Superplastic behaviour of annealed AA 8090 Al-Li alloy

Abstract

High temperature flow behaviour and microstructural evolution were investigated in an annealed AA 8090 Al - Li alloy over the temperature range 623 - 803 K and strain rate range ~ 6 × 10⁶ - 3 × 10² s^-1. Stress - strain rate data, obtained using a differential strain rate test technique and plotted in log - log scale, exhibited three regions I, II, and III, with increasing strain rate. In these regions, the values of strain rate sensitivity index m and the activation energy for deformation were determined to be 0.17, 0.43, and 0.17; and 758.8, 93.3, and 184.3 kJ mol^-1, respectively. The stress - strain curves obtained from constant strain rate tests exhibited flow hardening at lower strain rates and higher temperatures whereas flow softening occurred at higher strain rates and lower temperatures. The microstructural evolution revealed the dominance of grain growth under the former conditions and dynamic recrystallisation under the latter conditions. Ductility and m were found to increase with temperature, with the maximum values of 402% and 0.55, respectively, at a temperature 803 K and strain rate 2 × 10^-4 s^-1.     

Effect of nitrogen content on dynamic strain ageing behaviour of type 316LN austenitic stainless steel during tensile deformation

Abstract

The effect of nitrogen content on the dynamic strain ageing (DSA) behaviour of type 316LN austenitic stainless steel has been studied. The nitrogen content was varied from 0·07 to 0·22 wt-%. The tensile tests were carried out over a temperature range of 300–1123 K and at three strain rates in the range 3×10^?3–3×10^?5 s^?1. Serration was observed in the load elongation curves in the intermediate test temperature range and has been considered due to DSA phenomenon. The critical strain to onset of serrated flow increased with increase in nitrogen content and strain rate. The temperature for onset of DSA and the temperature of disappearance of DSA were found to increase with the increase in nitrogen content. The variations in tensile strength and work hardening rate of the steel with temperature exhibit peak values in the intermediate temperature range and have been attributed due to DSA phenomenon. The activation energy for DSA, estimated based on the temperature and strain rate dependences of the strain to onset of serrated flow, was found to increase from 111 to 218 kJ mol^?1 with the increase in nitrogen content from 0·07 to 0·22 wt-% and the increase has been attributed to the possible enhanced interaction of the DSA causing interstitial nitrogen with substitutional chromium.     

Superplastic behaviour of as received superplastic forming grade IN718 superalloy

Abstract

Tensile specimens of superplastic forming grade IN718 superalloy, containing banded microstructure in the as received state, were deformed at high temperatures T to investigate the stress σ versus strain rate ? · behaviour, the nature of the stress versus strain ? curves, ductility, and microstructure upon failure. The log σ–log ? · plot for the ? · range ～5 × 10^-6–3 × 10^-2 s^-1 at T = 1173–1248 K exhibited a strain rate sensitivity index m = 0·62 at low strain rates and m = 0·26 at high strain rates, representing region II and III behaviour, respectively. The activation energies were estimated to be 308 and 353 kJ mol^-1, respectively. All the σ–? curves, obtained at ? · = 1 × 10^-4 s^-1 for the temperature range 1173–1273 K, and at T = 1198 K for the strain rate range 1 × 10^-4–1 × 10^-2 s^-1, exhibited initial flow hardening, followed by flow softening. The microstructures revealed dynamic recrystallisation, grain growth, cavitation, and a variation in the amount of second phase particles. Grain growth and cavitation were found to increase with temperature in region II. Excessive grain growth at 1273 K led to the elimination of region II. Grain growth and cavitation were both found to be less pronounced as the strain rate increased in region III.     

