4 to 3×10^?2 s^?1. A maximum elongation of 373·3% was obtained at 910°C and 3×10^?4 s^?1. In addition, the alloy microstructure comprises α and β phases during plastic deformation. The primary α-grains aggregate and merge to form new crystal grains with irregular grain boundaries because of dynamic recrystallisation. Furthermore, the primary α phase content gradually decreases with increasing temperature. The resulting microstructure after deformation and double annealing is a duplex microstructure comprising a primary equiaxed α phase and a β-transformed lamellar structure. The acicular α phase transformed from the β phase is mutually interlaced as a basketweave structure after deformation at 930°C and double annealing.     

The superplasticity and microstructure evolution of TC11 titanium alloy

The superplasticity is the capability of some metallic materials to exhibit very highly tensile elongation before failure. The superplastic tensile tests were carried out at various deformation conditions in this paper to investigate the superplastic behaviors and microstructure evolution of TC11 titanium alloy. The results indicate that the smaller the grain size, the better the superplasticity is, and the wider the superplastic temperature and strain rate is, in which the superplastic temperature is ranging from 1023 to 1223 K and the strain rate is ranging from 4.4 × 10^?5 to 1.1 × 10^?2 s^?1. The maximum tensile elongation is 1260% at the optimum deformation conditions (1173 K and 2.2 × 10^?4 s^?1). For further enhancing the superplasticity of TC11 titanium alloy, the novel tensile method of maximum m superplastic deformation is adopted in the paper. Compared with the conventional tensile methods, the excellent superplasticity of TC11 titanium alloy has been found with its maximum elongation of 2300%.     

Superplasticity and production of mechanically milled Ti–6Al–4V–0.5B

Abstract

Superplastic forming of conventional titanium alloy sheet is limited commercially by the relatively long cycle times imposed by the high temperatures and slow strain rates required. In order to minimise cycle times material with a fine grain size is required to allow either, an increase in the forming rate or a reduction in the deformation temperature. This study details the manufacture of Ti–6Al–4V–0.5B powder with a nanocrystalline grain size, which was produced by mechanical milling. The material was consolidated by hot isostatic pressing at a range of temperatures during which ～2.5 vol.-%TiB was formed by an in situ reaction between the titanium and boron. The TiB particles limited the growth of the grain size in the titanium from the nanocrystalline structure in the powder to sizes in the range 600 nm–4 µm after consolidation. The consolidated material was hot tensile tested at a range of temperatures and strain rates. A superplastic elongation of 310%was achieved when testing at 900°C at a strain rate of 6×10^-2 s^-1 compared with 220% for conventional Ti–6Al–4V sheet. However, extensive cavitation, induced by the presence of argon, occurred during high temperature deformation and limited the superplastic extensions achieved.