Experimental Characterisation of Aluminium 6082 at Varying Temperature and Strain Rate

Abstract: This article describes the experimental methodology used in overcoming the challenges of performing tests and recording results on specimens, which are suitable for such a wide range of test conditions. Uniaxial tensile tests were conducted on aluminium alloy 6082‐T352 at varying temperatures and strain rates to validate testing techniques and to determine the effect of these parameters upon this material. The applied strain rate varied over several orders of magnitude – using a screw‐driven tensometer for quasi‐static loading (6.9 × 10^?4 s^?1), a hydraulic piston rig for moderate strain rate (4.0 × 10¹ s^?1) and a tensile Hopkinson bar for high strain rate (1.5 × 10³ s^?1). Temperature was varied using a heat gun, and the air temperature was measured using a thermocouple in the hot air stream. Specimen temperature is determined by finite element modelling, and this correlates well with other work. Although it would have been possible to improve the design of individual tests for specific test conditions, an important objective was to conduct the entire series of tests in as consistent a manner as possible. The procedure for characterising the stress–strain behaviour for this material under these different loading conditions is also considered in some detail, as the real material behaviour deviates from simplified elasto‐plastic material models. Results presented for Al 6082 samples show a slight increase in yield stress with increasing strain rate, and a decrease in yield stress with increasing temperature.     

Hydrogen redistribution under stress-induced diffusion and corresponding fracture behaviour of a structural steel

ABSTRACT

Hydrogen redistribution under stress-induced hydrogen diffusion and corresponding fracture behaviour of a 960?MPa grade martensitic steel were studied. Slow strain rate tensile (SSRT) tests after hydrogen pre-charging were performed and the fracture surface was observed and analysed. The strain rate ranged from 10^?6 to 10^?4?s^?1. In the pre-charged sample with a certain hydrogen content of 0.62?ppm, hydrogen distribution was homogeneous before the SSRT test. After tensile testing, brittle fracture features appeared in the centre of the fracture surface, while ductile features appeared in the surrounding area. Brittle region size increased with the strain rate slowing down in the range from 10^?4 to 5?×?10^?6?s^?1, while it stabilised at the strain rate slower than 5?×?10^?6?s^?1. Relationship between the strain rate and the brittle region size was established and discussed based on the present data of hydrogen content in the material.

This paper is part of a thematic issue on Hydrogen in Metallic Alloys     

Deformation mechanism transition in Fe–17Mn–0.4C–0.06V TWIP steel with different strain rates

The dynamic tensile behaviour and deformation mechanism of the Fe–17Mn–0.4C–0.06V twinning-induced plasticity (TWIP) steel were investigated over a wide range of strain rates from 10^?4 to 10³ s^?1. With increasing strain rate, the stacking fault energy increased due to the increase of adiabatic heating temperature, ΔT. At 10^?4 to 10¹ s^?1, the transformation-induced plasticity (TRIP) effect coexisted with the TWIP effect and weakened with increasing strain rate. With the increase of strain rate in the range of 10^?1 to 10¹ s^?1, the TWIP effect strengthened gradually and intersected deformation twins were formed. When the strain rate was higher than 10¹ s^?1, the TRIP effect disappeared and the twinning was inhibited since the adiabatic heating effect elevated.     

High strain rate superplasticity of TiNP/2014Al composite

Superplasticity of the TiN_p/2014AI composite prepared by powder metallurgy method was investigated by tensile tests conducted at different temperatures (773, 798, 818 and 838 K) with different strain rates range from 1·7×10° to 1·7×10^?3s^?1. Results show that a maximum elongation of 351% is achieved at 818 K and 3·3·10^?1s^?1. At different deformation temperatures, the curves of m value can be divided into two stages with the variation of strain rate and the critical strain rate is 10^?1 s^?1. Superplastic deformation activation energy in the TiN_p/2014AI composite is 417 kJ mol^?1, which is related to liquid phase formation at triple points of grain boundaries and interfaces between the matrix and the reinforcement. Superplastic deformation mechanism of the TiN_p/2014AI composite is grain boundary sliding accommodate mechanism when the strain rate is lower than 10^?1 s^?1, and transfers to grain boundary sliding accommodation mechanism plus liquid phase helper accommodation mechanism when the strain rate is higher than 10^?1 s^?1     

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.     