Effects of directional grain structure on impact properties and dislocation substructure of 6061-T6 aluminium alloy

Abstract

The effects of the grain structure direction on the impact properties and dislocation substructure of 6061-T6 aluminium alloy are investigated under room temperature conditions and strain rates of 1×10³, 3×10³ and 5×10³ s^?1 using a split-Hopkinson pressure bar system. The impact tests are performed using specimens machined from rolled 6061-T6 plates in the longitudinal, transverse and through thickness directions respectively. The results show that for all specimens, the flow stress increases with increasing strain rate. Furthermore, for all strain rates, the highest flow stress occurs in the transverse specimen. For strain rates of 1×10³ and 3×10³ s^?1, the flow stress in the through thickness specimen is greater than that in the longitudinal specimen. However, at a strain rate of 5×10³ s^?1, the flow stress in the longitudinal specimen is higher than that in the through thickness specimen due to a greater dislocation multiplication rate. For all three grain structure directions, the strain rate sensitivity increases with increasing strain rate, but decreases with increasing true strain. The highest strain rate sensitivity is observed in the longitudinal specimen at strain rates of 3×10³ to 5×10³ s^?1. The dislocation density increases markedly with increasing strain rate. Moreover, the square root of the dislocation density varies as a linear function of the flow stress in accordance with the Bailey–Hirsch relationship. The strengthening effect produced by the increased dislocation density is particularly evident in the transverse specimen, followed by the longitudinal specimen and the through thickness specimen.     

Superplasticity in a 7055 aluminum alloy subjected to intense plastic deformation

Abstract

Superplasticity in a 7055 aluminum alloy subjected to intense plastic straining through equal channel angular extrusion (ECAE) was studied in tension over a range of strain rates from 1.4 × 10^-5 to 5.6 × 10^-2 s^-1 in the temperature interval 300 - 450 °C. The alloy had a grain size of ~ 1 μm. A maximum elongation to failure of ~750% occurred at a temperature of 425 °C and an initial strain rate of 5.6 × 10^-4 s^-1, with a strain rate sensitivity coefficient m of about 0.46. The highest m value was ~0.5 at a strain rate of 1.4 × 10^-3 s^-1 and T≥ 425 °C. Moderate superplastic properties with a total elongation of about 435% and m of ~0.4 were recorded in the temperature interval 350 - 400 °C; no cavitation was found. It was shown that the main feature of superplastic behaviour of the ECAE processed 7055 aluminum alloy is a low yield stress and strong strain hardening during the initial stages of superplastic deformation. Comparing the present results with the superplastic behaviour of the 7055 Al subjected to thermomechanical processing (TMP), the highest tensile elongation in the ECAE processed material occurred at lower temperatures because ECAE produces a finer grained structure.     

Comparisons of deformation and fracture behaviour of PC/ABS blend and ABS copolymer under dynamic shear loading

Abstract

The dynamic shear deformation and fracture characteristics of PC/ABS blend and ABS copolymer with regard to the relation between mechanical properties and strain rate, are studied experimentally using a torsional split Hopkinson bar at room temperature under strain rates ranging from 8 × 10² s^-1 to 3.4 × 10³ s^-1. Fracture phenomena are analysed by scanning electron microscopy and correlated with macroscopic behaviour. The relative properties and fracture mechanism of both polymers are also compared. Results show that strain rate enhances shear strength of both PC/ABS blend and ABS, but fracture shear strain tends to decrease with increasing strain rate. ABS exhibits better ductility and lower shear strength. For both polymers, strain rate sensitivity increases with increasing range of strain rate, while an inverse tendency occurs for activation volume. Higher strain rate sensitivity and lower activation volume are found in PC/ABS blend. PC/ABS blend fracture is dominated by mixed shearing and tearing, but ABS fracture shows only shearing. Due to the increasing deformation heat, fracture surface viscoplastic flow for both polymers increases with increasing strain rate, inducing lower flow resistance and lower fracture strain at higher stain rates. The viscoplastic flow behaviour in ABS is more active.     

