Superplastic properties of a TiAl based alloy with a duplex microstructure

The superplastic properties of a engineering TiAl based alloy with a duplex microstructure were investigated with respect to the effect of testing temperatures ranging from 950°C to 1075°C and strain rates ranging from 8 × 10^–5 s^–1 to 2 × 10^–3 s^–1. A maximum elongation of 467% was achieved at 1050°C and at a strain rate of 8 × 10^–5 s^–1. The apparent activation energy was calculated to be 345 kJ/mol. Also, the dependence of the strain rate sensitivity values on strain during superplastic deformation was examined through the jump strain rate tests, and microstructural analysis was performed after superplastic deformation. It is concluded that superplasticity of the alloy at relatively low temperature and relatively high strain rate results from dynamic recrystallization, and grain boundary sliding and associated accommodation mechanism is related to superplasticity at higher temperature and lower strain rate.     

A new grain-refinement process for superplasticity of high-strength 7075 aluminium alloy

The superplasticity of high strength 7075 aluminium alloy has been improved to a great extent by the new thermomechanical treatment proposed. This treatment (TMPA) includes solution treatment, overageing, warm-rolling deformation, recrystallization and an artificial ageing process. The maximum elongation may be up to 2100% under deformation at an initial strain rate of 8.33×10^–4s^–1 and a temperature of 510 °C, which is much higher than reported before. Observation of the microstructure changes revealed that the excellent superplastic elongation of the alloy seems mainly to be due to a decrease in the grain growth rate of the alloy and a reduction in the number of cavities nucleated during superplastic deformation.     

The development of internal cavitation in a superplastic zinc–aluminum alloy processed by ECAP

A Zn-22% Al eutectoid alloy was processed by Equal-Channel Angular Pressing (ECAP) to produce an ultrafine grain size and then pulled in tension at elevated temperatures to evaluate the role of internal cavitation under superplastic conditions. Tensile testing yielded a highest elongation of 2,230% at a strain rate of 1.0 × 10^?2 s^?1 at 473 K representing high strain rate superplasticity. Quantitative cavity measurements were taken to investigate the significance of the internal cavities formed during superplastic deformation. The results demonstrate that cavity nucleation occurs continuously throughout superplastic flow, and there is a transition in the cavity growth mechanism from superplastic diffusion growth at the smaller cavity sizes to plasticity-controlled growth at the larger sizes.     

High-strain-rate superplastic deformation behavior of a powder metallurgy-processed 2124 Al alloy

High-strain-rate superplastic behavior of a powder-metallurgy processed 2124 alloy prepared through extrusion at a high ratio of 70 : 1 was investigated. A maximum tensile elongation of 700% was obtained at 823 K and at a strain rate of 10^–2 s^–1. Deformation behavior of this alloy was similar to those reported for other many HSR superplastic materials. Incorporation of threshold stress into the constitutive equation reveals that the true stress exponent is 2 and true activation energy for plastic flow is comparable to that for lattice diffusion in pure aluminum. Comparison of the present alloy with the 2124 Al composite indicates that the composite is weaker than the unreinforced alloy in the temperature range where grain boundary sliding is rate-controlled.     

On the evaluation of parameters of the constitutive equation for 7475 Al alloy

From the mechanical data on 7475 Al alloy, it is evident that flow stress is significantly dependent on the strain during superplastic flow. This is due to its ability to strain-harden during superplasticity. The rate of increase in the flow stress is much higher at 457° C than at 517° C. This gives rise to non-unique values for the parameters of the constitutive equation. At 457° C, whereas the stress exponent (n) and activation energy for superplastic flow at 1 × 10^–4 sec^–1 increase only slightly with strain, the grain size sensitivity parameter (p) and structure parameter (A) decrease significantly with strain. These changes in the constitutive parameters are associated with dislocation activity occurring within the grain interior, leading to grain elongation without significant changes in the grain size, through the parameter, (b/d) ^p , of the constitutive equation.     