Effect of telmperature and strain rate on tensile behaviour of M250 maraging steel

Abstract

The effect of temperature and strain rate on the 0·2% yield strength, ultimate tensile strength, and percentage elongation of M250 maraging steel was investigated under uniaxial tensile conditions in the temperature range from 25 (room temperature) to 550°C and strain rate range 10^?4–10^?1 S^?l. Up to 400°C the steel shows essentially strain rate insensitive behaviour with a gradual decrease in the 0·2% yield strength and ultimate tensile strength. The elongation remains constant at all strain rates up to 300°C. Fractographic analysis indicates that the increasing strain rate induces strain constraint resulting in an increased dimple size. An elongated structure was observed at temperatures above 400°C. X-ray diffraction reveals the presence of reverted austenite in the specimens tested at 550°C.

MST/3263     

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.     

Influence of strain rate on production of deformation induced ferrite and hot ductility of steels

Abstract

Compression testing was used to explore the influence of strain rate on the formation of deformation induced ferrite. Samples of a 0·4%C–1·4%Mn plain C–Mn steel were heated to 1225°C, cooled to test temperatures in the range 1100–610°C, and then given a true strain of 0·6, at strain rates of3 × 10^?2, 3 × 10^?3, and 3 × 10^?4 S^?1. At the lowest strain rate it wasfound that the strain to peak stress decreased with decreasing temperature in the range 750–610°C. This behaviour is related to the formation of thin films of the softer deformation induced ferrite at the γ grain boundaries at the higher temperatures, and spheroidisation at the lower temperatures. More normal stress–strain curves were observed at the higher strain rates, as raising the strain rate prevents the formation of deformation induced ferrite and delays spheroidisation. The strain rate was also found to have an important influence on the extent of recovery in the deformation induced ferrite; the lowest strain rate enabling full recovery and or recrystallisation to occur, thus keeping the film soft. This behaviour is shown to account for the poor hot tensile ductility at the lowest strain rates. Raising the strain rate in this temperature range improves the ductility because work hardening takes place, raising the strength of the ferrite closer to that of the y, thus preventing strain concentration from occurring.

MST/1934     

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     

Yield strength anomaly and dynamic strain ageing behaviour of recently developed advanced ultra-supercritical boiler grade wrought Ni-based superalloy IN 740H

The yield strength anomaly (YSA) and dynamic strain ageing (DSA) behaviour of advanced ultra-supercritical boiler grade wrought nickel-based superalloy IN 740H is studied by conducting tensile tests in temperature range 28–930°C and by employing strain rates 1 × 10^?2, 1 × 10^?3, 1 × 10^?4 and 1 × 10^?5 s^?1 followed by extensive electron microscopic examination. Increase in yield strength accompanied by impairment of ductility indicates that YSA exists in alloy IN 740H in temperature range of 650–760°C. The electron microscopic observation confirms that YSA is due to pinning of dislocations by γ′ precipitates and shearing of γ′ precipitates in IN 740H. DSA is observed in the temperature range of 200–500°C and is predominant at 300°C. The nature of serrated plastic flow due to DSA is dependent on the temperature and strain rate.     

Study on mechanical properties and constitutive model of 5052 aluminium alloy

The stress–strain relationship of 5052 aluminium alloy was investigated via quasi-static tensile tests and split Hopkinson pressure bar tests. The specimens were exposed to various temperatures (25–500°C) and strain rates (10^?4–0.7?×?10^4?s^?1). At strain rates ranging from 0.001 to 3000?s^?1, the material underwent significant work hardening. When the strain rate exceeded 5000?s^?1, the work hardening effect decreased and the flow stress was relatively constant. The Johnson–Cook constitutive model was modified to describe the deformation behaviour of the material subjected to high temperatures and strain rates. The accuracy of the modified model was verified through ballistic impact testing.     

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.     

High Strain Rate Compressive Tests on Wood

Abstract: The penetrating split Hopkinson pressure bar was used to study the response of dry maple wood under high strain rate impact load. Using longer bar and shorter specimens utilised the assumption of one‐dimensional stress waves travelling along the bars and specimen because the experiment fulfilled the ratio of diameter to length of bars condition in Kolsky bar experiments. The stress–strain relationships and behaviour of the fibre structure materials’ failure were investigated during the compressive dynamic tests at strain rates between 950¹ and 2000 s^?1. The mechanics of dynamic failure was studied and it was confirmed that deformation of specimen is a linear function of energy absorption by specimens.     

Thermal Effects During Tensile Deformation of Ti‐6Al‐4V at Different Strain Rates

An in‐depth analysis of the effect of heat generated by plastic work on the observed tensile behaviour of Ti­6Al­4V at different strain rates is presented. Special emphasis is put on the transition from isothermal to adiabatic conditions and how this transition is affected by several process parameters such as material properties, environmental conditions and sample geometry. Experiments are performed in isothermal conditions at moderate temperatures, from ?10 to 70 °C, as well as at strain rates from quasi‐static speeds to more than 1000 s^?1 using a split Hopkinson tensile bar setup. This experimental data is used in conjunction with numerical simulations to determine the evolution of temperature during the experiments and the temperature and strain rate sensitivity of the material, as well as the Taylor–Quinney coefficient. Finally, a full model of the material behaviour is presented and used to define clear limits for adiabatic and isothermal conditions.     

