Average Strain Rate Sensitivity Research Articles

Superplastic Deformation Behavior of 2 mm Ti60 Rolled Sheet in Air Environment

The superplastic deformation behavior of 2 mm Ti60 sheet is studied, and the constitutive equation of superplastic deformation is established. The results show that when the strain rate is 5.00 × 10−3 s−1 and the temperature is 950 °C, the maximum superplastic elongation reaches 400%. Through the analysis of the true strain–true stress curve, it is found that the average apparent activation energy (Q) is 490.783 kJ mol−1 and the average strain rate sensitivity (m) is 0.49. Dynamic spheroidization (DG) promotes the transformation of lath α phase to equiaxed α phase. Dynamic recrystallization (DRX) promotes the generation of high‐angle grain boundaries (θ > 15°). After deformation, the strength of R‐type textures changes significantly. From the grip to the tip, the strength of R‐type texture gradually weakens, which is mainly caused by the rotation of grains during deformation. The deformation mechanism of Ti60 sheet is dominated by grain boundary sliding, and is coordinated by grain growth, DG, DRX, dislocation motion, and grain rotation.

Designed for resistance to puncture: The dynamic response of fish scales

Natural dermal armors are serving as a source of inspiration in the pursuit of “next-generation” structural materials. Although the dynamic strain response of these materials is arguably the most relevant to their performance as armors, limited work has been performed in this area. Here, uniaxial tension and transverse puncture tests were performed on specimens obtained from the scales of Asian carp over strain rates spanning seven decades, from 10−4 to 103 s−1. The importance of anatomical variations was explored by comparing the performance of scales from the head, middle and tail regions. In both loading orientations, the scales exhibited a significant increase in the resistance to failure with loading rate. The rate sensitivity was substantially higher for transverse loading than for in-plane tension, with average strain rate sensitivity exponents for measures of the toughness of 0.35 and 0.08, respectively. Spatial variations in the properties were largest in the puncture responses, and scales from the head region exhibited the greatest resistance to puncture overall. The results suggest that the layered microstructure of fish scales is most effective at resisting puncture, rather than in-plane tension, and its effectiveness increases with rate of loading. X-ray microCT showed that delamination of plies in the internal elasmodine and stretching of the fibrils were key mechanisms of energy dissipation in response to puncture loading. Understanding contributions from the microstructure to this behavior could guide the development of flexible engineered laminates for penetration resistance and other related applications.

Deformation behaviour and microstructures of semi-solid A356.2 alloy prepared by radial forging process during high solid fraction compression

Radial forging was introduced to the strain-induced step in the strain-induced melt activation process to prepare high-quality semi-solid A356.2 billet for the high solid fraction compression. Then, the deformation behaviour and microstructures at different compression velocities, temperatures and deformation zones were investigated. The results showed that radial forging can induce enough strain at 60% reduction of area to prepare ideal semi-solid microstructure. The microstructure had no obvious improvement at 75% reduction of area because the distortion energy may be saturated at 60%. During compression tests, the flow stress was sensitive to compression velocity ( Vc) but was insensitive to holding temperature ( Th), and it obeyed the power law [Formula: see text]. The average strain rate sensitivity was 0.2757, and the average apparent activation energy was 254.5 kJ/mol. The compression sample can be divided into hard deformation zone, transition deformation zone, severe deformation zone and free deformation zone. Different zones had different predominant deformation mechanisms which not only affected the morphology of the grains but also codetermined the flow stress. With increasing Vc and Th, the average particle size was increased from hard deformation zone, reached the maximum at severe deformation zone and then decreased at free deformation zone, while the opposite was the case for shape factor. At each deformation zone, the average particle size was increased while the shape factor was decreased with the increasing Vc because higher Vc caused more grains to merge into larger and more irregular grains, while the opposite was the case for the increasing Th because higher Th generated more liquid which helped to isolate the contacted particles and improve their spheroidization degree.

Superplastic behavior and microstructure evolution of a new Al-Mg-Sc-Zr alloy subjected to a simple thermomechanical processing

A new Al-Mg-0.15% Sc-0.10% Zr (wt%) alloy sheet with an average (sub)grain size of ~2.25µm was processed by a simple thermomechanical processing. Excellent superplastic (elongations of ≥800%) can be achieved at a temperature range of 450–500°C and a high strain rate range of 1×10−2–1×10−1s−1. A maximum elongation of ~1579% was obtained at 475°C and a high strain rate of 5×10−2s−1. Electron back scatter diffraction analysis and transmission electron microscopy results showed that superior superplastic ductility of the Al-Mg-Sc-Zr alloy can be ascribed to the complete transformation of low angle grain boundaries to high angle grain boundaries due to the occurrence of continuous dynamic recrystallization and the presence of stable coherent Al3ScxZr1−x particles that effectively impede the growth of the grains during superplastic deformation. Besides, strong β-fiber rolling textures gradually weakened, and random textures were predominant in the superplastic deformed alloy. Analyses on the superplastic data revealed that the average strain rate sensitivity parameter and the average activation energy of the Al-Mg-Sc-Zr alloy were ~0.48 and ~84.4kJ/mol–1, respectively. All results indicated that the main superplastic deformation mechanism was grain boundary sliding.

