Microstructural evolution after thermomechanical processing in an equiatomic,single-phase CoCrFeMnNi high-entropy alloy with special focus on twin boundaries

The FCC-structured equiatomic CoCrFeMnNi high-entropy alloy was produced by arc melting and drop casting. After homogenization, the drop-cast ingots were cold rolled to sheets with six different final thicknesses (thickness reductions of 21, 41, 61, 84, 92 and 96%). Samples were cut from the rolled sheets and annealed for 1 h at temperatures between 400 and 1000 °C. The recrystallization temperature was then determined as a function of cold work by means of scanning electron microscopy and electron backscatter diffraction measurements. Additionally, Vickers indentation was performed on these samples. It was found that the microhardness first tends to increase slightly upon annealing below the recrystallization temperature but then drops steeply for higher annealing temperatures due to the onset of recrystallization. To study grain growth kinetics, samples that underwent 96% cold rolling were first recrystallized for 1 h at 800 °C, which is the lowest temperature at which complete recrystallization occurs, and then annealed at temperatures between 800 and 1150 °C for various times. The grain growth exponent was determined to be approximately n = 3, and the activation energy Q = 325 kJ/mol, both of which agree well with published values for this alloy. EBSD measurements were made in the as-recrystallized and grain growth samples to analyze the annealing twins. The density of annealing twins in the grain growth samples was found to depend only on grain size, i.e., it was independent of annealing temperature and time. No such correlation could be found for the as-recrystallized samples. These observations are discussed in the framework of existing theories for the formation of annealing twins.     

Prediction of internal stresses during growth of first- and second-generation twins in Mg and Mg alloys

The present work is dedicated to the development of a mean-field continuum mechanics method capable of predicting internal stresses in parent and twin phases during first- and second-generation twinning. For that purpose, a generalized Tanaka-Mori scheme in heterogeneous elastic media with plastic incompatibilities is developed. The work is applied to the case of first- and second-generation twinning in hexagonal close packed magnesium. In the case of first-generation twinning, the model is capable of predicting the trends in the development of back-stresses within the twin domain. A parametric study is performed to explain the roles of grain and twin shape and of relative volume twin volume fractions on the magnitude and directions of the back-stresses. In addition, applying the methodology to the case of second-generation twinning allows identification, in exact agreement with experimental observations, of the most likely second-generation twin variants to grow in a primary twin domain.     

State-of-the-knowledge on TWIP steel

Abstract

High Mn twinning induced plasticity (TWIP) steel is a new type of structural steel, characterised by both high strength and superior formability. TWIP steel offers an extraordinary opportunity to adjust the mechanical properties of steel by modifying the strain hardening. The use of TWIP steel may therefore lead to a considerable lightweighting of steel components, a reduction of material use and an improved press forming behaviour. These key advantages will help implement current automotive vehicle design trends which emphasise a reduction of greenhouse gas emissions and lowering of fuel consumption. In addition, high strength TWIP steel will effectively contribute to weight containment in vehicles equipped with hybrid and electric motors, as these are considerably heavier than conventional motors. The present review addresses all aspects of the physical metallurgy of the high strength TWIP steel with a special emphasis on the properties and key advantages of TWIP sheet steel products relevant to automotive applications.     

Anisotropic thermal expansion behaviors of copper matrix in β-eucryptite/copper composite

A β-eucryptite/copper composite was fabricated by spark plasma sintering process. The thermal expansion behaviors of Cu matrix of the composite were studied by in situ X-ray diffraction during heating process. The results show that Cu matrix exhibits anisotropic thermal expansion behaviors for different crystallographic directions, the expansion of Cu{1 1 1} plane is linear in the temperature range from 20 °C to 300 °C and the expansion of Cu{2 0 0} is nonlinear with a inflection at about 180 °C. The microstructures of Cu matrix before and after thermal expansion testing were investigated using transmission electronic microscope. The anisotropic thermal expansion behavior is related to the deformation twinning formed in the matrix during heating process. At the same time, the deformation twinning of Cu matrix makes the average coefficient of thermal expansion of the composite increase.     

A Method to Distinguish Twinning Variants and Second Twins by EBSD Measurements

This paper proposes a direct and simple method to identify the second twins from twinning variants by electron back scattered diffraction （EBSD）. To clarify the orientation relationship between the neighboring crystals, the misorientation calculation by MATLAB software based on the EBSD data is also used. This method is generally applicable to predict the variants occurring in the nucleation and growth of the recrystallization or phase transformation process.     

Continuation of twins across grain boundary in extruded Mg–3Al–1Zn alloy

Abstract

Paired twins in an extruded Mg–3Al–1Zn alloy are investigated by using electron backscattered diffraction in the current paper. The results show that these paired twins are discovered at low misorientation grain boundaries. The twin variant (1–102)[?1101] is operated in the paired twins. Additionally, a macroscopic angle exists in the paired twins and is determined by the c axis misorientation of the grains.     

On dynamic deformation behavior of WE43 magnesium alloy sheet under shock loading conditions

In the present study, the texture evolution, microstructure and mechanical behavior of WE43 magnesium sheet at high strain rates are investigated. Samples cut along the rolling direction (RD), 45° from the RD, transverse direction (TD) and perpendicular to the RD-TD plane were tested at strain rates of 800, 1200 and 1400 s⁻¹ using Split Hopkinson Pressure Bar. It is observed that after shock loading, the initial weak texture converts to a weak (00.2) basal texture in all samples. Besides, it is found that the strength and ductility increase and twinning fraction decreases with increase in strain rate. Moreover, another effect of increase in strain rate is found to be the higher activation of pyramidal 〈c + a〉 slip systems. In addition, degree of stress and strain anisotropy is low particularly at higher strain rates, which is mainly related to the weak initial texture of the samples. A viscoplastic self-consistent model with a tangent approach is used to analyze the deformation mechanism during shock loading.     

Effect of strain rate on microstructure evolution of a nickel-based superalloy during hot deformation

The hot deformation behavior of a nickel-based superalloy was investigated by means of isothermal compression tests in the strain rate range of 0.001–10 s⁻¹ at 1110 °C. Transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) technique were used to study the effect of strain rate on the microstructure evolution of the alloy during hot deformation. The results revealed that the dynamic recrystallization (DRX) process was stimulated at high strain rates ($\dot{\epsilon }⩾5{\text{s}}^{-1}$) due to the high dislocation density and adiabatic temperature rise. Meanwhile, high nucleation of DRX and low grain growth led to the fine DRX grains. In the strain rate rage of 0.001–1 s⁻¹, the volume fraction of DRX grains increased with the decreasing strain rate, and the grain growth gradually governed the DRX process. Moreover, the strain rate has an important effect on DDRX and CDRX during hot deformation. On the other hand, particular attention was also paid to the evolution of twin boundaries during hot deformation. It was found that there was a lower fraction of Σ3 boundaries at the intermediate strain rate of 1 s⁻¹, while the fractions of Σ3 boundaries were much higher at both the lower strain rates ($\dot{\epsilon }⩽0.1{\text{s}}^{-1}$) and higher strain rates ($\dot{\epsilon }⩾5{\text{s}}^{-1}$).