Microstructural development and superplasticity in Al–Li 8090 alloy

Abstract

The mechanical behaviour of an Al–Li–Mg–Cu–Zr 8090 alloy at a temperature of 515°C and strain rates in the range 10^?4?10^?2s^?1 was measured by tensile testing. The greatest strain rate sensitivity was measured in the middle of that strain rate range, and did not change significantly with strain. Large abrupt changes in strain rate during testing showed that the strain rate history had a significant effect, especially slow prestraining which gave a relative increase in flow stress and a reduction in rate sensitivity compared with testing at a constant rate to the same strain. The evolution of grain size was measured, and there was evidence that this aspect of the material microstructure could be used to explain the observed behaviour. This view was reinforced by the ability of a transition model of superplasticity, together with a simple model of the evolution of grain size distribution, to reproduce the essential features observed in testing with large changes in strain rate.

MST/3351     

Processing and superplastic properties of fine grained Si3N4/Al–Mg–Si composites

Abstract

Composites with an Al–Mg–Si alloy matrix containing 20 vol.-% of either Si₃N₄ whiskers or Si₃N₄ particulates were extruded at 773 K with a reduction ratio of 100: 1, and tensile experiments were performed under conditions of constant true strain rate. Recrystallisation and dynamic precipitation occurred during hot extrusion so that very small grain sizes of less than ~ 3 ;amp;#x03BC;m were produced. The extruded composites showed superplastic behaviour at high strain rates (above 10^?1 S^?1). The high strain rate superplasticity of the composites is attributed to the very small grain sizes. Internal cavities developed during straining and density studies revealed that the rate of increase of the extent of cavitation was lower at a temperature slightly above the partial melting temperature than at a temperature lower than the partial melting temperature. It is concluded that the presence of a liquid phase restricts the development of cavities because the liquid phase serves to relax the stress concentrations.

MST/3139     

Tensile stress–strain and work hardening behaviour of 316LN austenitic stainless steel

Abstract

The true stress (σ)–true plastic strain (?) data of a type 316LN austenitic stainless steel tested at nominal strain rates in the range 3×10^-5–3×10^-3 s^-1 and temperatures of 300–1123 K were analysed in terms of flow relationships proposed by Hollomon, Ludwik, Swift, Voce, and Ludwigson. The applicability of the particular flow relationship is discussed in terms of ‘complete’ and ‘applicable’ range fits of the experimental σ–? data. At all strain rates, in the case of the complete range fit, the Ludwigson equation followed the stress–strain data most closely at 300 K, while in the temperature range 523–773 K, the flow behaviour was described equally well by both the Ludwigson and Voce equations. In the temperature range 823–1023 K, the Voce equation described the flow behaviour most accurately in the case of the complete range fit of σ–? data at all strain rates. The analysis of σ–? data employing the Ludwigson equation in the applicable range fit covering low and intermediate strains, and the Hollomon equation at high strains provided close simulation of the observed flow behaviour in the temperature range 823–1023 K. At high temperatures of 1073 and 1123 K, the Ludwigson equation reduces to the Hollomon equation. The variations in different flow parameters of the Ludwigson and Voce equations with temperature and strain rate exhibited anomalous behaviour at intermediate temperatures because of dynamic strain aging.     

Dynamic deformation behavior of ultrafine grained aluminum produced by ARB and subsequent annealing

Commercial purity aluminum (1100-Al) sheets with various grain sizes, ranging from 0.2 to 10 μm, were fabricated through accumulative roll bonding (ARB) and subsequent annealing at various temperatures. Mechanical properties of these materials were examined at various strain rates ranging from 10^?2 to 10³ s^?1 (from quasi-static deformation to dynamic deformation). Yield strength of the UFG specimens did not change so much when the strain rate changed. Yielding behavior of the UFG Al with grain size of 1.4 µm was characterized by yield-drop phenomenon, which appeared at higher strain rate. It was found that strain-hardening of the Al matrix was significantly enhanced at high strain rates, which was independent of the grain size. Uniform elongation increased with increasing strain rate in the specimens with the grain size larger than 1 µm, while post-uniform elongation increased with increasing strain rate in the submicrometer grain-sized specimens. Consequently, total elongation of all specimens was improved as the strain rate increased.     

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.