Superplasticity and deformation mechanisms in NiAl(- Mo) alloy

Abstract

The mechanical behaviour and microstructural evolution in tension of NiAl - 9Mo eutectic alloy at 1100 ° C and strain rates from 10^-5 to 10^-3 s^-1 have been investigated. High values of strain rate sensitivity index, but relatively small elongations between 150 and 200%, have been observed. Tensile specimens with various strains were quenched in water to preserve the dislocation structures for TEM examination. The TEM results show that the dislocation configuration and density change significantly with an increase in strain rate, and therefore correspond to different deformation mechanisms. At a low strain rate (5.5 × 10^-5 s^-1), the dislocation density is relatively low and dislocation activity is regarded as an accommodation mechanism for grain boundary sliding. In contrast, the high density of dislocations as well as clear subboundaries observed in grains at a high strain rate (5.5 × 10^-4 s^-1) suggest that the dislocations are active directly in response to the applied stress as well as participating in the relaxation process of grain boundary sliding by subboundary formation. Thus, grain boundary sliding is mainly responsible for superplastic deformation at a low strain rate, while superlastic deformation at a high strain rate is controlled by the combined operation of both grain boundary sliding and dynamic recovery.     

Characterisation of dynamic recrystallisation in nickel using processing map for hot deformation

Abstract

The hot deformation behaviour of polycrystalline nickel has been characterised in the temperature range 750–1200°C and strain rate range 0·0003–100 s^?1 using processing maps developed on the basis of the dynamic materials model. The efficiency of power dissipation, given by [2m/(m+1)], where m is the strain rate sensitivity, is plotted as a function of temperature and strain rate to obtain a processing map. A domain of dynamic recrystallisation has been identified, with a peak efficiency of 31% occurring at 925°C and 1 s^?1. The published results are in agreement with the predictions of the processing map. The variations of efficiency of power dissipation with temperature and strain rate in the dynamic recrystallisation domain are identical to the corresponding variations of hot ductility. The stress–strain curves exhibited a single peak in the dynamic recrystallisation domain, whereas multiple peaks and ‘drooping’ stress–strain curves were observed at lower and higher strain rates, respectively. The results are explained on the basis of a simple model which considers dynamic recrystallisation in terms of rates of interface formation (nucleation) and migration (growth). It is shown that dynamic recrystallisation in nickel is controlled by the rate of nucleation, which is slower than the rate of migration. The rate of nucleation itself depends on the process of thermal recovery by climb, which in turn depends on self-diffusion.

MST/1524     

Microstructural evolution and plastic stability during superplastic flow in a 7475 aluminium alloy

Abstract

Superplastic behaviour and microstructural evolution were examined at 788 K for strain rates in the range 2 × 10^-4–2 × 10^-3 s^-1 in a 7475 aluminium alloy of nominal composition Al–(1·2–1·9)Cu–(5·2–6·2)Zn–(1· 9–2·6)Mg (wt-%). In addition, the variation of the strain hardening and plastic stability parameters with strain was investigated based on experimental grain growth and cavitation data. The strain hardening parameter at 2 × 10^-4 s^-1 was high over a wide range of strain because of the high grain growth rate. Decrease in the strain hardening parameter due to cavitation was negligible. The highest plastic stability parameter was attained at 2 × 10^-4 s^-1, although the strain rate sensitivity was the lowest for the strain rate range investigated. This demonstrates the influence of grain growth on high plastic stability during superplastic deformation.     

Temperature and strain rate effect on tensile properties of 9Cr–1·8W–0·5Mo–VNb steel

Abstract

Tensile tests have been carried out on 9Cr–1·8W–0·5Mo–VNb steel (grade 92) over wide ranges of temperature (300–923 K) and strain rate (3×10^?3–3×10^?5 s^?1). The tensile strength of the steel decreased slowly with temperature at relatively lower temperature range, whereas rapidly in the higher temperature range with a plateau in the intermediate temperature range. The decrease in strain rate decreased the tensile strength of the steel both at lower and higher temperature ranges. Elongation to fracture and reduction in area increased with increase in temperatures and decrease in strain rate at higher temperature regime with a plateau in the intermediate temperature regime. The ductile mode of tensile failure has been observed in the investigated temperatures and strain rates. The plateau in the variation of tensile strength with temperature, the negative strain rate sensitivity of tensile strength and minimum in ductility of the steel in the intermediate temperature range are considered as a consequence of dynamic strain ageing. The rapid decrease in tensile strengths and increase in ductility at high temperatures have been attributed to the dynamic recovery.     