The effect of boron addition on brittle-to-ductile transition temperature and its strain rate sensitivity in gamma titanium aluminide

Temperature dependence of tensile properties ofTi–47Al–2Mn–2Nb–0.8TiB₂ alloy was investigated andbrittle-to-ductile transition temperature (T _BD) wasevaluated accordingly within the strain rate range from 10^–5 to10^–1 s^–1. T _BD and its strain rate sensitivity inTi-47Al-2Mn-2Nb-0.8TiB₂ alloy were compared withthose in Ti-47Al-2Mn-2Nb alloy. It is found that theminor addition of 1.0 at% boron reduces T _BD by more than100 K and that T _BD in both alloys shows a positivesensitivity to the strain rate. But the B-doped alloy has a lower BDTactivation energy (256 kJ/mol) than that of B-free alloy(324 kJ/mol). The effect of boron on T _BD and its strain ratesensitivity is attributed to the reduction in the grain size.     

Mesoscopic grain boundary sliding as the rate controlling process for high strain rate superplastic deformation

Important features observed during high strain rate superplastic deformation are enumerated. Starting from the premise that the phenomenon of structural superplasticity in different classes of materials results when grain boundary sliding that develops to a mesoscopic scale (defined to be of the order of a grain diameter or more) controls the rate of flow, the particular case of high strain rate superplasticity is explained. The rate equation developed is validated using experimental results concerning 5 alloy systems in which an ultra-fine grain size is developed by thermomechanical processing and retained in a similar condition during superplastic deformation by fine, grain boundary pinning particles and 3 alloy composites in which the volume fraction of the reinforcing constituent is significant (15–25%). It is demonstrated that the analysis results in estimates for the externally measured strain rates that are within a factor of two, in addition to providing a physically meaningful free energy of activation for the rate controlling process. This approach explains superplastic flow in different classes of materials in terms of a single rate controlling mechanism of deformation, viz., mesoscopic grain boundary sliding, with the help of a few constants that have the same values for all systems. The system-dependent variables of threshold stress needed for the onset of mesoscopic boundary sliding and free energy of activation are obtained directly from superplasticity stress–strain rate data, without external inputs.     

Microstructure and superplasticity of laser welded Ti–6Al–4V alloy

In this paper laser beam welding (LBW) was used to join Ti–6Al–4V alloy as a pre-forming operation before superplastic deformation (SPF) process. Superplastic deformation behavior of laser welded Ti–6Al–4V alloy was investigated. The results indicated that the welded Ti–6Al–4V alloy had good superplasticity when deformed at temperature range of 870–920 °C and strain rate range of 10⁻³–10⁻² s⁻¹, and the elongation was 233–397%. The microstructure observation indicated that dynamic recrystallization happened in the weld bead, and the acicular structure of weld bead was transforming into equiaxed grains during tensile process.     

Superplastic deformation of the Pb-Sn eutectic

Dynamic observations of grain-boundary sliding during superplastic flow of the Pb-Sn eutectic are reported. These observations confirm the postulate that the dominant deformation mode during superplastic flow is grain-boundary sliding with localized deformation necessary to maintain grain coherency. Extensive grain-boundary sliding is also observed when the strain-rate and/or grain size is outside the superplastic flow regime. Stress relaxation tests were also carried out on the Pb-Sn eutectic. These tests provide data on the activation energy (45±5 kJ mol^–1), grain-size dependence (d ^–3), and stress dependence of superplastic flow in this alloy. A threshold stress of 1.3×10⁶N m^–2 for the onset of superplastic deformation is also observed.     

