Effect of multipeak dynamic recrystallisation on structure of deformed and annealed copper

Abstract

Compression tests were carried out on fine grained copper at 870 K and at a constant true strain rate of 1·4×10^?3 s^?1. Under these conditions, well defined flow stress oscillations followed by steady state flow stress are obtained. Grain size measurements of as deformed material revealed non-monotonic grain coarsening when stress oscillations appear. It was found that grain coarsening is most effective when the flow stress decreases after the first flow stress peak. Periodic flow stress is accompanied by periodic grain coarsening until the latter becomes practically independent of strain when the steady state flow stress region is attained. The structural effects of static processes (recovery and recrystallisation) in dynamically recrystallised material were examined closely. According to the model of periodic dynamic recrystallisation, one would expect periodic changes of the driving force for static restoration processes (mainly metadynamic and/or static recrystallisation). From the present work, conclusions are drawn that are contrary to this concept of structural softening. The critical strain leading to grain coarsening during post-deformation annealing of hot deformed copper was found to be significantly less than the strain corresponding to the first flow stress peak. For higher strains, the grain size of dynamically recrystallised copper was found to be highly stable during annealing for 7 h at 870 K.

MST/978     

Influence of strain and strain rate on microstructural evolution during superplasticity of Mg–Al–Zn sheet

Strain-induced abnormal grain growth was observed along the gage length during high-temperature uniaxial tensile testing of rolled Mg–Al–Zn (AZ31) sheet. Effective strain and strain rates in biaxial forming of AZ31 sheets also affected the nature of grain growth in the formed sheet. For the uniaxial testing done at 400 °C and a strain rate of 10^?1 s^?1, abnormal grain growth was prevalent in the gage sections that experienced true strain values between 0.2 and 1.0. Biaxial forming of AZ31 at 5 × 10^?2 s^?1 and 400 °C also exhibited abnormal grain growth at the cross sections which experienced a true strain of 1.7. Uniaxially tested sample at 400 °C and a strain rate of 10^?3 s^?1, however, showed no abnormal grain growth in the gage sections which experienced true local strain values ranging from 1.0 to 2.3. The normalized flow stress versus temperature and grain size compensated strain rate plot showed that the deformation kinetics of the current AZ31 alloy was similar to that reported in the literature for AZ31 alloys. Orientation image microscopy (OIM) was used to study the texture evolution, grain size, and grain boundary misorientation during uniaxial and biaxial forming. Influence of deformation parameters, namely strain rate, strain, and temperature on grain growth and refinement were discussed with the help of OIM results.     

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.     

Influence of strain rate on ultimate tensile stress of coarse-grained and ultrafine-grained copper

In the present paper OFHC (oxygen free high conductivity) copper was tested by static and dynamic tensile tests at room temperature owing to strain rate investigation. Because of coarse-grained (CG) and ultrafine-grained (UFG) microstructure observation the copper was subjected to drawing and ECAP processes. The investigation of strain rate and microstructure was focused on the ultimate tensile stress (UTS) after the tensile tests. Following this study, it was found that strain rate is an important characteristic influencing the mechanical properties of copper. The ultimate tensile stress grew with strain rate increasing and this effect is more visible at high strain rates ( ~ 10² s⁻¹). Moreover, it was revealed that strain rate hasn't got any influence on the failure mechanism of the copper on the other hand it has an influence on the values of dimple size. While strain rate increases the dimple size decreases.     

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     

Evaluating the microstructure and mechanical properties of friction stir processed Al–Si alloy

In this study, mechanical behaviour and microstructural evolution in friction stir processing (FSP) of casting hypereutectic A390 aluminium alloy have been investigated. The mechanical behaviour of FSP samples was investigated by measuring the strain rate sensitivity using shear punch testing. The room-temperature shear punch tests were conducted at shear strain rates in the range of 10^?4–10^?1?s^?1. The results indicate that the strain rate sensitivity index increases from about 0.015 to 0.120 for as-cast A390 after third FSP pass and then experiences a further growth in FSP passes. The increase in the grain size and CuAl₂ intermetallic particle size result in a reduction in strain sensitivity index as well as shear strength after third FSP pass.     