Processing parameters for the mechanical working of 9 Cr–1 Mo steel: processing maps approach

Abstract

Processing and instability maps using a dynamic materials model have been developed for 9Cr–1Mo steel in the temperature range 850 to 1200°C and strain rate range 0.001–100 s^–1 with a view to optimising its hot workability. The efficiency of power dissipation increased with increase in temperature and decrease in strain rate. The 9Cr–1Mo material exhibited two dynamic recrystallisation domains, one with a peak efficiency of 37% occurring at 950°C and 0.001 s^–1 and the other with a peak efficiency of 35% occurring at 1200°C and 0.1 s^–1. These results are in good agreement with those found in industry.     

Thermomechanical fatigue evaluation and life prediction of 316L(N) stainless steel

An attempt has been made to understand the thermomechanical fatigue (TMF) behaviour of a nitrogen-alloyed type 316L austenitic stainless steel under different temperature domains. Smooth, hollow specimens were subjected to in-phase (IP) and out-of-phase (OP) thermal–mechanical cycling in air under a mechanical strain control mode, at a strain rate of 6.4 × 10^?5 s^?1 and a strain amplitude of ±0.4%. For the sake of comparison, total strain controlled low cycle fatigue (LCF) tests were also performed at the peak temperatures of TMF cycling on similar specimens employing the same strain rate and strain amplitude. Life was found to depend on the thermal/mechanical phasing and temperature. Creep was found to contribute to life reduction in IP tests when the peak temperature of cycling was above 600 °C. A few TMF tests were performed in vacuum in order to assess environmental influence on life. Thermomechanical fatigue cycling led to the development of significant amounts of mean stresses and the stress response was generally higher compared to that of LCF tests at the peak cyclic temperatures. Also, the isothermal tests at the peak temperature of TMF cycling resulted in lower lives compared to those obtained under TMF. An attempt was made to predict the TMF life using the isothermal database and satisfactory predictions were achieved using the Ostergren’s frequency modified damage function (FMDF) approach.     

Influence of deformation induced ferrite,grain boundary sliding,and dynamic recrystallisation on hot ductility of 0·1–0·75%C steels

Abstract

The influence of C on hot ductility in the temperature range 600–1000°C has been examined for three C contents (0·1, 0·4, and 0·75 wt-%). Using a strain rate of 3 × 10^?3 s^?1, tensile specimens were heated to 1330°C before cooling to the test temperature. For the 0·4%C steel, two further strain rates of 3 × 10^?2 and 3 × 10^?4 s^?1 were examined. At the strain rate of 3 × 10^?3 s^?1, increasing the C content shifted the low ductility trough to lower temperatures in accordance with the trough being controlled by the γ–α transformation. Thin films of the softer deformation induced ferrite formed around the γ grain boundaries and allowed strain concentration to occur. Recovery to higher ductility at high temperatures occurred when these films could no longer form (i.e. above Ae₃) and dynamic recrystallisation was possible. The thin films of deformation induced ferrite suppressed dynamic recrystallisation in these coarse grained steels when tested at low strain rates. Recovery of ductility at the low temperature side of the trough in the 0·1%C steel corresponded to the presence of a large volume fraction of ferrite, this being the more ductile phase. For the 0·4%C steel decreasing the strain rate to 3 × 10^?4 s^?1 resulted in a very wide trough – extended to both higher and lower temperatures compared with the other strain rates. The high temperature extension was due to grain boundary sliding in the γ. Recovery of the ductility only occurred when dynamic recrystallisation was possible and this occurred at high temperatures. At the low temperature end, thin films of deformation induced ferrite were present and recovery did not occur until the temperature was sufficiently low to prevent strain concentration from occurring at the boundaries. Of the two intergranular modes of failure grain boundary sliding produced superior ductility. At the higher strain rates there was less grain boundary sliding, which led to a lower temperature for dynamic recrystallisation. Higher strain rates also increased the rate of work hardening of deformation induced ferrite, reducing the strain concentration at the boundaries. Ductility started to recover immediately below Ae₃, resulting in very narrow troughs. Finally, it was shown that the 2% strain that occurs during the straightening operation in continuous casting is sufficient to form deformation induced ferrite in steel containing 0·1%C.