Effect of Sc and Zr additions on grain stability and superplasticity of the simple thermal–mechanical processed Al–Zn–Mg alloy sheet

Effect of scandium and zirconium on grain stability and superplastic ductility in the simple thermal–mechanical processed Al–Zn–Mg alloys was investigated. Tensile testing revealed that the Al–Zn–Mg alloy without Sc and Zr additions showed no superplasticity because of the larger grain size (>10μm) and the poor stability of the microcrystalline structure during superplastic deformation. However, the Al–Zn–Mg–0.25Sc–0.10Zr (wt%) alloy exhibited excellent superplastic (elongations of ≥500%) at a wide temperature range of 450–550°C and high strain rate range of 5×10−3–5×10−2s−1, and the maximum elongation of ~1523% was achieved at 500°C and 1×10−2s−1. Electron back scatter diffraction analysis and transmission electron microscopy results showed that superior superplastic ductility of the Al–Zn–Mg–0.25Sc–0.10Zr alloy can be ascribed to the complete transformation of low angle grain boundaries to high angle grain boundaries due to the occurrence of dynamic recrystallization and the presence of coherent Al3ScxZr1−x particles that effectively impede the growth of the grains during superplastic deformation. Besides, strong β-fiber rolling textures gradually weakened, and random textures were predominant in the superplastic deformed alloy. Analyses on the superplastic data revealed that the average strain rate sensitivity and the average activation energy of the Al–Zn–Mg–0.25Sc–0.10Zr alloy were ~0.37 and ~84.5kJ/mol, respectively. All results indicated that the main superplastic deformation mechanism was grain boundary sliding.

Hot compression deformation of an Mg–2.54Nd–0.26Zn–0.32Zr alloy

Abstract The hot compression deformation of an Mg–2.54Nd–0.26Zn–0.32Zr cast alloy was investigated in the 25–400°C temperature range at initial strain rates from 10−4 s−1 to 10−2 s−1. It was found that strain rate has little effect on true stress when the material was compressed below 250°C. Above this temperature, high strain rates resulted in an increased true stress. The average strain rate sensitivity exponent at higher temperature (300–400°C) was determined to be 0.24 and the calculated activation energy was 185 kJ · mol−1, which is higher than that of lattice self diffusion of pure Mg. The constitutive equation was established and the Zener-Hollomon parameter was calculated as a function of peak stress. The microstructure of a sample tested at 300°C was analyzed by optical microscopy.

Strain rate sensitivity and activation volume of Cu/Ni metallic multilayer thin films measured via micropillar compression

Micropillar compression testing with repeated jumps in strain rate is used to circumvent inherent difficulties associated with nanoindentation and tensile testing of free-standing films. Application to sputtered 21 nm/21 nm Cu/Ni multilayer thin films with a cube-on-cube texture reveals an average strain rate sensitivity (m = 0.014) and activation volume (V = 17 b3), comparable to nanocrystalline face-centered cubic metals. Yet, m increases by ∼50% and V decreases by 70% with increasing strain, opposite to trends reported for nanotwinned Cu. The large, strain-dependent shifts in m and V are dependent on the underlying misfit dislocation structure of Cu/Ni interfaces.

Nanoindentation strain rate sensitivity of thermo-oxidized PMR-15 polyimide

This article reports the study of rate-dependent mechanical properties of thermal-oxidized PMR-15 polyimide resins with a nanoindenter. A series of PMR-15 resin specimens have been isothermally aged at various temperatures, times, and pressures. The strain rate sensitivity of oxidized surface layer obtained at each aging condition has been determined from nanoindentation creep experiments using constant displacement-rate \( \left( {\dot{h}} \right) \) method. Results show that the average strain rate sensitivity in the oxidized surface layer is notably higher than that in the unoxidized interior, indicating that the oxidized surface layer has limited ductility and thus is susceptible to fracture. Effects of aging environments (time, temperature, and pressure) on mechanical properties are also examined. After passing the initial oxidation stage, the change in strain rate sensitivity become insignificant and less sensitive to aging parameters.

Flow behaviour and microstructure development of forged Ti-25Al-10Nb-3V-1Mo (super α2)

Samples of super α 2(Ti-25Al-10Nb-3V-1Mo) have been hot upset forged after heating to either the ( α 2 + β)- or the β-phase field. Two different die temperatures and three strain rates, 1 s −1, 10 s −1 and 100 s −, were used. Detailed microstructural observations have been carried out on these forged samples. It was found that at low strain rate (<10 s −) flow softening of a super α 2 in the ( α 2 + β)-phase field is mainly associated with deformation heating, which causes some structural changes and dynamic recovery. At the higher strain rate (> 10 s −1) dynamic recrystallization is significant. The flow softening in the β-phase field consists of dynamic recovery and recrystallization coupled with a certain amount of grain boundary sliding or migration at both low and high strain rate. The average activation energies for deformation were calculated from the forging flow stress curves as 414 kJ mol −1 in the ( α 2 + β)-phase field and 235 kJ mol −1 in the β-phase field. These values confirmed that the controlling mechanism of deformation is dynamic recovery and recrystallization. Average strain rate sensitivity parameters of 0.29 and 0.36 were also obtained in the (α + β)- and β-phase field.

The activation energy for superplastic flow in Al-6Cu-0.4Zr

A study of the activation energies for superplastic flow in an essentially single phase Al-6Cu-0.4Zr alloy has been made in the temperature range 430 to 490° C. Straight line Arrhenius plots for bothQσ and\(Q_{\dot \in }\) were obtained in Regions I, II and III. In all cases the ratio\(Q_{\dot \in } /Q_\sigma\) corresponded well to the average strain rate sensitivity as determined by both change rate testing and from the slope of the Inσ versus In\(\dot \in\) curves. Values ofQσ of 35.2, 19.0 and 20.1 kcal mol−1 were obtained in Regions I, II and III respectively. These values were expected to be close to the true activation energies, and corresponded to the measured lattice and predicted grain boundary diffusion activation energies. These energies, together with microstructural observations made on deformed material, were used to identify possible deformation mechanisms.