High strain rate superplastic flow stress and post-deformation mechanical properties of mechanically alloyed 2024 aluminium alloy reinforced with SiC particles

Abstract

Composites consisting of 2024 aluminium alloys reinforced with volume fractions of 0, 5, 10, and 15 vol.-% of SiC particles were fabricatedfrom the mechanically alloyed powders by an optimised hot compaction and prestraining process. Fine and equiaxed grain structures with grain sizes of <1 μm were observed within the matrix of each alloy. The composite specimens were compressed at temperatures between 733 and 813 K with a wide strain rate range from 10^?3 to 10 s^?1. Two strain rate regions with different slopes from ～ 5 × 10^?1 s^?1 were found in log (true stress–log (strain rate) curves. In the lower strain rate region of each alloy, the strain rate sensitivity values m were 0.03–0.16. The threshold stress σ_th for each alloy was estimated using an extrapolation procedure. A linear relationship was found between <disp-formula><graphic href="splitsection10-m1.tif"/></disp-formula> and σ_th where V_f is the volume fraction of SiC particles. In the higher strain rate region of each alloy, m values greater than 0.3 were obtained at 773 K, which is very close to the solidus temperature of 775 K for 2024 aluminium alloy. Moreover, the maximum yield strength and elongation for each alloy at room temperature were also obtained in the specimens compressed at 773 K. Thus, it was found that the optimum temperature for the high strain rate superplastic processing of the composites was just below the solidus temperature of the 2024 aluminium alloy. The grain coarsening resulted in the decrease of post-deformation strength and ductility as well as the m value in hot compression above the solidus temperature.     

The tensile properties of a superplastic aluminium bronze

The high temperature tensile properties of a micrograin Cu-9.5% Al-4% Fe alloy, which is superplastic at 800° C, have been determined. Elongations at fracture of greater than 700% are achieved when the nominal strain-rate is in the range 3.9×10^–2 min^–1 to 7.9×10^–2 min^–1. The nature of plastic instability in superplastic materials is considered and it is shown that the amount of strain at the onset of plastic instability is inversely related to the applied strain-rate and is relatively independent of the strain-rate sensitivity exponent, m. The onset of plastic instability during a tensile test results in an increase of local strain-rate at the point of minimum cross-section and this, together with the existence of a triaxial stress state in the necked region, may produce errors in the m versus strain-rate plot if m is determined by the change-rate method. The initial strain-rate for maximum elongation is lower than the strain-rate for maximum m. This may be ascribed either to the influence of plastic instability or the formation of cavities at the higher strain-rates.     

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.     

Superplasticity of mullite-zirconia composite

Tension tests of mullite-zirconia composite were conducted at elevated temperature. A superplastic elongation of 122% could be achieved at an initial strain rate of 2.86×10^–5s^–1 at 1550°C. Strain hardening was observed at strain rates from 1.42×10^–4 to 2.86×10^s–5s^–1 at 1550°C. The addition of zirconia grains to the mullite matrix increased the creep rate of the composite.     

Superplastic deformation and microstructure evolution in PM IN-100 superalloy

The superplastic deformation behaviour of PM IN-100 alloys consolidated by hot isostatic pressing (HIP) was investigated in compression tests at temperatures between 1323 and 1373K. The microstructural changes were observed using scanning electron microscopy. In the high strain rate region, grain refinement occurs due to dynamic recrystallization, resulting in the work softening type stress-strain curves. At low strain rates, grain growth occurs during deformation corresponding to work hardening. The strain rate sensitivity index,m, reaches a maximum value (m = 0.6) at the optimum strain rate which depends on the test temperature. The grain size dependence coefficient,p, was determined to be 2.0. The activation energy for deformation was 348kJ mol^–1. The rate-controlling mechanism of superplasticity in as HI Ped IN-100 seems to be the grain-boundary sliding controlled by volume diffusion rather than grain-boundary diffusion.     

Plastic deformation behavior in dual-phase Ni–31Al intermetallics at elevated temperature

Plastic deformation behavior of dual-phase Ni–31Al intermetallics at elevated temperature was examined. It was found that the alloy exhibited good plasticity under an initial strain rate of 1.25 × 10⁻⁴ s⁻¹ to 8 × 10⁻³ s⁻¹ in a temperature range of 950–1075 °C. A maximum elongation of 281.3% was obtained under an initial strain rate of 5 × 10⁻⁴ s⁻¹ at 1000 °C. The strain rate sensitivity, m value was correlated with temperature and initial strain rate, being in the range of 0.241–0.346. During plastic deformation, both the two phases Ni₃Al and NiAl in dual-phase Ni–31Al could co-deform without any void formation or debonding, the initial coarse microstructure became much finer after plastic deformation. Dislocation played an important role during the plastic deformation in dual-phase Ni–31Al alloy, the deformation mechanism in dual-phase Ni–31Al could be explained by continuous dynamic recovery and recrystallization.     