Developing superplasticity in a magnesium AZ31 alloy by ECAP

The processing of a magnesium AZ31 alloy by equal-channel angular pressing refines the grain size to ~2.2 μm, but annealing for 30 min at 673 K coarsens the grains to ~6.0 μm. Despite this microstructural instability, the alloy is superplastic when pulled in tension at temperatures in the range of 623–723 K with elongations up to >1000% at strain rates at and below 10^?4 s^?1. Experiments within the superplastic regime show the strain rate sensitivity is ~0.5 and the activation energy is close to the value for grain boundary diffusion. It is demonstrated by calculation that the experimental results are in good agreement with a model for superplasticity based on grain boundary sliding.     

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 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     

A constitutive model for fcc crystals with application to polycrystalline OFHC copper

Based on the results of a series of experiments on commercially pure OFHC copper (an fcc polycrystal), a physically based, rate- and temperature-dependent constitutive model is proposed for fcc single crystals. Using this constitutive model and the Taylor averaging method, numerical calculations are performed to simulate the experimental results for polycrystalline OFHC copper. The model calculation is based on a new efficient algorithm which has been successfully used to simulate the flow stress of polycrystalline tantalum over broad ranges of temperature, strain rate, and strain (Nemat-Nasser, S., Okinaka, T., Ni, L., 1998. J. Mech. Phys. Solids 46, 1009). The model effectively simulates a large body of experimental data, over a broad range of strain rates (0.001–8000 s⁻¹), and temperatures (77–1096 K), with strains close to 100%. Few adjustable constitutive parameters of the model are fixed at the outset for a given material. All other involved constitutive parameters are estimated based on the crystal structure and the physics of the plastic flow.     

Microstructure evolution of ZK40 magnesium alloy during high strain rate compression deformation at elevated temperatures

Abstract

Microstructure evolution of the homogenised ZK40 magnesium alloy was investigated during compression in the temperature range of 250–400°C and at the strain rate range of 0·01–50 s^?1. At a higher strain rate (?10 s^?1), dynamic recrystallisation developed extensively at grain boundaries and twins, resulting in a more homogeneous microstructure than the other conditions. The hot deformation characteristics of ZK40 exhibited an abnormal relationship with the strain rate, i.e., the hot workability increased with increasing the strain rate. However, the dynamic recrystallisation grain size was almost the same with increasing the temperature at the strain rate of 10 s^?1, while it increased obviously at the strain rates of 20 and 50 s^?1. Therefore, hot deformation at the strain rate of 10 s^?1 and temperature range of 250–400°C was desirable and feasible for the ZK40 alloy.     

Flow stress and recrystallisation during hot deformation of Cu–9%Sn alloys

Abstract

Compression tests were carried out on two compositions of Cu–Sn bronze: Cu–9·2Sn and Cu–9·1Sn–0·26Zn (wt-%). The experiments were performed at temperatures from ambient up to 750°C and at nominal (initial) strain rates in the range 10^-3 to 10^-1 s^-1. The measured data were converted into true stress–true strain curves; these displayed yield drops as well as single peaks (or maxima) at higher temperatures and lower strain rates. The mean rate sensitivity applicable to the curves was 0·25. Optical metallography indicated that dynamic recrystallisation of the ‘grain refinement’ type was taking place at the higher temperatures and proceeded by necklace formation. Electron backscattered diffraction measurements were also carried out; these revealed that twinning plays an important role in these materials. The present results show that the progress of recrystallisation is considerably slower than in OFHC copper and that the recrystallised grain size is appreciably finer. These observations, taken together, all indicate that the high temperature flow behaviour of the tin bronzes is controlled by solute drag and is not of the conventional ‘pure metal’ type.     

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.     

Strain-rate and grain-size effect on substructures and mechanical properties in OFHC copper during tension

The combined effect of grain size (recrystallized grains of 34, 86, 105 and 128 m) and strain rate (0.01, 0.05, 0.25, 2.5 and 5 min^–1) on the evolution of dislocation substructures and mechanical properties in oxygen-free high conductivity (OFHC) copper during room-temperature tensile testing has been studied. Under identical conditions of deformation, the flow stress values for smaller grain size were higher than those for larger grain sizes with the exception in the case of 86 m which has been attributed to the inhomogeneous substructural developments in the microstructures. The cell size decreases monotonically with increase in per cent strain indicating no signs of cell size saturation. The effect of strain rate on the development of dislocation substructures at constant strain is such that the cell size decreases initially but increases with further increase in strain rate for smaller grain sizes of 34 and 86 m while a reverse trend has been observed for larger grain sizes of 105 and 128 m. A graph of the cell size strengthening coefficient, k, and the strain rate shows three distinct stages in the curves for different grain sizes.     

