Jinxu Liu

Dynamic Recrystallization in the Shear Bands of Tungsten Heavy Alloy Processed by Hot-Hydrostatic Extrusion and Hot Torsion

Abstract Hot-hydrostatic extrusion and hot torsion (HE+HT) were applied to the processing of the as-sintered tungsten heavy alloy (WHA). The dynamic mechanical properties of the WHA processed by HE+HT and the microstructural evolution within adiabatic shear band (ASB) were systematically investigated. The results show that the WHA susceptibility to ASB is much improved, and localized shear bands can be formed in specimens. TEM analysis of ASB shows that the multiplication and rearrangement of dislocation play dominate roles in the dynamic recrystallization (DRX) process within ASB of WHA, and the nano-scaled equiaxed-recrystallized grains within ASB can be formed via repeating the processes including dislocation multiplication, dislocation rearrangement and dislocation annihilation. The investigation result of observed DRX of the WHA firmly supports the rotational dynamic recrystallization (RDR) mechanism in metals.

Adiabatic shear banding of hot-rolling Ti–6Al–4V alloy subjected to dynamic shearing and uniaxial dynamic compression

Effect of stress state including dynamic shearing and uniaxial dynamic compression on adiabatic shear banding (ASBing) of hot-rolling Ti–6Al–4V (TC4) alloy was investigated. The absorbed energy of specimen before failure was calculated to evaluate the susceptibility to adiabatic shear band (ASB) of TC4 alloy quantitatively. Results show that the susceptibility to ASB of hot-rolling TC4 alloy exhibits obvious anisotropy under both dynamic shearing and uniaxial dynamic compression conditions, but the anisotropy of susceptibility to ASB under dynamic shearing condition exhibits an opposite tendency with that under uniaxial dynamic compression condition. Under the condition of uniaxial dynamic compression, material shows the highest susceptibility to ASB when loaded along transverse direction (TD) of the hot-rolling TC4, while the lowest susceptibility when loaded along rolling direction (RD). However, under the condition of dynamic shearing, the material behaves in the opposite way, demonstrating the lowest susceptibility when loaded along TD of the hot-rolling TC4, while the highest susceptibility when loaded along RD.

Adiabatic shear banding of hot-extruded tungsten heavy alloy under cryogenic temperature

The effect of cryogenic temperature on adiabatic shear banding (ASBing) of tungsten heavy alloy (WHA) processed by hot-hydrostatic extrusion was investigated. Results show that, when the initial temperature is decreased, the dynamic flow stress, the critical failure time, and the dynamic failure energy of specimens show an increasing tendency, while the susceptibility to ASB of WHA shows a decreasing tendency, which is characterized by decreased shear strain and increased width of shear bands. Microstructure analysis shows that the number of microcracks within ASB exhibits an increasing tendency with decreased initial temperature, and the dynamic recrystallization (DRX) process within ASB is evidently suppressed at the lower temperature. As a result of the lower temperature, the motion and rearrangement of dislocation are effectively suppressed, which is mainly responsible for the incomplete DRX process within ASB and decreases susceptibility to ASB.

Bending mechanical property and failure mechanisms of woven carbon fiber-reinforced aluminum alloy composite

Copper-coated woven carbon fiber-reinforced aluminum alloy composite was prepared by spark plasma sintering (SPS). Microstructure, three-point bending mechanical property, and the failure mechanisms of the composite were investigated. Microstructure observation shows that the carbon fibers bond compactly with matrix alloy. Compared with the matrix aluminum alloy, the bending strength, ductility, fracture energy, and cracking resistance of the composite are evidently improved. Microstructure analyses reveal that the high specific strength of carbon fibers and transfer of stress from matrix alloy to carbon fibers are responsible for the increase of the composite bending strength. The expanding of cracks is restrained, and cracking resistance of the composite is improved by adding woven carbon fiber. Attributed to the carbon fibers’ debonding, cracks deflection, and multipath propagation mechanisms, the fracture energy of the composite increases.

Dynamic behaviors of fiber reinforced aerogel and Mg/aerogel composites

The dynamic behaviors of glass fiber reinforced silica aerogel and the Mg/aerogel composites are experimentally investigated using spilt Hopkinson pressure bars. For the purpose of comparison,dynamic responses of hydrogel are also investigated. Incident wave shaping experiments are designed to investigate the shaping effects of aerogel and Mg/aerogel structures. Results show that the fiber reinforced aerogel exhibits significant strain rate strengthening and strain hardening behavior. The strength of the investigated aerogel exhibits evidently size effect: strength of aerogel shows an increasing tendency with the size of sample due to the special nano-porous network structure of aerogel. The incident wave shaped by the composite structure of Mg/aerogel is obviously weaker than that of single magnesium alloy or aerogel, indicating a better protective capacity of Mg/aerogel composite structures.

Strain rate-dependent and temperature- dependent compressive properties of 2DCf/SiC Composite

Effects of strain rate and temperature on dynamic behaviors of 2DCf/SiC composite were investigated by improved Split Hopkinson Pressure Bars (SHPB) using pulse shaper. Results show that the shape of incident wave changes from rectangle to triangle after using pulse shaper in SHPB testing. The dynamic compressive strength of the composite increases with strain rate increasing from 500s–1 to 1700s–1, while shows decreasing tendency with strain rate rising from 1700s–1 to 2750s–1, indicating a maximum dynamic strength at strain rate of about 1700s–1. The 2DCf/SiC composite exhibits higher strength and better ductility at elevated temperature in the range of 460°C~500°C compared with that at room temperature.

Pulse-series 1.57 μm optical parametric oscillator pumped by a Q-switched Nd:YAG laser with a variable reflectivity mirror

A method of generating 1.57 μm pulse-series eye-safe laser based on optical parametric oscillator (OPO) was developed. The 1.57 μm pulse-series OPO was pumped by a Q-switched Nd:YAG laser with a variable reflectivity unstable resonator. Every pulse-series includes three micro-pulses, and the pulse repetition rates, pulse numbers and pulse intervals were adjustable. The maximum output energy of the 3-micro-pulse-series was 274 mJ. The optical-optical efficiency of the OPO was about 40%.