Superplastic behaviour of YBa2Cu3O7−x /Ag superconductors

High-temperature deformation characteristics of YBa₂Cu₃O_7–x oxide (YBCO) and YBa₂Cu₃O_7–x /Ag composite (YBCO/Ag) in uniaxial compression have been investigated. A compression test was carried out at temperatures from 780–930°C at initial strain rates between 10^–6 and 10^–4 s^–1. YBCO/Ag composites with fine, dense and equiaxed grains were compressed over 120% with no indication of failure at higher temperatures, and the strain-rate sensitivity exponent, m, was found to be about 0.42–0.46 between 890 and 930°C. They are considered to be one indication of superplasticity. The activation energy for deformation was 500–580 KJ mol^–1. The specimens suffered grain growth slightly during the deformation at 930°C and the majority of growth might be a function of exposure time, temperature and silver content, but each grain maintained the equiaxed shape after extensive superplastic deformation. This is consistent with a grain-boundary sliding mechanism. The silver at grain boundaries acts to decrease the activation energy for deformation and promote the grain-boundary sliding.     

Effect of inverted pressurisation profile on deformation characteristics of 5083 aluminium alloy during superplastic forming

Abstract

Decreasing the cycle time for superplastic forming of a commercially available superplastic 5083 aluminium alloy has been studied in the present work by use of an inverted pressurisation profile. A right cylindrical cup with a depth/ diameter ratio of 0·5 could be superplastically gas pressure formed in less than 100 s. The deformation behaviour was similar to that of constant strain forming during the free bulging stage. In this stage, a stress state gradient from the pole to the edge of the formed dome was observed. Plasticity controlled growth of cavities was thought to be the mechanism for the increase of cavity volume fraction during forming. After the centre point of the deformed sheet touched the die surface, the metal flow pattern was found to be different from that of the traditional approach. The minimum thickness was not located at around the bottom corner of the cylindrical cup rather it was located ~ 7.5 mm away from the bottom centre of the cup with radius 20 mm. Significant cavity nucleation and coalescence caused higher cavity growth rates at large strains, owing to the continuous increase in strain rate resulting from the imposed pressurisation profile.     

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.     

Effect of superplastic deformation on the grain size and tensile properties of Al-6.2Zn-2.5Mg-1.7Cu (7010) alloy sheet

An Al-Zn-Mg alloy (7010) was cold-rolled and annealed to produce a small recrystallized grain size, and superplastically deformed in the temperature range 475 to 520° C at strain rates to 2.8×10^–3 sec^–1. At 500° C and sec^–1 superplastic elongations up to 350% were obtained, but above about 60% elongation the residual room-temperature tensile properties after heat treatment decreased due to increasing grain-boundary cavitation. Grain growth rates were increased by superplastic strain.     

The rate dependent response of a titanium alloy subjected to quasi-static loading in ambient environment

A dual phase Ti-6A1-4V alloy was tested in uniaxial tension over a large quasi-static loading range (10^–5–10^–1 s^–1) in ambient environment. As strain rate increases, strength of the alloy was found to increase at the expense of ductility. In the low strain-rate region, strain rate sensitivity of the material experienced a gradual decrease during plastic deformation. In the high strain-rate region, strain-rate sensitivity of the material was largely constant for most part of the plastic deformation. The different rate dependent behaviours are believed to be caused by a change of governing plastic deformation mechanism from dislocation slip at low strain rates to twinning at the highest strain rate. Strong fractographic and metallographic evidence was obtained to understand the micromechanisms of plastic deformation.