Chengwen Tan

Plastic Deformation Mechanisms of AZ31 Magnesium Alloy under High Strain Rate Compression

The twinning and slip activities of AZ31 magnesium alloy sheet at a strain rate of 1200 s^(-1) were investigated. Dynamically mechanical properties of various oriented samples were measured using Split Hopkinson Pressure Bar (SHPB). Optical microscope observations reveal that the dominant deformation mechanism is twinning for 90°oriented sample, and is slip for 45° and 0° oriented samples. TEM analysis for samples at a strain of 0.3% shows that the main deformation mechanisms for 90°, 45° and 0° oriented sample are {10(average)12} ＜10(average)11＞ and {10(average)11}＜10(average)12＞ twinning, basal slip and non basal slip, respectively. The main features of the true stress-true strain curves can be explained based on deformation mechanism analysis.

Microstructure and mechanical properties of Mg-Gd-Y-Zr alloy processed by equal channel angular pressing

Abstract Microstructure evolution and texture development and their effects on mechanical properties of a Mg-Gd-Y-Zr alloy during equal channel angular pressing (ECAP) were investigated. It is found that the microstructure is still inhomogeneous after four passes, and two zones, namely the fine grain zone (FGZ) and the coarse grain zone (CGZ) are formed. The grain refinement occurs mainly by particle-stimulated nucleation (PSN) mechanism, which led to a more random texture after four passes of ECAP. In the ECAP-processed alloy, the strength did not increase while the ductility was enhanced dramatically compared with the as-received condition. The change of ductility of this alloy was discussed in terms of texture and second phase particles.

Dynamic fracture of Ti-6Al-4V alloy in Taylor impact test

Abstract The dynamic fracture behaviors of Ti-6Al-4V alloy at high strain rate loading were investigated systemically through Taylor impact test, over the range of impact velocities from 145 m/s to 306 m/s. The critical impact velocity of fracture ranges from 217 m/s to 236 m/s. Smooth surfaces and ductile dimple areas were observed on the fracture surfaces. As the impact velocity reached 260 m/s, the serious melting regions were also observed on the fracture surfaces. Self-organization of cracks emerges when the impact velocity reaches 260 m/s, while some special cracks whose “tips” are not sharp but arc and smooth, and without any evidence of deformation or adiabatic shear band were also observed on the impact end surfaces. Examination of the sections of these special cracks reveals that the cracks expand along the two maximum shear stress directions respectively, and finally intersect as a tridimensional “stagger ridge” structure.

Adiabatic shear fracture in Ti-6Al-4V alloy

Abstract Separated specimens of Ti-6Al-4V alloy were dynamically loaded at a strain rate of 3 900 s −1 using a split Hopkinson pressure bar (SHPB) apparatus. The fracture features of the separated specimens were investigated by a scanning electron microscope. The results show that adiabatic shear failure occurs in the tested specimens, and two typical areas (dimple and smooth areas) with different features are alternatively distributed on the whole fracture surface. The dimple areas originate from voids generation and coalescence, exhibiting ductile fracture characteristics. Simultaneously, ultrafine grains (UFGs) and microcracks among grains are observed on the smooth areas, indicating that the emergence of UFG areas is caused by the propagation of microcracks along grain boundaries and exhibits brittle fracture characteristics. Fracture occurring in adiabatic shear bands is not uniform and ultimate rupture is resulted from ductile and brittle fracture modes.

Quasi-static and dynamic tensile behaviors in electron beam welded Ti-6Al-4V alloy

Abstract The quasi-static and dynamic tensile behaviors in electron beam welded (EBW) Ti-6Al-4V alloy were investigated at strain rates of 10−3 and 103 s−1, respectively, by materials test system (MTS) and reconstructive Hopkinson bars apparatus. The microstructures of the base metal (BM) and the welded metal (WM) were observed with optical microscope. The fracture characteristics of the BM and WM were characterized with scanning electronic microscope. In Ti-6Al-4V alloy joint, the flow stress of WM is higher than that of BM, while the fracture strain of WM is less than that of BM at strain rates of 103 and 10−3 s−1, respectively. The fracture strain of WM has apparent improvement when the strain rate rises from 10−3 to 103 s−1, while the fracture strain of BM almost has no change. At the same time, the fracture mode of WM alters from brittle to ductile fracture, which causes improvement of the fracture strain of WM.

Shock-Induced Mechanical Response and Substructural Evolution of Ti–6Al–4V Alloy

The effects of shock stress amplitude on the post-shock mechanical response and substructural evolution of Ti–6Al–4V alloy are investigated within the impact stress range of 6–10 GPa. The reload yield behavior of post-shock Ti–6Al–4V does not exhibit enhanced shock-induced strengthening at an effective strain level even if the shock stress achieves 10 GPa. The residual substructures of post-shock Ti–6Al–4V are examined by transmission electron microscopy. Results reveal that planar slip is the dominant deformation mechanism of this alloy during shock loading pulse. Dislocations tangle and form developed dislocation clusters (planar slip bands) with increased impact stress. The lack of dislocation cells or cell-like structures, high-density twins and additional strengthening phases limits the shock-induced strengthening effect in post-shock materials. However, dislocation multiplication and tangles lead to increased yield strength and strain hardening rate of reloaded materials.

Simulation analysis for the safety protection of cervical vertebra under unusual landing impact

The objective of this study was to analyse and evaluate the protection function and feasibility of two types of human neck safety equipment under unusual landings. Human head and neck dynamic responses were simulated by constructing a mechanical model of the human-seat system, which was equipped with two kinds of head support systems. The force and moment in the human neck were calculated. The protection function of the head support systems was evaluated according to the injury criteria HIC (head injury criterion) and Nij (neck injury criterion). Two kinds of safety equipment were used under unusual landing. The results of the experiment showed that Nij is smaller than the injury criterion limits under normal landings, and that the safety equipment provided effective protection. The safety equipment was confirmed to reduce the degree of neck injuries resulting from unusual landing impacts.

Microstructure and properties of atmospheric pressure chemical vapor deposition of tungsten carbide

Tungsten carbide / tungsten coating is prepared from tungsten hexafluoride (WF6), hydrogen (H2) and dimethyl ether (DME) mixtures by chemical vapor deposition (CVD) under atmospheric pressure conditions. The cross-sectional structure and the surface morphology of the coating were observed using optical microscope and scanning electron microscopy. It is found that the tungsten layer is columnar and tungsten carbide / tungsten coating is fine grain lamellar structure. The preferential growth of tungsten was proposed to be effect of DME, and the {100} preferred orientation gradually disappeared by XRD. That the pre-deposition of tungsten on the substrate can improve the coverage of the tungsten carbide coating was observed. Finally, the cross section of the micrograph was measured at 28.77 GPa with a nano-indentation.

Porous tungsten prepared by atmospheric-pressure chemical vapor deposition with WF6 and its characterization

Porous tungsten (W) is used in aeronautic and aerospace engineering, power electronics field and metallurgical industry. In this study, porous W with 98wt% W was prepared on a carbon foam substrate by atmospheric-pressure chemical vapor deposition (CVD) with tungsten fluoride (WF6) as the precursor. The porous W with 78.1346% porosity displayed a pure α-W phase and the uniform surface. The mode pore diameter of porous W is 208.0 µm. In a compression test, the fracture strength of porous W is 20.3 MPa.