Low cycle fatigue of an extruded Mg–3Nd–0.2Zn–0.5Zr magnesium alloy

The purpose of this study was to evaluate strain-controlled cyclic deformation behavior of an extruded Mg–3Nd–0.2Zn–0.5Zr (NZ30K) magnesium alloy. The microstructure of this alloy consisted of a bimodal microstructure with equiaxed recrystallized grains and unrecrystallized coarse grains along with a large number of smaller second-phase particles present inside the grains and larger particles along the grain boundaries alongside a characteristic precipitate free zone (PFZ). The average grain size was about approximately 5–7 μm. It was observed that unlike the higher RE-containing Mg–10Gd–3Y–0.5Zr (GW103K) magnesium alloy, the NZ30K alloy exhibited asymmetrical hysteresis loops in tension and compression in the fully reversed strain-control tests at a strain ratio of R_ε = −1. This was mainly due to the presence of relatively stronger crystallographic texture, PFZ, and the resultant twinning–detwinning activities during cyclic deformation. While this alloy exhibited cyclic softening at lower strain amplitudes and cyclic hardening at higher strain amplitudes, it had an equivalent fatigue life to that of other extruded Mg alloys. Fatigue crack was observed to initiate from the specimen surface with some isolated facets of the cleavage-like planes near the initiation site. Crack propagation was basically characterized by serrated fatigue striations.     

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.     

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.     

Work-hardening and plastic deformation behavior of Ti-based bulk metallic glass composites with bimodal sized B2 particles

In this work, the role of individual B2 particles with bimodal length scale on work-hardening and plastic deformation behaviors of Ti-based bulk metallic glass composites has been studied by systemic microstructural and mechanical investigations. At the early stage of plastic deformation, work-hardening characteristic was clearly observed. This work-hardening behavior can be supported by the martensitic transformation and the deformation induced twinning in both small- and large-sized B2 particles during deformation. On progress of plastic deformation after work-hardening, small-sized B2 particles (1–10 μm) were penetrated by propagation of main shear bands while large-sized B2 particles (100–200 μm) were severely interacted with shear bands leading to formation of multiple shear bands and impeding the propagation of principal shear bands. This reveals that each B2 particle with different length scale plays a distinct role on the stage of plastic deformation depending on the particle size.     

Temperature Effects on the Microstructures of Mg-Gd-Y Alloy Processed by Multi-direction Impact Forging

A high strain rate multi-directional impact forging(MDIF) was applied to a solutionized Mg-Gd-Y-Zr alloy in the temperature range of 350-500℃.Results demonstrate that the dominant deformation mode is twinning at a temperature below 400℃,whereas at a medium temperature of 450℃ considerable continuous dynamic recrystallization was promoted by{10-12} extension twins.At a higher temperature of 500℃,twinning activation was suppressed.New DRX grains were observed but their sizes were much bigger than those resulting from the MDIFed 50 passes at 450℃,which are ascribed to the larger grain boundary mobility and atomic diffusion at 500℃.Moreover,a non-basal weak texture was gained afterward MDIF at each temperature,which is credited to the MDIF process and the minor strain applied in each pass.     

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.     

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.     

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.