MST/1809     

Assessment and modelling of isothermal forging of intermetallic compounds Part 1 – TiAl

Abstract

In this, the first of four papers concerned with the isothermal forging of intermetallic compounds, Ti–48Al–2Mn–2Nb (at.-%), an alloy based on the γ-TiAl intermetallic phase, has been deformed over the temperature and strain rate ranges 1050–1125°C and 3·0 × 10^-4–3·0 × 10^-2 s^-1 respectively. Examination of the stress–strain curves shows an increase in flow softening behaviour with increasing temperature and decreasing strain rate, contrary to what might have been expected. Forged microstructures indicate that grain refinement via dynamic recrystallisation has occurred, resulting in a fine, almost fully γ microstructure. Constitutive data calculated from initial stress–strain curves (for example activation energy of deformation and strain rate sensitivity) have been used to model deformation behaviour with a reasonable degree of success.     

Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 1—Mechanical behaviour

Abstract

A split Hopkinson bar is used to investigate the effects of prestrain and strain rate on the dynamic mechanical behaviour of 304L stainless steel, and these results are correlated with microstructure and fracture characteristics. Annealed 304L stainless steel is prestrained to strains of 0·15, 0·3, and 0·5, then machined as cylindrical compression specimens. Dynamic mechanical tests are performed at strain rates ranging from 10² to 5 × 10³ s^-1 at room temperature, with true stains varying from 0·1 to 0·3. It was found that 304L stainless steel is sensitive to applied prestrain and strain rate, with flow stress increasing with increasing prestrain and strain rate. Work hardening rate, strain rate sensitivity, and activation volume depend strongly on the variation of prestrain, strain, and strain rate. At larger prestrain and higher strain rate, work hardening rate decreases rapidly owing to greater heat deformation enhancement of plastic flow instability at dynamic loading. Strain rate sensitivity increases with increasing prestrain and work hardening stress (σ-σ_y). However, activation volume exhibits the reverse tendency. Catastrophic fracture is found only for 0·5 prestrain, 0·3 strain, and strain rate of 4·8 × 10³ s^-1. Large prestrain increases the resistance to plastic flow but decreases fracture elongation. Optical microscopy and SEM fracture feature observations reveal adiabatic shear band formation is the dominant fracture mechanism. Adiabatic shear band void and crack formation is along the direction of maximum shear stress and induces specimen fracture.     

Dynamic recrystallisation of as cast austenite in 18–8 stainless steel

Abstract

Dynamic recrystallisation behaviour of an as cast 0Cr18Ni9Ti stainless steel during hot deformation was investigated by hot compression test at a temperature range of 950–1200°C and strain rate of 5 × 10^-3–1 × 10^-1 s^-1. Change of austenite grain size owing to dynamic recrystallisation was also studied by microstructural observation. The experimental results showed that the hot deformation conditions, such as temperature, strain, and strain rate determine the dynamic recrystallisation behaviour for the as cast stainless steel, and the dynamically recrystallised grain size is determined by the deformation conditions and is independent of the strain.     