Copper-based materials formed by high rate sputtering

This paper presents the results of microstructure and property determinations made on pure and alloyed copper sputter deposited at rates of 20–25 nm s^-1. For each material, the effects of substrate temperature and post-deposition annealing on the grain size and tensile properties were investigated. Fully dense coherent structures were obtained at all substrate temperatures. The absence of the porous poorly bonded structures obtained at temperatures in the range 0.1–0.2T_m by other workers is attributed to the low gas pressure, the normal incidence adatoms and the polished substrates used in the present study.All materials exhibited high tensile strenghts when deposited at 0.2T_m Values of 700 MPa were observed with pure copper, while precipitation-hardening and dispersion-hardening alloys produced values of 1000MPa. The principal difference between these materials was in the stability of their as-deposited structure and their strength during post-deposition annealing. For example, pure copper was unstable to recrystallization at room temperature (0.2T_m), while the dispersion- strengthened alloys exhibited only minor softening at 0.8T_m and very little grain growth at 0.95T_m. The mechanical properties are discussed in relation to the microstructure produced by high rate deposition.     

High Temperature Deformation and Microstructural Features of TXA321 Magnesium Alloy: Correlations with Processing Map

The hot deformation of cast TXA321 alloy has been studied in the temperature range 300–500 °C and in the strain rate range 0.0003–10 s^?1 by developing a processing map. The map exhibited four domains in the temperature and strain rate ranges: (1) 300–325 °C and 0.0003–0.001 s^?1, (2) 325–430 °C and 0.001–0.04 s^?1, (3) 430–500 °C and 0.01–0.5 s^?1, and (4) 430–500 °C and 0.0003–0.002 s^?1. The first three domains represent dynamic recrystallization, resulting in finer grain sizes in the first two domains and coarser in the third domain. In the fourth domain, the alloy exhibited grain boundary sliding resulting in intercrystalline cracking in tension and is not useful for its hot working. Two regimes of flow instability were identified at higher strain rates, one at temperatures <380 °C and the other at >480 °C.     

Effect of strain rate on microstructures and mechanical properties of Fe–18Cr–8Ni steel

The effect of strain rate on deformation microstructures and mechanical properties of Fe–18Cr–8Ni austenitic stainless steel was investigated at strain rates of from 10^?3 to 100?s^?1. The results indicated that the deformation mechanism of steel changes from transformation induced plasticity (TRIP) to TRIP?+?twinning induced plasticity (TWIP) effect when the strain rate is increased from 10^?3 to 100?s^?1. The yield strength of steel increases gradually with strain rate increased, while the tensile strength and elongation first decreases and then increases slowly. The changes in tensile strength and elongation are due to the change of deformation mechanism with the strain rate increased.     

Principles of superplasticity in ultrafine-grained materials

Ultrafine-grained materials are attractive for achieving superplastic elongations provided the grains are reasonably stable at elevated temperatures. Since the strain rate in superplasticity varies inversely with the grain size raised to a power of two, a reduction in grain size to the submicrometer level leads to the occurrence of superplastic flow within the region of high strain rate superplasticity at strain rates >10⁻² s⁻¹. This paper tabulates and examines the various reports of superplasticity in ultrafine-grained materials. It is shown that these materials exhibit many characteristics similar to conventional superplastic alloys including strain rates that are consistent with the standard model for superplastic flow and the development of internal cavitation during the flow process.     

Strain rate and gage length effects on tensile behavior of Kevlar 49 single yarn

In the present paper, Kevlar® 49 single yarns with different gage lengths were tested under both quasi-static loading at a strain rate of 4.2 × 10^?4 s^?1 using a MTS load frame and dynamic tensile loading over a strain rate range of 20–100 s^?1 using a servo-hydraulic high-rate testing system. The experimental results showed that the material mechanical properties are dependent on gage length and strain rate. Young’s modulus, tensile strength, maximum strain and toughness increase with increasing strain rate under dynamic loading; however the tensile strength decreases with increasing gage length under quasi-static loading. Weibull statistics were used to quantify the degree of variability in yarn strength at different gage lengths and strain rates. This data was then used to build an analytical model simulating the stress–strain response of single yarn under dynamic loading. The model predictions agree reasonably well with the experimental data.