Analysis of high flow stress and microstructural evolution of TC6 titanium alloy during isothermal forging

Abstract

Elevated temperature true stress – strain curves have been determined for the isothermal deformation of a TC6 titanium alloy using hot compression testing in the deformation temperature range 800 – 1040°C, strain rate range 0.001 – 50 s^-1 and reduction in height of 30 – 50%. The experimental results show that the flow stress of TC6 titanium alloy is strongly dependent on process parameters, especially on the deformation temperature and strain rate. The peak stress and steady stress of such an alloy have the same characterisation, which increases with higher strain rate and lower deformation temperature. During isothermal forging, microstructural characterisation, including volume fraction, grain size, and grain pattern of prior α phase, varies with different temperatures, height reductions, and 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.     

Effects of strain rate and temperature on dynamic mechanical behaviour and microstructural evolution in aluminium–scandium (Al–Sc) alloy

Abstract

The present study applies a compressive split Hopkinson bar to investigate the mechanical response, microstructural evolution and fracture characteristics of an aluminium–scandium (Al–Sc) alloy at temperatures ranging from ? 100 to 300°C and strain rates of 1·2 × 10³, 3·2×10³ and 5·8 × 10³ s^?1. The relationship between the dynamic mechanical behaviour of the Al–Sc alloy and its microstructural characteristics is explored. The fracture features and microstructural evolution are observed using scanning and transmission electron microscopy techniques. The stress–strain relationships indicate that the flow stress, work hardening rate and strain rate sensitivity increase with increasing strain rate, but decrease with increasing temperature. Conversely, the activation volume and activation energy increase as the temperature increases or the strain rate decreases. Additionally, the fracture strain reduces with increasing strain rate and decreasing temperature. The Zerilli–Armstrong fcc constitutive model is used to describe the plastic deformation behaviour of the Al–Sc alloy, and the error between the predicted flow stress and the measured stress is found to be less than 5%. The fracture analysis results reveal that cracks initiate and propagate in the shear bands of the Al–Sc alloy specimens and are responsible for their ultimate failure. However, at room temperature, under a low strain rate of 1·2 × 10³ s^?1 and at a high experimental temperature of 300°C under all three tested strain rates, the specimens do not fracture, even under large strain deformations. Scanning electron microscopy observations show that the surfaces of the fractured specimens are characterised by transgranular dimpled features, which are indicative of ductile fracture. The depth and density of these dimples are significantly influenced by the strain rate and temperature. The transmission electron microscopy structural observations show the precipitation of Al₃Sc particles in the matrix and at the grain boundaries. These particles suppress dislocation motion and result in a strengthening effect. The transmission electron microscopy analysis also reveals that the dislocation density increases, but the dislocation cell size decreases, with increasing strain rate for a constant level of strain. However, a higher temperature causes the dislocation density to decrease, thereby increasing the dislocation cell size.     

Effect of aging on high strain rate and high temperature properties of 7075 aluminium alloy

Abstract

The high strain rate and high temperature properties of as cast and aged 7075 aluminium alloy were examined by metallographic observation and by means of a split Hopkinson bar test at temperatures between 25 and 300°C and at strain rates of 1·3 × 10³ and 3·1 × 10³ s^-1. The effect of aging, as well as strain rate and temperature, on the dynamic mechanical response, microstructure evolution, and fracture characteristics are presented. The compressive stress–strain response of as cast and aged 7075 alloy is found to depend strongly on both the applied strain rate and the test temperature. However, the aged material is generally found to be stronger than the as cast material. The work hardening rate is seen to decrease with increasing strain, strain rate, and temperature, and its value is higher in the aged material than in the as cast material. Microscopic observation shows that aging, strain rate, and temperature have a significant influence on the microstructural evolution and the changes in grain morphologies. The average grain size can be expressed by a Hall–Petch type relationship after impact deformation. Fracture surface examination revealed that a high strain rate favours the formation of deformed shear bands that are precursors to crack formation and fracture. The aged material has a better ductility owing to the higher percentage of transgranular fracture and an increased density of microdimples.